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Network Working Group                                           S. HaresRequest for Comments: 1575                                  Merit/NSFNETObsoletes:1139                                             C. WittbrodtCategory: Standards Track                    Stanford University/BARRNet                                                           February 1994An Echo Function for CLNP (ISO 8473)Status 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.Abstract   This memo defines an echo function for the connection-less network   layer protocol.  The mechanism that is mandated here is in the final   process of being standardized by ISO as "Amendment X: Addition of an   Echo function to ISO 8473" an integral part of Version 2 of ISO 8473.Table of ContentsSection 1. Conventions .................................2Section 2. Introduction ................................2Section 3. The Generic Echo Function ...................3Section 3.1 The Echo-Request ...........................3Section 3.2 The Echo-Response ..........................3Section 4. The Implementation Mechanism ................4Section 4.1 The Echo-Request ...........................4Section 4.2 The Echo-Response ..........................4Section 5. Implementation Notes ........................4Section 5.1 Discarding Packets .........................4Section 5.2 Error Report Flag ..........................4Section 5.3 Use of the Lifetime Field ..................5Section 5.4 Echo-request function ......................5Section 5.5 Echo-response function .....................6Section 5.6 Use of the Priority Option .................8Section 5.7 Use of the Source Route Option .............8Section 5.8 Transmission of Multiple Echo-Requests .....9Section 6. Security Considerations .....................9Section 7. Authors' Addresses ..........................9Section 8. References ..................................9Hares & Wittbrodt                                               [Page 1]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 19941.  Conventions   The following language conventions are used in the items of   specification in this document:      o MUST, SHALL, or MANDATORY -- the item is an absolute        requirement of the specification.      o SHOULD or RECOMMENDED -- the item should generally be followed        for all but exceptional circumstances.      o MAY or OPTIONAL -- the item is truly optional and may be        followed or ignored according to the needs of the implementor.2.  Introduction   The OSI Connection-less network layer protocol (ISO 8473) defines a   means for transmitting and relaying data and error protocol data   units, (PDUs) or preferably, packets through an OSI internet.   Unfortunately, the world that these packets travel through is   imperfect.  Gateways and links may fail.  This memo defines an echo   function to be used in the debugging and testing of the OSI network   layer.  Hosts and routers which support the OSI network layer MUST be   able to originate an echo packet as well as respond when an echo is   received.   Network management protocols can be used to determine the state of a   gateway or link.  However, since these protocols themselves utilize a   protocol that may experience packet loss, it cannot be guaranteed   that the network management applications can be utilized.  A simple   mechanism in the network layer is required so that systems can be   probed to determine if the lowest levels of the networking software   are operating correctly.  This mechanism is not intended to compete   with or replace network management; rather it should be viewed as an   addition to the facilities offered by network management.   The code-path consideration requires that the echo path through a   system be identical (or very close) to the path used by normal data.   An echo path must succeed and fail in unison with the normal data   path or else it will not provide a useful diagnostic tool.   Previous drafts describing an echo function for CLNP offered two   implementation alternatives (seeRFC 1139).  Although backward   compatibility is an important consideration whenever a change is made   to a protocol, it is more important at this point that the echo   mechanisms used on the Internet interoperate.  For this reason, this   memo defines one implementation mechanism (consistent with one of the   previous drafts).Hares & Wittbrodt                                               [Page 2]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 19943.  The Generic Echo Function   The following section describes the echo function in a generic   fashion.  This memo defines an echo-request entity.  The function of   the echo-request entity is to accept an incoming echo-request packet,   perform some processing, and generate an echo-response packet.  The   echo implementation may be thought of as an entity that coexists with   the network layer.  Subsequent sections will detail the   implementation mechanism.   For the purposes of this memo, the term "ping" shall be used to mean   the act of transmitting an echo-request packet to a remote system   (with the expectation that an echo-response packet will be sent back   to the transmitter).3.1.  The Echo-Request   When a system decides to ping a remote system, an echo-request is   built.  