ARMWARE RFC Archive <- RFC Index (7701..7800)

RFC 7761

(also STD 83)

Obsoletes RFC 4601

Internet Engineering Task Force (IETF)                         B. Fenner
Request for Comments: 7761                               Arista Networks
STD: 83                                                       M. Handley
Obsoletes: 4601                                                      UCL
Category: Standards Track                                    H. Holbrook
ISSN: 2070-1721                                              I. Kouvelas
                                                         Arista Networks
                                                               R. Parekh
                                                     Cisco Systems, Inc.
                                                                Z. Zhang
                                                        Juniper Networks
                                                                L. Zheng
                                                     Huawei Technologies
                                                              March 2016

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

Abstract

   This document specifies Protocol Independent Multicast - Sparse Mode
   (PIM-SM).  PIM-SM is a multicast routing protocol that can use the
   underlying unicast routing information base or a separate multicast-
   capable routing information base.  It builds unidirectional shared
   trees rooted at a Rendezvous Point (RP) per group, and it optionally
   creates shortest-path trees per source.

   This document obsoletes RFC 4601 by replacing it, addresses the
   errata filed against it, removes the optional (*,*,RP), PIM Multicast
   Border Router features and authentication using IPsec that lack
   sufficient deployment experience (see Appendix A), and moves the PIM
   specification to Internet Standard.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7761.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................5
   2. Terminology .....................................................5
      2.1. Definitions ................................................5
      2.2. Pseudocode Notation ........................................7
   3. PIM-SM Protocol Overview ........................................7
      3.1. Phase One: RP Tree .........................................8
      3.2. Phase Two: Register-Stop ...................................9
      3.3. Phase Three: Shortest-Path Tree ...........................10
      3.4. Source-Specific Joins .....................................10
      3.5. Source-Specific Prunes ....................................11
      3.6. Multi-Access Transit LANs .................................11
      3.7. RP Discovery ..............................................12
   4. Protocol Specification .........................................12
      4.1. PIM Protocol State ........................................13
           4.1.1. General-Purpose State ..............................14
           4.1.2. (*,G) State ........................................15
           4.1.3. (S,G) State ........................................17
           4.1.4. (S,G,rpt) State ....................................19
           4.1.5. State Summarization Macros .........................20
      4.2. Data Packet Forwarding Rules ..............................24
           4.2.1. Last-Hop Switchover to the SPT .....................27
           4.2.2. Setting and Clearing the (S,G) SPTbit ..............27
      4.3. Designated Routers (DRs) and Hello Messages ...............29
           4.3.1. Sending Hello Messages .............................29
           4.3.2. DR Election ........................................31
           4.3.3. Reducing Prune Propagation Delay on LANs ...........33
           4.3.4. Maintaining Secondary Address Lists ................36
      4.4. PIM Register Messages .....................................37
           4.4.1. Sending Register Messages from the DR ..............38
           4.4.2. Receiving Register Messages at the RP ..............43

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      4.5. PIM Join/Prune Messages ...................................44
           4.5.1. Receiving (*,G) Join/Prune Messages ................45
           4.5.2. Receiving (S,G) Join/Prune Messages ................50
           4.5.3. Receiving (S,G,rpt) Join/Prune Messages ............54
           4.5.4. Sending (*,G) Join/Prune Messages ..................61
           4.5.5. Sending (S,G) Join/Prune Messages ..................65
           4.5.6. (S,G,rpt) Periodic Messages ........................71
           4.5.7. State Machine for (S,G,rpt) Triggered Messages .....72
      4.6. PIM Assert Messages .......................................76
           4.6.1. (S,G) Assert Message State Machine .................77
           4.6.2. (*,G) Assert Message State Machine .................85
           4.6.3. Assert Metrics .....................................93
           4.6.4. AssertCancel Messages ..............................94
           4.6.5. Assert State Macros ................................95
      4.7. PIM Bootstrap and RP Discovery ............................98
           4.7.1. Group-to-RP Mapping ................................99
           4.7.2. Hash Function .....................................100
      4.8. Source-Specific Multicast ................................101
           4.8.1. Protocol Modifications for SSM Destination
                  Addresses .........................................102
           4.8.2. PIM-SSM-Only Routers ..............................102
      4.9. PIM Packet Formats .......................................104
           4.9.1. Encoded Source and Group Address Formats ..........105
           4.9.2. Hello Message Format ..............................108
           4.9.3. Register Message Format ...........................111
           4.9.4. Register-Stop Message Format ......................113
           4.9.5. Join/Prune Message Format .........................114
                  4.9.5.1. Group Set Source List Rules ..............117
                  4.9.5.2. Group Set Fragmentation ..................120
           4.9.6. Assert Message Format .............................121
      4.10. PIM Timers ..............................................122
      4.11. Timer Values ............................................124
   5. IANA Considerations ...........................................130
      5.1. PIM Address Family .......................................130
      5.2. PIM Hello Options ........................................130
   6. Security Considerations .......................................131
      6.1. Attacks Based on Forged Messages .........................131
           6.1.1. Forged Link-Local Messages ........................131
           6.1.2. Forged Unicast Messages ...........................132
      6.2. Non-cryptographic Authentication Mechanisms ..............132
      6.3. Authentication ...........................................133
      6.4. Denial-of-Service Attacks ................................133
   7. References ....................................................133
      7.1. Normative References .....................................133
      7.2. Informative References ...................................134
   Appendix A. Functionality Removed from RFC 4601 ..................136
   Acknowledgements .................................................136
   Authors' Addresses ...............................................136

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RFC 7761             PIM-SM Specification (Revised)           March 2016

List of Figures (Shown in Tabular Form)

   Figure 1. Per-(S,G) Register State Machine at a DR ................39
   Figure 2. Downstream Per-Interface (*,G) State Machine ............47
   Figure 3. Downstream Per-Interface (S,G) State Machine ............51
   Figure 4. Downstream Per-Interface (S,G,rpt) State Machine ........56
   Figure 5. Upstream (*,G) State Machine ............................62
   Figure 6. Upstream (S,G) State Machine ............................66
   Figure 7. Upstream (S,G,rpt) State Machine for Triggered
             Messages ................................................72
   Figure 8. Per-Interface (S,G) Assert State Machine ................78
   Figure 9. Per-interface (*,G) Assert State Machine ................87

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RFC 7761             PIM-SM Specification (Revised)           March 2016

1.  Introduction

   This document specifies a protocol for efficiently routing multicast
   groups that may span wide-area (and inter-domain) internets.  This
   protocol is called Protocol Independent Multicast - Sparse Mode
   (PIM-SM) because, although it may use the underlying unicast routing
   to provide reverse-path information for multicast tree building, it
   is not dependent on any particular unicast routing protocol.

   PIM-SM Version 2 was specified in RFC 4601 as a Proposed Standard.
   This document is intended to address the reported errata and to
   remove the optional (*,*,RP), PIM Multicast Border Router features
   and authentication using IPsec that lacks sufficient deployment
   experience, to advance PIM-SM to Internet Standard.

   This document specifies the same protocol as RFC 4601, and
   implementations per the specification in this document will be able
   to interoperate successfully with implementations per RFC 4601.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2.1.  Definitions

   The following terms have special significance for PIM-SM:

   Rendezvous Point (RP)
      An RP is a router that has been configured to be used as the root
      of the non-source-specific distribution tree for a multicast
      group.  Join messages from receivers for a group are sent towards
      the RP, and data from senders is sent to the RP so that receivers
      can discover who the senders are and start to receive traffic
      destined for the group.

   Designated Router (DR)
      A shared-media LAN like Ethernet may have multiple PIM-SM routers
      connected to it.  A single one of these routers, the DR, will act
      on behalf of directly connected hosts with respect to the PIM-SM
      protocol.  A single DR is elected per interface (LAN or otherwise)
      using a simple election process.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   MRIB
      Multicast Routing Information Base.  This is the multicast
      topology table, which is typically derived from the unicast
      routing table, or routing protocols such as Multiprotocol BGP
      (MBGP) that carry multicast-specific topology information.  In
      PIM-SM, the MRIB is used to decide where to send Join/Prune
      messages.  A secondary function of the MRIB is to provide routing
      metrics for destination addresses; these metrics are used when
      sending and processing Assert messages.

   RPF Neighbor
      RPF stands for "Reverse Path Forwarding".  The RPF Neighbor of a
      router with respect to an address is the neighbor that the MRIB
      indicates should be used to forward packets to that address.  In
      the case of a PIM-SM multicast group, the RPF neighbor is the
      router that a Join message for that group would be directed to, in
      the absence of modifying Assert state.

   TIB
      Tree Information Base.  This is the collection of state at a PIM
      router that has been created by receiving PIM Join/Prune messages,
      PIM Assert messages, and Internet Group Management Protocol (IGMP)
      or Multicast Listener Discovery (MLD) information from local
      hosts.  It essentially stores the state of all multicast
      distribution trees at that router.

   MFIB
      Multicast Forwarding Information Base.  The TIB holds all the
      state that is necessary to forward multicast packets at a router.
      However, although this specification defines forwarding in terms
      of the TIB, to actually forward packets using the TIB is very
      inefficient.  Instead, a real router implementation will normally
      build an efficient MFIB from the TIB state to perform forwarding.
      How this is done is implementation-specific and is not discussed
      in this document.

   Upstream
      Towards the root of the tree.  The root of the tree may be either
      the source or the RP, depending on the context.

   Downstream
      Away from the root of the tree.

   GenID
      Generation Identifier, used to detect reboots.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

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 is the elements of set A that are not in set B.

      NULL    is the empty set or list.

   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.

   Unless otherwise noted, operations specified by statements having
   multiple (+) and (-) operators should be evaluated from left to
   right, i.e., A (+) B (-) C is the set resulting from union of sets A
   and B minus elements in set C.

3.  PIM-SM Protocol Overview

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

   PIM relies on an underlying topology-gathering protocol to populate a
   routing table with routes.  This routing table is called the
   Multicast Routing Information Base (MRIB).  The routes in this table
   may be taken directly from the unicast routing table, or they may be
   different and provided by a separate routing protocol such as MBGP
   [10].  Regardless of how it is created, the primary role of the MRIB
   in the PIM protocol is to provide the next-hop router along a
   multicast-capable path to each destination subnet.  The MRIB is used
   to determine the next-hop neighbor to which any PIM Join/Prune
   message is sent.  Data flows along the reverse path of the Join
   messages.  Thus, in contrast to the unicast RIB, which specifies the
   next hop that a data packet would take to get to some subnet, the
   MRIB gives reverse-path information and indicates the path that a
   multicast data packet would take from its origin subnet to the router
   that has the MRIB.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Like all multicast routing protocols that implement the service model
   from RFC 1112 [3], PIM-SM must be able to route data packets from
   sources to receivers without either the sources or receivers knowing
   a priori of the existence of the others.  This is essentially done in
   three phases, although as senders and receivers may come and go at
   any time, all three phases may occur simultaneously.

3.1.  Phase One: RP Tree

   In phase one, a multicast receiver expresses its interest in
   receiving traffic destined for a multicast group.  Typically, it does
   this using IGMP [2] or MLD [4], but other mechanisms might also serve
   this purpose.  One of the receiver's local routers is elected as the
   Designated Router (DR) for that subnet.  On receiving the receiver's
   expression of interest, the DR then sends a PIM Join message towards
   the RP for that multicast group.  This Join message is known as a
   (*,G) Join because it joins group G for all sources to that group.
   The (*,G) Join travels hop-by-hop towards the RP for the group, and
   in each router it passes through, multicast tree state for group G is
   instantiated.  Eventually, the (*,G) Join either reaches the RP or
   reaches a router that already has (*,G) Join state for that group.
   When many receivers join the group, their Join messages converge on
   the RP and form a distribution tree for group G that is rooted at the
   RP.  This is known as the RP Tree (RPT), and is also known as the
   shared tree because it is shared by all sources sending to that
   group.  Join messages are resent periodically so long as the receiver
   remains in the group.  When all receivers on a leaf-network leave the
   group, the DR will send a PIM (*,G) Prune message towards the RP for
   that multicast group.  However, if the Prune message is not sent for
   any reason, the state will eventually time out.

   A multicast data sender just starts sending data destined for a
   multicast group.  The sender's local router (DR) takes those data
   packets, unicast-encapsulates them, and sends them directly to the
   RP.  The RP receives these encapsulated data packets, decapsulates
   them, and forwards them onto the shared tree.  The packets then
   follow the (*,G) multicast tree state in the routers on the RP Tree,
   being replicated wherever the RP Tree branches, and eventually
   reaching all the receivers for that multicast group.  The process of
   encapsulating data packets to the RP is called registering, and the
   encapsulation packets are known as PIM Register packets.

   At the end of phase one, multicast traffic is flowing encapsulated to
   the RP, and then natively over the RP tree to the multicast
   receivers.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

3.2.  Phase Two: Register-Stop

   Register-encapsulation of data packets is inefficient for two
   reasons:

   o  Encapsulation and decapsulation may be relatively expensive
      operations for a router to perform, depending on whether or not
      the router has appropriate hardware for these tasks.

   o  Traveling all the way to the RP, and then back down the shared
      tree may result in the packets traveling a relatively long
      distance to reach receivers that are close to the sender.  For
      some applications, this increased latency or bandwidth consumption
      is undesirable.

   Although Register-encapsulation may continue indefinitely, for these
   reasons, the RP will normally choose to switch to native forwarding.
   To do this, when the RP receives a register-encapsulated data packet
   from source S on group G, it will normally initiate an (S,G) source-
   specific Join towards S.  This Join message travels hop-by-hop
   towards S, instantiating (S,G) multicast tree state in the routers
   along the path.  (S,G) multicast tree state is used only to forward
   packets for group G if those packets come from source S.  Eventually
   the Join message reaches S's subnet or a router that already has
   (S,G) multicast tree state, and then packets from S start to flow
   following the (S,G) tree state towards the RP.  These data packets
   may also reach routers with (*,G) state along the path towards the
   RP; if they do, they can shortcut onto the RP tree at this point.

   While the RP is in the process of joining the source-specific tree
   for S, the data packets will continue being encapsulated to the RP.
   When packets from S also start to arrive natively at the RP, the RP
   will be receiving two copies of each of these packets.  At this
   point, the RP starts to discard the encapsulated copy of these
   packets, and it sends a Register-Stop message back to S's DR to
   prevent the DR from unnecessarily encapsulating the packets.

   At the end of phase two, traffic will be flowing natively from S
   along a source-specific tree to the RP, and from there along the
   shared tree to the receivers.  Where the two trees intersect, traffic
   may transfer from the source-specific tree to the RP tree and thus
   avoid taking a long detour via the RP.

   Note that a sender may start sending before or after a receiver joins
   the group, and thus phase two may happen before the shared tree to
   the receiver is built.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

3.3.  Phase Three: Shortest-Path Tree

   Although having the RP join back towards the source removes the
   encapsulation overhead, it does not completely optimize the
   forwarding paths.  For many receivers, the route via the RP may
   involve a significant detour when compared with the shortest path
   from the source to the receiver.

   To obtain lower latencies or more efficient bandwidth utilization, a
   router on the receiver's LAN, typically the DR, may optionally
   initiate a transfer from the shared tree to a source-specific
   shortest-path tree (SPT).  To do this, it issues an (S,G) Join
   towards S.  This instantiates state in the routers along the path to
   S.  Eventually, this join either reaches S's subnet or reaches a
   router that already has (S,G) state.  When this happens, data packets
   from S start to flow following the (S,G) state until they reach the
   receiver.

   At this point, the receiver (or a router upstream of the receiver)
   will be receiving two copies of the data: one from the SPT and one
   from the RPT.  When the first traffic starts to arrive from the SPT,
   the DR or upstream router starts to drop the packets for G from S
   that arrive via the RP tree.  In addition, it sends an (S,G) Prune
   message towards the RP.  This is known as an (S,G,rpt) Prune.  The
   Prune message travels hop-by-hop, instantiating state along the path
   towards the RP indicating that traffic from S for G should NOT be
   forwarded in this direction.  The prune is propagated until it
   reaches the RP or a router that still needs the traffic from S for
   other receivers.

   By now, the receiver will be receiving traffic from S along the
   shortest-path tree between the receiver and S.  In addition, the RP
   is receiving the traffic from S, but this traffic is no longer
   reaching the receiver along the RP tree.  As far as the receiver is
   concerned, this is the final distribution tree.

3.4.  Source-Specific Joins

   IGMPv3 permits a receiver to join a group and specify that it only
   wants to receive traffic for a group if that traffic comes from a
   particular source.  If a receiver does this, and no other receiver on
   the LAN requires all the traffic for the group, then the DR may omit
   performing a (*,G) join to set up the shared tree, and instead issue
   a source-specific (S,G) join only.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   The range of multicast addresses from 232.0.0.0 to 232.255.255.255 is
   currently set aside for source-specific multicast in IPv4.  For
   groups in this range, receivers should only issue source-specific
   IGMPv3 joins.  If a PIM router receives a non-source-specific join
   for a group in this range, it should ignore it.

3.5.  Source-Specific Prunes

   IGMPv3 also permits a receiver to join a group and to specify that it
   only wants to receive traffic for a group if that traffic does not
   come from a specific source or sources.  In this case, the DR will
   perform a (*,G) join as normal, but may combine this with an
   (S,G,rpt) prune for each of the sources the receiver does not wish to
   receive.

3.6.  Multi-Access Transit LANs

   The overview so far has concerned itself with point-to-point transit
   links.  However, using multi-access LANs such as Ethernet for transit
   is not uncommon.  This can cause complications for three reasons:

   o  Two or more routers on the LAN may issue (*,G) Joins to different
      upstream routers on the LAN because they have inconsistent MRIB
      entries regarding how to reach the RP.  Both paths on the RP tree
      will be set up, causing two copies of all the shared tree traffic
      to appear on the LAN.

   o  Two or more routers on the LAN may issue (S,G) Joins to different
      upstream routers on the LAN because they have inconsistent MRIB
      entries regarding how to reach source S.  Both paths on the
      source-specific tree will be set up, causing two copies of all the
      traffic from S to appear on the LAN.

   o  A router on the LAN may issue a (*,G) Join to one upstream router
      on the LAN, and another router on the LAN may issue an (S,G) Join
      to a different upstream router on the same LAN.  Traffic from S
      may reach the LAN over both the RPT and the SPT.  If the receiver
      behind the downstream (*,G) router doesn't issue an (S,G,rpt)
      prune, then this condition would persist.

   All of these problems are caused by there being more than one
   upstream router with join state for the group or source-group pair.
   PIM does not prevent such duplicate joins from occurring; instead,
   when duplicate data packets appear on the LAN from different routers,
   these routers notice this and then elect a single forwarder.  This
   election is performed using PIM Assert messages, which resolve the
   problem in favor of the upstream router that has (S,G) state; or, if

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   neither router or both routers have (S,G) state, then the problem is
   resolved in favor of the router with the best metric to the RP for RP
   trees, or the best metric to the source for source-specific trees.

   These Assert messages are also received by the downstream routers on
   the LAN, and these cause subsequent Join messages to be sent to the
   upstream router that won the Assert.

3.7.  RP Discovery

   PIM-SM routers need to know the address of the RP for each group for
   which they have (*,G) state.  This address is obtained automatically
   (e.g., embedded-RP), through a bootstrap mechanism, or through static
   configuration.

   One dynamic way to do this is to use the Bootstrap Router (BSR)
   mechanism [11].  One router in each PIM domain is elected the BSR
   through a simple election process.  All the routers in the domain
   that are configured to be candidates to be RPs periodically unicast
   their candidacy to the BSR.  From the candidates, the BSR picks an
   RP-set, and periodically announces this set in a Bootstrap message.
   Bootstrap messages are flooded hop-by-hop throughout the domain until
   all routers in the domain know the RP-Set.

   To map a group to an RP, a router hashes the group address into the
   RP-set using an order-preserving hash function (one that minimizes
   changes if the RP-Set changes).  The resulting RP is the one that it
   uses as the RP for that group.

4.  Protocol Specification

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

   o  Section 4.1 details the protocol state stored.

   o  Section 4.2 specifies the data packet forwarding rules.

   o  Section 4.3 specifies Designated Router (DR) election and the
      rules for sending and processing Hello messages.

   o  Section 4.4 specifies the PIM Register generation and processing
      rules.

   o  Section 4.5 specifies the PIM Join/Prune generation and processing
      rules.

   o  Section 4.6 specifies the PIM Assert generation and processing
      rules.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   o  Section 4.7 specifies the RP discovery mechanisms.

   o  Section 4.8 describes PIM-SSM, the subset of PIM required to
      support Source-Specific Multicast.

   o  Section 4.9 specifies the PIM packet formats.

   o  Section 4.10 provides a summary of PIM-SM timers, and Section 4.11
      provides their default values.

4.1.  PIM Protocol State

   This section specifies all the protocol state that a PIM
   implementation should maintain in order to function correctly.  We
   term this state the Tree Information Base (TIB), as it holds the
   state of all the multicast distribution trees at this router.  In
   this specification, we define PIM 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.

   Although we specify precisely the state to be kept, this does not
   mean that an implementation of PIM-SM needs 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-SM implementation
   is free to hold whatever internal state it requires and will still be
   conformant with this specification so long as it results in the same
   externally visible protocol behavior as an abstract router that holds
   the following state.

   We divide TIB state into three sections:

   (*,G) state
        State that maintains the RP tree for G.

   (S,G) state
        State that maintains a source-specific tree for source S and
        group G.

   (S,G,rpt) state
        State that maintains source-specific information about source S
        on the RP tree for G.  For example, if a source is being
        received on the source-specific tree, it will normally have been
        pruned off the RP tree.  This prune state is (S,G,rpt) state.

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   The state that should be kept is described below.  Of course,
   implementations will only maintain state when it is relevant to
   forwarding operations; for example, the "NoInfo" state might be
   assumed from the lack of other state information rather than being
   held explicitly.

4.1.1.  General-Purpose State

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

   For each interface:

      o  Effective Override Interval

      o  Effective Propagation Delay

      o  Suppression state: One of {"Enable", "Disable"}

      Neighbor State:

         For each neighbor:

         o  Information from neighbor's Hello

         o  Neighbor's GenID.

         o  Neighbor Liveness Timer (NLT)

      Designated Router (DR) State:

         o  Designated Router's IP Address

         o  DR's DR Priority

   The Effective Override Interval, the Effective Propagation Delay, and
   the Interface suppression state are described in Section 4.3.3.
   Designated Router state is described in Section 4.3.

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4.1.2.  (*,G) State

   For every group G, a router keeps the following state:

   (*,G) state:
        For each interface:

             Local Membership:
                  State: One of {"NoInfo", "Include"}

             PIM (*,G) Join/Prune State:

                  o  State: One of {"NoInfo" (NI), "Join" (J),
                     "Prune-Pending" (PP)}

                  o  Prune-Pending Timer (PPT)

                  o  Join/Prune Expiry Timer (ET)

             (*,G) Assert Winner State

                  o  State: One of {"NoInfo" (NI), "I lost Assert" (L),
                     "I won Assert" (W)}

                  o  Assert Timer (AT)

                  o  Assert winner's IP Address (AssertWinner)

                  o  Assert winner's Assert Metric (AssertWinnerMetric)

        Not interface specific:

             Upstream (*,G) Join/Prune State:

                  o  State: One of {"NotJoined(*,G)", "Joined(*,G)"}

             o  Upstream Join/Prune Timer (JT)

             o  Last RP Used

             o  Last RPF Neighbor towards RP that was used

   Local membership is the result of the local membership mechanism
   (such as IGMP or MLD) running on that interface.  It need not be kept
   if this router is not the DR on that interface unless this router won
   a (*,G) assert on this interface for this group, although
   implementations may optionally keep this state in case they become
   the DR or assert winner.  It is RECOMMENDED to store this information

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   if possible, as it reduces latency converging to stable operating
   conditions after a failure causing a change of DR.  This information
   is used by the pim_include(*,G) macro described in Section 4.1.5.

   PIM (*,G) Join/Prune state is the result of receiving PIM (*,G)
   Join/Prune messages on this interface and is specified in
   Section 4.5.1.  The state is used by the macros that calculate the
   outgoing interface list in Section 4.1.5, and in the JoinDesired(*,G)
   macro (defined in Section 4.5.4) that is used in deciding whether a
   Join(*,G) should be sent upstream.

   (*,G) Assert Winner state is the result of sending or receiving (*,G)
   Assert messages on this interface.  It is specified in Section 4.6.2.

   The upstream (*,G) Join/Prune State reflects the state of the
   upstream (*,G) state machine described in Section 4.5.4.

   The upstream (*,G) Join/Prune Timer is used to send out periodic
   Join(*,G) messages, and to override Prune(*,G) messages from peers on
   an upstream LAN interface.

   The last RP used must be stored because if the RP changes, then state
   must be torn down and rebuilt for groups whose RP changes.

   The last RPF neighbor towards the RP is stored because if the MRIB
   changes, then the RPF neighbor towards the RP may change.  If it does
   so, then we need to trigger a new Join(*,G) to the new upstream
   neighbor and a Prune(*,G) to the old upstream neighbor.  Similarly,
   if a router detects through a changed GenID in a Hello message that
   the upstream neighbor towards the RP has rebooted, then it SHOULD
   re-instantiate state by sending a Join(*,G).  These mechanisms are
   specified in Section 4.5.4.

