Movatterモバイル変換

Network Working Group                                   F. Baker, EditorRequest for Comments: 1812                                 Cisco SystemsObsoletes:1716,1009                                          June 1995Category: Standards TrackRequirements for IP Version 4 RoutersStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.PREFACE   This document is an updated version ofRFC 1716, the historical   Router Requirements document.  That RFC preserved the significant   work that went into the working group, but failed to adequately   describe current technology for the IESG to consider it a current   standard.   The current editor had been asked to bring the document up to date,   so that it is useful as a procurement specification and a guide to   implementors.  In this, he stands squarely on the shoulders of those   who have gone before him, and depends largely on expert contributors   for text.  Any credit is theirs; the errors are his.   The content and form of this document are due, in large part, to the   working group's chair, and document's original editor and author:   Philip Almquist.  It is also largely due to the efforts of its   previous editor, Frank Kastenholz.  Without their efforts, this   document would not exist.Table of Contents1. INTRODUCTION ........................................61.1 Reading this Document ..............................81.1.1 Organization .....................................81.1.2 Requirements .....................................91.1.3 Compliance .......................................101.2 Relationships to Other Standards ...................111.3 General Considerations .............................121.3.1 Continuing Internet Evolution ....................121.3.2 Robustness Principle .............................131.3.3 Error Logging ....................................14Baker                       Standards Track                     [Page 1]

RFC 1812         Requirements for IP Version 4 Routers         June 19951.3.4 Configuration ....................................141.4 Algorithms .........................................162. INTERNET ARCHITECTURE ...............................162.1 Introduction .......................................162.2 Elements of the Architecture .......................172.2.1 Protocol Layering ................................172.2.2 Networks .........................................192.2.3 Routers ..........................................202.2.4 Autonomous Systems ...............................212.2.5 Addressing Architecture ..........................212.2.5.1 Classical IP Addressing Architecture ...........212.2.5.2 Classless Inter Domain Routing (CIDR) ..........232.2.6 IP Multicasting ..................................242.2.7 Unnumbered Lines and Networks Prefixes ...........252.2.8 Notable Oddities .................................262.2.8.1 Embedded Routers ...............................262.2.8.2 Transparent Routers ............................272.3 Router Characteristics .............................282.4 Architectural Assumptions ..........................313. LINK LAYER ..........................................323.1 INTRODUCTION .......................................323.2 LINK/INTERNET LAYER INTERFACE ......................333.3 SPECIFIC ISSUES ....................................343.3.1 Trailer Encapsulation ............................343.3.2 Address Resolution Protocol - ARP ................343.3.3 Ethernet and 802.3 Coexistence ...................353.3.4 Maximum Transmission Unit - MTU ..................353.3.5 Point-to-Point Protocol - PPP ....................353.3.5.1 Introduction ...................................363.3.5.2 Link Control Protocol (LCP) Options ............363.3.5.3 IP Control Protocol (IPCP) Options .............383.3.6 Interface Testing ................................384. INTERNET LAYER - PROTOCOLS ..........................394.1 INTRODUCTION .......................................394.2 INTERNET PROTOCOL - IP .............................394.2.1 INTRODUCTION .....................................394.2.2 PROTOCOL WALK-THROUGH ............................404.2.2.1 Options:RFC 791 Section 3.2 ...................404.2.2.2 Addresses in Options:RFC 791 Section 3.1 ......424.2.2.3 Unused IP Header Bits:RFC 791 Section 3.1 .....434.2.2.4 Type of Service:RFC 791 Section 3.1 ...........444.2.2.5 Header Checksum:RFC 791 Section 3.1 ...........44   4.2.2.6 Unrecognized Header Options:RFC 791,           Section 3.1 ....................................444.2.2.7 Fragmentation:RFC 791 Section 3.2 .............454.2.2.8 Reassembly:RFC 791 Section 3.2 ................464.2.2.9 Time to Live:RFC 791 Section 3.2 ..............464.2.2.10 Multi-subnet Broadcasts:RFC 922 ..............47Baker                       Standards Track                     [Page 2]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.2.11 Addressing:RFC 791 Section 3.2 ...............474.2.3 SPECIFIC ISSUES ..................................504.2.3.1 IP Broadcast Addresses .........................504.2.3.2 IP Multicasting ................................504.2.3.3 Path MTU Discovery .............................514.2.3.4 Subnetting .....................................514.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ...........524.3.1 INTRODUCTION .....................................524.3.2 GENERAL ISSUES ...................................534.3.2.1 Unknown Message Types ..........................534.3.2.2 ICMP Message TTL ...............................534.3.2.3 Original Message Header ........................534.3.2.4 ICMP Message Source Address ....................534.3.2.5 TOS and Precedence .............................544.3.2.6 Source Route ...................................544.3.2.7 When Not to Send ICMP Errors ...................554.3.2.8 Rate Limiting ..................................564.3.3 SPECIFIC ISSUES ..................................564.3.3.1 Destination Unreachable ........................564.3.3.2 Redirect .......................................574.3.3.3 Source Quench ..................................574.3.3.4 Time Exceeded ..................................584.3.3.5 Parameter Problem ..............................584.3.3.6 Echo Request/Reply .............................584.3.3.7 Information Request/Reply ......................594.3.3.8 Timestamp and Timestamp Reply ..................594.3.3.9 Address Mask Request/Reply .....................614.3.3.10 Router Advertisement and Solicitations ........624.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ..........625. INTERNET LAYER - FORWARDING .........................635.1 INTRODUCTION .......................................635.2 FORWARDING WALK-THROUGH ............................635.2.1 Forwarding Algorithm .............................635.2.1.1 General ........................................645.2.1.2 Unicast ........................................645.2.1.3 Multicast ......................................655.2.2 IP Header Validation .............................675.2.3 Local Delivery Decision ..........................695.2.4 Determining the Next Hop Address .................715.2.4.1 IP Destination Address .........................725.2.4.2 Local/Remote Decision ..........................725.2.4.3 Next Hop Address ...............................745.2.4.4 Administrative Preference ......................775.2.4.5 Load Splitting .................................795.2.5 Unused IP Header Bits:RFC-791 Section 3.1 .......79   5.2.6 Fragmentation and Reassembly:RFC-791,         Section 3.2 ......................................805.2.7 Internet Control Message Protocol - ICMP .........80Baker                       Standards Track                     [Page 3]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.2.7.1 Destination Unreachable ........................805.2.7.2 Redirect .......................................825.2.7.3 Time Exceeded ..................................845.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........845.3 SPECIFIC ISSUES ....................................855.3.1 Time to Live (TTL) ...............................855.3.2 Type of Service (TOS) ............................865.3.3 IP Precedence ....................................875.3.3.1 Precedence-Ordered Queue Service ...............885.3.3.2 Lower Layer Precedence Mappings ................895.3.3.3 Precedence Handling For All Routers ............905.3.4 Forwarding of Link Layer Broadcasts ..............925.3.5 Forwarding of Internet Layer Broadcasts ..........925.3.5.1 Limited Broadcasts .............................935.3.5.2 Directed Broadcasts ............................935.3.5.3 All-subnets-directed Broadcasts ................945.3.5.4  Subnet-directed Broadcasts ....................945.3.6 Congestion Control ...............................945.3.7 Martian Address Filtering ........................965.3.8 Source Address Validation ........................975.3.9 Packet Filtering and Access Lists ................975.3.10 Multicast Routing ...............................985.3.11 Controls on Forwarding ..........................985.3.12 State Changes ...................................995.3.12.1 When a Router Ceases Forwarding ...............995.3.12.2 When a Router Starts Forwarding ...............1005.3.12.3 When an Interface Fails or is Disabled ........1005.3.12.4 When an Interface is Enabled ..................1005.3.13 IP Options ......................................1015.3.13.1 Unrecognized Options ..........................1015.3.13.2 Security Option ...............................1015.3.13.3 Stream Identifier Option ......................1015.3.13.4 Source Route Options ..........................1015.3.13.5 Record Route Option ...........................1025.3.13.6 Timestamp Option ..............................1026. TRANSPORT LAYER .....................................1036.1 USER DATAGRAM PROTOCOL - UDP .......................1036.2 TRANSMISSION CONTROL PROTOCOL - TCP ................1047. APPLICATION LAYER - ROUTING PROTOCOLS ...............1067.1 INTRODUCTION .......................................1067.1.1 Routing Security Considerations ..................1067.1.2 Precedence .......................................1077.1.3 Message Validation ...............................1077.2 INTERIOR GATEWAY PROTOCOLS .........................1077.2.1 INTRODUCTION .....................................1077.2.2 OPEN SHORTEST PATH FIRST - OSPF ..................108   7.2.3 INTERMEDIATE SYSTEM TO  INTERMEDIATE  SYSTEM  -         DUAL IS-IS .......................................108Baker                       Standards Track                     [Page 4]

RFC 1812         Requirements for IP Version 4 Routers         June 19957.3  EXTERIOR GATEWAY PROTOCOLS ........................1097.3.1  INTRODUCTION ....................................1097.3.2 BORDER GATEWAY PROTOCOL - BGP ....................1097.3.2.1 Introduction ...................................1097.3.2.2 Protocol Walk-through ..........................1107.3.3 INTER-AS ROUTING WITHOUT AN  EXTERIOR  PROTOCOL         ..................................................1107.4 STATIC ROUTING .....................................1117.5 FILTERING OF ROUTING INFORMATION ...................1127.5.1 Route Validation .................................1137.5.2 Basic Route Filtering ............................1137.5.3 Advanced Route Filtering .........................1147.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........1148. APPLICATION LAYER - NETWORK  MANAGEMENT  PROTOCOLS      .....................................................1158.1 The Simple Network Management Protocol - SNMP ......1158.1.1 SNMP Protocol Elements ...........................1158.2 Community Table ....................................1168.3 Standard MIBS ......................................1188.4 Vendor Specific MIBS ...............................1198.5 Saving Changes .....................................1209. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS .........1209.1 BOOTP ..............................................1209.1.1 Introduction .....................................1209.1.2 BOOTP Relay Agents ...............................12110. OPERATIONS AND MAINTENANCE .........................12210.1 Introduction ......................................12210.2 Router Initialization .............................12310.2.1 Minimum Router Configuration ....................12310.2.2 Address and Prefix Initialization ...............12410.2.3 Network Booting using BOOTP and TFTP ............12510.3 Operation and Maintenance .........................12610.3.1 Introduction ....................................12610.3.2 Out Of Band Access ..............................12710.3.2 Router O&M Functions ............................12710.3.2.1 Maintenance - Hardware Diagnosis ..............12710.3.2.2 Control - Dumping and Rebooting ...............12710.3.2.3 Control - Configuring the Router ..............12810.3.2.4 Net Booting of System Software ................12810.3.2.5 Detecting and responding to misconfiguration            ...............................................12910.3.2.6 Minimizing Disruption .........................13010.3.2.7 Control - Troubleshooting Problems ............13010.4 Security Considerations ...........................13110.4.1 Auditing and Audit Trails .......................13110.4.2 Configuration Control ...........................13211. REFERENCES .........................................133   APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ......145Baker                       Standards Track                     [Page 5]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   APPENDIX B. GLOSSARY ...................................146   APPENDIX C. FUTURE DIRECTIONS ..........................152   APPENDIX D. Multicast Routing Protocols ................154D.1 Introduction .......................................154   D.2 Distance  Vector  Multicast  Routing  Protocol  -       DVMRP ..............................................154D.3 Multicast Extensions to OSPF - MOSPF ...............154D.4 Protocol Independent Multicast - PIM ...............155   APPENDIX E Additional Next-Hop  Selection  Algorithms        ...................................................155E.1. Some Historical Perspective .......................155E.2. Additional Pruning Rules ..........................157E.3 Some Route Lookup Algorithms .......................159E.3.1 The Revised Classic Algorithm ....................159E.3.2 The Variant Router Requirements Algorithm ........160E.3.3 The OSPF Algorithm ...............................160E.3.4 The Integrated IS-IS Algorithm ...................162   Security Considerations ................................163   APPENDIX F: HISTORICAL ROUTING PROTOCOLS ...............164F.1 EXTERIOR GATEWAY PROTOCOL - EGP ....................164F.1.1 Introduction .....................................164F.1.2 Protocol Walk-through ............................165F.2 ROUTING INFORMATION PROTOCOL - RIP .................167F.2.1 Introduction .....................................167F.2.2 Protocol Walk-Through ............................167F.2.3 Specific Issues ..................................172F.3 GATEWAY TO GATEWAY PROTOCOL - GGP ..................173   Acknowledgments ........................................173   Editor's Address .......................................1751. INTRODUCTION  This memo replaces forRFC 1716, "Requirements for Internet Gateways"  ([INTRO:1]).  This memo defines and discusses requirements for devices that perform  the network layer forwarding function of the Internet protocol suite.  The Internet community usually refers to such devices as IP routers or  simply routers; The OSI community refers to such devices as  intermediate systems.  Many older Internet documents refer to these  devices as gateways, a name which more recently has largely passed out  of favor to avoid confusion with application gateways.  An IP router can be distinguished from other sorts of packet switching  devices in that a router examines the IP protocol header as part of  the switching process.  It generally removes the Link Layer header a  message was received with, modifies the IP header, and replaces the  Link Layer header for retransmission.Baker                       Standards Track                     [Page 6]

RFC 1812         Requirements for IP Version 4 Routers         June 1995  The authors of this memo recognize, as should its readers, that many  routers support more than one protocol.  Support for multiple protocol  suites will be required in increasingly large parts of the Internet in  the future.  This memo, however, does not attempt to specify Internet  requirements for protocol suites other than TCP/IP.  This document enumerates standard protocols that a router connected to  the Internet must use, and it incorporates by reference the RFCs and  other documents describing the current specifications for these  protocols.  It corrects errors in the referenced documents and adds  additional discussion and guidance for an implementor.  For each protocol, this memo also contains an explicit set of  requirements, recommendations, and options.  The reader must  understand that the list of requirements in this memo is incomplete by  itself.  The complete set of requirements for an Internet protocol  router is primarily defined in the standard protocol specification  documents, with the corrections, amendments, and supplements contained  in this memo.  This memo should be read in conjunction with the Requirements for  Internet Hosts RFCs ([INTRO:2] and [INTRO:3]).  Internet hosts and  routers must both be capable of originating IP datagrams and receiving  IP datagrams destined for them.  The major distinction between  Internet hosts and routers is that routers implement forwarding  algorithms, while Internet hosts do not require forwarding  capabilities.  Any Internet host acting as a router must adhere to the  requirements contained in this memo.  The goal of open system interconnection dictates that routers must  function correctly as Internet hosts when necessary.  To achieve this,  this memo provides guidelines for such instances.  For simplification  and ease of document updates, this memo tries to avoid overlapping  discussions of host requirements with [INTRO:2] and [INTRO:3] and  incorporates the relevant requirements of those documents by  reference.  In some cases the requirements stated in [INTRO:2] and  [INTRO:3] are superseded by this document.  A good-faith implementation of the protocols produced after careful  reading of the RFCs should differ from the requirements of this memo  in only minor ways.  Producing such an implementation often requires  some interaction with the Internet technical community, and must  follow good communications software engineering practices.  In many  cases, the requirements in this document are already stated or implied  in the standard protocol documents, so that their inclusion here is,  in a sense, redundant.  They were included because some past  implementation has made the wrong choice, causing problems of  interoperability, performance, and/or robustness.Baker                       Standards Track                     [Page 7]

RFC 1812         Requirements for IP Version 4 Routers         June 1995  This memo includes discussion and explanation of many of the  requirements and recommendations.  A simple list of requirements would  be dangerous, because:  o Some required features are more important than others, and some     features are optional.  o Some features are critical in some applications of routers but     irrelevant in others.  o There may be valid reasons why particular vendor products that are     designed for restricted contexts might choose to use different     specifications.  However, the specifications of this memo must be followed to meet the  general goal of arbitrary router interoperation across the diversity  and complexity of the Internet.  Although most current implementations  fail to meet these requirements in various ways, some minor and some  major, this specification is the ideal towards which we need to move.  These requirements are based on the current level of Internet  architecture.  This memo will be updated as required to provide  additional clarifications or to include additional information in  those areas in which specifications are still evolving.1.1 Reading this Document1.1.1 Organization  This memo emulates the layered organization used by [INTRO:2] and  [INTRO:3].  Thus, Chapter 2 describes the layers found in the Internet  architecture.  Chapter 3 covers the Link Layer.  Chapters 4 and 5 are  concerned with the Internet Layer protocols and forwarding algorithms.  Chapter 6 covers the Transport Layer.  Upper layer protocols are  divided among Chapters 7, 8, and 9.  Chapter 7 discusses the protocols  which routers use to exchange routing information with each other.  Chapter 8 discusses network management.  Chapter 9 discusses other  upper layer protocols.  The final chapter covers operations and  maintenance features.  This organization was chosen for simplicity,  clarity, and consistency with the Host Requirements RFCs.  Appendices  to this memo include a bibliography, a glossary, and some conjectures  about future directions of router standards.  In describing the requirements, we assume that an implementation  strictly mirrors the layering of the protocols.  However, strict  layering is an imperfect model, both for the protocol suite and for  recommended implementation approaches.  Protocols in different layers  interact in complex and sometimes subtle ways, and particularBaker                       Standards Track                     [Page 8]

RFC 1812         Requirements for IP Version 4 Routers         June 1995  functions often involve multiple layers.  There are many design  choices in an implementation, many of which involve creative breaking  of strict layering.  Every implementor is urged to read [INTRO:4] and  [INTRO:5].  Each major section of this memo is organized into the following  subsections:  (1) Introduction  (2) Protocol Walk-Through - considers the protocol specification       documents section-by-section, correcting errors, stating       requirements that may be ambiguous or ill-defined, and providing       further clarification or explanation.  (3) Specific Issues - discusses protocol design and implementation       issues that were not included in the walk-through.  Under many of the individual topics in this memo, there is  parenthetical material labeled DISCUSSION or IMPLEMENTATION.  This  material is intended to give a justification, clarification or  explanation to the preceding requirements text.  The implementation  material contains suggested approaches that an implementor may want to  consider.  The DISCUSSION and IMPLEMENTATION sections are not part of  the standard.1.1.2 Requirements  In this memo, the words that are used to define the significance of  each particular requirement are capitalized.  These words are:  o MUST     This word means that the item is an absolute requirement of the     specification.  Violation of such a requirement is a fundamental     error; there is no case where it is justified.  o MUST IMPLEMENT     This phrase means that this specification requires that the item be     implemented, but does not require that it be enabled by default.  o MUST NOT     This phrase means that the item is an absolute prohibition of the     specification.  o SHOULD     This word means that there may exist valid reasons in particular     circumstances to ignore this item, but the full implications should     be understood and the case carefully weighed before choosing aBaker                       Standards Track                     [Page 9]

RFC 1812         Requirements for IP Version 4 Routers         June 1995     different course.  o SHOULD IMPLEMENT     This phrase is similar in meaning to SHOULD, but is used when we     recommend that a particular feature be provided but does not     necessarily recommend that it be enabled by default.  o SHOULD NOT     This phrase means that there may exist valid reasons in particular     circumstances when the described behavior is acceptable or even     useful.  Even so, the full implications should be understood and     the case carefully weighed before implementing any behavior     described with this label.  o MAY     This word means that this item is truly optional.  One vendor may     choose to include the item because a particular marketplace     requires it or because it enhances the product, for example;     another vendor may omit the same item.1.1.3 Compliance  Some requirements are applicable to all routers.  Other requirements  are applicable only to those which implement particular features or  protocols.  In the following paragraphs, relevant refers to the union  of the requirements applicable to all routers and the set of  requirements applicable to a particular router because of the set of  features and protocols it has implemented.  Note that not all Relevant requirements are stated directly in this  memo.  Various parts of this memo incorporate by reference sections of  the Host Requirements specification, [INTRO:2] and [INTRO:3].  For  purposes of determining compliance with this memo, it does not matter  whether a Relevant requirement is stated directly in this memo or  merely incorporated by reference from one of those documents.  An implementation is said to be conditionally compliant if it  satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT  requirements.  An implementation is said to be unconditionally  compliant if it is conditionally compliant and also satisfies all the  Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements.  An  implementation is not compliant if it is not conditionally compliant  (i.e., it fails to satisfy one or more of the Relevant MUST, MUST  IMPLEMENT, or MUST NOT requirements).  This specification occasionally indicates that an implementation  SHOULD implement a management variable, and that it SHOULD have a  certain default value.  An unconditionally compliant implementationBaker                       Standards Track                    [Page 10]

RFC 1812         Requirements for IP Version 4 Routers         June 1995  implements the default behavior, and if there are other implemented  behaviors implements the variable.  A conditionally compliant  implementation clearly documents what the default setting of the  variable is or, in the absence of the implementation of a variable,  may be construed to be.  An implementation that both fails to  implement the variable and chooses a different behavior is not  compliant.  For any of the SHOULD and SHOULD NOT requirements, a router may  provide a configuration option that will cause the router to act other  than as specified by the requirement.  Having such a configuration  option does not void a router's claim to unconditional compliance if  the option has a default setting, and that setting causes the router  to operate in the required manner.  Likewise, routers may provide, except where explicitly prohibited by  this memo, options which cause them to violate MUST or MUST NOT  requirements.  A router that provides such options is compliant  (either fully or conditionally) if and only if each such option has a  default setting that causes the router to conform to the requirements  of this memo.  Please note that the authors of this memo, although  aware of market realities, strongly recommend against provision of  such options.  Requirements are labeled MUST or MUST NOT because  experts in the field have judged them to be particularly important to  interoperability or proper functioning in the Internet.  Vendors  should weigh carefully the customer support costs of providing options  that violate those rules.  Of course, this memo is not a complete specification of an IP router,  but rather is closer to what in the OSI world is called a profile.  For example, this memo requires that a number of protocols be  implemented.  Although most of the contents of their protocol  specifications are not repeated in this memo, implementors are  nonetheless required to implement the protocols according to those  specifications.1.2 Relationships to Other Standards  There are several reference documents of interest in checking the  status of protocol specifications and standardization:    o INTERNET OFFICIAL PROTOCOL STANDARDS       This document describes the Internet standards process and lists       the standards status of the protocols.  As of this writing, the       current version of this document is STD 1,RFC 1780, [ARCH:7].       This document is periodically re-issued.  You should always       consult an RFC repository and use the latest version of this       document.Baker                       Standards Track                    [Page 11]

RFC 1812         Requirements for IP Version 4 Routers         June 1995    o Assigned Numbers       This document lists the assigned values of the parameters used in       the various protocols.  For example, it lists IP protocol codes,       TCP port numbers, Telnet Option Codes, ARP hardware types, and       Terminal Type names.  As of this writing, the current version of       this document is STD 2,RFC 1700, [INTRO:7].  This document is       periodically re-issued.  You should always consult an RFC       repository and use the latest version of this document.    o Host Requirements       This pair of documents reviews the specifications that apply to       hosts and supplies guidance and clarification for any       ambiguities.  Note that these requirements also apply to routers,       except where otherwise specified in this memo.  As of this       writing, the current versions of these documents areRFC 1122 andRFC 1123 (STD 3), [INTRO:2] and [INTRO:3].    o Router Requirements (formerly Gateway Requirements)       This memo.   Note that these documents are revised and updated at different times;   in case of differences between these documents, the most recent must   prevail.   These and other Internet protocol documents may be obtained from the:                               The InterNIC                              DS.INTERNIC.NET                  InterNIC Directory and Database Service                             info@internic.net                              +1-908-668-6587                       URL:http://ds.internic.net/1.3 General Considerations   There are several important lessons that vendors of Internet software   have learned and which a new vendor should consider seriously.1.3.1 Continuing Internet Evolution   The enormous growth of the Internet has revealed problems of   management and scaling in a large datagram based packet communication   system.  These problems are being addressed, and as a result there   will be continuing evolution of the specifications described in this   memo.  New routing protocols, algorithms, and architectures are   constantly being developed.  New internet layer protocols, and   modifications to existing protocols, are also constantly being   devised.  Routers play a crucial role in the Internet, and the numberBaker                       Standards Track                    [Page 12]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   of routers deployed in the Internet is much smaller than the number   of hosts.  Vendors should therefore expect that router standards will   continue to evolve much more quickly than host standards.  These   changes will be carefully planned and controlled since there is   extensive participation in this planning by the vendors and by the   organizations responsible for operation of the networks.   Development, evolution, and revision are characteristic of computer   network protocols today, and this situation will persist for some   years.  A vendor who develops computer communications software for   the Internet protocol suite (or any other protocol suite!) and then   fails to maintain and update that software for changing   specifications is going to leave a trail of unhappy customers.  The   Internet is a large communication network, and the users are in   constant contact through it.  Experience has shown that knowledge of   deficiencies in vendor software propagates quickly through the   Internet technical community.1.3.2 Robustness Principle   At every layer of the protocols, there is a general rule (from   [TRANS:2] by Jon Postel) whose application can lead to enormous   benefits in robustness and interoperability:                      Be conservative in what you do,                be liberal in what you accept from others.   Software should be written to deal with every conceivable error, no   matter how unlikely.  Eventually a packet will come in with that   particular combination of errors and attributes, and unless the   software is prepared, chaos can ensue.  It is best to assume that the   network is filled with malevolent entities that will send packets   designed to have the worst possible effect.  This assumption will   lead to suitably protective design.  The most serious problems in the   Internet have been caused by unforeseen mechanisms triggered by low   probability events; mere human malice would never have taken so   devious a course!   Adaptability to change must be designed into all levels of router   software.  As a simple example, consider a protocol specification   that contains an enumeration of values for a particular header field   - e.g., a type field, a port number, or an error code; this   enumeration must be assumed to be incomplete.  If the protocol   specification defines four possible error codes, the software must   not break when a fifth code is defined.  An undefined code might be   logged, but it must not cause a failure.Baker                       Standards Track                    [Page 13]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   The second part of the principal is almost as important: software on   hosts or other routers may contain deficiencies that make it unwise   to exploit legal but obscure protocol features.  It is unwise to   stray far from the obvious and simple, lest untoward effects result   elsewhere.  A corollary of this is watch out for misbehaving hosts;   router software should be prepared to survive in the presence of   misbehaving hosts.  An important function of routers in the Internet   is to limit the amount of disruption such hosts can inflict on the   shared communication facility.1.3.3 Error Logging   The Internet includes a great variety of systems, each implementing   many protocols and protocol layers, and some of these contain bugs   and misguided features in their Internet protocol software.  As a   result of complexity, diversity, and distribution of function, the   diagnosis of problems is often very difficult.   Problem diagnosis will be aided if routers include a carefully   designed facility for logging erroneous or strange events.  It is   important to include as much diagnostic information as possible when   an error is logged.  In particular, it is often useful to record the   header(s) of a packet that caused an error.  However, care must be   taken to ensure that error logging does not consume prohibitive   amounts of resources or otherwise interfere with the operation of the   router.   There is a tendency for abnormal but harmless protocol events to   overflow error logging files; this can be avoided by using a circular   log, or by enabling logging only while diagnosing a known failure.   It may be useful to filter and count duplicate successive messages.   One strategy that seems to work well is to both:   o Always count abnormalities and make such counts accessible through      the management protocol (see Chapter 8); and   o Allow the logging of a great variety of events to be selectively      enabled.  For example, it might useful to be able to log      everything or to log everything for host X.   This topic is further discussed in [MGT:5].1.3.4 Configuration   In an ideal world, routers would be easy to configure, and perhaps   even entirely self-configuring.  However, practical experience in the   real world suggests that this is an impossible goal, and that many   attempts by vendors to make configuration easy actually cause   customers more grief than they prevent.  As an extreme example, aBaker                       Standards Track                    [Page 14]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   router designed to come up and start routing packets without   requiring any configuration information at all would almost certainly   choose some incorrect parameter, possibly causing serious problems on   any networks unfortunate enough to be connected to it.   Often this memo requires that a parameter be a configurable option.   There are several reasons for this.  In a few cases there currently   is some uncertainty or disagreement about the best value and it may   be necessary to update the recommended value in the future.  In other   cases, the value really depends on external factors - e.g., the   distribution of its communication load, or the speeds and topology of   nearby networks - and self-tuning algorithms are unavailable and may   be insufficient.  In some cases, configurability is needed because of   administrative requirements.   Finally, some configuration options are required to communicate with   obsolete or incorrect implementations of the protocols, distributed   without sources, that persist in many parts of the Internet.  To make   correct systems coexist with these faulty systems, administrators   must occasionally misconfigure the correct systems.  This problem   will correct itself gradually as the faulty systems are retired, but   cannot be ignored by vendors.   When we say that a parameter must be configurable, we do not intend   to require that its value be explicitly read from a configuration   file at every boot time.  For many parameters, there is one value   that is appropriate for all but the most unusual situations.  In such   cases, it is quite reasonable that the parameter default to that   value if not explicitly set.   This memo requires a particular value for such defaults in some   cases.  The choice of default is a sensitive issue when the   configuration item controls accommodation of existing, faulty,   systems.  If the Internet is to converge successfully to complete   interoperability, the default values built into implementations must   implement the official protocol, not misconfigurations to accommodate   faulty implementations.  Although marketing considerations have led   some vendors to choose misconfiguration defaults, we urge vendors to   choose defaults that will conform to the standard.   Finally, we note that a vendor needs to provide adequate   documentation on all configuration parameters, their limits and   effects.Baker                       Standards Track                    [Page 15]

RFC 1812         Requirements for IP Version 4 Routers         June 19951.4 Algorithms   In several places in this memo, specific algorithms that a router   ought to follow are specified.  These algorithms are not, per se,   required of the router.  A router need not implement each algorithm   as it is written in this document.  Rather, an implementation must   present a behavior to the external world that is the same as a   strict, literal, implementation of the specified algorithm.   Algorithms are described in a manner that differs from the way a good   implementor would implement them.  For expository purposes, a style   that emphasizes conciseness, clarity, and independence from   implementation details has been chosen.  A good implementor will   choose algorithms and implementation methods that produce the same   results as these algorithms, but may be more efficient or less   general.   We note that the art of efficient router implementation is outside   the scope of this memo.2. INTERNET ARCHITECTURE   This chapter does not contain any requirements.  However, it does   contain useful background information on the general architecture of   the Internet and of routers.   General background and discussion on the Internet architecture and   supporting protocol suite can be found in the DDN Protocol Handbook   [ARCH:1]; for background see for example [ARCH:2], [ARCH:3], and   [ARCH:4].  The Internet architecture and protocols are also covered   in an ever-growing number of textbooks, such as [ARCH:5] and   [ARCH:6].2.1 Introduction   The Internet system consists of a number of interconnected packet   networks supporting communication among host computers using the   Internet protocols.  These protocols include the Internet Protocol   (IP), the Internet Control Message Protocol (ICMP), the Internet   Group Management Protocol (IGMP), and a variety transport and   application protocols that depend upon them.  As was described in   Section [1.2], the Internet Engineering Steering Group periodically   releases an Official Protocols memo listing all the Internet   protocols.   All Internet protocols use IP as the basic data transport mechanism.   IP is a datagram, or connectionless, internetwork service and   includes provision for addressing, type-of-service specification,Baker                       Standards Track                    [Page 16]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   fragmentation and reassembly, and security.  ICMP and IGMP are   considered integral parts of IP, although they are architecturally   layered upon IP.  ICMP provides error reporting, flow control,   first-hop router redirection, and other maintenance and control   functions.  IGMP provides the mechanisms by which hosts and routers   can join and leave IP multicast groups.   Reliable data delivery is provided in the Internet protocol suite by   Transport Layer protocols such as the Transmission Control Protocol   (TCP), which provides end-end retransmission, resequencing and   connection control.  Transport Layer connectionless service is   provided by the User Datagram Protocol (UDP).2.2 Elements of the Architecture2.2.1 Protocol Layering   To communicate using the Internet system, a host must implement the   layered set of protocols comprising the Internet protocol suite.  A   host typically must implement at least one protocol from each layer.   The protocol layers used in the Internet architecture are as follows   [ARCH:7]:   o Application Layer      The Application Layer is the top layer of the Internet protocol      suite.  The Internet suite does not further subdivide the      Application Layer, although some application layer protocols do      contain some internal sub-layering.  The application layer of the      Internet suite essentially combines the functions of the top two      layers - Presentation and Application - of the OSI Reference Model      [ARCH:8].  The Application Layer in the Internet protocol suite      also includes some of the function relegated to the Session Layer      in the OSI Reference Model.      We distinguish two categories of application layer protocols: user      protocols that provide service directly to users, and support      protocols that provide common system functions.  The most common      Internet user protocols are:      - Telnet (remote login)      - FTP (file transfer)      - SMTP (electronic mail delivery)      There are a number of other standardized user protocols and many      private user protocols.Baker                       Standards Track                    [Page 17]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      Support protocols, used for host name mapping, booting, and      management include SNMP, BOOTP, TFTP, the Domain Name System (DNS)      protocol, and a variety of routing protocols.      Application Layer protocols relevant to routers are discussed in      chapters 7, 8, and 9 of this memo.   o Transport Layer      The Transport Layer provides end-to-end communication services.      This layer is roughly equivalent to the Transport Layer in the OSI      Reference Model, except that it also incorporates some of OSI's      Session Layer establishment and destruction functions.      There are two primary Transport Layer protocols at present:      - Transmission Control Protocol (TCP)      - User Datagram Protocol (UDP)      TCP is a reliable connection-oriented transport service that      provides end-to-end reliability, resequencing, and flow control.      UDP is a connectionless (datagram) transport service.  Other      transport protocols have been developed by the research community,      and the set of official Internet transport protocols may be      expanded in the future.      Transport Layer protocols relevant to routers are discussed in      Chapter 6.   o Internet Layer      All Internet transport protocols use the Internet Protocol (IP) to      carry data from source host to destination host.  IP is a      connectionless or datagram internetwork service, providing no      end-to-end delivery guarantees.  IP datagrams may arrive at the      destination host damaged, duplicated, out of order, or not at all.      The layers above IP are responsible for reliable delivery service      when it is required.  The IP protocol includes provision for      addressing, type-of-service specification, fragmentation and      reassembly, and security.      The datagram or connectionless nature of IP is a fundamental and      characteristic feature of the Internet architecture.      The Internet Control Message Protocol (ICMP) is a control protocol      that is considered to be an integral part of IP, although it is      architecturally layered upon IP - it uses IP to carry its data      end-to-end.  ICMP provides error reporting, congestion reporting,      and first-hop router redirection.Baker                       Standards Track                    [Page 18]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      The Internet Group Management Protocol (IGMP) is an Internet layer      protocol used for establishing dynamic host groups for IP      multicasting.      The Internet layer protocols IP, ICMP, and IGMP are discussed in      chapter 4.   o Link Layer      To communicate on a directly connected network, a host must      implement the communication protocol used to interface to that      network.  We call this a Link Layer protocol.      Some older Internet documents refer to this layer as the Network      Layer, but it is not the same as the Network Layer in the OSI      Reference Model.      This layer contains everything below the Internet Layer and above      the Physical Layer (which is the media connectivity, normally      electrical or optical, which encodes and transports messages).      Its responsibility is the correct delivery of messages, among      which it does not differentiate.      Protocols in this Layer are generally outside the scope of      Internet standardization; the Internet (intentionally) uses      existing standards whenever possible.  Thus, Internet Link Layer      standards usually address only address resolution and rules for      transmitting IP packets over specific Link Layer protocols.      Internet Link Layer standards are discussed in chapter 3.2.2.2 Networks   The constituent networks of the Internet system are required to   provide only packet (connectionless) transport.  According to the IP   service specification, datagrams can be delivered out of order, be   lost or duplicated, and/or contain errors.   For reasonable performance of the protocols that use IP (e.g., TCP),   the loss rate of the network should be very low.  In networks   providing connection-oriented service, the extra reliability provided   by virtual circuits enhances the end-end robustness of the system,   but is not necessary for Internet operation.   Constituent networks may generally be divided into two classes:     o Local-Area Networks (LANs)        LANs may have a variety of designs.  LANs normally cover a small        geographical area (e.g., a single building or plant site) and        provide high bandwidth with low delays.  LANs may be passiveBaker                       Standards Track                    [Page 19]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        (similar to Ethernet) or they may be active (such as ATM).     o Wide-Area Networks (WANs)        Geographically dispersed hosts and LANs are interconnected by        wide-area networks, also called long-haul networks.  These        networks may have a complex internal structure of lines and        packet-switches, or they may be as simple as point-to-point        lines.2.2.3 Routers   In the Internet model, constituent networks are connected together by   IP datagram forwarders which are called routers or IP routers.  In   this document, every use of the term router is equivalent to IP   router.  Many older Internet documents refer to routers as gateways.   Historically, routers have been realized with packet-switching   software executing on a general-purpose CPU.  However, as custom   hardware development becomes cheaper and as higher throughput is   required, special purpose hardware is becoming increasingly common.   This specification applies to routers regardless of how they are   implemented.   A router connects to two or more logical interfaces, represented by   IP subnets or unnumbered point to point lines (discussed in section   [2.2.7]).  Thus, it has at least one physical interface.  Forwarding   an IP datagram generally requires the router to choose the address   and relevant interface of the next-hop router or (for the final hop)   the destination host.  This choice, called relaying or forwarding   depends upon a route database within the router.  The route database   is also called a routing table or forwarding table.  The term   "router" derives from the process of building this route database;   routing protocols and configuration interact in a process called   routing.   The routing database should be maintained dynamically to reflect the   current topology of the Internet system.  A router normally   accomplishes this by participating in distributed routing and   reachability algorithms with other routers.   Routers provide datagram transport only, and they seek to minimize   the state information necessary to sustain this service in the   interest of routing flexibility and robustness.   Packet switching devices may also operate at the Link Layer; such   devices are usually called bridges.  Network segments that are   connected by bridges share the same IP network prefix forming a   single IP subnet.  These other devices are outside the scope of thisBaker                       Standards Track                    [Page 20]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   document.2.2.4 Autonomous Systems   An Autonomous System (AS) is a connected segment of a network   topology that consists of a collection of subnetworks (with hosts   attached) interconnected by a set of routes.  The subnetworks and the   routers are expected to be under the control of a single operations   and maintenance (O&M) organization.  Within an AS routers may use one   or more interior routing protocols, and sometimes several sets of   metrics.  An AS is expected to present to other ASs an appearence of   a coherent interior routing plan, and a consistent picture of the   destinations reachable through the AS.  An AS is identified by an   Autonomous System number.   The concept of an AS plays an important role in the Internet routing   (seeSection 7.1).2.2.5 Addressing Architecture   An IP datagram carries 32-bit source and destination addresses, each   of which is partitioned into two parts - a constituent network prefix   and a host number on that network.  Symbolically:      IP-address ::= { <Network-prefix>, <Host-number> }   To finally deliver the datagram, the last router in its path must map   the Host-number (or rest) part of an IP address to the host's Link   Layer address.2.2.5.1 Classical IP Addressing Architecture   Although well documented elsewhere [INTERNET:2], it is useful to   describe the historical use of the network prefix.  The language   developed to describe it is used in this and other documents and   permeates the thinking behind many protocols.   The simplest classical network prefix is the Class A, B, C, D, or E   network prefix.  These address ranges are discriminated by observing   the values of the most significant bits of the address, and break the   address into simple prefix and host number fields.  This is described   in [INTERNET:18].  In short, the classification is:        0xxx - Class A - general purpose unicast addresses with standard        8 bit prefix        10xx - Class B - general purpose unicast addresses with standard        16 bit prefixBaker                       Standards Track                    [Page 21]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        110x - Class C - general purpose unicast addresses with standard        24 bit prefix        1110 - Class D - IP Multicast Addresses - 28 bit prefix, non-        aggregatable        1111 - Class E - reserved for experimental use   This simple notion has been extended by the concept of subnets.   These were introduced to allow arbitrary complexity of interconnected   LAN structures within an organization, while insulating the Internet   system against explosive growth in assigned network prefixes and   routing complexity.  Subnets provide a multi-level hierarchical   routing structure for the Internet system.  The subnet extension,   described in [INTERNET:2], is a required part of the Internet   architecture.  The basic idea is to partition the <Host-number> field   into two parts: a subnet number, and a true host number on that   subnet:      IP-address ::=        { <Network-number>, <Subnet-number>, <Host-number> }   The interconnected physical networks within an organization use the   same network prefix but different subnet numbers.  The distinction   between the subnets of such a subnetted network is not normally   visible outside of that network.  Thus, routing in the rest of the   Internet uses only the <Network-prefix> part of the IP destination   address.  Routers outside the network treat <Network-prefix> and   <Host-number> together as an uninterpreted rest part of the 32-bit IP   address.  Within the subnetted network, the routers use the extended   network prefix:      { <Network-number>, <Subnet-number> }   The bit positions containing this extended network number have   historically been indicated by a 32-bit mask called the subnet mask.   The <Subnet-number> bits SHOULD be contiguous and fall between the   <Network-number> and the <Host-number> fields.  More up to date   protocols do not refer to a subnet mask, but to a prefix length; the   "prefix" portion of an address is that which would be selected by a   subnet mask whose most significant bits are all ones and the rest are   zeroes.  The length of the prefix equals the number of ones in the   subnet mask.  This document assumes that all subnet masks are   expressible as prefix lengths.   The inventors of the subnet mechanism presumed that each piece of an   organization's network would have only a single subnet number.  In   practice, it has often proven necessary or useful to have several   subnets share a single physical cable.  For this reason, routers   should be capable of configuring multiple subnets on the sameBaker                       Standards Track                    [Page 22]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   physical interfaces, and treat them (from a routing or forwarding   perspective) as though they were distinct physical interfaces.2.2.5.2 Classless Inter Domain Routing (CIDR)   The explosive growth of the Internet has forced a review of address   assignment policies.  The traditional uses of general purpose (Class   A, B, and C) networks have been modified to achieve better use of   IP's 32-bit address space.  Classless Inter Domain Routing (CIDR)   [INTERNET:15] is a method currently being deployed in the Internet   backbones to achieve this added efficiency.  CIDR depends on   deploying and routing to arbitrarily sized networks.  In this model,   hosts and routers make no assumptions about the use of addressing in   the internet.  The Class D (IP Multicast) and Class E (Experimental)   address spaces are preserved, although this is primarily an   assignment policy.   By definition, CIDR comprises three elements:     o topologically significant address assignment,     o routing protocols that are capable of aggregating network layer        reachability information, and     o consistent forwarding algorithm ("longest match").   The use of networks and subnets is now historical, although the   language used to describe them remains in current use.  They have   been replaced by the more tractable concept of a network prefix.  A   network prefix is, by definition, a contiguous set of bits at the   more significant end of the address that defines a set of systems;   host numbers select among those systems.  There is no requirement   that all the internet use network prefixes uniformly.  To collapse   routing information, it is useful to divide the internet into   addressing domains.  Within such a domain, detailed information is   available about constituent networks; outside it, only the common   network prefix is advertised.   The classical IP addressing architecture used addresses and subnet   masks to discriminate the host number from the network prefix.  With   network prefixes, it is sufficient to indicate the number of bits in   the prefix.  Both representations are in common use.  Architecturally   correct subnet masks are capable of being represented using the   prefix length description.  They comprise that subset of all possible   bits patterns that have     o a contiguous string of ones at the more significant end,     o a contiguous string of zeros at the less significant end, and     o no intervening bits.Baker                       Standards Track                    [Page 23]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Routers SHOULD always treat a route as a network prefix, and SHOULD   reject configuration and routing information inconsistent with that   model.      IP-address ::= { <Network-prefix>, <Host-number> }   An effect of the use of CIDR is that the set of destinations   associated with address prefixes in the routing table may exhibit   subset relationship.  A route describing a smaller set of   destinations (a longer prefix) is said to be more specific than a   route describing a larger set of destinations (a shorter prefix);   similarly, a route describing a larger set of destinations (a shorter   prefix) is said to be less specific than a route describing a smaller   set of destinations (a longer prefix).  Routers must use the most   specific matching route (the longest matching network prefix) when   forwarding traffic.2.2.6 IP Multicasting   IP multicasting is an extension of Link Layer multicast to IP   internets.  Using IP multicasts, a single datagram can be addressed   to multiple hosts without sending it to all.  In the extended case,   these hosts may reside in different address domains.  This collection   of hosts is called a multicast group.  Each multicast group is   represented as a Class D IP address.  An IP datagram sent to the   group is to be delivered to each group member with the same best-   effort delivery as that provided for unicast IP traffic.  The sender   of the datagram does not itself need to be a member of the   destination group.   The semantics of IP multicast group membership are defined in   [INTERNET:4].  That document describes how hosts and routers join and   leave multicast groups.  It also defines a protocol, the Internet   Group Management Protocol (IGMP), that monitors IP multicast group   membership.   Forwarding of IP multicast datagrams is accomplished either through   static routing information or via a multicast routing protocol.   Devices that forward IP multicast datagrams are called multicast   routers.  They may or may not also forward IP unicasts.  Multicast   datagrams are forwarded on the basis of both their source and   destination addresses.  Forwarding of IP multicast packets is   described in more detail in Section [5.2.1].Appendix D discusses   multicast routing protocols.Baker                       Standards Track                    [Page 24]