All fields of the packet header are assigned normal values   (see implementation specific sections for more information).  The   address of the system to be pinged is inserted as the destination   NSAP address.  The rules of segmentation defined for a data (DT)   packet also apply to the echo-request packet.   The echo-request is switched through the network toward its   destination.  (An echo packet must follow the same path as CLNP data   packet with the same options in the CLNP header.)  Upon reaching the   destination system, the packet is processed according to normal   processing rules.  At the end of the input processing, the echo-   request packet is delivered to the echo-request entity.   The echo-request entity will build and dispatch the echo-response   packet.  This is a new packet.  Except as noted below, this second   packet is built using the normal construction procedures.  The   destination address of the echo-response packet is taken from the   source address of the echo-request packet.  Most options present in   the echo-request packet are copied into the echo-response packet (see   implementation notes for more information).3.2.  The Echo-Response   The entire echo-request packet is included in the data portion of the   echo-response packet.  This includes the echo-request packet header   as well as any data that accompanies the echo-request packet.  The   entire echo-request packet is included in the echo-response so that   fields such as the echo-request lifetime may be examined when the   response is received.  After the echo-response packet is built, it is   transmitted toward the new destination (the original source of theHares & Wittbrodt                                               [Page 3]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 1994   echo-request).  The rules of segmentation defined for a data packet   also apply to the echo-response packet.   The echo-response packet is relayed through the network toward its   destination. (A echo response packet must follow the same path as a   CLNP data packet with the same options in the CLNP header.)  Upon   reaching its destination, it is processed by the packet input   function and delivered to the entity that created the echo-request.4.  The Implementation Mechanism   The implementation mechanism defines two new 8473 packet types: ERQ   (echo-request) and ERP (echo-response).  With the exception of a new   type code, these packets will be identical to the date packet in   every respect.4.1.  The Echo-Request   The type code for the echo-request packet is decimal 30.4.2.  The Echo-Response   The type code for the echo-response packet is decimal 31.5.  Implementation Notes   The following notes are an integral part of memo.  It is important   that implementors take heed of these points.5.1.  Discarding Packets   The rules used for discarding a data packet (ISO 8473,Section 6.9 -Section 6.10) are applied when an echo-request or echo-response is   discarded.5.2.  Error Report Flag   The error report flag may be set on the echo-request packet, the   echo-response packet, or both.  If an echo-request is discarded, the   associated error-report (ER) packet will be sent to the echo-request   source address on the originating machine.  If an echo-response is   discarded, the associated error-report packet will be sent to the   echo-response source address.  In general, this will be the   destination address of the echo-request entity.  It should be noted   that the echo-request entity and the originator of the echo-request   packet are not required to process error-report packets.Hares & Wittbrodt                                               [Page 4]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 19945.3.  Use of the Lifetime Field   The lifetime field of the echo-request and echo-response packets   should be set to the value normally used for a data packet.  Note:   although this memo does not prohibit the generation of a packet with   a smaller-than-normal lifetime field, this memo explicitly does not   attempt to define a mechanism for varying the lifetime field set in   the echo-response packet.  This memo recommends the lifetime value   that would under normal circumstances by used when sending a data   packet.5.4.  Echo-request function   This function is invoked by system management to obtain information   about the dynamic state of the Network layer with respect to (a) the   reachability of specific network-entities, and (b) the   characteristics of the path or paths that can be created between   network-entities through the operation of Network layer routing   functions.  When invoked, the echo-request function causes an echo-   request (ERQ) packet to be created.  The echo-request packet shall be   constructed and processed by ISO 8473 network-entities in end systems   and intermediate systems in exactly the same way as the data packet,   with the following caveats:      a) The information available to the packet composition function      (ISO 8473) consists of current state, local information, and      information supplied by system management.      b) The source and destination address fields of the echo-request      packet shall contain, respectively, a Network entity title (NET)      of the originating network-entity and a Network entity title of      the destination network-entity (which may be in either an end      system or an intermediate system).  