INTERNET DRAFT                                            Venu Hemige
                                                            Alcatel
                                                         Alcatel-Lucent
  Internet Engineering Task Force                         Yetik Serbest
  Document:                                                        AT&T
 draft-ietf-l2vpn-vpls-pim-snooping-00.txt
  draft-ietf-l2vpn-vpls-pim-snooping-01.txt                     Ray Qiu
                                                       Suresh Boddapati
                                                            Alcatel
                                                         Alcatel-Lucent
  Category: Informational
  Expires: January September 2007

                        PIM Snooping over VPLS

Status of this memo

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  This document is an Internet-Draft and is in full conformance with
  Sections 5 and 6 of RFC 3667 and Section 5 of RFC 3668.

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Abstract

  In Virtual Private LAN Service (VPLS), as also in IEEE Bridged
  Networks, the switches simply flood multicast traffic on all ports in
  the LAN by default. IGMP Snooping is commonly deployed to ensure
  multicast traffic is not forwarded on ports without IGMP receivers.
  The procedures and recommendations for IGMP Snooping are defined in
  [IGMP-SNOOP]. But when any protocol other than IGMP is used, the
  common practice is to simply flood multicast traffic to all ports.
  PIM-SM, PIM-SSM, PIM-BIDIR are widely deployed routing protocols. PIM
  Snooping procedures are important to restrict multicast traffic to
  only the routers interested in receiving such traffic.

  While most of the PIM Snooping procedures defined here also apply to
  IEEE Bridged Networks, VPLS demands certain special procedures due to
  the split-horizon rules that require the Provider Edge (PE) devices
  to co-operate. This document describes the procedures and
  recommendations for PIM-Snooping in VPLS to facilitate replication to
  only those ports behind which there are interested PIM routers and/or
  IGMP hosts.

  This document also describes procedures for PIM Proxy. PIM Proxy is
  required on PEs for VPLS Multicast to work correctly when Join
  suppression is enabled in the VPLS. PIM Proxy also helps scale VPLS
  Multicast much better than just PIM Snooping.

  Conventions used in this document

  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 [RFC 2119].

Table of Contents

   1. Introduction.............................................3      Introduction .............................................3
   1.1. Assumptions............................................4    Assumptions...............................................4
   1.2. Definitions............................................4    PIM Snooping and PIM Proxy Complexity.....................4
   1.3.    Definitions...............................................5
   2.      Multicast Traffic over VPLS..............................5 VPLS...............................6
   2.1.    Constraining of IP Multicast in a VPLS.................6 VPLS....................6
   2.2.    IPv6 Considerations....................................7 Considerations.......................................7
   2.3.    PIM-SM (*,*,RP) Considerations.........................7 Considerations............................7
   2.4.    PIM Packet Types to Snoop.................................8
   2.5.    PIM Snooping vs PIM Proxy..............................7
  2.4.1. Proxy.................................8
   2.5.1.  Differences between PIM Snooping and PIM Proxy.......8 Proxy............9
   2.5.2.  PIM Control Message Latency...............................9
   2.5.3.  When to Snoop and When to Proxy......................... 10
   3.      PIM Snooping for VPLS....................................9 VPLS................................... 10
   3.1.    General Rules for PIM Snooping in VPLS.................9 VPLS.................. 11
   3.1.1.  Snooping PIM Packets................................10 Packets ................................... 11
   3.2.    Discovering PIM Routers...............................10 Routers................................. 11
   3.3.    PIM-SM and PIM-SSM....................................11 PIM-SSM...................................... 12
   3.3.1.  Building PIM-SM Snooping States.....................11 States......................... 13
   3.3.2. Explaination  Explanation for per (S,G,N) states.................13 states...................... 15
   3.3.3.  Receiving (*,G) PIM-SM Join/Prune Messages..........13 Messages.............. 15
   3.3.4.  Receiving (S,G) PIM-SM Join/Prune Messages..........16 Messages.............. 18
   3.3.5.  Receiving (S,G,rpt) Join/Prune Messages.............17 Messages................. 19
   3.3.6.  Sending (*,G) Join/Prune Messages...................17 Messages....................... 19
   3.3.7.  Sending (S,G) Join/Prune Messages...................18 Messages....................... 20
   3.3.8.  Sending PIM Join/Prune message upstream.............18 upstream................. 20
   3.3.8.1. Sending Triggered vs Refresh Join/Prune messages....... 20
   3.3.9.  Triggering ASSERT Election in PIM-SM................19 PIM-SM.................... 21
   3.4.    Bidirectional-PIM (PIM-BIDIR).........................22 (PIM-BIDIR)........................... 21
   3.4.1.  Building PIM-BIDIR Snooping States..................23 States...................... 22
   3.5. PIM-DM................................................23    PIM-DM.................................................. 22
   3.5.1.  Building PIM-DM Snooping States.....................23 States......................... 23
   3.5.2.  PIM-DM Downstream Per-Port PIM(S,G,N) State Machine.23 Machine .... 23
   3.5.3.  Triggering ASSERT election in PIM-DM................24 PIM-DM.................... 23
   3.6.    PIM Proxy.............................................24 Proxy............................................... 24
   3.6.1.  Downstream PIM Proxy behavior.......................24 behavior........................... 24
   3.6.2.  Upstream PIM Proxy behavior.........................25 behavior............................. 25
   3.6.3.  Source IP Address in Proxy PIM Join/Prune Packets....... 25
   3.7.    Directly Connected Multicast Source...................26 Source .................... 25
   3.8.    Data Forwarding Rules.................................26 Rules................................... 26
   3.8.1.  PIM-SM Data Forwarding Rules........................27 Rules ........................... 26
   3.8.2.  PIM-BIDIR Data Forwarding Rules.....................28 Rules......................... 27
   3.8.3.  PIM-DM Data Forwarding Rules........................28 Rules............................ 28
   4.      IANA Considerations.....................................29 Considerations..................................... 28
   5.      Security Considerations.................................29 Considerations................................. 29
   6. References..............................................29
  6.1.      Acknowledgements........................................ 29
   7.      References.............................................. 29
   7.1.    Normative References..................................29
  6.2. References ................................... 29
   7.2.    Informative References................................30 References.................................. 29
   Appendix A. Example Network Scenario............................ 31
   Appendix A.1 PIM-Snooping Example............................... 31
   Appendix A.2 PIM Proxy Example with (S,G) / (*,G) interaction... 33

1. Introduction

  In Virtual Private LAN Service (VPLS), the Provider Edge (PE) devices
  provide a logical interconnect such that Customer Edge (CE) devices
  belonging to a specific VPLS instance appear to be connected by a
  single LAN. Forwarding information base for particular VPLS instance
  is populated dynamically by source MAC address learning.  This is a
  straightforward solution to support unicast traffic, with reasonable
  flooding for unicast unknown traffic.  Since a VPLS provides LAN
  emulation for IEEE bridges as wells as for routers, the unicast and
  multicast traffic need to follow the same path for layer-2 protocols
  to work properly.  As such, multicast traffic is treated as broadcast
  traffic and is flooded to every site in the VPLS instance.

  VPLS solutions (i.e., [VPLS-LDP] and [VPLS-BGP]) perform replication
  for multicast traffic at the ingress PE devices.  When replicated at  As stated in the ingress PE, multicast traffic wastes bandwidth when:
     1.
  VPLS Multicast traffic is sent to sites Requirements draft [VPLS-MCAST-REQ], there are two
  issues with no members,
     2. Pseudo wires to different sites go through a shared path, and
     3. VPLS Multicast today:
      A. Multicast traffic is forwarded along a shortest path tree as
        opposed replicated to the minimum cost spanning tree. non-member sites.
      B. Replication of PWs on shared physical path.

  This document is addressing the first problem solves Issue A of [VPLS-MCAST-REQ] by IGMP and PIM
 snooping. Problems #2 and #3 are orthogonal ensuring that IP
  multicast traffic is not forwarded to #1 and non-member sites. Issue B is
  outside the scope of this document. The different mechanisms to
  tunnel IP multicast traffic in a VPLS from the ingress PE to the
  egress PEs are discussed in [VPLS-MCAST-TREES].

 Using VPLS The solution in conjunction this
  document when combined with IGMP and/or PIM snooping the solutions proposed in the working
  group to solve Issue B will provide a complete VPLS Multicast
  solution set.

  Using IGMP/PIM Snooping in VPLS has the following advantages:
       - It improves IP Multicast bandwidth usage in the VPLS core by
       ensuring traffic is replicated only to support IP multicast efficiently (not PEs with member sites.
       Note that this is not necessarily optimum, as there can still be
       bandwidth waste if traffic from a PE to other PE(s) is not
       forwarded along a minimum cost spanning tree.), tree.
       - It prevents sending multicast traffic to sites with no
        members. non-member sites.

  Procedures for IGMP Snooping are specified in [IGMP-SNOOP]. This
  document describes the procedures for Protocol Independent Multicast
  (PIM) snooping over VPLS for efficient distribution of IP multicast
  traffic. It also describes the rules when both IGMP and PIM are
  active in a VPLS instance.

  This document also describes procedures for PIM Proxy. PIM Proxy is
  required on PEs for VPLS Multicast to work correctly when Join
  suppression is enabled in the VPLS. PIM Proxy also helps scale VPLS
  Multicast much better than just PIM Snooping.

1.1. Assumptions

  Since this draft describes the procedures for PIM Snooping and PIM
  Proxy, the draft assumes that the reader has a good understanding of
  the PIM protocols. The text in this draft is written in the same
  style as the PIM RFCs to help correlate the concepts and to make it
  easier to follow. In order to avoid replicating the text relating to
  PIM protocol handling here, this draft assumes that the user will
  infer such detail from the PIM RFC referenced in this document.
  Deviations in protocol handling specific to PIM Snooping and PIM
  Proxy are specified in this draft. There could be cross references
  into definitions of macros and procedures from the PIM RFCs.

1.2. Definitions

 There are several definitions referenced in this document that are
 well described in the PIM RFCs [PIM-SM, PIM-BIDIR, PIM-DM].

 The following definitions Snooping and abbreviations are used throughout this
 document:

      - A port is defined as either an attachment circuit (AC) or a
        Pseudo-Wire (PW).
      - When we say a PIM message is 'received' on a port, it means
        any one of the following:
        o that a Proxy Complexity

  The PIM Snooping switch snooped the and PIM message.
        o that Proxy solutions described here requires a
  switch to examine and operate on only PIM Hello and PIM Join/Prune
  packets. The switch does not need to examine any other PIM packets.

  The switch does not need to have any routing tables like is required
  in PIM Multicast Routing. It knows how to forward Join/Prunes by
  looking at the Upstream Neighbor field in the Join/Prune packets.

  The switch does not need to know about Rendezvous Points (RP) and
  does not have to maintain any RP Set. All that is transparent to a
  PIM Snooping switch.

  Most of the procedures in PIM Snooping and PIM Proxy in the handling
  of PIM Hellos and PIM Join/Prune packets are very similar to that of
  a PIM Router.