RFC 1812         Requirements for IP Version 4 Routers         June 19952.2.7 Unnumbered Lines and Networks Prefixes   Traditionally, each network interface on an IP host or router has its   own IP address.  This can cause inefficient use of the scarce IP   address space, since it forces allocation of an IP network prefix to   every point-to-point link.   To solve this problem, a number of people have proposed and   implemented the concept of unnumbered point to point lines.  An   unnumbered point to point line does not have any network prefix   associated with it.  As a consequence, the network interfaces   connected to an unnumbered point to point line do not have IP   addresses.   Because the IP architecture has traditionally assumed that all   interfaces had IP addresses, these unnumbered interfaces cause some   interesting dilemmas.  For example, some IP options (e.g., Record   Route) specify that a router must insert the interface address into   the option, but an unnumbered interface has no IP address.  Even more   fundamental (as we shall see in chapter 5) is that routes contain the   IP address of the next hop router.  A router expects that this IP   address will be on an IP (sub)net to which the router is connected.   That assumption is of course violated if the only connection is an   unnumbered point to point line.   To get around these difficulties, two schemes have been conceived.   The first scheme says that two routers connected by an unnumbered   point to point line are not really two routers at all, but rather two   half-routers that together make up a single virtual router.  The   unnumbered point to point line is essentially considered to be an   internal bus in the virtual router.  The two halves of the virtual   router must coordinate their activities in such a way that they act   exactly like a single router.   This scheme fits in well with the IP architecture, but suffers from   two important drawbacks.  The first is that, although it handles the   common case of a single unnumbered point to point line, it is not   readily extensible to handle the case of a mesh of routers and   unnumbered point to point lines.  The second drawback is that the   interactions between the half routers are necessarily complex and are   not standardized, effectively precluding the connection of equipment   from different vendors using unnumbered point to point lines.   Because of these drawbacks, this memo has adopted an alternate   scheme, which has been invented multiple times but which is probably   originally attributable to Phil Karn.  In this scheme, a router that   has unnumbered point to point lines also has a special IP address,   called a router-id in this memo.  The router-id is one of theBaker                       Standards Track                    [Page 25]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   router's IP addresses (a router is required to have at least one IP   address).  This router-id is used as if it is the IP address of all   unnumbered interfaces.2.2.8 Notable Oddities2.2.8.1 Embedded Routers   A router may be a stand-alone computer system, dedicated to its IP   router functions.  Alternatively, it is possible to embed router   functions within a host operating system that supports connections to   two or more networks.  The best-known example of an operating system   with embedded router code is the Berkeley BSD system.  The embedded   router feature seems to make building a network easy, but it has a   number of hidden pitfalls:   (1) If a host has only a single constituent-network interface, it        should not act as a router.        For example, hosts with embedded router code that gratuitously        forward broadcast packets or datagrams on the same net often        cause packet avalanches.   (2) If a (multihomed) host acts as a router, it is subject to the        requirements for routers contained in this document.        For example, the routing protocol issues and the router control        and monitoring problems are as hard and important for embedded        routers as for stand-alone routers.        Internet router requirements and specifications may change        independently of operating system changes.  An administration        that operates an embedded router in the Internet is strongly        advised to maintain and update the router code.  This might        require router source code.   (3) When a host executes embedded router code, it becomes part of the        Internet infrastructure.  Thus, errors in software or        configuration can hinder communication between other hosts.  As        a consequence, the host administrator must lose some autonomy.        In many circumstances, a host administrator will need to disable        router code embedded in the operating system.  For this reason,        it should be straightforward to disable embedded router        functionality.Baker                       Standards Track                    [Page 26]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (4) When a host running embedded router code is concurrently used for        other services, the Operation and Maintenance requirements for        the two modes of use may conflict.        For example, router O&M will in many cases be performed remotely        by an operations center; this may require privileged system        access that the host administrator would not normally want to        distribute.2.2.8.2 Transparent Routers   There are two basic models for interconnecting local-area networks   and wide-area (or long-haul) networks in the Internet.  In the first,   the local-area network is assigned a network prefix and all routers   in the Internet must know how to route to that network.  In the   second, the local-area network shares (a small part of) the address   space of the wide-area network.  Routers that support this second   model are called address sharing routers or transparent routers.  The   focus of this memo is on routers that support the first model, but   this is not intended to exclude the use of transparent routers.   The basic idea of a transparent router is that the hosts on the   local-area network behind such a router share the address space of   the wide-area network in front of the router.  In certain situations   this is a very useful approach and the limitations do not present   significant drawbacks.   The words in front and behind indicate one of the limitations of this   approach: this model of interconnection is suitable only for a   geographically (and topologically) limited stub environment.  It   requires that there be some form of logical addressing in the network   level addressing of the wide-area network.  IP addresses in the local   environment map to a few (usually one) physical address in the wide-   area network.  This mapping occurs in a way consistent with the { IP   address <-> network address } mapping used throughout the wide-area   network.   Multihoming is possible on one wide-area network, but may present   routing problems if the interfaces are geographically or   topologically separated.  Multihoming on two (or more) wide-area   networks is a problem due to the confusion of addresses.   The behavior that hosts see from other hosts in what is apparently   the same network may differ if the transparent router cannot fully   emulate the normal wide-area network service.  For example, the   ARPANET used a Link Layer protocol that provided a Destination Dead   indication in response to an attempt to send to a host that was off-   line.  However, if there were a transparent router between theBaker                       Standards Track                    [Page 27]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   ARPANET and an Ethernet, a host on the ARPANET would not receive a   Destination Dead indication for Ethernet hosts.2.3 Router Characteristics   An Internet router performs the following functions:   (1) Conforms to specific Internet protocols specified in this        document, including the Internet Protocol (IP), Internet Control        Message Protocol (ICMP), and others as necessary.   (2) Interfaces to two or more packet networks.  For each connected        network the router must implement the functions required by that        network.  These functions typically include:        o Encapsulating and decapsulating the IP datagrams with the           connected network framing (e.g., an Ethernet header and           checksum),        o Sending and receiving IP datagrams up to the maximum size           supported by that network, this size is the network's Maximum           Transmission Unit or MTU,        o Translating the IP destination address into an appropriate           network-level address for the connected network (e.g., an           Ethernet hardware address), if needed, and        o Responding to network flow control and error indications, if           any.        See chapter 3 (Link Layer).   (3) Receives and forwards Internet datagrams.  Important issues in        this process are buffer management, congestion control, and        fairness.        o Recognizes error conditions and generates ICMP error and           information messages as required.        o Drops datagrams whose time-to-live fields have reached zero.        o Fragments datagrams when necessary to fit into the MTU of the           next network.        See chapter 4 (Internet Layer - Protocols) and chapter 5        (Internet Layer - Forwarding) for more information.Baker                       Standards Track                    [Page 28]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (4) Chooses a next-hop destination for each IP datagram, based on the        information in its routing database.  See chapter 5 (Internet        Layer - Forwarding) for more information.   (5) (Usually) supports an interior gateway protocol (IGP) to carry        out distributed routing and reachability algorithms with the        other routers in the same autonomous system.  In addition, some        routers will need to support an exterior gateway protocol (EGP)        to exchange topological information with other autonomous        systems.  See chapter 7 (Application Layer - Routing Protocols)        for more information.   (6) Provides network management and system support facilities,        including loading, debugging, status reporting, exception        reporting and control.  See chapter 8 (Application Layer -        Network Management Protocols) and chapter 10 (Operation and        Maintenance) for more information.   A router vendor will have many choices on power, complexity, and   features for a particular router product.  It may be helpful to   observe that the Internet system is neither homogeneous nor fully   connected.  For reasons of technology and geography it is growing   into a global interconnect system plus a fringe of LANs around the   edge.  More and more these fringe LANs are becoming richly   interconnected, thus making them less out on the fringe and more   demanding on router requirements.   o The global interconnect system is composed of a number of wide-area      networks to which are attached routers of several Autonomous      Systems (AS); there are relatively few hosts connected directly to      the system.   o Most hosts are connected to LANs.  Many organizations have clusters      of LANs interconnected by local routers.  Each such cluster is      connected by routers at one or more points into the global      interconnect system.  If it is connected at only one point, a LAN      is known as a stub network.   Routers in the global interconnect system generally require:   o Advanced Routing and Forwarding Algorithms      These routers need routing algorithms that are highly dynamic,      impose minimal processing and communication burdens, and offer      type-of-service routing.  Congestion is still not a completely      resolved issue (see Section [5.3.6]).  Improvements in these areas      are expected, as the research community is actively working on      these issues.Baker                       Standards Track                    [Page 29]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o High Availability      These routers need to be highly reliable, providing 24 hours a      day, 7 days a week service.  Equipment and software faults can      have a wide-spread (sometimes global) effect.  In case of failure,      they must recover quickly.  In any environment, a router must be      highly robust and able to operate, possibly in a degraded state,      under conditions of extreme congestion or failure of network      resources.   o Advanced O&M Features      Internet routers normally operate in an unattended mode.  They      will typically be operated remotely from a centralized monitoring      center.  They need to provide sophisticated means for monitoring      and measuring traffic and other events and for diagnosing faults.   o High Performance      Long-haul lines in the Internet today are most frequently full      duplex 56 KBPS, DS1 (1.544 Mbps), or DS3 (45 Mbps) speeds.  LANs,      which are half duplex multiaccess media, are typically Ethernet      (10Mbps) and, to a lesser degree, FDDI (100Mbps).  However,      network media technology is constantly advancing and higher speeds      are likely in the future.   The requirements for routers used in the LAN fringe (e.g., campus   networks) depend greatly on the demands of the local networks.  These   may be high or medium-performance devices, probably competitively   procured from several different vendors and operated by an internal   organization (e.g., a campus computing center).  The design of these   routers should emphasize low average latency and good burst   performance, together with delay and type-of-service sensitive   resource management.  In this environment there may be less formal   O&M but it will not be less important.  The need for the routing   mechanism to be highly dynamic will become more important as networks   become more complex and interconnected.  Users will demand more out   of their local connections because of the speed of the global   interconnects.   As networks have grown, and as more networks have become old enough   that they are phasing out older equipment, it has become increasingly   imperative that routers interoperate with routers from other vendors.   Even though the Internet system is not fully interconnected, many   parts of the system need to have redundant connectivity.  Rich   connectivity allows reliable service despite failures of   communication lines and routers, and it can also improve service byBaker                       Standards Track                    [Page 30]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   shortening Internet paths and by providing additional capacity.   Unfortunately, this richer topology can make it much more difficult   to choose the best path to a particular destination.2.4 Architectural Assumptions   The current Internet architecture is based on a set of assumptions   about the communication system.  The assumptions most relevant to   routers are as follows:   o The Internet is a network of networks.      Each host is directly connected to some particular network(s); its      connection to the Internet is only conceptual.  Two hosts on the      same network communicate with each other using the same set of      protocols that they would use to communicate with hosts on distant      networks.   o Routers do not keep connection state information.      To improve the robustness of the communication system, routers are      designed to be stateless, forwarding each IP packet independently      of other packets.  As a result, redundant paths can be exploited      to provide robust service in spite of failures of intervening      routers and networks.      All state information required for end-to-end flow control and      reliability is implemented in the hosts, in the transport layer or      in application programs.  All connection control information is      thus co-located with the end points of the communication, so it      will be lost only if an end point fails.  Routers control message      flow only indirectly, by dropping packets or increasing network      delay.      Note that future protocol developments may well end up putting      some more state into routers.  This is especially likely for      multicast routing, resource reservation, and flow based      forwarding.   o Routing complexity should be in the routers.      Routing is a complex and difficult problem, and ought to be      performed by the routers, not the hosts.  An important objective      is to insulate host software from changes caused by the inevitable      evolution of the Internet routing architecture.Baker                       Standards Track                    [Page 31]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o The system must tolerate wide network variation.      A basic objective of the Internet design is to tolerate a wide      range of network characteristics - e.g., bandwidth, delay, packet      loss, packet reordering, and maximum packet size.  Another      objective is robustness against failure of individual networks,      routers, and hosts, using whatever bandwidth is still available.      Finally, the goal is full open system interconnection: an Internet      router must be able to interoperate robustly and effectively with      any other router or Internet host, across diverse Internet paths.      Sometimes implementors have designed for less ambitious goals.      For example, the LAN environment is typically much more benign      than the Internet as a whole; LANs have low packet loss and delay      and do not reorder packets.  Some vendors have fielded      implementations that are adequate for a simple LAN environment,      but work badly for general interoperation.  The vendor justifies      such a product as being economical within the restricted LAN      market.  However, isolated LANs seldom stay isolated for long.      They are soon connected to each other, to organization-wide      internets, and eventually to the global Internet system.  In the      end, neither the customer nor the vendor is served by incomplete      or substandard routers.      The requirements in this document are designed for a full-function      router.  It is intended that fully compliant routers will be      usable in almost any part of the Internet.3. LINK LAYER   Although [INTRO:1] covers Link Layer standards (IP over various link   layers, ARP, etc.), this document anticipates that Link-Layer   material will be covered in a separate Link Layer Requirements   document.  A Link-Layer Requirements document would be applicable to   both hosts and routers.  Thus, this document will not obsolete the   parts of [INTRO:1] that deal with link-layer issues.3.1 INTRODUCTION   Routers have essentially the same Link Layer protocol requirements as   other sorts of Internet systems.  These requirements are given in   chapter 3 of Requirements for Internet Gateways [INTRO:1].  A router   MUST comply with its requirements and SHOULD comply with its   recommendations.  Since some of the material in that document has   become somewhat dated, some additional requirements and explanations   are included below.Baker                       Standards Track                    [Page 32]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      It is expected that the Internet community will produce a      Requirements for Internet Link Layer standard which will supersede      both this chapter and the chapter entitled "INTERNET LAYER      PROTOCOLS" in [INTRO:1].3.2 LINK/INTERNET LAYER INTERFACE   This document does not attempt to specify the interface between the   Link Layer and the upper layers.  However, note well that other parts   of this document, particularly chapter 5, require various sorts of   information to be passed across this layer boundary.   This section uses the following definitions:   o Source physical address      The source physical address is the Link Layer address of the host      or router from which the packet was received.   o Destination physical address      The destination physical address is the Link Layer address to      which the packet was sent.   The information that must pass from the Link Layer to the   Internetwork Layer for each received packet is:   (1) The IP packet [5.2.2],   (2) The length of the data portion (i.e., not including the Link-        Layer framing) of the Link Layer frame [5.2.2],   (3) The identity of the physical interface from which the IP packet        was received [5.2.3], and   (4) The classification of the packet's destination physical address        as a Link Layer unicast, broadcast, or multicast [4.3.2],        [5.3.4].   In addition, the Link Layer also should provide:   (5) The source physical address.   The information that must pass from the Internetwork Layer to the   Link Layer for each transmitted packet is:Baker                       Standards Track                    [Page 33]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (1) The IP packet [5.2.1]   (2) The length of the IP packet [5.2.1]   (3) The destination physical interface [5.2.1]   (4) The next hop IP address [5.2.1]   In addition, the Internetwork Layer also should provide:   (5) The Link Layer priority value [5.3.3.2]   The Link Layer must also notify the Internetwork Layer if the packet   to be transmitted causes a Link Layer precedence-related error   [5.3.3.3].3.3 SPECIFIC ISSUES3.3.1 Trailer Encapsulation   Routers that can connect to ten megabit Ethernets MAY be able to   receive and forward Ethernet packets encapsulated using the trailer   encapsulation described in [LINK:1].  However, a router SHOULD NOT   originate trailer encapsulated packets.  A router MUST NOT originate   trailer encapsulated packets without first verifying, using the   mechanism described in [INTRO:2], that the immediate destination of   the packet is willing and able to accept trailer-encapsulated   packets.  A router SHOULD NOT agree (using these mechanisms) to   accept trailer-encapsulated packets.3.3.2 Address Resolution Protocol - ARP   Routers that implement ARP MUST be compliant and SHOULD be   unconditionally compliant with the requirements in [INTRO:2].   The link layer MUST NOT report a Destination Unreachable error to IP   solely because there is no ARP cache entry for a destination; it   SHOULD queue up to a small number of datagrams breifly while   performing the ARP request/reply sequence, and reply that the   destination is unreachable to one of the queued datagrams only when   this proves fruitless.   A router MUST not believe any ARP reply that claims that the Link   Layer address of another host or router is a broadcast or multicast   address.Baker                       Standards Track                    [Page 34]

RFC 1812         Requirements for IP Version 4 Routers         June 19953.3.3 Ethernet and 802.3 Coexistence   Routers that can connect to ten megabit Ethernets MUST be compliant   and SHOULD be unconditionally compliant with the Ethernet   requirements of [INTRO:2].3.3.4 Maximum Transmission Unit - MTU   The MTU of each logical interface MUST be configurable within the   range of legal MTUs for the interface.   Many Link Layer protocols define a maximum frame size that may be   sent.  In such cases, a router MUST NOT allow an MTU to be set which   would allow sending of frames larger than those allowed by the Link   Layer protocol.  However, a router SHOULD be willing to receive a   packet as large as the maximum frame size even if that is larger than   the MTU.   DISCUSSION      Note that this is a stricter requirement than imposed on hosts by      [INTRO:2], which requires that the MTU of each physical interface      be configurable.      If a network is using an MTU smaller than the maximum frame size      for the Link Layer, a router may receive packets larger than the      MTU from misconfigured and incompletely initialized hosts.  The      Robustness Principle indicates that the router should successfully      receive these packets if possible.3.3.5 Point-to-Point Protocol - PPP   Contrary to [INTRO:1], the Internet does have a standard point to   point line protocol: the Point-to-Point Protocol (PPP), defined in   [LINK:2], [LINK:3], [LINK:4], and [LINK:5].   A point to point interface is any interface that is designed to send   data over a point to point line.  Such interfaces include telephone,   leased, dedicated or direct lines (either 2 or 4 wire), and may use   point to point channels or virtual circuits of multiplexed interfaces   such as ISDN.  They normally use a standardized modem or bit serial   interface (such as RS-232, RS-449 or V.35), using either synchronous   or asynchronous clocking.  Multiplexed interfaces often have special   physical interfaces.   A general purpose serial interface uses the same physical media as a   point to point line, but supports the use of link layer networks as   well as point to point connectivity.  Link layer networks (such as   X.25 or Frame Relay) use an alternative IP link layer specification.Baker                       Standards Track                    [Page 35]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Routers that implement point to point or general purpose serial   interfaces MUST IMPLEMENT PPP.   PPP MUST be supported on all general purpose serial interfaces on a   router.  The router MAY allow the line to be configured to use point   to point line protocols other than PPP.  Point to point interfaces   SHOULD either default to using PPP when enabled or require   configuration of the link layer protocol before being enabled.   General purpose serial interfaces SHOULD require configuration of the   link layer protocol before being enabled.3.3.5.1 Introduction   This section provides guidelines to router implementors so that they   can ensure interoperability with other routers using PPP over either   synchronous or asynchronous links.   It is critical that an implementor understand the semantics of the   option negotiation mechanism.  Options are a means for a local device   to indicate to a remote peer what the local device will accept from   the remote peer, not what it wishes to send.  It is up to the remote   peer to decide what is most convenient to send within the confines of   the set of options that the local device has stated that it can   accept.  Therefore it is perfectly acceptable and normal for a remote   peer to ACK all the options indicated in an LCP Configuration Request   (CR) even if the remote peer does not support any of those options.   Again, the options are simply a mechanism for either device to   indicate to its peer what it will accept, not necessarily what it   will send.3.3.5.2 Link Control Protocol (LCP) Options   The PPP Link Control Protocol (LCP) offers a number of options that   may be negotiated.  These options include (among others) address and   control field compression, protocol field compression, asynchronous   character map, Maximum Receive Unit (MRU), Link Quality Monitoring   (LQM), magic number (for loopback detection), Password Authentication   Protocol (PAP), Challenge Handshake Authentication Protocol (CHAP),   and the 32-bit Frame Check Sequence (FCS).   A router MAY use address/control field compression on either   synchronous or asynchronous links.  A router MAY use protocol field   compression on either synchronous or asynchronous links.  A router   that indicates that it can accept these compressions MUST be able to   accept uncompressed PPP header information also.Baker                       Standards Track                    [Page 36]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      These options control the appearance of the PPP header.  Normally      the PPP header consists of the address, the control field, and the      protocol field.  The address, on a point to point line, is 0xFF,      indicating "broadcast".  The control field is 0x03, indicating      "Unnumbered Information." The Protocol Identifier is a two byte      value indicating the contents of the data area of the frame.  If a      system negotiates address and control field compression it      indicates to its peer that it will accept PPP frames that have or      do not have these fields at the front of the header.  It does not      indicate that it will be sending frames with these fields removed.      Protocol field compression, when negotiated, indicates that the      system is willing to receive protocol fields compressed to one      byte when this is legal.  There is no requirement that the sender      do so.      Use of address/control field compression is inconsistent with the      use of numbered mode (reliable) PPP.   IMPLEMENTATION      Some hardware does not deal well with variable length header      information.  In those cases it makes most sense for the remote      peer to send the full PPP header.  Implementations may ensure this      by not sending the address/control field and protocol field      compression options to the remote peer.  Even if the remote peer      has indicated an ability to receive compressed headers there is no      requirement for the local router to send compressed headers.   A router MUST negotiate the Asynchronous Control Character Map (ACCM)   for asynchronous PPP links, but SHOULD NOT negotiate the ACCM for   synchronous links.  If a router receives an attempt to negotiate the   ACCM over a synchronous link, it MUST ACKnowledge the option and then   ignore it.   DISCUSSION      There are implementations that offer both synchronous and      asynchronous modes of operation and may use the same code to      implement the option negotiation.  In this situation it is      possible that one end or the other may send the ACCM option on a      synchronous link.   A router SHOULD properly negotiate the maximum receive unit (MRU).   Even if a system negotiates an MRU smaller than 1,500 bytes, it MUST   be able to receive a 1,500 byte frame.   A router SHOULD negotiate and enable the link quality monitoring   (LQM) option.Baker                       Standards Track                    [Page 37]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      This memo does not specify a policy for deciding whether the      link's quality is adequate.  However, it is important (see Section      [3.3.6]) that a router disable failed links.   A router SHOULD implement and negotiate the magic number option for   loopback detection.   A router MAY support the authentication options (PAP - Password   Authentication Protocol, and/or CHAP - Challenge Handshake   Authentication Protocol).   A router MUST support 16-bit CRC frame check sequence (FCS) and MAY   support the 32-bit CRC.3.3.5.3 IP Control Protocol (IPCP) Options   A router MAY offer to perform IP address negotiation.  A router MUST   accept a refusal (REJect) to perform IP address negotiation from the   peer.   Routers operating at link speeds of 19,200 BPS or less SHOULD   implement and offer to perform Van Jacobson header compression.   Routers that implement VJ compression SHOULD implement an   administrative control enabling or disabling it.3.3.6 Interface Testing   A router MUST have a mechanism to allow routing software to determine   whether a physical interface is available to send packets or not; on   multiplexed interfaces where permanent virtual circuits are opened   for limited sets of neighbors, the router must also be able to   determine whether the virtual circuits are viable.  A router SHOULD   have a mechanism to allow routing software to judge the quality of a   physical interface.  A router MUST have a mechanism for informing the   routing software when a physical interface becomes available or   unavailable to send packets because of administrative action.  A   router MUST have a mechanism for informing the routing software when   it detects a Link level interface has become available or   unavailable, for any reason.   DISCUSSION      It is crucial that routers have workable mechanisms for      determining that their network connections are functioning      properly.  Failure to detect link loss, or failure to take the      proper actions when a problem is detected, can lead to black      holes.Baker                       Standards Track                    [Page 38]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      The mechanisms available for detecting problems with network      connections vary considerably, depending on the Link Layer      protocols in use and the interface hardware.  The intent is to      maximize the capability to detect failures within the Link-Layer      constraints.4. INTERNET LAYER - PROTOCOLS4.1 INTRODUCTION   This chapter and chapter 5 discuss the protocols used at the Internet   Layer: IP, ICMP, and IGMP.  Since forwarding is obviously a crucial   topic in a document discussing routers, chapter 5 limits itself to   the aspects of the protocols that directly relate to forwarding.  The   current chapter contains the remainder of the discussion of the   Internet Layer protocols.4.2 INTERNET PROTOCOL - IP4.2.1 INTRODUCTION   Routers MUST implement the IP protocol, as defined by [INTERNET:1].   They MUST also implement its mandatory extensions: subnets (defined   in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and   Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).   Router implementors need not consider compliance with the section of   [INTRO:2] entitled "Internet Protocol -- IP," as that section is   entirely duplicated or superseded in this document.  A router MUST be   compliant, and SHOULD be unconditionally compliant, with the   requirements of the section entitled "SPECIFIC ISSUES" relating to IP   in [INTRO:2].   In the following, the action specified in certain cases is to   silently discard a received datagram.  This means that the datagram   will be discarded without further processing and that the router will   not send any ICMP error message (see Section [4.3]) as a result.   However, for diagnosis of problems a router SHOULD provide the   capability of logging the error (see Section [1.3.3]), including the   contents of the silently discarded datagram, and SHOULD count   datagrams discarded.Baker                       Standards Track                    [Page 39]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.2 PROTOCOL WALK-THROUGHRFC 791 [INTERNET:1] is the specification for the Internet Protocol.4.2.2.1 Options:RFC 791 Section 3.2   In datagrams received by the router itself, the IP layer MUST   interpret IP options that it understands and preserve the rest   unchanged for use by higher layer protocols.   Higher layer protocols may require the ability to set IP options in   datagrams they send or examine IP options in datagrams they receive.   Later sections of this document discuss specific IP option support   required by higher layer protocols.   DISCUSSION      Neither this memo nor [INTRO:2] define the order in which a      receiver must process multiple options in the same IP header.      Hosts and routers originating datagrams containing multiple      options must be aware that this introduces an ambiguity in the      meaning of certain options when combined with a source-route      option.   Here are the requirements for specific IP options:   (a) Security Option        Some environments require the Security option in every packet        originated or received.  Routers SHOULD IMPLEMENT the revised        security option described in [INTERNET:5].   DISCUSSION      Note that the security options described in [INTERNET:1] andRFC1038 ([INTERNET:16]) are obsolete.   (b) Stream Identifier Option         This option is obsolete; routers SHOULD NOT place this option         in a datagram that the router originates.  This option MUST be         ignored in datagrams received by the router.   (c) Source Route Options         A router MUST be able to act as the final destination of a         source route.  If a router receives a packet containing a         completed source route, the packet has reached its final         destination.  In such an option, the pointer points beyond the         last field and the destination address in the IP headerBaker                       Standards Track                    [Page 40]

