RFC 2117 (rfc2117) - Page 2 of 66

Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification

RFC 2117                         PIM-SM                       June 1997


1 Introduction

   This document describes a protocol for efficiently routing to
   multicast groups that may span wide-area (and inter-domain)
   internets.  We refer to the approach as Protocol Independent
   Multicast--Sparse Mode (PIM-SM) because it is not dependent on any
   particular unicast routing protocol, and because it is designed to
   support sparse groups as defined in [1][2]. This document describes
   the protocol details.  For the motivation behind the design and a
   description of the architecture, see [1][2]. Section 2 summarizes
   PIM-SM operation.  It describes the protocol from a network
   perspective, in particular, how the participating routers interact to
   create and maintain the multicast distribution tree.  Section 3
   describes PIM-SM operations from the perspective of a single router
   implementing the protocol; this section constitutes the main body of
   the protocol specification.  It is organized according to PIM-SM
   message type; for each message type we describe its contents, its
   generation, and its processing.

   Sections 3.8 and 3.9 summarize the timers and flags referred to
   throughout this document. Section 4 provides packet format details.

   The most significant functional changes since the January '95 version
   involve the Rendezvous Point-related mechanisms, several resulting
   simplifications to the protocol, and removal of the PIM-DM protocol
   details to a separate document [3] (for clarity).

2 PIM-SM Protocol Overview

   In this section we provide an overview of the architectural
   components of PIM-SM.

   A router receives explicit Join/Prune messages from those neighboring
   routers that have downstream group members. The router then forwards
   data packets addressed to a multicast group, G, only onto those
   interfaces on which explicit joins have been received. Note that all
   routers mentioned in this document are assumed to be PIM-SM capable,
   unless otherwise specified.

   A Designated Router (DR) sends periodic Join/Prune messages toward a
   group-specific Rendezvous Point (RP) for each group for which it has
   active members. Each router along the path toward the RP builds a
   wildcard (any-source) state for the group and sends Join/Prune
   messages on toward the RP. We use the term route entry to refer to
   the state maintained in a router to represent the distribution tree.
   A route entry may include such fields as the source address, the
   group address, the incoming interface from which packets are
   accepted, the list of outgoing interfaces to which packets are sent,



Estrin, et. al.               Experimental