  The solutions described here provide complete separation of control
  and data planes.

  A PIM Proxy solution minimizes the control plane messages received at
  CE routers by proxying one message upstream on behalf of a large
  number of downstream CEs. As such control plane messaging is very
  similar to that of a PIM Router.

1.3. Definitions

  There are several definitions referenced in this document that are
  well described in the PIM RFCs [PIM-SM, PIM-BIDIR, PIM-DM].

  The following definitions and abbreviations are used throughout this
  document:

       - A port is defined as either an attachment circuit (AC) or a
          Pseudo-Wire (PW).
       - When we say a PIM message is 'received' on a port, it means
          any one of the following:
         o that a PIM Snooping switch snooped the PIM message.
         o that a PIM message was received via LDP on a PW if LDP (as
            defined in [VPLS-MCAST-LDP]) is used for propogating
            multicast states among the PEs.

  Abbreviations used in the document:

       - S: IP Address of the Multicast Source.
       - G: IP Address of the Multicast Group.
       - N: Upstream Neighbor field in a Join/Prune/Graft message.
       - Rport(X): Rport(N): Port on which neighbor X N is learnt

  Other definitions are explained in the sections where they are
  introduced.

2. Multicast Traffic over VPLS

  In VPLS, if a PE receives a frame from an Attachment Circuit (AC)
  with no matching entry in the forwarding information base for that
  particular VPLS instance, it floods the frame to all other PEs (which
  are part of this VPLS instance) and to directly connected ACs (other
  than the one that the frame is received from).  The flooding of a
  frame occurs when:
       - The destination MAC address has not been learned,
       - The destination MAC address is a broadcast address,
       - The destination MAC address is a multicast address.

  Malicious attacks (e.g., receiving unknown frames constantly) aside,
  the first situation is handled by VPLS solutions as long as
  destination MAC address can be learned.  After that point on, the
  frames will not be flooded.  A PE is REQUIRED to have safeguards,
  such as unknown unicast limiting and MAC table limiting, against
  malicious unknown unicast attacks.

  There is no way around flooding broadcast frames.  To prevent runaway
  broadcast traffic from adversely affecting the VPLS service and the
  SP network, a PE is REQUIRED to have tools to rate limit the
  broadcast traffic as well.

  Similar to broadcast frames, multicast frames are flooded as well, as
  a PE cannot know where multicast members reside.  Rate limiting
  multicast traffic, while possible, should be should be done carefully
  since several network control protocols relies on multicast.  For one
  thing, layer-2 and layer-3 protocols utilize multicast for their
  operation.  For instance, Bridge Protocol Data Units (BPDUs) use an
  IEEE assigned all bridges multicast MAC address, and OSPF is
  multicast to all OSPF routers multicast MAC address.  If the rate-
  limiting of multicast traffic is not done properly, the customer
  network will experience instability and poor performance.  For the
  other, it is not straightforward to determine the right rate limiting
  parameters for multicast.

  A VPLS solution MUST NOT affect the operation of customer layer-2
  protocols (e.g., BPDUs).  Additionally, a VPLS solution MUST NOT
  affect the operation of layer-3 protocols.

  In the following section, we describe procedures to constrain the
  flooding of IP multicast traffic in a VPLS.

2.1. Constraining of IP Multicast in a VPLS

  For a PE in a VPLS (a layer-2 device) to constrain IP multicast
  traffic, it needs to be able to learn which CEs are interested in
  receiving multicast traffic for what flows.

  The most obvious solution is to snoop IP multicast control traffic at
  the PEs. Snooping as a solution to constrain multicast traffic makes
  sense under the following circumstances:

       - The CE-CE protocol the PEs snoop is a popular and widely
          deployed protocol.
       - It does not require any changes on the CEs and it should be
          completely transparent to the CEs.

 Using VPLS in conjunction with IGMP and/or

  IGMP/MLD and PIM snooping has are the
 following advantages:
      - It improves VPLS to support popular IP multicast efficiently (not
        necessarily optimum, as there can still be bandwidth waste if
        traffic from a PE to other PE(s) is not forwarded along a
        minimum cost spanning tree.),
      - It prevents sending multicast traffic to sites with no
        members. Multicast Routing protocols
  today. Other routing protocols such as DVMRP or MOSPF are outside the
  scope of this document.

  This document describes the guidelines for PIM Snooping and PIM Proxy
  in VPLS. The specifications in this document could be used for either
  PIM Snooping or PIM Proxy. The PIM Proxy solution is described in
  section 3.6 Differences that need to be observed while implementing
  one or the other and recommendations on which method to employ in
  different scenarios are noted in section 2.5We will largely refer to
  PIM "Snooping" in this document. Unless specifically specified, the
  same procedures should apply to a Proxy solution as well.

  In the following sub-sections, we provide some guidelines for the
  implementation of PIM snooping in VPLS. Snooping techniques need to
  be employed on ACs at the downstream PEs. Snooping techniques can
  also be employed on PWs at the upstream PEs. This may work well for
  small to medium scale deployments. However, if there are a large
  number of VPLS instances with a large number of PEs per instances,
  then the amount of snooping required at the upstream PEs can
  overwhelm the upstream PEs. In [VPLS-MCAST-LDP] and [VPLS-MCAST-BGP],
  procedures are defined to exchange multicast membership information
  between the PEs using LDP or BGP. Using a reliable mechanism like LDP
  or BGP allows the upstream PEs to eliminate the requirement to snoop
  on PWs. It also eliminates the need to refresh multicast states on
  the upstream PEs.

 This document also describes the guidelines for PIM Proxy in VPLS.
 The specifications in this document could be used for either PIM
 Snooping or PIM Proxy. The PIM Proxy solution is described in section
 3.6.  Differences that need to be observed while implementing one or
 the other and recommendations on which method to employ in different
 scenarios are noted in section 2.4.

2.2. IPv6 Considerations

  In VPLS, PEs forward Ethernet frames received from CEs and as such
  are agnostic of the layer-3 protocol used by the CEs.  However, as an
  IGMP and PIM snooping switch, the PE would have to look deeper into
  the IP and IGMP/PIM packets and build snooping state based on that.
  The PIM Protocol specifications handle both IPv4 and IPv6. The
  specification for PIM Snooping in this draft can be applied to both
  IPv4 and IPv6 payloads.

2.3. PIM-SM (*,*,RP) Considerations

  This draft does not address (*,*,RP) states in the VPLS network.
  Although [PIM-SM] specifies that routers MUST support (*,*,RP)
  states, there are very few implementations that actually support it
  in actual deployments. Given the complexity of supporting (*,*,RP)
  states and knowing that there is little to no use to supporting it,
  this draft omits the specification relating to (*,*,RP) support.

2.4. PIM Packet Types to Snoop

  A PIM Snooping switch need only snoop on PIM Hellos and PIM
  Join/Prune packets. All other PIM packets can be transparently
  flooded unexamined.

2.5. PIM Snooping vs PIM Proxy

  PIM Snooping switches simply snoop on PIM packets as they are being
  forwarded in the VPLS. As such it truly provides transparent LAN
  services since no customer packets are modified or consumed or new
  packets introduced in the VPLS. It is also slightly simpler to
  implement than PIM Proxy. However for PIM Snooping to work correctly,
  it is a requirement that CE routers MUST disable Join suppression in
  the VPLS.

  Given that a large number of existing CE deployments do not support
  disabling of Join suppression and given the operational complexity
  for a provider to manage disabling of Join suppression in the VPLS,
  it becomes a difficult solution to deploy. Another disadvantage of
  PIM Snooping as a solution is that it does not scale as well as PIM
  Proxy. If there are a large number of CEs in a VPLS, then every CE
  will see every other CE's Join/Prune messages.

  PIM Proxy on the PEs has the advantage that it does not require Join
  suppression to be disabled in the VPLS. Multicast as a VPLS service
  can be very easily be provided without requiring any changes on the
  CE routers. It also helps scale VPLS Multicast very well since the
  PEs intelligently forward only one Join/Prune message for a given
  flow and only to the upstream CE.

  PIM Proxy as a solution however loses the transparency argument since
  Join/Prunes could get modified or even consumed at a PE. Also, new
  packets could get introduced in the VPLS. However, this loss of
  transparency is limited to PIM control Join/Prune packets. It is in the
  interest of optimizing multicast in the VPLS and helping a VPLS
  network scale much better. Data traffic will still be completely
  transparent.

  Both PIM Snooping and PIM Proxy procedures can be used in conjunction
  with [VPLS-MCAST-LDP] for propogating multicast states among the PEs.
  If [VPLS-MCAST-LDP] is used for propogating multicast states among
  the PEs, then both PIM Snooping and PIM Proxy switches do not process
  any PIM packets arriving on a PW.

2.4.1.

2.5.1. Differences between PIM Snooping and PIM Proxy

  For PIM-SM and PIM-BIDIR, a PIM Snooping/Proxy Switch only needs to
  examine PIM Hello and Join/Prune messages. PIM Proxy for PIM-DM is
  for future study and is not currently specified in this draft.

  The proxy proposal is to perform proxy of only the Join/Prune
 messages while snooping Hello
  messages. PIM Hello messages are snooped by both PIM Snooping and PIM
  Proxy switches.

  Details on the PIM Proxy solution are discussed in section 3.6. 3.6 This
  section is presented here to say that most of the procedures to
  follow (unless explicitly specified) are common to both PIM Snooping
  and PIM Proxy.

  Differences between a PIM Snooping switch and a PIM Proxy switch can
  be summarized as the following:

      +------------------------------|--------------------------------+
      |     PIM Snooping             |       PIM Proxy                |
      +==============================|================================+
      | 1. PIM Snooping switches     | 1. PIM Proxy switches also     |
      |    snoop Hello and Join/Prune|    snoop PIM Hello messages    |
      |    messages while they are   |    while they are transparently|
      |    transparently flooded in  |    flooded in the VPLS. But    |
      |    the VPLS.                 |    they consume PIM Join/Prune |
      |                              |    messages and do not flood   |
      |                              |    them as is in the VPLS.     |
      +------------------------------|--------------------------------+
      | 2. PIM Snooping switches do  | 2. PIM Proxy switches may      |
      |    not originate any PIM     |    originate new or modified   |
      |    packets. They may however |    PIM packets. Hello and Join/Prune    |
      |    originate PIM messages to |    packets.                    |
      |    be sent via LDP on PWs.   |                                |
      +------------------------------|--------------------------------+

  Other than the above simple differences, most of the procedures are
  common to PIM Snooping and PIM Proxy. There are additional
  simplifications to PIM Snooping that can be made if [VPLS-MCAST-LDP]
  is not used for PE-PE communication, but otherwise the procedures for
  PIM Snooping and PIM Proxy are mostly the same. In the text to
  follow, we describe the text as procedures for PIM Snooping. "Snooping". Unless
 otherwise specified,
  specifically stated otherwise, such procedures apply to PIM Proxy as
  well.

3.

