RFC 1333 (rfc1333) - Page 2 of 15
PPP Link Quality Monitoring
Alternative Format: Original Text Document
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