RFC1377

Network Working Group D. Katz Request for Comments: 1377 cisco

                                                       November 1992

      The 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 [email protected] mailing list. Table of Contents 1. Introduction .......................................... 2 1.1 OSI Network Layer Protocols over PPP .................. 2 2. A PPP Network Control Protocol (NCP) for OSI .......... 5 2.1 Sending OSI NPDUs ..................................... 6 2.2 NPDU Alignment ........................................ 6 2.3 Network Layer Addressing Information .................. 6 3. OSINLCP Configuration Options ......................... 7 3.1 Align-NPDU ............................................ 7 REFERENCES ................................................... 9 ACKNOWLEDGEMENTS ............................................. 9 SECURITY CONSIDERATIONS ...................................... 10 CHAIR'S ADDRESS .............................................. 10 AUTHOR'S ADDRESS ............................................. 10

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).

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.

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 is

  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.

  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.

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 other

  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.

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.

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.

Network Layer Addressing Information

OSINLCP does not define a separate configuration option for the exchange of OSI Network Layer address information. Instead, the ES-

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.

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-NPDU

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 by

     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.

References [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 implementation

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: [email protected] Author'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: [email protected]