RFC 1812         Requirements for IP Version 4 Routers         June 1995         addresses the router.  The option as received (the recorded         route) MUST be passed up to the transport layer (or to ICMP         message processing).         In the general case, a correct response to a source-routed         datagram traverses the same route.  A router MUST provide a         means whereby transport protocols and applications can reverse         the source route in a received datagram.  This reversed source         route MUST be inserted into datagrams they originate (see         [INTRO:2] for details) when the router is unaware of policy         constraints.  However, if the router is policy aware, it MAY         select another path.         Some applications in the router MAY require that the user be         able to enter a source route.         A router MUST NOT originate a datagram containing multiple         source route options.  What a router should do if asked to         forward a packet containing multiple source route options is         described in Section [5.2.4.1].         When a source route option is created (which would happen when         the router is originating a source routed datagram or is         inserting a source route option as a result of a special         filter), it MUST be correctly formed even if it is being         created by reversing a recorded route that erroneously includes         the source host (see case (B) in the discussion below).   DISCUSSION      Suppose a source routed datagram is to be routed from sourceS to      destination D via routers G1, G2, Gn.  Source S constructs a      datagram with G1's IP address as its destination address, and a      source route option to get the datagram the rest of the way to its      destination.  However, there is an ambiguity in the specification      over whether the source route option in a datagram sent out by S      should be (A) or (B):      (A): {>>G2, G3, ... Gn, D} <--- CORRECT      (B): {S, >>G2, G3, ... Gn, D} <---- WRONG      (where >> represents the pointer).  If (A) is sent, the datagram      received at D will contain the option: {G1, G2, ... Gn >>}, with S      and D as the IP source and destination addresses.  If (B) were      sent, the datagram received at D would again contain S and D as      the same IP source and destination addresses, but the option would      be: {S, G1, ...Gn >>}; i.e., the originating host would be the      first hop in the route.Baker                       Standards Track                    [Page 41]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (d) Record Route Option         Routers MAY support the Record Route option in datagrams         originated by the router.   (e) Timestamp Option         Routers MAY support the timestamp option in datagrams         originated by the router.  The following rules apply:         o When originating a datagram containing a Timestamp Option, a            router MUST record a timestamp in the option if            - Its Internet address fields are not pre-specified or            - Its first pre-specified address is the IP address of the               logical interface over which the datagram is being sent               (or the router's router-id if the datagram is being sent               over an unnumbered interface).         o If the router itself receives a datagram containing a            Timestamp Option, the router MUST insert the current time            into the Timestamp Option (if there is space in the option            to do so) before passing the option to the transport layer            or to ICMP for processing.  If space is not present, the            router MUST increment the Overflow Count in the option.         o A timestamp value MUST follow the rules defined in [INTRO:2].   IMPLEMENTATION      To maximize the utility of the timestamps contained in the      timestamp option, the timestamp inserted should be, as nearly as      practical, the time at which the packet arrived at the router.      For datagrams originated by the router, the timestamp inserted      should be, as nearly as practical, the time at which the datagram      was passed to the Link Layer for transmission.      The timestamp option permits the use of a non-standard time clock,      but the use of a non-synchronized clock limits the utility of the      time stamp.  Therefore, routers are well advised to implement the      Network Time Protocol for the purpose of synchronizing their      clocks.4.2.2.2 Addresses in Options:RFC 791 Section 3.1   Routers are called upon to insert their address into Record Route,   Strict Source and Record Route, Loose Source and Record Route, or   Timestamp Options.  When a router inserts its address into such an   option, it MUST use the IP address of the logical interface on whichBaker                       Standards Track                    [Page 42]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   the packet is being sent.  Where this rule cannot be obeyed because   the output interface has no IP address (i.e., is an unnumbered   interface), the router MUST instead insert its router-id.  The   router's router-id is one of the router's IP addresses.  The Router   ID may be specified on a system basis or on a per-link basis.  Which   of the router's addresses is used as the router-id MUST NOT change   (even across reboots) unless changed by the network manager.   Relevant management changes include reconfiguration of the router   such that the IP address used as the router-id ceases to be one of   the router's IP addresses.  Routers with multiple unnumbered   interfaces MAY have multiple router-id's.  Each unnumbered interface   MUST be associated with a particular router-id.  This association   MUST NOT change (even across reboots) without reconfiguration of the   router.   DISCUSSION      This specification does not allow for routers that do not have at      least one IP address.  We do not view this as a serious      limitation, since a router needs an IP address to meet the      manageability requirements of Chapter [8] even if the router is      connected only to point-to-point links.   IMPLEMENTATION      One possible method of choosing the router-id that fulfills this      requirement is to use the numerically smallest (or greatest) IP      address (treating the address as a 32-bit integer) that is      assigned to the router.4.2.2.3 Unused IP Header Bits:RFC 791 Section 3.1   The IP header contains two reserved bits: one in the Type of Service   byte and the other in the Flags field.  A router MUST NOT set either   of these bits to one in datagrams originated by the router.  A router   MUST NOT drop (refuse to receive or forward) a packet merely because   one or more of these reserved bits has a non-zero value; i.e., the   router MUST NOT check the values of thes bits.   DISCUSSION      Future revisions to the IP protocol may make use of these unused      bits.  These rules are intended to ensure that these revisions can      be deployed without having to simultaneously upgrade all routers      in the Internet.Baker                       Standards Track                    [Page 43]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.2.4 Type of Service:RFC 791 Section 3.1   The Type-of-Service byte in the IP header is divided into three   sections: the Precedence field (high-order 3 bits), a field that is   customarily called Type of Service or TOS (next 4 bits), and a   reserved bit (the low order bit).   Rules governing the reserved bit were described in Section [4.2.2.3].   A more extensive discussion of the TOS field and its use can be found   in [ROUTE:11].   The description of the IP Precedence field is superseded by Section   [5.3.3].RFC 795, Service Mappings, is obsolete and SHOULD NOT be   implemented.4.2.2.5 Header Checksum:RFC 791 Section 3.1   As stated in Section [5.2.2], a router MUST verify the IP checksum of   any packet that is received, and MUST discard messages containing   invalid checksums.  The router MUST NOT provide a means to disable   this checksum verification.   A router MAY use incremental IP header checksum updating when the   only change to the IP header is the time to live.  This will reduce   the possibility of undetected corruption of the IP header by the   router.  See [INTERNET:6] for a discussion of incrementally updating   the checksum.   IMPLEMENTATION      A more extensive description of the IP checksum, including      extensive implementation hints, can be found in [INTERNET:6] and      [INTERNET:7].4.2.2.6 Unrecognized Header Options:RFC 791 Section 3.1   A router MUST ignore IP options which it does not recognize.  A   corollary of this requirement is that a router MUST implement the End   of Option List option and the No Operation option, since neither   contains an explicit length.   DISCUSSION      All future IP options will include an explicit length.Baker                       Standards Track                    [Page 44]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.2.7 Fragmentation:RFC 791 Section 3.2   Fragmentation, as described in [INTERNET:1], MUST be supported by a   router.   When a router fragments an IP datagram, it SHOULD minimize the number   of fragments.  When a router fragments an IP datagram, it SHOULD send   the fragments in order.  A fragmentation method that may generate one   IP fragment that is significantly smaller than the other MAY cause   the first IP fragment to be the smaller one.   DISCUSSION      There are several fragmentation techniques in common use in the      Internet.  One involves splitting the IP datagram into IP      fragments with the first being MTU sized, and the others being      approximately the same size, smaller than the MTU.  The reason for      this is twofold.  The first IP fragment in the sequence will be      the effective MTU of the current path between the hosts, and the      following IP fragments are sized to minimize the further      fragmentation of the IP datagram.  Another technique is to split      the IP datagram into MTU sized IP fragments, with the last      fragment being the only one smaller, as described in [INTERNET:1].      A common trick used by some implementations of TCP/IP is to      fragment an IP datagram into IP fragments that are no larger than      576 bytes when the IP datagram is to travel through a router.      This is intended to allow the resulting IP fragments to pass the      rest of the path without further fragmentation.  This would,      though, create more of a load on the destination host, since it      would have a larger number of IP fragments to reassemble into one      IP datagram.  It would also not be efficient on networks where the      MTU only changes once and stays much larger than 576 bytes.      Examples include LAN networks such as an IEEE 802.5 network with a      MTU of 2048 or an Ethernet network with an MTU of 1500).      One other fragmentation technique discussed was splitting the IP      datagram into approximately equal sized IP fragments, with the      size less than or equal to the next hop network's MTU.  This is      intended to minimize the number of fragments that would result      from additional fragmentation further down the path, and assure      equal delay for each fragment.      Routers SHOULD generate the least possible number of IP fragments.      Work with slow machines leads us to believe that if it is      necessary to fragment messages, sending the small IP fragment      first maximizes the chance of a host with a slow interface of      receiving all the fragments.Baker                       Standards Track                    [Page 45]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.2.8 Reassembly:RFC 791 Section 3.2   As specified in the corresponding section of [INTRO:2], a router MUST   support reassembly of datagrams that it delivers to itself.4.2.2.9 Time to Live:RFC 791 Section 3.2   Time to Live (TTL) handling for packets originated or received by the   router is governed by [INTRO:2]; this section changes none of its   stipulations.  However, since the remainder of the IP Protocol   section of [INTRO:2] is rewritten, this section is as well.   Note in particular that a router MUST NOT check the TTL of a packet   except when forwarding it.   A router MUST NOT originate or forward a datagram with a Time-to-Live   (TTL) value of zero.   A router MUST NOT discard a datagram just because it was received   with TTL equal to zero or one; if it is to the router and otherwise   valid, the router MUST attempt to receive it.   On messages the router originates, the IP layer MUST provide a means   for the transport layer to set the TTL field of every datagram that   is sent.  When a fixed TTL value is used, it MUST be configurable.   The number SHOULD exceed the typical internet diameter, and current   wisdom suggests that it should exceed twice the internet diameter to   allow for growth.  Current suggested values are normally posted in   the Assigned Numbers RFC.  The TTL field has two functions: limit the   lifetime of TCP segments (seeRFC 793 [TCP:1], p. 28), and terminate   Internet routing loops.  Although TTL is a time in seconds, it also   has some attributes of a hop-count, since each router is required to   reduce the TTL field by at least one.   TTL expiration is intended to cause datagrams to be discarded by   routers, but not by the destination host.  Hosts that act as routers   by forwarding datagrams must therefore follow the router's rules for   TTL.   A higher-layer protocol may want to set the TTL in order to implement   an "expanding scope" search for some Internet resource.  This is used   by some diagnostic tools, and is expected to be useful for locating   the "nearest" server of a given class using IP multicasting, for   example.  A particular transport protocol may also want to specify   its own TTL bound on maximum datagram lifetime.   A fixed default value must be at least big enough for the Internet   "diameter," i.e., the longest possible path.  A reasonable value isBaker                       Standards Track                    [Page 46]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   about twice the diameter, to allow for continued Internet growth.  As   of this writing, messages crossing the United States frequently   traverse 15 to 20 routers; this argues for a default TTL value in   excess of 40, and 64 is a common value.4.2.2.10 Multi-subnet Broadcasts:RFC 922   All-subnets broadcasts (called multi-subnet broadcasts in   [INTERNET:3]) have been deprecated.  See Section [5.3.5.3].4.2.2.11 Addressing:RFC 791 Section 3.2   As noted in 2.2.5.1, there are now five classes of IP addresses:   Class A through Class E.  Class D addresses are used for IP   multicasting [INTERNET:4], while Class E addresses are reserved for   experimental use.  The distinction between Class A, B, and C   addresses is no longer important; they are used as generalized   unicast network prefixes with only historical interest in their   class.   An IP multicast address is a 28-bit logical address that stands for a   group of hosts, and may be either permanent or transient.  Permanent   multicast addresses are allocated by the Internet Assigned Number   Authority [INTRO:7], while transient addresses may be allocated   dynamically to transient groups.  Group membership is determined   dynamically using IGMP [INTERNET:4].   We now summarize the important special cases for general purpose   unicast IP addresses, using the following notation for an IP address:    { <Network-prefix>, <Host-number> }   and the notation -1 for a field that contains all 1 bits and the   notation 0 for a field that contains all 0 bits.   (a) { 0, 0 }        This host on this network.  It MUST NOT be used as a source        address by routers, except the router MAY use this as a source        address as part of an initialization procedure (e.g., if the        router is using BOOTP to load its configuration information).        Incoming datagrams with a source address of { 0, 0 } which are        received for local delivery (see Section [5.2.3]), MUST be        accepted if the router implements the associated protocol and        that protocol clearly defines appropriate action to be taken.        Otherwise, a router MUST silently discard any locally-delivered        datagram whose source address is { 0, 0 }.Baker                       Standards Track                    [Page 47]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      Some protocols define specific actions to take in response to a      received datagram whose source address is { 0, 0 }.  Two examples      are BOOTP and ICMP Mask Request.  The proper operation of these      protocols often depends on the ability to receive datagrams whose      source address is { 0, 0 }.  For most protocols, however, it is      best to ignore datagrams having a source address of { 0, 0 } since      they were probably generated by a misconfigured host or router.      Thus, if a router knows how to deal with a given datagram having a      { 0, 0 } source address, the router MUST accept it.  Otherwise,      the router MUST discard it.   See also Section [4.2.3.1] for a non-standard use of { 0, 0 }.   (b) { 0, <Host-number> }         Specified host on this network.  It MUST NOT be sent by routers         except that the router MAY use this as a source address as part         of an initialization procedure by which the it learns its own         IP address.   (c) { -1, -1 }         Limited broadcast.  It MUST NOT be used as a source address.         A datagram with this destination address will be received by         every host and router on the connected physical network, but         will not be forwarded outside that network.   (d) { <Network-prefix>, -1 }         Directed Broadcast - a broadcast directed to the specified         network prefix.  It MUST NOT be used as a source address.  A         router MAY originate Network Directed Broadcast packets.  A         router MUST receive Network Directed Broadcast packets; however         a router MAY have a configuration option to prevent reception         of these packets.  Such an option MUST default to allowing         reception.    (e) { 127, <any> }         Internal host loopback address.  Addresses of this form MUST         NOT appear outside a host.    The <Network-prefix> is administratively assigned so that its value    will be unique in the routing domain to which the device is    connected.Baker                       Standards Track                    [Page 48]

RFC 1812         Requirements for IP Version 4 Routers         June 1995    IP addresses are not permitted to have the value 0 or -1 for the    <Host-number> or <Network-prefix> fields except in the special cases    listed above.  This implies that each of these fields will be at    least two bits long.   DISCUSSION      Previous versions of this document also noted that subnet numbers      must be neither 0 nor -1, and must be at least two bits in length.      In a CIDR world, the subnet number is clearly an extension of the      network prefix and cannot be interpreted without the remainder of      the prefix.  This restriction of subnet numbers is therefore      meaningless in view of CIDR and may be safely ignored.   For further discussion of broadcast addresses, see Section [4.2.3.1].   When a router originates any datagram, the IP source address MUST be   one of its own IP addresses (but not a broadcast or multicast   address).  The only exception is during initialization.   For most purposes, a datagram addressed to a broadcast or multicast   destination is processed as if it had been addressed to one of the   router's IP addresses; that is to say:   o A router MUST receive and process normally any packets with a      broadcast destination address.   o A router MUST receive and process normally any packets sent to a      multicast destination address that the router has asked to      receive.   The term specific-destination address means the equivalent local IP   address of the host.  The specific-destination address is defined to   be the destination address in the IP header unless the header   contains a broadcast or multicast address, in which case the   specific-destination is an IP address assigned to the physical   interface on which the datagram arrived.   A router MUST silently discard any received datagram containing an IP   source address that is invalid by the rules of this section.  This   validation could be done either by the IP layer or (when appropriate)   by each protocol in the transport layer.  As with any datagram a   router discards, the datagram discard SHOULD be counted.   DISCUSSION      A misaddressed datagram might be caused by a Link Layer broadcast      of a unicast datagram or by another router or host that is      confused or misconfigured.Baker                       Standards Track                    [Page 49]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.2.3 SPECIFIC ISSUES4.2.3.1 IP Broadcast Addresses   For historical reasons, there are a number of IP addresses (some   standard and some not) which are used to indicate that an IP packet   is an IP broadcast.  A router   (1) MUST treat as IP broadcasts packets addressed to 255.255.255.255        or { <Network-prefix>, -1 }.   (2) SHOULD silently discard on receipt (i.e., do not even deliver to        applications in the router) any packet addressed to 0.0.0.0 or {        <Network-prefix>, 0 }.  If these packets are not silently        discarded, they MUST be treated as IP broadcasts (see Section        [5.3.5]).  There MAY be a configuration option to allow receipt        of these packets.  This option SHOULD default to discarding        them.   (3) SHOULD (by default) use the limited broadcast address        (255.255.255.255) when originating an IP broadcast destined for        a connected (sub)network (except when sending an ICMP Address        Mask Reply, as discussed in Section [4.3.3.9]).  A router MUST        receive limited broadcasts.   (4) SHOULD NOT originate datagrams addressed to 0.0.0.0 or {        <Network-prefix>, 0 }.  There MAY be a configuration option to        allow generation of these packets (instead of using the relevant        1s format broadcast).  This option SHOULD default to not        generating them.   DISCUSSION      In the second bullet, the router obviously cannot recognize      addresses of the form { <Network-prefix>, 0 } if the router has no      interface to that network prefix.  In that case, the rules of the      second bullet do not apply because, from the point of view of the      router, the packet is not an IP broadcast packet.4.2.3.2 IP Multicasting   An IP router SHOULD satisfy the Host Requirements with respect to IP   multicasting, as specified in [INTRO:2].  An IP router SHOULD support   local IP multicasting on all connected networks.  When a mapping from   IP multicast addresses to link-layer addresses has been specified   (see the various IP-over-xxx specifications), it SHOULD use that   mapping, and MAY be configurable to use the link layer broadcast   instead.  On point-to-point links and all other interfaces,   multicasts are encapsulated as link layer broadcasts.  Support forBaker                       Standards Track                    [Page 50]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   local IP multicasting includes originating multicast datagrams,   joining multicast groups and receiving multicast datagrams, and   leaving multicast groups.  This implies support for all of   [INTERNET:4] including IGMP (see Section [4.4]).   DISCUSSION      Although [INTERNET:4] is entitled Host Extensions for IP      Multicasting, it applies to all IP systems, both hosts and      routers.  In particular, since routers may join multicast groups,      it is correct for them to perform the host part of IGMP, reporting      their group memberships to any multicast routers that may be      present on their attached networks (whether or not they themselves      are multicast routers).      Some router protocols may specifically require support for IP      multicasting (e.g., OSPF [ROUTE:1]), or may recommend it (e.g.,      ICMP Router Discovery [INTERNET:13]).4.2.3.3 Path MTU Discovery   To eliminate fragmentation or minimize it, it is desirable to know   what is the path MTU along the path from the source to destination.   The path MTU is the minimum of the MTUs of each hop in the path.   [INTERNET:14] describes a technique for dynamically discovering the   maximum transmission unit (MTU) of an arbitrary internet path.  For a   path that passes through a router that does not support   [INTERNET:14], this technique might not discover the correct Path   MTU, but it will always choose a Path MTU as accurate as, and in many   cases more accurate than, the Path MTU that would be chosen by older   techniques or the current practice.   When a router is originating an IP datagram, it SHOULD use the scheme   described in [INTERNET:14] to limit the datagram's size.  If the   router's route to the datagram's destination was learned from a   routing protocol that provides Path MTU information, the scheme   described in [INTERNET:14] is still used, but the Path MTU   information from the routing protocol SHOULD be used as the initial   guess as to the Path MTU and also as an upper bound on the Path MTU.4.2.3.4 Subnetting   Under certain circumstances, it may be desirable to support subnets   of a particular network being interconnected only through a path that   is not part of the subnetted network.  This is known as discontiguous   subnetwork support.   Routers MUST support discontiguous subnetworks.Baker                       Standards Track                    [Page 51]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   IMPLEMENTATION      In classical IP networks, this was very difficult to achieve; in      CIDR networks, it is a natural by-product.  Therefore, a router      SHOULD NOT make assumptions about subnet architecture, but SHOULD      treat each route as a generalized network prefix.   DISCUSSION The Internet has been growing at a tremendous rate of      late.  This has been placing severe strains on the IP addressing      technology.  A major factor in this strain is the strict IP      Address class boundaries.  These make it difficult to efficiently      size network prefixes to their networks and aggregate several      network prefixes into a single route advertisement.  By      eliminating the strict class boundaries of the IP address and      treating each route as a generalized network prefix, these strains      may be greatly reduced.      The technology for currently doing this is Classless Inter Domain      Routing (CIDR) [INTERNET:15].   For similar reasons, an address block associated with a given network   prefix could be subdivided into subblocks of different sizes, so that   the network prefixes associated with the subblocks would have   different length.  For example, within a block whose network prefix   is 8 bits long, one subblock may have a 16 bit network prefix,   another may have an 18 bit network prefix, and a third a 14 bit   network prefix.   Routers MUST support variable length network prefixes in both their   interface configurations and their routing databases.4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP4.3.1 INTRODUCTION   ICMP is an auxiliary protocol, which provides routing, diagnostic and   error functionality for IP.  It is described in [INTERNET:8].  A   router MUST support ICMP.   ICMP messages are grouped in two classes that are discussed in the   following sections:   ICMP error messages:   Destination UnreachableSection 4.3.3.1   RedirectSection 4.3.3.2   Source QuenchSection 4.3.3.3   Time ExceededSection 4.3.3.4   Parameter ProblemSection 4.3.3.5Baker                       Standards Track                    [Page 52]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   ICMP query messages:   EchoSection 4.3.3.6   InformationSection 4.3.3.7   TimestampSection 4.3.3.8   Address MaskSection 4.3.3.9   Router DiscoverySection 4.3.3.10   General ICMP requirements and discussion are in the next section.4.3.2 GENERAL ISSUES4.3.2.1 Unknown Message Types   If an ICMP message of unknown type is received, it MUST be passed to   the ICMP user interface (if the router has one) or silently discarded   (if the router does not have one).4.3.2.2 ICMP Message TTL   When originating an ICMP message, the router MUST initialize the TTL.   The TTL for ICMP responses must not be taken from the packet that   triggered the response.4.3.2.3 Original Message Header   Historically, every ICMP error message has included the Internet   header and at least the first 8 data bytes of the datagram that   triggered the error.  This is no longer adequate, due to the use of   IP-in-IP tunneling and other technologies.  Therefore, the ICMP   datagram SHOULD contain as much of the original datagram as possible   without the length of the ICMP datagram exceeding 576 bytes.  The   returned IP header (and user data) MUST be identical to that which   was received, except that the router is not required to undo any   modifications to the IP header that are normally performed in   forwarding that were performed before the error was detected (e.g.,   decrementing the TTL, or updating options).  Note that the   requirements of Section [4.3.3.5] supersede this requirement in some   cases (i.e., for a Parameter Problem message, if the problem is in a   modified field, the router must undo the modification).  See Section   [4.3.3.5]).4.3.2.4 ICMP Message Source Address   Except where this document specifies otherwise, the IP source address   in an ICMP message originated by the router MUST be one of the IP   addresses associated with the physical interface over which the ICMP   message is transmitted.  If the interface has no IP addressesBaker                       Standards Track                    [Page 53]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   associated with it, the router's router-id (see Section [5.2.5]) is   used instead.4.3.2.5 TOS and Precedence   ICMP error messages SHOULD have their TOS bits set to the same value   as the TOS bits in the packet that provoked the sending of the ICMP   error message, unless setting them to that value would cause the ICMP   error message to be immediately discarded because it could not be   routed to its destination.  Otherwise, ICMP error messages MUST be   sent with a normal (i.e., zero) TOS.  An ICMP reply message SHOULD   have its TOS bits set to the same value as the TOS bits in the ICMP   request that provoked the reply.   ICMP Source Quench error messages, if sent at all, MUST have their IP   Precedence field set to the same value as the IP Precedence field in   the packet that provoked the sending of the ICMP Source Quench   message.  All other ICMP error messages (Destination Unreachable,   Redirect, Time Exceeded, and Parameter Problem) SHOULD have their   precedence value set to 6 (INTERNETWORK CONTROL) or 7 (NETWORK   CONTROL).  The IP Precedence value for these error messages MAY be   settable.   An ICMP reply message MUST have its IP Precedence field set to the   same value as the IP Precedence field in the ICMP request that   provoked the reply.4.3.2.6 Source Route   If the packet which provokes the sending of an ICMP error message   contains a source route option, the ICMP error message SHOULD also   contain a source route option of the same type (strict or loose),   created by reversing the portion before the pointer of the route   recorded in the source route option of the original packet UNLESS the   ICMP error message is an ICMP Parameter Problem complaining about a   source route option in the original packet, or unless the router is   aware of policy that would prevent the delivery of the ICMP error   message.   DISCUSSION      In environments which use the U.S.  Department of Defense security      option (defined in [INTERNET:5]), ICMP messages may need to      include a security option.  Detailed information on this topic      should be available from the Defense Communications Agency.Baker                       Standards Track                    [Page 54]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.3.2.7 When Not to Send ICMP Errors   An ICMP error message MUST NOT be sent as the result of receiving:   o An ICMP error message, or   o A packet which fails the IP header validation tests described in      Section [5.2.2] (except where that section specifically permits      the sending of an ICMP error message), or   o A packet destined to an IP broadcast or IP multicast address, or   o A packet sent as a Link Layer broadcast or multicast, or   o A packet whose source address has a network prefix of zero or is an      invalid source address (as defined in Section [5.3.7]), or   o Any fragment of a datagram other then the first fragment (i.e., a      packet for which the fragment offset in the IP header is nonzero).   Furthermore, an ICMP error message MUST NOT be sent in any case where   this memo states that a packet is to be silently discarded.   NOTE: THESE RESTRICTIONS TAKE PRECEDENCE OVER ANY REQUIREMENT   ELSEWHERE IN THIS DOCUMENT FOR SENDING ICMP ERROR MESSAGES.   DISCUSSION      These rules aim to prevent the broadcast storms that have resulted      from routers or hosts returning ICMP error messages in response to      broadcast packets.  For example, a broadcast UDP packet to a non-      existent port could trigger a flood of ICMP Destination      Unreachable datagrams from all devices that do not have a client      for that destination port.  On a large Ethernet, the resulting      collisions can render the network useless for a second or more.      Every packet that is broadcast on the connected network should      have a valid IP broadcast address as its IP destination (see      Section [5.3.4] and [INTRO:2]).  However, some devices violate      this rule.  To be certain to detect broadcast packets, therefore,      routers are required to check for a link-layer broadcast as well      as an IP-layer address.   IMPLEMENTATION+ This requires that the link layer inform the IP layer      when a link-layer broadcast packet has been received; see Section      [3.1].Baker                       Standards Track                    [Page 55]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.3.2.8 Rate Limiting   A router which sends ICMP Source Quench messages MUST be able to   limit the rate at which the messages can be generated.  A router   SHOULD also be able to limit the rate at which it sends other sorts   of ICMP error messages (Destination Unreachable, Redirect, Time   Exceeded, Parameter Problem).  The rate limit parameters SHOULD be   settable as part of the configuration of the router.  How the limits   are applied (e.g., per router or per interface) is left to the   implementor's discretion.   DISCUSSION      Two problems for a router sending ICMP error message are:      (1) The consumption of bandwidth on the reverse path, and      (2) The use of router resources (e.g., memory, CPU time)      To help solve these problems a router can limit the frequency with      which it generates ICMP error messages.  For similar reasons, a      router may limit the frequency at which some other sorts of      messages, such as ICMP Echo Replies, are generated.   IMPLEMENTATION      Various mechanisms have been used or proposed for limiting the      rate at which ICMP messages are sent:      (1) Count-based - for example, send an ICMP error message for           every N dropped packets overall or per given source host.           This mechanism might be appropriate for ICMP Source Quench,           if used, but probably not for other types of ICMP messages.      (2) Timer-based - for example, send an ICMP error message to a           given source host or overall at most once per T milliseconds.      (3) Bandwidth-based - for example, limit the rate at which ICMP           messages are sent over a particular interface to some           fraction of the attached network's bandwidth.4.3.3 SPECIFIC ISSUES4.3.3.1 Destination Unreachable   If a router cannot forward a packet because it has no routes at all   (including no default route) to the destination specified in the   packet, then the router MUST generate a Destination Unreachable, Code   0 (Network Unreachable) ICMP message.  If the router does have routes   to the destination network specified in the packet but the TOS   specified for the routes is neither the default TOS (0000) nor the   TOS of the packet that the router is attempting to route, then theBaker                       Standards Track                    [Page 56]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   router MUST generate a Destination Unreachable, Code 11 (Network   Unreachable for TOS) ICMP message.   If a packet is to be forwarded to a host on a network that is   directly connected to the router (i.e., the router is the last-hop   router) and the router has ascertained that there is no path to the   destination host then the router MUST generate a Destination   Unreachable, Code 1 (Host Unreachable) ICMP message.  If a packet is   to be forwarded to a host that is on a network that is directly   connected to the router and the router cannot forward the packet   because no route to the destination has a TOS that is either equal to   the TOS requested in the packet or is the default TOS (0000) then the   router MUST generate a Destination Unreachable, Code 12 (Host   Unreachable for TOS) ICMP message.   DISCUSSION      The intent is that a router generates the "generic" host/network      unreachable if it has no path at all (including default routes) to      the destination.  If the router has one or more paths to the      destination, but none of those paths have an acceptable TOS, then      the router generates the "unreachable for TOS" message.4.3.3.2 Redirect   The ICMP Redirect message is generated to inform a local host that it   should use a different next hop router for certain traffic.   Contrary to [INTRO:2], a router MAY ignore ICMP Redirects when   choosing a path for a packet originated by the router if the router   is running a routing protocol or if forwarding is enabled on the   router and on the interface over which the packet is being sent.4.3.3.3 Source Quench   A router SHOULD NOT originate ICMP Source Quench messages.  As   specified in Section [4.3.2], a router that does originate Source   Quench messages MUST be able to limit the rate at which they are   generated.   DISCUSSION      Research seems to suggest that Source Quench consumes network      bandwidth but is an ineffective (and unfair) antidote to      congestion.  See, for example, [INTERNET:9] and [INTERNET:10].      Section [5.3.6] discusses the current thinking on how routers      ought to deal with overload and network congestion.   A router MAY ignore any ICMP Source Quench messages it receives.Baker                       Standards Track                    [Page 57]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      A router itself may receive a Source Quench as the result of      originating a packet sent to another router or host.  Such      datagrams might be, e.g., an EGP update sent to another router, or      a telnet stream sent to a host.  A mechanism has been proposed      ([INTERNET:11], [INTERNET:12]) to make the IP layer respond      directly to Source Quench by controlling the rate at which packets      are sent, however, this proposal is currently experimental and not      currently recommended.4.3.3.4 Time Exceeded   When a router is forwarding a packet and the TTL field of the packet   is reduced to 0, the requirements of section [5.2.3.8] apply.   When the router is reassembling a packet that is destined for the   router, it is acting as an Internet host.  [INTRO:2]'s reassembly   requirements therefore apply.   When the router receives (i.e., is destined for the router) a Time   Exceeded message, it MUST comply with [INTRO:2].4.3.3.5 Parameter Problem   A router MUST generate a Parameter Problem message for any error not   specifically covered by another ICMP message.  The IP header field or   IP option including the byte indicated by the pointer field MUST be   included unchanged in the IP header returned with this ICMP message.   Section [4.3.2] defines an exception to this requirement.   A new variant of the Parameter Problem message was defined in   [INTRO:2]:        Code 1 = required option is missing.   DISCUSSION      This variant is currently in use in the military community for a      missing security option.4.3.3.6 Echo Request/Reply   A router MUST implement an ICMP Echo server function that receives   Echo Requests sent to the router, and sends corresponding Echo   Replies.  A router MUST be prepared to receive, reassemble and echo   an ICMP Echo Request datagram at least as the maximum of 576 and the   MTUs of all the connected networks.   The Echo server function MAY choose not to respond to ICMP echo   requests addressed to IP broadcast or IP multicast addresses.Baker                       Standards Track                    [Page 58]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   A router SHOULD have a configuration option that, if enabled, causes   the router to silently ignore all ICMP echo requests; if provided,   this option MUST default to allowing responses.   DISCUSSION      The neutral provision about responding to broadcast and multicast      Echo Requests derives from [INTRO:2]'s "Echo Request/Reply"      section.   As stated in Section [10.3.3], a router MUST also implement a   user/application-layer interface for sending an Echo Request and   receiving an Echo Reply, for diagnostic purposes.  All ICMP Echo   Reply messages MUST be passed to this interface.   The IP source address in an ICMP Echo Reply MUST be the same as the   specific-destination address of the corresponding ICMP Echo Request   message.   Data received in an ICMP Echo Request MUST be entirely included in   the resulting Echo Reply.   If a Record Route and/or Timestamp option is received in an ICMP Echo   Request, this option (these options) SHOULD be updated to include the   current router and included in the IP header of the Echo Reply   message, without truncation.  Thus, the recorded route will be for   the entire round trip.   If a Source Route option is received in an ICMP Echo Request, the   return route MUST be reversed and used as a Source Route option for   the Echo Reply message, unless the router is aware of policy that   would prevent the delivery of the message.4.3.3.7 Information Request/Reply   A router SHOULD NOT originate or respond to these messages.   DISCUSSION      The Information Request/Reply pair was intended to support self-      configuring systems such as diskless workstations, to allow them      to discover their IP network prefixes at boot time.  However,      these messages are now obsolete.  The RARP and BOOTP protocols      provide better mechanisms for a host to discover its own IP      address.4.3.3.8 Timestamp and Timestamp Reply   A router MAY implement Timestamp and Timestamp Reply.  If they are   implemented then:Baker                       Standards Track                    [Page 59]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o The ICMP Timestamp server function MUST return a Timestamp Reply to      every Timestamp message that is received.  It SHOULD be designed      for minimum variability in delay.   o An ICMP Timestamp Request message to an IP broadcast or IP      multicast address MAY be silently discarded.   o The IP source address in an ICMP Timestamp Reply MUST be the same      as the specific-destination address of the corresponding Timestamp      Request message.   o If a Source Route option is received in an ICMP Timestamp Request,      the return route MUST be reversed and used as a Source Route      option for the Timestamp Reply message, unless the router is aware      of policy that would prevent the delivery of the message.   o If a Record Route and/or Timestamp option is received in a      Timestamp Request, this (these) option(s) SHOULD be updated to      include the current router and included in the IP header of the      Timestamp Reply message.   o If the router provides an application-layer interface for sending      Timestamp Request messages then incoming Timestamp Reply messages      MUST be passed up to the ICMP user interface.   The preferred form for a timestamp value (the standard value) is   milliseconds since midnight, Universal Time.  However, it may be   difficult to provide this value with millisecond resolution.  For   example, many systems use clocks that update only at line frequency,   50 or 60 times per second.  Therefore, some latitude is allowed in a   standard value:   (a) A standard value MUST be updated at least 16 times per second        (i.e., at most the six low-order bits of the value may be        undefined).   (b) The accuracy of a standard value MUST approximate that of        operator-set CPU clocks, i.e., correct within a few minutes.   IMPLEMENTATION      To meet the second condition, a router may need to query some time      server when the router is booted or restarted.  It is recommended      that the UDP Time Server Protocol be used for this purpose.  A      more advanced implementation would use the Network Time Protocol      (NTP) to achieve nearly millisecond clock synchronization;      however, this is not required.Baker                       Standards Track                    [Page 60]

RFC 1812         Requirements for IP Version 4 Routers         June 19954.3.3.9 Address Mask Request/Reply   A router MUST implement support for receiving ICMP Address Mask   Request messages and responding with ICMP Address Mask Reply   messages.  These messages are defined in [INTERNET:2].   A router SHOULD have a configuration option for each logical   interface specifying whether the router is allowed to answer Address   Mask Requests for that interface; this option MUST default to   allowing responses.  A router MUST NOT respond to an Address Mask   Request before the router knows the correct address mask.   A router MUST NOT respond to an Address Mask Request that has a   source address of 0.0.0.0 and which arrives on a physical interface   that has associated with it multiple logical interfaces and the   address masks for those interfaces are not all the same.   A router SHOULD examine all ICMP Address Mask Replies that it   receives to determine whether the information it contains matches the   router's knowledge of the address mask.  If the ICMP Address Mask   Reply appears to be in error, the router SHOULD log the address mask   and the sender's IP address.  A router MUST NOT use the contents of   an ICMP Address Mask Reply to determine the correct address mask.   Because hosts may not be able to learn the address mask if a router   is down when the host boots up, a router MAY broadcast a gratuitous   ICMP Address Mask Reply on each of its logical interfaces after it   has configured its own address masks.  However, this feature can be   dangerous in environments that use variable length address masks.   Therefore, if this feature is implemented, gratuitous Address Mask   Replies MUST NOT be broadcast over any logical interface(s) which   either:   o Are not configured to send gratuitous Address Mask Replies.  Each      logical interface MUST have a configuration parameter controlling      this, and that parameter MUST default to not sending the      gratuitous Address Mask Replies.   o Share subsuming (but not identical) network prefixes and physical      interface.   The { <Network-prefix>, -1 } form of the IP broadcast address MUST be   used for broadcast Address Mask Replies.   DISCUSSION      The ability to disable sending Address Mask Replies by routers is      required at a few sites that intentionally lie to their hosts      about the address mask.  The need for this is expected to go awayBaker                       Standards Track                    [Page 61]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      as more and more hosts become compliant with the Host Requirements      standards.      The reason for both the second bullet above and the requirement      about which IP broadcast address to use is to prevent problems      when multiple IP network prefixes are in use on the same physical      network.4.3.3.10 Router Advertisement and Solicitations   An IP router MUST support the router part of the ICMP Router   Discovery Protocol [INTERNET:13] on all connected networks on which   the router supports either IP multicast or IP broadcast addressing.   The implementation MUST include all the configuration variables   specified for routers, with the specified defaults.   DISCUSSION      Routers are not required to implement the host part of the ICMP      Router Discovery Protocol, but might find it useful for operation      while IP forwarding is disabled (i.e., when operating as a host).   DISCUSSION We note that it is quite common for hosts to use RIP      Version 1 as the router discovery protocol.  Such hosts listen to      RIP traffic and use and use information extracted from that      traffic to discover routers and to make decisions as to which      router to use as a first-hop router for a given destination.      While this behavior is discouraged, it is still common and      implementors should be aware of it.4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP   IGMP [INTERNET:4] is a protocol used between hosts and multicast   routers on a single physical network to establish hosts' membership   in particular multicast groups.  Multicast routers use this   information, in conjunction with a multicast routing protocol, to   support IP multicast forwarding across the Internet.   A router SHOULD implement the host part of IGMP.Baker                       Standards Track                    [Page 62]