2.5.2. PIM Control Message Latency

  A PIM Snooping for VPLS

 IGMP snooping procedures described in [IGMP-SNOOP] provide efficient
 delivery of IP multicast traffic in a or PIM Proxy switch snoops on PIM Hello packets while
  transparently flooding it in the VPLS. As such there is no latency
  introduced by the VPLS in the delivery of PIM Hello packets to remote
  CEs in the VPLS.

  A PIM Proxy switch consumes PIM Join/Prune packets and generates
  proxy Join/Prune packets to be sent upstream. This can result in
  additional latency for a downstream CE to receive multicast traffic
  after it has sent a Join. When a downstream CE prunes a multicast
  stream, the traffic should stop flowing to the CE with no additional
  latency introduced by the VPLS.

  A PIM Snooping switch snoops on PIM Join/Prune packets while
  transparently flooding them in the VPLS. There is no latency
  introduced by the VPLS in the delivery of PIM Join/Prune packets when
  PIM Snooping is employed.

2.5.3. When to Snoop and When to Proxy

  Explicit Tracking (ET) is enabled in a VPLS when all PIM CE Routers
  in the VPLS advertise Tracking Support in their PIM Hello messages.
  If even one does not advertise Tracking Support, then all PIM CE
  routers disable ET in the VPLS. When ET is enabled, it implies that
  Join Suppression is disabled and vice versa.

  PIM Snooping PEs can determine if ET is enabled or disabled in a VPLS
  by examining PIM Hellos. If ET is disabled, PIM Proxy MUST be used.
  If ET is enabled, PIM Snooping SHOULD be used.

3. PIM Snooping for VPLS

  IGMP snooping procedures described in [IGMP-SNOOP] provide efficient
  delivery of IP multicast traffic in a given VPLS service when end
  stations are connected to the VPLS.  However, when VPLS is offered as
  a WAN service it is likely that the CE devices are routers and would
  run PIM between them.  To provide efficient IP multicasting in such
  cases, it is necessary that the PE routers offering the VPLS service
  do PIM snooping.

  PIM is a multicast routing protocol, which runs exclusively between
  routers. PIM shares many of the common characteristics of a routing
  protocol, such as discovery messages (e.g., neighbor discovery using
  Hello messages), topology information (e.g., multicast tree), and
  error detection and notification (e.g., dead timer and designated
  router election).  On the other hand, PIM does not participate in any
  kind of exchange of databases, as it uses the unicast routing table
  to provide reverse path information for building multicast trees.
  There are a few variants of PIM.  In PIM-DM ([PIM-DM]), multicast
  data is pushed towards the members similar to broadcast mechanism.
  PIM-DM constructs a separate delivery tree for each multicast group.
  As opposed to PIM-DM, other PIM flavors (PIM-SM [PIM-SM], PIM-SSM
  [PIM-SSM], and PIM-BIDIR [PIM-BIDIR]) invoke a pull methodology
  instead of push technique.

  PIM routers periodically exchange Hello messages to discover and
  maintain stateful sessions with neighbors.  After neighbors are
  discovered, PIM routers can signal their intentions to join or prune
  specific multicast groups.  This is accomplished by having downstream
  routers send an explicit Join/Prune message (for the sake of
  generalization, consider Graft messages for PIM-DM as Join messages)
  to the upstream routers.  The Join/Prune message can be group
  specific (*,G) or group and source specific (S,G).

  In PIM snooping, a PE snoops on the PIM message exchange exchanged between
  routers, and builds its multicast states.

  Based on the multicast states, it forwards IP multicast traffic
  accordingly to avoid unnecessary flooding.

  In the following sub-sections, snooping mechanisms for each variety
  of PIM are specified.

3.1. General Rules for PIM Snooping in VPLS

  The following rules for the correct operation of IGMP/PIM PIM snooping MUST be
  followed.

       - IGMP messages, PIM messages and multicast data traffic forwarded by PEs MUST
          follow the split-horizon rule for mesh PWs as defined in
          [VPLS-LDP].
       - IGMP/PIM PIM snooping states in a PE MUST be per VPLS instance.
       - Multicast traffic MUST be replicated per PW and AC basis,
          i.e., even if there are more than one PIM neighbor behind a
          PW/AC, only one replication MUST be sent to that PW/AC.

3.1.1. Snooping PIM Packets

  PIM-SM and PIM-BIDIR snooping PEs need to snoop on just the PIM Hello
  and PIM Join/Prune messages to build its multicast states.

       - PIM-DM snooping PEs have to also snoop on PIM Graft and PIM
          State Refresh messages.

3.2. Discovering PIM Routers

  A PIM Snooping PE MUST snoop on PIM Hellos received on ACs and PWs.
  i.e. the PE transparently floods the PIM Hello while snooping on it.
  PIM Hellos are used by the snooping switch to discover PIM routers
  and their characteristics.

  For each neighbor discovered by a PE, it includes an entry in the PIM
  Neighbor Database with the following fields:

       - Layer 2 encapsulation for the Router sending the PIM Hello.
       - IP Address and address family of the Router sending the PIM
          Hello.
       - Port (AC / PW) on which the PIM Hello was received.
       - Hello TLVs

  The PE should be able to interpret and act on Hello TLVs currently
  defined in the PIM RFCs. The TLVs of particular interest in this
  document are:

       - Hello-Hold-Time
       - Tracking Support
       - DR Priority

  Please refer to [PIM-SM] for a list of the Hello TLVs.

  When a PIM Hello is received, the PE MUST reset the neighbor-expiry-
  timer to Hello-Hold-Time. If a PE does not receive a Hello message
  from a router within Hello-Hold-Time, the PE MUST remove that
  neighbor from its PIM Neighbor Database. If a PE receives a Hello
  message from a router with Hello-Hold-Time value set to zero, the PE
  MUST remove that router from the PIM snooping state immediately.

  From the PIM Neighbor Database, a PE MUST be able to use the
  procedures defined in [PIM-SM] to identify the PIM Designated Router
  in the VPLS instance. It should also be able to determine if Tracking
  Support is active in the VPLS instance.

3.3. PIM-SM and PIM-SSM

  The key characteristic of PIM-SM and PIM-SSM is explicit join
  behavior.  In this model, multicast traffic is only forwarded to
  locations that specifically request it.  The root node of a tree is
  the Rendezvous Point (RP) in case of a shared tree (PIM-SM only) or
  the first hop router that is directly connected to the multicast
  source in the case of a shortest path tree. All the procedures
  described in this section apply to both PIM-SM and PIM-SSM, except
  for the fact that there is no (*,G) state in PIM-SSM.

  The procedures to discover PIM-SM routers in a VPLS instance are as
  described in section 3.2. 3.2

3.3.1. Building PIM-SM Snooping States

  PIM-SM and PIM-SSM Snooping states are built by snooping on the PIM-
  SM Join/Prune messages received on AC/PWs.
  The downstream state machine of a PIM-SM snooping switch very closely
  resembles the downstream state machine of PIM-SM routers. The
  downstream state consists of:

  Per downstream (Port, *, G):
       - DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
          Pending" (PP) }

  Per downstream (Port, *, G, N):
       - Prune Pending Timer (PPT(N))
       - Join Expiry Timer (ET(N))

  Per downstream (Port, S, G):
       - DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
          Pending" (PP) }

  Per downstream (Port, S, G, N):
       - Prune Pending Timer (PPT(N))
       - Join Expiry Timer (ET(N))

  Per downstream (Port, S, G, rpt):
       - DownstreamJPRptState: One of { "NoInfo" (NI), "Pruned" (P),
          "Prune Pending" (PP) }

  Per downstream (Port, S, G, rpt, N):
       - Prune Pending Timer (PPT(N))
       - Join Expiry Timer (ET(N))

    Where S is the address of the multicast source, G is the Group
  address and N is the upstream neighbor field in the Join/Prune
  message. Notice that unlike on PIM-SM routers where PPT and ET are
  per (Interface, S, G), PIM Snooping switches have to maintain PPT and
  ET per (Port, S, G, N). The reasons for this are explained in section
 3.3.2.
  3.3.2

  Apart from the above states, we define the following state
  summarization macros.

  UpstreamNeighbors(*,G): If there is one or more Join(*,G) received on
  any port with upstream neighbor N and ET(N) is active, then N is
  added to UpstreamNeighbors(*,G). This set is used to determine if a
  Join(*,G) or a Prune(*,G) with upstream neighbor N needs to be sent
  upstream.

  UpstreamNeighbors(S,G): If there is one or more Join(S,G) received on
  any port with upstream neighbor N and ET(N) is active, then N is
  added to UpstreamNeighbors(S,G). This set is used to determine if a
  Join(S,G) or a Prune(S,G) with upstream neighbor N needs to be sent
  upstream.

 UpstreamPorts(G):

  UpstreamPorts(*,G): This is the set of all Rport(N) ports on which the Upstream
 Neighbors where N is
  in UpstreamNeighbors(*,G) and the UpstreamNeighbors(S,G)
 for the given G are learnt. set UpstreamNeighbors(*,G). Multicast Streams forwarded using
  a (*,G) match MUST be forwarded to these ports in addition to
  downstream ports. So UpstreamPorts(*,G) MUST be added to
  OutgoingPortList(*,G).

  UpstreamPorts(S,G): This is the set of all Rport(N) ports where N is used
  in section 3.3.8. the set UpstreamNeighbors(S,G). UpstreamPorts(S,G) MUST be added
  to
 determine OutgoingPortList(S,G).

  UpstreamPorts(S,G,rpt): If PruneDesired(S,G,rpt) becomes true, then
  this set is set to UpstreamPorts(*,G). Otherwise, this set is empty.
  UpstreamPorts(*,G) (-) UpstreamPorts(S,G,rpt) MUST be added to
  OutgoingPortList(S,G).

  See section 3.8.1 on Data Forwarding Rules for the ports specification on which
  how OutgoingPortList(S,G) is calculated.

  UpstreamPorts(G): This set is the union of all the UpstreamPorts(S,G)
  and UpstreamPorts(*,G) for a given G. Proxy (S,G) Join/Prune and
  (*,G) Join/Prune messages must be sent. Data
 traffic MUST also be forwarded to these ports sent to facilitate assert
 election a subset of
  UpstreamPorts(G) as specified in the VPLS. section 3.3.8.

  PWPorts: This is the set of all PWs.

  OutgoingPortList(*,G): This is the set of all ports to which traffic
  needs to be forwarded on a (*,G) match. Split Horizon rules apply as
  noted in section 3.8. 3.8

  OutgoingPortList(S,G): This is the set of all ports to which traffic
  needs to be forwarded on an (S,G) match. Split Horizon rules apply as
  noted in section 3.8. 3.8

  NumETsActive(Port,*,G): Number of (Port,*,G,N) entries that have
  Expiry Timer running. This macro keeps track of the number of
  Join(*,G)s that are received on this Port with different upstream
  neighbors.

  NumETsActive(Port,S,G): Number of (Port,S,G,N) entries that have
  Expiry Timer running. This macro keeps track of the number of
  Join(*,G)s that are received on this Port with different upstream
  neighbors.

