Network Working Group                                         M. Handley
Request for Comments: 5015                                           UCL
Category: Standards Track                                    I. Kouvelas
                                                             T. Speakman
                                                                   Cisco
                                                             L. Vicisano
                                                        Digital Fountain
                                                            October 2007

Bidirectional Protocol Independent Multicast (BIDIR-PIM)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document discusses Bidirectional PIM (BIDIR-PIM), a variant of
   PIM Sparse-Mode that builds bidirectional shared trees connecting
   multicast sources and receivers.  Bidirectional trees are built using
   a fail-safe Designated Forwarder (DF) election mechanism operating on
   each link of a multicast topology.  With the assistance of the DF,
   multicast data is natively forwarded from sources to the Rendezvous-
   Point (RP) and hence along the shared tree to receivers without
   requiring source-specific state.  The DF election takes place at RP
   discovery time and provides the route to the RP, thus eliminating the
   requirement for data-driven protocol events.

   1. Introduction ....................................................3
   2. Terminology .....................................................4
      2.1. Definitions ................................................4
      2.2. Pseudocode Notation ........................................6
   3. Protocol Specification ..........................................6
      3.1. BIDIR-PIM Protocol State ...................................7
           3.1.1. General Purpose State ...............................8
           3.1.2. RPA State ...........................................8
           3.1.3. Group State .........................................9
           3.1.4. State Summarization Macros .........................10
      3.2. PIM Neighbor Discovery ....................................11
      3.3. Data Packet Forwarding Rules ..............................11
           3.3.1. Upstream Forwarding at RP ..........................12
           3.3.2. Source-Only Branches ...............................12
           3.3.3. Directly Connected Sources .........................13
      3.4. PIM Join/Prune Messages ...................................13
           3.4.1. Receiving (*,G) Join/Prune Messages ................13
           3.4.2. Sending Join/Prune Messages ........................16
      3.5. Designated Forwarder (DF) Election ........................18
           3.5.1. DF Requirements ....................................18
           3.5.2. DF Election Description ............................19
                  3.5.2.1. Bootstrap Election ........................20
                  3.5.2.2. Loser Metric Changes ......................20
                  3.5.2.3. Winner Metric Changes .....................21
                  3.5.2.4. Winner Loses Path .........................22
                  3.5.2.5. Late Router Starting Up ...................22
                  3.5.2.6. Winner Dies ...............................22
           3.5.3. Election Protocol Specification ....................22
                  3.5.3.1. Election State ............................22
                  3.5.3.2. Election Messages .........................23
                  3.5.3.3. Election Events ...........................24
                  3.5.3.4. Election Actions ..........................25
                  3.5.3.5. Election State Transitions ................26
           3.5.4. Election Reliability Enhancements ..................30
           3.5.5. Missing Pass .......................................30
           3.5.6. Periodic Winner Announcement .......................30
      3.6. Timers, Counters, and Constants ...........................31
      3.7. BIDIR-PIM Packet Formats ..................................34
           3.7.1. DF Election Packet Formats .........................34
           3.7.2. Backoff Message ....................................36
           3.7.3. Pass Message .......................................36
           3.7.4. Bidirectional Capable PIM-Hello Option .............37
   4. RP Discovery ...................................................37
   5. Security Considerations ........................................38
      5.1. Attacks Based on Forged Messages ..........................38
           5.1.1. Election of an Incorrect DF ........................38

           5.1.2. Preventing Election Convergence ....................39
      5.2. Non-Cryptographic Authentication Mechanisms ...............39
           5.2.1. Basic Access Control ...............................39
      5.3. Authentication Using IPsec ................................40
      5.4. Denial-of-Service Attacks .................................40
   6. IANA Considerations ............................................40
   7. Acknowledgments ................................................40
   8. Normative References ...........................................40
   9. Informative References .........................................41
List of Figures
   Figure 1. Downstream group per-interface state machine ............15
   Figure 2. Upstream group state machine ............................17
   Figure 3. Designated Forwarder election state machine .............27

1. Introduction

   This document specifies Bidirectional PIM (BIDIR-PIM), a variant of
   PIM Sparse-Mode (PIM-SM) [4] that builds bidirectional shared trees
   connecting multicast sources and receivers.

   PIM-SM constructs unidirectional shared trees that are used to
   forward data from senders to receivers of a multicast group.  PIM-SM
   also allows the construction of source-specific trees, but this
   capability is not related to the protocol described in this document.

