





Network Working Group                                           A. Adams
Request for Comments: 3973                          NextHop Technologies
Category: Experimental                                       J. Nicholas
                                                                ITT A/CD
                                                               W. Siadak
                                                    NextHop Technologies
                                                            January 2005


         Protocol Independent Multicast - Dense Mode (PIM-DM):
                    Protocol Specification (Revised)

Status of This Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies Protocol Independent Multicast - Dense Mode
   (PIM-DM).  PIM-DM is a multicast routing protocol that uses the
   underlying unicast routing information base to flood multicast
   datagrams to all multicast routers.  Prune messages are used to
   prevent future messages from propagating to routers without group
   membership information.




















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . .  4
       2.2.  Pseudocode Notation  . . . . . . . . . . . . . . . . . .  5
   3.  PIM-DM Protocol Overview . . . . . . . . . . . . . . . . . . .  5
   4.  Protocol Specification . . . . . . . . . . . . . . . . . . . .  6
       4.1.  PIM Protocol State . . . . . . . . . . . . . . . . . . .  7
             4.1.1.  General Purpose State  . . . . . . . . . . . . .  7
             4.1.2.  (S,G) State  . . . . . . . . . . . . . . . . . .  8
             4.1.3.  State Summarization Macros . . . . . . . . . . .  8
       4.2.  Data Packet Forwarding Rules . . . . . . . . . . . . . . 10
       4.3.  Hello Messages . . . . . . . . . . . . . . . . . . . . . 11
             4.3.1.  Sending Hello Messages . . . . . . . . . . . . . 11
             4.3.2.  Receiving Hello Messages . . . . . . . . . . . . 11
             4.3.3.  Hello Message Hold Time  . . . . . . . . . . . . 12
             4.3.4.  Handling Router Failures . . . . . . . . . . . . 12
             4.3.5.  Reducing Prune Propagation Delay on LANs . . . . 13
       4.4.  PIM-DM Prune, Join, and Graft Messages . . . . . . . . . 13
             4.4.1.  Upstream Prune, Join, and Graft Messages . . . . 14
                     4.4.1.1.  Transitions from the Forwarding
                               (F) State  . . . . . . . . . . . . . . 17
                     4.4.1.2.  Transitions from the Pruned
                               (P) State  . . . . . . . . . . . . . . 18
                     4.4.1.3.  Transitions from the AckPending
                               (AP) State . . . . . . . . . . . . . . 19
             4.4.2.  Downstream Prune, Join, and Graft Messages . . . 21
                     4.4.2.1.  Transitions from the NoInfo State  . . 23
                     4.4.2.2.  Transitions from the PrunePending
                               (PP) State . . . . . . . . . . . . . . 24
                     4.4.2.3.  Transitions from the Prune
                               (P) State  . . . . . . . . . . . . . . 25
       4.5.  State Refresh  . . . . . . . . . . . . . . . . . . . . . 26
             4.5.1.  Forwarding of State Refresh Messages . . . . . . 26
             4.5.2.  State Refresh Message Origination  . . . . . . . 28
                     4.5.2.1.  Transitions from the NotOriginator
                               (NO) State . . . . . . . . . . . . . . 29
                     4.5.2.2.  Transitions from the Originator
                               (O) State  . . . . . . . . . . . . . . 29











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       4.6.  PIM Assert Messages  . . . . . . . . . . . . . . . . . . 30
             4.6.1.  Assert Metrics . . . . . . . . . . . . . . . . . 30
             4.6.2.  AssertCancel Messages  . . . . . . . . . . . . . 31
             4.6.3.  Assert State Macros  . . . . . . . . . . . . . . 32
             4.6.4.  (S,G) Assert Message State Machine . . . . . . . 32
                     4.6.4.1.  Transitions from NoInfo State  . . . . 34
                     4.6.4.2.  Transitions from Winner State  . . . . 35
                     4.6.4.3.  Transitions from Loser State . . . . . 36
             4.6.5.  Rationale for Assert Rules . . . . . . . . . . . 38
       4.7.  PIM Packet Formats . . . . . . . . . . . . . . . . . . . 38
             4.7.1.  PIM Header . . . . . . . . . . . . . . . . . . . 38
             4.7.2.  Encoded Unicast Address  . . . . . . . . . . . . 39
             4.7.3.  Encoded Group Address  . . . . . . . . . . . . . 40
             4.7.4.  Encoded Source Address . . . . . . . . . . . . . 41
             4.7.5.  Hello Message Format . . . . . . . . . . . . . . 42
                     4.7.5.1.  Hello Hold Time Option . . . . . . . . 43
                     4.7.5.2.  LAN Prune Delay Option . . . . . . . . 43
                     4.7.5.3.  Generation ID Option . . . . . . . . . 44
                     4.7.5.4.  State Refresh Capable Option . . . . . 44
             4.7.6.  Join/Prune Message Format  . . . . . . . . . . . 45
             4.7.7.  Assert Message Format  . . . . . . . . . . . . . 47
             4.7.8.  Graft Message Format . . . . . . . . . . . . . . 48
             4.7.9.  Graft Ack Message Format . . . . . . . . . . . . 48
             4.7.10. State Refresh Message Format . . . . . . . . . . 48
       4.8.  PIM-DM Timers  . . . . . . . . . . . . . . . . . . . . . 50
   5.  Protocol Interaction Considerations  . . . . . . . . . . . . . 53
       5.1.  PIM-SM Interactions  . . . . . . . . . . . . . . . . . . 53
       5.2.  IGMP Interactions  . . . . . . . . . . . . . . . . . . . 54
       5.3.  Source Specific Multicast (SSM) Interactions . . . . . . 54
       5.4.  Multicast Group Scope Boundary Interactions  . . . . . . 54
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 54
       6.1.  PIM Address Family . . . . . . . . . . . . . . . . . . . 54
       6.2.  PIM Hello Options  . . . . . . . . . . . . . . . . . . . 55
   7.  Security Considerations. . . . . . . . . . . . . . . . . . . . 55
       7.1.  Attacks Based on Forged Messages . . . . . . . . . . . . 55
       7.2.  Non-cryptographic Authentication Mechanisms  . . . . . . 56
       7.3.  Authentication Using IPsec . . . . . . . . . . . . . . . 56
       7.4.  Denial of Service Attacks  . . . . . . . . . . . . . . . 58
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 58
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 58
       9.2.  Informative References . . . . . . . . . . . . . . . . . 59
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 60
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 61







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1.  Introduction

   This specification defines a multicast routing algorithm for
   multicast groups that are densely distributed across a network.  This
   protocol does not have a topology discovery mechanism often used by a
   unicast routing protocol.  It employs the same packet formats sparse
   mode PIM (PIM-SM) uses.  This protocol is called PIM - Dense Mode.
   The foundation of this design was largely built on Deering's early
   work on IP multicast routing [12].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to
   be interpreted as described in RFC 2119 [11] and indicate requirement
   levels for compliant PIM-DM implementations.

2.1.  Definitions

   Multicast Routing Information Base (MRIB)
     This is the multicast topology table, which is typically derived
     from the unicast routing table, or from routing protocols such as
     MBGP that carry multicast-specific topology information.  PIM-DM
     uses the MRIB to make decisions regarding RPF interfaces.

   Tree Information Base (TIB)
     This is the collection of state maintained by a PIM router and
     created by receiving PIM messages and IGMP information from local
     hosts.  It essentially stores the state of all multicast
     distribution trees at that router.

   Reverse Path Forwarding (RPF)
     RPF is a multicast forwarding mode in which a data packet is
     accepted for forwarding only if it is received on an interface used
     to reach the source in unicast.

   Upstream Interface
     Interface toward the source of the datagram.  Also known as the RPF
     Interface.

   Downstream Interface
     All interfaces that are not the upstream interface, including the
     router itself.

   (S,G) Pair
     Source S and destination group G associated with an IP packet.





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2.2.  Pseudocode Notation

   We use set notation in several places in this specification.

   A (+) B
     is the union of two sets, A and B.

   A (-) B
     are the elements of set A that are not in set B.

   NULL
     is the empty set or list.

   Note that operations MUST be conducted in the order specified.  This
   is due to the fact that (-) is not a true difference operator,
   because B is not necessarily a subset of A.  That is, A (+) B (-) C =
   A (-) C (+) B is not a true statement unless C is a subset of both A
   and B.

   In addition, we use C-like syntax:

     =   denotes assignment of a variable.
     ==  denotes a comparison for equality.
     !=  denotes a comparison for inequality.

   Braces { and } are used for grouping.

3.  PIM-DM Protocol Overview

   This section provides an overview of PIM-DM behavior.  It is intended
   as an introduction to how PIM-DM works and is NOT definitive.  For
   the definitive specification, see Section 4, Protocol Specification.

   PIM-DM assumes that when a source starts sending, all downstream
   systems want to receive multicast datagrams.  Initially, multicast
   datagrams are flooded to all areas of the network.  PIM-DM uses RPF
   to prevent looping of multicast datagrams while flooding.  If some
   areas of the network do not have group members, PIM-DM will prune off
   the forwarding branch by instantiating prune state.

   Prune state has a finite lifetime.  When that lifetime expires, data
   will again be forwarded down the previously pruned branch.

   Prune state is associated with an (S,G) pair.  When a new member for
   a group G appears in a pruned area, a router can "graft" toward the
   source S for the group, thereby turning the pruned branch back into a
   forwarding branch.




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   The broadcast of datagrams followed by pruning of unwanted branches
   is often referred to as a flood and prune cycle and is typical of
   dense mode protocols.

   To minimize repeated flooding of datagrams and subsequent pruning
   associated with a particular (S,G) pair, PIM-DM uses a state refresh
   message.  This message is sent by the router(s) directly connected to
   the source and is propagated throughout the network.  When received
   by a router on its RPF interface, the state refresh message causes an
   existing prune state to be refreshed.

   Compared with multicast routing protocols with built-in topology
   discovery mechanisms (e.g., DVMRP [13]), PIM-DM has a simplified
   design and is not hard-wired into a specific topology discovery
   protocol.  However, this simplification does incur more overhead by
   causing flooding and pruning to occur on some links that could be
   avoided if sufficient topology information were available; i.e., to
   decide whether an interface leads to any downstream members of a
   particular group.  Additional overhead is chosen in favor of the
   simplification and flexibility gained by not depending on a specific
   topology discovery protocol.

   PIM-DM differs from PIM-SM in two essential ways: 1) There are no
   periodic joins transmitted, only explicitly triggered prunes and
   grafts.  2) There is no Rendezvous Point (RP).  This is particularly
   important in networks that cannot tolerate a single point of failure.
   (An RP is the root of a shared multicast distribution tree.  For more
   details, see [4]).

4.  Protocol Specification

   The specification of PIM-DM is broken into several parts:

   * Section 4.1 details the protocol state stored.
   * Section 4.2 specifies the data packet forwarding rules.
   * Section 4.3 specifies generation and processing of Hello messages.
   * Section 4.4 specifies the Join, Prune, and Graft generation and
                 processing rules.
   * Section 4.5 specifies the State Refresh generation and forwarding
                 rules.
   * Section 4.6 specifies the Assert generation and processing rules.
   * Section 4.7 gives details on PIM-DM Packet Formats.
   * Section 4.8 summarizes PIM-DM timers and their defaults.








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4.1.  PIM Protocol State

   This section specifies all the protocol states that a PIM-DM
   implementation should maintain to function correctly.  We term this
   state the Tree Information Base or TIB, as it holds the state of all
   the multicast distribution trees at this router.  In this
   specification, we define PIM-DM mechanisms in terms of the TIB.
   However, only a very simple implementation would actually implement
   packet forwarding operations in terms of this state.  Most
   implementations will use this state to build a multicast forwarding
   table, which would then be updated when the relevant state in the TIB
   changes.

   Unlike PIM-SM, PIM-DM does not maintain a keepalive timer associated
   with each (S,G) route.  Within PIM-DM, route and state information
   associated with an (S,G) entry MUST be maintained as long as any
   timer associated with that (S,G) entry is active.  When no timer
   associated with an (S,G) entry is active, all information concerning
   that (S,G) route may be discarded.

   Although we precisely specify the state to be kept, this does not
   mean that an implementation of PIM-DM has to hold the state in this
   form.  This is actually an abstract state definition, which is needed
   in order to specify the router's behavior.  A PIM-DM implementation
   is free to hold whatever internal state it requires and will still be
   conformant with this specification as long as it results in the same
   externally visible protocol behavior as an abstract router that holds
   the following state.

