RFC 1663 (rfc1663) - Page 2 of 8
PPP Reliable Transmission
Alternative Format: Original Text Document
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