RFC 1989 (rfc1989) - Page 2 of 16

PPP Link Quality Monitoring

RFC 1989              PPP Link Quality Monitoring            August 1996


1.  Introduction

   In order to establish communications over a point-to-point link, each
   end of the PPP link must first send LCP packets to configure the data
   link during Link Establishment phase.  During the Authentication and
   Network-Layer Protocol phases, the link may be tested to determine if
   quality is sufficient for operation.  This testing is completely
   optional.

   If an implementation desires that the peer use some specific link
   quality monitoring protocol, then it MUST negotiate the use of that
   protocol using the Quality-Protocol Configuration Option during Link
   Establishment phase.

   The negotiation mechanism is independent in each direction.  However,
   if the peer agrees to send Quality-Protocol packets, it MUST
   correctly process such packets on reception, even if it does not
   request such packets or implement a monitoring policy.

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.  For example, routers
   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 ensures that two implementations
   with different policies may communicate and interoperate.





Simpson                     Standards Track