RFC 3208 (rfc3208) - Page 3 of 111

PGM Reliable Transport Protocol Specification

RFC 3208            PGM Reliable Transport Protocol        December 2001


1.  Introduction and Overview

   A variety of reliable protocols have been proposed for multicast data
   delivery, each with an emphasis on particular types of applications,
   network characteristics, or definitions of reliability ([1], [2],
   [3], [4]).  In this tradition, Pragmatic General Multicast (PGM) is a
   reliable transport protocol for applications that require ordered or
   unordered, duplicate-free, multicast data delivery from multiple
   sources to multiple receivers.

   PGM is specifically intended as a workable solution for multicast
   applications with basic reliability requirements rather than as a
   comprehensive solution for multicast applications with sophisticated
   ordering, agreement, and robustness requirements.  Its central design
   goal is simplicity of operation with due regard for scalability and
   network efficiency.

   PGM has no notion of group membership.  It simply provides reliable
   multicast data delivery within a transmit window advanced by a source
   according to a purely local strategy.  Reliable delivery is provided
   within a source's transmit window from the time a receiver joins the
   group until it departs.  PGM guarantees that a receiver in the group
   either receives all data packets from transmissions and repairs, or
   is able to detect unrecoverable data packet loss.  PGM supports any
   number of sources within a multicast group, each fully identified by
   a globally unique Transport Session Identifier (TSI), but since these
   sources/sessions operate entirely independently of each other, this
   specification is phrased in terms of a single source and extends
   without modification to multiple sources.

   More specifically, PGM is not intended for use with applications that
   depend either upon acknowledged delivery to a known group of
   recipients, or upon total ordering amongst multiple sources.

   Rather, PGM is best suited to those applications in which members may
   join and leave at any time, and that are either insensitive to
   unrecoverable data packet loss or are prepared to resort to
   application recovery in the event.  Through its optional extensions,
   PGM provides specific mechanisms to support applications as disparate
   as stock and news updates, data conferencing, low-delay real-time
   video transfer, and bulk data transfer.

   In the following text, transport-layer originators of PGM data
   packets are referred to as sources, transport-layer consumers of PGM
   data packets are referred to as receivers, and network-layer entities
   in the intervening network are referred to as network elements.





Speakman, et. al.             Experimental