RFC 1663 (rfc1663) - Page 2 of 8

PPP Reliable Transmission

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