RFC 1812         Requirements for IP Version 4 Routers         June 19955. INTERNET LAYER - FORWARDING5.1 INTRODUCTION   This section describes the process of forwarding packets.5.2 FORWARDING WALK-THROUGH   There is no separate specification of the forwarding function in IP.   Instead, forwarding is covered by the protocol specifications for the   internet layer protocols ([INTERNET:1], [INTERNET:2], [INTERNET:3],   [INTERNET:8], and [ROUTE:11]).5.2.1 Forwarding Algorithm   Since none of the primary protocol documents describe the forwarding   algorithm in any detail, we present it here.  This is just a general   outline, and omits important details, such as handling of congestion,   that are dealt with in later sections.   It is not required that an implementation follow exactly the   algorithms given in sections [5.2.1.1], [5.2.1.2], and [5.2.1.3].   Much of the challenge of writing router software is to maximize the   rate at which the router can forward packets while still achieving   the same effect of the algorithm.  Details of how to do that are   beyond the scope of this document, in part because they are heavily   dependent on the architecture of the router.  Instead, we merely   point out the order dependencies among the steps:   (1) A router MUST verify the IP header, as described in section        [5.2.2], before performing any actions based on the contents of        the header.  This allows the router to detect and discard bad        packets before the expenditure of other resources.   (2) Processing of certain IP options requires that the router insert        its IP address into the option.  As noted in Section [5.2.4],        the address inserted MUST be the address of the logical        interface on which the packet is sent or the router's router-id        if the packet is sent over an unnumbered interface.  Thus,        processing of these options cannot be completed until after the        output interface is chosen.   (3) The router cannot check and decrement the TTL before checking        whether the packet should be delivered to the router itself, for        reasons mentioned in Section [4.2.2.9].   (4) More generally, when a packet is delivered locally to the router,        its IP header MUST NOT be modified in any way (except that aBaker                       Standards Track                    [Page 63]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        router may be required to insert a timestamp into any Timestamp        options in the IP header).  Thus, before the router determines        whether the packet is to be delivered locally to the router, it        cannot update the IP header in any way that it is not prepared        to undo.5.2.1.1 General   This section covers the general forwarding algorithm.  This algorithm   applies to all forms of packets to be forwarded: unicast, multicast,   and broadcast.   (1) The router receives the IP packet (plus additional information        about it, as described in Section [3.1]) from the Link Layer.   (2) The router validates the IP header, as described in Section        [5.2.2].  Note that IP reassembly is not done, except on IP        fragments to be queued for local delivery in step (4).   (3) The router performs most of the processing of any IP options.  As        described in Section [5.2.4], some IP options require additional        processing after the routing decision has been made.   (4) The router examines the destination IP address of the IP        datagram, as described in Section [5.2.3], to determine how it        should continue to process the IP datagram.  There are three        possibilities:        o The IP datagram is destined for the router, and should be           queued for local delivery, doing reassembly if needed.        o The IP datagram is not destined for the router, and should be           queued for forwarding.        o The IP datagram should be queued for forwarding, but (a copy)           must also be queued for local delivery.5.2.1.2 Unicast   Since the local delivery case is well covered by [INTRO:2], the   following assumes that the IP datagram was queued for forwarding.  If   the destination is an IP unicast address:   (5) The forwarder determines the next hop IP address for the packet,        usually by looking up the packet's destination in the router's        routing table.  This procedure is described in more detail in        Section [5.2.4].  This procedure also decides which networkBaker                       Standards Track                    [Page 64]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        interface should be used to send the packet.   (6) The forwarder verifies that forwarding the packet is permitted.        The source and destination addresses should be valid, as        described in Section [5.3.7] and Section [5.3.4] If the router        supports administrative constraints on forwarding, such as those        described in Section [5.3.9], those constraints must be        satisfied.   (7) The forwarder decrements (by at least one) and checks the        packet's TTL, as described in Section [5.3.1].   (8) The forwarder performs any IP option processing that could not be        completed in step 3.   (9) The forwarder performs any necessary IP fragmentation, as        described in Section [4.2.2.7].  Since this step occurs after        outbound interface selection (step 5), all fragments of the same        datagram will be transmitted out the same interface.   (10) The forwarder determines the Link Layer address of the packet's        next hop.  The mechanisms for doing this are Link Layer-        dependent (see chapter 3).   (11) The forwarder encapsulates the IP datagram (or each of the        fragments thereof) in an appropriate Link Layer frame and queues        it for output on the interface selected in step 5.   (12) The forwarder sends an ICMP redirect if necessary, as described        in Section [4.3.3.2].5.2.1.3 Multicast   If the destination is an IP multicast, the following steps are taken.   Note that the main differences between the forwarding of IP unicasts   and the forwarding of IP multicasts are   o IP multicasts are usually forwarded based on both the datagram's      source and destination IP addresses,   o IP multicast uses an expanding ring search,   o IP multicasts are forwarded as Link Level multicasts, and   o ICMP errors are never sent in response to IP multicast datagrams.Baker                       Standards Track                    [Page 65]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Note that the forwarding of IP multicasts is still somewhat   experimental.  As a result, the algorithm presented below is not   mandatory, and is provided as an example only.   (5a) Based on the IP source and destination addresses found in the        datagram header, the router determines whether the datagram has        been received on the proper interface for forwarding.  If not,        the datagram is dropped silently.  The method for determining        the proper receiving interface depends on the multicast routing        algorithm(s) in use.  In one of the simplest algorithms, reverse        path forwarding (RPF), the proper interface is the one that        would be used to forward unicasts back to the datagram source.   (6a) Based on the IP source and destination addresses found in the        datagram header, the router determines the datagram's outgoing        interfaces.  To implement IP multicast's expanding ring search        (see [INTERNET:4]) a minimum TTL value is specified for each        outgoing interface.  A copy of the multicast datagram is        forwarded out each outgoing interface whose minimum TTL value is        less than or equal to the TTL value in the datagram header, by        separately applying the remaining steps on each such interface.   (7a) The router decrements the packet's TTL by one.   (8a) The forwarder performs any IP option processing that could not        be completed in step (3).   (9a) The forwarder performs any necessary IP fragmentation, as        described in Section [4.2.2.7].   (10a) The forwarder determines the Link Layer address to use in the        Link Level encapsulation.  The mechanisms for doing this are        Link Layer-dependent.  On LANs a Link Level multicast or        broadcast is selected, as an algorithmic translation of the        datagrams' IP multicast address.  See the various IP-over-xxx        specifications for more details.   (11a) The forwarder encapsulates the packet (or each of the fragments        thereof) in an appropriate Link Layer frame and queues it for        output on the appropriate interface.Baker                       Standards Track                    [Page 66]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.2.2 IP Header Validation   Before a router can process any IP packet, it MUST perform a the   following basic validity checks on the packet's IP header to ensure   that the header is meaningful.  If the packet fails any of the   following tests, it MUST be silently discarded, and the error SHOULD   be logged.   (1) The packet length reported by the Link Layer must be large enough        to hold the minimum length legal IP datagram (20 bytes).   (2) The IP checksum must be correct.   (3) The IP version number must be 4.  If the version number is not 4        then the packet may be another version of IP, such as IPng or        ST-II.   (4) The IP header length field must be large enough to hold the        minimum length legal IP datagram (20 bytes = 5 words).   (5) The IP total length field must be large enough to hold the IP        datagram header, whose length is specified in the IP header        length field.   A router MUST NOT have a configuration option that allows disabling   any of these tests.   If the packet passes the second and third tests, the IP header length   field is at least 4, and both the IP total length field and the   packet length reported by the Link Layer are at least 16 then,   despite the above rule, the router MAY respond with an ICMP Parameter   Problem message, whose pointer points at the IP header length field   (if it failed the fourth test) or the IP total length field (if it   failed the fifth test).  However, it still MUST discard the packet   and still SHOULD log the error.   These rules (and this entire document) apply only to version 4 of the   Internet Protocol.  These rules should not be construed as   prohibiting routers from supporting other versions of IP.   Furthermore, if a router can truly classify a packet as being some   other version of IP then it ought not treat that packet as an error   packet within the context of this memo.   IMPLEMENTATION      It is desirable for purposes of error reporting, though not always      entirely possible, to determine why a header was invalid.  There      are four possible reasons:Baker                       Standards Track                    [Page 67]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      o The Link Layer truncated the IP header      o The datagram is using a version of IP other than the standard         one (version 4).      o The IP header has been corrupted in transit.      o The sender generated an illegal IP header.      It is probably desirable to perform the checks in the order      listed, since we believe that this ordering is most likely to      correctly categorize the cause of the error.  For purposes of      error reporting, it may also be desirable to check if a packet      that fails these tests has an IP version number indicating IPng or      ST-II; these should be handled according to their respective      specifications.   Additionally, the router SHOULD verify that the packet length   reported by the Link Layer is at least as large as the IP total   length recorded in the packet's IP header.  If it appears that the   packet has been truncated, the packet MUST be discarded, the error   SHOULD be logged, and the router SHOULD respond with an ICMP   Parameter Problem message whose pointer points at the IP total length   field.   DISCUSSION      Because any higher layer protocol that concerns itself with data      corruption will detect truncation of the packet data when it      reaches its final destination, it is not absolutely necessary for      routers to perform the check suggested above to maintain protocol      correctness.  However, by making this check a router can simplify      considerably the task of determining which hop in the path is      truncating the packets.  It will also reduce the expenditure of      resources down-stream from the router in that down-stream systems      will not need to deal with the packet.   Finally, if the destination address in the IP header is not one of   the addresses of the router, the router SHOULD verify that the packet   does not contain a Strict Source and Record Route option.  If a   packet fails this test (if it contains a strict source route option),   the router SHOULD log the error and SHOULD respond with an ICMP   Parameter Problem error with the pointer pointing at the offending   packet's IP destination address.   DISCUSSION      Some people might suggest that the router should respond with a      Bad Source Route message instead of a Parameter Problem message.      However, when a packet fails this test, it usually indicates aBaker                       Standards Track                    [Page 68]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      protocol error by the previous hop router, whereas Bad Source      Route would suggest that the source host had requested a      nonexistent or broken path through the network.5.2.3 Local Delivery Decision   When a router receives an IP packet, it must decide whether the   packet is addressed to the router (and should be delivered locally)   or the packet is addressed to another system (and should be handled   by the forwarder).  There is also a hybrid case, where certain IP   broadcasts and IP multicasts are both delivered locally and   forwarded.  A router MUST determine which of the these three cases   applies using the following rules.   o An unexpired source route option is one whose pointer value does      not point past the last entry in the source route.  If the packet      contains an unexpired source route option, the pointer in the      option is advanced until either the pointer does point past the      last address in the option or else the next address is not one of      the router's own addresses.  In the latter (normal) case, the      packet is forwarded (and not delivered locally) regardless of the      rules below.   o The packet is delivered locally and not considered for forwarding      in the following cases:      - The packet's destination address exactly matches one of the         router's IP addresses,      - The packet's destination address is a limited broadcast address         ({-1, -1}), or      - The packet's destination is an IP multicast address which is         never forwarded (such as 224.0.0.1 or 224.0.0.2) and (at least)         one of the logical interfaces associated with the physical         interface on which the packet arrived is a member of the         destination multicast group.   o The packet is passed to the forwarder AND delivered locally in the      following cases:      - The packet's destination address is an IP broadcast address that         addresses at least one of the router's logical interfaces but         does not address any of the logical interfaces associated with         the physical interface on which the packet arrivedBaker                       Standards Track                    [Page 69]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      - The packet's destination is an IP multicast address which is         permitted to be forwarded (unlike 224.0.0.1 and 224.0.0.2) and         (at least) one of the logical interfaces associated with the         physical interface on which the packet arrived is a member of         the destination multicast group.   o The packet is delivered locally if the packet's destination address      is an IP broadcast address (other than a limited broadcast      address) that addresses at least one of the logical interfaces      associated with the physical interface on which the packet      arrived.  The packet is ALSO passed to the forwarder unless the      link on which the packet arrived uses an IP encapsulation that      does not encapsulate broadcasts differently than unicasts (e.g.,      by using different Link Layer destination addresses).   o The packet is passed to the forwarder in all other cases.   DISCUSSION      The purpose of the requirement in the last sentence of the fourth      bullet is to deal with a directed broadcast to another network      prefix on the same physical cable.  Normally, this works as      expected: the sender sends the broadcast to the router as a Link      Layer unicast.  The router notes that it arrived as a unicast, and      therefore must be destined for a different network prefix than the      sender sent it on.  Therefore, the router can safely send it as a      Link Layer broadcast out the same (physical) interface over which      it arrived.  However, if the router can't tell whether the packet      was received as a Link Layer unicast, the sentence ensures that      the router does the safe but wrong thing rather than the unsafe      but right thing.   IMPLEMENTATION      As described in Section [5.3.4], packets received as Link Layer      broadcasts are generally not forwarded.  It may be advantageous to      avoid passing to the forwarder packets it would later discard      because of the rules in that section.      Some Link Layers (either because of the hardware or because of      special code in the drivers) can deliver to the router copies of      all Link Layer broadcasts and multicasts it transmits.  Use of      this feature can simplify the implementation of cases where a      packet has to both be passed to the forwarder and delivered      locally, since forwarding the packet will automatically cause the      router to receive a copy of the packet that it can then deliver      locally.  One must use care in these circumstances to prevent      treating a received loop-back packet as a normal packet that was      received (and then being subject to the rules of forwarding,      etc.).Baker                       Standards Track                    [Page 70]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      Even without such a Link Layer, it is of course hardly necessary      to make a copy of an entire packet to queue it both for forwarding      and for local delivery, though care must be taken with fragments,      since reassembly is performed on locally delivered packets but not      on forwarded packets.  One simple scheme is to associate a flag      with each packet on the router's output queue that indicates      whether it should be queued for local delivery after it has been      sent.5.2.4 Determining the Next Hop Address   When a router is going to forward a packet, it must determine whether   it can send it directly to its destination, or whether it needs to   pass it through another router.  If the latter, it needs to determine   which router to use.  This section explains how these determinations   are made.   This section makes use of the following definitions:   o LSRR - IP Loose Source and Record Route option   o SSRR - IP Strict Source and Record Route option   o Source Route Option - an LSRR or an SSRR   o Ultimate Destination Address - where the packet is being sent to:      the last address in the source route of a source-routed packet, or      the destination address in the IP header of a non-source-routed      packet   o Adjacent - reachable without going through any IP routers   o Next Hop Address - the IP address of the adjacent host or router to      which the packet should be sent next   o IP Destination Address - the ultimate destination address, except      in source routed packets, where it is the next address specified      in the source route   o Immediate Destination - the node, System, router, end-system, or      whatever that is addressed by the IP Destination Address.Baker                       Standards Track                    [Page 71]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.2.4.1 IP Destination Address   If:   o the destination address in the IP header is one of the addresses of      the router,   o the packet contains a Source Route Option, and   o the pointer in the Source Route Option does not point past the end      of the option,   then the next IP Destination Address is the address pointed at by the   pointer in that option.  If:   o the destination address in the IP header is one of the addresses of      the router,   o the packet contains a Source Route Option, and   o the pointer in the Source Route Option points past the end of the      option,   then the message is addressed to the system analyzing the message.   A router MUST use the IP Destination Address, not the Ultimate   Destination Address (the last address in the source route option),   when determining how to handle a packet.   It is an error for more than one source route option to appear in a   datagram.  If it receives such a datagram, it SHOULD discard the   packet and reply with an ICMP Parameter Problem message whose pointer   points at the beginning of the second source route option.5.2.4.2 Local/Remote Decision   After it has been determined that the IP packet needs to be forwarded   according to the rules specified in Section [5.2.3], the following   algorithm MUST be used to determine if the Immediate Destination is   directly accessible (see [INTERNET:2]).   (1) For each network interface that has not been assigned any IP       address (the unnumbered lines as described in Section [2.2.7]),       compare the router-id of the other end of the line to the IP       Destination Address.  If they are exactly equal, the packet can       be transmitted through this interface.Baker                       Standards Track                    [Page 72]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      In other words, the router or host at the remote end of the line      is the destination of the packet or is the next step in the source      route of a source routed packet.   (2) If no network interface has been selected in the first step, for       each IP address assigned to the router:   (a) isolate the network prefix used by the interface.   IMPLEMENTATION      The result of this operation will usually have been computed and      saved during initialization.   (b) Isolate the corresponding set of bits from the IP Destination      Address of the packet.   (c) Compare the resulting network prefixes.  If they are equal to      each other, the packet can be transmitted through the      corresponding network interface.   (3) If the destination was neither the router-id of a neighbor on an       unnumbered interface nor a member of a directly connected network       prefix, the IP Destination is accessible only through some other       router.  The selection of the router and the next hop IP address       is described in Section [5.2.4.3].  In the case of a host that is       not also a router, this may be the configured default router.   Ongoing work in the IETF [ARCH:9, NRHP] considers some cases such as   when multiple IP (sub)networks are overlaid on the same link layer   network.  Barring policy restrictions, hosts and routers using a   common link layer network can directly communicate even if they are   not in the same IP (sub)network, if there is adequate information   present.  The Next Hop Routing Protocol (NHRP) enables IP entities to   determine the "optimal" link layer address to be used to traverse   such a link layer network towards a remote destination.   (4) If the selected "next hop" is reachable through an interface   configured to use NHRP, then the following additional steps apply:     (a) Compare the IP Destination Address to the destination addresses        in the NHRP cache.  If the address is in the cache, then send        the datagram to the corresponding cached link layer address.     (b) If the address is not in the cache, then construct an NHRP        request packet containing the IP Destination Address.  This        message is sent to the NHRP server configured for that        interface.  This may be a logically separate process or entity        in the router itself.Baker                       Standards Track                    [Page 73]

RFC 1812         Requirements for IP Version 4 Routers         June 1995     (c) The NHRP server will respond with the proper link layer address        to use to transmit the datagram and subsequent datagrams to the        same destination.  The system MAY transmit the datagram(s) to        the traditional "next hop" router while awaiting the NHRP reply.5.2.4.3 Next Hop Address   EDITORS+COMMENTS      The router applies the algorithm in the previous section to      determine if the IP Destination Address is adjacent.  If so, the      next hop address is the same as the IP Destination Address.      Otherwise, the packet must be forwarded through another router to      reach its Immediate Destination.  The selection of this router is      the topic of this section.      If the packet contains an SSRR, the router MUST discard the packet      and reply with an ICMP Bad Source Route error.  Otherwise, the      router looks up the IP Destination Address in its routing table to      determine an appropriate next hop address.   DISCUSSION      Per the IP specification, a Strict Source Route must specify a      sequence of nodes through which the packet must traverse; the      packet must go from one node of the source route to the next,      traversing intermediate networks only.  Thus, if the router is not      adjacent to the next step of the source route, the source route      can not be fulfilled.  Therefore, the router rejects such with an      ICMP Bad Source Route error.   The goal of the next-hop selection process is to examine the entries   in the router's Forwarding Information Base (FIB) and select the best   route (if there is one) for the packet from those available in the   FIB.   Conceptually, any route lookup algorithm starts out with a set of   candidate routes that consists of the entire contents of the FIB.   The algorithm consists of a series of steps that discard routes from   the set.  These steps are referred to as Pruning Rules.  Normally,   when the algorithm terminates there is exactly one route remaining in   the set.  If the set ever becomes empty, the packet is discarded   because the destination is unreachable.  It is also possible for the   algorithm to terminate when more than one route remains in the set.   In this case, the router may arbitrarily discard all but one of them,   or may perform "load-splitting" by choosing whichever of the routes   has been least recently used.   With the exception of rule 3 (Weak TOS), a router MUST use the   following Pruning Rules when selecting a next hop for a packet.  If aBaker                       Standards Track                    [Page 74]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   router does consider TOS when making next-hop decisions, the Rule 3   must be applied in the order indicated below.  These rules MUST be   (conceptually) applied to the FIB in the order that they are   presented.  (For some historical perspective, additional pruning   rules, and other common algorithms in use, seeAppendix E.)   DISCUSSION      Rule 3 is optional in that Section [5.3.2] says that a router only      SHOULD consider TOS when making forwarding decisions.      (1) Basic Match           This rule discards any routes to destinations other than the           IP Destination Address of the packet.  For example, if a           packet's IP Destination Address is 10.144.2.5, this step           would discard a route to net 128.12.0.0/16 but would retain           any routes to the network prefixes 10.0.0.0/8 and           10.144.0.0/16, and any default routes.           More precisely, we assume that each route has a destination           attribute, called route.dest and a corresponding prefix           length, called route.length, to specify which bits of           route.dest are significant.  The IP Destination Address of           the packet being forwarded is ip.dest.  This rule discards           all routes from the set of candidates except those for which           the most significant route.length bits of route.dest and           ip.dest are equal.           For example, if a packet's IP Destination Address is           10.144.2.5 and there are network prefixes 10.144.1.0/24,           10.144.2.0/24, and 10.144.3.0/24, this rule would keep only           10.144.2.0/24; it is the only route whose prefix has the same           value as the corresponding bits in the IP Destination Address           of the packet.      (2) Longest Match           Longest Match is a refinement of Basic Match, described           above.  After performing Basic Match pruning, the algorithm           examines the remaining routes to determine which among them           have the largest route.length values.  All except these are           discarded.           For example, if a packet's IP Destination Address is           10.144.2.5 and there are network prefixes 10.144.2.0/24,           10.144.0.0/16, and 10.0.0.0/8, then this rule would keep only           the first (10.144.2.0/24) because its prefix length is           longest.Baker                       Standards Track                    [Page 75]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      (3) Weak TOS           Each route has a type of service attribute, called route.tos,           whose possible values are assumed to be identical to those           used in the TOS field of the IP header.  Routing protocols           that distribute TOS information fill in route.tos           appropriately in routes they add to the FIB; routes from           other routing protocols are treated as if they have the           default TOS (0000).  The TOS field in the IP header of the           packet being routed is called ip.tos.           The set of candidate routes is examined to determine if it           contains any routes for which route.tos = ip.tos.  If so, all           routes except those for which route.tos = ip.tos are           discarded.  If not, all routes except those for which           route.tos = 0000 are discarded from the set of candidate           routes.           Additional discussion of routing based on Weak TOS may be           found in [ROUTE:11].   DISCUSSION      The effect of this rule is to select only those routes that have a      TOS that matches the TOS requested in the packet.  If no such      routes exist then routes with the default TOS are considered.      Routes with a non-default TOS that is not the TOS requested in the      packet are never used, even if such routes are the only available      routes that go to the packet's destination.     (4) Best Metric          Each route has a metric attribute, called route.metric, and a          routing domain identifier, called route.domain.  Each member          of the set of candidate routes is compared with each other          member of the set.  If route.domain is equal for the two          routes and route.metric is strictly inferior for one when          compared with the other, then the one with the inferior metric          is discarded from the set.  The determination of inferior is          usually by a simple arithmetic comparison, though some          protocols may have structured metrics requiring more complex          comparisons.     (5) Vendor Policy          Vendor Policy is sort of a catch-all to make up for the fact          that the previously listed rules are often inadequate to          choose from the possible routes.  Vendor Policy pruning rules          are extremely vendor-specific.  See section [5.2.4.4].     This algorithm has two distinct disadvantages.  Presumably, a     router implementor might develop techniques to deal with theseBaker                       Standards Track                    [Page 76]

RFC 1812         Requirements for IP Version 4 Routers         June 1995     disadvantages and make them a part of the Vendor Policy pruning     rule.     (1) IS-IS and OSPF route classes are not directly handled.     (2) Path properties other than type of service (e.g., MTU) are          ignored.     It is also worth noting a deficiency in the way that TOS is     supported: routing protocols that support TOS are implicitly     preferred when forwarding packets that have non-zero TOS values.     The Basic Match and Longest Match pruning rules generalize the     treatment of a number of particular types of routes.  These routes     are selected in the following, decreasing, order of preference:     (1) Host Route: This is a route to a specific end system.     (2) Hierarchical Network Prefix Routes: This is a route to a          particular network prefix.  Note that the FIB may contain          several routes to network prefixes that subsume each other          (one prefix is the other prefix with additional bits).  These          are selected in order of decreasing prefix length.     (5) Default Route: This is a route to all networks for which there          are no explicit routes.  It is by definition the route whose          prefix length is zero.     If, after application of the pruning rules, the set of routes is     empty (i.e., no routes were found), the packet MUST be discarded     and an appropriate ICMP error generated (ICMP Bad Source Route if     the IP Destination Address came from a source route option;     otherwise, whichever of ICMP Destination Host Unreachable or     Destination Network Unreachable is appropriate, as described in     Section [4.3.3.1]).5.2.4.4 Administrative Preference     One suggested mechanism for the Vendor Policy Pruning Rule is to     use administrative preference, which is a simple prioritization     algorithm.  The idea is to manually prioritize the routes that one     might need to select among.     Each route has associated with it a preference value, based on     various attributes of the route (specific mechanisms for assignment     of preference values are suggested below).  This preference value     is an integer in the range [0..255], with zero being the most     preferred and 254 being the least preferred.  255 is a specialBaker                       Standards Track                    [Page 77]

