RFC 1333 (rfc1333) - Page 2 of 15

PPP Link Quality Monitoring

RFC 1333              PPP Link Quality Monitoring               May 1992


2.  Link Quality Monitoring

   Data communications links are rarely perfect.  Packets can be dropped
   or corrupted for various reasons (line noise, equipment failure,
   buffer overruns, etc.).  Sometimes, it is desirable to determine
   when, and how often, the link is dropping data.  Routers, for
   example, may want to temporarily allow another route to take
   precedence.  An implementation may also have the option of
   disconnecting and switching to an alternate link.  The process of
   determining data loss is called "Link Quality Monitoring".

2.1.  Design Motivation

   There are many different ways to measure link quality, and even more
   ways to react to it.  Rather than specifying a single scheme, Link
   Quality Monitoring is divided into a "mechanism" and a "policy".  PPP
   fully specifies the "mechanism" for Link Quality Monitoring by
   defining the Link-Quality-Report (LQR) packet and specifying a
   procedure for its use.  PPP does NOT specify a Link Quality
   Monitoring "policy" -- how to judge link quality or what to do when
   it is inadequate.  That is left as an implementation decision, and
   can be different at each end of the link.  Implementations are
   allowed, and even encouraged, to experiment with various link quality
   policies.  The Link Quality Monitoring mechanism specification
   insures that two implementations with different policies may
   communicate and interoperate.

   To allow flexible policies to be implemented, the PPP Link Quality
   Monitoring mechanism measures data loss in units of packets, octets,
   and Link-Quality-Reports.  Each measurement is made separately for
   each half of the link, both inbound and outbound.  All measurements
   are communicated to both ends of the link so that each end of the
   link can implement its own link quality policy for both its outbound
   and inbound links.

   Finally, the Link Quality Monitoring protocol is designed to be
   implementable on many different kinds of systems.  Although it may be
   common to implement PPP (and especially Link Quality Monitoring) as a
   single software process, multi-process implementations with hardware
   support are also envisioned.  The PPP Link Quality Monitoring
   mechanism provides for this by careful definition of the Link-
   Quality-Report packet format, and by specifying reference points for
   all data transmission and reception measurements.

2.2.  Counters

   Each Link Quality Monitoring implementation maintains counts of the
   number of packets and octets transmitted and successfully received,



Simpson