   The shared tree for each multicast group is rooted at a multicast
   router called the Rendezvous Point (RP).  Different multicast groups
   can use separate RPs within a PIM domain.

   In unidirectional PIM-SM, there are two possible methods for
   distributing data packets on the shared tree.  These differ in the
   way packets are forwarded from a source to the RP:

   o Initially, when a source starts transmitting, its first hop router
     encapsulates data packets in special control messages (Registers)
     that are unicast to the RP.  After reaching the RP, the packets are
     decapsulated and distributed on the shared tree.

   o A transition from the above distribution mode can be made at a
     later stage.  This is achieved by building source-specific state on
     all routers along the path between the source and the RP.  This
     state is then used to natively forward packets from that source.

   Both of these mechanisms suffer from problems.  Encapsulation results
   in significant processing, bandwidth, and delay overheads.
   Forwarding using source-specific state has additional protocol and
   memory requirements.

   Bidirectional PIM dispenses with both encapsulation and source state
   by allowing packets to be natively forwarded from a source to the RP
   using shared tree state.  In contrast to PIM-SM, this mode of
   forwarding does not require any data-driven events.

   The protocol specification in this document assumes familiarity with
   the PIM-SM specification in [4].  Portions of the BIDIR-PIM protocol
   operation that are identical to that of PIM-SM are only defined by
   reference.

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
   indicate requirement levels for compliant BIDIR-PIM implementations.

2.1. Definitions

   This specification uses a number of terms to refer to the roles of
   routers participating in BIDIR-PIM.  The following terms have special
   significance for BIDIR-PIM:

   Multicast Routing Information Base (MRIB)
      The multicast topology table, which is typically derived from the
      unicast routing table, or routing protocols such as Multiprotocol
      BGP (MBGP) [8] that carry multicast-specific topology information.
      It is used by PIM for establishing the RPF interface (used in the
      forwarding rules).  In PIM-SM, the MRIB is also used to make
      decisions regarding where to forward Join/Prune messages, whereas
      in BIDIR-PIM, it is used as a source for routing metrics for the
      DF election process.

   Rendezvous Point Address (RPA)
      An RPA is an address that is used as the root of the distribution
      tree for a range of multicast groups.  The RPA must be routable
      from all routers in the PIM domain.  The RPA does not need to
      correspond to an address for an interface of a real router.  In
      this respect, BIDIR-PIM differs from PIM-SM, which requires an
      actual router to be configured as the Rendezvous Point (RP).  Join
      messages from receivers for a BIDIR-PIM group propagate hop-by-hop
      towards the RPA.

   Rendezvous Point Link (RPL)
      An RPL for a particular RPA is the physical link to which the RPA
      belongs.  In BIDIR-PIM, all multicast traffic to groups mapping to
      a specific RPA is forwarded on the RPL of that RPA.  The RPL is
      special within a BIDIR-PIM domain as it is the only link on which

      a Designated Forwarder election does not take place (see DF
      definition below).

   Upstream
      Towards the root (RPA) of the tree.  The direction used by packets
      traveling from sources to the RPL.

   Downstream
      Away from the root of the tree.  The direction on which packets
      travel from the RPL to receivers.

   Designated Forwarder (DF)
      The protocol presented in this document is largely based on the
      concept of a Designated Forwarder (DF).  A single DF exists for
      each RPA on every link within a BIDIR-PIM domain (this includes
      both multi-access and point-to-point links).  The only exception
      is the RPL on which no DF exists.  The DF is the router on the
      link with the best route to the RPA (determined by comparing MRIB
      provided metrics).  A DF for a given RPA is in charge of
      forwarding downstream traffic onto its link, and forwarding
      upstream traffic from its link towards the RPL.  It does this for
      all the bidirectional groups that map to the RPA.  The DF on a
      link is also responsible for processing Join messages from
      downstream routers on the link as well as ensuring that packets
      are forwarded to local receivers (discovered through a local
      membership mechanism such as MLD [3] or IGMP [2]).

   RPF Interface
      RPF stands for "Reverse Path Forwarding".  The RPF Interface of a
      router with respect to an address is the interface that the MRIB
      indicates should be used to reach that address.  In the case of a
      BIDIR-PIM multicast group, the RPF interface is determined by
      looking up the RPA in the MRIB.  The RPF information determines
      the interface of the router that would be used to send packets
      towards the RPL for the group.