 JoinAttributes(*,G): Join attributes [PIM-JOIN-ATTR]

  RpfVectorTlvs(*,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
  present in received Join(*,G) messages. If present, they must be
  copied to JoinAttributes(*,G).

 JoinAttributes(S,G): Join attributes [PIM-JOIN-ATTR] RpfVectorTlvs(*,G).

  RpfVectorTlvs(S,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
  present in received Join(S,G) messages. If present, they must be
  copied to JoinAttributes(S,G). RpfVectorTlvs(S,G).

  Since there are a few differences between the downstream state
  machines of PIM-SM Routers and PIM-SM snooping switches, we specify
  the details of the downstream state machine of PIM-SM snooping
  switches at the risk of repeating most of the text documented in
  [PIM-SM].

3.3.2. Explaination Explanation for per (S,G,N) states

  In PIM Routing protocols, states are built per (S,G). On a router, an
  (S,G) has only one RPF-Neighbor. However, a PIM Snooping switch does
  not have the Layer 3 routing information available to the routers in
  order to determine the RPF-Neighbor for a multicast flow. It merely
  discovers it by snooping the Join/Prune message. A PE could have
  snooped on two or more different Join/Prune messages for the same
  (S,G) that could have carried different Upstream-Neighbor fields.
  This could happen during transient network conditions or due to dual-
  homed sources. A PE cannot make assumptions on which one to pick, but
  instead must facilitate the CE routers decide which Upstream Neighbor
  gets elected the RPF-Neighbor. And for this purpose, the PE will have
  to track downstream and upstream Join/Prune states per (S,G,N).

3.3.3. Receiving (*,G) PIM-SM Join/Prune Messages

  A Join(*,G) or Prune(*,G) is "received" when the port on which it was
  received is not also the port on which the upstream-neighbor N of the
  Join/Prune(*,G) was learnt.

  When a router receives a Join(*,G) or a Prune(*,G) with upstream
  neighbor N, it must process the message as defined in the state
  machine below. Note that the macro computations of the various macros
  resulting from this state machine transition is exactly as specified
  in the PIM-SM RFC [PIM-SM].

  We define the following per-port (*,G,N) macro to help with the state
  machine below.

   Figure 1: Downstream per-port (*,G) state machine in tabular form

+---------------++----------------------------------------+
|               ||              Previous State            |
|               ++------------+--------------+------------+
|   Event       ||NoInfo (NI) | Join (J)     | Prune-Pend |
+---------------++------------+--------------+------------+
| Receive       ||-> J state  | -> J state   | -> J state |
| Join(*,G)     || Action     | Action       | Action     |
|               || RxJoin(N)  | RxJoin(N)    | RxJoin(N)  |
+---------------++------------+--------------+------------+
|Receive        || -          | -> PP state  | -> PP state|
|Prune(*,G) and ||            | Start PPT(N) |            |
|NumETsActive<=1||            |              |            |
+---------------++------------+--------------+------------+
|Receive        || -          | -> J state   | -          |
|Prune(*,G) and ||            | Start PPT(N) |            |
 NumETsActive>1 ||            |              |            |
+---------------++------------+--------------+------------+
|PPT(N) expires || -          | -> J state   | -> NI state|
|               ||            | Action       | Action     |
|               ||            | PPTExpiry(N) |PPTExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires  || -          | -> NI state  | -> NI state|
|and            ||            | Action       | Action     |
|NumETsActive<=1||            | ETExpiry(N)  | ETExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires  || -          | -> J state   | -> NI state|
|and            ||            | Action       | Action     |
|NumETsActive>1 ||            | ETExpiry(N)  | ETExpiry(N)|
+---------------++------------+--------------+------------+

Action RxJoin(N):

  If ET(N) is not already running, then start ET(N). Otherwise restart
  ET(N).
  If N is not already in UpstreamNeighbors(*,G), then add N to
  UpstreamNeighbors(*,G) and trigger a Join(*,G) with upstream neighbor
  N to be forwarded upstream as specified in section 3.3.8. 3.3.8
  Record N as RPF_Neighbor(*,G).
  If there are Join Attributes RPF Vector TLVs in the received (S,G) message and if the
 Join Attributes
  they are different from the recorded JoinAttributes(S,G), RpfVectorTlvs(S,G), then copy
  them into JoinAttributes(S,G). RpfVectorTlvs(S,G). Also trigger a Join(S,G) with upstream
  neighbor N to be forwarded upstream as specified in section 3.3.8. 3.3.8

Action PPTExpiry(N):

  Disable timers ET(N) and PPT(N). If there are no other (Port,*,G)
  states with NumETsActive(Port,*,G) > 0, then trigger a Prune(*,G)
  with upstream neighbor N to be forwarded upstream as specified in
  section 3.3.8. 3.3.8 Then delete N from UpstreamNeighbors(*,G).

  Send a Prune-Echo(*,G) with upstream-neighbor N on the downstream
  port.

Action ETExpiry(N):

  Disable timers ET(N) and PPT(N). If there are no other (Port,*,G)
  states with NumETsActive(Port,*,G) > 0, then trigger a Prune(*,G)
  with upstream neighbor N to be forwarded upstream as specified in
  section 3.3.8. 3.3.8 Then delete N from UpstreamNeighbors(*,G).

3.3.4. Receiving (S,G) PIM-SM Join/Prune Messages

  A Join(S,G) or Prune(S,G) is "received" when the port on which it was
  received is not also the port on which the upstream-neighbor N of the
  Join/Prune(S,G) was learnt.

  When a router receives a Join(S,G) or a Prune(S,G) with upstream
  neighbor N, it must process the message as defined in the state
  machine below. Note that the macro computations of the various macros
  resulting from this state machine transition is exactly as specified
  in the PIM-SM RFC [PIM-SM].

   Figure 2: Downstream per-port (S,G) state machine in tabular form

+---------------++----------------------------------------+
|               ||              Previous State            |
|               ++------------+--------------+------------+
|   Event       ||NoInfo (NI) | Join (J)     | Prune-Pend |
+---------------++------------+--------------+------------+
| Receive       ||-> J state  | -> J state   | -> J state |
| Join(S,G)     || Action     | Action       | Action     |
|               || RxJoin(N)  | RxJoin(N)    | RxJoin(N)  |
+---------------++------------+--------------+------------+
|Receive        || -          | -> PP state  | -> PP state|
|Prune (S,G) and||            | Start PPT(N) |            |
|NumETsActive<=1||            |              |            |
+---------------++------------+--------------+------------+
|Receive        || -          | -> J state   | -          |
|Prune(S,G) and ||            | Start PPT(N) |            |
 NumETsActive>1 ||            |              |            |
+---------------++------------+--------------+------------+
|PPT(N) expires || -          | -> J state   | -> NI state|
|               ||            | Action       | Action     |
|               ||            | PPTExpiry(N) |PPTExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires  || -          | -> NI state  | -> NI state|
|and            ||            | Action       | Action     |
|NumETsActive<=1||            | ETExpiry(N)  | ETExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires  || -          | -> J state   | -> NI state|
|and            ||            | Action       | Action     |
|NumETsActive>1 ||            | ETExpiry(N)  | ETExpiry(N)|
+---------------++------------+--------------+------------+

Action RxJoin(N):

  If ET(N) is not already running, then start ET(N). Otherwise, restart
  ET(N).

  If N is not already in UpstreamNeighbors(S,G), then add N to
  UpstreamNeighbors(S,G) and trigger a Join(S,G) with upstream neighbor
  N to be forwarded upstream as specified in section 3.3.8. 3.3.8
  Record N as RPF_Neighbor(S,G).
  If there are Join Attributes RPF Vector TLVs in the received (S,G) message and if the
 Join Attributes
  they are different from the recorded JoinAttributes(S,G), RpfVectorTlvs(S,G), then copy
  them into JoinAttributes(S,G). RpfVectorTlvs(S,G). Also trigger a Join(S,G) with upstream
  neighbor N to be forwarded upstream as specified in section 3.3.8. 3.3.8

Action PPTExpiry(N):

  Disable timers ET(N) and PPT(N). If there are no other (Port,S,G)
  states with NumETsActive(Port,S,G) > 0, then trigger a Prune(S,G)
  with upstream neighbor N to be forwarded upstream as specified in
  section 3.3.8. Then 3.3.8Then delete N from UpstreamNeighbors(S,G).

  Send a Prune-Echo(S,G) with upstream-neighbor N on the downstream
  port.

Action ETExpiry(N):

  Disable timers ET(N) and PPT(N). If there are no other (Port,S,G)
  states with NumETsActive(Port,S,G) > 0, then trigger a Prune(S,G)
  with upstream neighbor N to be forwarded upstream as specified in
  section 3.3.8. 3.3.8 Then delete N from UpstreamNeighbors(S,G).

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

A Join(S,G,rpt) or Prune(S,G,rpt) is "received" when the port on which
it was received is not also the port on which the upstream-neighbor N
of the Join/Prune(S,G,rpt) was learnt.

While it is important to ensure that the (S,G) and (*,G) state machines
allow for handling per (S,G,N) states, it is not as important for
(S,G,rpt) states. It suffices to say that the downstream (S,G,rpt)
state machine is the same as what is defined in section 4.5.4 of the
PIM-SM RFC [PIM-SM].

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

  A PIM Snooping Proxy PE MUST implement the Upstream (*,G) state machine for
  which the procedures are similar to what is defined in section 4.5.6
  of [PIM-SM]. Section 3.3.8. of 3.3.8of this draft specifies how the message
  should be sent.

  For the purposes of the Upstream (*,G) state machine, a Join(*,G) or
  Prune(*,G) message with upstream neighbor N is "seen" on a PIM
  Snooping switch if the port on which the message was received is also
  the port on which the upstream neighbor N was learnt.

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

  A PIM Snooping Proxy PE MUST implement the Upstream (S,G) state machine for
  which the procedures are similar to what is defined in section 4.5.6
  of [PIM-SM]. Section 3.3.8. of 3.3.8of this draft specifies how the message
  should be sent.

  For the purposes of the Upstream (S,G) state machine, a Join(*,G) or
  Prune(*,G) message with upstream neighbor N is "seen" on a PIM
  Snooping switch if the port on which the message was received is also
  the port on which the upstream neighbor N was learnt.

3.3.8. Sending PIM Join/Prune message upstream.

  Sending of PIM Join/Prune messages upstream is only required on a PIM
  Proxy Switch and not on a PIM Snooping Switch. This section applies
  only to a PIM Proxy Switch.

  The downstream Join/Prune state machines above describe when PIM
  Join/Prune packets must be forwarded upstream and with what upstream
  neighbor field. In order to correctly facilitate assert among the CE
  routers, such Join/Prunes need to sent not only towards the upstream
  neighbor, but also on certain PWs as described below. It is important
  to note that Join/Prune packets are sent to a subset of the ports in
  UpstreamPorts(G) and is not simply flooded to all PWs.
  If JoinAttributes(*,G) RpfVectorTlvs(*,G) is not empty, then it must be encoded in a
  Join(*,G) message sent upstream.

