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rfc:rfc7761

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.

Fenner, et al. Standards Track [Page 1] 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

Fenner, et al. Standards Track [Page 2] 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

Fenner, et al. Standards Track [Page 3] 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

Fenner, et al. Standards Track [Page 4] 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.

Fenner, et al. Standards Track [Page 5] 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.

Fenner, et al. Standards Track [Page 6] 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.

Fenner, et al. Standards Track [Page 7] 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.

Fenner, et al. Standards Track [Page 8] 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.

Fenner, et al. Standards Track [Page 9] 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.

Fenner, et al. Standards Track [Page 10] 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

Fenner, et al. Standards Track [Page 11] 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.

Fenner, et al. Standards Track [Page 12] 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.

Fenner, et al. Standards Track [Page 13] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 14] RFC 7761 PIM-SM Specification (Revised) March 2016

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

Fenner, et al. Standards Track [Page 15] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 16] RFC 7761 PIM-SM Specification (Revised) March 2016

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)

Fenner, et al. Standards Track [Page 17] RFC 7761 PIM-SM Specification (Revised) March 2016

           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

Fenner, et al. Standards Track [Page 18] RFC 7761 PIM-SM Specification (Revised) March 2016

 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)

Fenner, et al. Standards Track [Page 19] RFC 7761 PIM-SM Specification (Revised) March 2016

 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).

Fenner, et al. Standards Track [Page 20] RFC 7761 PIM-SM Specification (Revised) March 2016

 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

Fenner, et al. Standards Track [Page 21] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 22] RFC 7761 PIM-SM Specification (Revised) March 2016

 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

Fenner, et al. Standards Track [Page 23] RFC 7761 PIM-SM Specification (Revised) March 2016

 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).

Fenner, et al. Standards Track [Page 24] RFC 7761 PIM-SM Specification (Revised) March 2016

 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

Fenner, et al. Standards Track [Page 25] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 26] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 27] 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.

Fenner, et al. Standards Track [Page 28] 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.

Fenner, et al. Standards Track [Page 29] RFC 7761 PIM-SM Specification (Revised) March 2016

 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

Fenner, et al. Standards Track [Page 30] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 31] RFC 7761 PIM-SM Specification (Revised) March 2016

    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
   }

Fenner, et al. Standards Track [Page 32] 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.

Fenner, et al. Standards Track [Page 33] 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
   }

Fenner, et al. Standards Track [Page 34] 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
   }

Fenner, et al. Standards Track [Page 35] 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.

Fenner, et al. Standards Track [Page 36] 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.

Fenner, et al. Standards Track [Page 37] 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.

Fenner, et al. Standards Track [Page 38] RFC 7761 PIM-SM Specification (Revised) March 2016

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

+———-++———————————————————-+

Event
Prev StateRegister- 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
 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.

Fenner, et al. Standards Track [Page 40] 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.

Fenner, et al. Standards Track [Page 41] 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.

Fenner, et al. Standards Track [Page 42] 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

Fenner, et al. Standards Track [Page 43] 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

Fenner, et al. Standards Track [Page 44] 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).

Fenner, et al. Standards Track [Page 45] 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.

Fenner, et al. Standards Track [Page 46] 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.

Fenner, et al. Standards Track [Page 47] 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.

Fenner, et al. Standards Track [Page 48] 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.

Fenner, et al. Standards Track [Page 49] 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.

Fenner, et al. Standards Track [Page 50] 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.

Fenner, et al. Standards Track [Page 51] 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.

Fenner, et al. Standards Track [Page 52] 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.

Fenner, et al. Standards Track [Page 53] 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.

Fenner, et al. Standards Track [Page 54] 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.

Fenner, et al. Standards Track [Page 55] 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,

Fenner, et al. Standards Track [Page 56] 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.

Fenner, et al. Standards Track [Page 57] 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.

Fenner, et al. Standards Track [Page 58] 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.

Fenner, et al. Standards Track [Page 59] 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.

Fenner, et al. Standards Track [Page 60] 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).

Fenner, et al. Standards Track [Page 61] RFC 7761 PIM-SM Specification (Revised) March 2016

               Figure 5: Upstream (*,G) State Machine

+——————-++————————————————-+

Event
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

+———————————-+———————————–+

Fenner, et al. Standards Track [Page 62] 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).

Fenner, et al. Standards Track [Page 63] 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.

Fenner, et al. Standards Track [Page 64] 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.

Fenner, et al. Standards Track [Page 65] 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
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

+——————-+————————-+————————+

Fenner, et al. Standards Track [Page 66] 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.

Fenner, et al. Standards Track [Page 67] 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.

Fenner, et al. Standards Track [Page 68] 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.

Fenner, et al. Standards Track [Page 69] 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.