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4.1.3.  (S,G) State

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

   (S,G) state:

        For each interface:

             Local Membership:
                  State: One of {"NoInfo", "Include"}

             PIM (S,G) Join/Prune State:

                  o  State: One of {"NoInfo" (NI), "Join" (J),
                     "Prune-Pending" (PP)}

                  o  Prune-Pending Timer (PPT)

                  o  Join/Prune Expiry Timer (ET)

             (S,G) Assert Winner State

                  o  State: One of {"NoInfo" (NI), "I lost Assert" (L),
                     "I won Assert" (W)}

                  o  Assert Timer (AT)

                  o  Assert winner's IP Address (AssertWinner)

                  o  Assert winner's Assert Metric (AssertWinnerMetric)

        Not interface specific:

             Upstream (S,G) Join/Prune State:

                  o  State: One of {"NotJoined(S,G)", "Joined(S,G)"}

             o  Upstream (S,G) Join/Prune Timer (JT)

             o  Last RPF Neighbor towards S that was used

             o  SPTbit (indicates (S,G) state is active)

             o  (S,G) Keepalive Timer (KAT)

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             Additional (S,G) state at the DR:

                  o  Register state: One of {"Join" (J), "Prune" (P),
                     "Join-Pending" (JP), "NoInfo" (NI)}

                  o  Register-Stop Timer (RST)

   Local membership is the result of the local source-specific
   membership mechanism (such as IGMP Version 3) running on that
   interface and specifying that this particular source should be
   included.  As stored here, this state is the resulting state after
   any IGMPv3 inconsistencies have been resolved.  It need not be kept
   if this router is not the DR on that interface unless this router won
   an (S,G) assert on this interface for this group.  However, it is
   RECOMMENDED to store this information if possible, as it reduces
   latency converging to stable operating conditions after a failure
   causing a change of DR.  This information is used by the
   pim_include(S,G) macro described in Section 4.1.5.

   PIM (S,G) Join/Prune state is the result of receiving PIM (S,G)
   Join/Prune messages on this interface and is specified in
   Section 4.5.2.  The state is used by the macros that calculate the
   outgoing interface list in Section 4.1.5, and in the JoinDesired(S,G)
   macro (defined in Section 4.5.5) that is used in deciding whether a
   Join(S,G) should be sent upstream.

   (S,G) Assert Winner state is the result of sending or receiving (S,G)
   Assert messages on this interface.  It is specified in Section 4.6.1.

   The upstream (S,G) Join/Prune State reflects the state of the
   upstream (S,G) state machine described in Section 4.5.5.

   The upstream (S,G) Join/Prune Timer is used to send out periodic
   Join(S,G) messages, and to override Prune(S,G) messages from peers on
   an upstream LAN interface.

   The last RPF neighbor towards S is stored because if the MRIB
   changes, then the RPF neighbor towards S may change.  If it does so,
   then we need to trigger a new Join(S,G) to the new upstream neighbor
   and a Prune(S,G) to the old upstream neighbor.  Similarly, if the
   router detects through a changed GenID in a Hello message that the
   upstream neighbor towards S has rebooted, then it SHOULD
   re-instantiate state by sending a Join(S,G).  These mechanisms are
   specified in Section 4.5.5.

   The SPTbit is used to indicate whether forwarding is taking place on
   the (S,G) Shortest Path Tree (SPT) or on the (*,G) tree.  A router
   can have (S,G) state and still be forwarding on (*,G) state during

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   the interval when the source-specific tree is being constructed.
   When SPTbit is FALSE, only (*,G) forwarding state is used to forward
   packets from S to G.  When SPTbit is TRUE, both (*,G) and (S,G)
   forwarding state are used.

   The (S,G) Keepalive Timer is updated by data being forwarded using
   this (S,G) forwarding state.  It is used to keep (S,G) state alive in
   the absence of explicit (S,G) Joins.  Amongst other things, this is
   necessary for the so-called "turnaround rules" -- when the RP uses
   (S,G) joins to stop encapsulation, and then (S,G) prunes to prevent
   traffic from unnecessarily reaching the RP.

   On a DR, the (S,G) Register State is used to keep track of whether to
   encapsulate data to the RP on the Register Tunnel; the (S,G)
   Register-Stop Timer tracks how long before encapsulation begins again
   for a given (S,G).

4.1.4.  (S,G,rpt) State

   For every source/group pair (S,G) for which a router also has (*,G)
   state, it also keeps the following state:

   (S,G,rpt) state:

        For each interface:

             Local Membership:
                  State: One of {"NoInfo", "Exclude"}

             PIM (S,G,rpt) Join/Prune State:

                  o  State: One of {"NoInfo", "Pruned",
                     "Prune-Pending"}

                  o  Prune-Pending Timer (PPT)

                  o  Join/Prune Expiry Timer (ET)

        Not interface specific:

             Upstream (S,G,rpt) Join/Prune State:

                  o  State: One of {"RPTNotJoined(G)",
                     "NotPruned(S,G,rpt)", "Pruned(S,G,rpt)"}

                  o  Override Timer (OT)

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   Local membership is the result of the local source-specific
   membership mechanism (such as IGMPv3) running on that interface and
   specifying that although there is (*,G) Include state, this
   particular source should be excluded.  As stored here, this state is
   the resulting state after any IGMPv3 inconsistencies between LAN
   members have been resolved.  It need not be kept if this router is
   not the DR on that interface unless this router won a (*,G) assert on
   this interface for this group.  However, we RECOMMEND storing this
   information if possible, as it reduces latency converging to stable
   operating conditions after a failure causing a change of DR.  This
   information is used by the pim_exclude(S,G) macro described in
   Section 4.1.5.

   PIM (S,G,rpt) Join/Prune state is the result of receiving PIM
   (S,G,rpt) Join/Prune messages on this interface and is specified in
   Section 4.5.3.  The state is used by the macros that calculate the
   outgoing interface list in Section 4.1.5, and in the rules for adding
   Prune(S,G,rpt) messages to Join(*,G) messages specified in
   Section 4.5.6.

   The upstream (S,G,rpt) Join/Prune state is used along with the
   Override Timer to send the correct override messages in response to
   Join/Prune messages sent by upstream peers on a LAN.  This state and
   behavior are specified in Section 4.5.7.

4.1.5.  State Summarization Macros

   Using this state, we define the following "macro" definitions, which
   we will use in the descriptions of the state machines and pseudocode
   in the following sections.

   The most important macros are those that define the outgoing
   interface list (or "olist") for the relevant state.  An olist can be
   "immediate" if it is built directly from the state of the relevant
   type.  For example, the immediate_olist(S,G) is the olist that would
   be built if the router only had (S,G) state and no (*,G) or (S,G,rpt)
   state.  In contrast, the "inherited" olist inherits state from other
   types.  For example, the inherited_olist(S,G) is the olist that is
   relevant for forwarding a packet from S to G using both source-
   specific and group-specific state.

   There is no immediate_olist(S,G,rpt), as (S,G,rpt) state is negative
   state; it removes interfaces in the (*,G) olist from the olist that
   is actually used to forward traffic.  The inherited_olist(S,G,rpt) is
   therefore the olist that would be used for a packet from S to G
   forwarding on the RP tree.  It is a strict subset of
   immediate_olist(*,G).

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   Generally speaking, the inherited_olists are used for forwarding, and
   the immediate_olists are used to make decisions about state
   maintenance.

   immediate_olist(*,G) =
       joins(*,G) (+) pim_include(*,G) (-) lost_assert(*,G)

   immediate_olist(S,G) =
       joins(S,G) (+) pim_include(S,G) (-) lost_assert(S,G)

   inherited_olist(S,G,rpt) =
           ( joins(*,G) (-) prunes(S,G,rpt) )
       (+) ( pim_include(*,G) (-) pim_exclude(S,G))
       (-) ( lost_assert(*,G) (+) lost_assert(S,G,rpt) )

   inherited_olist(S,G) =
       inherited_olist(S,G,rpt) (+)
       joins(S,G) (+) pim_include(S,G) (-) lost_assert(S,G)

   The macros pim_include(*,G) and pim_include(S,G) indicate the
   interfaces to which traffic might be forwarded because of hosts that
   are local members on that interface.  Note that normally only the DR
   cares about local membership, but when an assert happens, the assert
   winner takes over responsibility for forwarding traffic to local
   members that have requested traffic on a group or source/group pair.

   pim_include(*,G) =
       { all interfaces I such that:
         ( ( I_am_DR( I ) AND lost_assert(*,G,I) == FALSE )
           OR AssertWinner(*,G,I) == me )
          AND  local_receiver_include(*,G,I) }

   pim_include(S,G) =
       { all interfaces I such that:
         ( (I_am_DR( I ) AND lost_assert(S,G,I) == FALSE )
           OR AssertWinner(S,G,I) == me )
          AND  local_receiver_include(S,G,I) }

   pim_exclude(S,G) =
       { all interfaces I such that:
         ( (I_am_DR( I ) AND lost_assert(*,G,I) == FALSE )
           OR AssertWinner(*,G,I) == me )
          AND  local_receiver_exclude(S,G,I) }

   The clause "local_receiver_include(S,G,I)" is true if the IGMP/MLD
   module or other local membership mechanism has determined that local
   members on interface I desire to receive traffic sent specifically by
   S to G.  "local_receiver_include(*,G,I)" is true if the IGMP/MLD

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   module or other local membership mechanism has determined that local
   members on interface I desire to receive all traffic sent to G
   (possibly excluding traffic from a specific set of sources).
   "local_receiver_exclude(S,G,I)" is true if
   "local_receiver_include(*,G,I)" is true but none of the local members
   desire to receive traffic from S.

   The set "joins(*,G)" is the set of all interfaces on which the router
   has received (*,G) Joins:

   joins(*,G) =
       { all interfaces I such that
         DownstreamJPState(*,G,I) is either Join or Prune-Pending }

   DownstreamJPState(*,G,I) is the state of the finite state machine in
   Section 4.5.1.

   The set "joins(S,G)" is the set of all interfaces on which the router
   has received (S,G) Joins:

   joins(S,G) =
       { all interfaces I such that
         DownstreamJPState(S,G,I) is either Join or Prune-Pending }

   DownstreamJPState(S,G,I) is the state of the finite state machine in
   Section 4.5.2.

   The set "prunes(S,G,rpt)" is the set of all interfaces on which the
   router has received (*,G) joins and (S,G,rpt) prunes:

   prunes(S,G,rpt) =
       { all interfaces I such that
         DownstreamJPState(S,G,rpt,I) is Prune or PruneTmp }

   DownstreamJPState(S,G,rpt,I) is the state of the finite state machine
   in Section 4.5.3.

   The set "lost_assert(*,G)" is the set of all interfaces on which the
   router has received (*,G) joins but has lost a (*,G) assert.  The
   macro lost_assert(*,G,I) is defined in Section 4.6.5.

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

   The set "lost_assert(S,G,rpt)" is the set of all interfaces on which
   the router has received (*,G) joins but has lost an (S,G) assert.
   The macro lost_assert(S,G,rpt,I) is defined in Section 4.6.5.

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

   The set "lost_assert(S,G)" is the set of all interfaces on which the
   router has received (S,G) joins but has lost an (S,G) assert.  The
   macro lost_assert(S,G,I) is defined in Section 4.6.5.

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

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

     neighbor RPF'(*,G) {
         if ( I_Am_Assert_Loser(*, G, RPF_interface(RP(G))) ) {
              return AssertWinner(*, G, RPF_interface(RP(G)) )
         } else {
              return NBR( RPF_interface(RP(G)), MRIB.next_hop( RP(G) ) )
         }
     }

     neighbor RPF'(S,G,rpt) {
         if( I_Am_Assert_Loser(S, G, RPF_interface(RP(G)) ) ) {
              return AssertWinner(S, G, RPF_interface(RP(G)) )
         } else {
              return RPF'(*,G)
         }
     }

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

   RPF'(*,G) and RPF'(S,G) indicate the neighbor from which data packets
   should be coming and to which joins should be sent on the RP tree and
   SPT, respectively.

   RPF'(S,G,rpt) is basically RPF'(*,G) modified by the result of an
   Assert(S,G) on RPF_interface(RP(G)).  In such a case, packets from S
   will be originating from a different router than RPF'(*,G).  If we

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   only have active (*,G) Join state, we need to accept packets from
   RPF'(S,G,rpt) and add a Prune(S,G,rpt) to the periodic Join(*,G)
   messages that we send to RPF'(*,G) (see Section 4.5.6).

   The function MRIB.next_hop( S ) returns an address of the next-hop
   PIM neighbor toward the host S, as indicated by the current MRIB.  If
   S is directly adjacent, then MRIB.next_hop( S ) returns NULL.  At the
   RP for G, MRIB.next_hop( RP(G)) returns NULL.

   The function NBR( I, A ) uses information gathered through PIM Hello
   messages to map the IP address A of a directly connected PIM neighbor
   router on interface I to the primary IP address of the same router
   (Section 4.3.4).  The primary IP address of a neighbor is the address
   that it uses as the source of its PIM Hello messages.  Note that a
   neighbor's IP address may be non-unique within the PIM neighbor
   database due to scope issues.  The address must, however, be unique
   amongst the addresses of all the PIM neighbors on a specific
   interface.

   I_Am_Assert_Loser(S, G, I) is true if the Assert state machine (in
   Section 4.6.1) for (S,G) on Interface I is in "I am Assert Loser"
   state.

   I_Am_Assert_Loser(*, G, I) is true if the Assert state machine (in
   Section 4.6.2) for (*,G) on Interface I is in "I am Assert Loser"
   state.

4.2.  Data Packet Forwarding Rules

   The PIM-SM 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).

      RP is the address of the Rendezvous Point for this group.

      RPF_interface(S) is the interface the MRIB indicates would be used
         to route packets to S.

      RPF_interface(RP) is the interface the MRIB indicates would be
         used to route packets to the RP, except at the RP when it is
         the decapsulation interface (the "virtual" interface on which
         Register packets are received).

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   First, we restart (or start) the Keepalive Timer if the source is on
   a directly connected subnet.

   Second, we check to see if the SPTbit should be set because we've now
   switched from the RP tree to the SPT.

   Next, we check to see whether the packet should be accepted based on
   TIB state and the interface that the packet arrived on.

   If the packet should be forwarded using (S,G) state, we then build an
   outgoing interface list for the packet.  If this list is not empty,
   then we restart the (S,G) state Keepalive Timer.

   If the packet should be forwarded using (*,G) state, then we just
   build an outgoing interface list for the packet.  We also check if we
   should initiate a switch to start receiving this source on a shortest
   path tree.

   Finally, we remove the incoming interface from the outgoing interface
   list we've created, and if the resulting outgoing interface list is
   not empty, we forward the packet out of those interfaces.

   On receipt of data from S to G on interface iif:
    if( DirectlyConnected(S) == TRUE AND iif == RPF_interface(S) ) {
         set KeepaliveTimer(S,G) to Keepalive_Period
         # Note: A register state transition or UpstreamJPState(S,G)
         # transition may happen as a result of restarting
         # KeepaliveTimer, and must be dealt with here.
    }

   if( iif == RPF_interface(S) AND UpstreamJPState(S,G) == Joined AND
      inherited_olist(S,G) != NULL ) {
          set KeepaliveTimer(S,G) to Keepalive_Period
   }

   Update_SPTbit(S,G,iif)
   oiflist = NULL

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   if( iif == RPF_interface(S) AND SPTbit(S,G) == TRUE ) {
      oiflist = inherited_olist(S,G)
   } else if( iif == RPF_interface(RP(G)) AND SPTbit(S,G) == FALSE ) {
     oiflist = inherited_olist(S,G,rpt)
     CheckSwitchToSpt(S,G)
   } else {
      # Note: RPF check failed.
      # A transition in an Assert finite state machine may cause an
      # Assert(S,G) or Assert(*,G) message to be sent out interface iif.
      # See Section 4.6 for details.
      if ( SPTbit(S,G) == TRUE AND iif is in inherited_olist(S,G) ) {
         send Assert(S,G) on iif
      } else if ( SPTbit(S,G) == FALSE AND
                  iif is in inherited_olist(S,G,rpt) ) {
         send Assert(*,G) on iif
      }
   }

   oiflist = oiflist (-) iif
   forward packet on all interfaces in oiflist

   This pseudocode employs several "macro" definitions:

   DirectlyConnected(S) is TRUE if the source S is on any subnet that is
   directly connected to this router (or for packets originating on this
   router).

   inherited_olist(S,G) and inherited_olist(S,G,rpt) are defined in
   Section 4.1.

   Basically, inherited_olist(S,G) is the outgoing interface list for
   packets forwarded on (S,G) state, taking into account (*,G) state,
   asserts, etc.

   inherited_olist(S,G,rpt) is the outgoing interface list for packets
   forwarded on (*,G) state, taking into account (S,G,rpt) prune state,
   asserts, etc.

   Update_SPTbit(S,G,iif) is defined in Section 4.2.2.

   CheckSwitchToSpt(S,G) is defined in Section 4.2.1.

   UpstreamJPState(S,G) is the state of the finite state machine in
   Section 4.5.5.

   Keepalive_Period is defined in Section 4.11.

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   Data-triggered PIM-Assert messages sent from the above forwarding
   code SHOULD be rate-limited in an implementation-dependent manner.

4.2.1.  Last-Hop Switchover to the SPT

   In Sparse-Mode PIM, last-hop routers join the shared tree towards the
   RP.  Once traffic from sources to joined groups arrives at a last-hop
   router, it has the option of switching to receive the traffic on a
   shortest path tree (SPT).

   The decision for a router to switch to the SPT is controlled as
   follows:

     void
     CheckSwitchToSpt(S,G) {
       if ( ( pim_include(*,G) (-) pim_exclude(S,G)
              (+) pim_include(S,G) != NULL )
            AND SwitchToSptDesired(S,G) ) {
              # Note: Restarting the KAT will result in the SPT switch.
              set KeepaliveTimer(S,G) to Keepalive_Period
       }
     }

   SwitchToSptDesired(S,G) is a policy function that is implementation
   defined.  An "infinite threshold" policy can be implemented by making
   SwitchToSptDesired(S,G) return false all the time.  A "switch on
   first packet" policy can be implemented by making
   SwitchToSptDesired(S,G) return true once a single packet has been
   received for the source and group.

4.2.2.  Setting and Clearing the (S,G) SPTbit

   The (S,G) SPTbit is used to distinguish whether to forward on (*,G)
   or on (S,G) state.  When switching from the RP tree to the source
   tree, there is a transition period when data is arriving due to
   upstream (*,G) state while upstream (S,G) state is being established,
   during which time a router should continue to forward only on (*,G)
   state.  This prevents temporary black holes that would be caused by
   sending a Prune(S,G,rpt) before the upstream (S,G) state has finished
   being established.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Thus, when a packet arrives, the (S,G) SPTbit is updated as follows:

     void
     Update_SPTbit(S,G,iif) {
       if ( iif == RPF_interface(S)
             AND JoinDesired(S,G) == TRUE
             AND ( DirectlyConnected(S) == TRUE
                   OR RPF_interface(S) != RPF_interface(RP(G))
                   OR inherited_olist(S,G,rpt) == NULL
                   OR ( ( RPF'(S,G) == RPF'(*,G) ) AND
                        ( RPF'(S,G) != NULL ) )
                   OR ( I_Am_Assert_Loser(S,G,iif) ) ) ) {
          Set SPTbit(S,G) to TRUE
       }
     }

   Additionally, a router can set SPTbit(S,G) to TRUE in other cases,
   such as when it receives an Assert(S,G) on RPF_interface(S) (see
   Section 4.6.1).

   JoinDesired(S,G) is defined in Section 4.5.5 and indicates whether we
   have the appropriate (S,G) Join state to wish to send a Join(S,G)
   upstream.

   Basically, Update_SPTbit(S,G,iif) will set the SPTbit if we have the
   appropriate (S,G) join state, and if the packet arrived on the
   correct upstream interface for S, and if one or more of the following
   conditions apply:

   1.  The source is directly connected, in which case the switch to the
       SPT is a no-op.

   2.  The RPF interface to S is different from the RPF interface to the
       RP.  The packet arrived on RPF_interface(S), and so the SPT must
       have been completed.

   3.  No one wants the packet on the RP tree.

   4.  RPF'(S,G) == RPF'(*,G).  In this case, the router will never be
       able to tell if the SPT has been completed, so it should just
       switch immediately.  The RPF'(S,G) != NULL check ensures that the
       SPTbit is set only if the RPF neighbor towards S is valid.

   In the case where the RPF interface is the same for the RP and for S,
   but RPF'(S,G) and RPF'(*,G) differ, we wait for an Assert(S,G), which
   indicates that the upstream router with (S,G) state believes the SPT
   has been completed.  However, item (3) above is needed because there
   may not be any (*,G) state to trigger an Assert(S,G) to happen.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   The SPTbit is cleared in the (S,G) upstream state machine (see
   Section 4.5.5) when JoinDesired(S,G) becomes FALSE.

4.3.  Designated Routers (DRs) and Hello Messages

   A shared-media LAN like Ethernet may have multiple PIM-SM routers
   connected to it.  A single one of these routers, the DR, will act on
   behalf of directly connected hosts with respect to the PIM-SM
   protocol.  Because the distinction between LANs and point-to-point
   interfaces can sometimes be blurred, and because routers may also
   have multicast host functionality, the PIM-SM specification makes no
   distinction between the two.  Thus, DR election will happen on all
   interfaces, LAN or otherwise.

   DR election is performed using Hello messages.  Hello messages are
   also the way that option negotiation takes place in PIM, so that
   additional functionality can be enabled, or parameters tuned.

4.3.1.  Sending Hello Messages

   PIM Hello messages are sent periodically on each PIM-enabled
   interface.  They allow a router to learn about the neighboring PIM
   routers on each interface.  Hello messages are also the mechanism
   used to elect a DR, and to negotiate additional capabilities.  A
   router must record the Hello information received from each PIM
   neighbor.

   Hello messages MUST be sent on all active interfaces, including
   physical point-to-point links, and are multicast to the
   'ALL-PIM-ROUTERS' group address ('224.0.0.13' for IPv4 and 'ff02::d'
   for IPv6).

   We note that some implementations do not send Hello messages on
   point-to-point interfaces.  This is non-compliant behavior.  A
   compliant PIM router MUST send Hello messages, even on point-to-point
   interfaces.

   A per-interface Hello Timer (HT(I)) is used to trigger sending Hello
   messages on each active interface.  When PIM is enabled on an
   interface or a router first starts, the Hello Timer of that interface
   is set to a random value between 0 and Triggered_Hello_Delay.  This
   prevents synchronization of Hello messages if multiple routers are
   powered on simultaneously.  After the initial randomized interval,
   Hello messages MUST be sent every Hello_Period seconds.  The
   Hello Timer SHOULD NOT be reset except when it expires.

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   Note that neighbors will not accept Join/Prune or Assert messages
   from a router unless they have first heard a Hello message from that
   router.  Thus, if a router needs to send a Join/Prune or Assert
   message on an interface on which it has not yet sent a Hello message
   with the currently configured IP address, then it MUST immediately
   send the relevant Hello message without waiting for the Hello Timer
   to expire, followed by the Join/Prune or Assert message.

   The DR Priority option allows a network administrator to give
   preference to a particular router in the DR election process by
   giving it a numerically larger DR Priority.  The DR Priority option
   SHOULD be included in every Hello message, even if no DR Priority is
   explicitly configured on that interface.  This is necessary because
   priority-based DR election is only enabled when all neighbors on an
   interface advertise that they are capable of using the DR Priority
   option.  The default priority is 1.

   The Generation Identifier (GenID) option SHOULD be included in all
   Hello messages.  The GenID option contains a randomly generated
   32-bit value that is regenerated each time PIM forwarding is started
   or restarted on the interface, including when the router itself
   restarts.  When a Hello message with a new GenID is received from a
   neighbor, any old Hello information about that neighbor SHOULD be
   discarded and superseded by the information from the new Hello
   message.  This may cause a new DR to be chosen on that interface.

   The LAN Prune Delay option SHOULD be included in all Hello messages
   sent on multi-access LANs.  This option advertises a router's
   capability to use values other than the defaults for the
   Propagation_Delay and Override_Interval, which affect the setting of
   the Prune-Pending, Upstream Join, and Override Timers (defined in
   Section 4.10).

   The Address List option advertises all the secondary addresses
   associated with the source interface of the router originating the
   message.  The option MUST be included in all Hello messages if there
   are secondary addresses associated with the source interface and MAY
   be omitted if no secondary addresses exist.

   To allow new or rebooting routers to learn of PIM neighbors quickly,
   when a Hello message is received from a new neighbor, or a Hello
   message with a new GenID is received from an existing neighbor, a new
   Hello message SHOULD be sent on this interface after a randomized
   delay between 0 and Triggered_Hello_Delay.  This triggered message
   need not change the timing of the scheduled periodic message.  If a
   router needs to send a Join/Prune to the new neighbor or send an
   Assert message in response to an Assert message from the new neighbor
   before this randomized delay has expired, then it MUST immediately

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   send the relevant Hello message without waiting for the Hello Timer
   to expire, followed by the Join/Prune or Assert message.  If it does
   not do this, then the new neighbor will discard the Join/Prune or
   Assert message.

   Before an interface goes down or changes primary IP address, a Hello
   message with a zero HoldTime SHOULD be sent immediately (with the old
   IP address if the IP address changed).  This will cause PIM neighbors
   to remove this neighbor (or its old IP address) immediately.  After
   an interface has changed its IP address, it MUST send a Hello message
   with its new IP address.  If an interface changes one of its
   secondary IP addresses, a Hello message with an updated Address List
   option and a non-zero HoldTime SHOULD be sent immediately.  This will
   cause PIM neighbors to update this neighbor's list of secondary
   addresses immediately.

4.3.2.  DR Election

   When a PIM Hello message is received on interface I, the following
   information about the sending neighbor is recorded:

      neighbor.interface
            The interface on which the Hello message arrived.

      neighbor.primary_ip_address
            The IP address that the PIM neighbor used as the source
            address of the Hello message.

      neighbor.genid
            The Generation ID of the PIM neighbor.

      neighbor.dr_priority
            The DR Priority field of the PIM neighbor, if it is present
            in the Hello message.

      neighbor.dr_priority_present
            A flag indicating if the DR Priority field was present in
            the Hello message.

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      neighbor.timeout
            A timer value to time out the neighbor state when it becomes
            stale, also known as the Neighbor Liveness Timer.