  If JoinAttributes(S,G) RpfVectorTlvs(S,G) is not empty, then it must be encoded in a
  Join(S,G) message sent upstream.

 If the

3.3.8.1.  Sending Triggered vs Refresh Join/Prune message messages

  A Join is a refresh join if it is being sent out as a result of upstream
  join timer expiry. If the join is being sent because there was a refresh Join(*,G)
 message, then send
  change in the refresh Join(*,G) downstream join/prune state machine, then it is a
  triggered join.

  If the Join/Prune message being sent out is a refresh Join(*,G)
  message, then send the refresh Join(*,G) on all ports in
  UpstreamPorts(G). The Upstream Neighbor field should be the recorded
  RPF_Neighbor(*,G).
  If the Join/Prune message being sent out is a refresh Join(S,G)
  message, then send the refresh Join(S,G) on all ports in
  UpstreamPorts(G). The Upstream Neighbor field should be the recorded
  RPF_Neighbor(S,G).

  If the Join/Prune message being sent out is a triggered Join/Prune
  message (due to an event in the downstream Join/Prune state machine),
  then the following rules apply. These rules apply to both (S,G) and
  (*,G) Join/Prune messages to be sent out:

       - The upstream neighbor field N in the Join/Prune to be sent is
          dictated by the downstream Join/Prune state machine
          transition.
       - If the downstream Join/Prune event was on an AC port, then
          send the upstream Join/Prune message to all PWs in
          UpstreamPorts(G). Send the Join/Prune message to Rport(N)
          also.
       - If the downstream Join/Prune event was on a PW port and if
          Rport(N) is a PW, then silently discard the Join/Prune
          message without sending it. If Rport(N) is an AC, then send
          the Join/Prune message on that AC.

 The source IP address in PIM packets sent upstream SHOULD be the
 address of a PIM neighbor in the VPLS. The address picked MUST NOT be
 the upstream neighbor field to be encoded in the packet. The layer 2
 encapsulation for the selected source IP address MUST be the
 encapsulation recorded in the PIM Neighbor database for that IP
 address.

3.3.9. Triggering ASSERT Election in PIM-SM

  In PIM-SM, there are scenarios where multiple routers could be
  forwarding the same multicast traffic on a LAN. When this happens,
  using PIM Assert Election process by sending PIM Assert Messages,
  routers ensure that only the Assert Winner forwards traffic on the
  LAN. In a typical LAN, the Assert Election is a data driven event and
  happens only if a router sees traffic on the interface to which it
  should be forwarding the traffic. Therefore, in the case of VPLS, in
  order to trigger Assert Election and stop duplicate traffic, it is
  necessary that two routers that are forwarding duplicate traffic for
  an (S,G)/(*,G) see each other's traffic.

  PIM Snooping switches must hence ensure that they not only forward
  multicast traffic for an (S,G) on the ports on which they snooped
  Joins(S,G)/Joins(*,G), but also on the ports on which such Joins were
  forwarded (i.e. towards the upstream neighbor(s)). Traffic should not
 be forwarded on the port on which it was received. So if two or more
  Joins(S,G) each carrying a different upstream neighbor field were
  snooped at a PE, then the ports on which such Joins were snooped
  along with the ports on which the upstream neighbors were learnt must
  be added to the outgoing port list.

  The above logic needs to be facilitated without breaking VPLS Split
  Horizon Rules dictate that Rules. i.e. traffic should not be forwarded on the port on
  which it was received. And traffic arriving on a PW MUST NOT be
  forwarded onto other PW(s). Let us consider The rules specified above in calculating
  the outgoing port list ensures this.

  An example network scenario is discussed in
 Figure 3: So at a downstream PE (say PE1), if a Join(S,G) Appendix A with
 Upstream Neighbor N1 was sent on one PW (say PW12) possible
  ASSERT Election scenarios.

3.4. Bidirectional-PIM (PIM-BIDIR)

  PIM-BIDIR is a variation of PIM-SM.  The main differences between
  PIM-SM and another
 Join(S,G) with Upstream Neighbor N2 was sent on another PW (say
 PW13), then duplicate traffic will arrive on both PW12 Bidirectional-PIM are as follows:

       - There are no source-based trees, and PW13. Due
 to VPLS Split Horizon Rules, source-specific
          multicast is not supported (i.e., no (S,G) states) in PIM-
          BIDIR.
       - Multicast traffic from PW12 cannot be forwarded
 onto PW13 and vice-versa. However, as long as can flow up the upstream PEs (PE2
 and PE3) also snoop shared tree in PIM-BIDIR.
       - To avoid forwarding loops, one router on each link is elected
          as the Joins/Prunes, then per the rules in the
 previous paragraph, they will add the PW towards both upstream
 neighbors N1 and N2 to the outgoing port list.

 Let us consider the scenario in Figure 3.

         Figure 3: An Example Scenario Designated Forwarder (DF) for Triggering Assert

                                     +------+ AC3 +------+
                                     |  PE2 |-----| CE3  |
                                    /|      |     |      |
                                   / +------+     +------+
                                  /     |             |
                                 /      |             |
                                /PW12   |             |
                               /        |          +-----+
                              /         |PW23      |  S  |
                             /          |          +-----+
                            /           |             |
                           /            |             |
                          /             |             |
   +------+     +------+ /           +------+     +------+
   | CE1  |     |  PE1 |/   PW13     |  PE3 |     | CE4  |
   |      |-----|      |-------------|      |-----|      |
   +------+ AC1 +------+             +------+ AC4 +------+
                    |
                    |AC2
                +------+
                | CE2  |
                |      |
                +------+

 In the scenario depicted each RP in Figure 3, both CE1 and CE2 have two ECMP
 routes to reach the source "S".  Hence, CE1 may pick CE3 as its next
 hop ("Upstream Neighbor"), and CE2 may pick CE4 as its next hop.  As
 a result, both CE1 and CE2 will receive duplicate traffic for a
 moment. If Assert procedures are not invoked by CE3 and CE4, the
 duplicate traffic on the LAN will persist forever. Following PIM-BIDIR.

  The main advantage of PIM-BIDIR is that it scales well for many-to-
  many applications.  However, the
 sequence lack of events source-based trees means
  that illustrates how duplicate multicast traffic is
 resolved. Note that this illustration assumes PIM Snooping in the
 VPLS with Join Suppression disabled forced to remain on the CEs. Procedures for PIM
 Proxy shared tree.

  The procedures to discover PIM-SM routers in a VPLS instance are slightly different and will be covered as
  described in section 3.6.

   1. CE1 sends a Join(S,G) with N=CE3.
   2. When PE1 snoops on the Join(S,G), it adds CE3 to its
      UpstreamNeighbors(S,G). It also adds AC1 and PW12 3.2 For PIM-BIDIR to its
      OutgoingPortList(S,G). UpstreamNeighbors(S,G) on PE1 =
      {CE3}. OutgoingPortList(S,G) on PE1 = {AC1, PW12}.
   3. PE1 floods the Join(S,G) in work properly, all routers
  within the VPLS. If using LDP (as
      explained in [VPLS-MCAST-LDP]), PE1 also sends domain must know the Join(S,G)
      via LDP on PW12 (the PW towards UpstreamNeighbors(S,G)).

   4. When PE2 receives address of the Join(S,G), it adds CE3 to its
      UpstreamNeighbors(S,G) and it also adds PW12 and AC3 to its
      OutgoingPortList(S,G). UpstreamNeighbors(S,G) on PE2 =
      {CE3}. OutgoingPortList(S,G) on PE2 = {PW12, AC3}. RP. During RP
  discovery time, PIM routers elect DF per subnet for each RP. The above
  algorithm to elect the DF is as follows: all that needs to occur PIM neighbors in most cases where there is no
 assert.

   5. CE2 sends a Join(S,G) with N=CE4.
   6. This results in PE1 adding CE4
  subnet advertise their unicast route to its
      UpstreamNeighbors(S,G). It also adds AC2 elect the RP and PW13 to its
      OutgoingPortList(S,G). UpstreamNeighbors(S,G) at PE1 =
      {CE3, CE4}. OutgoingPortList(S,G) on PE2 = {AC1, AC2, PW12,
      PW13}.
   7. PE1 floods the join in router
  with the VPLS. If using LDP, best route is elected.

  Snooping for PIM-BIDIR is much simpler than it is for PIM-SM. The
  complexity resulting from various combinations of (S,G), (*,G), IGMP
  and assert states makes PIM-SM procedures fairly complex. PIM-BIDIR
  has none of those issues since the
      Join was received PIM-BIDIR builds only (*,G) states and
  all routers on an AC AND since a neighbor was added
      to UpstreamNeighbors(S,G), it sends a Join(S,G) via LDP LAN agree on who the PWs towards UpstreamNeighbors(S,G) {PW12, PW13}.
   8. PE2 receives upstream neighbor, i.e. DF(RP)
  is. So the Join(S,G) and adds CE4 to
      UpstreamNeighbors(S,G). It also adds PW23 to its
      OutgoingPortList(S,G). UpstreamNeighbors(S,G) = {CE3, CE4}.
      OutgoingPortList(S,G) on PE2 = {PW12, AC3, PW23}.
   9. PE3 receives the Join(S,G) too. It adds CE4 to its
      UpstreamNeighbors(S,G). And it adds PW13 and AC4 to its
      OutgoingPortList(S,G). UpstreamNeighbors(S,G) = {CE4}.
      OutgoingPortList(S,G) on PE3 = {PW13, AC4}.

 So even before duplicate traffic starts flowing, the Outgoing
 interface list snooping procedures for PIM-BIDIR is very much like that
  on the PEs are (i.e., the forwarding plane):

 PE1: {AC1, AC2, PW12, PW13}
 PE2: {PW12, AC3, PW23}
 PE3: {PW13, AC4}

 By building such a forwarding state when Joins are processed, there
 needs to be no additional action taken by the PIM-BIDIR router [PIM-BIDIR].

3.4.1. Building PIM-BIDIR Snooping States

   The PEs when duplicate
 traffic is received. Traffic arriving from CE3 will be forwarded to
 CE4 thus allowing for CE3 and CE4 to transparently trigger assert
 election. This makes the MUST snoop on PIM Snooping completely a control-plane
 protocol without any data-plane interaction. When the assert election
 is complete, if CE3 becomes the assert winner, then

   10.  CE2 sends a Prune(S,G) with N=CE4 Hello and a Join(S,G) with
      N=CE3.
   11.  When PE1 snoops the Prune(S,G), it removes CE4 from its
      UpstreamNeighbors(S,G). It also removes AC2 PIM-BIDIR Join/Prune packets and PW13 from
      the OutgoingPortList(S,G).
   build states as described in [PIM-BIDIR]. The Join(S,G) will result PEs SHOULD simply
   flood all other PIM packet types without examining them.