4.1.1.  General Purpose State

   A router stores the following non-group-specific state:

   For each interface:
     Hello Timer (HT)
     State Refresh Capable
     LAN Delay Enabled
     Propagation Delay (PD)
     Override Interval (OI)

     Neighbor State:
       For each neighbor:
         Information from neighbor's Hello
         Neighbor's Gen ID.
         Neighbor's LAN Prune Delay
         Neighbor's Override Interval
         Neighbor's State Refresh Capability
         Neighbor Liveness Timer (NLT)



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4.1.2.  (S,G) State

   For every source/group pair (S,G), a router stores the following
   state:

   (S,G) state:
     For each interface:
       Local Membership:
         State: One of {"NoInfo", "Include"}

       PIM (S,G) Prune State:
         State: One of {"NoInfo" (NI), "Pruned" (P), "PrunePending"
                        (PP)}
                        Prune Pending Timer (PPT)
                        Prune Timer (PT)

       (S,G) Assert Winner State:
         State: One of {"NoInfo" (NI), "I lost Assert" (L), "I won
                        Assert" (W)}
         Assert Timer (AT)
         Assert winner's IP Address
         Assert winner's Assert Metric

     Upstream interface-specific:
       Graft/Prune State:
         State: One of {"NoInfo" (NI), "Pruned" (P), "Forwarding" (F),
                        "AckPending" (AP) }
         GraftRetry Timer (GRT)
         Override Timer (OT)
         Prune Limit Timer (PLT)

       Originator State:
         Source Active Timer (SAT)
         State Refresh Timer (SRT)

4.1.3.  State Summarization Macros

   Using the state defined above, the following "macros" are defined and
   will be used in the descriptions of the state machines and pseudocode
   in the following sections.

   The most important macros are those defining the outgoing interface
   list (or "olist") for the relevant state.

   immediate_olist(S,G) = pim_nbrs (-) prunes(S,G) (+)
                          (pim_include(*,G) (-) pim_exclude(S,G) ) (+)
                          pim_include(S,G) (-) lost_assert(S,G) (-)
                          boundary(G)



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   olist(S,G) = immediate_olist(S,G) (-) RPF_interface(S)

   The macros pim_include(*,G) and pim_include(S,G) indicate the
   interfaces to which traffic might or might not be forwarded because
   of hosts that are local members on those interfaces.

   pim_include(*,G) = {all interfaces I such that:
                       local_receiver_include(*,G,I)}
   pim_include(S,G) = {all interfaces I such that:
                       local_receiver_include(S,G,I)}
   pim_exclude(S,G) = {all interfaces I such that:
                       local_receiver_exclude(S,G,I)}

   The macro RPF_interface(S) returns the RPF interface for source S.
   That is to say, it returns the interface used to reach S as indicated
   by the MRIB.

   The macro local_receiver_include(S,G,I) is true if the IGMP module or
   other local membership mechanism ([1], [2], [3], [6]) has determined
   that there are local members on interface I that seek to receive
   traffic sent specifically by S to G.

   The macro local_receiver_include(*,G,I) is true if the IGMP module or
   other local membership mechanism has determined that there are local
   members on interface I that seek to receive all traffic sent to G.
   Note that this determination is expected to account for membership
   joins initiated on or by the router.

   The macro local_receiver_exclude(S,G,I) is true if
   local_receiver_include(*,G,I) is true but none of the local members
   seek to receive traffic from S.

   The set pim_nbrs is the set of all interfaces on which the router has
   at least one active PIM neighbor.

   The set prunes(S,G) is the set of all interfaces on which the router
   has received Prune(S,G) messages:

   prunes(S,G) = {all interfaces I such that
                  DownstreamPState(S,G,I) is in Pruned state}











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   The set lost_assert(S,G) is the set of all interfaces on which the
   router has lost an (S,G) Assert.

   lost_assert(S,G) = {all interfaces I such that
                       lost_assert(S,G,I) == TRUE}

   boundary(G) = {all interfaces I with an administratively scoped
                  boundary for group G}

   The following pseudocode macro definitions are also used in many
   places in the specification.  Basically RPF' is the RPF neighbor
   toward a source unless a PIM-DM Assert has overridden the normal
   choice of neighbor.

   neighbor RPF'(S,G) {
     if ( I_Am_Assert_loser(S, G, RPF_interface(S) )) {
       return AssertWinner(S, G, RPF_interface(S) )
     } else {
       return MRIB.next_hop( S )
     }
   }

   The macro I_Am_Assert_loser(S, G, I) is true if the Assert state
   machine (in Section 4.6) for (S,G) on interface I is in the "I am
   Assert Loser" state.

4.2.  Data Packet Forwarding Rules

   The PIM-DM packet forwarding rules are defined below in pseudocode.

   iif is the incoming interface of the packet.  S is the source address
   of the packet.  G is the destination address of the packet (group
   address).  RPF_interface(S) is the interface the MRIB indicates would
   be used to route packets to S.

   First, an RPF check MUST be performed to determine whether the packet
   should be accepted based on TIB state and the interface on which that
   the packet arrived.  Packets that fail the RPF check MUST NOT be
   forwarded, and the router will conduct an assert process for the
   (S,G) pair specified in the packet.  Packets for which a route to the
   source cannot be found MUST be discarded.

   If the RPF check has been passed, an outgoing interface list is
   constructed for the packet.  If this list is not empty, then the
   packet MUST be forwarded to all listed interfaces.  If the list is
   empty, then the router will conduct a prune process for the (S,G)
   pair specified in the packet.




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   Upon receipt of a data packet from S addressed to G on interface iif:

   if (iif == RPF_interface(S) AND UpstreamPState(S,G) != Pruned) {
       oiflist = olist(S,G)
   } else {
       oiflist = NULL
   }
   forward packet on all interfaces in oiflist

   This pseudocode employs the following "macro" definition:

   UpstreamPState(S,G) is the state of the Upstream(S,G) state machine
   in Section 4.4.1.

4.3.  Hello Messages

   This section describes the generation and processing of Hello
   messages.

4.3.1.  Sending Hello Messages

   PIM-DM uses Hello messages to detect other PIM routers.  Hello
   messages are sent periodically on each PIM enabled interface.  Hello
   messages are multicast to the ALL-PIM-ROUTERS group.  When PIM is
   enabled on an interface or when a router first starts, the Hello
   Timer (HT) MUST be set to random value between 0 and
   Triggered_Hello_Delay.  This prevents synchronization of Hello
   messages if multiple routers are powered on simultaneously.

   After the initial Hello message, a Hello message MUST be sent every
   Hello_Period.  A single Hello timer MAY be used to trigger sending
   Hello messages on all active interfaces.  The Hello Timer SHOULD NOT
   be reset except when it expires.

4.3.2.  Receiving Hello Messages

   When a Hello message is received, the receiving router SHALL record
   the receiving interface, the sender, and any information contained in
   recognized options.  This information is retained for a number of
   seconds in the Hold Time field of the Hello Message.  If a new Hello
   message is received from a particular neighbor N, the Neighbor
   Liveness Timer (NLT(N,I)) MUST be reset to the newly received Hello
   Holdtime.  If a Hello message is received from a new neighbor, the
   receiving router SHOULD send its own Hello message after a random
   delay between 0 and Triggered_Hello_Delay.






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4.3.3.  Hello Message Hold Time

   The Hold Time in the Hello Message should be set to a value that can
   reasonably be expected to keep the Hello active until a new Hello
   message is received.  On most links, this will be 3.5 times the value
   of Hello_Period.

   If the Hold Time is set to '0xffff', the receiving router MUST NOT
   time out that Hello message.  This feature might be used for on-
   demand links to avoid keeping the link up with periodic Hello
   messages.

   If a Hold Time of '0' is received, the corresponding neighbor state
   expires immediately.  When a PIM router takes an interface down or
   changes IP address, a Hello message with a zero Hold Time SHOULD be
   sent immediately (with the old IP address if the IP address is
   changed) to cause any PIM neighbors to remove the old information
   immediately.

4.3.4.  Handling Router Failures

   If a Hello message is received from an active neighbor with a
   different Generation ID (GenID), the neighbor has restarted and may
   not contain the correct (S,G) state.  A Hello message SHOULD be sent
   after a random delay between 0 and Triggered_Hello_Delay (see 4.8)
   before any other messages are sent.  If the neighbor is downstream,
   the router MAY replay the last State Refresh message for any (S,G)
   pairs for which it is the Assert Winner indicating Prune and Assert
   status to the downstream router.  These State Refresh messages SHOULD
   be sent out immediately after the Hello message.  If the neighbor is
   the upstream neighbor for an (S,G) entry, the router MAY cancel its
   Prune Limit Timer to permit sending a prune and reestablishing a
   Pruned state in the upstream router.

   Upon startup, a router MAY use any State Refresh messages received
   within Hello_Period of its first Hello message on an interface to
   establish state information.  The State Refresh source will be the
   RPF'(S), and Prune status for all interfaces will be set according to
   the Prune Indicator bit in the State Refresh message.  If the Prune
   Indicator is set, the router SHOULD set the PruneLimitTimer to
   Prune_Holdtime and set the PruneTimer on all downstream interfaces to
   the State Refresh's Interval times two.  The router SHOULD then
   propagate the State Refresh as described in Section 4.5.1.








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4.3.5.  Reducing Prune Propagation Delay on LANs

   If all routers on a LAN support the LAN Prune Delay option, then the
   PIM routers on that LAN will use the values received to adjust their
   J/P_Override_Interval on that interface and the interface is LAN
   Delay Enabled.  Briefly, to avoid synchronization of Prune Override
   (Join) messages when multiple downstream routers share a multi-access
   link, sending of these messages is delayed by a small random amount
   of time.  The period of randomization is configurable and has a
   default value of 3 seconds.

   Each router on the LAN expresses its view of the amount of
   randomization necessary in the Override Interval field of the LAN
   Prune Delay option.  When all routers on a LAN use the LAN Prune
   Delay Option, all routers on the LAN MUST set their Override_Interval
   to the largest Override value on the LAN.

   The LAN Delay inserted by a router in the LAN Prune Delay option
   expresses the expected message propagation delay on the link and
   SHOULD be configurable by the system administrator.  When all routers
   on a link use the LAN Prune Delay Option, all routers on the LAN MUST
   set Propagation Delay to the largest LAN Delay on the LAN.

   PIM implementers should enforce a lower bound on the permitted values
   for this delay to allow for scheduling and processing delays within
   their router.  Such delays may cause received messages to be
   processed later and triggered messages to be sent later than
   intended.  Setting this LAN Prune Delay to too low a value may result
   in temporary forwarding outages, because a downstream router will not
   be able to override a neighbor's prune message before the upstream
   neighbor stops forwarding.

4.4.  PIM-DM Prune, Join, and Graft Messages

   This section describes the generation and processing of PIM-DM Join,
   Prune, and Graft messages.  Prune messages are sent toward the
   upstream neighbor for S to indicate that traffic from S addressed to
   group G is not desired.  In the case of downstream routers A and B,
   where A wishes to continue receiving data and B does not, A will send
   a Join in response to B's Prune to override the Prune.  This is the
   only situation in PIM-DM in which a Join message is used.  Finally, a
   Graft message is used to re-join a previously pruned branch to the
   delivery tree.








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4.4.1.  Upstream Prune, Join, and Graft Messages

   The Upstream(S,G) state machine for sending Prune, Graft, and Join
   messages is given below.  There are three states.

     Forwarding (F)
       This is the starting state of the Upsteam(S,G) state machine.
       The state machine is in this state if it just started or if
       oiflist(S,G) != NULL.

     Pruned (P)
       The set, olist(S,G), is empty.  The router will not forward data
       from S addressed to group G.

     AckPending (AP)
       The router was in the Pruned(P) state, but a transition has
       occurred in the Downstream(S,G) state machine for one of this
       (S,G) entry's outgoing interfaces, indicating that traffic from S
       addressed to G should again be forwarded.  A Graft message has
       been sent to RPF'(S), but a Graft Ack message has not yet been
       received.