NOTE: A Network entity title      is syntactically indistinguishable from an NSAP address.  The      additional information in an NSAP address, if any, beyond that      which is present in a Network entity title, is relevant only to      the operation of the packet decomposition function in a      destination end system, and therefore is not needed for the      processing of an echo-request packet (from which no N-UNITDATA      indication is ever produced).  The fact that the source and      destination address fields of the echo-request packet contain NETs      rather than NSAP addresses therefore does not affect the      processing of an echo-request packet by any network-entity.      c) When an echo-request packet has reached its destination, as      determined by the Header processing (call HEADER FORMAT Analysis      function in ISO 8473), the echo-response function shall handle      this Network Protocol Data Units (NPDU) instead of the packetHares & Wittbrodt                                               [Page 5]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 1994      decomposition function.  In ISO 8473, the packet decomposition      function is like a decomposing fish on the sea shore - it takes a      packet down to its bare bones and processes it.      Also, it is up to each individual system whether or not handling      echo-request packets involves system management.  One example of      involving system management is the reporting reception of the echo      packets as some systems do with the ping packet.  Some systems      find this of value if they are being pinged to death.      d) The maximum length of the echo-request packet is equal to the      maximum length of the echo-response packet minus the maximum      length of the echo-response packet header.  This ensures that the      entire echo-request packet can be contained within the data field      of the echo-response packet (see ISO 8473).      e) The data part of the echo-request packet may, as a local      matter, contain zero or more octets with any values that fit      within the echo-request packet. (see (d) above for maximum length      of the echo-request packet). If the first octet of data is binary      1000 0001, then an echo-response header is contained in the echo-      request packet.  The existence of this header insures that a      router can formulate a standard echo-response packet.   Normally, the "more segmentation" flag in the encapsulated echo-   response packet header shall be zero, and the segmentation portion of   the encapsulated packet shall not be included.  The segmentation   length in the echo-response packet header shall be zero.   If the "more segmentation" flag is set in the encapsulated echo-   response packet header, then a segmentation length shall be filled in   and the segmentation part of the echo-response packet header will be   present in the echo-response header.  This same segmentation function   shall be present in the echo-response sent by the router.   NOTE: However, this formulated echo-response is not required between   any two systems.  With a common format for an echo-request packet   used in an environment such as the Internet, the echo-response header   may not be needed, and may in fact be unnecessary overhead.5.5.  Echo-response function   This function is performed by a network-entity when it has received   an echo-request packet that has reached its destination, as   determined by the Header format analysis function (ISO 8473 clause   6.3) that is, an echo-request packet which contains, in its   destination address field, a Network entity title that identifies the   network-entity.  When invoked, the echo-response function causes anHares & Wittbrodt                                               [Page 6]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 1994   echo-response (ERP) packet to be created.  The echo-response packet   shall be constructed and processed by ISO 8473 network-entities in   end systems and intermediate systems in exactly the same way as the   data packet, with the following caveats:      a) The information available to the packet composition function      consists of current state, local information, and information      contained in the corresponding echo-request packet.      b) The source address field of the echo-response packet shall      contain the value of the destination address field of the      corresponding echo-request packet.  The destination address field      of the echo-response packet shall contain the value of the source      address field of the corresponding echo-request packet.      c) The echo-request packet, in its entirety, shall be placed into      the data part of the echo-response packet.  The data part of the      echo-response packet shall contain only the corresponding echo-      request packet.      d) If the data part of the echo-request packet contains an echo-      response header, the packet composition function may, but is not      required to, use some or all of the information contained therein      to select values for the fields of the echo-response packet      header.  