            The Neighbor Liveness Timer (NLT(N,I)) is reset to
            Hello_Holdtime (from the Hello Holdtime option) whenever a
            Hello message is received containing a Holdtime option, or
            to Default_Hello_Holdtime if the Hello message does not
            contain the Holdtime option.

            Neighbor state is deleted when the neighbor timeout expires.

   The function for computing the DR on interface I is:

     host
     DR(I) {
         dr = me
         for each neighbor on interface I {
             if ( dr_is_better( neighbor, dr, I ) == TRUE ) {
                 dr = neighbor
             }
         }
         return dr
     }

   The function used for comparing DR "metrics" on interface I is:

     bool
     dr_is_better(a,b,I) {
         if( there is a neighbor n on I for which n.dr_priority_present
                 is false ) {
             return a.primary_ip_address > b.primary_ip_address
         } else {
             return ( a.dr_priority > b.dr_priority ) OR
                    ( a.dr_priority == b.dr_priority AND
                      a.primary_ip_address > b.primary_ip_address )
         }
     }

   The trivial function I_am_DR(I) is defined to aid readability:

     bool
     I_am_DR(I) {
        return DR(I) == me
     }

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   The DR Priority is a 32-bit unsigned number, and the numerically
   larger priority is always preferred.  A router's idea of the current
   DR on an interface can change when a PIM Hello message is received,
   when a neighbor times out, or when a router's own DR Priority
   changes.  If the router becomes the DR or ceases to be the DR, this
   will normally cause the DR Register state machine to change state.
   Subsequent actions are determined by that state machine.

      We note that some PIM implementations do not send Hello messages
      on point-to-point interfaces and thus cannot perform DR election
      on such interfaces.  This is non-compliant behavior.  DR election
      MUST be performed on ALL active PIM-SM interfaces.

4.3.3.  Reducing Prune Propagation Delay on LANs

   In addition to the information recorded for the DR Election, the
   following per-neighbor information is obtained from the LAN Prune
   Delay Hello option:

      neighbor.lan_prune_delay_present
            A flag indicating if the LAN Prune Delay option was present
            in the Hello message.

      neighbor.tracking_support
            A flag storing the value of the T bit in the LAN Prune Delay
            option if it is present in the Hello message.  This
            indicates the neighbor's capability to disable Join message
            suppression.

      neighbor.propagation_delay
            The Propagation Delay field of the LAN Prune Delay option
            (if present) in the Hello message.

      neighbor.override_interval
            The Override_Interval field of the LAN Prune Delay option
            (if present) in the Hello message.

   The additional state described above is deleted along with the DR
   neighbor state when the neighbor timeout expires.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Just like the DR Priority option, the information provided in the LAN
   Prune Delay option is not used unless all neighbors on a link
   advertise the option.  The function below computes this state:

     bool
     lan_delay_enabled(I) {
         for each neighbor on interface I {
             if ( neighbor.lan_prune_delay_present == false ) {
                 return false
             }
         }
         return true
     }

   The Propagation 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.  It is used
   by upstream routers to figure out how long they should wait for a
   Join override message before pruning an interface.

   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 as well as triggered messages to be sent later than
   intended.  Setting this Propagation 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.

   When all routers on a link are in a position to negotiate a
   Propagation Delay different from the default, the largest value from
   those advertised by each neighbor is chosen.  The function for
   computing the Effective Propagation Delay of interface I is:

     time_interval
     Effective_Propagation_Delay(I) {
         if ( lan_delay_enabled(I) == false ) {
             return Propagation_delay_default
         }
         delay = Propagation_Delay(I)
         for each neighbor on interface I {
             if ( neighbor.propagation_delay > delay ) {
                 delay = neighbor.propagation_delay
             }
         }
         return delay
     }

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   To avoid synchronization of override messages when multiple
   downstream routers share a multi-access link, the sending of such
   messages is delayed by a small random amount of time.  The period of
   randomization should represent the size of the PIM router population
   on the link.  Each router 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 link are in a position to negotiate an Override
   Interval different from the default, the largest value from those
   advertised by each neighbor is chosen.  The function for computing
   the Effective Override Interval of interface I is:

     time_interval
     Effective_Override_Interval(I) {
         if ( lan_delay_enabled(I) == false ) {
             return t_override_default
         }
         delay = Override_Interval(I)
         for each neighbor on interface I {
             if ( neighbor.override_interval > delay ) {
                 delay = neighbor.override_interval
             }
         }
         return delay
     }

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Although the mechanisms are not specified in this document, it is
   possible for upstream routers to explicitly track the join
   membership of individual downstream routers if Join suppression is
   disabled.  A router can advertise its willingness to disable Join
   suppression by using the T bit in the LAN Prune Delay Hello option.
   Unless all PIM routers on a link negotiate this capability, explicit
   tracking and the disabling of the Join suppression mechanism are not
   possible.  The function for computing the state of Suppression on
   interface I is:

     bool
     Suppression_Enabled(I) {
         if ( lan_delay_enabled(I) == false ) {
             return true
         }
         for each neighbor on interface I {
             if ( neighbor.tracking_support == false ) {
                 return true
             }
         }
         return false
     }

   Note that the setting of Suppression_Enabled(I) affects the value of
   t_suppressed (see Section 4.11).

4.3.4.  Maintaining Secondary Address Lists

   Communication of a router's interface secondary addresses to its PIM
   neighbors is necessary to provide the neighbors with a mechanism for
   mapping next_hop information obtained through their MRIB to a primary
   address that can be used as a destination for Join/Prune messages.
   The mapping is performed through the NBR macro.  The primary address
   of a PIM neighbor is obtained from the source IP address used in its
   PIM Hello messages.  Secondary addresses are carried within the Hello
   message in an Address List Hello option.  The primary address of the
   source interface of the router MUST NOT be listed within the Address
   List Hello option.

   In addition to the information recorded for the DR Election, the
   following per-neighbor information is obtained from the Address List
   Hello option:

      neighbor.secondary_address_list
            The list of secondary addresses used by the PIM neighbor on
            the interface through which the Hello message was
            transmitted.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   When processing a received PIM Hello message containing an Address
   List Hello option, the list of secondary addresses in the message
   completely replaces any previously associated secondary addresses for
   that neighbor.  If a received PIM Hello message does not contain an
   Address List Hello option, then all secondary addresses associated
   with the neighbor MUST be deleted.  If a received PIM Hello message
   contains an Address List Hello option that includes the primary
   address of the sending router in the list of secondary addresses
   (although this is not expected), then the addresses listed in the
   message, excluding the primary address, are used to update the
   associated secondary addresses for that neighbor.

   All the advertised secondary addresses in received Hello messages
   must be checked against those previously advertised by all other PIM
   neighbors on that interface.  If there is a conflict and the same
   secondary address was previously advertised by another neighbor, then
   only the most recently received mapping MUST be maintained, and an
   error message SHOULD be logged to the administrator in a rate-limited
   manner.

   Within one Address List Hello option, all the addresses MUST be of
   the same address family.  It is not permitted to mix IPv4 and IPv6
   addresses within the same message.  In addition, the address family
   of the fields in the message SHOULD be the same as the IP source and
   destination addresses of the packet header.

4.4.  PIM Register Messages

   The Designated Router (DR) on a LAN or point-to-point link
   encapsulates multicast packets from local sources to the RP for the
   relevant group unless it recently received a Register-Stop message
   for that (S,G) or (*,G) from the RP.  When the DR receives a
   Register-Stop message from the RP, it starts a Register-Stop Timer to
   maintain this state.  Just before the Register-Stop Timer expires,
   the DR sends a Null-Register message to the RP to allow the RP to
   refresh the Register-Stop information at the DR.  If the
   Register-Stop Timer actually expires, the DR will resume
   encapsulating packets from the source to the RP.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.4.1.  Sending Register Messages from the DR

   Every PIM-SM router has the capability to be a DR.  The state machine
   below is used to implement Register functionality.  For the purposes
   of specification, we represent the mechanism to encapsulate packets
   to the RP as a Register-Tunnel interface, which is added to or
   removed from the (S,G) olist.  The tunnel interface then takes part
   in the normal packet forwarding rules as specified in Section 4.2.

   If register state is maintained, it is maintained only for directly
   connected sources and is per-(S,G).  There are four states in the
   DR's per-(S,G) Register state machine:

      Join (J)
            The register tunnel is "joined" (the join is actually
            implicit, but the DR acts as if the RP has joined the DR on
            the tunnel interface).

      Prune (P)
            The register tunnel is "pruned" (this occurs when a
            Register-Stop is received).

      Join-Pending (JP)
            The register tunnel is pruned but the DR is contemplating
            adding it back.

      NoInfo (NI)
            No information.  This is the initial state, and the state
            when the router is not the DR.

   In addition, a Register-Stop Timer (RST) is kept if the state machine
   is not in the NoInfo state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

           Figure 1: Per-(S,G) Register State Machine at a DR

+----------++----------------------------------------------------------+
|          ||                          Event                           |
|          ++----------+-----------+-----------+-----------+-----------+
|Prev State||Register- | Could     | Could     | Register- | RP changed|
|          ||Stop Timer| Register  | Register  | Stop      |           |
|          ||expires   | ->True    | ->False   | received  |           |
+----------++----------+-----------+-----------+-----------+-----------+
|NoInfo    ||-         | -> J state| -         | -         | -         |
|(NI)      ||          | add reg   |           |           |           |
|          ||          | tunnel    |           |           |           |
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-         | -         | -> NI     | -> P state| -> J state|
|          ||          |           | state     |           |           |
|          ||          |           | remove reg| remove reg| update reg|
|Join (J)  ||          |           | tunnel    | tunnel;   | tunnel    |
|          ||          |           |           | set       |           |
|          ||          |           |           | Register- |           |
|          ||          |           |           | Stop      |           |
|          ||          |           |           | Timer(*)  |           |
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-> J state| -         | -> NI     | -> P state| -> J state|
|          ||          |           | state     |           |           |
|Join-     ||add reg   |           |           | set       | add reg   |
|Pending   ||tunnel    |           |           | Register- | tunnel;   |
|(JP)      ||          |           |           | Stop      | cancel    |
|          ||          |           |           | Timer(*)  | Register- |
|          ||          |           |           |           | Stop Timer|
+----------++----------+-----------+-----------+-----------+-----------+
|          ||-> JP     | -         | -> NI     | -         | -> J state|
|          ||state     |           | state     |           |           |
|          ||set       |           |           |           | add reg   |
|Prune (P) ||Register- |           |           |           | tunnel;   |
|          ||Stop      |           |           |           | cancel    |
|          ||Timer(**);|           |           |           | Register- |
|          ||send Null-|           |           |           | Stop Timer|
|          ||Register  |           |           |           |           |
+----------++----------+-----------+-----------+-----------+-----------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Notes:

   (*)  The Register-Stop Timer is set to a random value chosen
        uniformly from the interval ( 0.5 * Register_Suppression_Time,
        1.5 * Register_Suppression_Time) minus Register_Probe_Time.

        Subtracting off Register_Probe_Time is a bit unnecessary because
        it is really small compared to Register_Suppression_Time, but
        this was in the old specification and is kept for compatibility.

   (**) The Register-Stop Timer is set to Register_Probe_Time.

   The following three actions are defined:

   Add Register Tunnel

      A Register-Tunnel virtual interface, VI, is created (if it doesn't
      already exist) with its encapsulation target being RP(G).
      DownstreamJPState(S,G,VI) is set to Join state, causing the tunnel
      interface to be added to immediate_olist(S,G) and
      inherited_olist(S,G).

   Remove Register Tunnel

      VI is the Register-Tunnel virtual interface with encapsulation
      target of RP(G).  DownstreamJPState(S,G,VI) is set to NoInfo
      state, causing the tunnel interface to be removed from
      immediate_olist(S,G) and inherited_olist(S,G).  If
      DownstreamJPState(S,G,VI) is NoInfo for all (S,G), then VI can be
      deleted.

   Update Register Tunnel

      This action occurs when RP(G) changes.

      VI_old is the Register-Tunnel virtual interface with encapsulation
      target old_RP(G).  A Register-Tunnel virtual interface, VI_new, is
      created (if it doesn't already exist) with its encapsulation
      target being new_RP(G).  DownstreamJPState(S,G,VI_old) is set to
      NoInfo state, and DownstreamJPState(S,G,VI_new) is set to Join
      state.  If DownstreamJPState(S,G,VI_old) is NoInfo for all (S,G),
      then VI_old can be deleted.

      Note that we cannot simply change the encapsulation target of
      VI_old because not all groups using that encapsulation tunnel will
      have moved to the same new RP.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   CouldRegister(S,G)

      The macro "CouldRegister" in the state machine is defined as:

         bool CouldRegister(S,G) {
            return ( I_am_DR( RPF_interface(S) ) AND
                     KeepaliveTimer(S,G) is running AND
                     DirectlyConnected(S) == TRUE )
         }

      Note that on reception of a packet at the DR from a directly
      connected source, KeepaliveTimer(S,G) needs to be set by the
      packet forwarding rules before computing CouldRegister(S,G) in the
      register state machine, or the first packet from a source won't be
      registered.

   Encapsulating Data Packets in the Register Tunnel

      Conceptually, the Register Tunnel is an interface with a smaller
      MTU than the underlying IP interface towards the RP.  IP
      fragmentation on packets forwarded on the Register Tunnel is
      performed based upon this smaller MTU.  The encapsulating DR may
      perform Path MTU Discovery to the RP to determine the effective
      MTU of the tunnel.  Fragmentation for the smaller MTU should take
      both the outer IP header and the PIM register header overhead into
      account.  If a multicast packet is fragmented on the way into the
      Register Tunnel, each fragment is encapsulated individually so it
      contains IP, PIM, and inner IP headers.

      In IPv6, the DR MUST perform Path MTU Discovery, and an ICMP
      Packet Too Big message MUST be sent by the encapsulating DR if it
      receives a packet that will not fit in the effective MTU of the
      tunnel.  If the MTU between the DR and the RP results in the
      effective tunnel MTU being smaller than 1280 (the IPv6 minimum
      MTU), the DR MUST send Fragmentation Required messages with an MTU
      value of 1280 and MUST fragment its PIM register messages as
      required, using an IPv6 fragmentation header between the outer
      IPv6 header and the PIM Register header.

      The TTL of a forwarded data packet is decremented before it is
      encapsulated in the Register Tunnel.  The encapsulating packet
      uses the normal TTL that the router would use for any locally
      generated IP packet.

      The IP Explicit Congestion Notification (ECN) bits should be
      copied from the original packet to the IP header of the
      encapsulating packet.  They SHOULD NOT be set independently by the
      encapsulating router.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      The Diffserv Code Point (DSCP) should be copied from the original
      packet to the IP header of the encapsulating packet.  It MAY be
      set independently by the encapsulating router, based upon static
      configuration or traffic classification.  See [12] for more
      discussion on setting the DSCP on tunnels.

   Handling Register-Stop(*,G) Messages at the DR

      An old RP might send a Register-Stop message with the source
      address set to all zeros.  This was the normal course of action in
      RFC 2362 when the Register message matched against (*,G) state at
      the RP, and it was defined as meaning "stop encapsulating all
      sources for this group".  However, the behavior of such a
      Register-Stop(*,G) is ambiguous or incorrect in some
      circumstances.

      We specify that an RP should not send Register-Stop(*,G) messages,
      but for compatibility, a DR should be able to accept one if it is
      received.

      A Register-Stop(*,G) should be treated as a Register-Stop(S,G) for
      all (S,G) Register state machines that are not in the NoInfo
      state.  A router should not apply a Register-Stop(*,G) to sources
      that become active after the Register-Stop(*,G) was received.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.4.2.  Receiving Register Messages at the RP

   When an RP receives a Register message, the course of action is
   decided according to the following pseudocode:

   packet_arrives_on_rp_tunnel( pkt ) {
       if( outer.dst is not one of my addresses ) {
           drop the packet silently.
           # Note: This may be a spoofing attempt.
       }
       if( I_am_RP(G) AND outer.dst == RP(G) ) {
             sentRegisterStop = FALSE;
             if ( SPTbit(S,G) OR
              ( SwitchToSptDesired(S,G) AND
                ( inherited_olist(S,G) == NULL ))) {
               send Register-Stop(S,G) to outer.src
               sentRegisterStop = TRUE;
             }
             if ( SPTbit(S,G) OR SwitchToSptDesired(S,G) ) {
                  if ( sentRegisterStop == TRUE ) {
                       set KeepaliveTimer(S,G) to RP_Keepalive_Period;
                  } else {
                       set KeepaliveTimer(S,G) to Keepalive_Period;
                  }
             }
             if( !SPTbit(S,G) AND ! pkt.NullRegisterBit ) {
                  decapsulate and forward the inner packet to
                  inherited_olist(S,G,rpt) # Note (+)
             }
       } else {
           send Register-Stop(S,G) to outer.src
           # Note (*)
       }
   }

   outer.dst is the IP destination address of the encapsulating header.

   outer.src is the IP source address of the encapsulating header, i.e.,
   the DR's address.

   I_am_RP(G) is true if the group-to-RP mapping indicates that this
   router is the RP for the group.

   Note (*): This may block traffic from S for Register_Suppression_Time
             if the DR learned about a new group-to-RP mapping before
             the RP did.  However, this doesn't matter unless we figure
             out some way for the RP also to accept (*,G) joins when it
             doesn't yet realize that it is about to become the RP

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RFC 7761             PIM-SM Specification (Revised)           March 2016

             for G.  This will all get sorted out once the RP learns the
             new group-to-RP mapping.  We decided to do nothing about
             this and just accept the fact that PIM may suffer
             interrupted (*,G) connectivity following an RP change.

   Note (+): Implementations SHOULD NOT make this a special case, but
             SHOULD arrange that this path rejoin the normal packet
             forwarding path.  All of the appropriate actions from the
             "On receipt of data from S to G on interface iif"
             pseudocode in Section 4.2 should be performed.

   KeepaliveTimer(S,G) is restarted at the RP when packets arrive on the
   proper tunnel interface and the RP desires to switch to the SPT or
   the SPTbit is already set.  This may cause the upstream (S,G) state
   machine to trigger a join if the inherited_olist(S,G) is not NULL.

   An RP should preserve (S,G) state that was created in response to a
   Register message for at least ( 3 * Register_Suppression_Time );
   otherwise, the RP may stop joining (S,G) before the DR for S has
   restarted sending registers.  Traffic would then be interrupted until
   the Register-Stop Timer expires at the DR.

   Thus, at the RP, KeepaliveTimer(S,G) should be restarted to ( 3 *
   Register_Suppression_Time + Register_Probe_Time ).

   When forwarding a packet from the Register Tunnel, the TTL of the
   original data packet is decremented after it is decapsulated.

   The IP ECN bits should be copied from the IP header of the Register
   packet to the decapsulated packet.

   The DSCP should be copied from the IP header of the Register packet
   to the decapsulated packet.  The RP MAY retain the DSCP of the inner
   packet or re-classify the packet and apply a different DSCP.
   Scenarios where each of these might be useful are discussed in [12].

4.5.  PIM Join/Prune Messages

   A PIM Join/Prune message consists of a list of groups and a list of
   Joined and Pruned sources for each group.  When processing a received
   Join/Prune message, each Joined or Pruned source for a group is
   effectively considered individually, and applies to one or more of
   the following state machines.  When considering a Join/Prune message
   whose Upstream Neighbor Address field addresses this router, (*,G)
   Joins and Prunes can affect both the (*,G) and (S,G,rpt) downstream
   state machines, while (S,G), and (S,G,rpt) Joins and Prunes can only
   affect their respective downstream state machines.  When considering

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   a Join/Prune message whose Upstream Neighbor Address field addresses
   another router, most Join or Prune messages could affect each
   upstream state machine.

   In general, a PIM Join/Prune message should only be accepted for
   processing if it comes from a known PIM neighbor.  A PIM router hears
   about PIM neighbors through PIM Hello messages.  If a router receives
   a Join/Prune message from a particular IP source address and it has
   not seen a PIM Hello message from that source address, then the
   Join/Prune message SHOULD be discarded without further processing.
   In addition, if the Hello message from a neighbor was authenticated
   (see Section 6.3), then all Join/Prune messages from that neighbor
   MUST also be authenticated.

   We note that some older PIM implementations incorrectly fail to send
   Hello messages on point-to-point interfaces, so we also RECOMMEND
   that a configuration option be provided to allow interoperation with
   such older routers, but that this configuration option SHOULD NOT be
   enabled by default.

4.5.1.  Receiving (*,G) Join/Prune Messages

   When a router receives a Join(*,G), it must first check to see
   whether the RP in the message matches RP(G) (the router's idea of who
   the RP is).  If the RP in the message does not match RP(G), the
   Join(*,G) should be silently dropped.  (Note that other source list
   entries, such as (S,G,rpt) or (S,G), in the same Group-Specific Set
   should still be processed.)  If a router has no RP information (e.g.,
   has not recently received a BSR message), then it may choose to
   accept Join(*,G) and treat the RP in the message as RP(G).  Received
   Prune(*,G) messages are processed even if the RP in the message does
   not match RP(G).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   The per-interface state machine for receiving (*,G) Join/Prune
   messages is given below.  There are three states:

      NoInfo (NI)
            The interface has no (*,G) Join state and no timers running.

      Join (J)
            The interface has (*,G) Join state, which will cause the
            router to forward packets destined for G from this interface
            except if there is also (S,G,rpt) prune information (see
            Section 4.5.3) or the router lost an assert on this
            interface.

      Prune-Pending (PP)
            The router has received a Prune(*,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 Prune-Pending state functions
            exactly like the Join state.

   In addition, the state machine uses two timers:

      Expiry Timer (ET)
            This timer is restarted when a valid Join(*,G) is received.
            Expiry of the Expiry Timer causes the interface state to
            revert to NoInfo for this group.

      Prune-Pending Timer (PPT)
            This timer is set when a valid Prune(*,G) is received.
            Expiry of the Prune-Pending Timer causes the interface state
            to revert to NoInfo for this group.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

         Figure 2: Downstream Per-Interface (*,G) State Machine

+------------++--------------------------------------------------------+
|            ||                         Event                          |
|            ++-------------+--------------+-------------+-------------+
|Prev State  ||Receive      | Receive      | Prune-      | Expiry Timer|
|            ||Join(*,G)    | Prune(*,G)   | Pending     | Expires     |
|            ||             |              | Timer       |             |
|            ||             |              | Expires     |             |
+------------++-------------+--------------+-------------+-------------+
|            ||-> J state   | -> NI state  | -           | -           |
|NoInfo (NI) ||start Expiry |              |             |             |
|            ||Timer        |              |             |             |
+------------++-------------+--------------+-------------+-------------+
|            ||-> J state   | -> PP state  | -           | -> NI state |
|Join (J)    ||restart      | start Prune- |             |             |
|            ||Expiry Timer | Pending      |             |             |
|            ||             | Timer        |             |             |
+------------++-------------+--------------+-------------+-------------+
|Prune-      ||-> J state   | -> PP state  | -> NI state | -> NI state |
|Pending (PP)||restart      |              | Send Prune- |             |
|            ||Expiry Timer |              | Echo(*,G)   |             |
+------------++-------------+--------------+-------------+-------------+

   The transition events "Receive Join(*,G)" and "Receive Prune(*,G)"
   imply receiving a Join or Prune targeted to this router's primary IP
   address on the received interface.  If the upstream neighbor address
   field is not correct, these state transitions in this state machine
   MUST NOT occur, although seeing such a packet may cause state
   transitions in other state machines.

   On unnumbered interfaces on point-to-point links, the router's
   address should be the same as the source address it chose for the
   Hello message it sent over that interface.  However, on point-to-
   point links it is RECOMMENDED that for backwards compatibility PIM
   Join/Prune messages with an upstream neighbor address field of all
   zeros also be accepted.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from NoInfo State

   When in NoInfo state, the following event may trigger a transition:

      Receive Join(*,G)
            A Join(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (*,G) downstream state machine on interface I
            transitions to the Join state.  The Expiry Timer (ET) is
            started and set to the HoldTime from the triggering
            Join/Prune message.

   Transitions from Join State

   When in Join state, the following events may trigger a transition:

      Receive Join(*,G)
            A Join(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (*,G) downstream state machine on interface I remains in
            Join state, and the Expiry Timer (ET) is restarted.  The ET
            is set to the maximum of its current value and the HoldTime
            from the triggering Join/Prune message.

      Receive Prune(*,G)
            A Prune(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (*,G) downstream state machine on interface I
            transitions to the Prune-Pending state.  The
            Prune-Pending Timer is started.  It is set to the
            J/P_Override_Interval(I) if the router has more than one
            neighbor on that interface; otherwise, it is set to zero,
            causing it to expire immediately.

      Expiry Timer Expires
            The Expiry Timer for the (*,G) downstream state machine on
            interface I expires.

            The (*,G) downstream state machine on interface I
            transitions to the NoInfo state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from Prune-Pending State

   When in Prune-Pending state, the following events may trigger a
   transition:

      Receive Join(*,G)
            A Join(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (*,G) downstream state machine on interface I
            transitions to the Join state.  The Prune-Pending Timer is
            canceled (without triggering an expiry event).  The
            Expiry Timer (ET) is restarted and is then set to the
            maximum of its current value and the HoldTime from the
            triggering Join/Prune message.

      Expiry Timer Expires
            The Expiry Timer for the (*,G) downstream state machine on
            interface I expires.

            The (*,G) downstream state machine on interface I
            transitions to the NoInfo state.

      Prune-Pending Timer Expires
            The Prune-Pending Timer for the (*,G) downstream state
            machine on interface I expires.

            The (*,G) downstream state machine on interface I
            transitions to the NoInfo state.  A PruneEcho(*,G) is sent
            onto the subnet connected to interface I.