   In addition, there are three state-machine-specific timers:

     GraftRetry Timer (GRT(S,G))
       This timer is set when a Graft is sent upstream.  If a
       corresponding GraftAck is not received before the timer expires,
       then another Graft is sent, and the GraftRetry Timer is reset.
       The timer is stopped when a Graft Ack message is received.  This
       timer is normally set to Graft_Retry_Period (see 4.8).

     Override Timer (OT(S,G))
       This timer is set when a Prune(S,G) is received on the upstream
       interface where olist(S,G) != NULL.  When the timer expires, a
       Join(S,G) message is sent on the upstream interface.  This timer
       is normally set to t_override (see 4.8).

     Prune Limit Timer (PLT(S,G))
       This timer is used to rate-limit Prunes on a LAN.  It is only
       used when the Upstream(S,G) state machine is in the Pruned state.
       A Prune cannot be sent if this timer is running.  This timer is
       normally set to t_limit (see 4.8).









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          +-------------+                        +-------------+
          |             |     olist == NULL      |             |
          |   Forward   |----------------------->|   Pruned    |
          |             |                        |             |
          +-------------+                        +-------------+
               ^   |                                  ^   |
               |   |                                  |   |
               |   |RPF`(S) Changes      olist == NULL|   |
               |   |                                  |   |
               |   |         +-------------+          |   |
               |   +-------->|             |----------+   |
               |             | AckPending  |              |
               +-------------|             |<-------------+
             Rcv GraftAck OR +-------------+ olist != NULL
           Rcv State Refresh
              With (P==0) OR
          S Directly Connect

                Figure 1: Upstream Interface State Machine

   In tabular form, the state machine is defined as follows:

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
|                               +------------+------------+------------+
|            Event              | Forwarding |   Pruned   | AckPending |
+-------------------------------+------------+------------+------------+
| Data packet arrives on        | ->P Send   | ->P Send   | N/A        |
| RPF_Interface(S) AND          | Prune(S,G) | Prune(S,G) |            |
| olist(S,G) == NULL AND        |Set PLT(S,G)|Set PLT(S,G)|            |
| PLT(S,G) not running          |            |            |            |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received   | ->F  Set   | ->P Reset  |->AP  Set   |
| from RPF`(S) AND              |    OT(S,G) |  PLT(S,G)  |    OT(S,G) |
| Prune Indicator == 1          |            |            |            |
+-------------------------------+------------+------------+------------+
| State Refresh(S,G) received   | ->F        | ->P Send   |->F Cancel  |
| from RPF`(S) AND              |            | Prune(S,G) |  GRT(S,G)  |
| Prune Indicator == 0 AND      |            |Set PLT(S,G)|            |
| PLT(S,G) not running          |            |            |            |
+-------------------------------+------------+------------+------------+










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+-------------------------------+--------------------------------------+
|                               |            Previous State            |
+                               +------------+------------+------------+
|            Event              | Forwarding |   Pruned   | AckPending |
+-------------------------------+------------+------------+------------+
| See Join(S,G) to RPF'(S)      | ->F Cancel | ->P        |->AP Cancel |
|                               |    OT(S,G) |            |    OT(S,G) |
+-------------------------------+------------+------------+------------+
| See Prune(S,G)                | ->F Set    | ->P        |->AP Set    |
|                               |    OT(S,G) |            |    OT(S,G) |
+-------------------------------+------------+------------+------------+
| OT(S,G) Expires               | ->F Send   | N/A        |->AP Send   |
|                               |  Join(S,G) |            |  Join(S,G) |
+-------------------------------+------------+------------+------------+
| olist(S,G)->NULL              | ->P Send   | N/A        |->P Send    |
|                               | Prune(S,G) |            | Prune(S,G) |
|                               |Set PLT(S,G)|            |Set PLT(S,G)|
|                               |            |            | Cancel     |
|                               |            |            | GRT(S,G)   |
+-------------------------------+------------+------------+------------+
| olist(S,G)->non-NULL          | N/A        | ->AP Send  | N/A        |
|                               |            | Graft(S,G) |            |
|                               |            |Set GRT(S,G)|            |
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND           | ->AP Send  | ->AP Send  |->AP Send   |
| olist(S,G) != NULL            | Graft(S,G) | Graft(S,G) | Graft(S,G) |
|                               |Set GRT(S,G)|Set GRT(S,G)|Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| RPF'(S) Changes AND           | ->P        | ->P Cancel |->P Cancel  |
| olist(S,G) == NULL            |            |  PLT(S,G)  |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+
| S becomes directly connected  | ->F        | ->P        |->F Cancel  |
|                               |            |            |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+
| GRT(S,G) Expires              | N/A        | N/A        |->AP Send   |
|                               |            |            | Graft(S,G) |
|                               |            |            |Set GRT(S,G)|
+-------------------------------+------------+------------+------------+
| Receive GraftAck(S,G) from    | ->F        | ->P        |->F Cancel  |
| RPF'(S)                       |            |            |  GRT(S,G)  |
+-------------------------------+------------+------------+------------+

   The transition event "RcvGraftAck(S,G)" implies receiving a Graft Ack
   message targeted to this router's address on the incoming interface
   for the (S,G) entry.  If the destination address is not correct, the
   state transitions in this state machine must not occur.





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4.4.1.1.  Transitions from the Forwarding (F) State

   When the Upstream(S,G) state machine is in the Forwarding (F) state,
   the following events may trigger a transition:

     Data Packet arrives on RPF_Interface(S) AND olist(S,G) == NULL AND
     S NOT directly connected
       The Upstream(S,G) state machine MUST transition to the Pruned (P)
       state, send a Prune(S,G) to RPF'(S), and set PLT(S,G) to t_limit
       seconds.

     State Refresh(S,G) Received from RPF'(S)
       The Upstream(S,G) state machine remains in a Forwarding state.
       If the received State Refresh has the Prune Indicator bit set to
       one, this router must override the upstream router's Prune state
       after a short random interval.  If OT(S,G) is not running and the
       Prune Indicator bit equals one, the router MUST set OT(S,G) to
       t_override seconds.

     See Join(S,G) to RPF'(S)
       This event is only relevant if RPF_interface(S) is a shared
       medium.  This router sees another router on RPF_interface(S) send
       a Join(S,G) to RPF'(S,G).  If the OT(S,G) is running, then it
       means that the router had scheduled a Join to override a
       previously received Prune.  Another router has responded more
       quickly with a Join, so the local router SHOULD cancel its
       OT(S,G), if it is running.  The Upstream(S,G) state machine
       remains in the Forwarding (F) state.

     See Prune(S,G) AND S NOT directly connected
       This event is only relevant if RPF_interface(S) is a shared
       medium.  This router sees another router on RPF_interface(S) send
       a Prune(S,G).  As this router is in Forwarding state, it must
       override the Prune after a short random interval.  If OT(S,G) is
       not running, the router MUST set OT(S,G) to t_override seconds.
       The Upstream(S,G) state machine remains in Forwarding (F) state.

     OT(S,G) Expires AND S NOT directly connected
       The OverrideTimer (OT(S,G)) expires.  The router MUST send a
       Join(S,G) to RPF'(S) to override a previously detected prune.
       The Upstream(S,G) state machine remains in the Forwarding (F)
       state.

     olist(S,G) -> NULL AND S NOT directly connected
       The Upstream(S,G) state machine MUST transition to the Pruned (P)
       state, send a Prune(S,G) to RPF'(S), and set PLT(S,G) to t_limit
       seconds.




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     RPF'(S) Changes AND olist(S,G) is non-NULL AND S NOT directly
     connected
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine MUST transition to the AckPending (AP) state, unicast a
       Graft to the new RPF'(S), and set the GraftRetry Timer (GRT(S,G))
       to Graft_Retry_Period.

     RPF'(S) Changes AND olist(S,G) is NULL
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine MUST transition to the Pruned (P) state.

4.4.1.2.  Transitions from the Pruned (P) State

   When the Upstream(S,G) state machine is in the Pruned (P) state, the
   following events may trigger a transition:

     Data arrives on RPF_interface(S) AND PLT(S,G) not running AND S NOT
     directly connected
       Either another router on the LAN desires traffic from S addressed
       to G or a previous Prune was lost.  To prevent generating a
       Prune(S,G) in response to every data packet, the PruneLimit Timer
       (PLT(S,G)) is used.  Once the PLT(S,G) expires, the router needs
       to send another prune in response to a data packet not received
       directly from the source.  A Prune(S,G) MUST be sent to RPF'(S),
       and the PLT(S,G) MUST be set to t_limit.

     State Refresh(S,G) Received from RPF'(S)
       The Upstream(S,G) state machine remains in a Pruned state.  If
       the State Refresh has its Prune Indicator bit set to zero and
       PLT(S,G) is not running, a Prune(S,G) MUST be sent to RPF'(S),
       and the PLT(S,G) MUST be set to t_limit.  If the State Refresh
       has its Prune Indicator bit set to one, the router MUST reset
       PLT(S,G) to t_limit.

     See Prune(S,G) to RPF'(S)
       A Prune(S,G) is seen on RPF_interface(S) to RPF'(S).  The
       Upstream(S,G) state machine stays in the Pruned (P) state.  The
       router MAY reset its PLT(S,G) to the value in the Holdtime field
       of the received message if it is greater than the current value
       of the PLT(S,G).

     olist(S,G)->non-NULL AND S NOT directly connected
       The set of interfaces defined by the olist(S,G) macro becomes
       non-empty, indicating that traffic from S addressed to group G
       must be forwarded.  The Upstream(S,G) state machine MUST cancel
       PLT(S,G), transition to the AckPending (AP) state and unicast a



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       Graft message to RPF'(S).  The Graft Retry Timer (GRT(S,G)) MUST
       be set to Graft_Retry_Period.

     RPF'(S) Changes AND olist(S,G) == non-NULL AND S NOT directly
     connected
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine MUST cancel PLT(S,G), transition to the AckPending (AP)
       state, send a Graft unicast to the new RPF'(S), and set the
       GraftRetry Timer (GRT(S,G)) to Graft_Retry_Period.

     RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine stays in the Pruned (P) state and MUST cancel the
       PLT(S,G) timer.

     S becomes directly connected
       Unicast routing changed so that S is directly connected.  The
       Upstream(S,G) state machine remains in the Pruned (P) state.

4.4.1.3.  Transitions from the AckPending (AP) State

   When the Upstream(S,G) state machine is in the AckPending (AP) state,
   the following events may trigger a transition:

     State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 1
       The Upstream(S,G) state machine remains in an AckPending state.
       The router must override the upstream router's Prune state after
       a short random interval.  If OT(S,G) is not running and the Prune
       Indicator bit equals one, the router MUST set OT(S,G) to
       t_override seconds.

     State Refresh(S,G) Received from RPF'(S) with Prune Indicator == 0
       The router MUST cancel its GraftRetry Timer (GRT(S,G)) and
       transition to the Forwarding (F) state.

     See Join(S,G) to RPF'(S,G)
       This event is only relevant if RPF_interface(S) is a shared
       medium.  This router sees another router on RPF_interface(S) send
       a Join(S,G) to RPF'(S,G).  If the OT(S,G) is running, then it
       means that the router had scheduled a Join to override a
       previously received Prune.  Another router has responded more
       quickly with a Join, so the local router SHOULD cancel its
       OT(S,G), if it is running.  The Upstream(S,G) state machine
       remains in the AckPending (AP) state.





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     See Prune(S,G)
       This event is only relevant if RPF_interface(S) is a shared
       medium.  This router sees another router on RPF_interface(S) send
       a Prune(S,G).  As this router is in AckPending (AP) state, it
       must override the Prune after a short random interval.  If
       OT(S,G) is not running, the router MUST set OT(S,G) to t_override
       seconds.  The Upstream(S,G) state machine remains in AckPending
       (AP) state.

     OT(S,G) Expires
       The OverrideTimer (OT(S,G)) expires.  The router MUST send a
       Join(S,G) to RPF'(S).  The Upstream(S,G) state machine remains in
       the AckPending (AP) state.

     olist(S,G) -> NULL
       The set of interfaces defined by the olist(S,G) macro becomes
       null, indicating that traffic from S addressed to group G should
       no longer be forwarded.  The Upstream(S,G) state machine MUST
       transition to the Pruned (P) state.  A Prune(S,G) MUST be
       multicast to the RPF_interface(S), with RPF'(S) named in the
       upstream neighbor field.  The GraftRetry Timer (GRT(S,G)) MUST be
       cancelled, and PLT(S,G) MUST be set to t_limit seconds.

