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Network Working Group                                            D. KatzRequest for Comments: 1377                                         cisco                                                           November 1992The PPP OSI Network Layer Control Protocol (OSINLCP)Status of this Memo   This RFC specifies an IAB standards track protocol for the Internet   community, and requests discussion and suggestions for improvements.   Please refer to the current edition of the "IAB Official Protocol   Standards" for the standardization state and status of this protocol.   Distribution of this memo is unlimited.Abstract   The Point-to-Point Protocol (PPP) [1] provides a standard method of   encapsulating Network Layer protocol information over point-to-point   links.  PPP also defines an extensible Link Control Protocol, and   proposes a family of Network Control Protocols (NCPs) for   establishing and configuring different network-layer protocols.   This document defines the NCP for establishing and configuring OSI   Network Layer Protocols.   This memo is the product of the Point-to-Point Protocol Working Group   of the Internet Engineering Task Force (IETF).  Comments on this memo   should be submitted to the ietf-ppp@ucdavis.edu mailing list.Table of Contents1.     Introduction ..........................................21.1    OSI Network Layer Protocols over PPP ..................22.     A PPP Network Control Protocol (NCP) for OSI ..........52.1    Sending OSI NPDUs .....................................62.2    NPDU Alignment ........................................62.3    Network Layer Addressing Information ..................63.     OSINLCP Configuration Options .........................73.1    Align-NPDU ............................................7   REFERENCES ...................................................9   ACKNOWLEDGEMENTS .............................................9   SECURITY CONSIDERATIONS ......................................10   CHAIR'S ADDRESS ..............................................10   AUTHOR'S ADDRESS .............................................10Katz                                                            [Page 1]

RFC 1377                      PPP OSINLCP                  November 19921.  Introduction   PPP has three main components:      1. A method for encapsulating datagrams over serial links.      2. A Link Control Protocol (LCP) for establishing, configuring,         and testing the data-link connection.      3. A family of Network Control Protocols (NCPs) for establishing         and configuring different network-layer protocols.   In order to establish communications over a point-to-point link, each   end of the PPP link must first send LCP packets to configure and test   the data link.  After the link has been established and optional   facilities have been negotiated as needed by the LCP, PPP must send   NCP packets to choose and configure one or more network-layer   protocols.  Once each of the chosen network-layer protocols has been   configured, datagrams from each network-layer protocol can be sent   over the link.   The link will remain configured for communications until explicit LCP   or NCP packets close the link down, or until some external event   occurs (an inactivity timer expires or network administrator   intervention).1.1.  OSI Network Layer Protocols over PPP   A number of protocols have been defined for the Network Layer of OSI,   including the Connectionless Network Layer Protocol (CLNP, ISO 8473)   [3], the End System to Intermediate System routing protocol (ES-IS,   ISO 9542) [4], the Intermediate System to Intermediate System routing   protocol (IS-IS, ISO 10589) [5], and the Inter-Domain Routeing   Protocol (IDRP, CD 10747) [6].  Generally, these protocols were   designed to run over non-reliable data link protocols such as PPP.   Network Layer Protocol Identifier (NLPID)      OSI Network Layer protocols can be discriminated according to the      first octet in each Network Protocol Data Unit (NPDU, that is,      packet), known as the Network Layer Protocol Identifier (NLPID),      which is defined in ISO/TR 9577 [7].  This allows the various      protocols to be run over a common data link without any      discriminator below the network layer.Katz                                                            [Page 2]