   RPF Neighbor
      The RPF Neighbor of a router with respect to an address is the
      neighbor that the MRIB indicates should be used to reach that
      address.  Note that in BIDIR-PIM, the RPF neighbor for a group is
      not necessarily the router on the RPF interface that Join messages
      for that group would be directed to (Join messages are only
      directed to the DF on the RPF interface for the group).

   Tree Information Base (TIB)
      This is the collection of state at a PIM router that has been
      created by receiving PIM Join/Prune messages, PIM DF election
      messages, and IGMP or MLD information from local hosts.  It

      essentially stores the state of all multicast distribution trees
      at that router.

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

2.2. Pseudocode Notation

   We use set notation in several places in this specification.

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

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

   NULL
       is the empty set or list.

   In addition, we use C-like syntax:

   =   denotes assignment of a variable.

   ==  denotes a comparison for equality.

   !=  denotes a comparison for inequality.

   Braces { and } are used for grouping.

3. Protocol Specification

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

   o Section 3.1 details the protocol state stored.

   o Section 3.2 defines the BIDIR-PIM extensions to the PIM-SM [4]
     neighbor discovery mechanism.

   o Section 3.3 specifies the data packet forwarding rules.

   o Section 3.4 specifies the BIDIR-PIM Join/Prune generation and
     processing rules.

   o Section 3.5 specifies the Designated Forwarder (DF) election.

   o Section 3.7 specifies the PIM packet formats.

   o Section 3.6 summarizes BIDIR-PIM timers and gives their default
     values.

3.1. BIDIR-PIM Protocol State

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

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

   We divide TIB state into two sections:

   RPA state
      State that maintains the DF election information for each RPA.

   Group state
      State that maintains a group-specific tree for groups that map to
      a given RPA.

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

3.1.1. General Purpose State

   A router holds the following state that is not specific to an RPA or
   group:

      Neighbor State:

         For each neighbor:

            o Neighbor's Gen ID

            o Neighbor liveness timer (NLT)

            o Other information from neighbor's Hello

   For more information on Hello information, look at Section 3.2 as
   well as the PIM-SM specification in [4].

3.1.2. RPA State

   A router maintains a multicast-group to RPA mapping, which is built
   through static configuration or by using an automatic RP discovery
   mechanism like BSR or AUTO-RP (see Section 4).  For each BIDIR-PIM
   RPA, a router holds the following state:

      o RPA (actual address)

      Designated Forwarder (DF) State:

           For each router interface:

           Acting DF information:

              o DF IP Address

              o DF metric

           Election information:

              o Election State

              o DF election-Timer (DFT)

              o Message-Count (MC)

                Current best offer:

                o IP address of best offering router

o Best offering router metric

Designated Forwarder state is described in Section 3.5.

3.1.3. Group State

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

         Group state:

            For each interface:

            Local Membership:

               o State: One of {"NoInfo", "Include"}

            PIM Join/Prune State:

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

               o PrunePendingTimer (PPT)

               o Join/Prune Expiry Timer (ET)

         Not interface specific:

            o Upstream Join/Prune Timer (JT)

            o Last RPA Used

   Local membership is the result of the local membership mechanism
   (such as IGMP [2]) running on that interface.  This information is
   used by the pim_include(*,G) macro described in Section 3.1.4.

   PIM Join/Prune state is the result of receiving PIM (*,G) Join/Prune
   messages on this interface, and is specified in Section 3.4.1.  The
   state is used by the macros that calculate the outgoing interface
   list in Section 3.1.4, and in the JoinDesired(G) macro (defined in
   Section 3.4.2) that is used in deciding whether a Join(*,G) should be
   sent upstream.

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

   The last RPA used must be stored because if the group to RPA mapping
   changes (see RP Set changes in [4]), then state must be torn down and
   rebuilt for groups whose RPA changes.

3.1.4. State Summarization Macros

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

    olist(G) =
       RPF_interface(RPA(G)) (+) joins(G) (+) pim_include(G)

   RPF_interface(RPA) is the interface the MRIB indicates would be used
   to route packets to RPA.  The olist(G) is the list of interfaces on
   which packets to group G must be forwarded.

