rfc1663.txt

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Network Working Group                                           D. Rand
Request for Comments: 1663                                       Novell
Category: Standards Track                                     July 1994


                       PPP Reliable Transmission

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

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   This document defines a method for negotiating and using Numbered-
   Mode, as defined by ISO 7776 [2], to provide a reliable serial link.

   This document is the product of the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the [e-mail] mailing list.

Table of Contents

   1.     Introduction ..........................................    1
   2.     Physical Layer Requirements ...........................    2
   3.     The Data Link Layer ...................................    2
   3.1       Frame Format .......................................    2
   4.     Configuration Option Format ...........................    4
   5.     Numbered-Mode Operation ...............................    5
   5.1       Single Link ........................................    6
   5.2       Inverse Multiplexing ...............................    6
   5.3       Using Multi-Link Procedure... ......................    7
   5.4       LAPB Parameter defaults ............................    8
   SECURITY CONSIDERATIONS ......................................    9
   REFERENCES ...................................................    9
   ACKNOWLEDGEMENTS .............................................    9
   CHAIR'S ADDRESS ..............................................   10
   AUTHOR'S ADDRESS .............................................   10







Rand                                                            [Page 1]

RFC 1663               PPP Reliable Transmission               July 1994


1.  Introduction

   By default, PPP packets over HDLC framed links consist of
   "connectionless" datagrams.  If reliable transmission over the HDLC
   link is desired, the implementation MUST specify the Numbered-Mode
   Configuration Option during Link Establishment phase.

   Generally, serial link reliability is not a major issue.  The
   architecture of protocols used in datagram networking presume
   best-effort non-sequential delivery.  When errors are detected,
   datagrams
   are discarded.

   However, in certain circumstances, it is advisable to provide a
   reliable link, at least for a subset of the messages.  The most
   obvious case is when the link is compressed.  Since the dictionary is
   recovered from the compressed data stream, and a lost datagram
   corrupts the dictionary, datagrams must not be lost.  Not all
   compression types will require a reliable data stream, since the cost
   to detect and reset a corrupt dictionary is small.

   The ISO 7776 LAPB can be used guarantee delivery.  This is referred
   to in this document as "Numbered Mode" to distinguish it from the use
   of "Unnumbered Information", which is standard PPP framing practice.

   Where multiple parallel links are used to emulate a single link of
   higher speed, Bridged traffic, Source Routed traffic, and traffic
   subjected to Van Jacobsen TCP/IP header compression must be delivered
   to the higher layer in a certain sequence.  However, the fact of the
   links being relatively asynchronous makes traffic ordering uncertain.

   The ISO 7776 Multi-Link Procedure MAY be used to restore order.
   Implementation of the ISO Multi-Link Procedure is deprecated.  It is
   recommended that the PPP multilink procedure [4] be used instead.

2.  Physical Layer Requirements

   PPP Reliable Transmission imposes the same requirements that are
   described in "PPP in HDLC Framing" [3], with the following
   exceptions.

   Control Signals

      While PPP does not normally require the use of control signals,
      implementation of Numbered-Mode LAPB or LAPD requires the
      provision of control signals, which indicate when the link has
      become connected or disconnected.  These in turn provide the Up
      and Down events to the LCP state machine.



Rand                                                            [Page 2]

RFC 1663               PPP Reliable Transmission               July 1994


3.  The Data Link Layer

   Numbered-Mode affects only the Address and Control fields.  The
   remainder of the frame conforms to the framing in use for PPP.

   The Address Field of the frame MUST take the value announced in the
   Numbered-Mode Configuration Option, and the Control Field MAY take
   any value valid in ISO 7776.

   Once the link enters Numbered-Mode, Numbered-Mode MUST be used on all
   frames, as some implementations do not support the use of the
   Unnumbered-Information control field or the use of the All-Stations
   address intermixed with Numbered-Mode frames.

3.1.  Frame Format

   The following frame format is valid under Numbered-Mode.  The fields
   are transmitted from left to right.

   Numbered Mode
           +----------+----------+----------+
           |   Flag   | Address  | Control  |
           | 01111110 |1-2 octets|1-2 octets|
           +----------+----------+----------+
           +----------+-------------+---------+
           | Protocol | Information | Padding |
           |1-2 octets|      *      |    *    |
           +----------+-------------+---------+
           +----------+----------+-----------------
           |   FCS    |   Flag   | Inter-frame Fill
           | 16 bits  | 01111110 | or next Address
           +----------+----------+-----------------

   The Protocol, Information and Padding fields are described in the
   Point-to-Point Protocol Encapsulation [1].  The FCS and Flag Sequence
   fields are described in "PPP in HDLC Framing" [3].

4.  Configuration Option Format

   Description

      The LCP Numbered-Mode Configuration Option negotiates the use of
      Numbered-Mode on the link.  By default or ultimate disagreement,
      Unnumbered-Mode is used.

   A summary of the Numbered-Mode Configuration Option format is shown
   below.  The fields are transmitted from left to right.




Rand                                                            [Page 3]

RFC 1663               PPP Reliable Transmission               July 1994


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |    Window     |   Address...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      11

   Length

      >= 4

   Window

      A value between 1 and 127.  This indicates the number of frames
      the receiver will buffer, which is the maximum number that the
      sender should send without receiving an acknowledgement.  If
      window < 8, then modulo 8 sequencing is used on the link.
      Otherwise, modulo 128 sequencing is used.

      It is conceivable and legal that differing window values might be
      announced.  However, it is not permitted for one system to use
      modulo 8 sequencing and the other to use modulo 128.  Therefore,
      the rule is: a Configure-Nak may reduce the window but may not
      increase it.

   Address

      An HDLC Address as specified in ISO 3309.  ISO 7776 specifies four
      of the possible values: 1 and 3 for single link operation, 7 and
      15 for the Multi-Link Procedure.  Other values consistent with ISO
      3309 are considered legal.

      Implementation of the Multi-Link Procedure is optional; A

      Configure-Nak may therefore force a change from MLP to single link
      mode, but not the reverse.

      Should the address be zero upon receipt, the receiver MUST
      Configure-Nak with an appropriate address.  If both peers send
      address zero, the system advertising the numerically smaller
      window will select the smaller address.  If both windows are the
      same size, a random choice MUST be made; when good sources of
      randomness are used, the link will converge in a reasonable time.





Rand                                                            [Page 4]