   PIM Proxy Rules specified in AC2
      added back section 2.5can be applied to OutgoingPortList(S,G). OutgoingPortList(S,G)
      on PE1 = {AC1, AC2, PW12}.
   12.  PE1 floods the messages in the VPLS. If LDP PIM-BIDR
   also. Only additional requirement is used,
      since that if the Prune was received on an AC and since Upstream Port of a neighbor
      was removed from UpstreamNeighbors(S,G),
   PIM-BIDIR group is a PW, then the Prune(S,G)
      will proxy PIM Join/Prune packet MUST
   be sent on the PWs towards UpstreamNeighbors(S,G)
      {PW12, PW13}. all PWs.

3.5. PIM-DM

  The Join(S,G) need not be sent characteristics of PIM-DM is flood and prune behavior.  Shortest
  path trees are built as a multicast source starts transmitting.

  The procedures to {PW12}
      since the CE3 was already discover PIM-DM routers are as explained in UpstreamNeighbors(S,G).
   13.  PE2 receives section
  3.2

3.5.1. Building PIM-DM Snooping States

  PIM-DM Snooping states are built by snooping on the Prune directed to CE4. As a result, PE2
      removes CE4 from its UpstreamNeighbors(S,G). It also
      removes PW23 from its OutgoingPortList(S,G).
      UpstreamNeighbors(S,G) PIM-DM Join,
  Prune, Graft and State Refresh messages received on PE2 = {CE3}.
      OutgoingPortList(S,G) AC/PWs and State-
  Refresh Messages sent on PE2 = {PW12, AC3}.
   14.  PE3 receives the Prune too. As AC/PWs. By snooping on these PIM-DM
  messages, a result, PE3 removes CE4
      from its UpstreamNeighbors(S,G). It also removes PW13 and
      AC4 from its OutgoingPortList(S,G). So, PE3 purges state
      for that (S,G).

 After assert election, the forwarding state should be:

 PE1: {AC1, PW12}
 PE2: {PW12, AC2}
 PE3: {}

 Other more complex duplicate traffic scenarios can exist due to PE builds the
 existence of (S,G) and/or (*,G) states and/or IGMP receiver following states in
 a VPLS. The inheritance rules in PIM-SM and the rules specified in
 this draft should ensure that assert per (S,G,N) where S is triggered among the CEs in
 all scenarios.

3.4. Bidirectional-PIM (PIM-BIDIR)
 PIM-BIDIR is a variation
  address of PIM-SM.  The main differences between
 PIM-SM and Bidirectional-PIM are as follows:
      - There are no source-based trees, and source-specific the multicast source, G is not supported (i.e., no (S,G) states) in PIM-
        BIDIR.
      - Multicast traffic can flow up the shared tree in PIM-BIDIR.
      - To avoid forwarding loops, one router on each link Group address and N is elected
        as the Designated Forwarder (DF) for each RP in PIM-BIDIR.

 The main advantage of PIM-BIDIR is that it scales well for many-to-
 many applications.  However, the lack of source-based trees means
 that multicast traffic is forced
  upstream neighbor to remain on which Prunes/Grafts are sent by downstream CEs:

  Per PIM (S,G,N):

      Per Port PIM (S,G,N) Prune State:
       - DownstreamPState(S,G,N,Port): One of {"NoInfo" (NI), "Pruned"
          (P), "PrunePending" (PP)}
       - Prune Pending Timer (PPT)
       - Prune Timer (PT)
       - Upstream Port (valid if the shared tree. PIM(S,G,N) Prune State is
          "Pruned").

3.5.2.     PIM-DM Downstream Per-Port PIM(S,G,N) State Machine

  The procedures to discover PIM-SM routers in a VPLS instance are downstream per-port PIM(S,G,N) state machine is as
 described defined in
  section 3.2.  For PIM-BIDIR 4.4.2 of [PIM-DM] with a few changes relevant to work properly, all
 routers within the domain must know PIM
  Snooping. When reading section 4.4.2 of [PIM-DM] for the address purposes of
  PIM-Snooping please be aware that the RP. During RP
 discovery time, PIM routers elect DF downstream states are built per subnet for each RP. The
 algorithm to elect the DF is
  (S, G, N, Downstream-Port} in PIM-Snooping and not per {Downstream-
  Interface, S, G} as follows: all PIM neighbors in a
 subnet advertise their unicast route to elect the RP and PIM-DM router. As noted in the router
 with previous
  section 3.5.1, the best route is elected.

 Snooping for PIM-BIDIR is much simpler than it is for PIM-SM. The
 complexity resulting from various combinations of (S,G), (*,G), IGMP
 and assert states makes PIM-SM procedures fairly complex. PIM-BIDIR
 has none of those issues since PIM-BIDIR builds only (*,G) states (DownstreamPState) and
 all routers on a LAN agree on who the upstream neighbor, i.e. DF(RP)
 is. So the snooping procedures for PIM-BIDIR is very much like that
 on a PIM-BIDIR router [PIM-BIDIR].

3.4.1. Building PIM-BIDIR Snooping States
  The PEs MUST snoop on PIM-BIDIR Join/Prune messages timers (PPT and build states
  as described in [PIM-BIDIR]. The PEs SHOULD simply flood all other
  PIM packet types. Since there PT)
  are no special procedures required for
  PIM-BIDIR snooping, we simply refer the reader to the [PIM-BIDIR]
  draft.

3.5. per (S,G,N,P).

3.5.3. Triggering ASSERT election in PIM-DM
 The characteristics of

  Since PIM-DM is flood and prune behavior.  Shortest
 path trees are built as a multicast source starts transmitting.

 The procedures flood-and-prune protocol, traffic is flooded to discover PIM-DM all
  routers are as explained in section
 3.2.

3.5.1. Building PIM-DM Snooping States unless explicitly pruned. Since PIM-DM Snooping states are built by snooping routers do not prune
  on non-RPF interfaces, PEs should typically not receive Prunes on
  Rport(RPF-neighbor). So the PIM-DM Join,
 Prune, Graft and State Refresh messages asserting routers should typically be in
  pim_oiflist(S,G). In most cases, assert election should occur
  naturally without any special handling since data traffic will be
  forwarded to the asserting routers.

  However, there are some scenarios where a prune might be received on AC/PWs and State-
 Refresh Messages sent on AC/PWs. By snooping on these PIM-DM
 messages,
  a PE builds the following states per (S,G,N) where S is the
 address of the multicast source, G port which is also an upstream port (UP). If we prune the Group address and N is port from
  pim_oiflist(S,G), then it would not be possible for the
 upstream neighbor asserting
  routers to which Prunes/Grafts are sent by downstream CEs:

 Per PIM (S,G,N):

     Per Port PIM (S,G,N) Prune State:
      - DownstreamPState(S,G,N,Port): One of {"NoInfo" (NI), "Pruned"
        (P), "PrunePending" (PP)}
      - Prune Pending Timer (PPT)
      - Prune Timer (PT)
      - Upstream Port (valid if the PIM(S,G,N) Prune State is
        "Pruned").

3.5.2.     PIM-DM Downstream Per-Port PIM(S,G,N) State Machine

 The downstream per-port PIM(S,G,N) state machine is as defined in
 section 4.4.2 of [PIM-DM] with a few changes relevant to PIM
 Snooping. When reading section 4.4.2 of [PIM-DM] for the purposes of
 PIM-Snooping please be aware that the downstream states are built per
 (S, G, N, Downstream-Port} in PIM-Snooping and not per {Downstream-
 Interface, S, G} as in a PIM-DM router. As noted in the previous
 section 3.5.1. , the states (DownstreamPState) and timers (PPT and
 PT) are per (S,G,N,P).

3.5.3. Triggering ASSERT election in PIM-DM

 Since PIM-DM is a flood-and-prune protocol, traffic is flooded to all
 routers unless explicitly pruned. Since PIM-DM routers do not prune
 on non-RPF interfaces, PEs should typically not receive Prunes on
 Rport(RPF-neighbor). So the asserting routers should typically be in
 pim_oiflist(S,G). In most cases, assert election should occur
 naturally without any special handling since data traffic will be
 forwarded to the asserting routers.

 However, there are some scenarios where a prune might be received on
 a port which is also an upstream port (UP). If we prune the port from
 pim_oiflist(S,G), then it would not be possible for the asserting
 routers to determine if traffic arrived on their determine if traffic arrived on their downstream port.
  This can be fixed by adding pim_iifs(S,G) to pim_oiflist(S,G) so that
  data traffic flows to the UP ports.

3.6. PIM Proxy

  As noted earlier in section 2.4. , 2.5, PIM Snooping will work correctly
  only if Join Suppression is disabled in the VPLS. If Join Suppression
  is enabled in the VPLS, then PEs MUST do PIM Proxy for VPLS Multicast
  to work correctly.

  A PIM Proxy switch behaves like a PIM Router by doing most of the
  functionality of a PIM Router. The complexity however is much lesser
  on a switch since many of the issues that a PIM Router has to deal
  with are not relevant on a switch. A PIM Router needs to be able to
  build and maintain RP-Sets. They also have to deal with the Register
  and Assert State Machines. There are other complexities for a PIM
  Router resulting from inter-domain multicast. A PIM Snooping or PIM
  Proxy switch can be agnostic of all of this. All that a PIM Proxy
  switch cares about is building multicast states using PIM Hellos and
  PIM Join/Prune message. As such it's complexity is greatly reduced.

  Other than the procedures defined here, the rest of the procedures
  that apply to PIM Snooping apply to PIM Proxy as well.

3.6.1. Downstream PIM Proxy behavior

  A PIM-SM or PIM-BIDIR Proxy PE is interested in the Hello and
  Join/Prune messages. The proposed PIM Proxy solution for PIM-SM and
  PIM-BIDIR is to proxy only Join/Prune messages. PIM Proxy for PIM-DM
  is for future study.

  PIM Hellos MUST be snooped while being flooded in the VPLS. i.e. PIM
  Hellos MUST NOT be consumed at a PE and regenerated.

  PIM Join/Prune messages arriving at an AC MUST be consumed. If [VPLS-
  MCAST-LDP] is not used to distribute multicast states among the PEs,
  then PIM Join/Prune messages arriving at a PW MUST also be consumed.

  All other PIM packet types are flooded in the VPLS without needing
  observation.

  Performing only proxy of Join/Prune messages keeps the switch
  behavior very similar to that of a PIM router without introducing too
  much additional complexity. It keeps the PIM Proxy solution fairly
  simple. Since Join/Prunes are forwarded by a PE along the slow-path
  and all other PIM packet types are forwarded along the fast-path, it
  is very likely that packets forwarded along the fast-path will arrive
  "ahead" of Join/Prune packets at a CE router (note the stress on the
  fact that fast-path messages will never arrive after Join/Prunes). Of
  particular importance are Hello packets sent along the fast-path. We
  can construct a variety of scenarios resulting in out of order
  delivery of Hellos and Join/Prune messages. However, there should be
  no deviation from normal expected behavior observed at the CE router
  receiving these messages out of order.