RFC 1377                      PPP OSINLCP                  November 1992   Inactive Network Layer Protocol      ISO/TR 9577 reserves a NLPID value of zero to represent the      "Inactive Network Layer Protocol", as defined in ISO 8473.  The      inactive network layer protocol MUST NOT be used over PPP.  This      assures that whichever OSI network layer protocol is used will      have a non-zero NLPID value.   Connection-Oriented Network Protocol      The OSI Connection-Oriented Network Protocol (ISO 8208) [8],      effectively the Packet Layer of CCITT X.25, is intended to be run      over a reliable data link, such as IEEE 802.2 type II or LAPB.      Therefore, the unreliable data link service provided by PPP is not      appropriate for use with ISO 8208.   ConnectionLess Network Protocol (CLNP)      The ConnectionLess Network Protocol offers a simple non-reliable      datagram service very similar to IP, and is designed to run over a      non-reliable data link service, such as provided by PPP.   End-System to Intermediate-System Protocol (ES-IS)      ES Hellos and IS Hellos are retransmitted on a periodic timer-      driven basis (based on expiration of the "Configuration Timer").      The resulting ES and IS configuration information is invalidated      on a timer driven basis, based on expiration of the "Holding      Timer" for each piece of information.  The value of a Holding      Timer is set by the source of the information, and transmitted in      the Holding Time field of the appropriate ES-IS packet.  ISO 9542      recommends that the holding time field is set to approximately      twice the Configuration Timer parameter, such that even if every      other Hello packet is lost the configuration information will be      retained (implying that the Holding Timer is actually set to      slightly more than twice the Configuration Timer).      Generally, the recommendation in ISO 9542 is sufficient for PPP      links.  For very unreliable links, it may be necessary to set the      Holding Timer to be slightly more than three times the      Configuration Timer to ensure that loss of configuration      information is an unusual event.      Redirect information is not transmitted on point-to-point links,      but may be transmitted on general topology subnetworks, which in      turn may make use of PPP.  Redirect information is sent on a      event-driven basis (based on a CLNP packet being forwarded by a      router out the incoming interface), but redirect information isKatz                                                            [Page 3]

RFC 1377                      PPP OSINLCP                  November 1992      invalidated on a timer-driven basis.  Loss of a single redirect      may result in a subsequent data packet being sent to the same      incorrect router, which will re-issue the redirect.  This operates      in the same manner as ICMP redirects for IP packets, and does not      pose any problem for operation over PPP links.   Intermediate-System to Intermediate-System Protocol (IS-IS)      IS-IS allows for broadcast links (typically LANs), point-to-point      links (such as PPP), and general topology links (such as X.25      networks) which are modelled as a collection of point-to-point      links.      There are four types of IS-IS packets: IS-IS Hello Packets, Link      State Packets (LSPs), Complete Sequence Number Packets (CSNPs),      and Partial Sequence Number Packets (PSNPs).      IS-IS Hello messages are transmitted periodically on point-to-      point links (based on expiration of the "ISISHello" timer).      Routers expect to receive IS-IS Hello packets periodically.      Specifically, the IS-IS Hello packet specifies a "Holding Time".      If no subsequent IS-IS Hello is received over the corresponding      link for the specified time period, then the neighboring router is      assumed to have been disconnected or to be down.  It is highly      undesireable for links to "flap" up and down unnecessarily, which      implies that the holding time needs to be large enough that a link      is very unlikely to be declared down due to a failure to receive      an IS-IS Hello.  This implies that running IS-IS over unreliable      data links requires the Holding time to be greater than "k" times      the ISISHello timer, where k is chosen such that the loss of k      consecutive IS-IS Hello's is rare.  If the quality of the link is      poor, then the Holding Time will need to be increased or the      "ISISHello" time decreased.      LSPs are acknowledged by the IS-IS protocol (via use of partial      sequence number packets).  A lost LSP will be recovered from with      no problem provided that PPP links are treated the same way as      other point-to-point links.  On those rare occasions where a      partial sequence number packet is lost, this might result in the      retransmission of a link state packet over a single link, but will      not impact the correct operation of the routing algorithm.      CSNPs are sent upon link startup on a point-to-point link.  This      does not need to be changed for PPP.  If a CSNP fragment is lost      upon startup it is merely loss of an optimization -- LSPs that did      not need to be transmitted over the link will be transmitted.  If      a periodic CSNP fragment is lost it merely means that detection of      low probability database corruption will take longer.Katz                                                            [Page 4]