            The action "Send PruneEcho(*,G)" is triggered when the
            router stops forwarding on an interface as a result of a
            prune.  A PruneEcho(*,G) is simply a Prune(*,G) message sent
            by the upstream router on a LAN with its own address in the
            Upstream Neighbor Address field.  Its purpose is to add
            additional reliability so that if a Prune that should have
            been overridden by another router is lost locally on the
            LAN, then the PruneEcho may be received and cause the
            override to happen.  A PruneEcho(*,G) need not be sent on an
            interface that contains only a single PIM neighbor during
            the time this state machine was in Prune-Pending state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.5.2.  Receiving (S,G) Join/Prune Messages

   The per-interface state machine for receiving (S,G) Join/Prune
   messages is given below and is almost identical to that for (*,G)
   messages.  There are three states:

      NoInfo (NI)
            The interface has no (S,G) Join state and no (S,G) timers
            running.

      Join (J)
            The interface has (S,G) Join state, which will cause the
            router to forward packets from S destined for G from this
            interface if the (S,G) state is active (the SPTbit is set)
            except if the router lost an assert on this interface.

      Prune-Pending (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 Prune-Pending state functions
            exactly like the Join state.

   In addition, there are two timers:

      Expiry Timer (ET)
            This timer is set when a valid Join(S,G) is received.
            Expiry of the Expiry Timer causes this state machine to
            revert to NoInfo state.

      Prune-Pending Timer (PPT)
            This timer is set when a valid Prune(S,G) is received.
            Expiry of the Prune-Pending Timer causes this state machine
            to revert to NoInfo state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

         Figure 3: Downstream Per-Interface (S,G) State Machine

+------------++--------------------------------------------------------+
|            ||                         Event                          |
|            ++-------------+--------------+-------------+-------------+
|Prev State  ||Receive      | Receive      | Prune-      | Expiry Timer|
|            ||Join(S,G)    | Prune(S,G)   | Pending     | Expires     |
|            ||             |              | Timer       |             |
|            ||             |              | Expires     |             |
+------------++-------------+--------------+-------------+-------------+
|            ||-> J state   | -> NI state  | -           | -           |
|NoInfo (NI) ||start Expiry |              |             |             |
|            ||Timer        |              |             |             |
+------------++-------------+--------------+-------------+-------------+
|            ||-> J state   | -> PP state  | -           | -> NI state |
|Join (J)    ||restart      | start Prune- |             |             |
|            ||Expiry Timer | Pending      |             |             |
|            ||             | Timer        |             |             |
+------------++-------------+--------------+-------------+-------------+
|Prune-      ||-> J state   | -> PP state  | -> NI state | -> NI state |
|Pending (PP)||restart      |              | Send Prune- |             |
|            ||Expiry Timer |              | Echo(S,G)   |             |
+------------++-------------+--------------+-------------+-------------+

   The transition events "Receive Join(S,G)" and "Receive Prune(S,G)"
   imply receiving a Join or Prune targeted to this router's primary IP
   address on the received interface.  If the upstream neighbor address
   field is not correct, these state transitions in this state machine
   MUST NOT occur, although seeing such a packet may cause state
   transitions in other state machines.

   On unnumbered interfaces on point-to-point links, the router's
   address SHOULD be the same as the source address it chose for the
   Hello message it sent over that interface.  However, on point-to-
   point links it is RECOMMENDED that for backwards compatibility PIM
   Join/Prune messages with an upstream neighbor address field of all
   zeros also be accepted.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from NoInfo State

   When in NoInfo state, the following event may trigger a transition:

      Receive Join(S,G)
            A Join(S,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G) downstream state machine on interface I
            transitions to the Join state.  The Expiry Timer (ET) is
            started and set to the HoldTime from the triggering
            Join/Prune message.

   Transitions from Join State

   When in Join state, the following events may trigger a transition:

      Receive Join(S,G)
            A Join(S,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G) downstream state machine on interface I remains in
            Join state.  The Expiry Timer (ET) is restarted and is then
            set to the maximum of its current value and the HoldTime
            from the triggering Join/Prune message.

      Receive Prune(S,G)
            A Prune(S,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G) downstream state machine on interface I
            transitions to the Prune-Pending state.  The
            Prune-Pending Timer is started.  It is set to the
            J/P_Override_Interval(I) if the router has more than one
            neighbor on that interface; otherwise, it is set to zero,
            causing it to expire immediately.

      Expiry Timer Expires
            The Expiry Timer for the (S,G) downstream state machine on
            interface I expires.

            The (S,G) downstream state machine on interface I
            transitions to the NoInfo state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from Prune-Pending State

   When in Prune-Pending state, the following events may trigger a
   transition:

      Receive Join(S,G)
            A Join(S,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G) downstream state machine on interface I
            transitions to the Join state.  The Prune-Pending Timer is
            canceled (without triggering an expiry event).  The
            Expiry Timer (ET) is restarted and is then set to the
            maximum of its current value and the HoldTime from the
            triggering Join/Prune message.

      Expiry Timer Expires
            The Expiry Timer for the (S,G) downstream state machine on
            interface I expires.

            The (S,G) downstream state machine on interface I
            transitions to the NoInfo state.

      Prune-Pending Timer Expires
            The Prune-Pending Timer for the (S,G) downstream state
            machine on interface I expires.

            The (S,G) downstream state machine on interface I
            transitions to the NoInfo state.  A PruneEcho(S,G) is sent
            onto the subnet connected to interface I.

            The action "Send PruneEcho(S,G)" is triggered when the
            router stops forwarding on an interface as a result of a
            prune.  A PruneEcho(S,G) is simply a Prune(S,G) message sent
            by the upstream router on a LAN with its own address in the
            Upstream Neighbor Address field.  Its purpose is to add
            additional reliability so that if a Prune that should have
            been overridden by another router is lost locally on the
            LAN, then the PruneEcho may be received and cause the
            override to happen.  A PruneEcho(S,G) need not be sent on an
            interface that contains only a single PIM neighbor during
            the time this state machine was in Prune-Pending state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.5.3.  Receiving (S,G,rpt) Join/Prune Messages

   The per-interface state machine for receiving (S,G,rpt) Join/Prune
   messages is given below.  There are five states:

      NoInfo (NI)
            The interface has no (S,G,rpt) Prune state and no (S,G,rpt)
            timers running.

      Prune (P)
            The interface has (S,G,rpt) Prune state, which will cause
            the router not to forward packets from S destined for G from
            this interface even though the interface has active (*,G)
            Join state.

      Prune-Pending (PP)
            The router has received a Prune(S,G,rpt) 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 Prune-Pending state functions
            exactly like the NoInfo state.

      PruneTmp (P')
            This state is a transient state that for forwarding purposes
            behaves exactly like the Prune state.  A (*,G) Join has been
            received (which may cancel the (S,G,rpt) Prune).  As we
            parse the Join/Prune message from top to bottom, we first
            enter this state if the message contains a (*,G) Join.
            Later in the message, we will normally encounter an
            (S,G,rpt) prune to reinstate the Prune state.  However, if
            we reach the end of the message without encountering such an
            (S,G,rpt) prune, then we will revert to NoInfo state in this
            state machine.

            As no time is spent in this state, no timers can expire.

      Prune-Pending-Tmp (PP')
            This state is a transient state that is identical to P'
            except that it is associated with the PP state rather than
            the P state.  For forwarding purposes, PP' behaves exactly
            like the PP state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   In addition, there are two timers:

      Expiry Timer (ET)
            This timer is set when a valid Prune(S,G,rpt) is received.
            Expiry of the Expiry Timer causes this state machine to
            revert to NoInfo state.

      Prune-Pending Timer (PPT)
            This timer is set when a valid Prune(S,G,rpt) is received.
            Expiry of the Prune-Pending Timer causes this state machine
            to move on to Prune state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

       Figure 4: Downstream Per-Interface (S,G,rpt) State Machine

+----------++----------------------------------------------------------+
|          ||                          Event                           |
|          ++---------+----------+----------+--------+--------+--------+
|Prev      ||Receive  | Receive  | Receive  | End of | Prune- | Expiry |
|State     ||Join(*,G)| Join     | Prune    | Message| Pending| Timer  |
|          ||         | (S,G,rpt)| (S,G,rpt)|        | Timer  | Expires|
|          ||         |          |          |        | Expires|        |
+----------++---------+----------+----------+--------+--------+--------+
|          ||-        | -        | -> PP    | -      | -      | -      |
|          ||         |          | state    |        |        |        |
|          ||         |          | start    |        |        |        |
|NoInfo    ||         |          | Prune-   |        |        |        |
|(NI)      ||         |          | Pending  |        |        |        |
|          ||         |          | Timer;   |        |        |        |
|          ||         |          | start    |        |        |        |
|          ||         |          | Expiry   |        |        |        |
|          ||         |          | Timer    |        |        |        |
+----------++---------+----------+----------+--------+--------+--------+
|          ||-> P'    | -> NI    | -> P     | -      | -      | -> NI  |
|          ||state    | state    | state    |        |        | state  |
|Prune (P) ||         |          | restart  |        |        |        |
|          ||         |          | Expiry   |        |        |        |
|          ||         |          | Timer    |        |        |        |
+----------++---------+----------+----------+--------+--------+--------+
|Prune-    ||-> PP'   | -> NI    | -        | -      | -> P   | -      |
|Pending   ||state    | state    |          |        | state  |        |
|(PP)      ||         |          |          |        |        |        |
+----------++---------+----------+----------+--------+--------+--------+
|          ||-        | -        | -> P     | -> NI  | -      | -      |
|PruneTmp  ||         |          | state    | state  |        |        |
|(P')      ||         |          | restart  |        |        |        |
|          ||         |          | Expiry   |        |        |        |
|          ||         |          | Timer    |        |        |        |
+----------++---------+----------+----------+--------+--------+--------+
|          ||-        | -        | -> PP    | -> NI  | -      | -      |
|Prune-    ||         |          | state    | state  |        |        |
|Pending-  ||         |          | restart  |        |        |        |
|Tmp (PP') ||         |          | Expiry   |        |        |        |
|          ||         |          | Timer    |        |        |        |
+----------++---------+----------+----------+--------+--------+--------+

   The transition events "Receive Join(S,G,rpt)", "Receive
   Prune(S,G,rpt)", and "Receive Join(*,G)" imply receiving a Join or
   Prune targeted to this router's primary IP address on the received
   interface.  If the upstream neighbor address field is not correct,

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   these state transitions in this state machine MUST NOT occur,
   although seeing such a packet may cause state transitions in other
   state machines.

   On unnumbered interfaces on point-to-point links, the router's
   address should be the same as the source address it chose for the
   Hello message it sent over that interface.  However, on point-to-
   point links it is RECOMMENDED that PIM Join/Prune messages with an
   upstream neighbor address field of all zeros also be accepted.

   Transitions from NoInfo State

   When in NoInfo (NI) state, the following event may trigger a
   transition:

      Receive Prune(S,G,rpt)
            A Prune(S,G,rpt) is received on interface I with its
            Upstream Neighbor Address set to the router's primary IP
            address on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the Prune-Pending state.  The Expiry Timer
            (ET) is started and set to the HoldTime from the triggering
            Join/Prune message.  The Prune-Pending Timer is started.  It
            is set to the J/P_Override_Interval(I) if the router has
            more than one neighbor on that interface; otherwise, it is
            set to zero, causing it to expire immediately.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from Prune-Pending State

   When in Prune-Pending (PP) state, the following events may trigger a
   transition:

      Receive Join(*,G)
            A Join(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the Prune-Pending-Tmp state whilst the
            remainder of the compound Join/Prune message containing the
            Join(*,G) is processed.

      Receive Join(S,G,rpt)
            A Join(S,G,rpt) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the NoInfo state.  The ET and PPT are
            canceled.

      Prune-Pending Timer Expires
            The Prune-Pending Timer for the (S,G,rpt) downstream state
            machine on interface I expires.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the Prune state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from Prune State

   When in Prune (P) state, the following events may trigger a
   transition:

      Receive Join(*,G)
            A Join(*,G) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the PruneTmp state whilst the remainder of
            the compound Join/Prune message containing the Join(*,G) is
            processed.

      Receive Join(S,G,rpt)
            A Join(S,G,rpt) is received on interface I with its Upstream
            Neighbor Address set to the router's primary IP address
            on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the NoInfo state.  The ET and PPT are
            canceled.

      Receive Prune(S,G,rpt)
            A Prune(S,G,rpt) is received on interface I with its
            Upstream Neighbor Address set to the router's primary IP
            address on I.

            The (S,G,rpt) downstream state machine on interface I
            remains in Prune state.  The Expiry Timer (ET) is restarted
            and is then set to the maximum of its current value and the
            HoldTime from the triggering Join/Prune message.

      Expiry Timer Expires
            The Expiry Timer for the (S,G,rpt) downstream state machine
            on interface I expires.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the NoInfo state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from Prune-Pending-Tmp State

   When in Prune-Pending-Tmp (PP') state and processing a compound
   Join/Prune message, the following events may trigger a transition:

      Receive Prune(S,G,rpt)
            The compound Join/Prune message contains a Prune(S,G,rpt)
            that is received on interface I with its Upstream Neighbor
            Address set to the router's primary IP address on I.

            The (S,G,rpt) downstream state machine on interface I
            transitions back to the Prune-Pending state.  The
            Expiry Timer (ET) is restarted and is then set to the
            maximum of its current value and the HoldTime from the
            triggering Join/Prune message.

      End of Message
            The end of the compound Join/Prune message is reached.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the NoInfo state.  The ET and PPT are
            canceled.

   Transitions from PruneTmp State

   When in PruneTmp (P') state and processing a compound Join/Prune
   message, the following events may trigger a transition:

      Receive Prune(S,G,rpt)
            The compound Join/Prune message contains a Prune(S,G,rpt).

            The (S,G,rpt) downstream state machine on interface I
            transitions back to the Prune state.  The Expiry Timer (ET)
            is restarted and is then set to the maximum of its current
            value and the HoldTime from the triggering Join/Prune
            message.

      End of Message
            The end of the compound Join/Prune message is reached.

            The (S,G,rpt) downstream state machine on interface I
            transitions to the NoInfo state.  ET is canceled.

   Note: Receiving a Prune(*,G) does not affect the (S,G,rpt) downstream
   state machine.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.5.4.  Sending (*,G) Join/Prune Messages

   The per-interface state machines for (*,G) hold join state from
   downstream PIM routers.  This state then determines whether a router
   needs to propagate a Join(*,G) upstream towards the RP.

   If a router wishes to propagate a Join(*,G) upstream, it must also
   watch for messages on its upstream interface from other routers on
   that subnet, and these may modify its behavior.  If it sees a
   Join(*,G) to the correct upstream neighbor, it should suppress its
   own Join(*,G).  If it sees a Prune(*,G) to the correct upstream
   neighbor, it should be prepared to override that prune by sending a
   Join(*,G) almost immediately.  Finally, if it sees the Generation ID
   (see Section 4.3) of the correct upstream neighbor change, it knows
   that the upstream neighbor has lost state, and it should be prepared
   to refresh the state by sending a Join(*,G) almost immediately.

   If a (*,G) Assert occurs on the upstream interface, and this changes
   this router's idea of the upstream neighbor, it should be prepared to
   ensure that the Assert winner is aware of downstream routers by
   sending a Join(*,G) almost immediately.

   In addition, if the MRIB changes to indicate that the next hop
   towards the RP has changed, and either the upstream interface changes
   or there is no Assert winner on the upstream interface, the router
   should prune off from the old next hop and join towards the new
   next hop.

   The upstream (*,G) state machine only contains two states:

      Not Joined
            The downstream state machines indicate that the router does
            not need to join the RP tree for this group.

      Joined
            The downstream state machines indicate that the router
            should join the RP tree for this group.

   In addition, one timer JT(*,G) is kept that is used to trigger the
   sending of a Join(*,G) to the upstream next hop towards the RP,
   RPF'(*,G).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

                 Figure 5: Upstream (*,G) State Machine

+-------------------++-------------------------------------------------+
|                   ||                      Event                      |
|  Prev State       ++------------------------+------------------------+
|                   ||   JoinDesired(*,G)     |    JoinDesired(*,G)    |
|                   ||   ->True               |    ->False             |
+-------------------++------------------------+------------------------+
|                   ||   -> J state           |    -                   |
|  NotJoined (NJ)   ||   Send Join(*,G);      |                        |
|                   ||   set Join Timer to    |                        |
|                   ||   t_periodic           |                        |
+-------------------++------------------------+------------------------+
|  Joined (J)       ||   -                    |    -> NJ state         |
|                   ||                        |    Send Prune(*,G);    |
|                   ||                        |    cancel Join Timer   |
+-------------------++------------------------+------------------------+

   In addition, we have the following transitions, which occur within
   the Joined state:

+----------------------------------------------------------------------+
|                        In Joined (J) State                           |
+----------------+-----------------+-----------------+-----------------+
|Timer Expires   | See Join(*,G)   | See Prune(*,G)  | RPF'(*,G)       |
|                | to RPF'(*,G)    | to RPF'(*,G)    | changes due to  |
|                |                 |                 | an Assert       |
+----------------+-----------------+-----------------+-----------------+
|Send            | Increase Join   | Decrease Join   | Decrease Join   |
|Join(*,G); set  | Timer to        | Timer to        | Timer to        |
|Join Timer to   | t_joinsuppress  | t_override      | t_override      |
|t_periodic      |                 |                 |                 |
+----------------+-----------------+-----------------+-----------------+

+----------------------------------------------------------------------+
|                         In Joined (J) State                          |
+----------------------------------+-----------------------------------+
|    RPF'(*,G) changes not         |       RPF'(*,G) GenID changes     |
|    due to an Assert              |                                   |
+----------------------------------+-----------------------------------+
|    Send Join(*,G) to new         |       Decrease Join Timer to      |
|    next hop; send                |       t_override                  |
|    Prune(*,G) to old next        |                                   |
|    hop; set Join Timer to        |                                   |
|    t_periodic                    |                                   |
+----------------------------------+-----------------------------------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   This state machine uses the following macro:

     bool JoinDesired(*,G) {
        if (immediate_olist(*,G) != NULL)
            return TRUE
        else
            return FALSE
     }

   JoinDesired(*,G) is true when the router has forwarding state that
   would cause it to forward traffic for G using shared tree state.
   Note that although JoinDesired is true, the router's sending of a
   Join(*,G) message may be suppressed by another router sending a
   Join(*,G) onto the upstream interface.

   Transitions from NotJoined State

   When the upstream (*,G) state machine is in NotJoined state, the
   following event may trigger a state transition:

      JoinDesired(*,G) becomes True
            The macro JoinDesired(*,G) becomes True, e.g., because the
            downstream state for (*,G) has changed so that at least one
            interface is in immediate_olist(*,G).

            The upstream (*,G) state machine transitions to the Joined
            state.  Send Join(*,G) to the appropriate upstream neighbor,
            which is RPF'(*,G).  Set the Join Timer (JT) to expire after
            t_periodic seconds.

   Transitions from Joined State

   When the upstream (*,G) state machine is in Joined state, the
   following events may trigger state transitions:

      JoinDesired(*,G) becomes False
            The macro JoinDesired(*,G) becomes False, e.g., because the
            downstream state for (*,G) has changed so no interface is in
            immediate_olist(*,G).

            The upstream (*,G) state machine transitions to the
            NotJoined state.  Send Prune(*,G) to the appropriate
            upstream neighbor, which is RPF'(*,G).  Cancel the
            Join Timer (JT).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      Join Timer Expires
            The Join Timer (JT) expires, indicating time to send a
            Join(*,G).

            Send Join(*,G) to the appropriate upstream neighbor, which
            is RPF'(*,G).  Restart the Join Timer (JT) to expire after
            t_periodic seconds.

      See Join(*,G) to RPF'(*,G)
            This event is only relevant if RPF_interface(RP(G)) is a
            shared medium.  This router sees another router on
            RPF_interface(RP(G)) send a Join(*,G) to RPF'(*,G).  This
            causes this router to suppress its own Join.

            The upstream (*,G) state machine remains in Joined state.

            Let t_joinsuppress be the minimum of t_suppressed and the
            HoldTime from the Join/Prune message triggering this event.
            If the Join Timer is set to expire in less than
            t_joinsuppress seconds, reset it so that it expires after
            t_joinsuppress seconds.  If the Join Timer is set to expire
            in more than t_joinsuppress seconds, leave it unchanged.

      See Prune(*,G) to RPF'(*,G)
            This event is only relevant if RPF_interface(RP(G)) is a
            shared medium.  This router sees another router on
            RPF_interface(RP(G)) send a Prune(*,G) to RPF'(*,G).  As
            this router is in Joined state, it must override the Prune
            after a short random interval.

            The upstream (*,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.  If the Join Timer is set to expire in
            less than t_override seconds, leave it unchanged.

      RPF'(*,G) changes due to an Assert
            The current next hop towards the RP changes due to an
            Assert(*,G) on the RPF_interface(RP(G)).

            The upstream (*,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.  If the Join Timer is set to expire in
            less than t_override seconds, leave it unchanged.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      RPF'(*,G) changes not due to an Assert
            An event occurred that caused the next hop towards the RP
            for G to change.  This may be caused by a change in the MRIB
            routing database or the group-to-RP mapping.  Note that this
            transition does not occur if an Assert is active and the
            upstream interface does not change.

            The upstream (*,G) state machine remains in Joined state.
            Send Join(*,G) to the new upstream neighbor, which is the
            new value of RPF'(*,G).  Send Prune(*,G) to the old upstream
            neighbor, which is the old value of RPF'(*,G).  Use the new
            value of RP(G) in the Prune(*,G) message or all zeros if
            RP(G) becomes unknown (old value of RP(G) may be used
            instead to improve behavior in routers implementing older
            versions of this specification).  Set the Join Timer (JT) to
            expire after t_periodic seconds.

      RPF'(*,G) GenID changes
            The Generation ID of the router that is RPF'(*,G) changes.
            This normally means that this neighbor has lost state, and
            so the state must be refreshed.

            The upstream (*,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.

4.5.5.  Sending (S,G) Join/Prune Messages

   The per-interface state machines for (S,G) hold join state from
   downstream PIM routers.  This state then determines whether a router
   needs to propagate a Join(S,G) upstream towards the source.

   If a router wishes to propagate a Join(S,G) upstream, it must also
   watch for messages on its upstream interface from other routers on
   that subnet, and these may modify its behavior.  If it sees a
   Join(S,G) to the correct upstream neighbor, it should suppress its
   own Join(S,G).  If it sees a Prune(S,G), Prune(S,G,rpt), or
   Prune(*,G) to the correct upstream neighbor towards S, it should be
   prepared to override that prune by scheduling a Join(S,G) to be sent
   almost immediately.  Finally, if it sees the Generation ID of its
   upstream neighbor change, it knows that the upstream neighbor has
   lost state, and it should refresh the state by scheduling a Join(S,G)
   to be sent almost immediately.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   If an (S,G) Assert occurs on the upstream interface, and this changes
   this router's idea of the upstream neighbor, it should be prepared to
   ensure that the Assert winner is aware of downstream routers by
   scheduling a Join(S,G) to be sent almost immediately.

   In addition, if MRIB changes cause the next hop towards the source to
   change, and either the upstream interface changes or there is no
   Assert winner on the upstream interface, the router should send a
   prune to the old next hop and a join to the new next hop.

   The upstream (S,G) state machine only contains two states:

      Not Joined
            The downstream state machines and local membership
            information do not indicate that the router needs to join
            the shortest-path tree for this (S,G).

      Joined
            The downstream state machines and local membership
            information indicate that the router should join the
            shortest-path tree for this (S,G).

   In addition, one timer JT(S,G) is kept that is used to trigger the
   sending of a Join(S,G) to the upstream next hop towards S, RPF'(S,G).

                 Figure 6: Upstream (S,G) State Machine

+-------------------+--------------------------------------------------+
|                   |                      Event                       |
|  Prev State       +-------------------------+------------------------+
|                   |   JoinDesired(S,G)      |   JoinDesired(S,G)     |
|                   |   ->True                |   ->False              |
+-------------------+-------------------------+------------------------+
|  NotJoined (NJ)   |   -> J state            |   -                    |
|                   |   Send Join(S,G);       |                        |
|                   |   set Join Timer to     |                        |
|                   |   t_periodic            |                        |
+-------------------+-------------------------+------------------------+
|  Joined (J)       |   -                     |   -> NJ state          |
|                   |                         |   Send Prune(S,G);     |
|                   |                         |   set SPTbit(S,G) to   |
|                   |                         |   FALSE; cancel Join   |
|                   |                         |   Timer                |
+-------------------+-------------------------+------------------------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   In addition, we have the following transitions, which occur within
   the Joined state:

+----------------------------------------------------------------------+
|                         In Joined (J) State                          |
+-----------------+-----------------+-----------------+----------------+
| Timer Expires   | See Join(S,G)   | See Prune(S,G)  | See Prune      |
|                 | to RPF'(S,G)    | to RPF'(S,G)    | (S,G,rpt) to   |
|                 |                 |                 | RPF'(S,G)      |
+-----------------+-----------------+-----------------+----------------+
| Send            | Increase Join   | Decrease Join   | Decrease Join  |
| Join(S,G); set  | Timer to        | Timer to        | Timer to       |
| Join Timer to   | t_joinsuppress  | t_override      | t_override     |
| t_periodic      |                 |                 |                |
+-----------------+-----------------+-----------------+----------------+

+----------------------------------------------------------------------+
|                        In Joined (J) State                           |
+-----------------+-----------------+----------------+-----------------+
| See Prune(*,G)  | RPF'(S,G)       | RPF'(S,G)      | RPF'(S,G)       |
| to RPF'(S,G)    | changes not     | GenID changes  | changes due to  |
|                 | due to an       |                | an Assert       |
|                 | Assert          |                |                 |
+-----------------+-----------------+----------------+-----------------+
| Decrease Join   | Send Join(S,G)  | Decrease Join  | Decrease Join   |
| Timer to        | to new next     | Timer to       | Timer to        |
| t_override      | hop; send       | t_override     | t_override      |
|                 | Prune(S,G) to   |                |                 |
|                 | old next hop;   |                |                 |
|                 | set Join Timer  |                |                 |
|                 | to t_periodic   |                |                 |
+-----------------+-----------------+----------------+-----------------+

   This state machine uses the following macro:

     bool JoinDesired(S,G) {
         return( immediate_olist(S,G) != NULL
                 OR ( KeepaliveTimer(S,G) is running
                      AND inherited_olist(S,G) != NULL ) )
     }

   JoinDesired(S,G) is true when the router has forwarding state that
   would cause it to forward traffic for G using source tree state.  The
   source tree state can be as a result of either active source-specific
   join state, or the (S,G) Keepalive Timer and active non-source-
   specific state.  Note that although JoinDesired is true, the router's
   sending of a Join(S,G) message may be suppressed by another router
   sending a Join(S,G) onto the upstream interface.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from NotJoined State

   When the upstream (S,G) state machine is in NotJoined state, the
   following event may trigger a state transition:

      JoinDesired(S,G) becomes True
            The macro JoinDesired(S,G) becomes True, e.g., because the
            downstream state for (S,G) has changed so that at least one
            interface is in inherited_olist(S,G).