   The macro pim_include(G) indicates the interfaces to which traffic
   might be forwarded because of hosts that are local members on that
   interface.

    pim_include(G) =
       { all interfaces I such that:
         I_am_DF(RPA(G),I) AND  local_receiver_include(G,I) }

   The clause "I_am_DF(RPA,I)" is TRUE if the router is in the Win or
   Backoff states in the DF election state machine (described in Section
   3.5) for the given RPA on interface I.  Otherwise, it is FALSE.

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

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

   joins(G) =
       { all interfaces I such that
         I_am_DF(RPA(G),I) AND
         DownstreamJPState(G,I) is either Joined or PrunePending }

   DownstreamJPState(G,I) is the state of the finite state machine in
   Section 3.4.1.

   RPF_DF(RPA) is the neighbor that Join messages must be sent to in
   order to build the group shared tree rooted at the RPL for the given
   RPA.  This is the Designated-Forwarder on the RPF_interface(RPA).

3.2. PIM Neighbor Discovery

   PIM routers exchange PIM-Hello messages with their neighboring PIM
   routers.  These messages are used to update the Neighbor State
   described in Section 3.1.  The procedures for generating and
   processing Hello messages as well as maintaining Neighbor State are
   specified in the PIM-SM [4] documentation.

   BIDIR-PIM introduces the Bidirectional Capable PIM-Hello option that
   MUST be included in all Hello messages from a BIDIR-PIM capable
   router.  The Bidirectional Capable option advertises the router's
   ability to participate in the BIDIR-PIM protocol.  The format of the
   Bidirectional Capable option is described in Section 3.7.

   If a BIDIR-PIM router receives a PIM-Hello message that does not
   contain the Bidirectional Capable option from one of its neighbors,
   the error must be logged to the router administrator in a rate-
   limited manner.

3.3. Data Packet Forwarding Rules

   For groups mapping to a given RPA, the following responsibilities are
   uniquely assigned to the DF for that RPA on each link:

   o The DF is the only router that forwards packets traveling
     downstream onto the link.

   o The DF is the only router that picks-up upstream traveling packets
     off the link to forward towards the RPL.

   Non-DF routers on a link, which use that link as their RPF interface
   to reach the RPA, may perform the following forwarding actions for
   bidirectional groups:

   o Forward packets from the link towards downstream receivers.

   o Forward packets from downstream sources onto the link (provided
     they are the DF for the downstream link from which the packet was
     picked-up).

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

      iif is the incoming interface of the packet.
      G is the destination address of the packet (group address).
      RPA is the Rendezvous Point Address for this group.

   First we check to see whether the packet should be accepted based on
   TIB state and the interface that the packet arrived on.  A packet is
   accepted if it arrives on the RPF interface to reach the RPA
   (downstream traveling packet) or if the router is the DF on the
   interface the packet arrives (upstream traveling packet).

   If the packet should be forwarded, we build an outgoing interface
   list for the packet.

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

   On receipt of data to G on interface iif:
    if( iif == RPF_interface(RPA) || I_am_DF(RPA,iif) ) {
       oiflist = olist(G) (-) iif
       forward packet on all interfaces in oiflist
    }

3.3.1. Upstream Forwarding at RP

   When configuring a BIDIR-PIM domain, it is possible to assign the
   Rendezvous Point Address (RPA) such that it does not belong to a
   physical box but instead is simply a routable address.  Routers that
   have interfaces on the RPL that the RPA belongs to will upstream
   forward traffic onto the link.  Joins from receivers in the domain
   will propagate hop-by-hop till they reach one of the routers
   connected to the RPL where they will terminate (as there will be no
   DF elected on the RPL).

   If instead the administrator chooses to configure the RPA to be the
   address of a physical interface of a specific router, then nothing
   changes.  That router must still upstream forward traffic on to the
   RPL and behave no differently than any other router with an interface
   on the RPL.

   To configure a BIDIR-PIM network to operate in a mode similar to that
   of PIM-SM where a single router (the RP) is acting as the root of the
   distribution tree, the RPA can be configured to be the loopback
   interface of a router.

3.3.2. Source-Only Branches

   Source-only branches of the distribution tree for a group G are
   branches that do not lead to any receivers, but that are used to
   forward packets traveling upstream from sources towards the RPL.
   Routers along source-only branches only have the RPF interface to the
   RPA in their olist for G, and hence do not need to maintain any group

   specific state.  Upstream forwarding can be performed using only RPA
   specific state.  An implementation may decide to maintain group state
   for source-only branches for accounting or performance reasons.
   However, doing so requires data-driven events (to discover the groups
   with active sources), thus sacrificing one of the main benefits of
   BIDIR-PIM.

