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RFC 2117

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

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

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

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.


   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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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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   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

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   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,

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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   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-

   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
   path. (In more complicated scenarios, other entries in the router
   have to be considered, as described in Section 3). Note that (S,G)

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   state must be maintained in each last-hop router that is responsible
   for initiating and maintaining an SP-tree. Even when (*,G) and (S,G)
   overlap, both states are needed to trigger the source-specific
   Join/Prune messages.  (S,G) state is kept alive by data packets
   arriving from that source. A timer, Entry-timer, is set for the (S,G)
   entry and this timer is restarted whenever data packets for (S,G) are
   forwarded out at least one oif, or Registers are sent.  When the
   Entry-timer expires, the state is deleted. The last-hop router is the
   router that delivers the packets to their ultimate end-system
   destination.  This is the router that monitors if there is group
   membership and joins or prunes the appropriate distribution trees in
   response.  In general the last-hop router is the Designated Router
   (DR) for the LAN. However, under various conditions described later,
   a parallel router connected to the same LAN may take over as the
   last-hop router in place of the DR.

   Only the RP and routers with local members can initiate switching to
   the SP-tree; intermediate routers do not. Consequently, last-hop
   routers create (S,G) state in response to data packets from the
   source, S; whereas intermediate routers only create (S,G) state in
   response to Join/Prune messages from downstream that have S in the
   Join list.

   The (S,G) entry is initialized with the SPT-bit cleared, indicating
   that the shortest path tree branch from S has not yet been setup
   completely, and the router can still accept packets from S that
   arrive on the (*,G) entry's indicated incoming interface (iif). Each
   PIM multicast entry has an associated incoming interface on which
   packets are expected to arrive.

   When a router with a (S,G) entry and a cleared SPT-bit starts to
   receive packets from the new source S on the iif for the (S,G) entry,
   and that iif differs from the (*,G) entry's iif, the router sets the
   SPT-bit, and sends a Join/Prune message towards the RP, indicating
   that the router no longer wants to receive packets from S via the
   shared RP-tree. The Join/Prune message sent towards the RP includes S
   in the prune list, with the RPT-bit set indicating that S's packets
   must not be forwarded down this branch of the shared tree. If the
   router receiving the Join/Prune message has (S,G) state (with or
   without the route entry's RPT-bit flag set), it deletes the arriving
   interface from the (S,G) oif list.  If the router has only (*,G)
   state, it creates an entry with the RPT-bit flag set to 1. For

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   brevity we refer to an (S,G) entry that has the RPT-bit flag set to 1
   as an (S,G)RPT-bit entry. This notational distinction is useful to
   point out the different actions taken for (S,G) entries depending on
   the setting of the RPT-bit flag. Note that a router can have no more
   than one active (S,G) entry for any particular S and G, at any
   particular time; whether the RPT-bit flag is set or not. In other
   words, a router never has both an (S,G) and an (S,G)RPT-bit entry for
   the same S and G at the same time. The Join/Prune message payload
   contains Multicast-Address=G, Join=NULL, Prune=S,RPT-bit.

   A new receiver may join an existing RP-tree on which source-specific
   prune state has been established (e.g., because downstream receivers
   have switched to SP-trees). In this case the prune state must be
   eradicated upstream of the new receiver to bring all sources' data
   packets down to the new receiver.  Therefore, when a (*,G) Join
   arrives at a router that has any (Si,G)RPT-bit entries (i.e., entries
   that cause the router to send source-specific prunes toward the RP),
   these entries must be updated upstream of the router so as to bring
   all sources' packets down to the new member. To accomplish this, each
   router that receives a (*,G) Join/Prune message updates all existing
   (S,G)RPT-bit entries. The router may also trigger a (*,G) Join/Prune
   message upstream to cause the same updating of RPT-bit settings
   upstream and pull down all active sources' packets. If the arriving
   (*,G) join has some sources included in its prune list, then the
   corresponding (S,G)RPT-bit entries are left unchanged (i.e., the
   RPT-bit remains set and no oif is added).

2.5 Steady state maintenance of distribution tree (i.e., router state)

   In the steady state each router sends periodic Join/Prune messages
   for each active PIM route entry; the Join/Prune messages are sent to
   the neighbor indicated in the corresponding entry. These messages are
   sent periodically to capture state, topology, and membership changes.
   A Join/Prune message is also sent on an event-triggered basis each
   time a new route entry is established for some new source (note that
   some damping function may be applied, e.g., a short delay to allow
   for merging of new Join information). Join/Prune messages do not
   elicit any form of explicit acknowledgment; routers recover from lost
   packets using the periodic refresh mechanism.

