RFC 2117 (rfc2117) - Page 2 of 66
Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification
Alternative Format: Original Text Document
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