RFC 1377                      PPP OSINLCP                  November 1992      PSNPs function as ACKs.  Loss of a PSNP may result in an      unnecessary retransmission of an LSP, but does not prevent correct      operation of the routing protocol.   Inter-Domain Routeing Protocol (IDRP)      IDRP expects to run over datagram links, but requires reliable      exchange of IDRP information.  For this reason, IDRP contains      built-in reliability mechanisms which ensure that packets will be      received correctly.2.  A PPP Network Control Protocol (NCP) for OSI   The OSI Network Layer Control Protocol (OSINLCP) is responsible for   configuring, enabling, and disabling the OSI protocol modules on both   ends of the point-to-point link.  OSINLCP uses the same packet   exchange machanism as the Link Control Protocol (LCP).  OSINLCP   packets may not be exchanged until PPP has reached the Network-Layer   Protocol phase.  OSINLCP packets received before this phase is   reached should be silently discarded.   The OSI Network Layer Control Protocol is exactly the same as the   Link Control Protocol [1] with the following exceptions:   Frame Modifications      The packet may utilize any modifications to the basic frame format      which have been negotiated during the Link Establishment phase.   Data Link Layer Protocol Field      Exactly one OSINLCP packet is encapsulated in the Information      field of a PPP Data Link Layer frame where the Protocol field      indicates type hex 8023 (OSI Network Layer Control Protocol).   Code field      Only Codes 1 through 7 (Configure-Request, Configure-Ack,      Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack      and Code-Reject) are used.  Other Codes should be treated as      unrecognized and should result in Code-Rejects.   Timeouts      OSINLCP packets may not be exchanged until PPP has reached the      Network-Layer Protocol phase.  An implementation should be      prepared to wait for Authentication and Link Quality Determination      to finish before timing out waiting for a Configure-Ack or otherKatz                                                            [Page 5]

RFC 1377                      PPP OSINLCP                  November 1992      response.  It is suggested that an implementation give up only      after user intervention or a configurable amount of time.   Configuration Option Types      OSINLCP has one Configuration Option, which is defined below.2.1.  Sending OSI NPDUs   Before any Network Protocol Data Units (NPDUs) may be communicated,   PPP must reach the Network-Layer Protocol phase, and the OSI Network   Layer Control Protocol must reach the Opened state.   Exactly one OSI NPDU is encapsulated in the Information field of a   PPP Data Link Layer frame where the Protocol field indicates type hex   0023 (OSI Network Layer).   The maximum length of an OSI NPDU transmitted over a PPP link is the   same as the maximum length of the Information field of a PPP data   link layer frame.  Larger NPDUs must be segmented as necessary.  If a   system wishes to avoid segmentation and reassembly, it should use   transport layer mechanisms to discourage others from sending large   PDUs.2.2.  NPDU Alignment   OSI protocols have peculiar alignment problems due to the fact that   they are often encapsulated in data link protocols with odd-length   headers, while PPP defaults to even-length headers.  A router   switching an OSI packet may find that the beginning of the packet   falls on an inconvenient memory boundary when the hardware used to   transmit the packet to its next hop requires a particular alignment.   This situation can be addressed by the use of leading zero padding.   When sending, an implementation MAY insert one to three octets of   zero between the PPP header and the OSI NPDU.  These zero octets   correspondingly reduce the maximum length of the NPDU that may be   transmitted.   On reception, any such leading zero octets (if present) MUST be   removed.  Regardless of whether leading zero padding is used, an   implementation MUST also be able to receive a PPP packet with any   arbitrary alignment of the NPDU.2.3.  Network Layer Addressing Information   OSINLCP does not define a separate configuration option for the   exchange of OSI Network Layer address information.  Instead, the ES-Katz                                                            [Page 6]

RFC 1377                      PPP OSINLCP                  November 1992   IS protocol, ISO 9542, should be used.  This protocol provides a   mechanism for determining the Network Layer address(es) of the   neighbor on the link, as well as determining if the neighbor is an   End System or an Intermediate System.   A draft addendum to ES-IS [9] is being defined in ISO to add support   for dynamic address assignment.  This addendum has currently passed   the formal "Committee Draft" (CD) letter ballot.3.  OSINLCP Configuration Options   OSINLCP Configuration Options allow negotiatiation of desirable   Internet Protocol parameters.  OSINLCP uses the same Configuration   Option format defined for LCP [1], with a separate set of Options.   The most up-to-date values of the OSINLCP Option Type field are   specified in the most recent "Assigned Numbers" RFC [2].  Current   values are assigned as follows:      1       Align-NPDU3.1.  Align-NPDU   Description      This Configuration Option provides a way for the receiver to      negotiate a particular alignment of the OSI NPDU.  Empirical      evidence suggests that the greatest time deficit for re-alignment      exists at the receiver.      The alignment is accomplished through combination of PPP header      compression with leading zero padding (see above).  It is      recommended that alignment be entirely through header compression      combinations whenever possible.  For example, an alignment of 3      could be achieved by combining uncompressed PPP Address and      Control fields (2 octets) with a compressed PPP Protocol field (1      octet).      This option is negotiated separately in each direction.  A      receiver which does not need alignment MUST NOT request the      option.  A sender which desires alignment prior to sending SHOULD      Configure-Nak with an appropriate value.         Implementation Note: In a complex environment, there might be         several conflicting needs for alignment.  It is recommended         that the receiver request alignment based on the needs of the         highest speed next hop link.  Also, greater efficiency might be         obtained by negotiating upstream the values requested byKatz                                                            [Page 7]