2.6 Obtaining RP information

   To obtain the RP information, all routers within a PIM domain collect
   Bootstrap messages. Bootstrap messages are sent hop-by-hop within the
   domain; the domain's bootstrap router (BSR) is responsible for
   originating the Bootstrap messages. Bootstrap messages are used to
   carry out a dynamic BSR election when needed and to distribute RP
   information in steady state.

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   A domain in this context is a contiguous set of routers that all
   implement PIM and are configured to operate within a common boundary
   defined by PIM Multicast Border Routers (PMBRs). PMBRs connect each
   PIM domain to the rest of the internet.

   Routers use a set of available RPs (called the {RP-Set}) distributed
   in Bootstrap messages to get the proper Group to RP mapping. The
   following paragraphs summarize the mechanism; details of the
   mechanism may be found in Sections 3.6 and Appendix 6.2. A (small)
   set of routers, within a domain, are configured as candidate BSRs
   and, through a simple election mechanism, a single BSR is selected
   for that domain. A set of routers within a domain are also configured
   as candidate RPs (C-RPs); typically these will be the same routers
   that are configured as C-BSRs.  Candidate RPs periodically unicast
   Candidate-RP-Advertisement messages (C-RP-Advs) to the BSR of that
   domain. C-RP-Advs include the address of the advertising C-RP, as
   well as an optional group address and a mask length field, indicating
   the group prefix(es) for which the candidacy is advertised.  The BSR
   then includes a set of these Candidate-RPs (the RP-Set), along with
   the corresponding group prefixes, in Bootstrap messages it
   periodically originates.  Bootstrap messages are distributed hop-by-
   hop throughout the domain.

   Routers receive and store Bootstrap messages originated by the BSR.
   When a DR gets a membership indication from IGMP for (or a data
   packet from) a directly connected host, for a group for which it has
   no entry, the DR uses a hash function to map the group address to one
   of the C-RPs whose Group-prefix includes the group (see Section 3.7).
   The DR then sends a Join/Prune message towards (or unicasts Registers
   to) that RP.

   The Bootstrap message indicates liveness of the RPs included therein.
   If an RP is included in the message, then it is tagged as `up' at the
   routers; while RPs not included in the message are removed from the
   list of RPs over which the hash algorithm acts. Each router continues
   to use the contents of the most recently received Bootstrap message
   until it receives a new Bootstrap message.

   If a PIM domain partitions, each area separated from the old BSR will
   elect its own BSR, which will distribute an RP-Set containing RPs
   that are reachable within that partition. When the partition heals,
   another election will occur automatically and only one of the BSRs
   will continue to send out Bootstrap messages. As is expected at the
   time of a partition or healing, some disruption in packet delivery
   may occur.  This time will be on the order of the region's round-trip
   time and the bootstrap router timeout value.

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2.7 Interoperation with dense mode  protocols such as DVMRP

   In order to interoperate with networks that run dense-mode,
   {broadcast and prune}, protocols, such as DVMRP, all packets
   generated within a PIM-SM region must be pulled out to that region's
   PIM Multicast Border Routers (PMBRs) and injected (i.e., broadcast)
   into the DVMRP network.  A PMBR is a router that sits at the boundary
   of a PIM-SM domain and interoperates with other types of multicast
   routers such as those that run DVMRP.  Generally a PMBR would speak
   both protocols and implement interoperability functions not required
   by regular PIM routers. To support interoperability, a special entry
   type, referred to as (*,*,RP), must be supported by all PIM routers.
   For this reason we include details about (*,*,RP) entry handling in
   this general PIM specification.

   A data packet will match on a (*,*,RP) entry if there is no more
   specific entry (such as (S,G) or (*,G)) and the destination group
   address in the packet maps to the RP listed in the (*,*,RP) entry. In
   this sense, a (*,*,RP) entry represents an aggregation of all the
   groups that hash to that RP. PMBRs initialize (*,*,RP) state for each
   RP in the domain's RPset. The (*,*,RP) state causes the PMBRs to send
   (*,*,RP) Join/Prune messages toward each of the active RPs in the
   domain.  As a result distribution trees are built that carry all data
   packets originated within the PIM domain (and sent to the RPs) down
   to the PMBRs.

