RFC 2117 (rfc2117) - Page 3 of 66

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

RFC 2117                         PIM-SM                       June 1997


   timers, flag bits, etc. The wildcard route entry's incoming interface
   points toward the RP; the outgoing interfaces point to the
   neighboring downstream routers that have sent Join/Prune messages
   toward the RP. This state creates a shared, RP-centered, distribution
   tree that reaches all group members. When a data source first sends
   to a group, its DR unicasts Register messages to the RP with the
   source's data packets encapsulated within. If the data rate is high,
   the RP can send source-specific Join/Prune messages back towards the
   source and the source's data packets will follow the resulting
   forwarding state and travel unencapsulated to the RP.  Whether they
   arrive encapsulated or natively, the RP forwards the source's
   decapsulated data packets down the RP-centered distribution tree
   toward group members.  If the data rate warrants it, routers with
   local receivers can join a source-specific, shortest path,
   distribution tree, and prune this source's packets off of the shared
   RP-centered tree. For low data rate sources, neither the RP, nor
   last-hop routers need join a source-specific shortest path tree and
   data packets can be delivered via the shared, RP-tree.

   The following subsections describe SM operation in more detail, in
   particular, the control messages, and the actions they trigger.

2.1 Local hosts joining a group


   In order to join a multicast group, G, a host conveys its membership
   information through the Internet Group Management Protocol (IGMP), as
   specified in [4][5], (see figure 1).  From this point on we refer to
   such a host as a receiver, R, (or member) of the group G.

   Note that all figures used in this section are for illustration and
   are not intended to be complete. For complete and detailed protocol
   action see Section 3.

      [Figures are present only in the postscript version]
      Fig. 1  Example: how a receiver joins, and sets up shared tree


   When a DR (e.g., router A in figure 1) gets a membership indication
   from IGMP for a new group, G, the DR looks up the associated RP. The
   DR creates a wildcard multicast route entry for the group, referred
   to here as a (*,G) entry; if there is no more specific match for a
   particular source, the packet will be forwarded according to this
   entry.







Estrin, et. al.               Experimental