RFC 1812         Requirements for IP Version 4 Routers         June 1995     value that means that the route should never be used.  The first     step in the Vendor Policy pruning rule discards all but the most     preferable routes (and always discards routes whose preference     value is 255).     This policy is not safe in that it can easily be misused to create     routing loops.  Since no protocol ensures that the preferences     configured for a router is consistent with the preferences     configured in its neighbors, network managers must exercise care in     configuring preferences.     o Address Match        It is useful to be able to assign a single preference value to        all routes (learned from the same routing domain) to any of a        specified set of destinations, where the set of destinations is        all destinations that match a specified network prefix.     o Route Class        For routing protocols which maintain the distinction, it is        useful to be able to assign a single preference value to all        routes (learned from the same routing domain) which have a        particular route class (intra-area, inter-area, external with        internal metrics, or external with external metrics).     o Interface        It is useful to be able to assign a single preference value to        all routes (learned from a particular routing domain) that would        cause packets to be routed out a particular logical interface on        the router (logical interfaces generally map one-to-one onto the        router's network interfaces, except that any network interface        that has multiple IP addresses will have multiple logical        interfaces associated with it).     o Source router        It is useful to be able to assign a single preference value to        all routes (learned from the same routing domain) that were        learned from any of a set of routers, where the set of routers        are those whose updates have a source address that match a        specified network prefix.     o Originating AS        For routing protocols which provide the information, it is        useful to be able to assign a single preference value to all        routes (learned from a particular routing domain) which        originated in another particular routing domain.  For BGP        routes, the originating AS is the first AS listed in the route's        AS_PATH attribute.  For OSPF external routes, the originating AS        may be considered to be the low order 16 bits of the route'sBaker                       Standards Track                    [Page 78]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        external route tag if the tag's Automatic bit is set and the        tag's Path Length is not equal to 3.     o External route tag        It is useful to be able to assign a single preference value to        all OSPF external routes (learned from the same routing domain)        whose external route tags match any of a list of specified        values.  Because the external route tag may contain a structured        value, it may be useful to provide the ability to match        particular subfields of the tag.     o AS path        It may be useful to be able to assign a single preference value        to all BGP routes (learned from the same routing domain) whose        AS path "matches" any of a set of specified values.  It is not        yet clear exactly what kinds of matches are most useful.  A        simple option would be to allow matching of all routes for which        a particular AS number appears (or alternatively, does not        appear) anywhere in the route's AS_PATH attribute.  A more        general but somewhat more difficult alternative would be to        allow matching all routes for which the AS path matches a        specified regular expression.5.2.4.5 Load Splitting     At the end of the Next-hop selection process, multiple routes may     still remain.  A router has several options when this occurs.  It     may arbitrarily discard some of the routes.  It may reduce the     number of candidate routes by comparing metrics of routes from     routing domains that are not considered equivalent.  It may retain     more than one route and employ a load-splitting mechanism to divide     traffic among them.  Perhaps the only thing that can be said about     the relative merits of the options is that load-splitting is useful     in some situations but not in others, so a wise implementor who     implements load-splitting will also provide a way for the network     manager to disable it.5.2.5 Unused IP Header Bits:RFC-791 Section 3.1     The IP header contains several reserved bits, in the Type of     Service field and in the Flags field.  Routers MUST NOT drop     packets merely because one or more of these reserved bits has a     non-zero value.     Routers MUST ignore and MUST pass through unchanged the values of     these reserved bits.  If a router fragments a packet, it MUST copy     these bits into each fragment.Baker                       Standards Track                    [Page 79]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      Future revisions to the IP protocol may make use of these unused      bits.  These rules are intended to ensure that these revisions can      be deployed without having to simultaneously upgrade all routers      in the Internet.5.2.6 Fragmentation and Reassembly:RFC-791 Section 3.2   As was discussed in Section [4.2.2.7], a router MUST support IP   fragmentation.   A router MUST NOT reassemble any datagram before forwarding it.   DISCUSSION      A few people have suggested that there might be some topologies      where reassembly of transit datagrams by routers might improve      performance.  The fact that fragments may take different paths to      the destination precludes safe use of such a feature.      Nothing in this section should be construed to control or limit      fragmentation or reassembly performed as a link layer function by      the router.      Similarly, if an IP datagram is encapsulated in another IP      datagram (e.g., it is tunnelled), that datagram is in turn      fragmented, the fragments must be reassembled in order to forward      the original datagram.  This section does not preclude this.5.2.7 Internet Control Message Protocol - ICMP   General requirements for ICMP were discussed in Section [4.3].  This   section discusses ICMP messages that are sent only by routers.5.2.7.1 Destination Unreachable   The ICMP Destination Unreachable message is sent by a router in   response to a packet which it cannot forward because the destination   (or next hop) is unreachable or a service is unavailable.  Examples   of such cases include a message addressed to a host which is not   there and therefore does not respond to ARP requests, and messages   addressed to network prefixes for which the router has no valid   route.   A router MUST be able to generate ICMP Destination Unreachable   messages and SHOULD choose a response code that most closely matches   the reason the message is being generated.   The following codes are defined in [INTERNET:8] and [INTRO:2]:Baker                       Standards Track                    [Page 80]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   0 = Network Unreachable - generated by a router if a forwarding path        (route) to the destination network is not available;   1 = Host Unreachable - generated by a router if a forwarding path        (route) to the destination host on a directly connected network        is not available (does not respond to ARP);   2 = Protocol Unreachable - generated if the transport protocol        designated in a datagram is not supported in the transport layer        of the final destination;   3 = Port Unreachable - generated if the designated transport protocol        (e.g., UDP) is unable to demultiplex the datagram in the        transport layer of the final destination but has no protocol        mechanism to inform the sender;   4 = Fragmentation Needed and DF Set - generated if a router needs to        fragment a datagram but cannot since the DF flag is set;   5 = Source Route Failed - generated if a router cannot forward a        packet to the next hop in a source route option;   6 = Destination Network Unknown - This code SHOULD NOT be generated        since it would imply on the part of the router that the        destination network does not exist (net unreachable code 0        SHOULD be used in place of code 6);   7 = Destination Host Unknown - generated only when a router can        determine (from link layer advice) that the destination host        does not exist;   11 = Network Unreachable For Type Of Service - generated by a router        if a forwarding path (route) to the destination network with the        requested or default TOS is not available;   12 = Host Unreachable For Type Of Service - generated if a router        cannot forward a packet because its route(s) to the destination        do not match either the TOS requested in the datagram or the        default TOS (0).   The following additional codes are hereby defined:   13 = Communication Administratively Prohibited - generated if a        router cannot forward a packet due to administrative filtering;   14 = Host Precedence Violation.  Sent by the first hop router to a        host to indicate that a requested precedence is not permitted        for the particular combination of source/destination host orBaker                       Standards Track                    [Page 81]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        network, upper layer protocol, and source/destination port;   15 = Precedence cutoff in effect.  The network operators have imposed        a minimum level of precedence required for operation, the        datagram was sent with a precedence below this level;   NOTE: [INTRO:2] defined Code 8 for source host isolated.  Routers   SHOULD NOT generate Code 8; whichever of Codes 0 (Network   Unreachable) and 1 (Host Unreachable) is appropriate SHOULD be used   instead.  [INTRO:2] also defined Code 9 for communication with   destination network administratively prohibited and Code 10 for   communication with destination host administratively prohibited.   These codes were intended for use by end-to-end encryption devices   used by U.S military agencies.  Routers SHOULD use the newly defined   Code 13 (Communication Administratively Prohibited) if they   administratively filter packets.   Routers MAY have a configuration option that causes Code 13   (Communication Administratively Prohibited) messages not to be   generated.  When this option is enabled, no ICMP error message is   sent in response to a packet that is dropped because its forwarding   is administratively prohibited.   Similarly, routers MAY have a configuration option that causes Code   14 (Host Precedence Violation) and Code 15 (Precedence Cutoff in   Effect) messages not to be generated.  When this option is enabled,   no ICMP error message is sent in response to a packet that is dropped   because of a precedence violation.   Routers MUST use Host Unreachable or Destination Host Unknown codes   whenever other hosts on the same destination network might be   reachable; otherwise, the source host may erroneously conclude that   all hosts on the network are unreachable, and that may not be the   case.   [INTERNET:14] describes a slight modification the form of Destination   Unreachable messages containing Code 4 (Fragmentation needed and DF   set).  A router MUST use this modified form when originating Code 4   Destination Unreachable messages.5.2.7.2 Redirect   The ICMP Redirect message is generated to inform a local host the it   should use a different next hop router for a certain class of   traffic.   Routers MUST NOT generate the Redirect for Network or Redirect for   Network and Type of Service messages (Codes 0 and 2) specified inBaker                       Standards Track                    [Page 82]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   [INTERNET:8].  Routers MUST be able to generate the Redirect for Host   message (Code 1) and SHOULD be able to generate the Redirect for Type   of Service and Host message (Code 3) specified in [INTERNET:8].   DISCUSSION      If the directly connected network is not subnetted (in the      classical sense), a router can normally generate a network      Redirect that applies to all hosts on a specified remote network.      Using a network rather than a host Redirect may economize slightly      on network traffic and on host routing table storage.  However,      the savings are not significant, and subnets create an ambiguity      about the subnet mask to be used to interpret a network Redirect.      In a CIDR environment, it is difficult to specify precisely the      cases in which network Redirects can be used.  Therefore, routers      must send only host (or host and type of service) Redirects.   A Code 3 (Redirect for Host and Type of Service) message is generated   when the packet provoking the redirect has a destination for which   the path chosen by the router would depend (in part) on the TOS   requested.   Routers that can generate Code 3 redirects (Host and Type of Service)   MUST have a configuration option (which defaults to on) to enable   Code 1 (Host) redirects to be substituted for Code 3 redirects.  A   router MUST send a Code 1 Redirect in place of a Code 3 Redirect if   it has been configured to do so.   If a router is not able to generate Code 3 Redirects then it MUST   generate Code 1 Redirects in situations where a Code 3 Redirect is   called for.   Routers MUST NOT generate a Redirect Message unless all the following   conditions are met:   o The packet is being forwarded out the same physical interface that      it was received from,   o The IP source address in the packet is on the same Logical IP      (sub)network as the next-hop IP address, and   o The packet does not contain an IP source route option.   The source address used in the ICMP Redirect MUST belong to the same   logical (sub)net as the destination address.   A router using a routing protocol (other than static routes) MUST NOT   consider paths learned from ICMP Redirects when forwarding a packet.   If a router is not using a routing protocol, a router MAY have aBaker                       Standards Track                    [Page 83]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   configuration that, if set, allows the router to consider routes   learned through ICMP Redirects when forwarding packets.   DISCUSSION      ICMP Redirect is a mechanism for routers to convey routing      information to hosts.  Routers use other mechanisms to learn      routing information, and therefore have no reason to obey      redirects.  Believing a redirect which contradicted the router's      other information would likely create routing loops.      On the other hand, when a router is not acting as a router, it      MUST comply with the behavior required of a host.5.2.7.3 Time Exceeded   A router MUST generate a Time Exceeded message Code 0 (In Transit)   when it discards a packet due to an expired TTL field.  A router MAY   have a per-interface option to disable origination of these messages   on that interface, but that option MUST default to allowing the   messages to be originated.5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP   IGMP [INTERNET:4] is a protocol used between hosts and multicast   routers on a single physical network to establish hosts' membership   in particular multicast groups.  Multicast routers use this   information, in conjunction with a multicast routing protocol, to   support IP multicast forwarding across the Internet.   A router SHOULD implement the multicast router part of IGMP.Baker                       Standards Track                    [Page 84]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.3 SPECIFIC ISSUES5.3.1 Time to Live (TTL)   The Time-to-Live (TTL) field of the IP header is defined to be a   timer limiting the lifetime of a datagram.  It is an 8-bit field and   the units are seconds.  Each router (or other module) that handles a   packet MUST decrement the TTL by at least one, even if the elapsed   time was much less than a second.  Since this is very often the case,   the TTL is effectively a hop count limit on how far a datagram can   propagate through the Internet.   When a router forwards a packet, it MUST reduce the TTL by at least   one.  If it holds a packet for more than one second, it MAY decrement   the TTL by one for each second.   If the TTL is reduced to zero (or less), the packet MUST be   discarded, and if the destination is not a multicast address the   router MUST send an ICMP Time Exceeded message, Code 0 (TTL Exceeded   in Transit) message to the source.  Note that a router MUST NOT   discard an IP unicast or broadcast packet with a non-zero TTL merely   because it can predict that another router on the path to the   packet's final destination will decrement the TTL to zero.  However,   a router MAY do so for IP multicasts, in order to more efficiently   implement IP multicast's expanding ring search algorithm (see   [INTERNET:4]).   DISCUSSION      The IP TTL is used, somewhat schizophrenically, as both a hop      count limit and a time limit.  Its hop count function is critical      to ensuring that routing problems can't melt down the network by      causing packets to loop infinitely in the network.  The time limit      function is used by transport protocols such as TCP to ensure      reliable data transfer.  Many current implementations treat TTL as      a pure hop count, and in parts of the Internet community there is      a strong sentiment that the time limit function should instead be      performed by the transport protocols that need it.      In this specification, we have reluctantly decided to follow the      strong belief among the router vendors that the time limit      function should be optional.  They argued that implementation of      the time limit function is difficult enough that it is currently      not generally done.  They further pointed to the lack of      documented cases where this shortcut has caused TCP to corrupt      data (of course, we would expect the problems created to be rare      and difficult to reproduce, so the lack of documented cases      provides little reassurance that there haven't been a number of      undocumented cases).Baker                       Standards Track                    [Page 85]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      IP multicast notions such as the expanding ring search may not      work as expected unless the TTL is treated as a pure hop count.      The same thing is somewhat true of traceroute.      ICMP Time Exceeded messages are required because the traceroute      diagnostic tool depends on them.      Thus, the tradeoff is between severely crippling, if not      eliminating, two very useful tools and avoiding a very rare and      transient data transport problem that may not occur at all.  We      have chosen to preserve the tools.5.3.2 Type of Service (TOS)      The Type-of-Service byte in the IP header is divided into three      sections: the Precedence field (high-order 3 bits), a field that      is customarily called Type of Service or "TOS (next 4 bits), and a      reserved bit (the low order bit).  Rules governing the reserved      bit were described in Section [4.2.2.3].  The Precedence field      will be discussed in Section [5.3.3].  A more extensive discussion      of the TOS field and its use can be found in [ROUTE:11].      A router SHOULD consider the TOS field in a packet's IP header      when deciding how to forward it.  The remainder of this section      describes the rules that apply to routers that conform to this      requirement.      A router MUST maintain a TOS value for each route in its routing      table.  Routes learned through a routing protocol that does not      support TOS MUST be assigned a TOS of zero (the default TOS).      To choose a route to a destination, a router MUST use an algorithm      equivalent to the following:      (1) The router locates in its routing table all available routes           to the destination (see Section [5.2.4]).      (2) If there are none, the router drops the packet because the           destination is unreachable.  See section [5.2.4].      (3) If one or more of those routes have a TOS that exactly matches           the TOS specified in the packet, the router chooses the route           with the best metric.      (4) Otherwise, the router repeats the above step, except looking           at routes whose TOS is zero.Baker                       Standards Track                    [Page 86]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      (5) If no route was chosen above, the router drops the packet           because the destination is unreachable.  The router returns           an ICMP Destination Unreachable error specifying the           appropriate code: either Network Unreachable with Type of           Service (code 11) or Host Unreachable with Type of Service           (code 12).   DISCUSSION      Although TOS has been little used in the past, its use by hosts is      now mandated by the Requirements for Internet Hosts RFCs      ([INTRO:2] and [INTRO:3]).  Support for TOS in routers may become      a MUST in the future, but is a SHOULD for now until we get more      experience with it and can better judge both its benefits and its      costs.      Various people have proposed that TOS should affect other aspects      of the forwarding function.  For example:      (1) A router could place packets that have the Low Delay bit set           ahead of other packets in its output queues.      (2) a router is forced to discard packets, it could try to avoid           discarding those which have the High Reliability bit set.      These ideas have been explored in more detail in [INTERNET:17] but      we don't yet have enough experience with such schemes to make      requirements in this area.5.3.3 IP Precedence      This section specifies requirements and guidelines for appropriate      processing of the IP Precedence field in routers.  Precedence is a      scheme for allocating resources in the network based on the      relative importance of different traffic flows.  The IP      specification defines specific values to be used in this field for      various types of traffic.      The basic mechanisms for precedence processing in a router are      preferential resource allocation, including both precedence-      ordered queue service and precedence-based congestion control, and      selection of Link Layer priority features.  The router also      selects the IP precedence for routing, management and control      traffic it originates.  For a more extensive discussion of IP      Precedence and its implementation see [FORWARD:6].      Precedence-ordered queue service, as discussed in this section,      includes but is not limited to the queue for the forwarding      process and queues for outgoing links.  It is intended that aBaker                       Standards Track                    [Page 87]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      router supporting precedence should also use the precedence      indication at whatever points in its processing are concerned with      allocation of finite resources, such as packet buffers or Link      Layer connections.  The set of such points is implementation-      dependent.   DISCUSSION      Although the Precedence field was originally provided for use in      DOD systems where large traffic surges or major damage to the      network are viewed as inherent threats, it has useful applications      for many non-military IP networks.  Although the traffic handling      capacity of networks has grown greatly in recent years, the      traffic generating ability of the users has also grown, and      network overload conditions still occur at times.  Since IP-based      routing and management protocols have become more critical to the      successful operation of the Internet, overloads present two      additional risks to the network:      (1) High delays may result in routing protocol packets being lost.           This may cause the routing protocol to falsely deduce a           topology change and propagate this false information to other           routers.  Not only can this cause routes to oscillate, but an           extra processing burden may be placed on other routers.      (2) High delays may interfere with the use of network management           tools to analyze and perhaps correct or relieve the problem           in the network that caused the overload condition to occur.      Implementation and appropriate use of the Precedence mechanism      alleviates both of these problems.5.3.3.1 Precedence-Ordered Queue Service   Routers SHOULD implement precedence-ordered queue service.   Precedence-ordered queue service means that when a packet is selected   for output on a (logical) link, the packet of highest precedence that   has been queued for that link is sent.  Routers that implement   precedence-ordered queue service MUST also have a configuration   option to suppress precedence-ordered queue service in the Internet   Layer.   Any router MAY implement other policy-based throughput management   procedures that result in other than strict precedence ordering, but   it MUST be configurable to suppress them (i.e., use strict ordering).   As detailed in Section [5.3.6], routers that implement precedence-   ordered queue service discard low precedence packets before   discarding high precedence packets for congestion control purposes.Baker                       Standards Track                    [Page 88]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Preemption (interruption of processing or transmission of a packet)   is not envisioned as a function of the Internet Layer.  Some   protocols at other layers may provide preemption features.5.3.3.2 Lower Layer Precedence Mappings   Routers that implement precedence-ordered queuing MUST IMPLEMENT, and   other routers SHOULD IMPLEMENT, Lower Layer Precedence Mapping.   A router that implements Lower Layer Precedence Mapping:   o MUST be able to map IP Precedence to Link Layer priority mechanisms      for link layers that have such a feature defined.   o MUST have a configuration option to select the Link Layer's default      priority treatment for all IP traffic   o SHOULD be able to configure specific nonstandard mappings of IP      precedence values to Link Layer priority values for each      interface.   DISCUSSION      Some research questions the workability of the priority features      of some Link Layer protocols, and some networks may have faulty      implementations of the link layer priority mechanism.  It seems      prudent to provide an escape mechanism in case such problems show      up in a network.      On the other hand, there are proposals to use novel queuing      strategies to implement special services such as multimedia      bandwidth reservation or low-delay service.  Special services and      queuing strategies to support them are current research subjects      and are in the process of standardization.      Implementors may wish to consider that correct link layer mapping      of IP precedence is required by DOD policy for TCP/IP systems used      on DOD networks.  Since these requirements are intended to      encourage (but not force) the use of precedence features in the      hope of providing better Internet service to all users, routers      supporting precedence-ordered queue service should default to      maintaining strict precedence ordering regardless of the type of      service requested.Baker                       Standards Track                    [Page 89]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.3.3.3 Precedence Handling For All Routers   A router (whether or not it employs precedence-ordered queue   service):   (1) MUST accept and process incoming traffic of all precedence levels        normally, unless it has been administratively configured to do        otherwise.   (2) MAY implement a validation filter to administratively restrict        the use of precedence levels by particular traffic sources.  If        provided, this filter MUST NOT filter out or cut off the        following sorts of ICMP error messages: Destination Unreachable,        Redirect, Time Exceeded, and Parameter Problem.  If this filter        is provided, the procedures required for packet filtering by        addresses are required for this filter also.   DISCUSSION      Precedence filtering should be applicable to specific      source/destination IP Address pairs, specific protocols, specific      ports, and so on.   An ICMP Destination Unreachable message with code 14 SHOULD be sent   when a packet is dropped by the validation filter, unless this has   been suppressed by configuration choice.   (3) MAY implement a cutoff function that allows the router to be set        to refuse or drop traffic with precedence below a specified        level.  This function may be activated by management actions or        by some implementation dependent heuristics, but there MUST be a        configuration option to disable any heuristic mechanism that        operates without human intervention.  An ICMP Destination        Unreachable message with code 15 SHOULD be sent when a packet is        dropped by the cutoff function, unless this has been suppressed        by configuration choice.        A router MUST NOT refuse to forward datagrams with IP precedence        of 6 (Internetwork Control) or 7 (Network Control) solely due to        precedence cutoff.  However, other criteria may be used in        conjunction with precedence cutoff to filter high precedence        traffic.   DISCUSSION      Unrestricted precedence cutoff could result in an unintentional      cutoff of routing and control traffic.  In the general case, host      traffic should be restricted to a value of 5 (CRITIC/ECP) or      below; this is not a requirement and may not be correct in certain      systems.Baker                       Standards Track                    [Page 90]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (4) MUST NOT change precedence settings on packets it did not        originate.   (5) SHOULD be able to configure distinct precedence values to be used        for each routing or management protocol supported (except for        those protocols, such as OSPF, which specify which precedence        value must be used).   (6) MAY be able to configure routing or management traffic precedence        values independently for each peer address.   (7) MUST respond appropriately to Link Layer precedence-related error        indications where provided.  An ICMP Destination Unreachable        message with code 15 SHOULD be sent when a packet is dropped        because a link cannot accept it due to a precedence-related        condition, unless this has been suppressed by configuration        choice.   DISCUSSION      The precedence cutoff mechanism described in (3) is somewhat      controversial.  Depending on the topological location of the area      affected by the cutoff, transit traffic may be directed by routing      protocols into the area of the cutoff, where it will be dropped.      This is only a problem if another path that is unaffected by the      cutoff exists between the communicating points.  Proposed ways of      avoiding this problem include providing some minimum bandwidth to      all precedence levels even under overload conditions, or      propagating cutoff information in routing protocols.  In the      absence of a widely accepted (and implemented) solution to this      problem, great caution is recommended in activating cutoff      mechanisms in transit networks.      A transport layer relay could legitimately provide the function      prohibited by (4) above.  Changing precedence levels may cause      subtle interactions with TCP and perhaps other protocols; a      correct design is a non-trivial task.      The intent of (5) and (6) (and the discussion of IP Precedence in      ICMP messages in Section [4.3.2]) is that the IP precedence bits      should be appropriately set, whether or not this router acts upon      those bits in any other way.  We expect that in the future      specifications for routing protocols and network management      protocols will specify how the IP Precedence should be set for      messages sent by those protocols.      The appropriate response for (7) depends on the link layer      protocol in use.  Typically, the router should stop trying to send      offensive traffic to that destination for some period of time, andBaker                       Standards Track                    [Page 91]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      should return an ICMP Destination Unreachable message with code 15      (service not available for precedence requested) to the traffic      source.  It also should not try to reestablish a preempted Link      Layer connection for some time.5.3.4 Forwarding of Link Layer Broadcasts   The encapsulation of IP packets in most Link Layer protocols (except   PPP) allows a receiver to distinguish broadcasts and multicasts from   unicasts simply by examining the Link Layer protocol headers (most   commonly, the Link Layer destination address).  The rules in this   section that refer to Link Layer broadcasts apply only to Link Layer   protocols that allow broadcasts to be distinguished; likewise, the   rules that refer to Link Layer multicasts apply only to Link Layer   protocols that allow multicasts to be distinguished.   A router MUST NOT forward any packet that the router received as a   Link Layer broadcast, unless it is directed to an IP Multicast   address.  In this latter case, one would presume that link layer   broadcast was used due to the lack of an effective multicast service.   A router MUST NOT forward any packet which the router received as a   Link Layer multicast unless the packet's destination address is an IP   multicast address.   A router SHOULD silently discard a packet that is received via a Link   Layer broadcast but does not specify an IP multicast or IP broadcast   destination address.   When a router sends a packet as a Link Layer broadcast, the IP   destination address MUST be a legal IP broadcast or IP multicast   address.5.3.5 Forwarding of Internet Layer Broadcasts   There are two major types of IP broadcast addresses; limited   broadcast and directed broadcast.  In addition, there are three   subtypes of directed broadcast: a broadcast directed to a specified   network prefix, a broadcast directed to a specified subnetwork, and a   broadcast directed to all subnets of a specified network.   Classification by a router of a broadcast into one of these   categories depends on the broadcast address and on the router's   understanding (if any) of the subnet structure of the destination   network.  The same broadcast will be classified differently by   different routers.   A limited IP broadcast address is defined to be all-ones: { -1, -1 }   or 255.255.255.255.Baker                       Standards Track                    [Page 92]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   A network-prefix-directed broadcast is composed of the network prefix   of the IP address with a local part of all-ones or { <Network-   prefix>, -1 }.  For example, a Class A net broadcast address is   net.255.255.255, a Class B net broadcast address is net.net.255.255   and a Class C net broadcast address is net.net.net.255 where net is a   byte of the network address.   The all-subnets-directed-broadcast is not well defined in a CIDR   environment, and was deprecated in version 1 of this memo.   As was described in Section [4.2.3.1], a router may encounter certain   non-standard IP broadcast addresses:   o 0.0.0.0 is an obsolete form of the limited broadcast address   o { <Network-prefix>, 0 } is an obsolete form of a network-prefix-      directed broadcast address.   As was described in that section, packets addressed to any of these   addresses SHOULD be silently discarded, but if they are not, they   MUST be treated according to the same rules that apply to packets   addressed to the non-obsolete forms of the broadcast addresses   described above.  These rules are described in the next few sections.5.3.5.1 Limited Broadcasts   Limited broadcasts MUST NOT be forwarded.  Limited broadcasts MUST   NOT be discarded.  Limited broadcasts MAY be sent and SHOULD be sent   instead of directed broadcasts where limited broadcasts will suffice.   DISCUSSION      Some routers contain UDP servers which function by resending the      requests (as unicasts or directed broadcasts) to other servers.      This requirement should not be interpreted as prohibiting such      servers.  Note, however, that such servers can easily cause packet      looping if misconfigured.  Thus, providers of such servers would      probably be well advised to document their setup carefully and to      consider carefully the TTL on packets that are sent.5.3.5.2 Directed Broadcasts   A router MUST classify as network-prefix-directed broadcasts all   valid, directed broadcasts destined for a remote network or an   attached nonsubnetted network.  Note that in view of CIDR, such   appear to be host addresses within the network prefix; we preclude   inspection of the host part of such network prefixes.  Given a route   and no overriding policy, then, a router MUST forward network-   prefix-directed broadcasts.  Network-Prefix-Directed broadcasts MAYBaker                       Standards Track                    [Page 93]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   be sent.   A router MAY have an option to disable receiving network-prefix-   directed broadcasts on an interface and MUST have an option to   disable forwarding network-prefix-directed broadcasts.  These options   MUST default to permit receiving and forwarding network-prefix-   directed broadcasts.   DISCUSSION      There has been some debate about forwarding or not forwarding      directed broadcasts.  In this memo we have made the forwarding      decision depend on the router's knowledge of the destination      network prefix.  Routers cannot determine that a message is      unicast or directed broadcast apart from this knowledge.  The      decision to forward or not forward the message is by definition      only possible in the last hop router.5.3.5.3 All-subnets-directed Broadcasts   The first version of this memo described an algorithm for   distributing a directed broadcast to all the subnets of a classical   network number.  This algorithm was stated to be "broken," and   certain failure cases were specified.   In a CIDR routing domain, wherein classical IP network numbers are   meaningless, the concept of an all-subnets-directed-broadcast is also   meaningless.  To the knowledge of the working group, the facility was   never implemented or deployed, and is now relegated to the dustbin of   history.5.3.5.4  Subnet-directed Broadcasts   The first version of this memo spelled out procedures for dealing   with subnet-directed-broadcasts.  In a CIDR routing domain, these are   indistinguishable from net-drected-broadcasts.  The two are therefore   treated together in section [5.3.5.2 Directed Broadcasts], and should   be viewed as network-prefix directed broadcasts.5.3.6 Congestion Control   Congestion in a network is loosely defined as a condition where   demand for resources (usually bandwidth or CPU time) exceeds   capacity.  Congestion avoidance tries to prevent demand from   exceeding capacity, while congestion recovery tries to restore an   operative state.  It is possible for a router to contribute to both   of these mechanisms.  A great deal of effort has been spent studying   the problem.  The reader is encouraged to read [FORWARD:2] for a   survey of the work.  Important papers on the subject includeBaker                       Standards Track                    [Page 94]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   [FORWARD:3], [FORWARD:4], [FORWARD:5], [FORWARD:10], [FORWARD:11],   [FORWARD:12], [FORWARD:13], [FORWARD:14], and [INTERNET:10], among   others.   The amount of storage that router should have available to handle   peak instantaneous demand when hosts use reasonable congestion   policies, such as described in [FORWARD:5], is a function of the   product of the bandwidth of the link times the path delay of the   flows using the link, and therefore storage should increase as this   Bandwidth*Delay product increases.  The exact function relating   storage capacity to probability of discard is not known.   When a router receives a packet beyond its storage capacity it must   (by definition, not by decree) discard it or some other packet or   packets.  Which packet to discard is the subject of much study but,   unfortunately, little agreement so far.  The best wisdom to date   suggests discarding a packet from the data stream most heavily using   the link.  However, a number of additional factors may be relevant,   including the precedence of the traffic, active bandwidth   reservation, and the complexity associated with selecting that   packet.   A router MAY discard the packet it has just received; this is the   simplest but not the best policy.  Ideally, the router should select   a packet from one of the sessions most heavily abusing the link,   given that the applicable Quality of Service policy permits this.  A   recommended policy in datagram environments using FIFO queues is to   discard a packet randomly selected from the queue (see [FORWARD:5]).   An equivalent algorithm in routers using fair queues is to discard   from the longest queue or that using the greatest virtual time (see   [FORWARD:13]).  A router MAY use these algorithms to determine which   packet to discard.   If a router implements a discard policy (such as Random Drop) under   which it chooses a packet to discard from a pool of eligible packets:   o If precedence-ordered queue service (described in Section      [5.3.3.1]) is implemented and enabled, the router MUST NOT discard      a packet whose IP precedence is higher than that of a packet that      is not discarded.   o A router MAY protect packets whose IP headers request the maximize      reliability TOS, except where doing so would be in violation of      the previous rule.   o A router MAY protect fragmented IP packets, on the theory that      dropping a fragment of a datagram may increase congestion by      causing all fragments of the datagram to be retransmitted by theBaker                       Standards Track                    [Page 95]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      source.   o To help prevent routing perturbations or disruption of management      functions, the router MAY protect packets used for routing      control, link control, or network management from being discarded.      Dedicated routers (i.e., routers that are not also general purpose      hosts, terminal servers, etc.) can achieve an approximation of      this rule by protecting packets whose source or destination is the      router itself.   Advanced methods of congestion control include a notion of fairness,   so that the 'user' that is penalized by losing a packet is the one   that contributed the most to the congestion.  No matter what   mechanism is implemented to deal with bandwidth congestion control,   it is important that the CPU effort expended be sufficiently small   that the router is not driven into CPU congestion also.   As described in Section [4.3.3.3], this document recommends that a   router SHOULD NOT send a Source Quench to the sender of the packet   that it is discarding.  ICMP Source Quench is a very weak mechanism,   so it is not necessary for a router to send it, and host software   should not use it exclusively as an indicator of congestion.5.3.7 Martian Address Filtering   An IP source address is invalid if it is a special IP address, as   defined in 4.2.2.11 or 5.3.7, or is not a unicast address.   An IP destination address is invalid if it is among those defined as   illegal destinations in 4.2.3.1, or is a Class E address (except   255.255.255.255).   A router SHOULD NOT forward any packet that has an invalid IP source   address or a source address on network 0.  A router SHOULD NOT   forward, except over a loopback interface, any packet that has a   source address on network 127.  A router MAY have a switch that   allows the network manager to disable these checks.  If such a switch   is provided, it MUST default to performing the checks.   A router SHOULD NOT forward any packet that has an invalid IP   destination address or a destination address on network 0.  A router   SHOULD NOT forward, except over a loopback interface, any packet that   has a destination address on network 127.  A router MAY have a switch   that allows the network manager to disable these checks.  If such a   switch is provided, it MUST default to performing the checks.   If a router discards a packet because of these rules, it SHOULD log   at least the IP source address, the IP destination address, and, ifBaker                       Standards Track                    [Page 96]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   the problem was with the source address, the physical interface on   which the packet was received and the Link Layer address of the host   or router from which the packet was received.5.3.8 Source Address Validation   A router SHOULD IMPLEMENT the ability to filter traffic based on a   comparison of the source address of a packet and the forwarding table   for a logical interface on which the packet was received.  If this   filtering is enabled, the router MUST silently discard a packet if   the interface on which the packet was received is not the interface   on which a packet would be forwarded to reach the address contained   in the source address.  In simpler terms, if a router wouldn't route   a packet containing this address through a particular interface, it   shouldn't believe the address if it appears as a source address in a   packet read from this interface.   If this feature is implemented, it MUST be disabled by default.   DISCUSSION      This feature can provide useful security improvements in some      situations, but can erroneously discard valid packets in      situations where paths are asymmetric.5.3.9 Packet Filtering and Access Lists   As a means of providing security and/or limiting traffic through   portions of a network a router SHOULD provide the ability to   selectively forward (or filter) packets.  If this capability is   provided, filtering of packets SHOULD be configurable either to   forward all packets or to selectively forward them based upon the   source and destination prefixes, and MAY filter on other message   attributes.  Each source and destination address SHOULD allow   specification of an arbitrary prefix length.   DISCUSSION      This feature can provide a measure of privacy, where systems      outside a boundary are not permitted to exchange certain protocols      with systems inside the boundary, or are limited as to which      systems they may communicate with.  It can also help prevent      certain classes of security breach, wherein a system outside a      boundary masquerades as a system inside the boundary and mimics a      session with it.   If supported, a router SHOULD be configurable to allow one of an   o Include list - specification of a list of message definitions to be      forwarded, or anBaker                       Standards Track                    [Page 97]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o Exclude list - specification of a list of message definitions NOT      to be forwarded.   A "message definition", in this context, specifies the source and   destination network prefix, and may include other identifying   information such as IP Protocol Type or TCP port number.   A router MAY provide a configuration switch that allows a choice   between specifying an include or an exclude list, or other equivalent   controls.   A value matching any address (e.g., a keyword any, an address with a   mask of all 0's, or a network prefix whose length is zero) MUST be   allowed as a source and/or destination address.   In addition to address pairs, the router MAY allow any combination of   transport and/or application protocol and source and destination   ports to be specified.   The router MUST allow packets to be silently discarded (i.e.,   discarded without an ICMP error message being sent).   The router SHOULD allow an appropriate ICMP unreachable message to be   sent when a packet is discarded.  The ICMP message SHOULD specify   Communication Administratively Prohibited (code 13) as the reason for   the destination being unreachable.   The router SHOULD allow the sending of ICMP destination unreachable   messages (code 13) to be configured for each combination of address   pairs, protocol types, and ports it allows to be specified.   The router SHOULD count and SHOULD allow selective logging of packets   not forwarded.5.3.10 Multicast Routing   An IP router SHOULD support forwarding of IP multicast packets, based   either on static multicast routes or on routes dynamically determined   by a multicast routing protocol (e.g., DVMRP [ROUTE:9]).  A router   that forwards IP multicast packets is called a multicast router.5.3.11 Controls on Forwarding   For each physical interface, a router SHOULD have a configuration   option that specifies whether forwarding is enabled on that   interface.  When forwarding on an interface is disabled, the router:Baker                       Standards Track                    [Page 98]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o MUST silently discard any packets which are received on that      interface but are not addressed to the router   o MUST NOT send packets out that interface, except for datagrams      originated by the router   o MUST NOT announce via any routing protocols the availability of      paths through the interface   DISCUSSION      This feature allows the network manager to essentially turn off an      interface but leaves it accessible for network management.      Ideally, this control would apply to logical rather than physical      interfaces.  It cannot, because there is no known way for a router      to determine which logical interface a packet arrived absent a      one-to-one correspondence between logical and physical interfaces.5.3.12 State Changes   During router operation, interfaces may fail or be manually disabled,   or may become available for use by the router.  Similarly, forwarding   may be disabled for a particular interface or for the entire router   or may be (re)enabled.  While such transitions are (usually)   uncommon, it is important that routers handle them correctly.5.3.12.1 When a Router Ceases Forwarding   When a router ceases forwarding it MUST stop advertising all routes,   except for third party routes.  It MAY continue to receive and use   routes from other routers in its routing domains.  If the forwarding   database is retained, the router MUST NOT cease timing the routes in   the forwarding database.  If routes that have been received from   other routers are remembered, the router MUST NOT cease timing the   routes that it has remembered.  It MUST discard any routes whose   timers expire while forwarding is disabled, just as it would do if   forwarding were enabled.   DISCUSSION      When a router ceases forwarding, it essentially ceases being a      router.  It is still a host, and must follow all of the      requirements of Host Requirements [INTRO:2].  The router may still      be a passive member of one or more routing domains, however.  As      such, it is allowed to maintain its forwarding database by      listening to other routers in its routing domain.  It may not,      however, advertise any of the routes in its forwarding database,      since it itself is doing no forwarding.  The only exception to      this rule is when the router is advertising a route that uses onlyBaker                       Standards Track                    [Page 99]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      some other router, but which this router has been asked to      advertise.   A router MAY send ICMP destination unreachable (host unreachable)   messages to the senders of packets that it is unable to forward.  It   SHOULD NOT send ICMP redirect messages.   DISCUSSION      Note that sending an ICMP destination unreachable (host      unreachable) is a router action.  This message should not be sent      by hosts.  This exception to the rules for hosts is allowed so      that packets may be rerouted in the shortest possible time, and so      that black holes are avoided.5.3.12.2 When a Router Starts Forwarding   When a router begins forwarding, it SHOULD expedite the sending of   new routing information to all routers with which it normally   exchanges routing information.5.3.12.3 When an Interface Fails or is Disabled   If an interface fails or is disabled a router MUST remove and stop   advertising all routes in its forwarding database that make use of   that interface.  It MUST disable all static routes that make use of   that interface.  If other routes to the same destination and TOS are   learned or remembered by the router, the router MUST choose the best   alternate, and add it to its forwarding database.  The router SHOULD   send ICMP destination unreachable or ICMP redirect messages, as   appropriate, in reply to all packets that it is unable to forward due   to the interface being unavailable.5.3.12.4 When an Interface is Enabled   If an interface that had not been available becomes available, a   router MUST reenable any static routes that use that interface.  If   routes that would use that interface are learned by the router, then   these routes MUST be evaluated along with all the other learned   routes, and the router MUST make a decision as to which routes should   be placed in the forwarding database.  The implementor is referred to   Chapter [7], Application Layer - Routing Protocols for further   information on how this decision is made.   A router SHOULD expedite the sending of new routing information to   all routers with which it normally exchanges routing information.Baker                       Standards Track                   [Page 100]

RFC 1812         Requirements for IP Version 4 Routers         June 19955.3.13 IP Options   Several options, such as Record Route and Timestamp, contain slots   into which a router inserts its address when forwarding the packet.   However, each such option has a finite number of slots, and therefore   a router may find that there is not free slot into which it can   insert its address.  No requirement listed below should be construed   as requiring a router to insert its address into an option that has   no remaining slot to insert it into.  Section [5.2.5] discusses how a   router must choose which of its addresses to insert into an option.5.3.13.1 Unrecognized Options Unrecognized IP options in forwarded   packets MUST be passed through unchanged.5.3.13.2 Security Option   Some environments require the Security option in every packet; such a   requirement is outside the scope of this document and the IP standard   specification.  Note, however, that the security options described in   [INTERNET:1] and [INTERNET:16] are obsolete.  Routers SHOULD   IMPLEMENT the revised security option described in [INTERNET:5].   DISCUSSION      Routers intended for use in networks with multiple security levels      should support packet filtering based on IPSO (RFC-1108) labels.      To implement this support, the router would need to permit the      router administrator to configure both a lower sensitivity limit      (e.g. Unclassified) and an upper sensitivity limit (e.g. Secret)      on each interface.  It is commonly but not always the case that      the two limits are the same (e.g. a single-level interface).      Packets caught by an IPSO filter as being out of range should be      silently dropped and a counter should note the number of packets      dropped because of out of range IPSO labels.5.3.13.3 Stream Identifier Option   This option is obsolete.  If the Stream Identifier option is present   in a packet forwarded by the router, the option MUST be ignored and   passed through unchanged.5.3.13.4 Source Route Options   A router MUST implement support for source route options in forwarded   packets.  A router MAY implement a configuration option that, when   enabled, causes all source-routed packets to be discarded.  However,   such an option MUST NOT be enabled by default.Baker                       Standards Track                   [Page 101]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      The ability to source route datagrams through the Internet is      important to various network diagnostic tools.  However, source      routing may be used to bypass administrative and security controls      within a network.  Specifically, those cases where manipulation of      routing tables is used to provide administrative separation in      lieu of other methods such as packet filtering may be vulnerable      through source routed packets.   EDITORS+COMMENTS      Packet filtering can be defeated by source routing as well, if it      is applied in any router except one on the final leg of the source      routed path.  Neither route nor packet filters constitute a      complete solution for security.5.3.13.5 Record Route Option   Routers MUST support the Record Route option in forwarded packets.   A router MAY provide a configuration option that, if enabled, will   cause the router to ignore (i.e., pass through unchanged) Record   Route options in forwarded packets.  If provided, such an option MUST   default to enabling the record-route.  This option should not affect   the processing of Record Route options in datagrams received by the   router itself (in particular, Record Route options in ICMP echo   requests will still be processed according to Section [4.3.3.6]).   DISCUSSION      There are some people who believe that Record Route is a security      problem because it discloses information about the topology of the      network.  Thus, this document allows it to be disabled.5.3.13.6 Timestamp Option   Routers MUST support the timestamp option in forwarded packets.  A   timestamp value MUST follow the rules given [INTRO:2].   If the flags field = 3 (timestamp and prespecified address), the   router MUST add its timestamp if the next prespecified address   matches any of the router's IP addresses.  It is not necessary that   the prespecified address be either the address of the interface on   which the packet arrived or the address of the interface over which   it will be sent.   IMPLEMENTATION      To maximize the utility of the timestamps contained in the      timestamp option, it is suggested that the timestamp inserted be,      as nearly as practical, the time at which the packet arrived atBaker                       Standards Track                   [Page 102]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      the router.  For datagrams originated by the router, the timestamp      inserted should be, as nearly as practical, the time at which the      datagram was passed to the network layer for transmission.   A router MAY provide a configuration option which, if enabled, will   cause the router to ignore (i.e., pass through unchanged) Timestamp   options in forwarded datagrams when the flag word is set to zero   (timestamps only) or one (timestamp and registering IP address).  If   provided, such an option MUST default to off (that is, the router   does not ignore the timestamp).  This option should not affect the   processing of Timestamp options in datagrams received by the router   itself (in particular, a router will insert timestamps into Timestamp   options in datagrams received by the router, and Timestamp options in   ICMP echo requests will still be processed according to Section   [4.3.3.6]).   DISCUSSION      Like the Record Route option, the Timestamp option can reveal      information about a network's topology.  Some people consider this      to be a security concern.6. TRANSPORT LAYER   A router is not required to implement any Transport Layer protocols   except those required to support Application Layer protocols   supported by the router.  In practice, this means that most routers   implement both the Transmission Control Protocol (TCP) and the User   Datagram Protocol (UDP).6.1 USER DATAGRAM PROTOCOL - UDP   The User Datagram Protocol (UDP) is specified in [TRANS:1].   A router that implements UDP MUST be compliant, and SHOULD be   unconditionally compliant, with the requirements of [INTRO:2], except   that:   o This specification does not specify the interfaces between the      various protocol layers.  Thus, a router's interfaces need not      comply with [INTRO:2], except where compliance is required for      proper functioning of Application Layer protocols supported by the      router.   o Contrary to [INTRO:2], an application SHOULD NOT disable generation      of UDP checksums.Baker                       Standards Track                   [Page 103]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      Although a particular application protocol may require that UDP      datagrams it receives must contain a UDP checksum, there is no      general requirement that received UDP datagrams contain UDP      checksums.  Of course, if a UDP checksum is present in a received      datagram, the checksum must be verified and the datagram discarded      if the checksum is incorrect.6.2 TRANSMISSION CONTROL PROTOCOL - TCP   The Transmission Control Protocol (TCP) is specified in [TRANS:2].   A router that implements TCP MUST be compliant, and SHOULD be   unconditionally compliant, with the requirements of [INTRO:2], except   that:   o This specification does not specify the interfaces between the      various protocol layers.  Thus, a router need not comply with the      following requirements of [INTRO:2] (except of course where      compliance is required for proper functioning of Application Layer      protocols supported by the router):      Use of Push:RFC-793 Section 2.8:           Passing a received PSH flag to the application layer is now           OPTIONAL.      Urgent Pointer:RFC-793 Section 3.1:           A TCP MUST inform the application layer asynchronously           whenever it receives an Urgent pointer and there was           previously no pending urgent data, or whenever the Urgent           pointer advances in the data stream.  There MUST be a way for           the application to learn how much urgent data remains to be           read from the connection, or at least to determine whether or           not more urgent data remains to be read.      TCP Connection Failures:           An application MUST be able to set the value for R2 for a           particular connection.  For example, an interactive           application might set R2 to ``infinity,'' giving the user           control over when to disconnect.      TCP Multihoming:           If an application on a multihomed host does not specify the           local IP address when actively opening a TCP connection, then           the TCP MUST ask the IP layer to select a local IP address           before sending the (first) SYN.  See the function           GET_SRCADDR() inSection 3.4.Baker                       Standards Track                   [Page 104]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      IP Options:           An application MUST be able to specify a source route when it           actively opens a TCP connection, and this MUST take           precedence over a source route received in a datagram.   o For similar reasons, a router need not comply with any of the      requirements of [INTRO:2].   o The requirements concerning the Maximum Segment Size Option in      [INTRO:2] are amended as follows: a router that implements the      host portion of MTU discovery (discussed in Section [4.2.3.3] of      this memo) uses 536 as the default value of SendMSS only if the      path MTU is unknown; if the path MTU is known, the default value      for SendMSS is the path MTU - 40.   o The requirements concerning the Maximum Segment Size Option in      [INTRO:2] are amended as follows: ICMP Destination Unreachable      codes 11 and 12 are additional soft error conditions.  Therefore,      these message MUST NOT cause TCP to abort a connection.   DISCUSSION      It should particularly be noted that a TCP implementation in a      router must conform to the following requirements of [INTRO:2]:      o Providing a configurable TTL.  [Time to Live:RFC-793 Section 3.9]      o Providing an interface to configure keep-alive behavior, if         keep-alives are used at all.  [TCP Keep-Alives]      o Providing an error reporting mechanism, and the ability to         manage it.  [Asynchronous Reports]      o Specifying type of service.  [Type-of-Service]      The general paradigm applied is that if a particular interface is      visible outside the router, then all requirements for the      interface must be followed.  For example, if a router provides a      telnet function, then it will be generating traffic, likely to be      routed in the external networks.  Therefore, it must be able to      set the type of service correctly or else the telnet traffic may      not get through.Baker                       Standards Track                   [Page 105]

RFC 1812         Requirements for IP Version 4 Routers         June 19957. APPLICATION LAYER - ROUTING PROTOCOLS7.1 INTRODUCTION   For technical, managerial, and sometimes political reasons, the   Internet routing system consists of two components - interior routing   and exterior routing.  The concept of an Autonomous System (AS), as   define inSection 2.2.4 of this document, plays a key role in   separating interior from an exterior routing, as this concept allows   to deliniate the set of routers where a change from interior to   exterior routing occurs.  An IP datagram may have to traverse the   routers of two or more Autonomous Systems to reach its destination,   and the Autonomous Systems must provide each other with topology   information to allow such forwarding.  Interior gateway protocols   (IGPs) are used to distribute routing information within an AS (i.e.,   intra-AS routing).  Exterior gateway protocols are used to exchange   routing information among ASs (i.e., inter-AS routing).7.1.1 Routing Security Considerations   Routing is one of the few places where the Robustness Principle (be   liberal in what you accept) does not apply.  Routers should be   relatively suspicious in accepting routing data from other routing   systems.   A router SHOULD provide the ability to rank routing information   sources from most trustworthy to least trustworthy and to accept   routing information about any particular destination from the most   trustworthy sources first.  This was implicit in the original   core/stub autonomous system routing model using EGP and various   interior routing protocols.  It is even more important with the   demise of a central, trusted core.   A router SHOULD provide a mechanism to filter out obviously invalid   routes (such as those for net 127).   Routers MUST NOT by default redistribute routing data they do not   themselves use, trust or otherwise consider valid.  In rare cases, it   may be necessary to redistribute suspicious information, but this   should only happen under direct intercession by some human agency.   Routers must be at least a little paranoid about accepting routing   data from anyone, and must be especially careful when they distribute   routing information provided to them by another party.  See below for   specific guidelines.Baker                       Standards Track                   [Page 106]

RFC 1812         Requirements for IP Version 4 Routers         June 19957.1.2 Precedence   Except where the specification for a particular routing protocol   specifies otherwise, a router SHOULD set the IP Precedence value for   IP datagrams carrying routing traffic it originates to 6   (INTERNETWORK CONTROL).   DISCUSSION      Routing traffic with VERY FEW exceptions should be the highest      precedence traffic on any network.  If a system's routing traffic      can't get through, chances are nothing else will.7.1.3 Message Validation   Peer-to-peer authentication involves several tests.  The application   of message passwords and explicit acceptable neighbor lists has in   the past improved the robustness of the route database.  Routers   SHOULD IMPLEMENT management controls that enable explicit listing of   valid routing neighbors.  Routers SHOULD IMPLEMENT peer-to-peer   authentication for those routing protocols that support them.   Routers SHOULD validate routing neighbors based on their source   address and the interface a message is received on; neighbors in a   directly attached subnet SHOULD be restricted to communicate with the   router via the interface that subnet is posited on or via unnumbered   interfaces.  Messages received on other interfaces SHOULD be silently   discarded.   DISCUSSION      Security breaches and numerous routing problems are avoided by      this basic testing.7.2 INTERIOR GATEWAY PROTOCOLS7.2.1 INTRODUCTION   An Interior Gateway Protocol (IGP) is used to distribute routing   information between the various routers in a particular AS.   Independent of the algorithm used to implement a particular IGP, it   should perform the following functions:   (1) Respond quickly to changes in the internal topology of an AS   (2) Provide a mechanism such that circuit flapping does not cause        continuous routing updates   (3) Provide quick convergence to loop-free routingBaker                       Standards Track                   [Page 107]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (4) Utilize minimal bandwidth   (5) Provide equal cost routes to enable load-splitting   (6) Provide a means for authentication of routing updates   Current IGPs used in the internet today are characterized as either   being based on a distance-vector or a link-state algorithm.   Several IGPs are detailed in this section, including those most   commonly used and some recently developed protocols that may be   widely used in the future.  Numerous other protocols intended for use   in intra-AS routing exist in the Internet community.   A router that implements any routing protocol (other than static   routes) MUST IMPLEMENT OSPF (see Section [7.2.2]).  A router MAY   implement additional IGPs.7.2.2 OPEN SHORTEST PATH FIRST - OSPF   Shortest Path First (SPF) based routing protocols are a class of   link-state algorithms that are based on the shortest-path algorithm   of Dijkstra.  Although SPF based algorithms have been around since   the inception of the ARPANET, it is only recently that they have   achieved popularity both inside both the IP and the OSI communities.   In an SPF based system, each router obtains the entire topology   database through a process known as flooding.  Flooding insures a   reliable transfer of the information.  Each router then runs the SPF   algorithm on its database to build the IP routing table.  The OSPF   routing protocol is an implementation of an SPF algorithm.  The   current version, OSPF version 2, is specified in [ROUTE:1].  Note   thatRFC-1131, which describes OSPF version 1, is obsolete.   Note that to comply with Section [8.3] of this memo, a router that   implements OSPF MUST implement the OSPF MIB [MGT:14].7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM - DUAL IS-IS   The American National Standards Institute (ANSI) X3S3.3 committee has   defined an intra-domain routing protocol.  This protocol is titled   Intermediate System to Intermediate System Routeing Exchange   Protocol.   Its application to an IP network has been defined in [ROUTE:2], and   is referred to as Dual IS-IS (or sometimes as Integrated IS-IS).   IS-IS is based on a link-state (SPF) routing algorithm and shares all   the advantages for this class of protocols.Baker                       Standards Track                   [Page 108]