   PMBRs are also responsible for delivering externally-generated
   packets to routers within the PIM domain. To do so, PMBRs initially
   encapsulate externally-originated packets (i.e., received on DVMRP
   interfaces) in Register messages and unicast them to the
   corresponding RP within the PIM domain. The Register message has a
   bit indicating that it was originated by a border router and the RP
   caches the originating PMBR's address in the route entry so that
   duplicate Registers from other PMBRs can be declined with a
   Register-Stop message.

   All PIM routers must be capable of supporting (*,*,RP) state and
   interpreting associated Join/Prune messages. We describe the handling
   of (*,*,RP) entries and messages throughout this document; however,
   detailed PIM Multicast Border Router (PMBR) functions will be
   specified in a separate interoperability document (see directory,

2.8 Multicast data packet processing

   Data packets are processed in a manner similar to other multicast
   schemes.  A router first performs a longest match on the source and
   group address in the data packet. A (S,G) entry is matched first if

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   one exists; a (*,G) entry is matched otherwise. If neither state
   exists, then a (*,*,RP) entry match is attempted as follows: the
   router hashes on G to identify the RP for group G, and looks for a
   (*,*,RP) entry that has this RP address associated with it.  If none
   of the above exists, then the packet is dropped. If a state is
   matched, the router compares the interface on which the packet
   arrived to the incoming interface field in the matched route entry.
   If the iif check fails the packet is dropped, otherwise the packet is
   forwarded to all interfaces listed in the outgoing interface list.

   Some special actions are needed to deliver packets continuously while
   switching from the shared to shortest-path tree. In particular, when
   a (S,G) entry is matched, incoming packets are forwarded as follows:

      1    If the SPT-bit is set, then:

           1    if the incoming interface is the same as a matching
                (S,G) iif, the packet is forwarded to the oif-list of

           2    if the incoming interface is different than a matching
                (S,G) iif , the packet is discarded.

      2    If the SPT-bit is cleared, then:

           1    if the incoming interface is the same as a matching
                (S,G) iif, the packet is forwarded to the oif-list of
                (S,G). In addition, the SPT bit is set for that entry
                if the incoming interface differs from the incoming
                interface of the (*,G) or (*,*,RP) entry.

           2    if the incoming interface is different than a matching
                (S,G) iif, the incoming interface is tested against a
                matching (*,G) or (*,*,RP) entry. If the iif is the
                same as one of those, the packet is forwarded to the
                oif-list of the matching entry.

           3    Otherwise the iif does not match any entry for G and
                the packet is discarded.

   Data packets never trigger prunes.  However, data packets may trigger
   actions that in turn trigger prunes. For example, when router B in
   figure 3 decides to switch to SP-tree at step 3, it creates a (S,G)
   entry with SPT-bit set to 0. When data packets from S arrive at

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   interface 2 of B, B sets the SPT-bit to 1 since the iif for (*,G) is
   different than that for (S,G). This triggers the sending of prunes
   towards the RP.

2.9 Operation over Multi-access Networks

   This section describes a few additional protocol mechanisms needed to
   operate PIM over multi-access networks: Designated Router election,
   Assert messages to resolve parallel paths, and the Join/Prune-
   Suppression-Timer to suppress redundant Joins on multi-access

   * Designated router election

   When there are multiple routers connected to a multi-access network,
   one of them must be chosen to operate as the designated router (DR)
   at any point in time.  The DR is responsible for sending triggered
   Join/Prune and Register messages toward the RP.

   A simple designated router (DR) election mechanism is used for both
   SM and traditional IP multicast routing.  Neighboring routers send
   Hello messages to each other. The sender with the largest IP address
   assumes the role of DR. Each router connected to the multi-access LAN
   sends the Hellos periodically in order to adapt to changes in router

   * Parallel paths to a source or the RP--Assert process

   If a router receives a multicast datagram on a multi-access LAN from
   a source whose corresponding (S,G) outgoing interface list includes
   the interface to that LAN, the packet must be a duplicate.  In this
   case a single forwarder must be elected.  Using Assert messages
   addressed to `' (ALL-PIM-ROUTERS group) on the LAN,
   upstream routers can resolve which one will act as the forwarder.
   Downstream routers listen to the Asserts so they know which one was
   elected, and therefore where to send subsequent Joins. Typically this
   is the same as the downstream router's RPF (Reverse Path Forwarding)
   neighbor; but there are circumstances where this might not be the
   case, e.g., when using multiple unicast routing protocols on that
   LAN. The RPF neighbor for a particular source (or RP) is the next-hop
   router to which packets are forwarded en route to that source (or
   RP); and therefore is considered a good path via which to accept
   packets from that source.