RFC 1377                      PPP OSINLCP                  November 1992         downstream PPP links, since those packets will not need a         change in alignment on transit.      The alignment request is advisory, and failure to agree on an      alignment MUST NOT prevent the OSINLCP from reaching the Opened      state.  By default, the alignment is done according to the needs      of the sender, and all receivers MUST be capable of accepting      packets with any alignment.         Vernacular: If you don't like this option, you can refuse to         negotiate it, and you can send whatever alignment you want.         However, if you accept the peer's alignment option, then you         MUST transmit packets with the agreed alignment.   A summary of the Align-NPDU Configuration Option format is shown   below.  The fields are transmitted from left to right.    0                   1                   2    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |     Type      |    Length     |   Alignment   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Type      1   Length      3   Alignment      This field specifies the offset of the beginning of the OSI NPDU      relative to the beginning of the PPP packet header (not including      any leading Flag Sequences).      A value of 1 through 4 requires an offset of that specific length,      modulo 4.  For example, a value of 1 would require no padding when      the PPP Address, Control, and Protocol fields are compressed.  One      octet of leading zero padding would be necessary when the PPP      header is full sized.      A value of 255 requests an offset of an odd length (1 or 3).  A      value of 254 requests an offset of an even length (2 or 4).  If      the sender is not capable of dynamically varying the amount of      padding, it MUST NAK with one of the two specific values.Katz                                                            [Page 8]

RFC 1377                      PPP OSINLCP                  November 1992References   [1] Simpson, W., "The Point-to-Point Protocol (PPP)",RFC 1331,       Daydreamer, May 1992.   [2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,RFC 1340,       USC/Information Sciences Institute, July 1992.   [3] ISO, "Information processing systems -- Data communications --       Protocol for providing the connectionless-mode network       service", ISO 8473, 1988.   [4] ISO, "Information processing systems -- Telecommunications and       information exchange between systems -- End system to       Intermediate system Routeing exchange protocol for use in       conjunction with the protocol for providing the connectionless-       mode network service (ISO 8473)", ISO 9542, 1988.   [5] ISO, "Information processing systems -- Telecommunications and       information exchange between systems -- Intermediate system to       Intermediate system Intra-Domain routeing exchange protocol for       use in conjunction with the protocol for providing the       connectionless-mode network service (ISO 8473)", ISO 10589,       1990.   [6] ISO, "Protocol for Exchange of Inter-domain Routeing       Information among Intermediate Systems to Support Forwarding of       ISO 8473 PDUs", ISO CD 10747, 1991.   [7] ISO, "Information technology -- Telecommunications and       information exchange between systems -- Protocol identification       in the network layer", ISO/IEC TR9577:1990.   [8] ISO, "Information processing systems -- Data communications --       X.25 packet level protocol for Data terminal equipment", ISO       8208, 1984.   [9] Taylor, E., "Addendum to ISO 9542 (PDAM 1 - Dynamic Discovery       of OSI NSAP Addresses by End Systems)", SC6/N7248.Acknowledgments   Some of the text in this document is taken from previous documents   produced by the Point-to-Point Protocol Working Group of the Internet   Engineering Task Force (IETF).   Special thanks to Ross Callon (DEC), and Cyndi Jung (3Com), for   contributions of text and design suggestions based on implementationKatz                                                            [Page 9]

RFC 1377                      PPP OSINLCP                  November 1992   experience.   Thanks also to Bill Simpson for his editing and formatting efforts,   both for this document and for PPP in general.Security Considerations   Security issues are not discussed in this memo.Chair's Address   The working group can be contacted via the current chair:   Brian Lloyd   Lloyd & Associates   3420 Sudbury Road   Cameron Park, California 95682   Phone: (916) 676-1147   EMail: brian@lloyd.comAuthor's Address   Questions about this memo can also be directed to:   Dave Katz   cisco Systems, Inc.   1525 O'Brien Dr.   Menlo Park, CA  94025   Phone: (415) 688-8284   EMail: dkatz@cisco.comKatz                                                           [Page 10]