RFC 1812         Requirements for IP Version 4 Routers         June 19957.3  EXTERIOR GATEWAY PROTOCOLS7.3.1  INTRODUCTION   Exterior Gateway Protocols are utilized for inter-Autonomous System   routing to exchange reachability information for a set of networks   internal to a particular autonomous system to a neighboring   autonomous system.   The area of inter-AS routing is a current topic of research inside   the Internet Engineering Task Force.  The Exterior Gateway Protocol   (EGP) described in Section [Appendix F.1] has traditionally been the   inter-AS protocol of choice, but is now historical.  The Border   Gateway Protocol (BGP) eliminates many of the restrictions and   limitations of EGP, and is therefore growing rapidly in popularity.   A router is not required to implement any inter-AS routing protocol.   However, if a router does implement EGP it also MUST IMPLEMENT BGP.   Although it was not designed as an exterior gateway protocol, RIP   (described in Section [7.2.4]) is sometimes used for inter-AS   routing.7.3.2 BORDER GATEWAY PROTOCOL - BGP7.3.2.1 Introduction   The Border Gateway Protocol (BGP-4) is an inter-AS routing protocol   that exchanges network reachability information with other BGP   speakers.  The information for a network includes the complete list   of ASs that traffic must transit to reach that network.  This   information can then be used to insure loop-free paths.  This   information is sufficient to construct a graph of AS connectivity   from which routing loops may be pruned and some policy decisions at   the AS level may be enforced.   BGP is defined by [ROUTE:4].  [ROUTE:5] specifies the proper usage of   BGP in the Internet, and provides some useful implementation hints   and guidelines.  [ROUTE:12] and [ROUTE:13] provide additional useful   information.   To comply with Section [8.3] of this memo, a router that implements   BGP is required to implement the BGP MIB [MGT:15].   To characterize the set of policy decisions that can be enforced   using BGP, one must focus on the rule that an AS advertises to its   neighbor ASs only those routes that it itself uses.  This rule   reflects the hop-by-hop routing paradigm generally used throughout   the current Internet.  Note that some policies cannot be supported by   the hop-by-hop routing paradigm and thus require techniques such asBaker                       Standards Track                   [Page 109]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   source routing to enforce.  For example, BGP does not enable one AS   to send traffic to a neighbor AS intending that traffic take a   different route from that taken by traffic originating in the   neighbor AS.  On the other hand, BGP can support any policy   conforming to the hop-by-hop routing paradigm.   Implementors of BGP are strongly encouraged to follow the   recommendations outlined inSection 6 of [ROUTE:5].7.3.2.2 Protocol Walk-through   While BGP provides support for quite complex routing policies (as an   example seeSection 4.2 in [ROUTE:5]), it is not required for all BGP   implementors to support such policies.  At a minimum, however, a BGP   implementation:   (1) SHOULD allow an AS to control announcements of the BGP learned        routes to adjacent AS's.  Implementations SHOULD support such        control with at least the granularity of a single network.        Implementations SHOULD also support such control with the        granularity of an autonomous system, where the autonomous system        may be either the autonomous system that originated the route,        or the autonomous system that advertised the route to the local        system (adjacent autonomous system).   (2) SHOULD allow an AS to prefer a particular path to a destination        (when more than one path is available).  Such function SHOULD be        implemented by allowing system administrator to assign weights        to Autonomous Systems, and making route selection process to        select a route with the lowest weight (where weight of a route        is defined as a sum of weights of all AS's in the AS_PATH path        attribute associated with that route).   (3) SHOULD allow an AS to ignore routes with certain AS's in the        AS_PATH path attribute.  Such function can be implemented by        using technique outlined in (2), and by assigning infinity as        weights for such AS's.  The route selection process must ignore        routes that have weight equal to infinity.7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL   It is possible to exchange routing information between two autonomous   systems or routing domains without using a standard exterior routing   protocol between two separate, standard interior routing protocols.   The most common way of doing this is to run both interior protocols   independently in one of the border routers with an exchange of route   information between the two processes.Baker                       Standards Track                   [Page 110]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   As with the exchange of information from an EGP to an IGP, without   appropriate controls these exchanges of routing information between   two IGPs in a single router are subject to creation of routing loops.7.4 STATIC ROUTING   Static routing provides a means of explicitly defining the next hop   from a router for a particular destination.  A router SHOULD provide   a means for defining a static route to a destination, where the   destination is defined by a network prefix.  The mechanism SHOULD   also allow for a metric to be specified for each static route.   A router that supports a dynamic routing protocol MUST allow static   routes to be defined with any metric valid for the routing protocol   used.  The router MUST provide the ability for the user to specify a   list of static routes that may or may not be propagated through the   routing protocol.  In addition, a router SHOULD support the following   additional information if it supports a routing protocol that could   make use of the information.  They are:   o TOS,   o Subnet Mask, or   o Prefix Length, or   o A metric specific to a given routing protocol that can import the      route.   DISCUSSION      We intend that one needs to support only the things useful to the      given routing protocol.  The need for TOS should not require the      vendor to implement the other parts if they are not used.      Whether a router prefers a static route over a dynamic route (or      vice versa) or whether the associated metrics are used to choose      between conflicting static and dynamic routes SHOULD be      configurable for each static route.      A router MUST allow a metric to be assigned to a static route for      each routing domain that it supports.  Each such metric MUST be      explicitly assigned to a specific routing domain.  For example:           route 10.0.0.0/8 via 192.0.2.3 rip metric 3           route 10.21.0.0/16 via 192.0.2.4 ospf inter-area metric 27           route 10.22.0.0/16 via 192.0.2.5 egp 123 metric 99Baker                       Standards Track                   [Page 111]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      It has been suggested that, ideally, static routes should have      preference values rather than metrics (since metrics can only be      compared with metrics of other routes in the same routing domain,      the metric of a static route could only be compared with metrics      of other static routes).  This is contrary to some current      implementations, where static routes really do have metrics, and      those metrics are used to determine whether a particular dynamic      route overrides the static route to the same destination.  Thus,      this document uses the term metric rather than preference.      This technique essentially makes the static route into a RIP      route, or an OSPF route (or whatever, depending on the domain of      the metric).  Thus, the route lookup algorithm of that domain      applies.  However, this is NOT route leaking, in that coercing a      static route into a dynamic routing domain does not authorize the      router to redistribute the route into the dynamic routing domain.      For static routes not put into a specific routing domain, the      route lookup algorithm is:      (1) Basic match      (2) Longest match      (3) Weak TOS (if TOS supported)      (4) Best metric (where metric are implementation-defined)      The last step may not be necessary, but it's useful in the case      where you want to have a primary static route over one interface      and a secondary static route over an alternate interface, with      failover to the alternate path if the interface for the primary      route fails.7.5 FILTERING OF ROUTING INFORMATION   Each router within a network makes forwarding decisions based upon   information contained within its forwarding database.  In a simple   network the contents of the database may be configured statically.   As the network grows more complex, the need for dynamic updating of   the forwarding database becomes critical to the efficient operation   of the network.   If the data flow through a network is to be as efficient as possible,   it is necessary to provide a mechanism for controlling the   propagation of the information a router uses to build its forwarding   database.  This control takes the form of choosing which sources ofBaker                       Standards Track                   [Page 112]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   routing information should be trusted and selecting which pieces of   the information to believe.  The resulting forwarding database is a   filtered version of the available routing information.   In addition to efficiency, controlling the propagation of routing   information can reduce instability by preventing the spread of   incorrect or bad routing information.   In some cases local policy may require that complete routing   information not be widely propagated.   These filtering requirements apply only to non-SPF-based protocols   (and therefore not at all to routers which don't implement any   distance vector protocols).7.5.1 Route Validation   A router SHOULD log as an error any routing update advertising a   route that violates the specifications of this memo, unless the   routing protocol from which the update was received uses those values   to encode special routes (such as default routes).7.5.2 Basic Route Filtering   Filtering of routing information allows control of paths used by a   router to forward packets it receives.  A router should be selective   in which sources of routing information it listens to and what routes   it believes.  Therefore, a router MUST provide the ability to   specify:   o On which logical interfaces routing information will be accepted      and which routes will be accepted from each logical interface.   o Whether all routes or only a default route is advertised on a      logical interface.   Some routing protocols do not recognize logical interfaces as a   source of routing information.  In such cases the router MUST provide   the ability to specify   o from which other routers routing information will be accepted.   For example, assume a router connecting one or more leaf networks to   the main portion or backbone of a larger network.  Since each of the   leaf networks has only one path in and out, the router can simply   send a default route to them.  It advertises the leaf networks to the   main network.Baker                       Standards Track                   [Page 113]

RFC 1812         Requirements for IP Version 4 Routers         June 19957.5.3 Advanced Route Filtering   As the topology of a network grows more complex, the need for more   complex route filtering arises.  Therefore, a router SHOULD provide   the ability to specify independently for each routing protocol:   o Which logical interfaces or routers routing information (routes)      will be accepted from and which routes will be believed from each      other router or logical interface,   o Which routes will be sent via which logical interface(s), and   o Which routers routing information will be sent to, if this is      supported by the routing protocol in use.   In many situations it is desirable to assign a reliability ordering   to routing information received from another router instead of the   simple believe or don't believe choice listed in the first bullet   above.  A router MAY provide the ability to specify:   o A reliability or preference to be assigned to each route received.      A route with higher reliability will be chosen over one with lower      reliability regardless of the routing metric associated with each      route.   If a router supports assignment of preferences, the router MUST NOT   propagate any routes it does not prefer as first party information.   If the routing protocol being used to propagate the routes does not   support distinguishing between first and third party information, the   router MUST NOT propagate any routes it does not prefer.   DISCUSSION      For example, assume a router receives a route to network C from      router R and a route to the same network from router S.  If router      R is considered more reliable than router S traffic destined for      network C will be forwarded to router R regardless of the route      received from router S.   Routing information for routes which the router does not use (router   S in the above example) MUST NOT be passed to any other router.7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE   Routers MUST be able to exchange routing information between separate   IP interior routing protocols, if independent IP routing processes   can run in the same router.  Routers MUST provide some mechanism for   avoiding routing loops when routers are configured for bi-directional   exchange of routing information between two separate interior routingBaker                       Standards Track                   [Page 114]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   processes.  Routers MUST provide some priority mechanism for choosing   routes from independent routing processes.  Routers SHOULD provide   administrative control of IGP-IGP exchange when used across   administrative boundaries.   Routers SHOULD provide some mechanism for translating or transforming   metrics on a per network basis.  Routers (or routing protocols) MAY   allow for global preference of exterior routes imported into an IGP.   DISCUSSION      Different IGPs use different metrics, requiring some translation      technique when introducing information from one protocol into      another protocol with a different form of metric.  Some IGPs can      run multiple instances within the same router or set of routers.      In this case metric information can be preserved exactly or      translated.      There are at least two techniques for translation between      different routing processes.  The static (or reachability)      approach uses the existence of a route advertisement in one IGP to      generate a route advertisement in the other IGP with a given      metric.  The translation or tabular approach uses the metric in      one IGP to create a metric in the other IGP through use of either      a function (such as adding a constant) or a table lookup.      Bi-directional exchange of routing information is dangerous      without control mechanisms to limit feedback.  This is the same      problem that distance vector routing protocols must address with      the split horizon technique and that EGP addresses with the      third-party rule.  Routing loops can be avoided explicitly through      use of tables or lists of permitted/denied routes or implicitly      through use of a split horizon rule, a no-third-party rule, or a      route tagging mechanism.  Vendors are encouraged to use implicit      techniques where possible to make administration easier for      network operators.8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS   Note that this chapter supersedes any requirements stated under   "REMOTE MANAGEMENT" in [INTRO:3].8.1 The Simple Network Management Protocol - SNMP8.1.1 SNMP Protocol Elements   Routers MUST be manageable by SNMP [MGT:3].  The SNMP MUST operate   using UDP/IP as its transport and network protocols.  Others MAY be   supported (e.g., see [MGT:25, MGT:26, MGT:27, and MGT:28]).  SNMPBaker                       Standards Track                   [Page 115]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   management operations MUST operate as if the SNMP was implemented on   the router itself.  Specifically, management operations MUST be   effected by sending SNMP management requests to any of the IP   addresses assigned to any of the router's interfaces.  The actual   management operation may be performed either by the router or by a   proxy for the router.   DISCUSSION      This wording is intended to allow management either by proxy,      where the proxy device responds to SNMP packets that have one of      the router's IP addresses in the packets destination address      field, or the SNMP is implemented directly in the router itself      and receives packets and responds to them in the proper manner.      It is important that management operations can be sent to one of      the router's IP Addresses.  In diagnosing network problems the      only thing identifying the router that is available may be one of      the router's IP address; obtained perhaps by looking through      another router's routing table.   All SNMP operations (get, get-next, get-response, set, and trap) MUST   be implemented.   Routers MUST provide a mechanism for rate-limiting the generation of   SNMP trap messages.  Routers MAY provide this mechanism through the   algorithms for asynchronous alert management described in [MGT:5].   DISCUSSION      Although there is general agreement about the need to rate-limit      traps, there is not yet consensus on how this is best achieved.      The reference cited is considered experimental.8.2 Community Table   For the purposes of this specification, we assume that there is an   abstract `community table' in the router.  This table contains   several entries, each entry for a specific community and containing   the parameters necessary to completely define the attributes of that   community.  The actual implementation method of the abstract   community table is, of course, implementation specific.   A router's community table MUST allow for at least one entry and   SHOULD allow for at least two entries.   DISCUSSION      A community table with zero capacity is useless.  It means that      the router will not recognize any communities and, therefore, all      SNMP operations will be rejected.Baker                       Standards Track                   [Page 116]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      Therefore, one entry is the minimal useful size of the table.      Having two entries allows one entry to be limited to read-only      access while the other would have write capabilities.   Routers MUST allow the user to manually (i.e., without using SNMP)   examine, add, delete and change entries in the SNMP community table.   The user MUST be able to set the community name or construct a MIB   view.  The user MUST be able to configure communities as read-only   (i.e., they do not allow SETs) or read-write (i.e., they do allow   SETs).   The user MUST be able to define at least one IP address to which   notifications are sent for each community or MIB view, if traps are   used.  These addresses SHOULD be definable on a community or MIB view   basis.  It SHOULD be possible to enable or disable notifications on a   community or MIB view basis.   A router SHOULD provide the ability to specify a list of valid   network managers for any particular community.  If enabled, a router   MUST validate the source address of the SNMP datagram against the   list and MUST discard the datagram if its address does not appear.   If the datagram is discarded the router MUST take all actions   appropriate to an SNMP authentication failure.   DISCUSSION      This is a rather limited authentication system, but coupled with      various forms of packet filtering may provide some small measure      of increased security.   The community table MUST be saved in non-volatile storage.   The initial state of the community table SHOULD contain one entry,   with the community name string public and read-only access.  The   default state of this entry MUST NOT send traps.  If it is   implemented, then this entry MUST remain in the community table until   the administrator changes it or deletes it.   DISCUSSION      By default, traps are not sent to this community.  Trap PDUs are      sent to unicast IP addresses.  This address must be configured      into the router in some manner.  Before the configuration occurs,      there is no such address, so to whom should the trap be sent?      Therefore trap sending to the public community defaults to be      disabled.  This can, of course, be changed by an administrative      operation once the router is operational.Baker                       Standards Track                   [Page 117]

RFC 1812         Requirements for IP Version 4 Routers         June 19958.3 Standard MIBS   All MIBS relevant to a router's configuration are to be implemented.   To wit:   o The System, Interface, IP, ICMP, and UDP groups of MIB-II [MGT:2]      MUST be implemented.   o The Interface Extensions MIB [MGT:18] MUST be implemented.   o The IP Forwarding Table MIB [MGT:20] MUST be implemented.   o If the router implements TCP (e.g., for Telnet) then the TCP group      of MIB-II [MGT:2] MUST be implemented.   o If the router implements EGP then the EGP group of MIB-II [MGT:2]      MUST be implemented.   o If the router supports OSPF then the OSPF MIB [MGT:14] MUST be      implemented.   o If the router supports BGP then the BGP MIB [MGT:15] MUST be      implemented.   o If the router has Ethernet, 802.3, or StarLan interfaces then the      Ethernet-Like MIB [MGT:6] MUST be implemented.   o If the router has 802.4 interfaces then the 802.4 MIB [MGT:7] MUST      be implemented.   o If the router has 802.5 interfaces then the 802.5 MIB [MGT:8] MUST      be implemented.   o If the router has FDDI interfaces that implement ANSI SMT 7.3 then      the FDDI MIB [MGT:9] MUST be implemented.   o If the router has FDDI interfaces that implement ANSI SMT 6.2 then      the FDDI MIB [MGT:29] MUST be implemented.   o If the router has interfaces that use V.24 signalling, such as RS-      232, V.10, V.11, V.35, V.36, or RS-422/423/449, then the RS-232      [MGT:10] MIB MUST be implemented.   o If the router has T1/DS1 interfaces then the T1/DS1 MIB [MGT:16]      MUST be implemented.   o If the router has T3/DS3 interfaces then the T3/DS3 MIB [MGT:17]      MUST be implemented.Baker                       Standards Track                   [Page 118]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o If the router has SMDS interfaces then the SMDS Interface Protocol      MIB [MGT:19] MUST be implemented.   o If the router supports PPP over any of its interfaces then the PPP      MIBs [MGT:11], [MGT:12], and [MGT:13] MUST be implemented.   o If the router supports RIP Version 2 then the RIP Version 2 MIB      [MGT:21] MUST be implemented.   o If the router supports X.25 over any of its interfaces then the      X.25 MIBs [MGT:22, MGT:23 and MGT:24] MUST be implemented.8.4 Vendor Specific MIBS   The Internet Standard and Experimental MIBs do not cover the entire   range of statistical, state, configuration and control information   that may be available in a network element.  This information is,   nevertheless, extremely useful.  Vendors of routers (and other   network devices) generally have developed MIB extensions that cover   this information.  These MIB extensions are called Vendor Specific   MIBs.   The Vendor Specific MIB for the router MUST provide access to all   statistical, state, configuration, and control information that is   not available through the Standard and Experimental MIBs that have   been implemented.  This information MUST be available for both   monitoring and control operations.   DISCUSSION      The intent of this requirement is to provide the ability to do      anything on the router through SNMP that can be done through a      console, and vice versa.  A certain minimal amount of      configuration is necessary before SNMP can operate (e.g., the      router must have an IP address).  This initial configuration can      not be done through SNMP.  However, once the initial configuration      is done, full capabilities ought to be available through network      management.   The vendor SHOULD make available the specifications for all Vendor   Specific MIB variables.  These specifications MUST conform to the SMI   [MGT:1] and the descriptions MUST be in the form specified in   [MGT:4].   DISCUSSION      Making the Vendor Specific MIB available to the user is necessary.      Without this information the users would not be able to configure      their network management systems to be able to access the Vendor      Specific parameters.  These parameters would then be useless.Baker                       Standards Track                   [Page 119]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      ne 2 The format of the MIB specification is also specified.      Parsers that read MIB specifications and generate the needed      tables for the network management station are available.  These      parsers generally understand only the standard MIB specification      format.8.5 Saving Changes   Parameters altered by SNMP MAY be saved to non-volatile storage.   DISCUSSION      Reasons why this requirement is a MAY:      o The exact physical nature of non-volatile storage is not         specified in this document.  Hence, parameters may be saved in         NVRAM/EEPROM, local floppy or hard disk, or in some TFTP file         server or BOOTP server, etc.  Suppose that this information is         in a file that is retrieved through TFTP.  In that case, a         change made to a configuration parameter on the router would         need to be propagated back to the file server holding the         configuration file.  Alternatively, the SNMP operation would         need to be directed to the file server, and then the change         somehow propagated to the router.  The answer to this problem         does not seem obvious.         This also places more requirements on the host holding the         configuration information than just having an available TFTP         server, so much more that its probably unsafe for a vendor to         assume that any potential customer will have a suitable host         available.      o The timing of committing changed parameters to non-volatile         storage is still an issue for debate.  Some prefer to commit         all changes immediately.  Others prefer to commit changes to         non-volatile storage only upon an explicit command.9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS   For all additional application protocols that a router implements,   the router MUST be compliant and SHOULD be unconditionally compliant   with the relevant requirements of [INTRO:3].9.1 BOOTP9.1.1 Introduction   The Bootstrap Protocol (BOOTP) is a UDP/IP-based protocol that allows   a booting host to configure itself dynamically and without userBaker                       Standards Track                   [Page 120]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   supervision.  BOOTP provides a means to notify a host of its assigned   IP address, the IP address of a boot server host, and the name of a   file to be loaded into memory and executed ([APPL:1]).  Other   configuration information such as the local prefix length or subnet   mask, the local time offset, the addresses of default routers, and   the addresses of various Internet servers can also be communicated to   a host using BOOTP ([APPL:2]).9.1.2 BOOTP Relay Agents   In many cases, BOOTP clients and their associated BOOTP server(s) do   not reside on the same IP (sub)network.  In such cases, a third-party   agent is required to transfer BOOTP messages between clients and   servers.  Such an agent was originally referred to as a BOOTP   forwarding agent.  However, to avoid confusion with the IP forwarding   function of a router, the name BOOTP relay agent has been adopted   instead.   DISCUSSION      A BOOTP relay agent performs a task that is distinct from a      router's normal IP forwarding function.  While a router normally      switches IP datagrams between networks more-or-less transparently,      a BOOTP relay agent may more properly be thought to receive BOOTP      messages as a final destination and then generate new BOOTP      messages as a result.  One should resist the notion of simply      forwarding a BOOTP message straight through like a regular packet.   This relay-agent functionality is most conveniently located in the   routers that interconnect the clients and servers (although it may   alternatively be located in a host that is directly connected to the   client (sub)net).   A router MAY provide BOOTP relay-agent capability.  If it does, it   MUST conform to the specifications in [APPL:3].   Section [5.2.3] discussed the circumstances under which a packet is   delivered locally (to the router).  All locally delivered UDP   messages whose UDP destination port number is BOOTPS (67) are   considered for special processing by the router's logical BOOTP relay   agent.   Sections [4.2.2.11] and [5.3.7] discussed invalid IP source   addresses.  According to these rules, a router must not forward any   received datagram whose IP source address is 0.0.0.0.  However,   routers that support a BOOTP relay agent MUST accept for local   delivery to the relay agent BOOTREQUEST messages whose IP source   address is 0.0.0.0.Baker                       Standards Track                   [Page 121]

RFC 1812         Requirements for IP Version 4 Routers         June 199510. OPERATIONS AND MAINTENANCE   This chapter supersedes any requirements of [INTRO:3] relating to   "Extensions to the IP Module."   Facilities to support operation and maintenance (O&M) activities form   an essential part of any router implementation.  Although these   functions do not seem to relate directly to interoperability, they   are essential to the network manager who must make the router   interoperate and must track down problems when it doesn't.  This   chapter also includes some discussion of router initialization and of   facilities to assist network managers in securing and accounting for   their networks.10.1 Introduction   The following kinds of activities are included under router O&M:   o Diagnosing hardware problems in the router's processor, in its      network interfaces, or in its connected networks, modems, or      communication lines.   o Installing new hardware   o Installing new software.   o Restarting or rebooting the router after a crash.   o Configuring (or reconfiguring) the router.   o Detecting and diagnosing Internet problems such as congestion,      routing loops, bad IP addresses, black holes, packet avalanches,      and misbehaved hosts.   o Changing network topology, either temporarily (e.g., to bypass a      communication line problem) or permanently.   o Monitoring the status and performance of the routers and the      connected networks.   o Collecting traffic statistics for use in (Inter-)network planning.   o Coordinating the above activities with appropriate vendors and      telecommunications specialists.   Routers and their connected communication lines are often operated as   a system by a centralized O&M organization.  This organization may   maintain a (Inter-)network operation center, or NOC, to carry out itsBaker                       Standards Track                   [Page 122]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   O&M functions.  It is essential that routers support remote control   and monitoring from such a NOC through an Internet path, since   routers might not be connected to the same network as their NOC.   Since a network failure may temporarily preclude network access, many   NOCs insist that routers be accessible for network management through   an alternative means, often dial-up modems attached to console ports   on the routers.   Since an IP packet traversing an internet will often use routers   under the control of more than one NOC, Internet problem diagnosis   will often involve cooperation of personnel of more than one NOC.  In   some cases, the same router may need to be monitored by more than one   NOC, but only if necessary, because excessive monitoring could impact   a router's performance.   The tools available for monitoring at a NOC may cover a wide range of   sophistication.  Current implementations include multi-window,   dynamic displays of the entire router system.  The use of AI   techniques for automatic problem diagnosis is proposed for the   future.   Router O&M facilities discussed here are only a part of the large and   difficult problem of Internet management.  These problems encompass   not only multiple management organizations, but also multiple   protocol layers.  For example, at the current stage of evolution of   the Internet architecture, there is a strong coupling between host   TCP implementations and eventual IP-level congestion in the router   system [OPER:1].  Therefore, diagnosis of congestion problems will   sometimes require the monitoring of TCP statistics in hosts.  There   are currently a number of R&D efforts in progress in the area of   Internet management and more specifically router O&M.  These R&D   efforts have already produced standards for router O&M.  This is also   an area in which vendor creativity can make a significant   contribution.10.2 Router Initialization10.2.1 Minimum Router Configuration   There exists a minimum set of conditions that must be satisfied   before a router may forward packets.  A router MUST NOT enable   forwarding on any physical interface unless either:   (1) The router knows the IP address and associated subnet mask or        network prefix length of at least one logical interface        associated with that physical interface, orBaker                       Standards Track                   [Page 123]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   (2) The router knows that the interface is an unnumbered interface        and knows its router-id.   These parameters MUST be explicitly configured:   o A router MUST NOT use factory-configured default values for its IP      addresses, prefix lengths, or router-id, and   o A router MUST NOT assume that an unconfigured interface is an      unnumbered interface.   DISCUSSION      There have been instances in which routers have been shipped with      vendor-installed default addresses for interfaces.  In a few      cases, this has resulted in routers advertising these default      addresses into active networks.10.2.2 Address and Prefix Initialization   A router MUST allow its IP addresses and their address masks or   prefix lengths to be statically configured and saved in non-volatile   storage.   A router MAY obtain its IP addresses and their corresponding address   masks dynamically as a side effect of the system initialization   process (seeSection 10.2.3]);   If the dynamic method is provided, the choice of method to be used in   a particular router MUST be configurable.   As was described in Section [4.2.2.11], IP addresses are not   permitted to have the value 0 or -1 in the <Host-number> or   <Network-prefix> fields.  Therefore, a router SHOULD NOT allow an IP   address or address mask to be set to a value that would make any of   the these fields above have the value zero or -1.   DISCUSSION      It is possible using arbitrary address masks to create situations      in which routing is ambiguous (i.e., two routes with different but      equally specific subnet masks match a particular destination      address).  This is one of the strongest arguments for the use of      network prefixes, and the reason the use of discontiguous subnet      masks is not permitted.   A router SHOULD make the following checks on any address mask it   installs:Baker                       Standards Track                   [Page 124]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   o The mask is neither all ones nor all zeroes (the prefix length is      neither zero nor 32).   o The bits which correspond to the network prefix part of the address      are all set to 1.   o The bits that correspond to the network prefix are contiguous.   DISCUSSION      The masks associated with routes are also sometimes called subnet      masks, this test should not be applied to them.10.2.3 Network Booting using BOOTP and TFTP   There has been much discussion of how routers can and should be   booted from the network.  These discussions have revolved around   BOOTP and TFTP.  Currently, there are routers that boot with TFTP   from the network.  There is no reason that BOOTP could not be used   for locating the server that the boot image should be loaded from.   BOOTP is a protocol used to boot end systems, and requires some   stretching to accommodate its use with routers.  If a router is using   BOOTP to locate the current boot host, it should send a BOOTP Request   with its hardware address for its first interface, or, if it has been   previously configured otherwise, with either another interface's   hardware address, or another number to put in the hardware address   field of the BOOTP packet.  This is to allow routers without hardware   addresses (like synchronous line only routers) to use BOOTP for   bootload discovery.  TFTP can then be used to retrieve the image   found in the BOOTP Reply.  If there are no configured interfaces or   numbers to use, a router MAY cycle through the interface hardware   addresses it has until a match is found by the BOOTP server.   A router SHOULD IMPLEMENT the ability to store parameters learned   through BOOTP into local non-volatile storage.  A router MAY   implement the ability to store a system image loaded over the network   into local stable storage.   A router MAY have a facility to allow a remote user to request that   the router get a new boot image.  Differentiation should be made   between getting the new boot image from one of three locations: the   one included in the request, from the last boot image server, and   using BOOTP to locate a server.Baker                       Standards Track                   [Page 125]

RFC 1812         Requirements for IP Version 4 Routers         June 199510.3 Operation and Maintenance10.3.1 Introduction   There is a range of possible models for performing O&M functions on a   router.  At one extreme is the local-only model, under which the O&M   functions can only be executed locally (e.g., from a terminal plugged   into the router machine).  At the other extreme, the fully remote   model allows only an absolute minimum of functions to be performed   locally (e.g., forcing a boot), with most O&M being done remotely   from the NOC.  There are intermediate models, such as one in which   NOC personnel can log into the router as a host, using the Telnet   protocol, to perform functions that can also be invoked locally.  The   local-only model may be adequate in a few router installations, but   remote operation from a NOC is normally required, and therefore   remote O&M provisions are required for most routers.   Remote O&M functions may be exercised through a control agent   (program).  In the direct approach, the router would support remote   O&M functions directly from the NOC using standard Internet protocols   (e.g., SNMP, UDP or TCP); in the indirect approach, the control agent   would support these protocols and control the router itself using   proprietary protocols.  The direct approach is preferred, although   either approach is acceptable.  The use of specialized host hardware   and/or software requiring significant additional investment is   discouraged; nevertheless, some vendors may elect to provide the   control agent as an integrated part of the network in which the   routers are a part.  If this is the case, it is required that a means   be available to operate the control agent from a remote site using   Internet protocols and paths and with equivalent functionality with   respect to a local agent terminal.   It is desirable that a control agent and any other NOC software tools   that a vendor provides operate as user programs in a standard   operating system.  The use of the standard Internet protocols UDP and   TCP for communicating with the routers should facilitate this.   Remote router monitoring and (especially) remote router control   present important access control problems that must be addressed.   Care must also be taken to ensure control of the use of router   resources for these functions.  It is not desirable to let router   monitoring take more than some limited fraction of the router CPU   time, for example.  On the other hand, O&M functions must receive   priority so they can be exercised when the router is congested, since   often that is when O&M is most needed.Baker                       Standards Track                   [Page 126]

RFC 1812         Requirements for IP Version 4 Routers         June 199510.3.2 Out Of Band Access   Routers MUST support Out-Of-Band (OOB) access.  OOB access SHOULD   provide the same functionality as in-band access.  This access SHOULD   implement access controls, to prevent unauthorized access.   DISCUSSION      This Out-Of-Band access will allow the NOC a way to access      isolated routers during times when network access is not      available.      Out-Of-Band access is an important management tool for the network      administrator.  It allows the access of equipment independent of      the network connections.  There are many ways to achieve this      access.  Whichever one is used it is important that the access is      independent of the network connections.  An example of Out-Of-Band      access would be a serial port connected to a modem that provides      dial up access to the router.      It is important that the OOB access provides the same      functionality as in-band access.  In-band access, or accessing      equipment through the existing network connection, is limiting,      because most of the time, administrators need to reach equipment      to figure out why it is unreachable.  In band access is still very      important for configuring a router, and for troubleshooting more      subtle problems.10.3.2 Router O&M Functions10.3.2.1 Maintenance - Hardware Diagnosis   Each router SHOULD operate as a stand-alone device for the purposes   of local hardware maintenance.  Means SHOULD be available to run   diagnostic programs at the router site using only on-site tools.  A   router SHOULD be able to run diagnostics in case of a fault.  For   suggested hardware and software diagnostics see Section [10.3.3].10.3.2.2 Control - Dumping and Rebooting   A router MUST include both in-band and out-of-band mechanisms to   allow the network manager to reload, stop, and restart the router.  A   router SHOULD also contain a mechanism (such as a watchdog timer)   which will reboot the router automatically if it hangs due to a   software or hardware fault.   A router SHOULD IMPLEMENT a mechanism for dumping the contents of a   router's memory (and/or other state useful for vendor debugging after   a crash), and either saving them on a stable storage device local toBaker                       Standards Track                   [Page 127]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   the router or saving them on another host via an up-line dump   mechanism such as TFTP (see [OPER:2], [INTRO:3]).10.3.2.3 Control - Configuring the Router   Every router has configuration parameters that may need to be set.   It SHOULD be possible to update the parameters without rebooting the   router; at worst, a restart MAY be required.  There may be cases when   it is not possible to change parameters without rebooting the router   (for instance, changing the IP address of an interface).  In these   cases, care should be taken to minimize disruption to the router and   the surrounding network.   There SHOULD be a way to configure the router over the network either   manually or automatically.  A router SHOULD be able to upload or   download its parameters from a host or another router.  A means   SHOULD be provided, either as an application program or a router   function, to convert between the parameter format and a human-   editable format.  A router SHOULD have some sort of stable storage   for its configuration.  A router SHOULD NOT believe protocols such as   RARP, ICMP Address Mask Reply, and MAY not believe BOOTP.   DISCUSSION      It is necessary to note here that in the future RARP, ICMP Address      Mask Reply, BOOTP and other mechanisms may be needed to allow a      router to auto-configure.  Although routers may in the future be      able to configure automatically, the intent here is to discourage      this practice in a production environment until auto-configuration      has been tested more thoroughly.  The intent is NOT to discourage      auto-configuration all together.  In cases where a router is      expected to get its configuration automatically it may be wise to      allow the router to believe these things as it comes up and then      ignore them after it has gotten its configuration.10.3.2.4 Net Booting of System Software      A router SHOULD keep its system image in local non-volatile      storage such as PROM, NVRAM, or disk.  It MAY also be able to load      its system software over the network from a host or another      router.      A router that can keep its system image in local non-volatile      storage MAY be configurable to boot its system image over the      network.  A router that offers this option SHOULD be configurable      to boot the system image in its non-volatile local storage if it      is unable to boot its system image over the network.Baker                       Standards Track                   [Page 128]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      It is important that the router be able to come up and run on its      own.  NVRAM may be a particular solution for routers used in large      networks, since changing PROMs can be quite time consuming for a      network manager responsible for numerous or geographically      dispersed routers.  It is important to be able to netboot the      system image because there should be an easy way for a router to      get a bug fix or new feature more quickly than getting PROMs      installed.  Also if the router has NVRAM instead of PROMs, it will      netboot the image and then put it in NVRAM.      Routers SHOULD perform some basic consistency check on any image      loaded, to detect and perhaps prevent incorrect images.   A router MAY also be able to distinguish between different   configurations based on which software it is running.  If   configuration commands change from one software version to another,   it would be helpful if the router could use the configuration that   was compatible with the software.10.3.2.5 Detecting and responding to misconfiguration   There MUST be mechanisms for detecting and responding to   misconfigurations.  If a command is executed incorrectly, the router   SHOULD give an error message.  The router SHOULD NOT accept a poorly   formed command as if it were correct.   DISCUSSION      There are cases where it is not possible to detect errors: the      command is correctly formed, but incorrect with respect to the      network.  This may be detected by the router, but may not be      possible.   Another form of misconfiguration is misconfiguration of the network   to which the router is attached.  A router MAY detect   misconfigurations in the network.  The router MAY log these findings   to a file, either on the router or a host, so that the network   manager will see that there are possible problems on the network.   DISCUSSION      Examples of such misconfigurations might be another router with      the same address as the one in question or a router with the wrong      address mask.  If a router detects such problems it is probably      not the best idea for the router to try to fix the situation.      That could cause more harm than good.Baker                       Standards Track                   [Page 129]