     RPF'(S) Changes AND olist(S,G) does not become NULL AND S NOT
     directly connected
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine stays in the AckPending (AP) state.  A Graft MUST be
       unicast to the new RPF'(S) and the GraftRetry Timer (GRT(S,G))
       reset to Graft_Retry_Period.

     RPF'(S) Changes AND olist(S,G) == NULL AND S NOT directly connected
       Unicast routing or Assert state causes RPF'(S) to change,
       including changes to RPF_Interface(S).  The Upstream(S,G) state
       machine MUST transition to the Pruned (P) state.  The GraftRetry
       Timer (GRT(S,G)) MUST be cancelled.

     S becomes directly connected
       Unicast routing has changed so that S is directly connected.  The
       GraftRetry Timer MUST be cancelled, and the Upstream(S,G) state
       machine MUST transition to the Forwarding(F) state.










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     GRT(S,G) Expires
       The GraftRetry Timer (GRT(S,G)) expires for this (S,G) entry.
       The Upstream(S,G) state machine stays in the AckPending (AP)
       state.  Another Graft message for (S,G) SHOULD be unicast to
       RPF'(S) and the GraftRetry Timer (GRT(S,G)) reset to
       Graft_Retry_Period.  It is RECOMMENDED that the router retry a
       configured number of times before ceasing retries.

     See GraftAck(S,G) from RPF'(S)
       A GraftAck is received from  RPF'(S).  The GraftRetry Timer MUST
       be cancelled, and the Upstream(S,G) state machine MUST transition
       to the Forwarding(F) state.

4.4.2.  Downstream Prune, Join, and Graft Messages

   The Prune(S,G) Downstream state machine for receiving Prune, Join and
   Graft messages on interface I is given below.  This state machine
   MUST always be in the NoInfo state on the upstream interface.  It
   contains three states.

     NoInfo(NI)
       The interface has no (S,G) Prune state, and neither the Prune
       timer (PT(S,G,I)) nor the PrunePending timer ((PPT(S,G,I)) is
       running.

     PrunePending(PP)
       The router has received a Prune(S,G) on this interface from a
       downstream neighbor and is waiting to see whether the prune will
       be overridden by another downstream router.  For forwarding
       purposes, the PrunePending state functions exactly like the
       NoInfo state.

     Pruned(P)
       The router has received a Prune(S,G) on this interface from a
       downstream neighbor, and the Prune was not overridden.  Data from
       S addressed to group G is no longer being forwarded on this
       interface.

   In addition, there are two timers:

     PrunePending Timer (PPT(S,G,I))
       This timer is set when a valid Prune(S,G) is received.  Expiry of
       the PrunePending Timer (PPT(S,G,I)) causes the interface to
       transition to the Pruned state.







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     Prune Timer (PT(S,G,I))
       This timer is set when the PrunePending Timer (PT(S,G,I))
       expires.  Expiry of the Prune Timer (PT(S,G,I)) causes the
       interface to transition to the NoInfo (NI) state, thereby
       allowing data from S addressed to group G to be forwarded on the
       interface.

            +-------------+                        +-------------+
            |             |      PPT Expires       |             |
            |PrunePending |----------------------->|   Pruned    |
            |             |                        |             |
            +-------------+                        +-------------+
                 |   ^                                      |
                 |   |                                      |
                 |   |Rcv Prune                             |
                 |   |                                      |
                 |   |         +-------------+              |
                 |   +---------|             |              |
                 |             |   NoInfo    |<-------------+
                 +------------>|             | Rcv Join/Graft OR
             Rcv Join/Graft OR +-------------+ PT Expires OR
           RPF_Interface(S)->I                 RPF_Interface(S)->I

                 Figure 2: Downstream Interface State Machine



























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   In tabular form, the state machine is as follows:

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
+                               +------------+------------+------------+
|            Event              |  No Info   | PrunePend  |   Pruned   |
+-------------------------------+------------+------------+------------+
| Receive Prune(S,G)            |->PP  Set   |->PP        |->P Reset   |
|                               | PPT(S,G,I) |            |  PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Join(S,G)             |->NI        |->NI Cancel |->NI Cancel |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Receive Graft(S,G)            |->NI Send   |->NI Send   |->NI Send   |
|                               |  GraftAck  |  GraftAck  |  GraftAck  |
|                               |            |  Cancel    |  Cancel    |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| PPT(S,G) Expires              | N/A        |->P Set     | N/A        |
|                               |            |  PT(S,G,I) |            |
+-------------------------------+------------+------------+------------+
| PT(S,G) Expires               | N/A        | N/A        |->NI        |
+-------------------------------+------------+------------+------------+
| RPF_Interface(S) becomes I    |->NI        |->NI Cancel |->NI Cancel |
|                               |            | PPT(S,G,I) |  PT(S,G,I) |
+-------------------------------+------------+------------+------------+
| Send State Refresh(S,G) out I |->NI        |->PP        |->P Reset   |
|                               |            |            |  PT(S,G,I) |
+-------------------------------+------------+------------+------------+

   The transition events "Receive Graft(S,G)", "Receive Prune(S,G)", and
   "Receive Join(S,G)" denote receiving a Graft, Prune, or Join message
   in which this router's address on I is contained in the message's
   upstream neighbor field.  If the upstream neighbor field does not
   match this router's address on I, then these state transitions in
   this state machine must not occur.

4.4.2.1.  Transitions from the NoInfo State

   When the Prune(S,G) Downstream state machine is in the NoInfo (NI)
   state, the following events may trigger a transition:

     Receive Prune(S,G)
       A Prune(S,G) is received on interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       PrunePending (PP) state.  The PrunePending Timer (PPT(S,G,I))
       MUST be set to J/P_Override_Interval if the router has more than



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       one neighbor on I.  If the router has only one neighbor on
       interface I, then it SHOULD set the PPT(S,G,I) to zero,
       effectively transitioning immediately to the Pruned (P) state.

     Receive Graft(S,G)
       A Graft(S,G) is received on the interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I stays in the NoInfo (NI)
       state.  A GraftAck(S,G) MUST be unicast to the originator of the
       Graft(S,G) message.

4.4.2.2.  Transitions from the PrunePending (PP) State

   When the Prune(S,G) downstream state machine is in the PrunePending
   (PP) state, the following events may trigger a transition.

     Receive Join(S,G)
       A Join(S,G) is received on interface I with the upstream neighbor
       field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       NoInfo (NI) state.  The PrunePending Timer (PPT(S,G,I)) MUST be
       cancelled.

     Receive Graft(S,G)
       A Graft(S,G) is received on interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       NoInfo (NI) state and MUST unicast a Graft Ack message to the
       Graft originator.  The PrunePending Timer (PPT(S,G,I)) MUST be
       cancelled.

     PPT(S,G,I) Expires
       The PrunePending Timer (PPT(S,G,I)) expires, indicating that no
       neighbors have overridden the previous Prune(S,G) message.  The
       Prune(S,G) Downstream state machine on interface I MUST
       transition to the Pruned (P) state.  The Prune Timer (PT(S,G,I))
       is started and MUST be initialized to the received
       Prune_Hold_Time minus J/P_Override_Interval.  A PruneEcho(S,G)
       MUST be sent on I if I has more than one PIM neighbor.  A
       PruneEcho(S,G) is simply a Prune(S,G) message multicast by the
       upstream router to a LAN, with itself as the Upstream Neighbor.
       Its purpose is to add additional reliability so that if a Join
       that should have overridden the Prune is lost locally on the LAN,
       the PruneEcho(S,G) may be received and trigger a new Join
       message.  A PruneEcho(S,G) is OPTIONAL on an interface with only
       one PIM neighbor.  In addition, the router MUST evaluate any
       possible transitions in the Upstream(S,G) state machine.




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     RPF_Interface(S) becomes interface I
       The upstream interface for S has changed.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       NoInfo (NI) state.  The PrunePending Timer (PPT(S,G,I)) MUST be
       cancelled.

4.4.2.3.  Transitions from the Prune (P) State

   When the Prune(S,G) Downstream state machine is in the Pruned (P)
   state, the following events may trigger a transition.

     Receive Prune(S,G)
       A Prune(S,G) is received on the interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I remains in the Pruned (P)
       state.  The Prune Timer (PT(S,G,I)) SHOULD be reset to the
       holdtime contained in the Prune(S,G) message if it is greater
       than the current value.

     Receive Join(S,G)
       A Join(S,G) is received on the interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       downstream state machine on interface I MUST transition to the
       NoInfo (NI) state.  The Prune Timer (PT(S,G,I)) MUST be
       cancelled.  The router MUST evaluate any possible transitions in
       the Upstream(S,G) state machine.

     Receive Graft(S,G)
       A Graft(S,G) is received on interface I with the upstream
       neighbor field set to the router's address on I.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       NoInfo (NI) state and send a Graft Ack back to the Graft's
       source.  The Prune Timer (PT(S,G,I)) MUST be cancelled.  The
       router MUST evaluate any possible transitions in the
       Upstream(S,G) state machine.

     PT(S,G,I) Expires
       The Prune Timer (PT(S,G,I)) expires, indicating that it is again
       time to flood data from S addressed to group G onto interface I.
       The Prune(S,G) Downstream state machine on interface I MUST
       transition to the NoInfo (NI) state.  The router MUST evaluate
       any possible transitions in the Upstream(S,G) state machine.

     RPF_Interface(S) becomes interface I
       The upstream interface for S has changed.  The Prune(S,G)
       Downstream state machine on interface I MUST transition to the
       NoInfo (NI) state.  The PruneTimer (PT(S,G,I)) MUST be cancelled.




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     Send State Refresh(S,G) out interface I
       The router has refreshed the Prune(S,G) state on interface I.
       The router MUST reset the Prune Timer (PT(S,G,I)) to the Holdtime
       from an active Prune received on interface I.  The Holdtime used
       SHOULD be the largest active one but MAY be the most recently
       received active Prune Holdtime.

4.5.  State Refresh

   This section describes the major portions of the state refresh
   mechanism.

4.5.1.  Forwarding of State Refresh Messages

   When a State Refresh message, SRM, is received, it is forwarded
   according to the following pseudo-code.

   if (iif != RPF_interface(S))
     return;
   if (RPF'(S) != srcaddr(SRM))
     return;
   if (StateRefreshRateLimit(S,G) == TRUE)
     return;

   for each interface I in pim_nbrs {
     if (TTL(SRM) == 0 OR (TTL(SRM) - 1) < Threshold(I))
       continue;     /* Out of TTL, skip this interface */
     if (boundary(I,G))
       continue;     /* This interface is scope boundary, skip it */
     if (I == iif)
       continue;     /* This is the incoming interface, skip it */
     if (lost_assert(S,G,I) == TRUE)
       continue;     /* Let the Assert Winner do State Refresh */

     Copy SRM to SRM';   /* Make a copy of SRM to forward */

     if (I contained in prunes(S,G)) {
       set Prune Indicator bit of SRM' to 1;

       if StateRefreshCapable(I) == TRUE
         set PT(S,G) to largest active holdtime read from a Prune
         message accepted on I;









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     } else {
       set Prune Indicator bit of SRM' to 0;
     }

     set srcaddr(SRM') to my_addr(I);
     set TTL of SRM' to TTL(SRM) - 1;
     set metric of SRM' to metric of unicast route used to reach S;
     set pref of SRM' to preference of unicast route used to reach S;
     set mask of SRM' to mask of route used to reach S;

     if (AssertState == NoInfo) {
       set Assert Override of SRM' to 1;
     } else {
       set Assert Override of SRM' to 0;
     }

     transmit SRM' on I;
   }

   The pseudocode above employs the following macro definitions.

   Boundary(I,G) is TRUE if an administratively scoped boundary for
   group G is configured on interface I.

   StateRefreshCapable(I) is TRUE if all neighbors on an interface use
   the State Refresh option.

   StateRefreshRateLimit(S,G) is TRUE if the time elapsed since the last
   received StateRefresh(S,G) is less than the configured
   RefreshLimitInterval.

   TTL(SRM) returns the TTL contained in the State Refresh Message, SRM.
   This is different from the TTL contained in the IP header.

   Threshold(I) returns the minimum TTL that a packet must have before
   it can be transmitted on interface I.

   srcaddr(SRM) returns the source address contained in the network
   protocol (e.g., IPv4) header of the State Refresh Message, SRM.

   my_addr(I) returns this node's network (e.g., IPv4) address on
   interface I.