   The upstream router elected is the one that has the shortest distance
   to the source. Therefore, when a packet is received on an outgoing
   interface a router sends an Assert message on the multi-access LAN
   indicating what metric it uses to reach the source of the data

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   packet.  The router with the smallest numerical metric (with ties
   broken by highest address) will become the forwarder. All other
   upstream routers will delete the interface from their outgoing
   interface list. The downstream routers also do the comparison in case
   the forwarder is different than the RPF neighbor.

   Associated with the metric is a metric preference value. This is
   provided to deal with the case where the upstream routers may run
   different unicast routing protocols. The numerically smaller metric
   preference is always preferred. The metric preference is treated as
   the high-order part of an assert metric comparison.  Therefore, a
   metric value can be compared with another metric value provided both
   metric preferences are the same.  A metric preference can be assigned
   per unicast routing protocol and needs to be consistent for all
   routers on the multi-access network.

   Asserts are also needed for (*,G) entries since an RP-Tree and an
   SP-Tree for the same group may both cross the same multi- access
   network. When an assert is sent for a (*,G) entry, the first bit in
   the metric preference (RPT-bit) is always set to 1 to indicate that
   this path corresponds to the RP tree, and that the match must be done
   on (*,G) if it exists. Furthermore, the RPT-bit is always cleared for
   metric preferences that refer to SP-tree entries; this causes an SP-
   tree path to always look better than an RP-tree path. When the SP-
   tree and RPtree cross the same LAN, this mechanism eliminates the
   duplicates that would otherwise be carried over the LAN.

   In case the packet, or the Assert message, matches  on  oif  for
   (*,*,RP) entry, a (*,G) entry is created, and asserts take place as
   if the matching state were (*,G).

   The DR may lose the (*,G) Assert process to another router on the LAN
   if there are multiple paths to the RP through the LAN.  From then on,
   the DR is no longer the last-hop router for local receivers and
   removes the LAN from its (*,G) oif list. The winning router becomes
   the last-hop router and is responsible for sending (*,G) join
   messages to the RP.

   * Join/Prune suppression

   Join/Prune suppression may be used on multi-access LANs to reduce
   duplicate control message overhead; it is not required for correct
   performance of the protocol. If a Join/Prune message arrives and
   matches on the incoming interface for an existing (S,G), (*,G), or
   (*,*,RP) route entry, and the Holdtime included in the Join/Prune
   message is greater than the recipient's own [Join/Prune-Holdtime]
   (with ties resolved in favor of the higher IP address), a timer (the
   Join/Prune-Suppression-timer) in the recipient's route entry may be

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   started to suppress further Join/Prune messages.  After this timer
   expires, the recipient triggers a Join/Prune message, and resumes
   sending periodic Join/Prunes, for this entry. The Join/Prune-
   Suppression-timer should be restarted each time a Join/Prune message
   is received with a higher Holdtime.

2.10 Unicast Routing Changes

   When unicast routing changes, an RPF check is done on all active
   (S,G), (*,G) and (*,*,RP) entries, and all affected expected incoming
   interfaces are updated.  In particular, if the new incoming interface
   appears in the outgoing interface list, it is deleted from the
   outgoing interface list. The previous incoming interface may be added
   to the outgoing interface list by a subsequent Join/Prune from
   downstream.  Join/Prune messages received on the current incoming
   interface are ignored.  Join/Prune messages received on new
   interfaces or existing outgoing interfaces are not ignored. Other
   outgoing interfaces are left as is until they are explicitly pruned
   by downstream routers or are timed out due to lack of appropriate
   Join/Prune messages. If the router has a (S,G) entry with the SPT-bit
   set, and the updated iif(S,G) does not differ from iif(*,G) or
   iif(*,*,RP), then the router resets the SPT-bit.

   The router must send a Join/Prune message with S in the Join list out
   any new incoming interfaces to inform upstream routers that it
   expects multicast datagrams over the interface.  It may also send a
   Join/Prune message with S in the Prune list out the old incoming
   interface, if the link is operational, to inform upstream routers
   that this part of the distribution tree is going away.

2.11 PIM-SM for Inter-Domain Multicast

   Future documents will address the use of PIM-SM as a backbone inter-
   domain multicast routing protocol. Design choices center primarily
   around the distribution and usage of RP information for wide area,
   inter-domain groups.

2.12 Security

   All PIM control messages may use IPsec [6] to address security
   concerns.  Security mechanisms are likely to be enhanced in the near

(page 13 continued on part 2)

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