            The upstream (S,G) state machine transitions to the Joined
            state.  Send Join(S,G) to the appropriate upstream neighbor,
            which is RPF'(S,G).  Set the Join Timer (JT) to expire after
            t_periodic seconds.

   Transitions from Joined State

   When the upstream (S,G) state machine is in Joined state, the
   following events may trigger state transitions:

      JoinDesired(S,G) becomes False
            The macro JoinDesired(S,G) becomes False, e.g., because the
            downstream state for (S,G) has changed so no interface is in
            inherited_olist(S,G).

            The upstream (S,G) state machine transitions to the
            NotJoined state.  Send Prune(S,G) to the appropriate
            upstream neighbor, which is RPF'(S,G).  Cancel the
            Join Timer (JT), and set SPTbit(S,G) to FALSE.

      Join Timer Expires
            The Join Timer (JT) expires, indicating time to send a
            Join(S,G).

            Send Join(S,G) to the appropriate upstream neighbor, which
            is RPF'(S,G).  Restart the Join Timer (JT) to expire after
            t_periodic seconds.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      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).  This causes this router to
            suppress its own Join.

            The upstream (S,G) state machine remains in Joined state.

            Let t_joinsuppress be the minimum of t_suppressed and the
            HoldTime from the Join/Prune message triggering this event.

            If the Join Timer is set to expire in less than
            t_joinsuppress seconds, reset it so that it expires after
            t_joinsuppress seconds.  If the Join Timer is set to expire
            in more than t_joinsuppress seconds, leave it unchanged.

      See Prune(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 Prune(S,G) to RPF'(S,G).  As this router is in Joined
            state, it must override the Prune after a short random
            interval.

            The upstream (S,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.

      See Prune(S,G,rpt) 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 Prune(S,G,rpt) to RPF'(S,G).  If the upstream router
            is an RFC-2362-compliant PIM router, then the Prune(S,G,rpt)
            will cause it to stop forwarding.  For backwards
            compatibility, this router should override the prune so that
            forwarding continues.

            The upstream (S,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      See Prune(*,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 Prune(*,G) to RPF'(S,G).  If the upstream router is
            an RFC-2362-compliant PIM router, then the Prune(*,G) will
            cause it to stop forwarding.  For backwards compatibility,
            this router should override the prune so that forwarding
            continues.

            The upstream (S,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.

      RPF'(S,G) changes due to an Assert
            The current next hop towards S changes due to an Assert(S,G)
            on the RPF_interface(S).

            The upstream (S,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.  If the Join Timer is set to expire in
            less than t_override seconds, leave it unchanged.

      RPF'(S,G) changes not due to an Assert
            An event occurred that caused the next hop towards S to
            change.  Note that this transition does not occur if an
            Assert is active and the upstream interface does not change.

            The upstream (S,G) state machine remains in Joined state.
            Send Join(S,G) to the new upstream neighbor, which is the
            new value of RPF'(S,G).  Send Prune(S,G) to the old upstream
            neighbor, which is the old value of RPF'(S,G).  Set the
            Join Timer (JT) to expire after t_periodic seconds.

      RPF'(S,G) GenID changes
            The Generation ID of the router that is RPF'(S,G) changes.
            This normally means that this neighbor has lost state, and
            so the state must be refreshed.

            The upstream (S,G) state machine remains in Joined state.
            If the Join Timer is set to expire in more than
            t_override seconds, reset it so that it expires after
            t_override seconds.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.5.6.  (S,G,rpt) Periodic Messages

   (S,G,rpt) Joins and Prunes are (S,G) Joins or Prunes sent on the RP
   tree with the RPT bit set, either to modify the results of (*,G)
   Joins, or to override the behavior of other upstream LAN peers.  The
   next section describes the rules for sending triggered messages.
   This section describes the rules for including a Prune(S,G,rpt)
   message with a Join(*,G).

   When a router is going to send a Join(*,G), it should use the
   following pseudocode, for each (S,G) for which it has state, to
   decide whether to include a Prune(S,G,rpt) in the compound Join/Prune
   message:

     if( SPTbit(S,G) == TRUE ) {
         # Note: If receiving (S,G) on the SPT, we only prune off the
         # shared tree if the RPF neighbors differ.
          if( RPF'(*,G) != RPF'(S,G) ) {
              add Prune(S,G,rpt) to compound message
          }
     } else if ( inherited_olist(S,G,rpt) == NULL ) {
       # Note: All (*,G) olist interfaces received RPT prunes for (S,G).
       add Prune(S,G,rpt) to compound message
     } else if ( RPF'(*,G) != RPF'(S,G,rpt) {
       # Note: We joined the shared tree, but there was an (S,G) assert
       # and the source tree RPF neighbor is different.
       add Prune(S,G,rpt) to compound message
     }

   Note that Join(S,G,rpt) is normally sent not as a periodic message,
   but only as a triggered message.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

4.5.7.  State Machine for (S,G,rpt) Triggered Messages

   The state machine for (S,G,rpt) triggered messages is required
   per-(S,G) when there is (*,G) join state at a router, and the router
   or any of its upstream LAN peers wishes to prune S off the RP tree.

   There are three states in the state machine.  One of the states is
   when there is no (*,G) join state at this router.  If there is (*,G)
   join state at the router, then the state machine must be at one of
   the other two states.  The three states are:

      Pruned(S,G,rpt)
         (*,G) Joined, but (S,G,rpt) pruned.

      NotPruned(S,G,rpt)
         (*,G) Joined, and (S,G,rpt) not pruned.

      RPTNotJoined(G)
         (*,G) has not been joined.

   In addition, there is an (S,G,rpt) Override Timer, OT(S,G,rpt), which
   is used to delay triggered Join(S,G,rpt) messages to prevent
   implosions of triggered messages.

   Figure 7: Upstream (S,G,rpt) State Machine for Triggered Messages

+------------++--------------------------------------------------------+
|            ||                           Event                        |
|            ++--------------+--------------+-------------+------------+
|Prev State  || PruneDesired | PruneDesired | RPTJoin     | inherited_ |
|            || (S,G,rpt)    | (S,G,rpt)    | Desired(G)  | olist      |
|            || ->True       | ->False      | ->False     | (S,G,rpt)  |
|            ||              |              |             | ->non-NULL |
+------------++--------------+--------------+-------------+------------+
|RPTNotJoined|| -> P state   | -            | -           | -> NP state|
|(G) (NJ)    ||              |              |             |            |
+------------++--------------+--------------+-------------+------------+
|Pruned      || -            | -> NP state  | -> NJ state | -          |
|(S,G,rpt)   ||              | Send Join    |             |            |
|(P)         ||              | (S,G,rpt)    |             |            |
+------------++--------------+--------------+-------------+------------+
|NotPruned   || -> P state   | -            | -> NJ state | -          |
|(S,G,rpt)   || Send Prune   |              | Cancel OT   |            |
|(NP)        || (S,G,rpt);   |              |             |            |
|            || cancel OT    |              |             |            |
+------------++--------------+--------------+-------------+------------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Additionally, we have the following transitions within the
   NotPruned(S,G,rpt) state, which are all used for prune override
   behavior.

+----------------------------------------------------------------------+
|                    In NotPruned(S,G,rpt) State                       |
+----------+--------------+--------------+--------------+--------------+
|Override  | See Prune    | See Join     | See Prune    | RPF'         |
|Timer     | (S,G,rpt) to | (S,G,rpt) to | (S,G) to     | (S,G,rpt) -> |
|expires   | RPF'         | RPF'         | RPF'         | RPF' (*,G)   |
|          | (S,G,rpt)    | (S,G,rpt)    | (S,G,rpt)    |              |
+----------+--------------+--------------+--------------+--------------+
|Send Join | OT = min(OT, | Cancel OT    | OT = min(OT, | OT = min(OT, |
|(S,G,rpt);| t_override)  |              | t_override)  | t_override)  |
|leave OT  |              |              |              |              |
|unset     |              |              |              |              |
+----------+--------------+--------------+--------------+--------------+

   Note that the min function in the above state machine considers a
   non-running timer to have an infinite value (e.g., min(not-running,
   t_override) = t_override).

   This state machine uses the following macros:

     bool RPTJoinDesired(G) {
       return (JoinDesired(*,G))
     }

   RPTJoinDesired(G) is true when the router has forwarding state that
   would cause it to forward traffic for G using (*,G) shared tree
   state.

     bool PruneDesired(S,G,rpt) {
          return ( RPTJoinDesired(G) AND
                   ( inherited_olist(S,G,rpt) == NULL
                     OR (SPTbit(S,G)==TRUE
                         AND (RPF'(*,G) != RPF'(S,G)) )))
     }

   PruneDesired(S,G,rpt) can only be true if RPTJoinDesired(G) is true.
   If RPTJoinDesired(G) is true, then PruneDesired(S,G,rpt) is true
   either if there are no outgoing interfaces that S would be forwarded
   on, or if the router has active (S,G) forwarding state but RPF'(*,G)
   != RPF'(S,G).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   The state machine contains the following transition events:

      See Join(S,G,rpt) to RPF'(S,G,rpt)
            This event is only relevant in the "Not Pruned" state.

            The router sees a Join(S,G,rpt) from someone else to
            RPF'(S,G,rpt), which is the correct upstream neighbor.  If
            we're in "NotPruned" state and the (S,G,rpt) Override Timer
            is running, then this is because we have been triggered to
            send our own Join(S,G,rpt) to RPF'(S,G,rpt).  Someone else
            beat us to it, so there's no need to send our own Join.

            The action is to cancel the Override Timer.

      See Prune(S,G,rpt) to RPF'(S,G,rpt)
            This event is only relevant in the "NotPruned" state.

            The router sees a Prune(S,G,rpt) from someone else to
            RPF'(S,G,rpt), which is the correct upstream neighbor.  If
            we're in the "NotPruned" state, then we want to continue to
            receive traffic from S destined for G, and that traffic is
            being supplied by RPF'(S,G,rpt).  Thus, we need to override
            the Prune.

            The action is to set the (S,G,rpt) Override Timer to the
            randomized prune-override interval, t_override.  However, if
            the Override Timer is already running, we only set the timer
            if doing so would set it to a lower value.  At the end of
            this interval, if no one else has sent a Join, then we will
            do so.

      See Prune(S,G) to RPF'(S,G,rpt)
            This event is only relevant in the "NotPruned" state.

            This transition and action are the same as the above
            transition and action, except that the Prune does not have
            the RPT bit set.  This transition is necessary to be
            compatible with routers implemented from RFC 2362 that don't
            maintain separate (S,G) and (S,G,rpt) state.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      The (S,G,rpt) prune Override Timer expires
            This event is only relevant in the "NotPruned" state.

            When the Override Timer expires, we must send a
            Join(S,G,rpt) to RPF'(S,G,rpt) to override the Prune message
            that caused the timer to be running.  We only send this if
            RPF'(S,G,rpt) equals RPF'(*,G); if this were not the case,
            then the Join might be sent to a router that does not have
            (*,G) Join state, and so the behavior would not be well
            defined.  If RPF'(S,G,rpt) is not the same as RPF'(*,G),
            then it may stop forwarding S.  However, if this happens,
            then the router will send an AssertCancel(S,G), which would
            then cause RPF'(S,G,rpt) to become equal to RPF'(*,G) (see
            below).

      RPF'(S,G,rpt) changes to become equal to RPF'(*,G)
            This event is only relevant in the "NotPruned" state.

            RPF'(S,G,rpt) can only be different from RPF'(*,G) if an
            (S,G) Assert has happened, which means that traffic from S
            is arriving on the SPT, and so Prune(S,G,rpt) will have been
            sent to RPF'(*,G).  When RPF'(S,G,rpt) changes to become
            equal to RPF'(*,G), we need to trigger a Join(S,G,rpt) to
            RPF'(*,G) to cause that router to start forwarding S again.

            The action is to set the (S,G,rpt) Override Timer to the
            randomized prune-override interval t_override.  However, if
            the timer is already running, we only set the timer if doing
            so would set it to a lower value.  At the end of this
            interval, if no one else has sent a Join, then we will
            do so.

      PruneDesired(S,G,rpt)->TRUE
            See macro above.  This event is relevant in the "NotPruned"
            and "RPTNotJoined(G)" states.

            The router wishes to receive traffic for G but does not wish
            to receive traffic from S destined for G.  This causes the
            router to transition into the Pruned state.

            If the router was previously in NotPruned state, then the
            action is to send a Prune(S,G,rpt) to RPF'(S,G,rpt), and to
            cancel the Override Timer.  If the router was previously in
            RPTNotJoined(G) state, then there is no need to trigger an
            action in this state machine because sending a
            Prune(S,G,rpt) is handled by the rules for sending the
            Join(*,G).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      PruneDesired(S,G,rpt)->FALSE
            See macro above.  This transition is only relevant in the
            "Pruned" state.

            If the router is in the Pruned(S,G,rpt) state, and
            PruneDesired(S,G,rpt) changes to FALSE, this could be
            because the router no longer has RPTJoinDesired(G) true, or
            it now wishes to receive traffic from S again.  If it is the
            former, then this transition should not happen, but instead
            the "RPTJoinDesired(G)->FALSE" transition should happen.
            Thus, this transition should be interpreted as
            "PruneDesired(S,G,rpt)->FALSE AND RPTJoinDesired(G)==TRUE".

            The action is to send a Join(S,G,rpt) to RPF'(S,G,rpt).

      RPTJoinDesired(G)->FALSE
            This event is relevant in the "Pruned" and "NotPruned"
            states.

            The router no longer wishes to receive any traffic destined
            for G on the RP Tree.  This causes a transition to the
            RPTNotJoined(G) state, and the Override Timer is canceled if
            it was running.  Any further actions are handled by the
            appropriate upstream state machine for (*,G).

      inherited_olist(S,G,rpt) becomes non-NULL
            This transition is only relevant in the RPTNotJoined(G)
            state.

            The router has joined the RP tree (handled by the (*,G)
            upstream state machine as appropriate) and wants to receive
            traffic from S.  This does not trigger any events in this
            state machine, but causes a transition to the
            NotPruned(S,G,rpt) state.

4.6.  PIM Assert Messages

   Where multiple PIM routers peer over a shared LAN, it is possible for
   more than one upstream router to have valid forwarding state for a
   packet, which can lead to packet duplication (see Section 3.6).  PIM
   does not attempt to prevent this from occurring.  Instead, it detects
   when this has happened and elects a single forwarder amongst the
   upstream routers to prevent further duplication.  This election is
   performed using PIM Assert messages.  Assert messages are also
   received by downstream routers on the LAN, and these cause subsequent
   Join/Prune messages to be sent to the upstream router that won the
   Assert.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   In general, a PIM Assert message should only be accepted for
   processing if it comes from a known PIM neighbor.  A PIM router hears
   about PIM neighbors through PIM Hello messages.  If a router receives
   an Assert message from a particular IP source address and it has not
   seen a PIM Hello message from that source address, then the Assert
   message SHOULD be discarded without further processing.  In addition,
   if the Hello message from a neighbor was authenticated (see
   Section 6.3), then all Assert messages from that neighbor MUST also
   be authenticated.

   We note that some older PIM implementations incorrectly fail to send
   Hello messages on point-to-point interfaces, so we also RECOMMEND
   that a configuration option be provided to allow interoperation with
   such older routers, but that this configuration option SHOULD NOT be
   enabled by default.

4.6.1.  (S,G) Assert Message State Machine

   The (S,G) Assert state machine for interface I is shown in Figure 8.
   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
            out of interface I.  Irrespective of whether it is the DR
            for I, while a router is the assert winner, it is also
            responsible for forwarding traffic onto I on behalf of local
            hosts on I that have made membership requests that
            specifically refer to S (and G).

      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.  If it is the DR on I, it is no longer
            responsible for forwarding traffic onto I to satisfy local
            hosts with membership requests that specifically refer to S
            and G.

   In addition, there is also an Assert Timer (AT) that is used to
   time out asserts on the assert losers and to resend asserts on the
   assert winner.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

           Figure 8: Per-Interface (S,G) Assert State Machine

+----------------------------------------------------------------------+
|                         In NoInfo (NI) State                         |
+---------------+-------------------+------------------+---------------+
| Receive       |  Receive Assert   |  Data arrives    |  Receive      |
| Inferior      |  with RPTbit      |  from S to G on  |  Acceptable   |
| Assert with   |  set and          |  I and           |  Assert with  |
| RPTbit clear  |  CouldAssert      |  CouldAssert     |  RPTbit clear |
|               |  (S,G,I)          |  (S,G,I)         |  and AssTrDes |
|               |                   |                  |  (S,G,I)      |
+---------------+-------------------+------------------+---------------+
| -> W state    |  -> W state       |  -> W state      |  -> L state   |
| [Actions A1]  |  [Actions A1]     |  [Actions A1]    |  [Actions A6] |
+---------------+-------------------+------------------+---------------+

+----------------------------------------------------------------------+
|                   In I Am Assert Winner (W) State                    |
+----------------+------------------+-----------------+----------------+
| Assert Timer   |   Receive        |  Receive        |  CouldAssert   |
| Expires        |   Inferior       |  Preferred      |  (S,G,I) ->    |
|                |   Assert         |  Assert         |  FALSE         |
+----------------+------------------+-----------------+----------------+
| -> W state     |   -> W state     |  -> L state     |  -> NI state   |
| [Actions A3]   |   [Actions A3]   |  [Actions A2]   |  [Actions A4]  |
+----------------+------------------+-----------------+----------------+

+---------------------------------------------------------------------+
|                   In I Am Assert Loser (L) State                    |
+-------------+-------------+-------------+-------------+-------------+
|Receive      |Receive      |Receive      |Assert Timer |Current      |
|Preferred    |Acceptable   |Inferior     |Expires      |Winner's     |
|Assert       |Assert with  |Assert or    |             |GenID        |
|             |RPTbit clear |Assert       |             |Changes or   |
|             |from Current |Cancel from  |             |NLT Expires  |
|             |Winner       |Current      |             |             |
|             |             |Winner       |             |             |
+-------------+-------------+-------------+-------------+-------------+
|-> L state   |-> L state   |-> NI state  |-> NI state  |-> NI state  |
|[Actions A2] |[Actions A2] |[Actions A5] |[Actions A5] |[Actions A5] |
+-------------+-------------+-------------+-------------+-------------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

+----------------------------------------------------------------------+
|                    In I Am Assert Loser (L) State                    |
+----------------+-----------------+------------------+----------------+
| AssTrDes       |  my_metric ->   |  RPF_interface   |  Receive       |
| (S,G,I) ->     |  better than    |  (S) stops       |  Join(S,G) on  |
| FALSE          |  winner's       |  being I         |  interface I   |
|                |  metric         |                  |                |
+----------------+-----------------+------------------+----------------+
| -> NI state    |  -> NI state    |  -> NI state     |  -> NI State   |
| [Actions A5]   |  [Actions A5]   |  [Actions A5]    |  [Actions A5]  |
+----------------+-----------------+------------------+----------------+

   Note that for reasons of compactness, "AssTrDes(S,G,I)" is used in
   the state machine table to refer to AssertTrackingDesired(S,G,I).

   Terminology:

      A "preferred assert" is one with a better metric than the current
      winner.

      An "acceptable assert" is one that has a better metric than
      my_assert_metric(S,G,I).  An assert is never considered acceptable
      if its metric is infinite.

      An "inferior assert" is one with a worse metric than
      my_assert_metric(S,G,I).  An assert is never considered inferior
      if my_assert_metric(S,G,I) is infinite.

   The state machine uses the following macros:

   CouldAssert(S,G,I) =
        SPTbit(S,G)==TRUE
        AND (RPF_interface(S) != I)
        AND (I in ( ( joins(*,G) (-) prunes(S,G,rpt) )
                    (+) ( pim_include(*,G) (-) pim_exclude(S,G) )
                    (-) lost_assert(*,G)
                    (+) joins(S,G) (+) pim_include(S,G) ) )

   CouldAssert(S,G,I) is true for downstream interfaces that would be in
   the inherited_olist(S,G) if (S,G) assert information was not taken
   into account.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   AssertTrackingDesired(S,G,I) =
        (I in ( joins(*,G) (-) prunes(S,G,rpt)
                (+) ( pim_include(*,G) (-) pim_exclude(S,G) )
                (-) lost_assert(*,G)
                (+) joins(S,G) ) )
        OR (local_receiver_include(S,G,I) == TRUE
            AND (I_am_DR(I) OR (AssertWinner(S,G,I) == me)))
        OR ((RPF_interface(S) == I) AND (JoinDesired(S,G) == TRUE))
        OR ((RPF_interface(RP(G)) == I) AND (JoinDesired(*,G) == TRUE)
            AND (SPTbit(S,G) == FALSE))

   AssertTrackingDesired(S,G,I) is true on any interface in which an
   (S,G) assert might affect the router's behavior on that interface.

   The first three lines of AssertTrackingDesired account for (*,G) join
   and local membership information received on I that might cause the
   router to be interested in asserts on I.

   The 4th line accounts for (S,G) join information received on I that
   might cause the router to be interested in asserts on I.

   The 5th and 6th lines account for (S,G) local membership information
   on I.  Note that we can't use the pim_include(S,G) macro, since it
   uses lost_assert(S,G,I) and would result in the router forgetting
   that it lost an assert if the only reason it was interested was local
   membership.  The AssertWinner(S,G,I) check forces an assert winner to
   keep on being responsible for forwarding as long as local receivers
   are present.  Removing this check would make the assert winner
   give up forwarding as soon as the information that originally caused
   it to forward went away, and the task of forwarding for local
   receivers would revert back to the DR.

   The last three lines account for the fact that a router must keep
   track of assert information on upstream interfaces in order to send
   joins and prunes to the proper neighbor.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from NoInfo State

   When in NoInfo state, the following events may trigger transitions:

      Receive Inferior Assert with RPTbit cleared
            An assert is received for (S,G) with the RPT bit cleared
            that is inferior to our own assert metric.  The RPT bit
            cleared indicates that the sender of the assert had (S,G)
            forwarding state on this interface.  If the assert is
            inferior to our metric, then we must also have (S,G)
            forwarding state (i.e., CouldAssert(S,G,I)==TRUE) as (S,G)
            asserts have priority over (*,G) asserts, and so we should
            be the assert winner.  We transition to the "I am Assert
            Winner" state and perform Actions A1 (below).

      Receive Assert with RPTbit set AND CouldAssert(S,G,I)==TRUE
            An assert is received for (S,G) on I with the RPT bit set
            (it is a (*,G) assert).  CouldAssert(S,G,I) is TRUE only if
            we have (S,G) forwarding state on this interface, so we
            should be the assert winner.  We transition to the "I am
            Assert Winner" state and perform Actions A1 (below).

      An (S,G) data packet arrives on interface I, AND
         CouldAssert(S,G,I)==TRUE
            An (S,G) data packet arrived on a downstream interface that
            is in our (S,G) outgoing interface list.  We optimistically
            assume that we will be the assert winner for this (S,G), and
            so we transition to the "I am Assert Winner" state and
            perform Actions A1 (below), which will initiate the assert
            negotiation for (S,G).

      Receive Acceptable Assert with RPT bit clear AND
         AssertTrackingDesired(S,G,I)==TRUE
            We're interested in (S,G) Asserts, either because I is a
            downstream interface for which we have (S,G) or (*,G)
            forwarding state, or because I is the upstream interface for
            S and we have (S,G) forwarding state.  The received assert
            has a better metric than our own, so we do not win the
            Assert.  We transition to "I am Assert Loser" and perform
            Actions A6 (below).

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from "I am Assert Winner" State

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

      Assert Timer Expires
            The (S,G) Assert Timer expires.  As we're in the Winner
            state, we must still have (S,G) forwarding state that is
            actively being kept alive.  We resend the (S,G) Assert and
            restart the Assert Timer (Actions A3 below).  Note that the
            assert winner's Assert Timer is engineered to expire shortly
            before timers on assert losers; this prevents unnecessary
            thrashing of the forwarder and periodic flooding of
            duplicate packets.

      Receive Inferior Assert
            We receive an (S,G) assert or (*,G) assert mentioning S that
            has a worse metric than our own.  Whoever sent the assert is
            in error, and so we resend an (S,G) Assert and restart the
            Assert Timer (Actions A3 below).

      Receive Preferred Assert
            We receive an (S,G) assert that has a better metric than our
            own.  We transition to "I am Assert Loser" state and perform
            Actions A2 (below).  Note that this may affect the value of
            JoinDesired(S,G) and PruneDesired(S,G,rpt), which could
            cause transitions in the upstream (S,G) or (S,G,rpt) state
            machines.