Fenner, et al. Standards Track [Page 70] 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.

Fenner, et al. Standards Track [Page 71] 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

+————++————–+————–+————-+————+

Fenner, et al. Standards Track [Page 72] 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).

Fenner, et al. Standards Track [Page 73] 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.

Fenner, et al. Standards Track [Page 74] 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).

Fenner, et al. Standards Track [Page 75] 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.

Fenner, et al. Standards Track [Page 76] 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.

Fenner, et al. Standards Track [Page 77] 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]

+————-+————-+————-+————-+————-+

Fenner, et al. Standards Track [Page 78] 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.

Fenner, et al. Standards Track [Page 79] 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.

Fenner, et al. Standards Track [Page 80] 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).

Fenner, et al. Standards Track [Page 81] 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.

Fenner, et al. Standards Track [Page 82] 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.

Fenner, et al. Standards Track [Page 83] 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).

Fenner, et al. Standards Track [Page 84] RFC 7761 PIM-SM Specification (Revised) March 2016

    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.

Fenner, et al. Standards Track [Page 85] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 86] RFC 7761 PIM-SM Specification (Revised) March 2016

         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]

+————-+————-+————-+————-+————-+

Fenner, et al. Standards Track [Page 87] RFC 7761 PIM-SM Specification (Revised) March 2016

+———————————————————————-+

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.

Fenner, et al. Standards Track [Page 88] RFC 7761 PIM-SM Specification (Revised) March 2016

 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).

Fenner, et al. Standards Track [Page 89] 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, 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).

Fenner, et al. Standards Track [Page 90] RFC 7761 PIM-SM Specification (Revised) March 2016

 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).

Fenner, et al. Standards Track [Page 92] RFC 7761 PIM-SM Specification (Revised) March 2016

    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()
       }
   }

Fenner, et al. Standards Track [Page 93] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 94] RFC 7761 PIM-SM Specification (Revised) March 2016

 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 )
     }
   }

Fenner, et al. Standards Track [Page 95] RFC 7761 PIM-SM Specification (Revised) March 2016

 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

Fenner, et al. Standards Track [Page 104] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 105] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 106] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 107] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 108] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 109] RFC 7761 PIM-SM Specification (Revised) March 2016

    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.

Fenner, et al. Standards Track [Page 110] RFC 7761 PIM-SM Specification (Revised) March 2016

 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                     .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fenner, et al. Standards Track [Page 111] RFC 7761 PIM-SM Specification (Revised) March 2016

 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)

Fenner, et al. Standards Track [Page 112] RFC 7761 PIM-SM Specification (Revised) March 2016

       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)            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fenner, et al. Standards Track [Page 113] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 114] RFC 7761 PIM-SM Specification (Revised) March 2016

  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)        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fenner, et al. Standards Track [Page 115] 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.

Fenner, et al. Standards Track [Page 116] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 117] RFC 7761 PIM-SM Specification (Revised) March 2016

    (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.

Fenner, et al. Standards Track [Page 118] RFC 7761 PIM-SM Specification (Revised) March 2016

 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)     ||      |       |           |           |       |       |
 +----------++------+-------+-----------+-----------+-------+-------+

Fenner, et al. Standards Track [Page 119] RFC 7761 PIM-SM Specification (Revised) March 2016

 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).

Fenner, et al. Standards Track [Page 120] RFC 7761 PIM-SM Specification (Revised) March 2016

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.

Fenner, et al. Standards Track [Page 121] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 122] RFC 7761 PIM-SM Specification (Revised) March 2016

 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)

Fenner, et al. Standards Track [Page 123] RFC 7761 PIM-SM Specification (Revised) March 2016

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.

+————————–+—————–+————————-+

Fenner, et al. Standards Track [Page 124] 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))".

Fenner, et al. Standards Track [Page 125] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 126] RFC 7761 PIM-SM Specification (Revised) March 2016

 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.

Fenner, et al. Standards Track [Page 127] 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.

Fenner, et al. Standards Track [Page 128] 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.

Fenner, et al. Standards Track [Page 129] RFC 7761 PIM-SM Specification (Revised) March 2016

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]).

Fenner, et al. Standards Track [Page 130] RFC 7761 PIM-SM Specification (Revised) March 2016

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.

Fenner, et al. Standards Track [Page 131] RFC 7761 PIM-SM Specification (Revised) March 2016

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.

Fenner, et al. Standards Track [Page 132] 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>.

Fenner, et al. Standards Track [Page 133] 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>.

Fenner, et al. Standards Track [Page 134] 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>.

Fenner, et al. Standards Track [Page 135] 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

Fenner, et al. Standards Track [Page 136] 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

Fenner, et al. Standards Track [Page 137]

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