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4.5.2.  State Refresh Message Origination

   This section describes the origination of State Refresh messages.
   These messages are generated periodically by the PIM-DM router
   directly connected to a source.  One Origination(S,G) state machine
   exists per (S,G) entry in a PIM-DM router.

   The Origination(S,G) state machine has the following states:

     NotOriginator(NO)
       This is the starting state of the Origination(S,G) state machine.
       While in this state, a router will not originate State Refresh
       messages for the (S,G) pair.

     Originator(O)
       When in this state the router will periodically originate State
       Refresh messages.  Only routers directly connected to S may
       transition to this state.

   In addition, there are two state machine specific timers:

     State Refresh Timer (SRT(S,G))
       This timer controls when State Refresh messages are generated.
       The timer is initially set when that Origination(S,G) state
       machine transitions to the O state.  It is cancelled when the
       Origination(S,G) state machine transitions to the NO state.  This
       timer is normally set to StateRefreshInterval (see 4.8).

     Source Active Timer (SAT(S,G))
       This timer is first set when the Origination(S,G) state machine
       transitions to the O state and is reset on the receipt of every
       data packet from S addressed to group G.  When it expires, the
       Origination(S,G) state machine transitions to the NO state.  This
       timer is normally set to SourceLifetime (see 4.8).

            +-------------+  Rcv Directly From S   +-------------+
            |             |----------------------->|             |
            |NotOriginator|                        | Originator  |
            |             |<-----------------------|             |
            +-------------+     SAT Expires OR     +-------------+
                             S NOT Direct Connect

                     Figure 3: State Refresh State Machine








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   In tabular form, the state machine is defined as follows:

+----------------------------------------------------------------------+
|                                  |           Previous State          |
|                                  +---------------+-------------------+
|            Event                 | NotOriginator |    Originator     |
+----------------------------------+---------------+-------------------+
| Receive Data from S AND          | ->O           | ->O Reset         |
| S directly connected             | Set SRT(S,G)  |     SAT(S,G)      |
|                                  | Set SAT(S,G)  |                   |
+----------------------------------+---------------+-------------------+
| SRT(S,G) Expires                 | N/A           | ->O    Send       |
|                                  |               | StateRefresh(S,G) |
|                                  |               |  Reset SRT(S,G)   |
+----------------------------------+---------------+-------------------+
| SAT(S,G) Expires                 | N/A           | ->NO  Cancel      |
|                                  |               |       SRT(S,G)    |
+----------------------------------+---------------+-------------------+
| S no longer directly connected   | ->NO          | ->NO              |
|                                  |               |   Cancel SRT(S,G) |
|                                  |               |   Cancel SAT(S,G) |
+----------------------------------+---------------+-------------------+

4.5.2.1.  Transitions from the NotOriginator (NO) State

   When the Originating(S,G) state machine is in the NotOriginator (NO)
   state, the following event may trigger a transition:

     Data Packet received from directly connected Source S addressed to
     group G
       The router MUST transition to an Originator (O) state, set
       SAT(S,G) to SourceLifetime, and set SRT(S,G) to
       StateRefreshInterval.  The router SHOULD record the TTL of the
       packet for use in State Refresh messages.

4.5.2.2.  Transitions from the Originator (O) State

   When the Originating(S,G) state machine is in the Originator (O)
   state, the following events may trigger a transition:

     Receive Data Packet from S addressed to G
       The router remains in the Originator (O) state and MUST reset
       SAT(S,G) to SourceLifetime.  The router SHOULD increase its
       recorded TTL to match the TTL of the packet, if the packet's TTL
       is larger than the previously recorded TTL.  A router MAY record
       the TTL based on an implementation specific sampling policy to
       avoid examining the TTL of every multicast packet it handles.




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     SRT(S,G) Expires
       The router remains in the Originator (O) state and MUST reset
       SRT(S,G) to StateRefreshInterval.  The router MUST also generate
       State Refresh messages for transmission, as described in the
       State Refresh Forwarding rules (Section 4.5.1), except for the
       TTL.  If the TTL of data packets from S to G are being recorded,
       then the TTL of each State Refresh message is set to the highest
       recorded TTL.  Otherwise, the TTL is set to the configured State
       Refresh TTL.  Let I denote the interface over which a State
       Refresh message is being sent.  If the Prune(S,G) Downstream
       state machine is in the Pruned (P) state, then the Prune-
       Indicator bit MUST be set to 1 in the State Refresh message being
       sent over I. Otherwise, the Prune-Indicator bit MUST be set to 0.

     SAT(S,G) Expires
       The router MUST cancel the SRT(S,G) timer and transition to the
       NotOriginator (NO) state.

     S is no longer directly connected
       The router MUST transition to the NotOriginator (NO) state and
       cancel both the SAT(S,G) and SRT(S,G).

4.6.  PIM Assert Messages

4.6.1.  Assert Metrics

   Assert metrics are defined as follows:

   struct assert_metric {
     metric_preference;
     route_metric;
     ip_address;
   };

   When assert_metrics are compared, the metric_preference and
   route_metric field are compared in order, where the first lower value
   wins.  If all fields are equal, the IP address of the router that
   sourced the Assert message is used as a tie-breaker, with the highest
   IP address winning.












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   An Assert metric for (S,G) to include in (or compare against) an
   Assert message sent on interface I should be computed by using the
   following pseudocode:

   assert_metric
   my_assert_metric(S,G,I) {
     if (CouldAssert(S,G,I) == TRUE) {
       return spt_assert_metric(S,G,I)
     } else {
       return infinite_assert_metric()
     }
   }

   spt_assert_metric(S,I) gives the Assert metric we use if we're
   sending an Assert based on active (S,G) forwarding state:

   assert_metric
   spt_assert_metric(S,I) {
     return {0,MRIB.pref(S),MRIB.metric(S),my_addr(I)}
   }

   MRIB.pref(X) and MRIB.metric(X) are the routing preference and
   routing metrics associated with the route to a particular (unicast)
   destination X, as determined by the MRIB.  my_addr(I) is simply the
   router's network (e.g., IP) address associated with the local
   interface I.

   infinite_assert_metric() gives the Assert metric we need to send an
   Assert but doesn't match (S,G) forwarding state:

   assert_metric
   infinite_assert_metric() {
     return {1,infinity,infinity,0}
   }

4.6.2.  AssertCancel Messages

   An AssertCancel(S,G) message is simply an Assert message for (S,G)
   with infinite metric.  The Assert winner sends this message when it
   changes its upstream interface to this interface.  Other routers will
   see this metric, causing those with forwarding state to send their
   own Asserts and re-establish an Assert winner.

   AssertCancel messages are simply an optimization.  The original
   Assert timeout mechanism will eventually allow a subnet to become
   consistent; the AssertCancel mechanism simply causes faster
   convergence.  No special processing is required for an AssertCancel
   message, as it is simply an Assert message from the current winner.



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4.6.3.  Assert State Macros

   The macro lost_assert(S,G,I), is used in the olist computations of
   Section 4.1.3, and is defined as follows:

   bool lost_assert(S,G,I) {
     if ( RPF_interface(S) == I ) {
       return FALSE
     } else {
       return (AssertWinner(S,G,I) != me  AND
               (AssertWinnerMetric(S,G,I) is better than
                spt_assert_metric(S,G,I)))
     }
   }

   AssertWinner(S,G,I) defaults to NULL, and AssertWinnerMetric(S,G,I)
   defaults to Infinity when in the NoInfo state.

4.6.4.  (S,G) Assert Message State Machine

   The (S,G) Assert state machine for interface I is shown in Figure 4.
   There are three states:

     NoInfo (NI)
       This router has no (S,G) Assert state on interface I.

     I am Assert Winner (W)
       This router has won an (S,G) Assert on interface I.  It is now
       responsible for forwarding traffic from S destined for G via
       interface I.

     I am Assert Loser (L)
       This router has lost an (S,G) Assert on interface I.  It must not
       forward packets from S destined for G onto interface I.

   In addition, an Assert Timer (AT(S,G,I)) is used to time out the
   Assert state.














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         +-------------+                        +-------------+
         |             | Rcv Pref Assert or SR  |             |
         |   Winner    |----------------------->|    Loser    |
         |             |                        |             |
         +-------------+                        +-------------+
              ^   |                                  ^   |
              |   |                Rcv Pref Assert or|   |
              |   |AT Expires OR        State Refresh|   |
              |   |CouldAssert->FALSE                |   |
              |   |                                  |   |
              |   |         +-------------+          |   |
              |   +-------->|             |----------+   |
              |             |   No Info   |              |
              +-------------|             |<-------------+
       Rcv Data from dnstrm +-------------+ Rcv Inf Assert from Win OR
     OR Rcv Inferior Assert                 Rcv Inf SR from Winner OR
         OR Rcv Inferior SR                 AT Expires OR
                                            CouldAssert Changes OR
                                            Winner's NLT Expires

                     Figure 4: Assert State Machine

   In tabular form, the state machine is defined as follows:

+-------------------------------+--------------------------------------+
|                               |            Previous State            |
|                               +------------+------------+------------+
|            Event              |  No Info   |   Winner   |    Loser   |
+-------------------------------+------------+------------+------------+
| An (S,G) Data packet received | ->W Send   | ->W Send   | ->L        |
| on downstream interface       | Assert(S,G)| Assert(S,G)|            |
|                               |    Set     |    Set     |            |
|                               |  AT(S,G,I) |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR   | N/A        | N/A        |->NI Cancel |
| State Refresh) from Assert    |            |            |  AT(S,G,I) |
| Winner                        |            |            |            |
+-------------------------------+--------------------------------------+
| Receive Inferior (Assert OR   | ->W Send   | ->W Send   | ->L        |
| State Refresh) from non-Assert| Assert(S,G)| Assert(S,G)|            |
| Winner AND CouldAssert==TRUE  |    Set     |    Set     |            |
|                               |  AT(S,G,I) |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+








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+-------------------------------+--------------------------------------+
|                               |            Previous State            |
|                               +------------+------------+------------+
|            Event              |  No Info   |   Winner   |    Loser   |
+-------------------------------+------------+------------+------------+
| Receive Preferred Assert OR   | ->L Send   | ->L Send   | ->L  Set   |
| State Refresh                 | Prune(S,G) | Prune(S,G) |  AT(S,G,I) |
|                               |    Set     |    Set     |            |
|                               |  AT(S,G,I) |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| Send State Refresh            | ->NI       | ->W Reset  | N/A        |
|                               |            |  AT(S,G,I) |            |
+-------------------------------+--------------------------------------+
| AT(S,G) Expires               | N/A        | ->NI       | ->NI       |
+-------------------------------+--------------------------------------+
| CouldAssert -> FALSE          | ->NI       |->NI Cancel |->NI Cancel |
|                               |            |  AT(S,G,I) |  AT(S,G,I) |
+-------------------------------+--------------------------------------+
| CouldAssert -> TRUE           | ->NI       | N/A        |->NI Cancel |
|                               |            |            |  AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Winner's NLT(N,I) Expires     | N/A        | N/A        |->NI Cancel |
|                               |            |            |  AT(S,G,I) |
+-------------------------------+--------------------------------------+
| Receive Prune(S,G), Join(S,G) | ->NI       | ->W        | ->L Send   |
| or Graft(S,G)                 |            |            | Assert(S,G)|
+-------------------------------+--------------------------------------+

   Terminology: A "preferred assert" is one with a better metric than
   the current winner.  An "inferior assert" is one with a worse metric
   than my_assert_metric(S,G,I).

   The state machine uses the following macro:

   CouldAssert(S,G,I) = (RPF_interface(S) != I)

4.6.4.1.  Transitions from NoInfo State

   In the NoInfo state, the following events may trigger transitions:

     An (S,G) data packet arrives on downstream interface I
       An (S,G) data packet arrived on a downstream interface.  It is
       optimistically assumed that this router will be the Assert winner
       for this (S,G).  The Assert state machine MUST transition to the
       "I am Assert Winner" state, send an Assert(S,G) to interface I,
       store its own address and metric as the Assert Winner, and set
       the Assert_Timer (AT(S,G,I) to Assert_Time, thereby initiating
       the Assert negotiation for (S,G).