3.3.3. Directly Connected Sources

    A major advantage of using a Designated Forwarder in BIDIR-PIM
   compared to PIM-SM is that special treatment is no longer required
   for sources that are directly connected to a router.  Data from such
   sources does not need to be differentiated from other multicast
   traffic and will automatically be picked up by the DF and forwarded
   upstream.  This removes the need for performing a directly-
   connected-source check for data to groups that do not have existing
   state.

3.4. PIM Join/Prune Messages

   BIDIR-PIM Join/Prune messages are used to construct group-specific
   distribution trees between receivers and the RPL.  Joins are
   originated by last-hop routers that are elected as the DF on an
   interface with directly connected receivers.  The Joins propagate
   hop-by-hop towards the RPA until they reach a router connected to the
   RPL.

   A BIDIR-PIM Join/Prune message consists of a list of Joined and
   Pruned Groups.  When processing a received Join/Prune message, each
   Joined or Pruned Group is effectively considered individually by
   applying the following state machines.  When considering a Join/Prune
   message whose PIM Destination field addresses this router, (*,G)
   Joins and Prunes can affect the downstream state machine.  When
   considering a Join/Prune message whose PIM Destination field
   addresses another router, most Join or Prune entries could affect the
   upstream state machine.

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

   When a router receives a Join(*,G) or Prune(*,G), it MUST first check
   to see whether the RP address in the message matches RPA(G) (the
   router's idea of what the Rendezvous Point Address is).  If the RP
   address in the message does not match RPA(G), the Join or Prune MUST
   be silently dropped.

   If a router has no RPA information for the group (e.g., has not
   recently received a BSR message), then it MAY choose to accept
   Join(*,G) or Prune(*,G) and treat the RP address in the message as

   RPA(G).  If the newly discovered RPA did not previously exist for any
   other group, a DF election has to be initiated.

   Note that a router will process a Join(*,G) targeted to itself even
   if it is not the DF for RP(G) on the interface on which the message
   was received.  This is an optimisation to eliminate the Join delay of
   one Join period (t_periodic) in the case where a new DF processes the
   received Pass and Join messages in reverse order.  The BIDIR-PIM
   forwarding logic will ensure that data packets are not forwarded on
   such an interface while the router is not the DF (unless it is the
   RPF interface towards the RPA).

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

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

      Join (J)
         The interface has (*,G) Join state.  If the router is the DF on
         this interface (I_am_DF(RPA(G),I) is TRUE), the Join state will
         cause us to forward packets destined for G on this interface.

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

   In addition, the state machine uses two timers:

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

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

   Figure 1: Downstream group per-interface state machine in tabular
             form

  +---------------++---------------------------------------------------+
  |               ||                    Prev State                     |
  |Event          ++---------------+-----------------+-----------------+
  |               || NoInfo (NI)   | Join (J)        | PrunePending    |
  |               ||               |                 | (PP)            |
  +---------------++---------------+-----------------+-----------------+
  |               || -> J state    | -> J state      | -> J state      |
  |Receive        || start Expiry  | restart Expiry  | restart Expiry  |
  |Join(*,G)      || Timer         | Timer           | Timer; stop     |
  |               ||               |                 | PrunePending-   |
  |               ||               |                 | Timer           |
  +---------------++---------------+-----------------+-----------------+
  |Receive        || -             | -> PP state     | -> PP state     |
  |Prune(*,G)     ||               | start Prune-    |                 |
  |               ||               | PendingTimer    |                 |
  +---------------++---------------+-----------------+-----------------+
  |PrunePending-  || -             | -               | -> NI state     |
  |Timer Expires  ||               |                 | Send Prune-     |
  |               ||               |                 | Echo(*,G)       |
  +---------------++---------------+-----------------+-----------------+
  |Expiry Timer   || -             | -> NI state     | -> NI state     |
  |Expires        ||               |                 |                 |
  +---------------++---------------+-----------------+-----------------+
  |Stop Being DF  || -             | -> NI state     | -> NI state     |
  |on I           ||               |                 |                 |
  +---------------++---------------+-----------------+-----------------+

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

   On unnumbered interfaces on point-to-point links, the router's
   address should be the same as the source address it chose for the
   Hello packet it sent over that interface.  However, on point-to-point
   links, we also RECOMMEND that PIM messages with a destination address
   of all zeros also be accepted.