RFC 1812         Requirements for IP Version 4 Routers         June 199510.3.2.6 Minimizing Disruption   Changing the configuration of a router SHOULD have minimal affect on   the network.  Routing tables SHOULD NOT be unnecessarily flushed when   a simple change is made to the router.  If a router is running   several routing protocols, stopping one routing protocol SHOULD NOT   disrupt other routing protocols, except in the case where one network   is learned by more than one routing protocol.   DISCUSSION      It is the goal of a network manager to run a network so that users      of the network get the best connectivity possible.  Reloading a      router for simple configuration changes can cause disruptions in      routing and ultimately cause disruptions to the network and its      users.  If routing tables are unnecessarily flushed, for instance,      the default route will be lost as well as specific routes to sites      within the network.  This sort of disruption will cause      significant downtime for the users.  It is the purpose of this      section to point out that whenever possible, these disruptions      should be avoided.10.3.2.7 Control - Troubleshooting Problems      (1) A router MUST provide in-band network access, but (except as           required by Section [8.2]) for security considerations this           access SHOULD be disabled by default.  Vendors MUST document           the default state of any in-band access.  This access SHOULD           implement access controls, to prevent unauthorized access.   DISCUSSION      In-band access primarily refers to access through the normal      network protocols that may or may not affect the permanent      operational state of the router.  This includes, but is not      limited to Telnet/RLOGIN console access and SNMP operations.      This was a point of contention between the operational out of the      box and secure out of The box contingents.  Any automagic access      to the router may introduce insecurities, but it may be more      important for the customer to have a router that is accessible      over the network as soon as it is plugged in.  At least one vendor      supplies routers without any external console access and depends      on being able to access the router through the network to complete      its configuration.      It is the vendors call whether in-band access is enabled by      default; but it is also the vendor's responsibility to make its      customers aware of possible insecurities.Baker                       Standards Track                   [Page 130]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      (2) A router MUST provide the ability to initiate an ICMP echo.           The following options SHOULD be implemented:           o Choice of data patterns           o Choice of packet size           o Record route           and the following additional options MAY be implemented:           o Loose source route           o Strict source route           o Timestamps      (3) A router SHOULD provide the ability to initiate a traceroute.           If traceroute is provided, then the 3rd party traceroute           SHOULD be implemented.   Each of the above three facilities (if implemented) SHOULD have   access restrictions placed on it to prevent its abuse by unauthorized   persons.10.4 Security Considerations10.4.1 Auditing and Audit Trails   Auditing and billing are the bane of the network operator, but are   the two features most requested by those in charge of network   security and those who are responsible for paying the bills.  In the   context of security, auditing is desirable if it helps you keep your   network working and protects your resources from abuse, without   costing you more than those resources are worth.   (1) Configuration Changes        Router SHOULD provide a method for auditing a configuration        change of a router, even if it's something as simple as        recording the operator's initials and time of change.   DISCUSSION      Configuration change logging (who made a configuration change,      what was changed, and when) is very useful, especially when      traffic is suddenly routed through Alaska on its way across town.      So is the ability to revert to a previous configuration.Baker                       Standards Track                   [Page 131]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      (2) Packet Accounting           Vendors should strongly consider providing a system for           tracking traffic levels between pairs of hosts or networks.           A mechanism for limiting the collection of this information           to specific pairs of hosts or networks is also strongly           encouraged.   DISCUSSION      A host traffic matrix as described above can give the network      operator a glimpse of traffic trends not apparent from other      statistics.  It can also identify hosts or networks that are      probing the structure of the attached networks - e.g., a single      external host that tries to send packets to every IP address in      the network address range for a connected network.      (3) Security Auditing           Routers MUST provide a method for auditing security related           failures or violations to include:           o Authorization Failures: bad passwords, invalid SNMP              communities, invalid authorization tokens,           o Violations of Policy Controls: Prohibited Source Routes,              Filtered Destinations, and           o Authorization Approvals: good passwords - Telnet in-band              access, console access.           Routers MUST provide a method of limiting or disabling such           auditing but auditing SHOULD be on by default.  Possible           methods for auditing include listing violations to a console           if present, logging or counting them internally, or logging           them to a remote security server through the SNMP trap           mechanism or the Unix logging mechanism as appropriate.  A           router MUST implement at least one of these reporting           mechanisms - it MAY implement more than one.10.4.2 Configuration Control   A vendor has a responsibility to use good configuration control   practices in the creation of the software/firmware loads for their   routers.  In particular, if a vendor makes updates and loads   available for retrieval over the Internet, the vendor should also   provide a way for the customer to confirm the load is a valid one,   perhaps by the verification of a checksum over the load.Baker                       Standards Track                   [Page 132]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      Many vendors currently provide short notice updates of their      software products through the Internet.  This a good trend and      should be encouraged, but provides a point of vulnerability in the      configuration control process.   If a vendor provides the ability for the customer to change the   configuration parameters of a router remotely, for example through a   Telnet session, the ability to do so SHOULD be configurable and   SHOULD default to off.  The router SHOULD require  valid   authentication before permitting remote reconfiguration.  This   authentication procedure SHOULD NOT transmit the authentication   secret over the network.  For example, if telnet is implemented, the   vendor SHOULD IMPLEMENT Kerberos, S-Key, or a similar authentication   procedure.   DISCUSSION      Allowing your properly identified network operator to twiddle with      your routers is necessary; allowing anyone else to do so is      foolhardy.   A router MUST NOT have undocumented back door access and master   passwords.  A vendor MUST ensure any such access added for purposes   of debugging or product development are deleted before the product is   distributed to its customers.   DISCUSSION      A vendor has a responsibility to its customers to ensure they are      aware of the vulnerabilities present in its code by intention -      e.g., in-band access.  Trap doors, back doors and master passwords      intentional or unintentional can turn a relatively secure router      into a major problem on an operational network.  The supposed      operational benefits are not matched by the potential problems.11. REFERENCES   Implementors should be aware that Internet protocol standards are   occasionally updated.  These references are current as of this   writing, but a cautious implementor will always check a recent   version of the RFC index to ensure that an RFC has not been updated   or superseded by another, more recent RFC.  Reference [INTRO:6]   explains various ways to obtain a current RFC index.   APPL:1.        Croft, B., and J.  Gilmore, "Bootstrap Protocol (BOOTP)",RFC951, Stanford University, Sun Microsystems, September 1985.Baker                       Standards Track                   [Page 133]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   APPL:2.        Alexander, S., and R.  Droms, "DHCP Options and BOOTP Vendor        Extensions",RFC 1533, Lachman Technology, Inc., Bucknell        University, October 1993.   APPL:3.        Wimer, W., "Clarifications and Extensions for the Bootstrap        Protocol",RFC 1542, Carnegie Mellon University, October 1993.   ARCH:1.        DDN Protocol Handbook, NIC-50004, NIC-50005, NIC-50006 (three        volumes), DDN Network Information Center, SRI International,        Menlo Park, California, USA, December 1985.   ARCH:2.        V.  Cerf and R.  Kahn, "A Protocol for Packet Network        Intercommunication", IEEE Transactions on Communication, May        1974.  Also included in [ARCH:1].   ARCH:3.        J.  Postel, C.  Sunshine, and D.  Cohen, "The ARPA Internet        Protocol", Computer Networks, volume 5, number 4, July 1981.        Also included in [ARCH:1].   ARCH:4.        B.  Leiner, J.  Postel, R.  Cole, and D.  Mills, :The DARPA        Internet Protocol Suite", Proceedings of INFOCOM '85, IEEE,        Washington, DC, March 1985.  Also in: IEEE Communications        Magazine, March 1985.  Also available from the Information        Sciences Institute, University of Southern California as        Technical Report ISI-RS-85-153.   ARCH:5.        D.  Comer, "Internetworking With TCP/IP Volume 1: Principles,        Protocols, and Architecture", Prentice Hall, Englewood Cliffs,        NJ, 1991.   ARCH:6.        W.  Stallings, "Handbook of Computer-Communications Standards        Volume 3: The TCP/IP Protocol Suite", Macmillan, New York, NY,        1990.   ARCH:7.        Postel, J., "Internet Official Protocol Standards", STD 1,RFC1780, Internet Architecture Board, March 1995.Baker                       Standards Track                   [Page 134]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   ARCH:8.        Information processing systems - Open Systems Interconnection -        Basic Reference Model, ISO 7489, International Standards        Organization, 1984.   ARCH:9        R.  Braden, J.  Postel, Y.  Rekhter, "Internet Architecture        Extensions for Shared Media", 05/20/1994   FORWARD:1.        IETF CIP Working Group (C. Topolcic, Editor), "Experimental        Internet Stream Protocol", Version 2 (ST-II),RFC 1190, October        1990.   FORWARD:2.        Mankin, A., and K.  Ramakrishnan, Editors, "Gateway Congestion        Control Survey",RFC 1254, MITRE, Digital Equipment Corporation,        August 1991.   FORWARD:3.        J.  Nagle, "On Packet Switches with Infinite Storage", IEEE        Transactions on Communications, volume COM-35, number 4, April        1987.   FORWARD:4.        R.  Jain, K.  Ramakrishnan, and D.  Chiu, "Congestion Avoidance        in Computer Networks With a Connectionless Network Layer",        Technical Report DEC-TR-506, Digital Equipment Corporation.   FORWARD:5.        V.  Jacobson, "Congestion Avoidance and Control", Proceedings of        SIGCOMM '88, Association for Computing Machinery, August 1988.   FORWARD:6.        W.  Barns, "Precedence and Priority Access Implementation for        Department of Defense Data Networks", Technical Report MTR-        91W00029, The Mitre Corporation, McLean, Virginia, USA, July        1991.   FORWARD:7        Fang, Chen, Hutchins, "Simulation Results of TCP Performance        over ATM with and without Flow Control", presentation to the ATM        Forum, November 15, 1993.   FORWARD:8        V.  Paxson, S.  Floyd "Wide Area Traffic: the Failure of Poisson        Modeling", short version in SIGCOMM '94.Baker                       Standards Track                   [Page 135]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   FORWARD:9        Leland, Taqqu, Willinger and Wilson, "On the Self-Similar Nature        of Ethernet Traffic", Proceedings of SIGCOMM '93, September,        1993.   FORWARD:10        S.  Keshav "A Control Theoretic Approach to Flow Control",        SIGCOMM 91, pages 3-16   FORWARD:11        K.K.  Ramakrishnan and R.  Jain, "A Binary Feedback Scheme for        Congestion Avoidance in Computer Networks", ACM Transactions of        Computer Systems, volume 8, number 2, 1980.   FORWARD:12        H.  Kanakia, P.  Mishara, and A.  Reibman].  "An adaptive        congestion control scheme for real-time packet video transport",        In Proceedings of ACM SIGCOMM 1994, pages 20-31, San Francisco,        California, September 1993.   FORWARD:13        A.  Demers, S.  Keshav, S.  Shenker, "Analysis and Simulation of        a Fair Queuing Algorithm",         93 pages 1-12   FORWARD:14        Clark, D., Shenker, S., and L.  Zhang, "Supporting Real-Time        Applications in an Integrated Services Packet Network:        Architecture and Mechanism", 92 pages 14-26   INTERNET:1.        Postel, J., "Internet Protocol", STD 5,RFC 791, USC/Information        Sciences Institute, September 1981.   INTERNET:2.        Mogul, J., and J.  Postel, "Internet Standard Subnetting        Procedure", STD 5,RFC 950, Stanford, USC/Information Sciences        Institute, August 1985.   INTERNET:3.        Mogul, J., "Broadcasting Internet Datagrams in the Presence of        Subnets", STD 5,RFC 922, Stanford University, October 1984.   INTERNET:4.        Deering, S., "Host Extensions for IP Multicasting", STD 5,RFC1112, Stanford University, August 1989.Baker                       Standards Track                   [Page 136]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   INTERNET:5.        Kent, S., "U.S.  Department of Defense Security Options for the        Internet Protocol",RFC 1108, BBN Communications, November 1991.   INTERNET:6.        Braden, R., Borman, D., and C.  Partridge, "Computing the        Internet Checksum",RFC 1071, USC/Information Sciences        Institute, Cray Research, BBN Communications, September 1988.   INTERNET:7.        Mallory T., and A.  Kullberg, "Incremental Updating of the        Internet Checksum",RFC 1141, BBN Communications, January 1990.   INTERNET:8.        Postel, J., "Internet Control Message Protocol", STD 5,RFC 792,        USC/Information Sciences Institute, September 1981.   INTERNET:9.        A.  Mankin, G.  Hollingsworth, G.  Reichlen, K.  Thompson, R.        Wilder, and R.  Zahavi, "Evaluation of Internet Performance -        FY89", Technical Report MTR-89W00216, MITRE Corporation,        February, 1990.   INTERNET:10.        G.  Finn, A "Connectionless Congestion Control Algorithm",        Computer Communications Review, volume 19, number 5, Association        for Computing Machinery, October 1989.   INTERNET:11.        Prue, W., and J. Postel, "The Source Quench Introduced Delay        (SQuID)",RFC 1016, USC/Information Sciences Institute, August        1987.   INTERNET:12.        McKenzie, A., "Some comments on SQuID",RFC 1018, BBN Labs,        August 1987.   INTERNET:13.        Deering, S., "ICMP Router Discovery Messages",RFC 1256, Xerox        PARC, September 1991.   INTERNET:14.        Mogul J., and S.  Deering, "Path MTU Discovery",RFC 1191,        DECWRL, Stanford University, November 1990.Baker                       Standards Track                   [Page 137]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   INTERNET:15        Fuller, V., Li, T., Yu, J., and K.  Varadhan, "Classless Inter-        Domain Routing (CIDR): an Address Assignment and Aggregation        Strategy"RFC 1519, BARRNet, cisco, Merit, OARnet, September        1993.   INTERNET:16        St.  Johns, M., "Draft Revised IP Security Option",RFC 1038,        IETF, January 1988.   INTERNET:17        Prue, W.,  and J.  Postel, "Queuing Algorithm to Provide Type-        of-service For IP Links",RFC 1046, USC/Information Sciences        Institute, February 1988.   INTERNET:18        Postel, J., "Address Mappings",RFC 796, USC/Information        Sciences Institute, September 1981.   INTRO:1.        Braden, R., and J.  Postel, "Requirements for Internet        Gateways", STD 4,RFC 1009, USC/Information Sciences Institute,        June 1987.   INTRO:2.        Internet Engineering Task Force (R. Braden, Editor),        "Requirements for Internet Hosts - Communication Layers", STD 3,RFC 1122, USC/Information Sciences Institute, October 1989.   INTRO:3.        Internet Engineering Task Force (R. Braden, Editor),        "Requirements for Internet Hosts - Application and Support", STD        3,RFC 1123, USC/Information Sciences Institute, October 1989.   INTRO:4.        Clark, D., "Modularity and Efficiency in Protocol        Implementations",RFC 817, MIT Laboratory for Computer Science,        July 1982.   INTRO:5.        Clark, D., "The Structuring of Systems Using Upcalls",        Proceedings of 10th ACM SOSP, December 1985.   INTRO:6.        Jacobsen, O.,  and J.  Postel, "Protocol Document Order        Information",RFC 980, SRI, USC/Information Sciences Institute,        March 1986.Baker                       Standards Track                   [Page 138]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   INTRO:7.        Reynolds, J.,  and J.  Postel, "Assigned Numbers", STD 2,RFC1700, USC/Information Sciences Institute, October 1994.  This        document is periodically updated and reissued with a new number.        It is wise to verify occasionally that the version you have is        still current.   INTRO:8.        DoD Trusted Computer System Evaluation Criteria, DoD publication        5200.28-STD, U.S.  Department of Defense, December 1985.   INTRO:9        Malkin, G., and T.  LaQuey Parker, Editors, "Internet Users'        Glossary", FYI 18,RFC 1392, Xylogics, Inc., UTexas, January        1993.   LINK:1.        Leffler, S., and M.  Karels, "Trailer Encapsulations",RFC 893,        University of California at Berkeley, April 1984.   LINK:2        Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,RFC1661, Daydreamer July 1994.   LINK:3        McGregor, G., "The PPP Internet Protocol Control Protocol        (IPCP)",RFC 1332, Merit May 1992.   LINK:4        Lloyd, B., and W.  Simpson, "PPP Authentication Protocols",RFC1334, L&A, Daydreamer, May 1992.   LINK:5        Simpson, W., "PPP Link Quality Monitoring",RFC 1333,        Daydreamer, May 1992.   MGT:1.        Rose, M., and K.  McCloghrie, "Structure and Identification of        Management Information of TCP/IP-based Internets", STD 16,RFC1155, Performance Systems International, Hughes LAN Systems, May        1990.   MGT:2.        McCloghrie, K., and M.  Rose (Editors), "Management Information        Base of TCP/IP-Based Internets: MIB-II", STD 16,RFC 1213,        Hughes LAN Systems, Inc., Performance Systems International,        March 1991.Baker                       Standards Track                   [Page 139]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   MGT:3.        Case, J., Fedor, M., Schoffstall, M., and J.  Davin, "Simple        Network Management Protocol", STD 15,RFC 1157, SNMP Research,        Performance Systems International, MIT Laboratory for Computer        Science, May 1990.   MGT:4.        Rose, M., and K.  McCloghrie (Editors), "Towards Concise MIB        Definitions", STD 16,RFC 1212, Performance Systems        International, Hughes LAN Systems, March 1991.   MGT:5.        Steinberg, L., "Techniques for Managing Asynchronously Generated        Alerts",RFC 1224, IBM Corporation, May 1991.   MGT:6.        Kastenholz, F., "Definitions of Managed Objects for the        Ethernet-like Interface Types",RFC 1398, FTP Software, Inc.,        January 1993.   MGT:7.        McCloghrie, K., and R. Fox "IEEE 802.4 Token Bus MIB",RFC 1230,        Hughes LAN Systems, Inc., Synoptics, Inc., May 1991.   MGT:8.        McCloghrie, K., Fox R., and E. Decker, "IEEE 802.5 Token Ring        MIB",RFC 1231, Hughes LAN Systems, Inc., Synoptics, Inc., cisco        Systems, Inc., February 1993.   MGT:9.        Case, J., and A.  Rijsinghani, "FDDI Management Information        Base",RFC 1512, The University of Tennesse and SNMP Research,        Digital Equipment Corporation, September 1993.   MGT:10.        Stewart, B., Editor "Definitions of Managed Objects for RS-232-        like Hardware Devices",RFC 1317, Xyplex, Inc., April 1992.   MGT:11.        Kastenholz, F., "Definitions of Managed Objects for the Link        Control Protocol of the Point-to-Point Protocol",RFC 1471, FTP        Software, Inc., June 1992.   MGT:12.        Kastenholz, F., "The Definitions of Managed Objects for the        Security Protocols of the Point-to-Point Protocol",RFC 1472,        FTP Software, Inc., June 1992.Baker                       Standards Track                   [Page 140]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   MGT:13.        Kastenholz, F., "The Definitions of Managed Objects for the IP        Network Control Protocol of the Point-to-Point Protocol",RFC1473, FTP Software, Inc., June 1992.   MGT:14.        Baker, F., and R.  Coltun, "OSPF Version 2 Management        Information Base",RFC 1253, ACC, Computer Science Center,        August 1991.   MGT:15.        Willis, S., and J.  Burruss, "Definitions of Managed Objects for        the Border Gateway Protocol (Version 3)",RFC 1269, Wellfleet        Communications Inc., October 1991.   MGT:16.        Baker, F., and J.  Watt, "Definitions of Managed Objects for the        DS1 and E1 Interface Types",RFC 1406, Advanced Computer        Communications, Newbridge Networks Corporation, January 1993.   MGT:17.        Cox, T., and K.  Tesink, Editors "Definitions of Managed Objects        for the DS3/E3 Interface Types",RFC 1407, Bell Communications        Research, January 1993.   MGT:18.        McCloghrie, K., "Extensions to the Generic-Interface MIB",RFC1229, Hughes LAN Systems, August 1992.   MGT:19.        Cox, T., and K.  Tesink, "Definitions of Managed Objects for the        SIP Interface Type",RFC 1304, Bell Communications Research,        February 1992.   MGT:20        Baker, F., "IP Forwarding Table MIB",RFC 1354, ACC, July 1992.   MGT:21.        Malkin, G., and F.  Baker, "RIP Version 2 MIB Extension",RFC1724, Xylogics, Inc., Cisco Systems, November 1994   MGT:22.        Throop, D., "SNMP MIB Extension for the X.25 Packet Layer",RFC1382, Data General Corporation, November 1992.Baker                       Standards Track                   [Page 141]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   MGT:23.        Throop, D., and F.  Baker, "SNMP MIB Extension for X.25 LAPB",RFC 1381, Data General Corporation, ACC, November 1992.   MGT:24.        Throop, D., and F.  Baker, "SNMP MIB Extension for MultiProtocol        Interconnect over X.25",RFC 1461, Data General Corporation, May        1993.   MGT:25.        Rose, M., "SNMP over OSI",RFC 1418, Dover Beach Consulting,        Inc., March 1993.   MGT:26.        Minshall, G., and M.  Ritter, "SNMP over AppleTalk",RFC 1419,        Novell, Inc., Apple Computer, Inc., March 1993.   MGT:27.        Bostock, S., "SNMP over IPX",RFC 1420, Novell, Inc., March        1993.   MGT:28.        Schoffstall, M., Davin, C., Fedor, M., and J.  Case, "SNMP over        Ethernet",RFC 1089, Rensselaer Polytechnic Institute, MIT        Laboratory for Computer Science, NYSERNet, Inc., University of        Tennessee at Knoxville, February 1989.   MGT:29.        Case, J., "FDDI Management Information Base",RFC 1285, SNMP        Research, Incorporated, January 1992.   OPER:1.        Nagle, J., "Congestion Control in IP/TCP Internetworks",RFC896, FACC, January 1984.   OPER:2.        Sollins, K., "TFTP Protocol (revision 2)",RFC 1350, MIT, July        1992.   ROUTE:1.        Moy, J., "OSPF Version 2",RFC 1583, Proteon, March 1994.   ROUTE:2.        Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual        Environments",RFC 1195, DEC, December 1990.Baker                       Standards Track                   [Page 142]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   ROUTE:3.        Hedrick, C., "Routing Information Protocol",RFC 1058, Rutgers        University, June 1988.   ROUTE:4.        Lougheed, K., and Y.  Rekhter, "A Border Gateway Protocol 3        (BGP-3)",RFC 1267, cisco, T.J. Watson Research Center, IBM        Corp., October 1991.   ROUTE:5.        Gross, P, and Y.  Rekhter, "Application of the Border Gateway        Protocol in the Internet",RFC 1772, T.J. Watson Research        Center, IBM Corp., MCI, March 1995.   ROUTE:6.        Mills, D., "Exterior Gateway Protocol Formal Specification",RFC904, UDEL, April 1984.   ROUTE:7.        Rosen, E., "Exterior Gateway Protocol (EGP)",RFC 827, BBN,        October 1982.   ROUTE:8.        Seamonson, L, and E.  Rosen, "STUB" "Exterior Gateway Protocol",RFC 888, BBN, January 1984.   ROUTE:9.        Waitzman, D., Partridge, C., and S.  Deering, "Distance Vector        Multicast Routing Protocol",RFC 1075, BBN, Stanford, November        1988.   ROUTE:10.        Deering, S., Multicast Routing in Internetworks and Extended        LANs, Proceedings of '88, Association for Computing Machinery,        August 1988.   ROUTE:11.        Almquist, P., "Type of Service in the Internet Protocol Suite",RFC 1349, Consultant, July 1992.   ROUTE:12.        Rekhter, Y., "Experience with the BGP Protocol",RFC 1266, T.J.        Watson Research Center, IBM Corp., October 1991.   ROUTE:13.        Rekhter, Y., "BGP Protocol Analysis",RFC 1265, T.J. Watson        Research Center, IBM Corp., October 1991.Baker                       Standards Track                   [Page 143]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   TRANS:1.        Postel, J., "User Datagram Protocol", STD 6,RFC 768,        USC/Information Sciences Institute, August 1980.   TRANS:2.        Postel, J., "Transmission Control Protocol", STD 7,RFC 793,        USC/Information Sciences Institute, September 1981.Baker                       Standards Track                   [Page 144]

RFC 1812         Requirements for IP Version 4 Routers         June 1995APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS   Subject to restrictions given below, a host MAY be able to act as an   intermediate hop in a source route, forwarding a source-routed   datagram to the next specified hop.   However, in performing this router-like function, the host MUST obey   all the relevant rules for a router forwarding source-routed   datagrams [INTRO:2].  This includes the following specific   provisions:   (A) TTL        The TTL field MUST be decremented and the datagram perhaps        discarded as specified for a router in [INTRO:2].   (B) ICMP Destination Unreachable        A host MUST be able to generate Destination Unreachable messages        with the following codes:        4 (Fragmentation Required but DF Set) when a source-routed          datagram cannot be fragmented to fit into the target network;        5 (Source Route Failed) when a source-routed datagram cannot be          forwarded, e.g., because of a routing problem or because the          next hop of a strict source route is not on a connected          network.   (C) IP Source Address        A source-routed datagram being forwarded MAY (and normally will)        have a source address that is not one of the IP addresses of the        forwarding host.   (D) Record Route Option        A host that is forwarding a source-routed datagram containing a        Record Route option MUST update that option, if it has room.   (E) Timestamp Option        A host that is forwarding a source-routed datagram containing a        Timestamp Option MUST add the current timestamp to that option,        according to the rules for this option.   To define the rules restricting host forwarding of source-routed   datagrams, we use the term local source-routing if the next hop will   be through the same physical interface through which the datagram   arrived; otherwise, it is non-local source-routing.   A host is permitted to perform local source-routing without   restriction.Baker                       Standards Track                   [Page 145]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   A host that supports non-local source-routing MUST have a   configurable switch to disable forwarding, and this switch MUST   default to disabled.   The host MUST satisfy all router requirements for configurable policy   filters [INTRO:2] restricting non-local forwarding.   If a host receives a datagram with an incomplete source route but   does not forward it for some reason, the host SHOULD return an ICMP   Destination Unreachable (code 5, Source Route Failed) message, unless   the datagram was itself an ICMP error message.APPENDIX B. GLOSSARY   This Appendix defines specific terms used in this memo.  It also   defines some general purpose terms that may be of interest.  See also   [INTRO:9] for a more general set of definitions.   Autonomous System (AS)        An Autonomous System (AS) is a connected segment of a network        topology that consists of a collection of subnetworks (with        hosts attached) interconnected by a set of routes.  The        subnetworks and the routers are expected to be under the control        of a single operations and maintenance (O&M) organization.        Within an AS routers may use one or more interior routing        protocols, and sometimes several sets of metrics.  An AS is        expected to present to other ASs an appearence of a coherent        interior routing plan, and a consistent picture of the        destinations reachable through the AS.  An AS is identified by        an Autonomous System number.   Connected Network        A network prefix to which a router is interfaced is often known        as a local network or the subnetwork of that router.  However,        these terms can cause confusion, and therefore we use the term        Connected Network in this memo.   Connected (Sub)Network        A Connected (Sub)Network is an IP subnetwork to which a router        is interfaced, or a connected network if the connected network        is not subnetted.  See also Connected Network.   Datagram        The unit transmitted between a pair of internet modules.  Data,        called datagrams, from sources to destinations.  The Internet        Protocol does not provide a reliable communication facility.        There are no acknowledgments either end-to-end or hop-by-hop.        There is no error no retransmissions.  There is no flow control.        See IP.Baker                       Standards Track                   [Page 146]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Default Route        A routing table entry that is used to direct any data addressed        to any network prefixes not explicitly listed in the routing        table.   Dense Mode        In multicast forwarding, two paradigms are possible: in Dense        Mode forwarding, a network multicast is forwarded as a data link        layer multicast to all interfaces except that on which it was        received, unless and until the router is instructed not to by a        multicast routing neighbor.  See Sparse Mode.   EGP        Exterior Gateway Protocol A protocol that distributes routing        information to the gateways (routers) which connect autonomous        systems.  See IGP.   EGP-2        Exterior Gateway Protocol version 2 This is an EGP routing        protocol developed to handle traffic between Autonomous Systems        in the Internet.   Forwarder        The logical entity within a router that is responsible for        switching packets among the router's interfaces.  The Forwarder        also makes the decisions to queue a packet for local delivery,        to queue a packet for transmission out another interface, or        both.   Forwarding        Forwarding is the process a router goes through for each packet        received by the router.  The packet may be consumed by the        router, it may be output on one or more interfaces of the        router, or both.  Forwarding includes the process of deciding        what to do with the packet as well as queuing it up for        (possible) output or internal consumption.   Forwarding Information Base (FIB)        The table containing the information necessary to forward IP        Datagrams, in this document, is called the Forwarding        Information Base.  At minimum, this contains the interface        identifier and next hop information for each reachable        destination network prefix.   Fragment        An IP datagram that represents a portion of a higher layer's        packet that was too large to be sent in its entirety over the        output network.Baker                       Standards Track                   [Page 147]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   General Purpose Serial Interface        A physical medium capable of connecting exactly two systems, and        therefore configurable as a point to point line, but also        configurable to support link layer networking using protocols        such as X.25 or Frame Relay.  A link layer network connects        another system to a switch, and a higher communication layer        multiplexes virtual circuits on the connection.  See Point to        Point Line.   IGP        Interior Gateway Protocol A protocol that distributes routing        information with an Autonomous System (AS).  See EGP.   Interface IP Address        The IP Address and network prefix length that is assigned to a        specific interface of a router.   Internet Address        An assigned number that identifies a host in an internet.  It        has two parts: an IP address and a prefix length.  The prefix        length indicates how many of the most specific bits of the        address constitute the network prefix.   IP        Internet Protocol The network layer protocol for the Internet.        It is a packet switching, datagram protocol defined inRFC 791.        IP does not provide a reliable communications facility; that is,        there are no end-to-end of hop-by-hop acknowledgments.   IP Datagram        An IP Datagram is the unit of end-to-end transmission in the        Internet Protocol.  An IP Datagram consists of an IP header        followed by all of higher-layer data (such as TCP, UDP, ICMP,        and the like).  An IP Datagram is an IP header followed by a        message.        An IP Datagram is a complete IP end-to-end transmission unit.        An IP Datagram is composed of one or more IP Fragments.        In this memo, the unqualified term Datagram should be understood        to refer to an IP Datagram.   IP Fragment        An IP Fragment is a component of an IP Datagram.  An IP Fragment        consists of an IP header followed by all or part of the higher-        layer of the original IP Datagram.        One or more IP Fragments comprises a single IP Datagram.Baker                       Standards Track                   [Page 148]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        In this memo, the unqualified term Fragment should be understood        to refer to an IP Fragment.   IP Packet        An IP Datagram or an IP Fragment.        In this memo, the unqualified term Packet should generally be        understood to refer to an IP Packet.   Logical [network] interface        We define a logical [network] interface to be a logical path,        distinguished by a unique IP address, to a connected network.   Martian Filtering        A packet that contains an invalid source or destination address        is considered to be martian and discarded.   MTU (Maximum Transmission Unit)        The size of the largest packet that can be transmitted or        received through a logical interface.  This size includes the IP        header but does not include the size of any Link Layer headers        or framing.   Multicast        A packet that is destined for multiple hosts.  See broadcast.   Multicast Address        A special type of address that is recognizable by multiple        hosts.        A Multicast Address is sometimes known as a Functional Address        or a Group Address.   Network Prefix        The portion of an IP Address that signifies a set of systems.        It is selected from the IP Address by logically ANDing a subnet        mask with the address, or (equivalently) setting the bits of the        address not among the most significant <prefix-length> bits of        the address to zero.   Originate        Packets can be transmitted by a router for one of two reasons:        1) the packet was received and is being forwarded or 2) the        router itself created the packet for transmission (such as route        advertisements).  Packets that the router creates for        transmission are said to originate at the router.Baker                       Standards Track                   [Page 149]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Packet        A packet is the unit of data passed across the interface between        the Internet Layer and the Link Layer.  It includes an IP header        and data.  A packet may be a complete IP datagram or a fragment        of an IP datagram.   Path        The sequence of routers and (sub-)networks that a packet        traverses from a particular router to a particular destination        host.  Note that a path is uni-directional; it is not unusual to        have different paths in the two directions between a given host        pair.   Physical Network        A Physical Network is a network (or a piece of an internet)        which is contiguous at the Link Layer.  Its internal structure        (if any) is transparent to the Internet Layer.        In this memo, several media components that are connected using        devices such as bridges or repeaters are considered to be a        single Physical Network since such devices are transparent to        the IP.   Physical Network Interface        This is a physical interface to a Connected Network and has a        (possibly unique) Link-Layer address.  Multiple Physical Network        Interfaces on a single router may share the same Link-Layer        address, but the address must be unique for different routers on        the same Physical Network.   Point to Point Line        A physical medium capable of connecting exactly two systems.  In        this document, it is only used to refer to such a line when used        to connect IP entities.  See General Purpose Serial Interface.   router        A special-purpose dedicated computer that connects several        networks.  Routers switch packets between these networks in a        process known as forwarding.  This process may be repeated        several times on a single packet by multiple routers until the        packet can be delivered to the final destination - switching the        packet from router to router to router...  until the packet gets        to its destination.   RPF        Reverse Path Forwarding - A method used to deduce the next hops        for broadcast and multicast packets.Baker                       Standards Track                   [Page 150]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Silently Discard        This memo specifies several cases where a router is to Silently        Discard a received packet (or datagram).  This means that the        router should discard the packet without further processing, and        that the router will not send any ICMP error message (see        Section [4.3.2]) as a result.  However, for diagnosis of        problems, the router should provide the capability of logging        the error (see Section [1.3.3]), including the contents of the        silently discarded packet, and should record the event in a        statistics counter.   Silently Ignore        A router is said to Silently Ignore an error or condition if it        takes no action other than possibly generating an error report        in an error log or through some network management protocol, and        discarding, or ignoring, the source of the error.  In        particular, the router does NOT generate an ICMP error message.   Sparse Mode        In multicast forwarding, two paradigms are possible: in Sparse        Mode forwarding, a network layer multicast datagram is forwarded        as a data link layer multicast frame to routers and hosts that        have asked for it.  The initial forwarding state is the inverse        of dense-mode in that it assumes no part  of the network wants        the data.  See Dense Mode.   Specific-destination address        This is defined to be the destination address in the IP header        unless the header contains an IP broadcast or IP multicast        address, in which case the specific-destination is an IP address        assigned to the physical interface on which the packet arrived.   subnet        A portion of a network, which may be a physically independent        network, which shares a network address with other portions of        the network and is distinguished by a subnet number.  A subnet        is to a network what a network is to an internet.   subnet number        A part of the internet address that designates a subnet.  It is        ignored for the purposes internet routing, but is used for        intranet routing.   TOS        Type Of Service A field in the IP header that represents the        degree of reliability expected from the network layer by the        transport layer or application.Baker                       Standards Track                   [Page 151]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   TTL        Time To Live A field in the IP header that represents how long a        packet is considered valid.  It is a combination hop count and        timer value.APPENDIX C. FUTURE DIRECTIONS   This appendix lists work that future revisions of this document may   wish to address.   In the preparation of Router Requirements, we stumbled across several   other architectural issues.  Each of these is dealt with somewhat in   the document, but still ought to be classified as an open issue in   the IP architecture.   Most of the he topics presented here generally indicate areas where   the technology is still relatively new and it is not appropriate to   develop specific requirements since the community is still gaining   operational experience.   Other topics represent areas of ongoing research and indicate areas   that the prudent developer would closely monitor.   (1) SNMP Version 2   (2) Additional SNMP MIBs   (7) More detailed requirements for leaking routes between routing        protocols   (8) Router system security   (9) Routing protocol security   (10) Internetwork Protocol layer security.  There has been extensive        work refining the security of IP since the original work writing        this document.  This security work should be included in here.   (12) Load Splitting   (13) Sending fragments along different paths   (15) Multiple logical (sub)nets on the same wire.  Router        Requirements does not require support for this.  We made some        attempt to identify pieces of the architecture (e.g., forwarding        of directed broadcasts and issuing of Redirects) where the        wording of the rules has to be done carefully to make the rightBaker                       Standards Track                   [Page 152]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        thing happen, and tried to clearly distinguish logical        interfaces from physical interfaces.  However, we did not study        this issue in detail, and we are not at all confident that all        the rules in the document are correct in the presence of        multiple logical (sub)nets on the same wire.   (15) Congestion control and resource management.  On the advice of        the IETF's experts (Mankin and Ramakrishnan) we deprecated        (SHOULD NOT) Source Quench and said little else concrete        (Section 5.3.6).   (16) Developing a Link-Layer requirements document that would be        common for both routers and hosts.   (17) Developing a common PPP LQM algorithm.   (18) Investigate of other information (above and beyond section        [3.2]) that passes between the layers, such as physical network        MTU, mappings of IP precedence to Link Layer priority values,        etc.   (19) Should the Link Layer notify IP if address resolution failed        (just like it notifies IP when there is a Link Layer priority        value problem)?   (20) Should all routers be required to implement a DNS resolver?   (21) Should a human user be able to use a host name anywhere you can        use an IP address when configuring the router?  Even in ping and        traceroute?   (22) Almquist's draft ruminations on the next hop and ruminations on        route leaking need to be reviewed, brought up to date, and        published.   (23) Investigation is needed to determine if a redirect message for        precedence is needed or not.  If not, are the type-of-service        redirects acceptable?   (24) RIPv2 and RIP+CIDR and variable length network prefixes.   (25) BGP-4 CIDR is going to be important, and everyone is betting on        BGP-4.  We can't avoid mentioning it.  Probably need to describe        the differences between BGP-3 and BGP-4, and explore upgrade        issues...   (26) Loose Source Route Mobile IP and some multicasting may require        this.  Perhaps it should be elevated to a SHOULD (per FredBaker                       Standards Track                   [Page 153]