      CouldAssert(S,G,I) -> FALSE
            Our (S,G) forwarding state or RPF interface changed so as to
            make CouldAssert(S,G,I) become false.  We can no longer
            perform the actions of the assert winner, and so we
            transition to NoInfo state and perform Actions A4 (below).
            This includes sending a "canceling assert" with an infinite
            metric.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Transitions from "I am Assert Loser" State

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

      Receive Preferred Assert
            We receive an assert that is better than that of the current
            assert winner.  We stay in Loser state and perform
            Actions A2 below.

      Receive Acceptable Assert with RPTbit clear from Current Winner
            We receive an assert from the current assert winner that is
            better than our own metric for this (S,G) (although the
            metric may be worse than the winner's previous metric).  We
            stay in Loser state and perform Actions A2 below.

      Receive Inferior Assert or Assert Cancel from Current Winner
            We receive an assert from the current assert winner that is
            worse than our own metric for this group (typically, because
            the winner's metric became worse or because it is an assert
            cancel).  We transition to NoInfo state, deleting the (S,G)
            assert information and allowing the normal PIM Join/Prune
            mechanisms to operate.  Usually, we will eventually
            re-assert and win when data packets from S have started
            flowing again.

      Assert Timer Expires
            The (S,G) Assert Timer expires.  We transition to NoInfo
            state, deleting the (S,G) assert information (Actions A5
            below).

      Current Winner's GenID Changes or NLT Expires
            The Neighbor Liveness Timer associated with the current
            winner expires or we receive a Hello message from the
            current winner reporting a different GenID from the one it
            previously reported.  This indicates that the current
            winner's interface or router has gone down (and may have
            come back up), and so we must assume that it no longer knows
            it was the winner.  We transition to the NoInfo state,
            deleting this (S,G) assert information (Actions A5 below).

      AssertTrackingDesired(S,G,I)->FALSE
            AssertTrackingDesired(S,G,I) becomes FALSE.  Our forwarding
            state has changed so that (S,G) Asserts on interface I are
            no longer of interest to us.  We transition to the NoInfo
            state, deleting the (S,G) assert information.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

      My metric becomes better than the assert winner's metric
            my_assert_metric(S,G,I) has changed so that now my assert
            metric for (S,G) is better than the metric we have stored
            for the current assert winner.  This might happen when the
            underlying routing metric changes, or when
            CouldAssert(S,G,I) becomes true, for example, when
            SPTbit(S,G) becomes true.  We transition to NoInfo state,
            delete this (S,G) assert state (Actions A5 below), and allow
            the normal PIM Join/Prune mechanisms to operate.  Usually,
            we will eventually re-assert and win when data packets from
            S have started flowing again.

      RPF_interface(S) stops being interface I
            Interface I used to be the RPF interface for S, and now it
            is not.  We transition to NoInfo state, deleting this (S,G)
            assert state (Actions A5 below).

      Receive Join(S,G) on Interface I
            We receive a Join(S,G) that has the Upstream Neighbor
            Address field set to my primary IP address on interface I.
            The action is to transition to NoInfo state, delete this
            (S,G) assert state (Actions A5 below), and allow the normal
            PIM Join/Prune mechanisms to operate.  If whoever sent the
            Join was in error, then the normal assert mechanism will
            eventually re-apply, and we will lose the assert again.
            However, whoever sent the assert may know that the previous
            assert winner has died, and so we may end up being the new
            forwarder.

   (S,G) Assert State Machine Actions

      A1: Send Assert(S,G).
          Set Assert Timer to (Assert_Time - Assert_Override_Interval).
          Store self as AssertWinner(S,G,I).
          Store spt_assert_metric(S,I) as AssertWinnerMetric(S,G,I).

      A2: Store new assert winner as AssertWinner(S,G,I) and assert
          winner metric as AssertWinnerMetric(S,G,I).
          Set Assert Timer to Assert_Time.

      A3: Send Assert(S,G).
          Set Assert Timer to (Assert_Time - Assert_Override_Interval).

      A4: Send AssertCancel(S,G).
          Delete assert information (AssertWinner(S,G,I) and
          AssertWinnerMetric(S,G,I) will then return to their default
          values).

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      A5: Delete assert information (AssertWinner(S,G,I) and
          AssertWinnerMetric(S,G,I) will then return to their default
          values).

      A6: Store new assert winner as AssertWinner(S,G,I) and assert
          winner metric as AssertWinnerMetric(S,G,I).
          Set Assert Timer to Assert_Time.
          If (I is RPF_interface(S)) AND (UpstreamJPState(S,G) ==
          Joined) set SPTbit(S,G) to TRUE.

   Note that some of these actions may cause the value of
   JoinDesired(S,G), PruneDesired(S,G,rpt), or RPF'(S,G) to change,
   which could cause further transitions in other state machines.

4.6.2.  (*,G) Assert Message State Machine

   The (*,G) Assert state machine for interface I is shown in Figure 9.
   There are three states:

      NoInfo (NI)
            This router has no (*,G) assert state on interface I.

      I am Assert Winner (W)
            This router has won a (*,G) assert on interface I.  It is
            now responsible for forwarding traffic destined for G onto
            interface I with the exception of traffic for which it has
            (S,G) "I am Assert Loser" state.  Irrespective of whether it
            is the DR for I, it is also responsible for handling the
            membership requests for G from local hosts on I.

      I am Assert Loser (L)
            This router has lost a (*,G) assert on interface I.  It must
            not forward packets for G onto interface I with the
            exception of traffic from sources for which it has (S,G) "I
            am Assert Winner" state.  If it is the DR, it is no longer
            responsible for handling the membership requests for group G
            from local hosts on I.

   In addition, there is also an Assert Timer (AT) that is used to time
   out asserts on the assert losers and to resend asserts on the assert
   winner.

   When an Assert message is received with a source address other than
   zero, a PIM implementation must first match it against the possible
   events in the (S,G) assert state machine and process any transitions
   and actions, before considering whether the Assert message matches
   against the (*,G) assert state machine.

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   It is important to note that NO TRANSITION CAN OCCUR in the (*,G)
   state machine as a result of receiving an Assert message unless the
   (S,G) assert state machine for the relevant S and G is in the
   "NoInfo" state after the (S,G) state machine has processed the
   message.  Also, NO TRANSITION CAN OCCUR in the (*,G) state machine as
   a result of receiving an assert message if that message triggers any
   change of state in the (S,G) state machine.  Obviously, when the
   source address in the received message is set to zero, an (S,G) state
   machine for the S and G does not exist and can be assumed to be in
   the "NoInfo" state.

   For example, if both the (S,G) and (*,G) assert state machines are in
   the NoInfo state when an Assert message arrives, and the message
   causes the (S,G) state machine to transition to either "W" or "L"
   state, then the assert will not be processed by the (*,G) assert
   state machine.

   Another example: if the (S,G) assert state machine is in "L" state
   when an assert message is received, and the assert metric in the
   message is worse than my_assert_metric(S,G,I), then the (S,G) assert
   state machine will transition to NoInfo state.  In such a case, if
   the (*,G) assert state machine were in NoInfo state, it might appear
   that it would transition to "W" state, but this is not the case
   because this message already triggered a transition in the (S,G)
   assert state machine.

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           Figure 9: Per-Interface (*,G) Assert State Machine

+----------------------------------------------------------------------+
|                         In NoInfo (NI) State                         |
+-----------------------+-----------------------+----------------------+
| Receive Inferior      |  Data arrives for G   |  Receive Acceptable  |
| Assert with RPTbit    |  on I and             |  Assert with RPTbit  |
| set and               |  CouldAssert          |  set and AssTrDes    |
| CouldAssert(*,G,I)    |  (*,G,I)              |  (*,G,I)             |
+-----------------------+-----------------------+----------------------+
| -> W state            |  -> W state           |  -> L state          |
| [Actions A1]          |  [Actions A1]         |  [Actions A2]        |
+-----------------------+-----------------------+----------------------+

+---------------------------------------------------------------------+
|                    In I Am Assert Winner (W) State                  |
+----------------+-----------------+-----------------+----------------+
| Assert Timer   |  Receive        |  Receive        |  CouldAssert   |
| Expires        |  Inferior       |  Preferred      |  (*,G,I) ->    |
|                |  Assert         |  Assert         |  FALSE         |
+----------------+-----------------+-----------------+----------------+
| -> W state     |  -> W state     |  -> L state     |  -> NI state   |
| [Actions A3]   |  [Actions A3]   |  [Actions A2]   |  [Actions A4]  |
+----------------+-----------------+-----------------+----------------+

+---------------------------------------------------------------------+
|                    In I Am Assert Loser (L) State                   |
+-------------+-------------+-------------+-------------+-------------+
|Receive      |Receive      |Receive      |Assert Timer |Current      |
|Preferred    |Acceptable   |Inferior     |Expires      |Winner's     |
|Assert with  |Assert from  |Assert or    |             |GenID        |
|RPTbit set   |Current      |Assert       |             |Changes or   |
|             |Winner with  |Cancel from  |             |NLT Expires  |
|             |RPTbit set   |Current      |             |             |
|             |             |Winner       |             |             |
+-------------+-------------+-------------+-------------+-------------+
|-> L state   |-> L state   |-> NI state  |-> NI state  |-> NI state  |
|[Actions A2] |[Actions A2] |[Actions A5] |[Actions A5] |[Actions A5] |
+-------------+-------------+-------------+-------------+-------------+

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+----------------------------------------------------------------------+
|                    In I Am Assert Loser (L) State                    |
+----------------+----------------+-----------------+------------------+
| AssTrDes       | my_metric ->   |  RPF_interface  |  Receive         |
| (*,G,I) ->     | better than    |  (RP(G)) stops  |  Join(*,G) on    |
| FALSE          | Winner's       |  being I        |  Interface I     |
|                | metric         |                 |                  |
+----------------+----------------+-----------------+------------------+
| -> NI state    | -> NI state    |  -> NI state    |  -> NI State     |
| [Actions A5]   | [Actions A5]   |  [Actions A5]   |  [Actions A5]    |
+----------------+----------------+-----------------+------------------+

   The state machine uses the following macros:

      CouldAssert(*,G,I) =
          ( I in ( joins(*,G) (+) pim_include(*,G)) )
          AND (RPF_interface(RP(G)) != I)

   CouldAssert(*,G,I) is true on downstream interfaces for which we have
   (*,G) join state, or local members that requested any traffic
   destined for G.

      AssertTrackingDesired(*,G,I) =
          CouldAssert(*,G,I)
          OR (local_receiver_include(*,G,I)==TRUE
              AND (I_am_DR(I) OR AssertWinner(*,G,I) == me))
          OR (RPF_interface(RP(G)) == I AND RPTJoinDesired(G))

   AssertTrackingDesired(*,G,I) is true on any interface on which a
   (*,G) assert might affect the router's behavior on that interface.

   Note that for reasons of compactness, "AssTrDes(*,G,I)" is used in
   the state machine table to refer to AssertTrackingDesired(*,G,I).

   Terminology:

      A "preferred assert" is one with a better metric than the current
      winner.

      An "acceptable assert" is one that has a better metric than
      my_assert_metric(*,G,I).  An assert is never considered acceptable
      if its metric is infinite.

      An "inferior assert" is one with a worse metric than
      my_assert_metric(*,G,I).  An assert is never considered inferior
      if my_assert_metric(*,G,I) is infinite.

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   Transitions from NoInfo State

   When in NoInfo state, the following events trigger transitions, but
   only if the (S,G) assert state machine is in NoInfo state before and
   after consideration of the received message:

      Receive Inferior Assert with RPTbit set AND
         CouldAssert(*,G,I)==TRUE
            An Inferior (*,G) assert is received for G on Interface I.
            If CouldAssert(*,G,I) is TRUE, then I is our downstream
            interface, and we have (*,G) forwarding state on this
            interface, so we should be the assert winner.  We transition
            to the "I am Assert Winner" state and perform Actions A1
            (below).

      A data packet destined for G arrives on interface I, AND
         CouldAssert(*,G,I)==TRUE
            A data packet destined for G arrived on a downstream
            interface that is in our (*,G) outgoing interface list.  We
            therefore believe we should be the forwarder for this (*,G),
            and so we transition to the "I am Assert Winner" state and
            perform Actions A1 (below).

      Receive Acceptable Assert with RPT bit set AND
         AssertTrackingDesired(*,G,I)==TRUE
            We're interested in (*,G) Asserts, either because I is a
            downstream interface for which we have (*,G) forwarding
            state, or because I is the upstream interface for RP(G) and
            we have (*,G) forwarding state.  We get a (*,G) Assert that
            has a better metric than our own, so we do not win the
            Assert.  We transition to "I am Assert Loser" and perform
            Actions A2 (below).

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   Transitions from "I am Assert Winner" State

   When in "I am Assert Winner" state, the following events trigger
   transitions, but only if the (S,G) assert state machine is in NoInfo
   state before and after consideration of the received message:

      Receive Inferior Assert
            We receive a (*,G) assert that has a worse metric than our
            own.  Whoever sent the assert has lost, and so we resend a
            (*,G) Assert and restart the Assert Timer (Actions A3
            below).

      Receive Preferred Assert
            We receive a (*,G) assert that has a better metric than our
            own.  We transition to "I am Assert Loser" state and perform
            Actions A2 (below).

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

      Assert Timer Expires
            The (*,G) Assert Timer expires.  As we're in the Winner
            state, then we must still have (*,G) forwarding state that
            is actively being kept alive.  To prevent unnecessary
            thrashing of the forwarder and periodic flooding of
            duplicate packets, we resend the (*,G) Assert and restart
            the Assert Timer (Actions A3 below).

      CouldAssert(*,G,I) -> FALSE
            Our (*,G) forwarding state or RPF interface changed so as to
            make CouldAssert(*,G,I) become false.  We can no longer
            perform the actions of the assert winner, and so we
            transition to NoInfo state and perform Actions A4 (below).

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   Transitions from "I am Assert Loser" State

   When in "I am Assert Loser" state, the following events trigger
   transitions, but only if the (S,G) assert state machine is in NoInfo
   state before and after consideration of the received message:

      Receive Preferred Assert with RPTbit set
            We receive a (*,G) assert that is better than that of the
            current assert winner.  We stay in Loser state and perform
            Actions A2 below.

      Receive Acceptable Assert from Current Winner with RPTbit set
            We receive a (*,G) assert from the current assert winner
            that is better than our own metric for this group (although
            the metric may be worse than the winner's previous metric).
            We stay in Loser state and perform Actions A2 below.

      Receive Inferior Assert or Assert Cancel from Current Winner
            We receive an assert from the current assert winner that is
            worse than our own metric for this group (typically because
            the winner's metric became worse or is now an assert
            cancel).  We transition to NoInfo state, delete this (*,G)
            assert state (Actions A5), and allow the normal PIM
            Join/Prune mechanisms to operate.  Usually, we will
            eventually re-assert and win when data packets for G have
            started flowing again.

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

      Assert Timer Expires
            The (*,G) Assert Timer expires.  We transition to NoInfo
            state and delete this (*,G) assert information (Actions A5).

      Current Winner's GenID Changes or NLT Expires
            The Neighbor Liveness Timer associated with the current
            winner expires or we receive a Hello message from the
            current winner reporting a different GenID from the one it
            previously reported.  This indicates that the current
            winner's interface or router has gone down (and may have
            come back up), and so we must assume that it no longer knows
            it was the winner.  We transition to the NoInfo state,
            deleting the (*,G) assert information (Actions A5).

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      AssertTrackingDesired(*,G,I)->FALSE
            AssertTrackingDesired(*,G,I) becomes FALSE.  Our forwarding
            state has changed so that (*,G) Asserts on interface I are
            no longer of interest to us.  We transition to NoInfo state
            and delete this (*,G) assert information (Actions A5).

      My metric becomes better than the assert winner's metric
            My routing metric, rpt_assert_metric(G,I), has changed so
            that now my assert metric for (*,G) is better than the
            metric we have stored for the current assert winner.  We
            transition to NoInfo state, delete this (*,G) assert state
            (Actions A5), and allow the normal PIM Join/Prune mechanisms
            to operate.  Usually, we will eventually re-assert and win
            when data packets for G have started flowing again.

      RPF_interface(RP(G)) stops being interface I
            Interface I used to be the RPF interface for RP(G), and now
            it is not.  We transition to NoInfo state and delete this
            (*,G) assert state (Actions A5).

      Receive Join(*,G) on interface I
            We receive a Join(*,G) that has the Upstream Neighbor
            Address field set to my primary IP address on interface I.
            The action is to transition to NoInfo state, delete this
            (*,G) assert state (Actions A5), and allow the normal PIM
            Join/Prune mechanisms to operate.  If whoever sent the Join
            was in error, then the normal assert mechanism will
            eventually re-apply, and we will lose the assert again.
            However, whoever sent the assert may know that the previous
            assert winner has died, so we may end up being the new
            forwarder.

   (*,G) Assert State Machine Actions

      A1: Send Assert(*,G).
          Set Assert Timer to (Assert_Time - Assert_Override_Interval).
          Store self as AssertWinner(*,G,I).
          Store rpt_assert_metric(G,I) as AssertWinnerMetric(*,G,I).

      A2: Store new assert winner as AssertWinner(*,G,I) and assert
          winner metric as AssertWinnerMetric(*,G,I).
          Set Assert Timer to Assert_Time.

      A3: Send Assert(*,G).
          Set Assert Timer to (Assert_Time - Assert_Override_Interval).

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      A4: Send AssertCancel(*,G).
          Delete assert information (AssertWinner(*,G,I) and
          AssertWinnerMetric(*,G,I) will then return to their default
          values).

      A5: Delete assert information (AssertWinner(*,G,I) and
          AssertWinnerMetric(*,G,I) will then return to their default
          values).

   Note that some of these actions may cause the value of
   JoinDesired(*,G) or RPF'(*,G) to change, which could cause further
   transitions in other state machines.

4.6.3.  Assert Metrics

   Assert metrics are defined as:

     struct assert_metric {
       rpt_bit_flag;
       metric_preference;
       route_metric;
       ip_address;
     };

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

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

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

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   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_ip_address(I)}
     }

   rpt_assert_metric(G,I) gives the assert metric we use if we're
   sending an assert based only on (*,G) forwarding state:

     assert_metric
     rpt_assert_metric(G,I) {
         return {1,MRIB.pref(RP(G)),MRIB.metric(RP(G)),my_ip_address(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_ip_address(I) is simply
   the router's primary IP address that is associated with the local
   interface I.

   infinite_assert_metric() is an assert metric that the router uses for
   an Assert that does not match either (S,G) or (*,G) forwarding state:

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

4.6.4.  AssertCancel Messages

   An AssertCancel message is simply an RPT Assert message but with an
   infinite metric.  It is sent by the assert winner when it deletes the
   forwarding state that had caused the assert to occur.  Other routers
   will see this metric, and it will cause any other router that has
   forwarding state to send its own assert, and to take over forwarding.

   An AssertCancel(S,G) is an infinite metric assert with the RPT bit
   set that names S as the source.

   An AssertCancel(*,G) is an infinite metric assert with the RPT bit
   set and the source set to zero.

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

4.6.5.  Assert State Macros

   The macros lost_assert(S,G,rpt,I), lost_assert(S,G,I), and
   lost_assert(*,G,I) are used in the olist computations of Section 4.1
   and are defined as:

     bool lost_assert(S,G,rpt,I) {
       if ( RPF_interface(RP(G)) == I  OR
            ( RPF_interface(S) == I AND SPTbit(S,G) == TRUE ) ) {
          return FALSE
       } else {
          return ( AssertWinner(S,G,I) != NULL AND
                   AssertWinner(S,G,I) != me )
       }
     }

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

   Note: The term "AssertWinnerMetric(S,G,I) is better than
   spt_assert_metric(S,I)" is required to correctly handle the
   transition phase when a router has (S,G) join state but has not yet
   set the SPTbit.  In this case, it needs to ignore the assert state if
   it will win the assert once the SPTbit is set.

     bool lost_assert(*,G,I) {
       if ( RPF_interface(RP(G)) == I ) {
          return FALSE
       } else {
          return ( AssertWinner(*,G,I) != NULL AND
                   AssertWinner(*,G,I) != me )
       }
     }

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   AssertWinner(S,G,I) is the IP source address of the Assert(S,G)
   packet that won an Assert.

   AssertWinner(*,G,I) is the IP source address of the Assert(*,G)
   packet that won an Assert.

   AssertWinnerMetric(S,G,I) is the Assert metric of the Assert(S,G)
   packet that won an Assert.

   AssertWinnerMetric(*,G,I) is the Assert metric of the Assert(*,G)
   packet that won an Assert.

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

   Summary of Assert Rules and Rationale

   This section summarizes the key rules for sending and reacting to
   asserts and the rationale for these rules.  This section is not
   intended to be and should not be treated as a definitive
   specification of protocol behavior.  The state machines and
   pseudocode should be consulted for that purpose.  Rather, this
   section is intended to document important aspects of the Assert
   protocol behavior and to provide information that may prove helpful
   to the reader in understanding and implementing this part of the
   protocol.

   1.  Behavior: Downstream neighbors send Join(*,G) and Join(S,G)
       periodic messages to the appropriate RPF' neighbor, i.e., the RPF
       neighbor as modified by the assert process.  They are not always
       sent to the RPF neighbor as indicated by the MRIB.  Normal
       suppression and override rules apply.

       Rationale: By sending the periodic and triggered Join messages to
       the RPF' neighbor instead of the RPF neighbor, the downstream
       router avoids re-triggering the Assert process with every Join.
       A side effect of sending Joins to the Assert winner is that
       traffic will not switch back to the "normal" RPF neighbor until
       the Assert times out.  This will not happen until data stops
       flowing, if item 8, below, is implemented.

   2.  Behavior: The assert winner for (*,G) acts as the local DR for
       (*,G) on behalf of IGMP/MLD members.

       Rationale: This is required to allow a single router to merge
       PIM and IGMP/MLD joins and leaves.  Without this, overrides
       don't work.

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   3.  Behavior: The assert winner for (S,G) acts as the local DR for
       (S,G) on behalf of IGMPv3 members.

       Rationale: Same rationale as for item 2.

   4.  Behavior: (S,G) and (*,G) prune overrides are sent to the RPF'
       neighbor and not to the regular RPF neighbor.

       Rationale: Same rationale as for item 1.

   5.  Behavior: An (S,G,rpt) prune override is not sent (at all) if
       RPF'(S,G,rpt) != RPF'(*,G).

       Rationale: This avoids keeping state alive on the (S,G) tree when
       only (*,G) downstream members are left.  Also, it avoids sending
       (S,G,rpt) joins to a router that is not on the (*,G) tree.  This
       behavior might be confusing, although this specification does
       indicate that such a join SHOULD be dropped.

   6.  Behavior: An assert loser that receives a Join(S,G) with an
       Upstream Neighbor Address that is its primary IP address on that
       interface expires the (S,G) Assert Timer.

       Rationale: This is necessary in order to have rapid convergence
       in the event that the downstream router that initially sent a
       join to the prior Assert winner has undergone a topology change.

   7.  Behavior: An assert loser that receives a Join(*,G) with an
       Upstream Neighbor Address that is its primary IP address on that
       interface expires the (*,G) Assert Timer and all (S,G) assert
       timers that do not have corresponding Prune(S,G,rpt) messages in
       the compound Join/Prune message.

       Rationale: Same rationale as for item 6.

   8.  Behavior: An assert winner for (*,G) or (S,G) sends a canceling
       assert when it is about to stop forwarding on a (*,G) or an (S,G)
       entry.  This behavior does not apply to (S,G,rpt).

       Rationale: This allows switching back to the shared tree after
       the last SPT router on the LAN leaves.  Doing this prevents
       downstream routers on the shared tree from keeping SPT state
       alive.

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   9.  Behavior: Resend the assert messages before timing out an assert.
       (This behavior is optional.)

       Rationale: This prevents the periodic duplicates that would
       otherwise occur each time that an assert times out and is then
       re-established.

   10. Behavior: When RPF'(S,G,rpt) changes to be the same as RPF'(*,G),
       we need to trigger a Join(S,G,rpt) to RPF'(*,G).

       Rationale: This allows switching back to the RPT after the last
       SPT member leaves.

4.7.  PIM Bootstrap and RP Discovery

   For correct operation, every PIM router within a PIM domain must be
   able to map a particular multicast group address to the same RP.  If
   this is not the case, then black holes may appear, where some
   receivers in the domain cannot receive some groups.  A domain in this
   context is a contiguous set of routers that all implement PIM and are
   configured to operate within a common boundary.

   A notable exception to this is where a PIM domain is broken up into
   multiple administrative scope regions; these are regions where a
   border has been configured so that a range of multicast groups will
   not be forwarded across that border.  For more information on
   Administratively Scoped IP Multicast, see RFC 2365.  The modified
   criteria for admin-scoped regions are that the region is convex with
   respect to forwarding based on the MRIB, and that all PIM routers
   within the scope region map scoped groups to the same RP within that
   region.

   This specification does not mandate the use of a single mechanism to
   provide routers with the information to perform the group-to-RP
   mapping.  Currently, four mechanisms are possible, and all four have
   associated problems:

   Static Configuration
         A PIM router MUST support the static configuration of group-to-
         RP mappings.  Such a mechanism is not robust to failures but
         does at least provide a basic interoperability mechanism.

   Embedded-RP
         Embedded-RP defines an address allocation policy in which the
         address of the Rendezvous Point (RP) is encoded in an IPv6
         multicast group address [16].

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   Cisco's Auto-RP
         Auto-RP uses a PIM Dense-Mode (PIM-DM) multicast group to
         announce group-to-RP mappings from a central location.  This
         mechanism is not useful if PIM Dense Mode is not being run in
         parallel with PIM Sparse Mode; it was only intended for use
         with PIM Sparse Mode Version 1.  No standard specification
         currently exists.