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     Receive Inferior (Assert OR State Refresh) AND
     CouldAssert(S,G,I)==TRUE
       An Assert or State Refresh is received for (S,G) that is inferior
       to our own assert metric on interface I. The Assert state machine
       MUST transition to the "I am Assert Winner" state, send an
       Assert(S,G) to interface I, store its own address and metric as
       the Assert Winner, and set the Assert Timer (AT(S,G,I)) to
       Assert_Time.

     Receive Preferred Assert or State Refresh
       The received Assert or State Refresh has a better metric than
       this router's, and therefore the Assert state machine MUST
       transition to the "I am Assert Loser" state and store the Assert
       Winner's address and metric.  If the metric was received in an
       Assert, the router MUST set the Assert Timer (AT(S,G,I)) to
       Assert_Time.  If the metric was received in a State Refresh, the
       router MUST set the Assert Timer (AT(S,G,I)) to three times the
       received State Refresh Interval.  If CouldAssert(S,G,I) == TRUE,
       the router MUST also multicast a Prune(S,G) to the Assert winner
       with a Prune Hold Time equal to the Assert Timer and evaluate any
       changes in its Upstream(S,G) state machine.

4.6.4.2.  Transitions from Winner State

   When in "I am Assert Winner" state, the following events trigger
   transitions:

     An (S,G) data packet arrives on downstream interface I
       An (S,G) data packet arrived on a downstream interface.  The
       Assert state machine remains in the "I am Assert Winner" state.
       The router MUST send an Assert(S,G) to interface I and set the
       Assert Timer (AT(S,G,I) to Assert_Time.

     Receive Inferior Assert or State Refresh
       An (S,G) Assert is received containing a metric for S that is
       worse than this router's metric for S.  Whoever sent the Assert
       is in error.  The router MUST send an Assert(S,G) to interface I
       and reset the Assert Timer (AT(S,G,I)) to Assert_Time.

     Receive Preferred Assert or State Refresh
       An (S,G) Assert or State Refresh is received that has a better
       metric than this router's metric for S on interface I.  The
       Assert state machine MUST transition to "I am Assert Loser" state
       and store the new Assert Winner's address and metric.  If the
       metric was received in an Assert, the router MUST set the Assert
       Timer (AT(S,G,I)) to Assert_Time.  If the metric was received in
       a State Refresh, the router MUST set the Assert Timer (AT(S,G,I))
       to three times the State Refresh Interval.  The router MUST also



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       multicast a Prune(S,G) to the Assert winner, with a Prune Hold
       Time equal to the Assert Timer, and evaluate any changes in its
       Upstream(S,G) state machine.

     Send State Refresh
       The router is sending a State Refresh(S,G) message on interface
       I.  The router MUST set the Assert Timer (AT(S,G,I)) to three
       times the State Refresh Interval contained in the State
       Refresh(S,G) message.

     AT(S,G,I) Expires
       The (S,G) Assert Timer (AT(S,G,I)) expires.  The Assert state
       machine MUST transition to the NoInfo (NI) state.

     CouldAssert(S,G,I) -> FALSE
       This router's RPF interface changed, making CouldAssert(S,G,I)
       false.  This router can no longer perform the actions of the
       Assert winner, so the Assert state machine MUST transition to
       NoInfo (NI) state, send an AssertCancel(S,G) to interface I,
       cancel the Assert Timer (AT(S,G,I)), and remove itself as the
       Assert Winner.

4.6.4.3.  Transitions from Loser State

   When in "I am Assert Loser" state, the following transitions can
   occur:

     Receive Inferior Assert or State Refresh from Current Winner
       An Assert or State Refresh is received from the current Assert
       winner that is worse than this router's metric for S (typically,
       the winner's metric became worse).  The Assert state machine MUST
       transition to NoInfo (NI) state and cancel AT(S,G,I).  The router
       MUST delete the previous Assert Winner's address and metric and
       evaluate any possible transitions to its Upstream(S,G) state
       machine.  Usually this router will eventually re-assert and win
       when data packets from S have started flowing again.

     Receive Preferred Assert or State Refresh
       An Assert or State Refresh is received that has a metric better
       than or equal to that of the current Assert winner.  The Assert
       state machine remains in Loser (L) state.  If the metric was
       received in an Assert, the router MUST set the Assert Timer
       (AT(S,G,I)) to Assert_Time.  If the metric was received in a
       State Refresh, the router MUST set the Assert Timer (AT(S,G,I))
       to three times the received State Refresh Interval.  If the
       metric is better than the current Assert Winner, the router MUST





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       store the address and metric of the new Assert Winner, and if
       CouldAssert(S,G,I) == TRUE, the router MUST multicast a
       Prune(S,G) to the new Assert winner.

     AT(S,G,I) Expires
       The (S,G) Assert Timer (AT(S,G,I)) expires.  The Assert state
       machine MUST transition to NoInfo (NI) state.  The router MUST
       delete the Assert Winner's address and metric.  If CouldAssert ==
       TRUE, the router MUST evaluate any possible transitions to its
       Upstream(S,G) state machine.

     CouldAssert -> FALSE
       CouldAssert has become FALSE because interface I has become the
       RPF interface for S.  The Assert state machine MUST transition to
       NoInfo (NI) state, cancel AT(S,G,I), and delete information
       concerning the Assert Winner on I.

     CouldAssert -> TRUE
       CouldAssert has become TRUE because interface I used to be the
       RPF interface for S, and now it is not.  The Assert state machine
       MUST transition to NoInfo (NI) state, cancel AT(S,G,I), and
       delete information concerning the Assert Winner on I.

     Current Assert Winner's NeighborLiveness Timer Expires
       The current Assert winner's NeighborLiveness Timer (NLT(N,I)) has
       expired.  The Assert state machine MUST transition to the NoInfo
       (NI) state, delete the Assert Winner's address and metric, and
       evaluate any possible transitions to its Upstream(S,G) state
       machine.

     Receive Prune(S,G), Join(S,G), or Graft(S,G)
       A Prune(S,G), Join(S,G), or Graft(S,G) message was received on
       interface I with its upstream neighbor address set to the
       router's address on I.  The router MUST send an Assert(S,G) on
       the receiving interface I to initiate an Assert negotiation.  The
       Assert state machine remains in the Assert Loser(L) state.  If a
       Graft(S,G) was received, the router MUST respond with a
       GraftAck(S,G).













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4.6.5.  Rationale for Assert Rules

   The following is a summary of the rules for generating and processing
   Assert messages.  It is not intended to be definitive (the state
   machines and pseudocode provide the definitive behavior).  Instead,
   it provides some rationale for the behavior.

   1. The Assert winner for (S,G) must act as the local forwarder for
      (S,G) on behalf of all downstream members.
   2. PIM messages are directed to the RPF' neighbor and not to the
      regular RPF neighbor.
   3. An Assert loser that receives a Prune(S,G), Join(S,G), or
      Graft(S,G) directed to it initiates a new Assert negotiation so
      that the downstream router can correct its RPF'(S).
   4. An Assert winner for (S,G) sends a cancelling assert when it is
      about to stop forwarding on an (S,G) entry.  Example: If a router
      is being taken down, then a canceling assert is sent.

4.7.  PIM Packet Formats

   All PIM-DM packets use the same format as PIM-SM packets.  In the
   event of a discrepancy, PIM-SM [4] should be considered the
   definitive specification.  All PIM control messages have IP protocol
   number 103.  All PIM-DM messages MUST be sent with a TTL of 1.  All
   PIM-DM messages except Graft and Graft Ack messages MUST be sent to
   the ALL-PIM-ROUTERS group.  Graft messages SHOULD be unicast to the
   RPF'(S).  Graft Ack messages MUST be unicast to the sender of the
   Graft.

   The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13.  The IPv6 ALL-PIM-
   ROUTERS group is 'ff02::d'.

4.7.1.  PIM Header

   All PIM control messages have the following header:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver PIM version number is 2.








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   Type
     Types for specific PIM messages.  Available types are as follows:
     0 = Hello
     1 = Register (PIM-SM only)
     2 = Register Stop (PIM-SM only)
     3 = Join/Prune
     4 = Bootstrap (PIM-SM only)
     5 = Assert
     6 = Graft
     7 = Graft Ack
     8 = Candidate RP Advertisement (PIM-SM only)
     9 = State Refresh

   Reserved
     Set to zero on transmission.  Ignored upon receipt.

   Checksum
     The checksum is the standard IP checksum; i.e., the 16 bit one's
     complement of the one's complement sum of the entire PIM message.
     For computing checksum, the checksum field is zeroed.

     For IPv6, the checksum also includes the IPv6 "pseudo-header", as
     specified in RFC 2460, Section 8.1 [5].

4.7.2.  Encoded Unicast Address

   An Encoded Unicast Address has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Addr Family  | Encoding Type |     Unicast Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

   Addr Family
     The PIM Address Family of the 'Unicast Address' field of this
     address.  Values 0 - 127 are as assigned by the IANA for Internet
     Address Families in [9].  Values 128 - 250 are reserved to be
     assigned by the IANA for PIM specific Address Families.  Values 251
     - 255 are designated for private use.  As there is no assignment
     authority for this space; collisions should be expected.

   Encoding Type
     The type of encoding used with a specific Address Family.  The
     value '0' is reserved for this field and represents the native
     encoding of the Address Family.





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   Unicast Address
     The unicast address as represented by the given Address Family and
     Encoding Type.

4.7.3.  Encoded Group Address

   An Encoded Group address has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Addr Family  | Encoding Type |B| Reserved  |Z|  Mask Len     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Group Multicast Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

   Addr Family
     As described above.

   Encoding Type
     As described above.

   B
     Indicates that the group range should use Bidirectional PIM [16].
     Transmitted as zero; ignored upon receipt.

   Reserved
     Transmitted as zero.  Ignored upon receipt.

   Z
     Indicates that the group range is an admin scope zone.  This is
     used in the Bootstrap Router Mechanism [18] only.  For all other
     purposes, this bit is set to zero and ignored on receipt.

   Mask Len
     The mask length field is 8 bits.  The value is the number of
     contiguous left justified one bits used as a mask, which, combined
     with the address, describes a range of addresses.  It is less than
     or equal to the address length in bits for the given Address Family
     and Encoding Type.  If the message is sent for a single address
     then the mask length MUST equal the address length.  PIM-DM routers
     MUST only send for a single address.

   Group Multicast Address
     The address of the multicast group.






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4.7.4.  Encoded Source Address

   An Encoded Source address has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Addr Family  | Encoding Type |  Rsrvd  |S|W|R|  Mask Len     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Source Address
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

   Addr Family
     As described above.

   Encoding Type
     As described above.

   Rsrvd
     Reserved.  Transmitted as zero.  Ignored upon receipt.

   S
     The Sparse Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.

   W
     The Wild Card Bit.  Set to 0 for PIM-DM.  Ignored upon receipt.

   R
     The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
     receipt.

   Mask Len
     As described above.  PIM-DM routers MUST only send for a single
     source address.

   Source Address
     The source address.














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4.7.5.  Hello Message Format

   The PIM Hello message, as defined by PIM-SM [4], has the following
   format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Option Type          |         Option Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Option Value                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Option Type          |         Option Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Option Value                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver, Type, Reserved, Checksum
     Described above.

   Option Type
     The type of option given in the Option Value field.  Available
     types are as follows:

       0              Reserved
       1              Hello Hold Time
       2              LAN Prune Delay
       3 - 16         Reserved
       17             To be assigned by IANA
       18             Deprecated and SHOULD NOT be used
       19             DR Priority (PIM-SM Only)
       20             Generation ID
       21             State Refresh Capable
       22             Bidir Capable
       23 - 65000     To be assigned by IANA
       65001 - 65535  Reserved for Private Use [9]

     Unknown options SHOULD be ignored.





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4.7.5.1.  Hello Hold Time Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Type = 1           |           Length = 2          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Hold Time          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Hold Time is the number of seconds a receiver MUST keep the neighbor
   reachable.  If the Hold Time is set to '0xffff', the receiver of this
   message never times out the neighbor.  This may be used with dial-
   on-demand links to avoid keeping the link up with periodic Hello
   messages.  Furthermore, if the Holdtime is set to '0', the
   information is timed out immediately.  The Hello Hold Time option
   MUST be used by PIM-DM routers.