RFC 1812         Requirements for IP Version 4 Routers         June 1995        Baker's Suggestion).APPENDIX D. Multicast Routing Protocols   Multicasting is a relatively new technology within the Internet   Protocol family.  It is not widely deployed or commonly in use yet.   Its importance, however, is expected to grow over the coming years.   This Appendix describes some of the technologies being investigated   for routing multicasts through the Internet.   A diligent implementor will keep abreast of developments in this area   to properly develop multicast facilities.   This Appendix does not specify any standards or requirements.D.1 Introduction   Multicast routing protocols enable the forwarding of IP multicast   datagrams throughout a TCP/IP internet.  Generally these algorithms   forward the datagram based on its source and destination addresses.   Additionally, the datagram may need to be forwarded to several   multicast group members, at times requiring the datagram to be   replicated and sent out multiple interfaces.   The state of multicast routing protocols is less developed than the   protocols available for the forwarding of IP unicasts.  Three   experimental multicast routing protocols have been documented for   TCP/IP.  Each uses the IGMP protocol (discussed in Section [4.4]) to   monitor multicast group membership.D.2 Distance Vector Multicast Routing Protocol - DVMRP   DVMRP, documented in [ROUTE:9], is based on Distance Vector or   Bellman-Ford technology.  It routes multicast datagrams only, and   does so within a single Autonomous System.  DVMRP is an   implementation of the Truncated Reverse Path Broadcasting algorithm   described in [ROUTE:10].  In addition, it specifies the tunneling of   IP multicasts through non-multicast-routing-capable IP domains.D.3 Multicast Extensions to OSPF - MOSPF   MOSPF, currently under development, is a backward-compatible addition   to OSPF that allows the forwarding of both IP multicasts and unicasts   within an Autonomous System.  MOSPF routers can be mixed with OSPF   routers within a routing domain, and they will interoperate in the   forwarding of unicasts.  OSPF is a link-state or SPF-based protocol.Baker                       Standards Track                   [Page 154]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   By adding link state advertisements that pinpoint group membership,   MOSPF routers can calculate the path of a multicast datagram as a   tree rooted at the datagram source.  Those branches that do not   contain group members can then be discarded, eliminating unnecessary   datagram forwarding hops.D.4 Protocol Independent Multicast - PIM   PIM, currently under development, is a multicast routing protocol   that runs over an existing unicast infrastructure.  PIM provides for   both dense and sparse group membership.  It is different from other   protocols, since it uses an explicit join model for sparse groups.   Joining occurs on a shared tree and can switch to a per-source tree.   Where bandwidth is plentiful and group membership is dense, overhead   can be reduced by flooding data out all links and later pruning   exception cases where there are no group members.APPENDIX E Additional Next-Hop Selection Algorithms   Section [5.2.4.3] specifies an algorithm that routers ought to use   when selecting a next-hop for a packet.   This appendix provides historical perspective for the next-hop   selection problem.  It also presents several additional pruning rules   and next-hop selection algorithms that might be found in the   Internet.   This appendix presents material drawn from an earlier, unpublished,   work by Philip Almquist; Ruminations on the Next Hop.   This Appendix does not specify any standards or requirements.E.1. Some Historical Perspective   It is useful to briefly review the history of the topic, beginning   with what is sometimes called the "classic model" of how a router   makes routing decisions.  This model predates IP.  In this model, a   router speaks some single routing protocol such as RIP.  The protocol   completely determines the contents of the router's Forwarding   Information Base (FIB).  The route lookup algorithm is trivial: the   router looks in the FIB for a route whose destination attribute   exactly matches the network prefix portion of the destination address   in the packet.  If one is found, it is used; if none is found, the   destination is unreachable.  Because the routing protocol keeps at   most one route to each destination, the problem of what to do when   there are multiple routes that match the same destination cannot   arise.Baker                       Standards Track                   [Page 155]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Over the years, this classic model has been augmented in small ways.   With the deployment of default routes, subnets, and host routes, it   became possible to have more than one routing table entry which in   some sense matched the destination.  This was easily resolved by a   consensus that there was a hierarchy of routes: host routes should be   preferred over subnet routes, subnet routes over net routes, and net   routes over default routes.   With the deployment of technologies supporting variable length subnet   masks (variable length network prefixes), the general approach   remained the same although its description became a little more   complicated; network prefixes were introduced as a conscious   simplification and regularization of the architecture.  We now say   that each route to a network prefix route has a prefix length   associated with it.  This prefix length indicates the number of bits   in the prefix.  This may also be represented using the classical   subnet mask.  A route cannot be used to route a packet unless each   significant bit in the route's network prefix matches the   corresponding bit in the packet's destination address.  Routes with   more bits set in their masks are preferred over routes that have   fewer bits set in their masks.  This is simply a generalization of   the hierarchy of routes described above, and will be referred to for   the rest of this memo as choosing a route by preferring longest   match.   Another way the classic model has been augmented is through a small   amount of relaxation of the notion that a routing protocol has   complete control over the contents of the routing table.  First,   static routes were introduced.  For the first time, it was possible   to simultaneously have two routes (one dynamic and one static) to the   same destination.  When this happened, a router had to have a policy   (in some cases configurable, and in other cases chosen by the author   of the router's software) which determined whether the static route   or the dynamic route was preferred.  However, this policy was only   used as a tie-breaker when longest match didn't uniquely determine   which route to use.  Thus, for example, a static default route would   never be preferred over a dynamic net route even if the policy   preferred static routes over dynamic routes.   The classic model had to be further augmented when inter-domain   routing protocols were invented.  Traditional routing protocols came   to be called "interior gateway protocols" (IGPs), and at each   Internet site there was a strange new beast called an "exterior   gateway", a router that spoke EGP to several "BBN Core Gateways" (the   routers that made up the Internet backbone at the time) at the same   time as it spoke its IGP to the other routers at its site.  Both   protocols wanted to determine the contents of the router's routing   table.  Theoretically, this could result in a router having threeBaker                       Standards Track                   [Page 156]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   routes (EGP, IGP, and static) to the same destination.  Because of   the Internet topology at the time, it was resolved with little debate   that routers would be best served by a policy of preferring IGP   routes over EGP routes.  However, the sanctity of longest match   remained unquestioned: a default route learned from the IGP would   never be preferred over a net route from learned EGP.   Although the Internet topology, and consequently routing in the   Internet, have evolved considerably since then, this slightly   augmented version of the classic model has survived intact to this   day in the Internet (except that BGP has replaced EGP).  Conceptually   (and often in implementation) each router has a routing table and one   or more routing protocol processes.  Each of these processes can add   any entry that it pleases, and can delete or modify any entry that it   has created.  When routing a packet, the router picks the best route   using longest match, augmented with a policy mechanism to break ties.   Although this augmented classic model has served us well, it has a   number of shortcomings:   o It ignores (although it could be augmented to consider) path      characteristics such as quality of service and MTU.   o It doesn't support routing protocols (such as OSPF and Integrated      IS-IS) that require route lookup algorithms different than pure      longest match.   o There has not been a firm consensus on what the tie-breaking      mechanism ought to be.  Tie-breaking mechanisms have often been      found to be difficult if not impossible to configure in such a way      that the router will always pick what the network manger considers      to be the "correct" route.E.2. Additional Pruning Rules      Section [5.2.4.3] defined several pruning rules to use to select      routes from the FIB.  There are other rules that could also be      used.      o OSPF Route Class         Routing protocols that have areas or make a distinction between         internal and external routes divide their routes into classes         by the type of information used to calculate the route.  A         route is always chosen from the most preferred class unless         none is available, in which case one is chosen from the second         most preferred class, and so on.  In OSPF, the classes (in         order from most preferred to least preferred) are intra-area,         inter-area, type 1 external (external routes with internal         metrics), and type 2 external.  As an additional wrinkle, aBaker                       Standards Track                   [Page 157]

RFC 1812         Requirements for IP Version 4 Routers         June 1995         router is configured to know what addresses ought to be         accessible using intra-area routes, and will not use inter-         area or external routes to reach these destinations even when         no intra-area route is available.         More precisely, we assume that each route has a class         attribute, called route.class, which is assigned by the routing         protocol.  The set of candidate routes is examined to determine         if it contains any for which route.class = intra-area.  If so,         all routes except those for which route.class = intra-area are         discarded.  Otherwise, router checks whether the packet's         destination falls within the address ranges configured for the         local area.  If so, the entire set of candidate routes is         deleted.  Otherwise, the set of candidate routes is examined to         determine if it contains any for which route.class = inter-         area.  If so, all routes except those for which route.class =         inter-area are discarded.  Otherwise, the set of candidate         routes is examined to determine if it contains any for which         route.class = type 1 external.  If so, all routes except those         for which route.class = type 1 external are discarded.      o IS-IS Route Class         IS-IS route classes work identically to OSPF's.  However, the         set of classes defined by Integrated IS-IS is different, such         that there isn't a one-to-one mapping between IS-IS route         classes and OSPF route classes.  The route classes used by         Integrated IS-IS are (in order from most preferred to least         preferred) intra-area, inter-area, and external.         The Integrated IS-IS internal class is equivalent to the OSPF         internal class.  Likewise, the Integrated IS-IS external class         is equivalent to OSPF's type 2 external class.  However,         Integrated IS-IS does not make a distinction between inter-area         routes and external routes with internal metrics - both are         considered to be inter-area routes.  Thus, OSPF prefers true         inter-area routes over external routes with internal metrics,         whereas Integrated IS-IS gives the two types of routes equal         preference.      o IDPR Policy         A specific case of Policy.  The IETF's Inter-domain Policy         Routing Working Group is devising a routing protocol called         Inter-Domain Policy Routing (IDPR) to support true policy-based         routing in the Internet.  Packets with certain combinations of         header attributes (such as specific combinations of source and         destination addresses or special IDPR source route options) are         required to use routes provided by the IDPR protocol.  Thus,         unlike other Policy pruning rules, IDPR Policy would have to beBaker                       Standards Track                   [Page 158]

RFC 1812         Requirements for IP Version 4 Routers         June 1995         applied before any other pruning rules except Basic Match.         Specifically, IDPR Policy examines the packet being forwarded         to ascertain if its attributes require that it be forwarded         using policy-based routes.  If so, IDPR Policy deletes all         routes not provided by the IDPR protocol.E.3 Some Route Lookup Algorithms      This section examines several route lookup algorithms that are in      use or have been proposed.  Each is described by giving the      sequence of pruning rules it uses.  The strengths and weaknesses      of each algorithm are presentedE.3.1 The Revised Classic Algorithm      The Revised Classic Algorithm is the form of the traditional      algorithm that was discussed in Section [E.1].  The steps of this      algorithm are:      1.  Basic match      2.  Longest match      3.  Best metric      4.  Policy      Some implementations omit the Policy step, since it is needed only      when routes may have metrics that are not comparable (because they      were learned from different routing domains).      The advantages of this algorithm are:      (1) It is widely implemented.      (2) Except for the Policy step (which an implementor can choose to           make arbitrarily complex) the algorithm is simple both to           understand and to implement.      Its disadvantages are:      (1) It does not handle IS-IS or OSPF route classes, and therefore           cannot be used for Integrated IS-IS or OSPF.      (2) It does not handle TOS or other path attributes.      (3) The policy mechanisms are not standardized in any way, and are           therefore are often implementation-specific.  This causes           extra work for implementors (who must invent appropriate           policy mechanisms) and for users (who must learn how to useBaker                       Standards Track                   [Page 159]

RFC 1812         Requirements for IP Version 4 Routers         June 1995           the mechanisms. This lack of a standardized mechanism also           makes it difficult to build consistent configurations for           routers from different vendors. This presents a significant           practical deterrent to multi-vendor interoperability.      (4) The proprietary policy mechanisms currently provided by           vendors are often inadequate in complex parts of the           Internet.      (5) The algorithm has not been written down in any generally           available document or standard.  It is, in effect, a part of           the Internet Folklore.E.3.2 The Variant Router Requirements Algorithm      Some Router Requirements Working Group members have proposed a      slight variant of the algorithm described in the Section      [5.2.4.3].  In this variant, matching the type of service      requested is considered to be more important, rather than less      important, than matching as much of the destination address as      possible.  For example, this algorithm would prefer a default      route that had the correct type of service over a network route      that had the default type of service, whereas the algorithm in      [5.2.4.3] would make the opposite choice.      The steps of the algorithm are:      1.  Basic match      2.  Weak TOS      3.  Longest match      4.  Best metric      5.  Policy      Debate between the proponents of this algorithm and the regular      Router Requirements Algorithm suggests that each side can show      cases where its algorithm leads to simpler, more intuitive routing      than the other's algorithm does.  This variant has the same set of      advantages and disadvantages that the algorithm specified in      [5.2.4.3] does, except that pruning on Weak TOS before pruning on      Longest Match makes this algorithm less compatible with OSPF and      Integrated IS-IS than the standard Router Requirements Algorithm.E.3.3 The OSPF Algorithm      OSPF uses an algorithm that is virtually identical to the Router      Requirements Algorithm except for one crucial difference: OSPF      considers OSPF route classes.Baker                       Standards Track                   [Page 160]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      The algorithm is:      1.  Basic match      2.  OSPF route class      3.  Longest match      4.  Weak TOS      5.  Best metric      6.  Policy      Type of service support is not always present.  If it is not      present then, of course, the fourth step would be omitted      This algorithm has some advantages over the Revised Classic      Algorithm:      (1) It supports type of service routing.      (2) Its rules are written down, rather than merely being a part of           the Internet folklore.      (3) It (obviously) works with OSPF.      However, this algorithm also retains some of the disadvantages of      the Revised Classic Algorithm:      (1) Path properties other than type of service (e.g., MTU) are           ignored.      (2) As in the Revised Classic Algorithm, the details (or even the           existence) of the Policy step are left to the discretion of           the implementor.      The OSPF Algorithm also has a further disadvantage (which is not      shared by the Revised Classic Algorithm).  OSPF internal (intra-      area or inter-area) routes are always considered to be superior to      routes learned from other routing protocols, even in cases where      the OSPF route matches fewer bits of the destination address.      This is a policy decision that is inappropriate in some networks.      Finally, it is worth noting that the OSPF Algorithm's TOS support      suffers from a deficiency in that routing protocols that support      TOS are implicitly preferred when forwarding packets that have      non-zero TOS values.  This may not be appropriate in some cases.Baker                       Standards Track                   [Page 161]

RFC 1812         Requirements for IP Version 4 Routers         June 1995E.3.4 The Integrated IS-IS Algorithm   Integrated IS-IS uses an algorithm that is similar to but not quite   identical to the OSPF Algorithm.  Integrated IS-IS uses a different   set of route classes, and differs slightly in its handling of type of   service.  The algorithm is:   1.  Basic Match   2.  IS-IS Route Classes   3.  Longest Match   4.  Weak TOS   5.  Best Metric   6.  Policy   Although Integrated IS-IS uses Weak TOS, the protocol is only capable   of carrying routes for a small specific subset of the possible values   for the TOS field in the IP header.  Packets containing other values   in the TOS field are routed using the default TOS.   Type of service support is optional; if disabled, the fourth step   would be omitted.  As in OSPF, the specification does not include the   Policy step.   This algorithm has some advantages over the Revised Classic   Algorithm:   (1) It supports type of service routing.   (2) Its rules are written down, rather than merely being a part of        the Internet folklore.   (3) It (obviously) works with Integrated IS-IS.   However, this algorithm also retains some of the disadvantages of the   Revised Classic Algorithm:   (1) Path properties other than type of service (e.g., MTU) are        ignored.   (2) As in the Revised Classic Algorithm, the details (or even the        existence) of the Policy step are left to the discretion of the        implementor.   (3) It doesn't work with OSPF because of the differences between IS-        IS route classes and OSPF route classes.  Also, because IS-IS        supports only a subset of the possible TOS values, some obvious        implementations of the Integrated IS-IS algorithm would not        support OSPF's interpretation of TOS.   The Integrated IS-IS Algorithm also has a further disadvantage (which   is not shared by the Revised Classic Algorithm): IS-IS internal   (intra-area or inter-area) routes are always considered to beBaker                       Standards Track                   [Page 162]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   superior to routes learned from other routing protocols, even in   cases where the IS-IS route matches fewer bits of the destination   address and doesn't provide the requested type of service.  This is a   policy decision that may not be appropriate in all cases.   Finally, it is worth noting that the Integrated IS-IS Algorithm's TOS   support suffers from the same deficiency noted for the OSPF   Algorithm.Security Considerations   Although the focus of this document is interoperability rather than   security, there are obviously many sections of this document that   have some ramifications on network security.   Security means different things to different people.  Security from a   router's point of view is anything that helps to keep its own   networks operational and in addition helps to keep the Internet as a   whole healthy.  For the purposes of this document, the security   services we are concerned with are denial of service, integrity, and   authentication as it applies to the first two.  Privacy as a security   service is important, but only peripherally a concern of a router -   at least as of the date of this document.   In several places in this document there are sections entitled ...   Security Considerations.  These sections discuss specific   considerations that apply to the general topic under discussion.   Rarely does this document say do this and your router/network will be   secure.  More likely, it says this is a good idea and if you do it,   it *may* improve the security of the Internet and your local system   in general.   Unfortunately, this is the state-of-the-art AT THIS TIME.  Few if any   of the network protocols a router is concerned with have reasonable,   built-in security features.  Industry and the protocol designers have   been and are continuing to struggle with these issues.  There is   progress, but only small baby steps such as the peer-to-peer   authentication available in the BGP and OSPF routing protocols.   In particular, this document notes the current research into   developing and enhancing network security.  Specific areas of   research, development, and engineering that are underway as of this   writing (December 1993) are in IP Security, SNMP Security, and common   authentication technologies.   Notwithstanding all the above, there are things both vendors and   users can do to improve the security of their router.  Vendors shouldBaker                       Standards Track                   [Page 163]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   get a copy of Trusted Computer System Interpretation [INTRO:8].  Even   if a vendor decides not to submit their device for formal   verification under these guidelines, the publication provides   excellent guidance on general security design and practices for   computing devices.APPENDIX F: HISTORICAL ROUTING PROTOCOLS   Certain routing protocols are common in the Internet, but the authors   of this document cannot in good conscience recommend their use.  This   is not because they do not work correctly, but because the   characteristics of the Internet assumed in their design (simple   routing, no policy, a single "core router" network under common   administration, limited complexity, or limited network diameter) are   not attributes of today's Internet.  Those parts of the Internet that   still use them are generally limited "fringe" domains with limited   complexity.   As a matter of good faith, collected wisdom concerning their   implementation is recorded in this section.F.1 EXTERIOR GATEWAY PROTOCOL - EGPF.1.1 Introduction   The Exterior Gateway Protocol (EGP) specifies an EGP that is used to   exchange reachability information between routers of the same or   differing autonomous systems.  EGP is not considered a routing   protocol since there is no standard interpretation (i.e. metric) for   the distance fields in the EGP update message, so distances are   comparable only among routers of the same AS.  It is however designed   to provide high-quality reachability information, both about neighbor   routers and about routes to non-neighbor routers.   EGP is defined by [ROUTE:6].  An implementor almost certainly wants   to read [ROUTE:7] and [ROUTE:8] as well, for they contain useful   explanations and background material.   DISCUSSION      The present EGP specification has serious limitations, most      importantly a restriction that limits routers to advertising only      those networks that are reachable from within the router's      autonomous system.  This restriction against propagating third      party EGP information is to prevent long-lived routing loops.      This effectively limits EGP to a two-level hierarchy.RFC-975 is not a part of the EGP specification, and should be      ignored.Baker                       Standards Track                   [Page 164]

RFC 1812         Requirements for IP Version 4 Routers         June 1995F.1.2 Protocol Walk-through      Indirect Neighbors:RFC-888, page 26         An implementation of EGP MUST include indirect neighbor         support.      Polling Intervals:RFC-904, page 10         The interval between Hello command retransmissions and the         interval between Poll retransmissions SHOULD be configurable         but there MUST be a minimum value defined.         The interval at which an implementation will respond to Hello         commands and Poll commands SHOULD be configurable but there         MUST be a minimum value defined.      Network Reachability:RFC-904, page 15   An implementation MUST default to not providing the external list of   routers in other autonomous systems; only the internal list of   routers together with the nets that are reachable through those   routers should be included in an Update Response/Indication packet.   However, an implementation MAY elect to provide a configuration   option enabling the external list to be provided.  An implementation   MUST NOT include in the external list routers that were learned   through the external list provided by a router in another autonomous   system.  An implementation MUST NOT send a network back to the   autonomous system from which it is learned, i.e.  it MUST do split-   horizon on an autonomous system level.   If more than 255 internal or 255 external routers need to be   specified in a Network Reachability update, the networks reachable   from routers that can not be listed MUST be merged into the list for   one of the listed routers.  Which of the listed routers is chosen for   this purpose SHOULD be user configurable, but SHOULD default to the   source address of the EGP update being generated.   An EGP update contains a series of blocks of network numbers, where   each block contains a list of network numbers reachable at a   particular distance through a particular router.  If more than 255   networks are reachable at a particular distance through a particular   router, they are split into multiple blocks (all of which have the   same distance).  Similarly, if more than 255 blocks are required to   list the networks reachable through a particular router, the router's   address is listed as many times as necessary to include all the   blocks in the update.Baker                       Standards Track                   [Page 165]

RFC 1812         Requirements for IP Version 4 Routers         June 1995Unsolicited Updates:RFC-904, page 16   If a network is shared with the peer, an implementation MUST send an   unsolicited update upon entry to the Up state if the source network   is the shared network.Neighbor Reachability:RFC-904, page 6, 13-15   The table on page 6 that describes the values of j and k (the   neighbor up and down thresholds) is incorrect.  It is reproduced   correctly here:      Name    Active  Passive Description      -----------------------------------------------       j         3       1    neighbor-up threshold       k         1       0    neighbor-down threshold   The value for k in passive mode also specified incorrectly inRFC-904, page 14 The values in parenthesis should read:      (j = 1, k = 0, and T3/T1 = 4)   As an optimization, an implementation can refrain from sending a   Hello command when a Poll is due.  If an implementation does so, it   SHOULD provide a user configurable option to disable this   optimization.Abort timer:RFC-904, pages 6, 12, 13   An EGP implementation MUST include support for the abort timer (as   documented insection 4.1.4 of RFC-904).  An implementation SHOULD   use the abort timer in the Idle state to automatically issue a Start   event to restart the protocol machine.  Recommended values are P4 for   a critical error (Administratively prohibited, Protocol Violation and   Parameter Problem) and P5 for all others.  The abort timer SHOULD NOT   be started when a Stop event was manually initiated (such as through   a network management protocol).Cease command received in Idle state:RFC-904, page 13   When the EGP state machine is in the Idle state, it MUST reply to   Cease commands with a Cease-ack response.Hello Polling Mode:RFC-904, page 11   An EGP implementation MUST include support for both active and   passive polling modes.Baker                       Standards Track                   [Page 166]

RFC 1812         Requirements for IP Version 4 Routers         June 1995Neighbor Acquisition Messages:RFC-904, page 18   As noted the Hello and Poll Intervals should only be present in   Request and Confirm messages.  Therefore the length of an EGP   Neighbor Acquisition Message is 14 bytes for a Request or Confirm   message and 10 bytes for a Refuse, Cease or Cease-ack message.   Implementations MUST NOT send 14 bytes for Refuse, Cease or Cease-ack   messages but MUST allow for implementations that send 14 bytes for   these messages.Sequence Numbers:RFC-904, page 10   Response or indication packets received with a sequence number not   equal to S MUST be discarded.  The send sequence number S MUST be   incremented just before the time a Poll command is sent and at no   other times.F.2 ROUTING INFORMATION PROTOCOL - RIPF.2.1 Introduction   RIP is specified in [ROUTE:3].  Although RIP is still quite important   in the Internet, it is being replaced in sophisticated applications   by more modern IGPs such as the ones described above.  A router   implementing RIP SHOULD implement RIP Version 2 [ROUTE:?], as it   supports CIDR routes.  If occasional access networking is in use, a   router implementing RIP SHOULD implement Demand RIP [ROUTE:?].   Another common use for RIP is as a router discovery protocol.   Section [4.3.3.10] briefly touches upon this subject.F.2.2 Protocol Walk-Through   Dealing with changes in topology: [ROUTE:3], page 11        An implementation of RIP MUST provide a means for timing out        routes.  Since messages are occasionally lost, implementations        MUST NOT invalidate a route based on a single missed update.        Implementations MUST by default wait six times the update        interval before invalidating a route.  A router MAY have        configuration options to alter this value.   DISCUSSION      It is important to routing stability that all routers in a RIP      autonomous system use similar timeout value for invalidating      routes, and therefore it is important that an implementation      default to the timeout value specified in the RIP specification.Baker                       Standards Track                   [Page 167]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      However, that timeout value is too conservative in environments      where packet loss is reasonably rare.  In such an environment, a      network manager may wish to be able to decrease the timeout period      to promote faster recovery from failures.   IMPLEMENTATION      There is a very simple mechanism that a router may use to meet the      requirement to invalidate routes promptly after they time out.      Whenever the router scans the routing table to see if any routes      have timed out, it also notes the age of the least recently      updated route that has not yet timed out.  Subtracting this age      from the timeout period gives the amount of time until the router      again needs to scan the table for timed out routes.Split Horizon: [ROUTE:3], page 14-15   An implementation of RIP MUST implement split horizon, a scheme used   for avoiding problems caused by including routes in updates sent to   the router from which they were learned.   An implementation of RIP SHOULD implement Split horizon with poisoned   reverse, a variant of split horizon that includes routes learned from   a router sent to that router, but sets their metric to infinity.   Because of the routing overhead that may be incurred by implementing   split horizon with poisoned reverse, implementations MAY include an   option to select whether poisoned reverse is in effect.  An   implementation SHOULD limit the time in which it sends reverse routes   at an infinite metric.   IMPLEMENTATION      Each of the following algorithms can be used to limit the time for      which poisoned reverse is applied to a route.  The first algorithm      is more complex but does a more thorough job of limiting poisoned      reverse to only those cases where it is necessary.      The goal of both algorithms is to ensure that poison reverse is      done for any destination whose route has changed in the last Route      Lifetime (typically 180 seconds), unless it can be sure that the      previous route used the same output interface.  The Route Lifetime      is used because that is the amount of time RIP will keep around an      old route before declaring it stale.      The time intervals (and derived variables) used in the following      algorithms are as follows:      Tu The Update Timer; the number of seconds between RIP updates.           This typically defaults to 30 seconds.Baker                       Standards Track                   [Page 168]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      Rl The Route Lifetime, in seconds.  This is the amount of time           that a route is presumed to be good, without requiring an           update.  This typically defaults to 180 seconds.      Ul The Update Loss; the number of consecutive updates that have to           be lost or fail to mention a route before RIP deletes the           route.  Ul is calculated to be (Rl/Tu)+1.  The +1 is to           account for the fact that the first time the ifcounter is           decremented will be less than Tu seconds after it is           initialized.  Typically, Ul will be 7: (180/30)+1.      In The value to set ifcounter to when a destination is newly           learned.  This value is Ul-4, where the 4 is RIP's garbage           collection timer/30      The first algorithm is:      - Associated with each destination is a counter, called the         ifcounter below.  Poison reverse is done for any route whose         destination's ifcounter is greater than zero.      - After a regular (not triggered or in response to a request)         update is sent, all the non-zero ifcounters are decremented by         one.      - When a route to a destination is created, its ifcounter is set         as follows:         - If the new route is superseding a valid route, and the old            route used a different (logical) output interface, then the            ifcounter is set to Ul.         - If the new route is superseding a stale route, and the old            route used a different (logical) output interface, then the            ifcounter is set to MAX(0, Ul - INT(seconds that the route            has been stale/Ut).         - If there was no previous route to the destination, the            ifcounter is set to In.         - Otherwise, the ifcounter is set to zero      - RIP also maintains a timer, called the resettimer below.  Poison         reverse is done on all routes whenever resettimer has not         expired (regardless of the ifcounter values).Baker                       Standards Track                   [Page 169]

RFC 1812         Requirements for IP Version 4 Routers         June 1995      - When RIP is started, restarted, reset, or otherwise has its         routing table cleared, it sets the resettimer to go off in Rl         seconds.      The second algorithm is identical to the first except that:      - The rules which set the ifcounter to non-zero values are changed         to always set it to Rl/Tu, and      - The resettimer is eliminated. Triggered updates: [ROUTE:3], page 15-16; page 29      Triggered updates (also called flash updates) are a mechanism for      immediately notifying a router's neighbors when the router adds or      deletes routes or changes their metrics.  A router MUST send a      triggered update when routes are deleted or their metrics are      increased.  A router MAY send a triggered update when routes are      added or their metrics decreased.      Since triggered updates can cause excessive routing overhead,      implementations MUST use the following mechanism to limit the      frequency of triggered updates:      (1) When a router sends a triggered update, it sets a timer to a           random time between one and five seconds in the future.  The           router must not generate additional triggered updates before           this timer expires.      (2) If the router would generate a triggered update during this           interval it sets a flag indicating that a triggered update is           desired.  The router also logs the desired triggered update.      (3) When the triggered update timer expires, the router checks the           triggered update flag.  If the flag is set then the router           sends a single triggered update which includes all the           changes that were logged.  The router then clears the flag           and, since a triggered update was sent, restarts this           algorithm.      (4) The flag is also cleared whenever a regular update is sent.      Triggered updates SHOULD include all routes that have changed      since the most recent regular (non-triggered) update.  Triggered      updates MUST NOT include routes that have not changed since the      most recent regular update.Baker                       Standards Track                   [Page 170]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   DISCUSSION      Sending all routes, whether they have changed recently or not, is      unacceptable in triggered updates because the tremendous size of      many Internet routing tables could otherwise result in      considerable bandwidth being wasted on triggered updates.Use of UDP: [ROUTE:3], page 18-19.   RIP packets sent to an IP broadcast address SHOULD have their initial   TTL set to one.   Note that to comply with Section [6.1] of this memo, a router SHOULD   use UDP checksums in RIP packets that it originates, MUST discard RIP   packets received with invalid UDP checksums, but MUST NOT discard   received RIP packets simply because they do not contain UDP   checksums.Addressing Considerations: [ROUTE:3], page 22   A RIP implementation SHOULD support host routes.  If it does not, it   MUST (as described on page 27 of [ROUTE:3]) ignore host routes in   received updates.  A router MAY log ignored hosts routes.   The special address 0.0.0.0 is used to describe a default route.  A   default route is used as the route of last resort (i.e., when a route   to the specific net does not exist in the routing table).  The router   MUST be able to create a RIP entry for the address 0.0.0.0.Input Processing - Response: [ROUTE:3], page 26   When processing an update, the following validity checks MUST be   performed:   o The response MUST be from UDP port 520.   o The source address MUST be on a directly connected subnet (or on a      directly connected, non-subnetted network) to be considered valid.   o The source address MUST NOT be one of the router's addresses.   DISCUSSION      Some networks, media, and interfaces allow a sending node to      receive packets that it broadcasts.  A router must not accept its      own packets as valid routing updates and process them.  The last      requirement prevents a router from accepting its own routing      updates and processing them (on the assumption that they were sent      by some other router on the network).Baker                       Standards Track                   [Page 171]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   An implementation MUST NOT replace an existing route if the metric   received is equal to the existing metric except in accordance with   the following heuristic.   An implementation MAY choose to implement the following heuristic to   deal with the above situation.  Normally, it is useless to change the   route to a network from one router to another if both are advertised   at the same metric.  However, the route being advertised by one of   the routers may be in the process of timing out.  Instead of waiting   for the route to timeout, the new route can be used after a specified   amount of time has elapsed.  If this heuristic is implemented, it   MUST wait at least halfway to the expiration point before the new   route is installed.F.2.3 Specific IssuesRIP Shutdown     An implementation of RIP SHOULD provide for a graceful shutdown     using the following steps:     (1) Input processing is terminated,     (2) Four updates are generated at random intervals of between two          and four seconds, These updates contain all routes that were          previously announced, but with some metric changes.  Routes          that were being announced at a metric of infinity should          continue to use this metric.  Routes that had been announced          with a non-infinite metric should be announced with a metric          of 15 (infinity - 1).   DISCUSSION      The metric used for the above really ought to be 16 (infinity);      setting it to 15 is a kludge to avoid breaking certain old hosts      that wiretap the RIP protocol.  Such a host will (erroneously)      abort a TCP connection if it tries to send a datagram on the      connection while the host has no route to the destination (even if      the period when the host has no route lasts only a few seconds      while RIP chooses an alternate path to the destination).RIP Split Horizon and Static Routes   Split horizon SHOULD be applied to static routes by default.  An   implementation SHOULD provide a way to specify, per static route,   that split horizon should not be applied to this route.Baker                       Standards Track                   [Page 172]

RFC 1812         Requirements for IP Version 4 Routers         June 1995F.3 GATEWAY TO GATEWAY PROTOCOL - GGP   The Gateway to Gateway protocol is considered obsolete and SHOULD NOT   be implemented.Acknowledgments   O that we now had here   But one ten thousand of those men in England   That do no work to-day!   What's he that wishes so?   My cousin Westmoreland? No, my fair cousin:   If we are mark'd to die, we are enow   To do our country loss; and if to live,   The fewer men, the greater share of honour.   God's will! I pray thee, wish not one man more.   By Jove, I am not covetous for gold,   Nor care I who doth feed upon my cost;   It yearns me not if men my garments wear;   Such outward things dwell not in my desires:   But if it be a sin to covet honour,   I am the most offending soul alive.   No, faith, my coz, wish not a man from England:   God's peace! I would not lose so great an honour   As one man more, methinks, would share from me   For the best hope I have. O, do not wish one more!   Rather proclaim it, Westmoreland, through my host,   That he which hath no stomach to this fight,   Let him depart; his passport shall be made   And crowns for convoy put into his purse:   We would not die in that man's company   That fears his fellowship to die with us.   This day is called the feast of Crispian:   He that outlives this day, and comes safe home,   Will stand a tip-toe when the day is named,   And rouse him at the name of Crispian.   He that shall live this day, and see old age,   Will yearly on the vigil feast his neighbours,   And say 'To-morrow is Saint Crispian:'   Then will he strip his sleeve and show his scars.   And say 'These wounds I had on Crispin's day.'   Old men forget: yet all shall be forgot,   But he'll remember with advantages   What feats he did that day: then shall our names.   Familiar in his mouth as household words   Harry the king, Bedford and Exeter,   Warwick and Talbot, Salisbury and Gloucester,Baker                       Standards Track                   [Page 173]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Be in their flowing cups freshly remember'd.   This story shall the good man teach his son;   And Crispin Crispian shall ne'er go by,   From this day to the ending of the world,   But we in it shall be remember'd;   We few, we happy few, we band of brothers;   For he to-day that sheds his blood with me   Shall be my brother; be he ne'er so vile,   This day shall gentle his condition:   And gentlemen in England now a-bed   Shall think themselves accursed they were not here,   And hold their manhoods cheap whiles any speaks   That fought with us upon Saint Crispin's day.                                   -- William Shakespeare   This memo is a product of the IETF's Router Requirements Working   Group.  A memo such as this one is of necessity the work of many more   people than could be listed here.  A wide variety of vendors, network   managers, and other experts from the Internet community graciously   contributed their time and wisdom to improve the quality of this   memo.  The editor wishes to extend sincere thanks to all of them.   The current editor also wishes to single out and extend his heartfelt   gratitude and appreciation to the original editor of this document;   Philip Almquist.  Without Philip's work, both as the original editor   and as the Chair of the working group, this document would not have   been produced.  He also wishes to express deep and heartfelt   gratitude to the previous editor, Frank Kastenholz.  Frank changed   the original document from a collection of information to a useful   description of IP technology - in his words, a "snapshot" of the   technology in 1991.  One can only hope that this snapshot, of the   technology in 1994, is as clear.   Philip Almquist, Jeffrey Burgan, Frank Kastenholz, and Cathy   Wittbrodt each wrote major chapters of this memo.  Others who made   major contributions to the document included Bill Barns, Steve   Deering, Kent England, Jim Forster, Martin Gross, Jeff Honig, Steve   Knowles, Yoni Malachi, Michael Reilly, and Walt Wimer.   Additional text came from Andy Malis, Paul Traina, Art Berggreen,   John Cavanaugh, Ross Callon, John Lekashman, Brian Lloyd, Gary   Malkin, Milo Medin, John Moy, Craig Partridge, Stephanie Price, Yakov   Rekhter, Steve Senum, Richard Smith, Frank Solensky, Rich Woundy, and   others who have been inadvertently overlooked.   Some of the text in this memo has been (shamelessly) plagiarized from   earlier documents, most notablyRFC-1122 by Bob Braden and the HostBaker                       Standards Track                   [Page 174]

RFC 1812         Requirements for IP Version 4 Routers         June 1995   Requirements Working Group, andRFC-1009 by Bob Braden and Jon   Postel.  The work of these earlier authors is gratefully   acknowledged.   Jim Forster was a co-chair of the Router Requirements Working Group   during its early meetings, and was instrumental in getting the group   off to a good start.  Jon Postel, Bob Braden, and Walt Prue also   contributed to the success by providing a wealth of good advice   before the group's first meeting.  Later on, Phill Gross, Vint Cerf,   and Noel Chiappa all provided valuable advice and support.   Mike St.  Johns coordinated the Working Group's interactions with the   security community, and Frank Kastenholz coordinated the Working   Group's interactions with the network management area.  Allison   Mankin and K.K.  Ramakrishnan provided expertise on the issues of   congestion control and resource allocation.   Many more people than could possibly be listed or credited here   participated in the deliberations of the Router Requirements Working   Group, either through electronic mail or by attending meetings.   However, the efforts of Ross Callon and Vince Fuller in sorting out   the difficult issues of route choice and route leaking are especially   acknowledged.   The editor thanks his employer, Cisco Systems, for allowing him to   spend the time necessary to produce the 1994 snapshot.Editor's Address   The address of the current editor of this document is      Fred Baker      Cisco Systems      519 Lado Drive      Santa Barbara, California 93111      USA      Phone:+1 805-681-0115      EMail: fred@cisco.comBaker                       Standards Track                   [Page 175]