  The other option for a PIM Proxy solution is to proxy both Hello and
  Join/Prune messages that a PE is interested in building states for.
  If Hellos are being proxied, then it becomes necessary that the PE
  proxy all other PIM packet types also. Because if Hellos are received
  after other packet types are received at a CE router, then bad things
  will happen. That means every PIM packet has to be sent along the
  slow-path. This greatly increases the complexity on the CE router, it
  is very compute intensive and does not scale well. Also, proxying
  Hellos will result in added latency to delivery of Hello messages to
  a CE and that affects multicast convergence in the VPLS.

3.6.2. Upstream PIM Proxy behavior

  Since a PIM Proxy switch consumes Join/Prune messages, it must also
  originate PIM Join/Prune messages to be sent upstream. If [VPLS-
  MCAST-LDP] is employed, then triggered Join/Prune messages are sent
  via LDP to forward PIM Join/Prunes on PWs. Join/Prune messages need
  not be refreshed on PWs when [VPLS-MCAST-LDP] is employed. On ACs,
  both triggered and refresh Join/Prunes are forwarded as PIM packets.

3.6.3. Source IP Address in Proxy PIM Join/Prune Packets

  The source IP address in PIM packets sent upstream SHOULD be the
  address of a PIM neighbor in the VPLS. The address picked MUST NOT be
  the upstream neighbor field to be encoded in the packet. The layer 2
  encapsulation for the selected source IP address MUST be the
  encapsulation recorded in the PIM Neighbor database for that IP
  address.

  If Explicit Tracking (ET) is disabled in the VPLS, then it does not
  matter what Source IP Address is picked in the packets sent upstream
  as long as we adhere to the rule in the previous paragraph. However, if
 Explicit Tracking

  If ET is enabled in the VPLS, then we cannot do this. In
 fact, enabled, it would be wrong to even enable PIM Proxy in the VPLS since
 the means that a CE routers may wish to receive the Join/Prunes from every router is interested in tracking
  every CE router. that wishes to join a stream. If a PE determines that Explicit Tracking ET is
 enabled in the VPLS,
  enabled, then it MUST disable PIM Proxy and SHOULD follow use PIM Snooping procedures instead. instead of PIM
  Proxy.

3.7. Directly Connected Multicast Source

  If there is a source in the CE network that connects directly into
  the VPLS instance, then multicast traffic from that source MUST be
  sent to all PIM routers on the VPLS instance apart from the igmp
  receivers in the VPLS.  If there is already (S,G) or (*,G) snooping
  state that is formed on any PE, this will not happen per the current
  forwarding rules and guidelines.  So, in order to determine if
  traffic needs to be flooded to all routers, a PE must be able to
  determine if the traffic came from a host on that LAN.  There are
  three ways to address this problem:

       - The PE would have to do ARP snooping to determine if a source
          is directly connected.
       - Another option is to have configuration on all PEs to say
          there are CE sources that are directly connected to the VPLS
          instance and disallow snooping for the groups for which the
          source is going to send traffic. This way traffic from that
          source to those groups will always be flooded within the
          provider network.
       - A third option is to require that sources of CE multicast
          routers must appear behind a router.

3.8. Data Forwarding Rules

  First we define the rules that are common to PIM-SM, PIM-BIDIR and
  PIM-DM PEs. Forwarding rules for each protocol type is specified in
  the sub-sections.

  If there is no matching forwarding state, then the PE MAY either
  discard the packet or send it towards all the snooped PIM CE routers
  or to a configured set of ports. How this is determined is outside
  the scope of this document.

  The following rules MUST be followed when forwarding multicast
  traffic in a VPLS:

       - Traffic arriving on a port MUST NOT be forwarded back onto
          the same port.
       - Due to VPLS Split-Horizon rules, traffic ingressing on a PW
          MUST NOT be forwarded to any other PW.

3.8.1. PIM-SM Data Forwarding Rules

  Per the rules in [PIM-SM] and per the additional rules specified in
  this document,

  OutgoingPortList(*,G) = inherited_olist(*,G) (+) UpstreamPorts(G) UpstreamPorts(*,G)
                         (+) Rport(PimDR)

  OutgoingPortList(S,G) = inherited_olist(S,G) (+) UpstreamPorts(G) UpstreamPorts(S,G)
                          (+) (UpstreamPorts(*,G) (-)
                               UpstreamPorts(S,G,rpt))
                          (+) Rport(PimDR)

  [PIM-SM] specifies how inherited_olist(*,G) and inherited_olist(S,G)
  are built. PimDR is the IP address of the PIM DR in the VPLS.

  The PIM-SM Snooping forwarding rules are defined below in pseudocode:

  BEGIN
     iif is the incoming port of the multicast packet.
     S is the Source IP Address of the multicast packet.
     G is the Destination IP Address of the multicast packet.

     If there is (S,G) state on the PE
     Then
         OutgoingPortList = OutgoingPortList(S,G)
     Else if there is (*,G) state on the PE
     Then
         OutgoingPortList = OutgoingPortList(*,G)
     Else
         OutgoingPortList = UserDefinedPortList
     Endif

     If iif is an AC
     Then
         OutgoingPortList = OutgoingPortList (-) iif
     Else
         ## iif is a PW
         OutgoingPortList = OutgoingPortList (-) PWPorts
     Endif

     Forward the packet to OutgoingPortList.
  END

  First if there is (S,G) state on the PE, then the set of outgoing
  ports is OutgoingPortList(S,G).

  Otherwise if there is (*,G) state on the PE, the set of outgoing
  ports is OutgoingPortList(*,G).

  The packet is forwarded to the selected set of outgoing ports while
  observing the rules above in section 3.8. 3.8

3.8.2. PIM-BIDIR Data Forwarding Rules

  The PIM-BIDIR Snooping forwarding rules are defined below in
  pseudocode:

  BEGIN
     iif is the incoming port of the multicast packet.
     G is the Destination IP Address of the multicast packet.

     If there is forwarding state for G
     Then
         OutgoingPortList = olist(G)
     Else
         OutgoingPortList = UserDefinedPortList
     Endif
     If iif is an AC
     Then
         OutgoingPortList = OutgoingPortList (-) iif
     Else
         ## iif is a PW
         OutgoingPortList = OutgoingPortList (-) PWPorts
     Endif

     Forward the packet to OutgoingPortList.
  END

  If there is forwarding state for G, then forward the packet to
  olist(G) while observing the rules above in section 3.8. 3.8

  [PIM-BIDIR] specifies how olist(G) is contructed.

3.8.3. PIM-DM Data Forwarding Rules

  The PIM-DM Snooping data forwarding rules are defined below in
  pseudocode:

  BEGIN
     iif is the incoming port of the multicast packet.
     S is the Source IP Address of the multicast packet.
     G is the Destination IP Address of the multicast packet.

     If there is (S,G) state on the PE
     Then
         OutgoingPortList = olist(S,G)
     Else
         OutgoingPortList = UserDefinedPortList
     Endif

     If iif is an AC
     Then
         OutgoingPortList = OutgoingPortList (-) iif
     Else
         ## iif is a PW
         OutgoingPortList = OutgoingPortList (-) PWPorts
     Endif

     Forward the packet to OutgoingPortList.
  END

  If there is forwarding state for (S,G), then forward the packet to
  olist(S,G) while observing the rules above in section 3.8. 3.8

  [PIM-DM] specifies how olist(S,G) is contructed.

4. IANA Considerations

  This document does not require any IANA assignments or action.

5. Security Considerations

  Security considerations provided in VPLS solution documents (i.e.,
  [VPLS-LDP] and [VPLS-BGP) apply to this document as well.

6. Acknowledgements

  Many members of the L2VPN and PIM working groups have contributed to
  and provided valuable comments and feedback to this draft, including
  Vach Kompella, Shane Amante, Sunil Khandekar, Rob Nath, Marc Lassere,
  Yuji Kamite, Yiqun Cai, Ali Sajassi, Jayant Kotalwar, Jozef Raets,
  Himanshu Shah (Ciena), Himanshu Shah (Alcatel-Lucent).

7. References

6.1.

7.1. Normative References

   [RFC 2119]       Bradner, S., "Key words for use in RFCs to Indicate
                    Requirement Levels", BCP 14, RFC 2119, March 1997.
   [PIM-DM]         Deering, S., et al. "Protocol Independent Multicast
                    Version 2 - Dense Mode Specification", RFC 3973,
                    January 2005.
   [PIM-SM]         Fenner, W, et al. "Protocol Independent Multicast-
                    Sparse Mode (PIM-SM): Protocol Specification
                    (Revised)", draft-ietf-pim-sm-v2-new-11.txt, April
                 2005. RFC 4601, August 2006.
   [PIM-SSM]        Holbrook, H., et al. "Source-Specific Multicast for
                    IP", work in progress RFC 4607, August 2006
   [PIM-BIDIR]      Handley, M., et al. "Bi-directional Protocol
                    Independent Multicast (BIDIR-PIM)", work in
                    progress
  [PIM-JOIN-ATTR]  Boers, A,
   [RPF-VECTOR]     IJ Wijnands, et al, "Format for using TLVs in PIM
                 messages", draft-ietf-pim-join-attributes-01.txt, "The RPF Vector TLV",
                    draft-ietf-pim-rpf-vector-03, Work in progress

6.2.

7.2. Informative References

   [VPLS-LDP]       Lasserre, M, et al. "Virtual Private LAN Services
                 over MPLS", work in progress
                    using LDP Signaling", RFC 4762, January 2007
   [VPLS-BGP]       Kompella, K, et al. "Virtual Private LAN Service",
                 work in progress Service
                    using BGP for Auto-Discovery and Signaling", RFC
                    4761, January 2007
   [IGMP-SNOOP]     Christensen, M., et al. "Considerations for IGMP
   [VPLS-MCAST-REQ] Kamite, Y, et al, "Requirements for Multicast
                    Support in Virtual Private LAN Services",
                    draft-ietf-l2vpn-vpls-mcast-reqts-03,
                    Work in Progress
   [VPLS-MCAST-LDP] Qui, R, Serbest, Y, et al, "Using LDP for VPLS
                    Multicast", draft-qiu-serbest-vpls-mcast-ldp-00.txt,
                    Work in progress
   [VPLS-MCAST-BGP] Aggarwal, R, et al, "Propagation of VPLS IP
                    Multicast Group Membership Information", draft-
                    raggarwa-l2vpn-vpls-mcast-ctrl-00.txt, Work in
                    progress
   [VPLS-MCAST-TREES] Aggarwal, R, et al. "Multicast in VPLS",
                    draft-raggarwa-l2vpn-vpls-mcast-01.txt,
                    Work in progress.

Appendix A.    Example Network Scenario

  Let us consider the scenario in Figure 3.