   Bootstrap Router (BSR)
         RFC 2362 specifies a bootstrap mechanism based on the automatic
         election of a BSR.  Any router in the domain that is configured
         to be a possible RP reports its candidacy to the BSR, and then
         a domain-wide flooding mechanism distributes the BSR's chosen
         set of RPs throughout the domain.  As specified in RFC 2362,
         the BSR mechanism is flawed in its handling of admin-scoped
         regions that are smaller than a PIM domain, but the mechanism
         does work for global-scoped groups.

   As far as PIM-SM is concerned, the only important requirement is that
   all routers in the domain (or admin scope zone for scoped regions)
   receive the same set of group-range-to-RP mappings.  This may be
   achieved through the use of any of these mechanisms, or through
   alternative mechanisms not currently specified.

   It must be operationally ensured that any RP address configured,
   learned, or advertised is reachable from all routers in the PIM
   domain.

4.7.1.  Group-to-RP Mapping

   Using one of the mechanisms described above, a PIM router receives
   one or more possible group-range-to-RP mappings.  Each mapping
   specifies a range of multicast groups (expressed as a group and mask)
   and the RP to which such groups should be mapped.  Each mapping may
   also have an associated priority.  It is possible to receive multiple
   mappings, all of which might match the same multicast group; this is
   the common case with the BSR mechanism.  The algorithm for performing
   the group-to-RP mapping is as follows:

   1.  Perform longest match on group range to obtain a list of RPs.

   2.  From this list of matching RPs, find the ones with highest
       priority.

       Eliminate any RPs from the list that have lower priorities.

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   3.  If only one RP remains in the list, use that RP.

   4.  If multiple RPs are in the list, use the PIM hash function to
       choose one.

   Thus, if two or more group-range-to-RP mappings cover a particular
   group, the one with the longest mask is the mapping to use.  If the
   mappings have the same mask length, then the one with the highest
   priority is chosen.  If there is more than one matching entry with
   the same longest mask and the priorities are identical, then a hash
   function (see Section 4.7.2) is applied to choose the RP.

   This algorithm is invoked by a DR when it needs to determine an RP
   for a given group, e.g., upon reception of a packet or IGMP/MLD
   membership indication for a group for which the DR does not know
   the RP.

   Furthermore, the mapping function is invoked by all routers upon
   receiving a (*,G) Join/Prune message.

   Note that if the set of possible group-range-to-RP mappings changes,
   each router will need to check whether any existing groups are
   affected.  This may, for example, cause a DR or acting DR to re-join
   a group, or cause it to restart register encapsulation to the new RP.

      Implementation note: The bootstrap mechanism described in RFC 2362
      omitted step 1 above.  However, of the implementations we are
      aware of, approximately half performed step 1 anyway.  Note that
      implementations of BSR that omit step 1 will not correctly
      interoperate with implementations of this specification when used
      with the BSR mechanism described in [11].

4.7.2.  Hash Function

   The hash function is used by all routers within a domain, to map a
   group to one of the RPs from the matching set of group-range-to-RP
   mappings (this set of mappings all have the same longest mask length
   and same highest priority).  The algorithm takes as input the group
   address, and the addresses of the candidate RPs from the mappings,
   and gives as output one RP address to be used.

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   The protocol requires that all routers hash to the same RP within a
   domain (except for transients).  The following hash function must be
   used in each router:

   1.  For RP addresses in the matching group-range-to-RP mappings,
       compute a value:

   Value(G,M,C(i))=
   (1103515245 * ((1103515245 * (G&M)+12345) XOR C(i)) + 12345) mod 2^31

       where C(i) is the RP address and M is a hash-mask.  If BSR is
       being used, the hash-mask is given in the Bootstrap messages.  If
       BSR is not being used, the alternative mechanism that supplies
       the group-range-to-RP mappings may supply the value, or else it
       defaults to a mask with the most significant 30 bits being one
       for IPv4 and the most significant 126 bits being one for IPv6.
       The hash-mask allows a small number of consecutive groups
       (e.g., 4) to always hash to the same RP.  For instance,
       hierarchically encoded data can be sent on consecutive group
       addresses to get the same delay and fate-sharing characteristics.

       For address families other than IPv4, a 32-bit digest to be used
       as C(i) and G must first be derived from the actual RP or group
       address.  Such a digest method must be used consistently
       throughout the PIM domain.  For IPv6 addresses, it is RECOMMENDED
       to use the equivalent IPv4 address for an IPv4-compatible
       address, and the exclusive-or of each 32-bit segment of the
       address for all other IPv6 addresses.  For example, the digest of
       the IPv6 address 3ffe:b00:c18:1::10 would be computed as
       0x3ffe0b00 ^ 0x0c180001 ^ 0x00000000 ^ 0x00000010,
       where the '^' symbol represents the exclusive-or operation.

   2.  The candidate RP with the highest resulting hash value is then
       the RP chosen by this hash function.  If more than one RP has the
       same highest hash value, the RP with the highest IP address is
       chosen.

4.8.  Source-Specific Multicast

   The Source-Specific Multicast (SSM) service model [6] can be
   implemented with a strict subset of the PIM-SM protocol mechanisms.
   Both regular IP Multicast and SSM semantics can coexist on a single
   router, and both can be implemented using the PIM-SM protocol.  A
   range of multicast addresses, currently 232.0.0.0/8 in IPv4 and
   ff3x::/32 for IPv6, is reserved for SSM, and the choice of semantics
   is determined by the multicast group address in both data packets and
   PIM messages.

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4.8.1.  Protocol Modifications for SSM Destination Addresses

   The following rules override the normal PIM-SM behavior for a
   multicast address G in the SSM range:

   o  A router MUST NOT send a (*,G) Join/Prune message for any reason.

   o  A router MUST NOT send an (S,G,rpt) Join/Prune message for any
      reason.

   o  A router MUST NOT send a Register message for any packet that is
      destined to an SSM address.

   o  A router MUST NOT forward packets based on (*,G) or (S,G,rpt)
      state.  The (*,G)- and (S,G,rpt)-related state summarization
      macros are NULL for any SSM address, for the purposes of packet
      forwarding.

   o  A router acting as an RP MUST NOT forward any Register-
      encapsulated packet that has an SSM destination address and SHOULD
      respond with a Register-Stop message to such a Register message.

   o  A router MAY optimize out the creation and maintenance of
      (S,G,rpt) and (*,G) state for SSM destination addresses -- this
      state is not needed for SSM packets.

   The last three rules are present to deal with SSM-unaware "legacy"
   routers that may be sending (*,G) and (S,G,rpt) Join/Prunes, or
   Register messages for SSM destination addresses.  Note that this
   specification does not attempt to aid an SSM-unaware "legacy" router
   with SSM operations.

4.8.2.  PIM-SSM-Only Routers

   An implementer may choose to implement only the subset of PIM
   Sparse Mode that provides SSM forwarding semantics.

   A PIM-SSM-only router MUST implement the following portions of this
   specification:

   o  Upstream (S,G) state machine (Section 4.5.5)

   o  Downstream (S,G) state machine (Section 4.5.2)

   o  (S,G) Assert state machine (Section 4.6.1)

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   o  Hello messages, neighbor discovery, and DR election (Section 4.3)

   o  Packet forwarding rules (Section 4.2)

   A PIM-SSM-only router does not need to implement the following
   protocol elements:

   o  Register state machine (Section 4.4)

   o  (*,G) and (S,G,rpt) downstream state machines (Sections 4.5.1 and
      4.5.3)

   o  (*,G) and (S,G,rpt) upstream state machines (Sections 4.5.4,
      4.5.6, and 4.5.7)

   o  (*,G) Assert state machine (Section 4.6.2)

   o  Bootstrap RP election (Section 4.7)

   o  Keepalive Timer

   o  SPTbit (Section 4.2.2)

   The Keepalive Timer should be treated as always running, and the
   SPTbit should be treated as always being set for an SSM address.
   Additionally, the packet forwarding rules of Section 4.2 can be
   simplified in a PIM-SSM-only router:

     oiflist = NULL

     if( iif == RPF_interface(S) AND UpstreamJPState(S,G) == Joined ) {
         oiflist = inherited_olist(S,G)
     } else if( iif is in inherited_olist(S,G) ) {
         send Assert(S,G) on iif
     }

     oiflist = oiflist (-) iif
     forward packet on all interfaces in oiflist

   This is nothing more than the reduction of the normal PIM-SM
   forwarding rule, with all (S,G,rpt) and (*,G) clauses replaced
   with NULL.

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4.9.  PIM Packet Formats

   This section describes the details of the packet formats for PIM
   control messages.

   All PIM control messages have IP protocol number 103.

   PIM messages are either unicast (e.g., Registers and Register-Stop)
   or multicast with TTL 1 to the 'ALL-PIM-ROUTERS' group (e.g.,
   Join/Prune, Asserts).  The source address used for unicast messages
   is a domain-wide reachable address; the source address used for
   multicast messages is the link-local address of the interface on
   which the message is being sent.

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

   The PIM header common to all PIM messages is:

    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.

   Type
         Types for specific PIM messages.  PIM Types are:

   Message Type                          Destination
   ---------------------------------------------------------------------
   0 = Hello                             Multicast to ALL-PIM-ROUTERS
   1 = Register                          Unicast to RP
   2 = Register-Stop                     Unicast to source of Register
                                            packet
   3 = Join/Prune                        Multicast to ALL-PIM-ROUTERS
   4 = Bootstrap                         Multicast to ALL-PIM-ROUTERS
   5 = Assert                            Multicast to ALL-PIM-ROUTERS
   6 = Graft (used in PIM-DM only)       Unicast to RPF'(S)
   7 = Graft-Ack (used in PIM-DM only)   Unicast to source of Graft
                                            packet
   8 = Candidate-RP-Advertisement        Unicast to Domain's BSR

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   Reserved
         Set to zero on transmission.  Ignored upon receipt.

   Checksum
         The checksum is a standard IP checksum, i.e., the 16-bit one's
         complement of the one's complement sum of the entire PIM
         message, excluding the "Multicast data packet" section of the
         Register message.  For computing the checksum, the checksum
         field is zeroed.  If the packet's length is not an integral
         number of 16-bit words, the packet is padded with a trailing
         byte of zero before performing the checksum.

         For IPv6, the checksum also includes the IPv6 "pseudo-header",
         as specified in RFC 2460, Section 8.1 [5].  This
         "pseudo-header" is prepended to the PIM header for the purposes
         of calculating the checksum.  The "Upper-Layer Packet Length"
         in the pseudo-header is set to the length of the PIM message,
         except in Register messages where it is set to the length of
         the PIM register header (8).  The Next Header value used in the
         pseudo-header is 103.

   If a message is received with an unrecognized PIM Ver or Type field,
   or if a message's destination does not correspond to the table above,
   the message MUST be discarded, and an error message SHOULD be logged
   to the administrator in a rate-limited manner.

4.9.1.  Encoded Source and Group Address Formats

   Encoded Unicast Address

   An encoded unicast address takes 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 [7].  Values 128-250 are reserved to be assigned by
         the IANA for PIM-specific Address Families.  Values 251 through
         255 are designated for Private Use.  As there is no assignment
         authority for this space, collisions should be expected.

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

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

   Encoded Group Address

   Encoded group addresses take 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
         Described above.

   Encoding Type
         Described above.

   [B]idirectional PIM
         Indicates that the group range uses Bidirectional PIM [13].
         For PIM-SM as defined in this specification, this bit MUST be
         zero.

   Reserved
         Transmitted as zero.  Ignored upon receipt.

   Admin Scope [Z]one
         Indicates that the group range is an admin scope zone.  This is
         used in the Bootstrap Router mechanism [11] only.  For all
         other purposes, this bit is set to zero and ignored on receipt.

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   Mask Len
         The Mask length field is 8 bits.  The value is the number of
         contiguous one bits that are left-justified and used as a mask;
         when combined with the group address, it describes a range of
         groups.  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 group, then the Mask length must equal the
         address length in bits for the given Address Family and
         Encoding Type (e.g., 32 for IPv4 native encoding, 128 for IPv6
         native encoding).

   Group multicast Address
         Contains the group address.

   Encoded Source Address

   An encoded source address takes 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
         Described above.

   Encoding Type
         Described above.

   Reserved
         Transmitted as zero, ignored on receipt.

   S     The Sparse bit is a 1-bit value, set to 1 for PIM-SM.  It is
         used for PIM Version 1 compatibility.

   W     The WC (or WildCard) bit is a 1-bit value for use with PIM
         Join/Prune messages (see Section 4.9.5.1).

   R     The RPT (or Rendezvous Point Tree) bit is a 1-bit value for use
         with PIM Join/Prune messages (see Section 4.9.5.1).  If the
         WC bit is 1, the RPT bit MUST be 1.

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   Mask Len
         The mask length field is 8 bits.  The value is the number of
         contiguous one bits that are left-justified and used as a mask;
         when combined with the source address, it describes a source
         subnet.  The mask length MUST be equal to the mask length in
         bits for the given Address Family and Encoding Type (32 for
         IPv4 native and 128 for IPv6 native).  A router SHOULD ignore
         any messages received with any other mask length.

   Source Address
         The source address.

4.9.2.  Hello Message Format

   A Hello message is sent periodically by routers on all interfaces.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OptionType           |         OptionLength          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          OptionValue                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          OptionType           |         OptionLength          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          OptionValue                          |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Version, Type, Reserved, Checksum
         Described in Section 4.9.

   OptionType
         The type of the option given in the following OptionValue
         field.

   OptionLength
         The length of the OptionValue field in bytes.

   OptionValue
         A variable-length field, carrying the value of the option.

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   The Option fields may contain the following values:

   o  OptionType 1: Holdtime

       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Holdtime             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Holdtime is the amount of time a receiver must keep the neighbor
      reachable, in seconds.  If the Holdtime 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.

      An implementation MAY provide a configuration mechanism to reject
      a Hello message with holdtime 0xffff, and/or provide a mechanism
      to remove a neighbor.

      Hello messages with a Holdtime value set to '0' are also sent by a
      router on an interface about to go down or changing IP address
      (see Section 4.3.1).  These are effectively goodbye messages, and
      the receiving routers SHOULD immediately time out the neighbor
      information for the sender.

   o  OptionType 2: LAN Prune Delay

       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|      Propagation_Delay      |      Override_Interval        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The LAN Prune Delay option is used to tune the prune propagation
      delay on multi-access LANs.  The T bit specifies the ability of
      the sending router to disable Join suppression.  Propagation_Delay
      and Override_Interval are time intervals in units of milliseconds.
      A router originating a LAN Prune Delay option on interface I sets
      the Propagation_Delay field to the configured value of
      Propagation_Delay(I) and the value of the Override_Interval field
      to the value of Override_Interval(I).  On a receiving router, the
      values of the fields are used to tune the value of the
      Effective_Override_Interval(I) and its derived timer values.

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      Section 4.3.3 describes how these values affect the behavior of a
      router.

   o  OptionTypes 3 through 16: Reserved; to be defined in future
      versions of this document.

   o  OptionType 18: Deprecated and should not be used.

   o  OptionType 19: DR Priority

       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 = 19            |          Length = 4           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         DR Priority                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      DR Priority is a 32-bit unsigned number and should be considered
      in the DR election as described in Section 4.3.2.

   o  OptionType 20: Generation ID

       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 32-bit 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.

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   o  OptionType 24: Address List

       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 = 24            |      Length = <Variable>      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Secondary Address 1 (Encoded-Unicast format)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                     ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Secondary Address N (Encoded-Unicast format)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The contents of the Address List Hello option are described in
      Section 4.3.4.  All addresses within a single Address List must
      belong to the same address family.

   OptionTypes 17 through 65000 are assigned by the IANA.
   OptionTypes 65001 through 65535 are reserved for Private Use,
   as defined in [9].

   Unknown options MUST be ignored and MUST NOT prevent a neighbor
   relationship from being formed.  The Holdtime option MUST be
   implemented; the DR Priority and Generation ID options SHOULD be
   implemented.  The Address List option MUST be implemented for IPv6.

4.9.3.  Register Message Format

   A Register message is sent by the DR to the RP when a multicast
   packet needs to be transmitted on the RP-tree.  The IP source address
   is set to the address of the DR, the destination address to the RP's
   address.  The IP TTL of the PIM packet is the system's normal
   unicast TTL.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |B|N|                       Reserved2                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                     Multicast data packet                     .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   PIM Version, Type, Reserved, Checksum
         Described in Section 4.9.  Note that in order to reduce
         encapsulation overhead, the checksum for Registers is done only
         on the first 8 bytes of the packet, including the PIM header
         and the next 4 bytes, excluding the data packet portion.  For
         interoperability reasons, a message carrying a checksum
         calculated over the entire PIM Register message should also be
         accepted.  When calculating the checksum, the IPv6
         pseudo-header "Upper-Layer Packet Length" is set to 8.

   B     The Border bit.  This specification deprecates the Border bit.
         A router MUST set the B bit to 0 on transmission and MUST
         ignore this bit on reception.

   N     The Null-Register bit.  Set to 1 by a DR that is probing the RP
         before expiring its local Register-Suppression Timer.  Set to 0
         otherwise.

   Reserved2
         Transmitted as zero, ignored on receipt.

   Multicast data packet
         The original packet sent by the source.  This packet must be of
         the same address family as the encapsulating PIM packet, e.g.,
         an IPv6 data packet must be encapsulated in an IPv6 PIM packet.
         Note that the TTL of the original packet is decremented before
         encapsulation, just like any other packet that is forwarded.
         In addition, the RP decrements the TTL after decapsulating,
         before forwarding the packet down the shared tree.

         For (S,G) Null-Registers, the Multicast data packet portion
         contains a dummy IP header with S as the source address
         and G as the destination address.  When generating an IPv4
         Null-Register message, the fields in the dummy IPv4 header
         SHOULD be filled in according to the following table.  Other
         IPv4 header fields may contain any value that is valid for
         that field.

         Field                  Value
         ---------------------------------------
         IP Version             4
         Header Length          5
         Checksum               Header checksum
         Fragmentation offset   0
         More Fragments         0
         Total Length           20
         IP Protocol            103 (PIM)

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         On receipt of an (S,G) Null-Register, if the Header Checksum
         field is non-zero, the recipient SHOULD check the checksum and
         discard Null-Registers that have a bad checksum.  The recipient
         SHOULD NOT check the value of any individual fields; a correct
         IP header checksum is sufficient.  If the Header Checksum field
         is zero, the recipient MUST NOT check the checksum.

         With IPv6, an implementation generates a dummy IP header
         followed by a dummy PIM header with values according to the
         following table in addition to the source and group.  Other
         IPv6 header fields may contain any value that is valid for that
         field.

         Header Field   Value
         --------------------------------------
         IP Version     6
         Next Header    103 (PIM)
         Length         4
         PIM Version    0
         PIM Type       0
         PIM Reserved   0
         PIM Checksum   PIM checksum, including
                        IPv6 "pseudo-header";
                        see Section 4.9

         On receipt of an IPv6 (S,G) Null-Register, if the dummy PIM
         header is present, the recipient SHOULD check the checksum and
         discard Null-Registers that have a bad checksum.

4.9.4.  Register-Stop Message Format

   A Register-Stop is unicast from the RP to the sender of the Register
   message.  The IP source address is the address to which the register
   was addressed.  The IP destination address is the source address of
   the register message.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Group Address (Encoded-Group format)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Source Address (Encoded-Unicast format)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   PIM Version, Type, Reserved, Checksum
         Described in Section 4.9.

   Group Address
         The group address from the multicast data packet in the
         Register.  The format for this address is described in
         Section 4.9.1.  Note that for Register-Stops the Mask Len field
         contains the full address length * 8 (e.g., 32 for IPv4 native
         encoding), if the message is sent for a single group.

   Source Address
         The host address of the source from the multicast data packet
         in the register.  The format for this address is given in the
         encoded unicast address in Section 4.9.1.  A special wildcard
         value consisting of an address field of all zeros can be used
         to indicate any source.

4.9.5.  Join/Prune Message Format

   A Join/Prune message is sent by routers towards upstream sources and
   RPs.  Joins are sent to build shared trees (RP trees) or source trees
   (SPT).  Prunes are sent to prune source trees when members leave
   groups as well as sources that do not use the shared tree.

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |   Reserved    |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Upstream Neighbor Address (Encoded-Unicast format)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Reserved     | Num groups    |          Holdtime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         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)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             .                                 |
   |                             .                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Joined Source Address n (Encoded-Source format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Pruned Source Address 1 (Encoded-Source format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             .                                 |
   |                             .                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Pruned Source Address n (Encoded-Source format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   PIM Version, Type, Reserved, Checksum
         Described in Section 4.9.

   Unicast Upstream Neighbor Address
         The primary address of the upstream neighbor that is the target
         of the message.  The format for this address is given in the
         encoded unicast address in Section 4.9.1.

   Reserved
         Transmitted as zero, ignored on receipt.

   Holdtime
         The amount of time a receiver MUST keep the Join/Prune state
         alive, in seconds.  If the Holdtime is set to '0xffff', the
         receiver of this message SHOULD hold the state until canceled
         by the appropriate canceling Join/Prune message, or timed out
         according to local policy.  This may be used with dial-on-
         demand links, to avoid keeping the link up with periodic
         Join/Prune messages.

         Note that the HoldTime MUST be larger than the
         J/P_Override_Interval(I).

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

   Multicast group address
         For format description, see Section 4.9.1.

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

   Joined Source Address 1 .. n
         This list contains the sources for a given group that the
         sending router will forward multicast datagrams from if
         received on the interface on which the Join/Prune message
         is sent.

         See Section 4.9.1 for the format description for the
         encoded source address.

   Number of Pruned Sources
         Number of pruned source addresses listed for a group.

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   Pruned Source Address 1 .. n
         This list contains the sources for a given group that the
         sending router does not want to forward multicast datagrams
         from when received on the interface on which the Join/Prune
         message is sent.

   Within one PIM Join/Prune message, all the Multicast Group addresses,
   Joined Source addresses, and Pruned Source addresses MUST be of the
   same address family.  It is NOT PERMITTED to mix IPv4 and IPv6
   addresses within the same message.  In addition, the address family
   of the fields in the message SHOULD be the same as the IP source and
   destination addresses of the packet.  This permits maximum
   implementation flexibility for dual-stack IPv4/IPv6 routers.  If a
   router receives a message with mixed family addresses, it SHOULD only
   process the addresses that are of the same family as the unicast
   upstream neighbor address.

4.9.5.1.  Group Set Source List Rules

   As described above, Join/Prune messages are composed of one or more
   group sets.  Each set contains two source lists: the Joined Sources
   and the Pruned Sources.  This section describes the different types
   of group sets and source list entries that can exist in a Join/Prune
   message.

   There is one valid group set type:

   Group-Specific Set
         A Group-Specific Set is represented by a valid IP multicast
         address in the group address field and the full length of the
         IP address in the mask length field of the Multicast Group
         Address.  Each Join/Prune message SHOULD NOT contain more than
         one group-specific set for the same IP multicast address.  Each
         group-specific set may contain (*,G), (S,G,rpt), and (S,G)
         source list entries in the Joined or Pruned lists.

      (*,G)
            The (*,G) source list entry is used in Join/Prune messages
            sent towards the RP for the specified group.  It expresses
            interest (or lack thereof) in receiving traffic sent to the
            group through the RP shared tree.  There MUST only be one
            such entry in both the Joined and Pruned lists of a group-
            specific set.

            (*,G) source list entries have the Source-Address set to the
            address of the RP for group G, the Source-Address Mask-Len
            set to the full length of the IP address, and both the WC
            and RPT bits of the encoded-source-address set.

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      (S,G,rpt)
            The (S,G,rpt) source list entry is used in Join/Prune
            messages sent towards the RP for the specified group.  It
            expresses interest (or lack thereof) in receiving traffic
            through the shared tree sent by the specified source to this
            group.  For each source address, the entry MUST exist in
            only one of the Joined and Pruned source lists of a group-
            specific set, but not both.

            (S,G,rpt) source list entries have the Source-Address set to
            the address of the source S, the Source-Address Mask-Len set
            to the full length of the IP address, and the WC bit cleared
            and the RPT bit set in the encoded source address.

      (S,G)
            The (S,G) source list entry is used in Join/Prune messages
            sent towards the specified source.  It expresses interest
            (or lack thereof) in receiving traffic through the shortest
            path tree sent by the source to the specified group.  For
            each source address, the entry MUST exist in only one of the
            Joined and Pruned source lists of a group-specific set, but
            not both.

            (S,G) source list entries have the Source-Address set to the
            address of the source S, the Source-Address Mask-Len set to
            the full length of the IP address, and both the WC and RPT
            bits of the encoded source address cleared.

   The rules described above are sufficient to prevent invalid
   combinations of source list entries in group-specific sets.  There
   are, however, a number of combinations that have a valid
   interpretation but that are not generated by the protocol as
   described in this specification:

   o  Combining a (*,G) Join and an (S,G,rpt) Join entry in the same
      message is redundant, as the (*,G) entry covers the information
      provided by the (S,G,rpt) entry.

   o  The same applies for a (*,G) Prune and an (S,G,rpt) Prune.

   o  The combination of a (*,G) Prune and an (S,G,rpt) Join is also not
      generated.  (S,G,rpt) Joins are only sent when the router is
      receiving all traffic for a group on the shared tree and it wishes
      to indicate a change for the particular source.  As a (*,G) prune
      indicates that the router no longer wishes to receive shared tree
      traffic, the (S,G,rpt) Join would be meaningless.

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   o  As Join/Prune messages are targeted to a single PIM neighbor,
      including both an (S,G) Join and an (S,G,rpt) Prune in the same
      message is usually redundant.  The (S,G) Join informs the neighbor
      that the sender wishes to receive the particular source on the
      shortest path tree.  It is therefore unnecessary for the router to
      say that it no longer wishes to receive it on the shared tree.
      However, there is a valid interpretation for this combination of
      entries.  A downstream router may have to instruct its upstream
      only to start forwarding a specific source once it has started
      receiving the source on the shortest-path tree.

   o  The combination of an (S,G) Prune and an (S,G,rpt) Join could
      possibly be used by a router to switch from receiving a particular
      source on the shortest-path tree back to receiving it on the
      shared tree (provided that the RPF neighbor for the shortest-path
      and shared trees is common).  However, Sparse-Mode PIM does not
      provide a mechanism for explicitly switching back to the shared
      tree.