4.7.5.2.  LAN Prune Delay Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Type = 2           |           Length = 4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |T|       LAN Prune Delay       |       Override Interval       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The LAN_Prune_Delay option is used to tune the prune propagation
   delay on multi-access LANs.  The T bit is used by PIM-SM and SHOULD
   be set to 0 by PIM-DM routers and ignored upon receipt.  The LAN
   Delay and Override Interval fields are time intervals in units of
   milliseconds and are used to tune the value of the J/P Override
   Interval and its derived timer values.  Section 4.3.5 describes how
   these values affect the behavior of a router.  The LAN Prune Delay
   SHOULD be used by PIM-DM routers.















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4.7.5.3.  Generation ID Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 20           |           Length = 4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Generation ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Generation ID is a random value for the interface on which the Hello
   message is sent.  The Generation ID is regenerated whenever PIM
   forwarding is started or restarted on the interface.  The Generation
   ID option MAY be used by PIM-DM routers.

4.7.5.4.  State Refresh Capable Option

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 21           |           Length = 4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Version = 1  |   Interval    |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Interval field is the router's configured State Refresh Interval
   in seconds.  The Reserved field is set to zero and ignored upon
   receipt.  The State Refresh Capable option MUST be used by State
   Refresh capable PIM-DM routers.






















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4.7.6.  Join/Prune Message Format

   PIM Join/Prune messages, as defined in PIM-SM [4], have the following
   format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Upstream Neighbor Address (Encoded Unicast Format)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Reserved    |  Num Groups   |          Hold Time            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Multicast Group Address 1 (Encoded Group Format)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Number of Joined Sources    |   Number of Pruned Sources    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Joined Source Address 1 (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Joined Source Address n (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Pruned Source Address 1 (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Pruned Source Address n (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Multicast Group Address m (Encoded Group Format)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Number of Joined Sources    |   Number of Pruned Sources    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Joined Source Address 1 (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Joined Source Address n (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Pruned Source Address 1 (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Pruned Source Address n (Encoded Source Format)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver, Type, Reserved, Checksum
     Described above.

   Upstream Neighbor Address
     The address of the upstream neighbor.  The format for this address
     is given in the Encoded Unicast address in Section 4.7.2.  PIM-DM
     routers MUST set this field to the RPF next hop.

   Reserved
     Transmitted as zero.  Ignored upon receipt.

   Hold Time
     The number of seconds a receiving PIM-DM router MUST keep a Prune
     state alive, unless removed by a Join or Graft message.  If the
     Hold Time is '0xffff', the receiver MUST NOT remove the Prune state
     unless a corresponding Join or Graft message is received.  The Hold
     Time is ignored in Join messages.

   Number of Groups
     Number of multicast group sets contained in the message.

   Multicast Group Address
     The multicast group address in the Encoded Multicast address format
     given in Section 4.7.3.

   Number of Joined Sources
     Number of Join source addresses listed for a given group.

   Number of Pruned Sources
     Number of Prune source addresses listed for a given group.








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   Join Source Address 1..n
     This list contains the sources from which the sending router wishes
     to continue to receive multicast messages for the given group on
     this interface.  The addresses use the Encoded Source address
     format given in Section 4.7.4.

   Prune Source Address 1..n
     This list contains the sources from which the sending router does
     not wish to receive multicast messages for the given group on this
     interface.  The addresses use the Encoded Source address format
     given in Section 4.7.4.

4.7.7.  Assert Message Format

   PIM Assert Messages, as defined in PIM-SM [4], have the following
   format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Multicast Group Address (Encoded Group Format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Source Address (Encoded Unicast Format)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                     Metric Preference                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver, Type, Reserved, Checksum
     Described above.

   Multicast Group Address
     The multicast group address in the Encoded Multicast address format
     given in Section 4.7.3.

   Source Address
     The source address in the Encoded Unicast address format given in
     Section 4.7.2.

   R
     The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
     receipt.






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   Metric Preference
     The preference value assigned to the unicast routing protocol that
     provided the route to the source.

   Metric
     The cost metric of the unicast route to the source.  The metric is
     in units applicable to the unicast routing protocol used.

4.7.8.  Graft Message Format

   PIM Graft messages use the same format as Join/Prune messages, except
   that the Type field is set to 6.  The source address MUST be in the
   Join section of the message.  The Hold Time field SHOULD be zero and
   SHOULD be ignored when a Graft is received.

4.7.9.  Graft Ack Message Format

   PIM Graft Ack messages are identical in format to the received Graft
   message, except that the Type field is set to 7.  The Upstream
   Neighbor Address field SHOULD be set to the sender of the Graft
   message and SHOULD be ignored upon receipt.

4.7.10.  State Refresh Message Format

   PIM State Refresh Messages have the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Multicast Group Address (Encoded Group Format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Source Address (Encoded Unicast Format)           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Originator Address (Encoded Unicast Format)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                     Metric Preference                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Masklen    |    TTL        |P|N|O|Reserved |   Interval    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver, Type, Reserved, Checksum
     Described above.





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   Multicast Group Address
     The multicast group address in the Encoded Multicast address format
     given in Section 4.7.3.

   Source Address
     The address of the data source in the Encoded Unicast address
     format given in Section 4.7.2.

   Originator Address
     The address of the first hop router in the Encoded Unicast address
     format given in Section 4.7.2.

   R
     The Rendezvous Point Tree bit.  Set to 0 for PIM-DM.  Ignored upon
     receipt.

   Metric Preference
     The preference value assigned to the unicast routing protocol that
     provided the route to the source.

   Metric
     The cost metric of the unicast route to the source.  The metric is
     in units applicable to the unicast routing protocol used.

   Masklen
     The length of the address mask of the unicast route to the source.

   TTL
     Time To Live of the State Refresh message.  Decremented each time
     the message is forwarded.  Note that this is different from the IP
     Header TTL, which is always set to 1.

   P
     Prune indicator flag.  This MUST be set to 1 if the State Refresh
     is to be sent on a Pruned interface.  Otherwise, it MUST be set to
     0.

   N
     Prune Now flag.  This SHOULD be set to 1 by the State Refresh
     originator on every third State Refresh message and SHOULD be
     ignored upon receipt.  This is for compatibility with earlier
     versions of state refresh.

   O
     Assert Override flag.  This SHOULD be set to 1 by upstream routers
     on a LAN if the Assert Timer (AT(S,G)) is not running and SHOULD be
     ignored upon receipt.  This is for compatibility with earlier
     versions of state refresh.



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   Reserved
     Set to zero and ignored upon receipt.

   Interval
     Set by the originating router to the interval (in seconds) between
     consecutive State Refresh messages for this (S,G) pair.

4.8.  PIM-DM Timers

   PIM-DM maintains the following timers.  All timers are countdown
   timers -- they are set to a value and count down to zero, at which
   point they typically trigger an action.  Of course they can just as
   easily be implemented as count-up timers, where the absolute expiry
   time is stored and compared against a real-time clock, but the
   language in this specification assumes that they count downward
   towards zero.

   Global Timers
     Hello Timer: HT

     Per interface (I):
       Per neighbor (N):
         Neighbor Liveness Timer: NLT(N,I)

       Per (S,G) Pair:
         (S,G) Assert Timer: AT(S,G,I)
         (S,G) Prune Timer: PT(S,G,I)
         (S,G) PrunePending Timer: PPT(S,G,I)

       Per (S,G) Pair:
         (S,G) Graft Retry Timer: GRT(S,G)
         (S,G) Upstream Override Timer: OT(S,G)
         (S,G) Prune Limit Timer: PLT(S,G)
         (S,G) Source Active Timer: SAT(S,G)
         (S,G) State Refresh Timer: SRT(S,G)
















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   When timer values are started or restarted, they are set to default
   values.  The following tables summarize those default values.

Timer Name: Hello Timer (HT)
+----------------------+--------+--------------------------------------+
| Value Name           | Value  | Explanation                          |
+----------------------+--------+--------------------------------------+
|Hello_Period          | 30 sec | Periodic interval for hello messages |
+----------------------+--------+--------------------------------------+
|Triggered_Hello_Delay | 5 sec  | Random interval for initial Hello    |
|                      |        | message on bootup or triggered Hello |
|                      |        | message to a rebooting neighbor      |
+----------------------+--------+--------------------------------------+

   Hello messages are sent on every active interface once every
   Hello_Period seconds.  At system power-up, the timer is initialized
   to rand(0,Triggered_Hello_Delay) to prevent synchronization.  When a
   new or rebooting neighbor is detected, a responding Hello is sent
   within rand(0,Triggered_Hello_Delay).

Timer Name: Neighbor Liveness Timer (NLT(N,I))
+-------------------+-----------------+--------------------------------+
| Value Name        | Value           | Explanation                    |
+-------------------+-----------------+--------------------------------+
| Hello Holdtime    | From message    | Hold Time from Hello Message   |
+-------------------+-----------------+--------------------------------+

Timer Name: PrunePending Timer (PPT(S,G,I))
+-----------------------+---------------+------------------------------+
| Value Name            | Value         | Explanation                  |
+-----------------------+---------------+------------------------------+
| J/P_Override_Interval | OI(I) + PD(I) | Short time after a Prune to  |
|                       |               | allow other routers on the   |
|                       |               | LAN to send a Join           |
+-----------------------+---------------+------------------------------+

   The J/P_Override_Interval is the sum of the interface's
   Override_Interval (OI(I)) and Propagation_Delay (PD(I)).  If all
   routers on a LAN are using the LAN Prune Delay option, both
   parameters MUST be set to the largest value on the LAN.  Otherwise,
   the Override_Interval (OI(I)) MUST be set to 2.5 seconds, and the
   Propagation_Delay (PD(I)) MUST be set to 0.5 seconds.









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Timer Name: Prune Timer (PT(S,G,I))
+----------------+----------------+------------------------------------+
| Value Name     | Value          | Explanation                        |
+----------------+----------------+------------------------------------+
| Prune Holdtime | From message   | Hold Time read from Prune Message  |
+----------------+----------------+------------------------------------+

Timer Name: Assert Timer (AT(S,G,I))
+--------------------------+---------+---------------------------------+
| Value Name               | Value   | Explanation                     |
+--------------------------+---------+---------------------------------+
| Assert Time              | 180 sec | Period after last assert before |
|                          |         | assert state is timed out       |
+--------------------------+---------+---------------------------------+

   Note that, for historical reasons, the Assert message lacks a
   Holdtime field.  Thus, changing the Assert Time from the default
   value is not recommended.  If all members of a LAN are state refresh
   enabled, the Assert Time will be three times the received
   RefreshInterval(S,G).

Timer Name: Graft Retry Timer (GRT(S,G))
+--------------------+-------+-----------------------------------------+
| Value Name         | Value | Explanation                             |
+--------------------+-------+-----------------------------------------+
| Graft_Retry_Period | 3 sec | In the absence of receipt of a GraftAck |
|                    |       | message, the time before retransmission |
|                    |       | of a Graft message                      |
+--------------------+-------+-----------------------------------------+

Timer Name: Upstream Override Timer (OT(S,G))
+------------+----------------+----------------------------------------+
| Value Name | Value          | Explanation                            |
+------------+----------------+----------------------------------------|
| t_override | rand(0, OI(I)) | Randomized delay to prevent response   |
|            |                | implosion when sending a join message  |
|            |                | to override someone else's prune       |
+------------+----------------+----------------------------------------+

   t_override is a random value between 0 and the interface's
   Override_Interval (OI(I)).  If all routers on a LAN are using the LAN
   Prune Delay option, the Override_Interval (OI(I)) MUST be set to the
   largest value on the LAN.  Otherwise, the Override_Interval (OI(I))
   MUST be set to 2.5 seconds.







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Timer Name: Prune Limit Timer (PLT(S,G))
+------------+--------------------+------------------------------------+
| Value Name | Value              | Explanation                        |
+------------+--------------------+------------------------------------|
| t_limit    | Default: 210 secs  | Used to prevent Prune storms on a  |
|            |                    | LAN                                |
+------------+--------------------+------------------------------------+

Timer Name: Source Active Timer (SAT(S,G))
+----------------+-------------------+---------------------------------+
| Value Name     | Value             | Explanation                     |
+----------------+-------------------+---------------------------------+
| SourceLifetime | Default: 210 secs | Period of time after receiving  |
|                |                   | a multicast message a directly  |
|                |                   | attached router will continue   |
|                |                   | to send State Refresh messages  |
+----------------+-------------------+---------------------------------+

Timer Name: State Refresh Timer (SRT(S,G))
+-----------------+------------------+---------------------------------+
| Value Name      | Value            | Explanation                     |
+-----------------+------------------+---------------------------------+
| RefreshInterval | Default: 60 secs | Interval between successive     |
|                 |                  | state refresh messages          |
+-----------------+------------------+---------------------------------+

5.  Protocol Interaction Considerations

   PIM-DM is designed to be independent of underlying unicast routing
   protocols and will interact only to the extent needed to perform RPF
   checks.  It is generally assumed that multicast area and autonomous
   system boundaries will correspond to the same boundaries for unicast
   routing, though a deployment that does not follow this assumption is
   not precluded by this specification.