          Figure 3: An Example Scenario for Triggering Assert

                                      +------+ AC3 +------+
                                      |  PE2 |-----| CE3  |
                                     /|      |     |      |
                                    / +------+     +------+
                                   /     |             |
                                  /      |             |
                                 /PW12   |             |
                                /        |          +-----+
                               /         |PW23      |  S  |
                              /          |          +-----+
                             /           |             |
                            /            |             |
                           /             |             |
    +------+     +------+ /           +------+     +------+
    | CE1  |     |  PE1 |/   PW13     |  PE3 |     | CE4  |
    |      |-----|      |-------------|      |-----|      |
    +------+ AC1 +------+             +------+ AC4 +------+
                     |
                     |AC2
                 +------+
                 | CE2  |
                 |      |
                 +------+

  In the scenario depicted in Figure 3, S is the source of a multicast
  stream (S,G). CE1 and CE2 both have two ECMP routes to reach the
  source.

  In the examples below, JT(Port,S,G,N) is the downstream Join Expiry
  Timer on the specified Port for the (S,G) with upstream neighbor N.

Appendix A.1   PIM-Snooping Example

    1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
    2. PE1 snoops on the Join(S,G) while flooding it in the VPLS. PE2
       and PE3 also snoop on the Join(S,G) while flooding it in the
       VPLS.

       The resulting states at the PEs is as follows:

          At PE1:
              JT(AC1,S,G,CE3)        = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

          At PE2:
              JT(PW12,S,G,CE3)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3:    PE3 ignores the Join(S,G) for the following
          reasons:
                 . It does not already have (S,G) state.
                 . The Join(S,G) was received on a PW and the Upstream
                    RPort is also a PW.

    3. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1

    4. Now CE2 sends a Join(S,G) with Upstream Neighbor(S,G) = CE4.
    5. All PEs snoop on the Join(S,G).

       The resulting states at the PEs:

          At PE1:
              JT(AC1,S,G,CE3)        = active
              JT(AC2,S,G,CE4)        = JP_HoldTime.
              UpstreamNeighbors(S,G) = { CE3, CE4 }
              UpstreamPorts(S,G)     = { PW12, PW13 }
              OutgoingPortList(S,G)  = { AC1, PW12, AC2, PW13 }

          At PE2:  Note: Since PE2 already has (S,G) state, it does not
                   ignore the Join(S,G) even though it received the
                   Join(S,G) on a PW and the Upstream Rport is a PW.
              JT(PW12,S,G,CE4)       = JP_HoldTime
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3, CE4 }
              UpstreamPorts(S,G)     = { AC3, PW23 }
              OutgoingPortList(S,G)  = { PW12, AC3, PW23 }

          At PE3:
              JT(PW13,S,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE4 }
              UpstreamPorts(S,G)     = { AC4 }
              OutgoingPortList(S,G)  = { PW13, AC4 }

    6. The multicast stream (S,G) flows into the VPLS from the two CEs
       CE3 and CE4. PE2 forwards the stream received from CE3 to PW23
       and PE3 forwards the stream to AC4. This facilitates the CE
       routers to trigger assert election. Let us say CE3 becomes the
       assert winner.

    7. CE3 sends an Assert message to the VPLS. The PEs flood the
       Assert message without examining it.
    8. CE4 stops sending the multicast stream to the VPLS.
    9. CE2 notices an RPF change due to Assert and sends a Prune(S,G)
       with Upstream Neighbor = CE4. CE2 also sends a Join(S,G) with
       Upstream Neighbor = CE3.
    10.  All the PEs start a prune-pend timer on the ports on which
       they received the Prune(S,G). When the prune-pend timer expires,
       all PEs will remove the downstream (S,G,CE4) states.

       Resulting states at the PEs:

          At PE1:
             JT(AC1,S,G,CE3)        = active
             UpstreamNeighbors(S,G) = { CE3 }
             UpstreamPorts(S,G)     = { PW12 }
             OutgoingPortList(S,G)  = { AC1, AC2, PW12 }

          At PE2:
             JT(PW12,S,G,CE3)       = active
             UpstreamNeighbors(S,G) = { CE3 }
             UpstreamPorts(S,G)     = { AC3 }
             OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3: no (S,G) state.

  Note that at the end of the assert election, there should be no
  duplicate traffic forwarded downstream and traffic should flow only
  on the desired path. Also note that there are no unnecessary (S,G)
  states on PE3 after the assert election.

Appendix A.2   PIM Proxy Example with (S,G) / (*,G) interaction

  In the same network, let us assume CE4 is the Upstream Neighbor
  towards the RP for G.

    1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
    2. PE1 consumes the Join(S,G). PE1 looks up the neighbor database
       and determines CE3 was learnt on PW12. PE1 sends a Proxy
       Join(S,G) to the resulting UpstreamPorts(G). i.e. it sends the
       proxy Join(S,G) on PW12.
    3. Likewise, PE2 consumes the Join(S,G) and sends a proxy Join(S,G)
       on AC3 with Upstream Neighbor = CE3.

       The resulting states at the PEs is as follows:

          At PE1:
              JT(AC1,S,G,CE3)        = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

          At PE2:
              JT(PW12,S,G,CE3)       = JP_HoldTime
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

          At PE3:    PE3 did not receive any PIM Join(S,G). So it has
                     no (S,G) state.

    4. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1.

    5. Now let us say CE1 sends a Join(*,G) towards CE4.
    6. PE1 consumes the Join(*,G). PE1 sends a Proxy Join(*,G) to the
       resulting UpstreamPorts(G). Since UpstreamPorts(G) now has both
       PW12 and PW13, the Join(*,G) gets sent on both PW12 and PW13.
       Note that the UpstreamPorts(S,G) and OutgoingPortList(S,G)
       inherit the corresponding (*,G) sets, but not vice versa.
    7. PE2 and PE3 perform a similar function. PE2 received the
       Join(*,G) on a PW and the Upstream Neighbor is also on a PW.
       Hence PE2 only adds UpstreamPorts(*,G) to OutgoingPortList(*,G)
       and not the downstream port PW12.

          At PE1:
              JT(AC1,S,G,CE3)        = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12, PW13 }
              OutgoingPortList(S,G)  = { AC1, PW12, PW13 }

              JT(AC1,*,G,CE4)        = JP_HoldTime.
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW13 }
              OutgoingPortList(*,G)  = { AC1, PW13 }

              UpstreamPorts(G)       = { PW12, PW13 }

          At PE2:
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { AC3, PW23 }
              OutgoingPortList(S,G)  = { PW12, AC3, PW23 }

              JT(PW12,*,G,CE4) = JP_HoldTime
              UpstreamNeighbors(*,G)      = { CE4 }
              UpstreamPorts(G)       = { PW23 }
              OutgoingPortList(*,G)       = { PW23 }

          At PE3:
              JT(PW13,*,G,CE4) = JP_HoldTime
              UpstreamNeighbors(*,G)      = { CE4 }
              UpstreamPorts(*,G)     = { AC4 }
              OutgoingPortList(*,G)       = { PW13, AC4 }

    8. The above state results in both (S,G) and (*,G) streams to be
       forwarded to AC1. The above state also results in the (S,G)
       stream to be forwarded from CE3 to CE4 resulting in an (S,G)
       assert election. Following the assert election, CE3 becomes the
       (S,G) assert winner. CE4 stops sending (S,G) stream down the
       RPT.
    9. CE1 notices an RPF change due to assert. It sends a
       Prune(S,G,rpt) with Upstream Neighbor = CE4.
    10.  PE1 consumes the Prune(S,G,rpt) and MLD Snooping Switches", work in progress
  [VPLS-MCAST-LDP] Qui, R, Serbest, Y, et al, "Using LDP for VPLS
                 Multicast", draft-qiu-serbest-vpls-mcast-ldp-00.txt,
                 Work forwards the
       Prune(S,G,rpt) to both PW12 and PW13. PE2 consumes the
       Prune(S,G,rpt) and updates its states. PE3 updates its states
       and forwards the Prune(S,G,rpt) on AC4.

          At PE1:
              JT(AC1,S,G,CE3)        = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(S,G)     = { PW12 }
              OutgoingPortList(S,G)  = { AC1, PW12 }

              JT(AC1,*,G,CE4)        = active.
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW13 }
              OutgoingPortList(*,G)  = { AC1, PW13 }

          At PE2:
              JT(PW12,S,G,CE3)       = active
              UpstreamNeighbors(S,G) = { CE3 }
              UpstreamPorts(*,G)     = { AC3 }
              OutgoingPortList(S,G)  = { PW12, AC3 }

              JT(PW12,*,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(*,G)     = { PW23 }
              OutgoingPortList(*,G)  = { PW23 }

          At PE3:
              JT(PW13,*,G,CE4)       = JP_HoldTime
              UpstreamNeighbors(*,G) = { CE4 }
              UpstreamPorts(G)       = { AC4 }
              OutgoingPortList(*,G)  = { PW13, AC4 }

  Even in progress
  [VPLS-MCAST-BGP] Aggarwal, R, et al, "Propagation this example, at the end of VPLS IP
                 Multicast Group Membership Information", draft-
                 raggarwa-l2vpn-vpls-mcast-ctrl-00.txt, Work in
                 progress
  [VPLS-MCAST-TREES] Aggarwal, R, et al. "Multicast the (S,G) / (*,G) assert
  election, there should be no duplicate traffic forwarded downstream
  and traffic should flow only to the desired CEs.

  Other more complex scenarios exist. This draft should addressin PIM-
  SM and the rules specified in VPLS",
                 draft-raggarwa-l2vpn-vpls-mcast-01.txt,
                 Work this draft should ensure that assert is
  triggered among the CEs in progress. all scenarios.

Authors' Addresses

  Venu Hemige
 Alcatel North America
  Alcatel-Lucent
  701 East Middlefield Rd.
  Mountain View, CA 94043
 Venu.hemige@alcatel.com
  Venu.hemige@alcatel-lucent.com

  Yetik Serbest
  AT&T Labs
  9505 Arboretum Blvd.
  Austin, TX 78759
 Yetik_serbest@labs.sbc.com
  Yetik_serbest@labs.att.com

  Ray Qiu
 Alcatel North America
  Alcatel-Lucent
  701 East Middlefield Rd.
  Mountain View, CA 94043
 Ray.Qiu@alcatel.com
  Ray.Qiu@alcatel-lucent.com

  Suresh Boddapati
 Alcatel North America
 701 East Middlefield Rd.
 Mountain View, CA 94043
 Suresh.boddapati@alcatel.com

 Rob Nath
 Riverstone Networks
 5200 Great America Parkway
 Santa Clara, CA 95054
 Rnath@riverstonenet.com
 Sunil Khandekar
 Alcatel North America
  Alcatel-Lucent
  701 East Middlefield Rd.
  Mountain View, CA 94043
 Sunil.khandekar@alcatel.com

 Vach Kompella
 Alcatel North America
 701 East Middlefield Rd.
 Mountain View, CA 94043
 Vach.kompella@alcatel.com

 Marc Lasserre
 Riverstone Networks
 Marc@riverstonenet.com

 Himanshu Shah
 Ciena
 hshah@ciena.com
  Suresh.boddapati@alcatel-lucent.com

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