   The rules are summarized in the table below.

   +----------++------+-------+-----------+-----------+-------+-------+
   |          ||Join  | Prune | Join      | Prune     | Join  | Prune |
   |          ||(*,G) | (*,G) | (S,G,rpt) | (S,G,rpt) | (S,G) | (S,G) |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Join      ||-     | no    | ?         | yes       | yes   | yes   |
   |(*,G)     ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Prune     ||no    | -     | ?         | ?         | yes   | yes   |
   |(*,G)     ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Join      ||?     | ?     | -         | no        | yes   | ?     |
   |(S,G,rpt) ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Prune     ||yes   | ?     | no        | -         | yes   | ?     |
   |(S,G,rpt) ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Join      ||yes   | yes   | yes       | yes       | -     | no    |
   |(S,G)     ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+
   |Prune     ||yes   | yes   | ?         | ?         | no    | -     |
   |(S,G)     ||      |       |           |           |       |       |
   +----------++------+-------+-----------+-----------+-------+-------+

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   yes   Allowed and expected.

   no    Combination is not allowed by the protocol and MUST NOT be
         generated by a router.  A router MAY accept these messages, but
         the result is undefined.  An error message MAY be logged to the
         administrator in a rate-limited manner.

   ?     Combination not expected by the protocol, but well defined.  A
         router MAY accept it but SHOULD NOT generate it.

   The order of source list entries in a group set source list is not
   important, except where limited by the packet format itself.

4.9.5.2.  Group Set Fragmentation

   When building a Join/Prune for a particular neighbor, a router should
   try to include in the message as much of the information it needs to
   convey to the neighbor as possible.  This implies adding one group
   set for each multicast group that has information pending
   transmission and within each set including all relevant source list
   entries.

   On a router with a large amount of multicast state, the number of
   entries that must be included may result in packets that are larger
   than the maximum IP packet size.  In most such cases, the information
   may be split into multiple messages.

   There is an exception with group sets that contain a (*,G) Joined
   source list entry.  The group set expresses the router's interest in
   receiving all traffic for the specified group on the shared tree, and
   it MUST include an (S,G,rpt) Pruned source list entry for every
   source that the router does not wish to receive.  This list of
   (S,G,rpt) Pruned source list entries MUST NOT be split in multiple
   messages.

   If only N (S,G,rpt) Prune entries fit into a maximum-sized Join/Prune
   message, but the router has more than N (S,G,rpt) Prunes to add, then
   the router MUST choose to include the first N (numerically smallest
   in network byte order) IP addresses, and the rest are ignored (not
   included).

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4.9.6.  Assert Message Format

   The Assert message is used to resolve forwarder conflicts between
   routers on a link.  It is sent when a router receives a multicast
   data packet on an interface on which the router would normally have
   forwarded that packet.  Assert messages may also be sent in response
   to an Assert message from another router.

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Group Address (Encoded-Group format)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Source Address (Encoded-Unicast format)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                      Metric Preference                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Version, Type, Reserved, Checksum
         Described in Section 4.9.

   Group Address
         The group address for which the router wishes to resolve the
         forwarding conflict.  This is an encoded group address, as
         specified in Section 4.9.1.

   Source Address
         Source address for which the router wishes to resolve the
         forwarding conflict.  The source address MAY be set to zero for
         (*,G) asserts (see below).  The format for this address is
         given in the encoded unicast address in Section 4.9.1.

   R     RPTbit is a 1-bit value.  The RPTbit is set to 1 for
         Assert(*,G) messages and 0 for Assert(S,G) messages.

   Metric Preference
         Preference value assigned to the unicast routing protocol that
         provided the route to the multicast source or Rendezvous Point.

   Metric
         The unicast routing table metric associated with the route used
         to reach the multicast source or Rendezvous Point.  The metric
         is in units applicable to the unicast routing protocol used.

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   Assert messages can be sent to resolve a forwarding conflict for all
   traffic to a given group or for a specific source and group.

   Assert(S,G)
         Source-specific asserts are sent by routers forwarding a
         specific source on the shortest-path tree (SPTbit is TRUE).
         (S,G) Asserts have the Group-Address field set to the group G
         and the Source-Address field set to the source S.  The RPTbit
         is set to 0, the Metric-Preference is set to MRIB.pref(S), and
         the Metric is set to MRIB.metric(S).

   Assert(*,G)
         Group-specific asserts are sent by routers forwarding data for
         the group and source(s) under contention on the shared tree.
         (*,G) asserts have the Group-Address field set to the group G.
         For data-triggered Asserts, the Source-Address field MAY be set
         to the IP source address of the data packet that triggered the
         Assert and is set to zero otherwise.  The RPTbit is set to 1,
         the Metric-Preference is set to MRIB.pref(RP(G)), and the
         Metric is set to MRIB.metric(RP(G)).

4.10.  PIM Timers

   PIM-SM maintains the following timers, as discussed in Section 4.11.
   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 downwards to zero.

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   Global Timers

   Per interface (I):

        Hello Timer: HT(I)

        Per neighbor (N):

             Neighbor Liveness Timer: NLT(N,I)

        Per Group (G):

             (*,G) Join Expiry Timer: ET(*,G,I)

             (*,G) Prune-Pending Timer: PPT(*,G,I)

             (*,G) Assert Timer: AT(*,G,I)

             Per Source (S):

                  (S,G) Join Expiry Timer: ET(S,G,I)

                  (S,G) Prune-Pending Timer: PPT(S,G,I)

                  (S,G) Assert Timer: AT(S,G,I)

                  (S,G,rpt) Prune Expiry Timer: ET(S,G,rpt,I)

                  (S,G,rpt) Prune-Pending Timer: PPT(S,G,rpt,I)

   Per Group (G):

        (*,G) Upstream Join Timer: JT(*,G)

        Per Source (S):

             (S,G) Upstream Join Timer: JT(S,G)

             (S,G) Keepalive Timer: KAT(S,G)

             (S,G,rpt) Upstream Override Timer: OT(S,G,rpt)

   At the DRs or relevant Assert Winners only:

        Per Source,Group pair (S,G):

             Register-Stop Timer: RST(S,G)

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4.11.  Timer Values

   When timers are started or restarted, they are set to default values.
   This section summarizes those default values.

   Note that protocol events or configuration may change the default
   value of a timer on a specific interface.  When timers are
   initialized in this document, the value specific to the interface in
   context must be used.

   Some of the timers listed below (Prune-Pending, Upstream Join,
   Upstream Override) can be set to values that depend on the settings
   of the Propagation_Delay and Override_Interval of the corresponding
   interface.  The default values for these are given below.

   Variable Name: Propagation_Delay(I)

+-------------------------------+--------------+----------------------+
|  Value Name                   |  Value       |  Explanation         |
+-------------------------------+--------------+----------------------+
|  Propagation_delay_default    |  0.5 secs    |  Expected            |
|                               |              |  propagation delay   |
|                               |              |  over the local      |
|                               |              |  link.               |
+-------------------------------+--------------+----------------------+

   The default value of the Propagation_delay_default is chosen to be
   relatively large to provide compatibility with older PIM
   implementations.

   Variable Name: Override_Interval(I)

+--------------------------+-----------------+-------------------------+
|  Value Name              |    Value        |    Explanation          |
+--------------------------+-----------------+-------------------------+
|  t_override_default      |    2.5 secs     |    Default delay        |
|                          |                 |    interval over        |
|                          |                 |    which to randomize   |
|                          |                 |    when scheduling a    |
|                          |                 |    delayed Join         |
|                          |                 |    message.             |
+--------------------------+-----------------+-------------------------+

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Timer Name: Hello Timer (HT(I))

+---------------------+--------+---------------------------------------+
|Value Name           | Value  | Explanation                           |
+---------------------+--------+---------------------------------------+
|Hello_Period         | 30 secs| Periodic interval for Hello messages. |
+---------------------+--------+---------------------------------------+
|Triggered_Hello_Delay| 5 secs | Randomized interval for initial Hello |
|                     |        | message on bootup or triggered Hello  |
|                     |        | message to a rebooting neighbor.      |
+---------------------+--------+---------------------------------------+

   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       |
+--------------------------+----------------------+--------------------+
| Default_Hello_Holdtime   |  3.5 * Hello_Period  |  Default holdtime  |
|                          |                      |  to keep neighbor  |
|                          |                      |  state alive       |
+--------------------------+----------------------+--------------------+
| Hello_Holdtime           |  from message        |  Holdtime from     |
|                          |                      |  Hello message     |
|                          |                      |  Holdtime option.  |
+--------------------------+----------------------+--------------------+

   The Holdtime in a Hello message should be set to
   (3.5 * Hello_Period), giving a default value of 105 seconds.

   Timer Names: Expiry Timer (ET(*,G,I), ET(S,G,I), ET(S,G,rpt,I))

+----------------+----------------+------------------------------------+
| Value Name     |  Value         |  Explanation                       |
+----------------+----------------+------------------------------------+
| J/P_HoldTime   |  from message  |  Holdtime from Join/Prune message  |
+----------------+----------------+------------------------------------+

   The value of J/P Holdtime that is included in Join/Prune messages is
   specified below, in the description of "Upstream Join Timer (JT(*,G),
   JT(S,G))".

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   Timer Names: Prune-Pending Timer (PPT(*,G,I), PPT(S,G,I),
   PPT(S,G,rpt,I))

+--------------------------+---------------------+---------------------+
|Value Name                | Value               | Explanation         |
+--------------------------+---------------------+---------------------+
|J/P_Override_Interval(I)  | Default:            | Short period after  |
|                          | Effective_          | a join or prune to  |
|                          | Propagation_        | allow other         |
|                          | Delay(I) +          | routers on the LAN  |
|                          | Effective_Override_ | to override the     |
|                          | Interval(I)         | join or prune       |
+--------------------------+---------------------+---------------------+

   Note that both Effective_Propagation_Delay(I) and
   Effective_Override_Interval(I) are interface-specific values that may
   change when Hello messages are received (see Section 4.3.3).

   Timer Names: Assert Timer (AT(*,G,I), AT(S,G,I))

+---------------------------+---------------------+--------------------+
| Value Name                | Value               | Explanation        |
+---------------------------+---------------------+--------------------+
| Assert_Override_Interval  | Default: 3 secs     | Short interval     |
|                           |                     | before an assert   |
|                           |                     | times out where    |
|                           |                     | the assert winner  |
|                           |                     | resends an Assert  |
|                           |                     | message            |
+---------------------------+---------------------+--------------------+
| Assert_Time               | Default: 180 secs   | 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.

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   Timer Names: Upstream Join Timer (JT(*,G), JT(S,G))

+-------------+--------------------+-----------------------------------+
|Value Name   | Value              | Explanation                       |
+-------------+--------------------+-----------------------------------+
|t_periodic   | Default: 60 secs   | Period between Join/Prune messages|
+-------------+--------------------+-----------------------------------+
|t_suppressed | rand(1.1 *         | Suppression period when someone   |
|             | t_periodic, 1.4 *  | else sends a J/P message so we    |
|             | t_periodic) when   | don't need to do so.              |
|             | Suppression_       |                                   |
|             | Enabled(I) is      |                                   |
|             | true, 0 otherwise  |                                   |
+-------------+--------------------+-----------------------------------+
|t_override   | rand(0, Effective_ | Randomized delay to prevent       |
|             | Override_          | response implosion when sending a |
|             | Interval(I))       | Join message to override someone  |
|             |                    | else's Prune message.             |
+-------------+--------------------+-----------------------------------+

   t_periodic may be set to take into account such things as the
   configured bandwidth and expected average number of multicast route
   entries for the attached network or link (e.g., the period would be
   longer for lower-speed links, or for routers in the center of the
   network that expect to have a larger number of entries).  If the
   Join/Prune-Period is modified during operation, these changes should
   be made relatively infrequently, and the router should continue to
   refresh at its previous Join/Prune-Period for at least
   Join/Prune-Holdtime, in order to allow the upstream router to adapt.

   The Holdtime specified in a Join/Prune message should be set to
   (3.5 * t_periodic).

   t_override depends on the Effective Override Interval of the upstream
   interface, which may change when Hello messages are received.

   t_suppressed depends on the Suppression State of the upstream
   interface (Section 4.3.3) and becomes zero when suppression is
   disabled.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Timer Name: Upstream Override Timer (OT(S,G,rpt))

+---------------+--------------------------+---------------------------+
| Value Name    | Value                    |  Explanation              |
+---------------+--------------------------+---------------------------+
| t_override    | see Upstream Join Timer  |  see Upstream Join Timer  |
+---------------+--------------------------+---------------------------+

   The Upstream Override Timer is only ever set to the t_override value;
   this value is defined earlier in this section, under "Timer Names:
   Upstream Join Timer (JT(*,G), JT(S,G))".

   Timer Name: Keepalive Timer (KAT(S,G))

+-----------------------+-----------------------+----------------------+
| Value Name            |  Value                |  Explanation         |
+-----------------------+-----------------------+----------------------+
| Keepalive_Period      |  Default: 210 secs    |  Period after last   |
|                       |                       |  (S,G) data packet   |
|                       |                       |  during which (S,G)  |
|                       |                       |  Join state will be  |
|                       |                       |  maintained even in  |
|                       |                       |  the absence of      |
|                       |                       |  (S,G) Join          |
|                       |                       |  messages.           |
+-----------------------+-----------------------+----------------------+
| RP_Keepalive_Period   |  ( 3 * Register_      |  As                  |
|                       |  Suppression_Time )   |  Keepalive_Period,   |
|                       |  + Register_          |  but at the RP when  |
|                       |  Probe_Time           |  a Register-Stop is  |
|                       |                       |  sent.               |
+-----------------------+-----------------------+----------------------+

   The normal keepalive period for the KAT(S,G) defaults to 210 seconds.
   However, at the RP, the keepalive period must be at least the
   Register_Suppression_Time, or the RP may time out the (S,G) state
   before the next Null-Register arrives.  Thus, the KAT(S,G) is set to
   max(Keepalive_Period, RP_Keepalive_Period) when a Register-Stop
   is sent.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Timer Name: Register-Stop Timer (RST(S,G))

+---------------------------+--------------------+---------------------+
|Value Name                 | Value              | Explanation         |
+---------------------------+--------------------+---------------------+
|Register_Suppression_Time  | Default: 60 secs   | Period during       |
|                           |                    | which a DR stops    |
|                           |                    | sending Register-   |
|                           |                    | encapsulated data   |
|                           |                    | to the RP after     |
|                           |                    | receiving a         |
|                           |                    | Register-Stop       |
|                           |                    | message.            |
+---------------------------+--------------------+---------------------+
|Register_Probe_Time        | Default: 5 secs    | Time before RST     |
|                           |                    | expires when a DR   |
|                           |                    | may send a Null-    |
|                           |                    | Register to the RP  |
|                           |                    | to cause it to      |
|                           |                    | resend a Register-  |
|                           |                    | Stop message.       |
+---------------------------+--------------------+---------------------+

   If the Register_Suppression_Time or the Register_Probe_Time is
   configured to values other than the defaults, it MUST be ensured that
   the value of the Register_Probe_Time is less than half the value of
   the Register_Suppression_Time to prevent a possible negative value in
   the setting of the Register-Stop Timer.

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5.  IANA Considerations

5.1.  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.  Because
   when the PIM packet format was designed only 15 values were assigned
   for Address Families, and large numbers of new Address Family values
   were not envisioned, 8 bits seemed large enough.  However, the IANA
   assigns Address Families in a 16-bit field.  Therefore, the PIM
   Address Family is allocated as follows:

      Values 0 through 127 are designated to have the same meaning as
      IANA-assigned Address Family Numbers [7].

      Values 128 through 250 are designated to be assigned for PIM by
      the IANA based upon IESG Approval, as defined in [9].

      Values 251 through 255 are designated for Private Use, as defined
      in [9].

5.2.  PIM Hello Options

   Values 17 through 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 [9].  Such assignments are valid for one year and may be
   renewed.  Permanent assignments require a specification (see
   "Specification Required" in [9]).

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6.  Security Considerations

   This section describes various possible security concerns related to
   the PIM-SM protocol.  The reader is referred to [8], [14], and [15]
   for further discussion of PIM-SM and multicast security.

   Note that PIM relies upon an MRIB populated outside of PIM;
   therefore, securing the sources of change to the MRIB is RECOMMENDED.

6.1.  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, including forged link-local (Join/Prune, Hello, and
   Assert) and forged unicast (Register and Register-Stop) messages.

6.1.1.  Forged Link-Local Messages

   Join/Prune, Hello, and Assert messages are all sent to the link-local
   ALL-PIM-ROUTERS multicast address and thus are not forwarded by a
   compliant router.  A forged message of this type 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 Join/Prune message can cause multicast traffic to be
       delivered to links where there are no legitimate requesters,
       potentially wasting bandwidth on that link.  A forged leave
       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 leave with a join before the upstream router
       stops forwarding data to the LAN.

   2.  By forging a Hello message, an unauthorized router can cause
       itself to be elected as the Designated Router on a LAN.  The
       Designated Router on a LAN is (in the absence of asserts)
       responsible for forwarding traffic to that LAN on behalf of any
       local members.  The Designated Router is also responsible for
       register-encapsulating to the RP any packets that are originated
       by hosts on the LAN.  Thus, the ability of local hosts to send
       and receive multicast traffic may be compromised by a forged
       Hello message.

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

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6.1.2.  Forged Unicast Messages

   Register messages and Register-Stop messages are forwarded by
   intermediate routers to their destination using normal IP forwarding.
   Without data origin authentication, an attacker who is located
   anywhere in the network may be able to forge a Register or
   Register-Stop message.  We next consider the effect of a forgery of
   each of these messages.

   1.  By forging a Register message, an attacker can cause the RP to
       inject forged traffic onto the shared multicast tree.

   2.  By forging a Register-Stop message, an attacker can prevent a
       legitimate DR from registering packets to the RP.  This can
       prevent local hosts on that LAN from sending multicast packets.

   The above two PIM messages are not changed by intermediate routers
   and need only be examined by the intended receiver.  Thus, these
   messages can be authenticated end-to-end.  Attacks on Register and
   Register-Stop messages do not apply to a PIM-SSM-only implementation,
   as these messages are not required for PIM-SSM.

6.2.  Non-cryptographic Authentication Mechanisms

   A PIM router SHOULD provide an option to limit the set of neighbors
   from which it will accept Join/Prune, Assert, and Hello messages.
   Either static configuration of IP addresses or an IPsec security
   association MAY be used.  Furthermore, a PIM router SHOULD NOT accept
   protocol messages from a router from which it has not yet received a
   valid Hello message.

   A Designated Router MUST NOT register-encapsulate a packet and send
   it to the RP unless the source address of the packet is a legal
   address for the subnet on which the packet was received.  Similarly,
   a Designated Router SHOULD NOT accept a Register-Stop packet whose IP
   source address is not a valid RP address for the local domain.

   An implementation SHOULD provide a mechanism to allow an RP to
   restrict the range of source addresses from which it accepts
   Register-encapsulated packets.

   All options that restrict the range of addresses from which packets
   are accepted MUST default to allowing all packets.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

6.3.  Authentication

   This document refers to RFC 5796 [8], which specifies mechanisms to
   authenticate PIM-SM link-local messages using the IPsec Encapsulating
   Security Payload (ESP) or (optionally) the Authentication Header
   (AH).  It also points out that non-link-local PIM-SM messages (i.e.,
   Register and Register-Stop messages) can be secured by a normal
   unicast IPsec Security Association (SA) between two communicants.

6.4.  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 traffic.  Authenticating PIM protocol traffic
   prevents some, but not all, of these attacks.  Two of the possible
   attacks include the following:

   o  Sending packets to many different group addresses quickly can be a
      denial-of-service attack in and of itself.  This will cause many
      register-encapsulated packets, loading the DR, the RP, and the
      routers between the DR and the RP.

   o  Forging Join messages can cause a multicast tree to get set up.
      A large number of forged joins can consume router resources and
      result in denial of service.

7.  References

7.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
        <http://www.rfc-editor.org/info/rfc2119>.

   [2]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
        Thyagarajan, "Internet Group Management Protocol, Version 3",
        RFC 3376, DOI 10.17487/RFC3376, October 2002,
        <http://www.rfc-editor.org/info/rfc3376>.

   [3]  Deering, S., "Host extensions for IP multicasting", STD 5,
        RFC 1112, DOI 10.17487/RFC1112, August 1989,
        <http://www.rfc-editor.org/info/rfc1112>.

   [4]  Deering, S., Fenner, W., and B. Haberman, "Multicast Listener
        Discovery (MLD) for IPv6", RFC 2710, DOI 10.17487/RFC2710,
        October 1999, <http://www.rfc-editor.org/info/rfc2710>.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   [5]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998,
        <http://www.rfc-editor.org/info/rfc2460>.

   [6]  Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
        RFC 4607, DOI 10.17487/RFC4607, August 2006,
        <http://www.rfc-editor.org/info/rfc4607>.

   [7]  IANA, "Address Family Numbers",
        <http://www.iana.org/assignments/address-family-numbers>.

   [8]  Atwood, W., Islam, S., and M. Siami, "Authentication and
        Confidentiality in Protocol Independent Multicast Sparse Mode
        (PIM-SM) Link-Local Messages", RFC 5796, DOI 10.17487/RFC5796,
        March 2010, <http://www.rfc-editor.org/info/rfc5796>.

   [9]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 5226,
        DOI 10.17487/RFC5226, May 2008,
        <http://www.rfc-editor.org/info/rfc5226>.

7.2.  Informative References

   [10] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
        Extensions for BGP-4", RFC 4760, DOI 10.17487/RFC4760,
        January 2007, <http://www.rfc-editor.org/info/rfc4760>.

   [11] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas, "Bootstrap
        Router (BSR) Mechanism for Protocol Independent Multicast
        (PIM)", RFC 5059, DOI 10.17487/RFC5059, January 2008,
        <http://www.rfc-editor.org/info/rfc5059>.

   [12] Black, D., "Differentiated Services and Tunnels", RFC 2983,
        DOI 10.17487/RFC2983, October 2000,
        <http://www.rfc-editor.org/info/rfc2983>.

   [13] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
        "Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
        RFC 5015, DOI 10.17487/RFC5015, October 2007,
        <http://www.rfc-editor.org/info/rfc5015>.

   [14] Savola, P., Lehtonen, R., and D. Meyer, "Protocol Independent
        Multicast - Sparse Mode (PIM-SM) Multicast Routing Security
        Issues and Enhancements", RFC 4609, DOI 10.17487/RFC4609,
        October 2006, <http://www.rfc-editor.org/info/rfc4609>.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   [15] Savola, P. and J. Lingard, "Host Threats to Protocol Independent
        Multicast (PIM)", RFC 5294, DOI 10.17487/RFC5294, August 2008,
        <http://www.rfc-editor.org/info/rfc5294>.

   [16] Savola, P. and B. Haberman, "Embedding the Rendezvous Point (RP)
        Address in an IPv6 Multicast Address", RFC 3956,
        DOI 10.17487/RFC3956, November 2004,
        <http://www.rfc-editor.org/info/rfc3956>.

   [17] Zheng, L., Zhang, J., and R. Parekh, "Survey Report on Protocol
        Independent Multicast - Sparse Mode (PIM-SM) Implementations and
        Deployments", RFC 7063, DOI 10.17487/RFC7063, December 2013,
        <http://www.rfc-editor.org/info/rfc7063>.

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RFC 7761             PIM-SM Specification (Revised)           March 2016

Appendix A.  Functionality Removed from RFC 4601

   Based on a survey of PIM implementations and deployments [17]
   conducted by the IETF PIM working group, the following functionality
   of RFC 4601 has been removed due to lack of sufficient implementation
   and deployment experience:

   o  (*,*,RP) State

   o  PIM Multicast Border Router (PMBR)

   o  Authentication using IPsec

Acknowledgements

   PIM-SM was designed over many years by a large group of people,
   including ideas, comments, and corrections from Deborah Estrin, Dino
   Farinacci, Ahmed Helmy, David Thaler, Steve Deering, Van Jacobson, C.
   Liu, Puneet Sharma, Liming Wei, Tom Pusateri, Tony Ballardie, Scott
   Brim, Jon Crowcroft, Paul Francis, Joel Halpern, Horst Hodel, Polly
   Huang, Stephen Ostrowski, Lixia Zhang, Girish Chandranmenon, Brian
   Haberman, Hal Sandick, Mike Mroz, Garry Kump, Pavlin Radoslavov, Mike
   Davison, James Huang, Christopher Thomas Brown, and James Lingard.

   Thanks are due to the American Licorice Company, for its obscure but
   possibly essential role in the creation of this document.

Authors' Addresses

   Bill Fenner
   Arista Networks

   Email: fenner@arista.com

   Mark Handley
   Department of Computer Science
   University College London
   Gower Street
   London WC1E 6BT
   United Kingdom

   Email: M.Handley@cs.ucl.ac.uk

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RFC 7761             PIM-SM Specification (Revised)           March 2016

   Hugh Holbrook
   Arista Networks
   5453 Great America Parkway
   Santa Clara, CA  95054

   Email: holbrook@arista.com

   Isidor Kouvelas
   Arista Networks
   5453 Great America Parkway
   Santa Clara, CA  95054

   Email: kouvelas@arista.com

   Rishabh Parekh
   Cisco Systems, Inc.
   170 W. Tasman Drive
   San Jose, CA  95134

   Email: riparekh@cisco.com

   Zhaohui Zhang
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886

   Email: zzhang@juniper.net

   Lianshu Zheng
   Huawei Technologies Co., Ltd
   Huawei Campus, 156 Beiqing Road, Hai-dian District
   Beijing  100089
   China

   Email: vero.zheng@huawei.com

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