In this case, however, the value of the lifetime field      contained in the echo-response packet header in the echo-request      packet data part must be used as the value of the lifetime field      in the echo-response packet.  The values of the segment length and      checksum fields shall be computed by the network-entity regardless      of the contents of those fields in the echo-response packet header      in the data part of the echo-request packet.      e) The options part of the echo-response packet may contain any      (or none) of the options described in ISO 8473 (but seeSection5.7 of this RFC).  The values for these options, if present, are      determined by the network-entity as a local matter.  They may be,      but are not required to be, either identical to or derived from      the corresponding options in the echo-request packet and/or the      echo-response packet header contained in the data part of the      echo-request packet (if present).  The source routing option in      the echo-response packet shall not be identical to (copied from)      the source routing option in the echo-request packet header.  If      the recording of route option in the echo-response packet is      identical to (copied from) the recording of route option in the      echo-request packet header, the second octet of the parameter      value field shall be set to the value 3.Hares & Wittbrodt                                               [Page 7]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 1994      f) It is a local matter whether or not the destination network-      entity performs the lifetime control function on an echo-request      packet before performing the echo-response function.  The      destination network-entity shall make the same decision in this      regard that it would make, as a local matter, for a data packet in      accordance with ISO 8473.5.6.  Use of the Priority Option      The 8473 priority function indicates the relative priority of      packet.  0 is normal and 14 is the highest.  Packets with higher      values will be transmitted before lower values.  The specific      action upon receiving a 8473 packet with the priority field set is      a "LOCAL MATTER".  These means, any two systems could do it      differently.      Hopefully, in the future, Internet routers will handle this as a      priority queueing function.  Some implementors consider the      priority queueing function to be a cap.  For example, if a router      is congested, all those packets with priorities higher than 20,      will be allowed through, and those with priority less than 20 will      be dropped.      In short, the basic function of priority has wide latitude in the      ISO specification.  This wide latitude of implementation needs to      be narrowed for implementations within a common network      environment such as the Internet.  The 8473 priority function is      rarely implemented in today's Internet.  The transmission of an      echo-request packet with a priority set may provided unexcepted      results until a more wide spread deployment of the priority      feature in 8473 capable routers and end systems.      However, if the priority function must be used it is the safest      value may be the value 0 - which indicates Normal priority.  It      most likely this value will follow the 8473 pathways.      In the future, as the implementation of the priority function      further Internet documents will need to deal with its expected      use.5.7.  Use of the Source Route Option      Use of the source route option in ISO 8473 may cause packets to      loop until their lifetime expires.  For this reason, this memo      recommends against the use of the source route option in either an      echo-request or echo-response packets.  If the source route option      is used to specify the route that the echo-request packet takes      toward its destination, this memo does not recommend the use of anHares & Wittbrodt                                               [Page 8]

RFC 1575          An Echo Function for CLNP (ISO 8473)     February 1994      automatically generated source route on the echo-response packet.5.8.  Transmission of Multiple Echo-Requests      The echo function may be utilized by more than one process on any      individual machine.  The mechanism by which multiple echo-requests      and echo-responses are correlated between multiple processes on a      single machine is a local matter and not defined by this memo.6.  Security Considerations      Security issues are not discussed in this memo.7.  Authors' Addresses      Susan K. Hares      MERIT/NSFNET      Internet Engineering      1075 Beal Avenue      Ann Arbor, MI 48109-2112      Phone: (313) 936-3000      EMail: skh@merit.edu      Cathy J. Wittbrodt      Stanford University/BARRNet      Networking Systems      Pine Hall 115      Stanford, CA 94305      Phone: (415) 725-5481      EMail: cjw@magnolia.Stanford.EDU8.  References   [1] ISO/IEC.  Protocol for Providing the Connectionless-mode Network       Service.  International Standard 8473, ISO/IEC JTC 1,       Switzerland, 1986.   [2] Hagens, R., "An Echo Function for ISO 8473",RFC 1139, IETF-OSI       Working Group, January 1990.   [3] ISO 8473-1993 Protocol for providing the connectionless-mode       network service, edition 2 (IS under preparation).Hares & Wittbrodt                                               [Page 9]