   In general, PIM-DM interactions with other multicast routing
   protocols should be in compliance with RFC 2715 [7].  Other specific
   interactions are noted below.

5.1.  PIM-SM Interactions

   PIM-DM is not intended to interact directly with PIM-SM, even though
   they share a common packet format.  It is particularly important to
   note that a router cannot differentiate between a PIM-DM neighbor and
   a PIM-SM neighbor based on Hello messages.






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   In the event that a PIM-DM router becomes a neighbor of a PIM-SM
   router, the two will effectively form a simplex link, with the PIM-DM
   router sending all multicast messages to the PIM-SM router while the
   PIM-SM router sends no multicast messages to the PIM-DM router.

   The common packet format permits a hybrid PIM-SM/DM implementation
   that would use PIM-SM when a rendezvous point is known and PIM-DM
   when one is not.  Such an implementation is outside the scope of this
   document.

5.2.  IGMP Interactions

   PIM-DM will forward received multicast data packets to neighboring
   host group members in all cases except when the PIM-DM router is in
   an Assert Loser state on that interface.  Note that a PIM Prune
   message is not permitted to prevent the delivery of messages to a
   network with group members.

   A PIM-DM Router MAY use the DR Priority option described in PIM-SM
   [14] to elect an IGMP v1 querier.

5.3.  Source Specific Multicast (SSM) Interactions

   PIM-DM makes no special considerations for SSM [15].  All Prunes and
   Grafts within the protocol are for a specific source, so no
   additional checks have to be made.

5.4.  Multicast Group Scope Boundary Interactions

   Although multicast group scope boundaries are generally identical to
   routing area boundaries, it is conceivable that a routing area might
   be partitioned for a particular multicast group.  PIM-DM routers MUST
   NOT send any messages concerning a particular group across that
   group's scope boundary.

6.  IANA Considerations

6.1.  PIM Address Family

   The PIM Address Family field was chosen to be 8 bits as a tradeoff
   between packet format and use of the IANA assigned numbers.  When the
   PIM packet format was designed, only 15 values were assigned for
   Address Families, and large numbers of new Address Families were not
   envisioned; 8 bits seemed large enough.  However, the IANA assigns
   Address Families in a 16 bit value.  Therefore, the PIM Address
   Family is allocated as follows:





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   Values 0 - 127 are designated to have the same meaning as IANA
   assigned Address Family Numbers [9].

   Values 128 - 250 are designated to be assigned by the IANA based on
   IESG approval, as defined in [8].

   Values 251 - 255 are designated for Private Use, as defined in [8].

6.2.  PIM Hello Options

   Values 17 - 65000 are to be assigned by the IANA.  Since the space is
   large, they may be assigned as First Come First Served, as defined in
   [8].  Assignments are valid for one year and may be renewed.
   Permanent assignments require a specification, as defined in [8].

7.  Security Considerations

   The IPsec authentication header [10] MAY be used to provide data
   integrity protection and groupwise data origin authentication of PIM
   protocol messages.  Authentication of PIM messages can protect
   against unwanted behaviors caused by unauthorized or altered PIM
   messages.  In any case, a PIM router SHOULD NOT accept and process
   PIM messages from neighbors unless a valid Hello message has been
   received from that neighbor.

   Note that PIM-DM has no rendezvous point, and therefore no single
   point of failure that may be vulnerable.  Because PIM-DM uses unicast
   routes provided by an unknown routing protocol, it may suffer
   collateral effects if the unicast routing protocol is attacked.

7.1.  Attacks Based on Forged Messages

   The extent of possible damage depends on the type of counterfeit
   messages accepted.  We next consider the impact of possible
   forgeries. A forged PIM-DM message is link local and can only reach a
   LAN if it was sent by a local host or if it was allowed onto the LAN
   by a compromised or non-compliant router.

   1. A forged Hello message can cause multicast traffic to be delivered
      to links where there are no legitimate requestors, potentially
      wasting bandwidth on that link.  On a multi-access LAN, the
      effects are limited without the capability to forge a Join
      message, as other routers will Prune the link if the traffic is
      not desired.

   2. A forged Join/Prune message can cause multicast traffic to be
      delivered to links where there are no legitimate requestors,
      potentially wasting bandwidth on that link.  A forged Prune



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      message on a multi-access LAN is generally not a significant
      attack in PIM, because any legitimately joined router on the LAN
      would override the Prune with a Join before the upstream router
      stops forwarding data to the LAN.

   3. A forged Graft message can cause multicast traffic to be delivered
      to links where there are no legitimate requestors, potentially
      wasting bandwidth on that link.  In principle, Graft messages
      could be sent multiple hops because they are unicast to the
      upstream router.  This should not be a problem, as the remote
      forger should have no way to get a Hello message to the target of
      the attack.  Without a valid Hello message, the receiving router
      SHOULD NOT accept the Graft.

   4. A forged GraftAck message has no impact, as it will be ignored
      unless the router has recently sent a Graft to its upstream
      router.

   5. By forging an Assert message on a multi-access LAN, an attacker
      could cause the legitimate forwarder to stop forwarding traffic to
      the LAN.  Such a forgery would prevent any hosts downstream of
      that LAN from receiving traffic.

   6. A forged State Refresh message on a multi-access LAN would have
      the same impact as a forged Assert message, having the same
      general functions.  In addition, forged State Refresh messages
      would be propagated downstream and might be used in a denial of
      service attack.  Therefore, a PIM-DM router SHOULD rate limit
      State Refresh messages propagated.

7.2.  Non-cryptographic Authentication Mechanisms

   A PIM-DM router SHOULD provide an option to limit the set of
   neighbors from which it will accept PIM-DM messages.  Either static
   configuration of IP addresses or an IPSec security association may be
   used.  All options that restrict the range of addresses from which
   packets are accepted MUST default to allowing all packets.

   Furthermore, a PIM router SHOULD NOT accept protocol messages from a
   router from which it has not yet received a valid Hello message.

7.3.  Authentication Using IPsec

   The IPSec [10] transport mode using the Authentication Header (AH) is
   the recommended method to prevent the above attacks in PIM.  The
   specific AH authentication algorithm and parameters, including the
   choice of authentication algorithm and the choice of key, are
   configured by the network administrator.  The Encapsulating Security



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   Payload (ESP) MAY also be used to provide both encryption and
   authentication of PIM protocol messages.  When IPsec authentication
   is used, a PIM router SHOULD reject (drop without processing) any
   unauthorized PIM protocol messages.

   To use IPSec, the administrator of a PIM network configures each PIM
   router with one or more Security Associations and associated Security
   Parameters Indices that are used by senders to authenticate PIM
   protocol messages and are used by receivers to authenticate received
   PIM protocol messages.  This document does not describe protocols for
   establishing Security Associations.  It assumes that manual
   configuration of Security Associations is performed, but it does not
   preclude the use of some future negotiation protocol such as GDOI
   [17] to establish Security Associations.

   The network administrator defines a Security Association (SA) and
   Security Parameters Index (SPI) to be used to authenticate all PIM-DM
   protocol messages from each router on each link in a PIM-DM domain.

   In order to avoid the problem of allocating individual keys for each
   neighbor on a link to each individual router, it is acceptable to
   establish only one authentication key for all PIM-DM routers on a
   link.  This will not specifically authenticate the individual router
   sending the message, but will ensure that the sender is a PIM-DM
   router on that link.  If this method is used, the receiver of the
   message MUST ignore the received sequence number, thus disabling
   anti-replay mechanisms.  The effects of disabling anti-replay
   mechanisms are essentially the same as the effects of forged
   messages, described in Section 7.1, with the additional protection
   that the forger can only reuse legitimate messages.

   The Security Policy Database at a PIM-DM router should be configured
   to ensure that all incoming and outgoing PIM-DM packets use the SA
   associated with the interface to which the packet is sent.  Note
   that, according to [10], there is nominally a different Security
   Association Database (SAD) for each router interface.  Thus, the
   selected Security Association for an inbound PIM-DM packet can vary
   depending on the interface on which the packet arrived.  This fact
   allows the network administrator to use different authentication
   methods for each link, even though the destination address is the
   same for most PIM-DM packets, regardless of interface.










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7.4.  Denial of Service Attacks

   There are a number of possible denial of service attacks against PIM
   that can be caused by generating false PIM protocol messages or even
   by generating false data traffic.  Authenticating PIM protocol
   traffic prevents some, but not all, of these attacks.  The possible
   attacks include the following:

   * Sending packets to many different group addresses quickly can
     amount to a denial of service attack in and of itself.  These
     messages will initially be flooded throughout the network before
     they are pruned back.  The maintenance of state machines and State
     Refresh messages will be a continual drain on network resources.

   * Forged State Refresh messages sent quickly could be propagated by
     downstream routers, creating a potential denial of service attack.
     Therefore, a PIM-DM router SHOULD limit the rate of State Refresh
     messages propagated.

8.  Acknowledgments

   The major features of PIM-DM were originally designed by Stephen
   Deering, Deborah Estrin, Dino Farinacci, Van Jacobson, Ahmed Helmy,
   David Meyer, and Liming Wei.  Additional features for state refresh
   were designed by Dino Farinacci, Isidor Kouvelas, and Kurt Windisch.
   This revision was undertaken to incorporate some of the lessons
   learned during the evolution of the PIM-SM specification and early
   deployments of PIM-DM.

   Thanks the PIM Working Group for their comments.

9.  References

9.1.  Normative References

   [1]  Deering, S., "Host extensions for IP multicasting", STD 5, RFC
        1112, August 1989.

   [2]  Fenner, W., "Internet Group Management Protocol, Version 2", RFC
        2236, November 1997.

   [3]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
        Thyagarajan, "Internet Group Management Protocol, Version 3",
        RFC 3376, October 2002.







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   [4]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
        Handley, M., Jacobson, V., Liu, C., Sharma, P., and L. Wei,
        "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol
        Specification", RFC 2362, June 1998.

   [5]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.

   [6]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
        Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [7]  Thaler, D., "Interoperability Rules for Multicast Routing
        Protocols", RFC 2715, October 1999.

   [8]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [9]  IANA, "Address Family Numbers", linked from
        http://www.iana.org/numbers.html.

   [10] Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [11] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

9.2.  Informative References

   [12] Deering, S.E., "Multicast Routing in a Datagram Internetwork",
        Ph.D. Thesis, Electrical Engineering Dept., Stanford University,
        December 1991.

   [13] Waitzman, D., Partridge, C., and S. Deering, "Distance Vector
        Multicast Routing Protocol", RFC 1075, November 1988.

   [14] Fenner,  W., Handley, M., Holbrook, H., and I. Kouvelas,
        "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
        Specification (Revised)", Work in Progress.

   [15] Holbrook, H. and B. Cain, "Source Specific Multicast for IP",
        Work in Progress.

   [16] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano, "Bi-
        directional Protocol Independent Multicast", Work in Progress.

   [17] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group
        Domain of Interpretation", RFC 3547, July 2003.




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   [18] Fenner, W., Handley, M., Kermode, R., and D. Thaler, "Bootstrap
        Router (BSR) Mechanism for PIM Sparse Mode", Work in Progress.

Authors' Addresses

   Andrew Adams
   NextHop Technologies
   825 Victors Way, Suite 100
   Ann Arbor, MI 48108-2738

   EMail: ala@nexthop.com


   Jonathan Nicholas
   ITT Industries
   Aerospace/Communications Division
   100 Kingsland Rd
   Clifton, NJ  07014

   EMail: jonathan.nicholas@itt.com


   William Siadak
   NextHop Technologies
   825 Victors Way, Suite 100
   Ann Arbor, MI 48108-2738

   EMail: wfs@nexthop.com























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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.






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