rfc2117.txt

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Network Working Group                                       D.  Estrin
Request for Comments: 2117                                         USC
Category: Experimental                                    D. Farinacci
                                                                 CISCO
                                                              A. Helmy
                                                                   USC
                                                             D. Thaler
                                                                 UMICH
                                                            S. Deering
                                                                 XEROX
                                                            M. Handley
                                                                   UCL
                                                           V. Jacobson
                                                                   LBL
                                                                C. Liu
                                                                   USC
                                                             P. Sharma
                                                                   USC
                                                                L. Wei
                                                                 CISCO
                                                             June 1997



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

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Acknowledgements

   The author list has been reordered to reflect the involvement in
   detailed editorial work on this specification document.  The first
   four authors are the primary editors and are listed alphabetically.
   The rest of the authors, also listed alphabetically, participated in
   all aspects of the architectural and detailed design but managed to
   get away without hacking the latex!









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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,



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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.







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RFC 2117                         PIM-SM                       June 1997


   The RP address is included in a special field in the route entry and
   is included in periodic upstream Join/Prune messages. The outgoing
   interface is set to that included in the IGMP membership indication
   for the new member.  The incoming interface is set to the interface
   used to send unicast packets to the RP.

   When there are no longer directly connected members for the group,
   IGMP notifies the DR.  If the DR has neither local members nor
   downstream receivers, the (*,G) state is deleted.

2.2 Establishing the RP-rooted shared tree

   Triggered by the (*,G) state, the DR creates a Join/Prune message
   with the RP address in its join list and the the wildcard bit (WC-
   bit) and RP-tree bit (RPT-bit) set to 1. The WC-bit indicates that
   any source may match and be forwarded according to this entry if
   there is no longer match; the RPT-bit indicates that this join is
   being sent up the shared, RP-tree. The prune list is left empty. When
   the RPT-bit is set to 1 it indicates that the join is associated with
   the shared RP-tree and therefore the Join/Prune message is propagated
   along the RP-tree. When the WC-bit is set to 1 it indicates that the
   address is an RP and the downstream receivers expect to receive
   packets from all sources via this (shared tree) path. The term RPT-
   bit is used to refer to both the RPT-bit flags associated with route
   entries, and the RPT-bit included in each encoded address in a
   Join/Prune message.

   Each upstream router creates or updates its multicast route entry for
   (*,G) when it receives a Join/Prune with the RPT-bit and WC-bit set.
   The interface on which the Join/Prune message arrived is added to the
   list of outgoing interfaces (oifs) for (*,G). Based on this entry
   each upstream router between the receiver and the RP sends a
   Join/Prune message in which the join list includes the RP. The packet
   payload contains Multicast-Address=G, Join=RP,WC-bit,RPT-bit,
   Prune=NULL.

2.3 Hosts sending to a group

   When a host starts sending multicast data packets to a group,
   initially its DR must deliver each packet to the RP for distribution
   down the RP-tree (see figure 2).  The sender's DR initially
   encapsulates each data packet in a Register message and unicasts it
   to the RP for that group. The RP decapsulates each Register message
   and forwards the enclosed data packet natively to downstream members
   on the shared RP-tree.

      [Figures are present only in the postscript version]
      Fig. 2  Example: a host sending to a group



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RFC 2117                         PIM-SM                       June 1997


   If the data rate of the source warrants the use of a source-specific
   shortest path tree (SPT), the RP may construct a new multicast route
   entry that is specific to the source, hereafter referred to as (S,G)
   state, and send periodic Join/Prune messages toward the source. Note
   that over time, the rules for when to switch can be modified without
   global coordination.  When and if the RP does switch to the SPT, the
   routers between the source and the RP build and maintain (S,G) state
   in response to these messages and send (S,G) messages upstream toward
   the source.

   The source's DR must stop encapsulating data packets in Registers
   when (and so long as) it receives Register-Stop messages from the RP.
   The RP triggers Register-Stop messages in response to Registers, if
   the RP has no downstream receivers for the group (or for that
   particular source), or if the RP has already joined the (S,G) tree
   and is receiving the data packets natively.  Each source's DR
   maintains, per (S,G), a Register-Suppression-timer.  The Register-
   Suppression-timer is started by the Register-Stop message; upon
   expiration, the source's DR resumes sending data packets to the RP,
   encapsulated in Register messages.

2.4 Switching from shared tree (RP-tree)  to  shortest  path  tree  (SP-
      tree)

   A router with directly-connected members first joins the shared RP-
   tree.  The router can switch to a source's shortest path tree (SP-
   tree) after receiving packets from that source over the shared RP-
   tree. The recommended policy is to initiate the switch to the SP-tree
   after receiving a significant number of data packets during a
   specified time interval from a particular source. To realize this
   policy the router can monitor data packets from sources for which it
   has no source-specific multicast route entry and initiate such an
   entry when the data rate exceeds the configured threshold.  As shown
   in figure 3, router `A' initiates a (S,G) state.

      [Figures are present only in the postscript version]
      Fig. 3  Example: Switching from shared tree to shortest path tree

   When a (S,G) entry is activated (and periodically so long as the
   state exists), a Join/Prune message is sent upstream towards the
   source, S, with S in the join list. The payload contains Multicast-
   Address=G, Join=S, Prune=NULL. When the (S,G) entry is created, the
   outgoing interface list is copied from (*,G), i.e., all local shared
   tree branches are replicated in the new shortest path tree. In this
   way when a data packet from S arrives and matches on this entry, all
   receivers will continue to receive the source's packets along this