   The transition event "Stop Being DF" implies a DF re-election taking
   place on this router interface for RPA(G) and the router changing
   status from being the active DF to being a non-DF router (the value
   of the I_am_DF macro changing to FALSE).

   When ExpiryTimer is started or restarted, it is set to the HoldTime
   from the Join/Prune message that triggered the timer.

   When PrunePendingTimer is started, it is set to the
   J/P_Override_Interval if the router has more than one neighbor on
   that interface; otherwise, it is set to zero causing it to expire
   immediately.

   The action "Send PruneEcho(*,G)" is triggered when the router stops
   forwarding on an interface as a result of a Prune.  A PruneEcho(*,G)
   is simply a Prune(*,G) message sent by the upstream router to itself
   on a LAN.  Its purpose is to add additional reliability so that if a
   Prune that should have been overridden by another router is lost
   locally on the LAN, then the PruneEcho may be received and cause the
   override to happen.  A PruneEcho(*,G) need not be sent when the
   router has only one neighbor on the link.

3.4.2. Sending Join/Prune Messages

   The downstream per-interface state machines described above hold Join
   state from downstream PIM routers.  This state then determines
   whether a router needs to propagate a Join(*,G) upstream towards the
   RPA.  Such Join(*,G) messages are sent on the RPF interface towards
   the RPA and are targeted at the DF on that interface.

   If a router wishes to propagate a Join(*,G) upstream, it must also
   watch for messages on its upstream interface from other routers on
   that subnet, and these may modify its behavior.  If it sees a
   Join(*,G) to the correct upstream neighbor, it should suppress its
   own Join(*,G).  If it sees a Prune(*,G) to the correct upstream
   neighbor, it should be prepared to override that Prune by sending a
   Join(*,G) almost immediately.  Finally, if it sees the Generation ID
   (see PIM-SM specification [4]) of the correct upstream neighbor
   change, it knows that the upstream neighbor has lost state, and it
   should be prepared to refresh the state by sending a Join(*,G) almost
   immediately.

   In addition, changes in the next hop towards the RPA trigger a Prune
   off from the old next hop and join towards the new next hop.  Such a
   change can be caused by the following two events:

      o The MRIB indicates that the RPF Interface towards the RPA has
        changed.  In this case the DF on the new RPF interface becomes
        the new RPF Neighbor.

      o There is a DF re-election on the RPF interface and a new router
        emerges as the DF.

  +--------------------------------------------------------------------+
  |                        In Joined (J) State                         |
  +-----------------------------------+--------------------------------+
  |    Change of RPF_DF(RPA(G))       |       RPF_DF(RPA(G)) GenID     |
  |                                   |       changes                  |
  +-----------------------------------+--------------------------------+
  |    Send Join(*,G) to new          |       Decrease Timer to        |
  |    DF; Send Prune(*,G) to         |       t_override               |
  |    old DF; set Timer to           |                                |
  |    t_periodic                     |                                |
  +-----------------------------------+--------------------------------+

  This state machine uses the following macro:

     bool JoinDesired(G) {
        if (olist(G) (-) RPF_interface(RPA(G))) != NULL
            return TRUE
        else
            return FALSE
     }

3.5. Designated Forwarder (DF) Election

   This section presents a fail-safe mechanism for electing a per-RPA
   designated router on each link in a BIDIR-PIM domain.  We call this
   router the Designated Forwarder (DF).  The DF election does not take
   place on the RPL for an RPA.

3.5.1. DF Requirements

   The DF election chooses the best router on a link to assume
   responsibility for forwarding traffic between the RPL and the link
   for the range of multicast groups served by the RPA.  Different
   multicast groups that share a common RPA share the same upstream
   direction.  Hence, the election of an upstream forwarder on each link
   does not have to be a group-specific decision but instead can be
   RPA-specific.  As the number of RPAs is typically small, the number
   of elections that have to be performed is significantly reduced by
   this observation.

   To optimise tree creation, it is desirable that the winner of the
   election process should be the router on the link with the "best"
   unicast routing metric (as reported by the MRIB) to reach the RPA.
   When comparing metrics from different unicast routing protocols, we
   use the same comparison rules used by the PIM-SM assert process [4].

   The election process needs to take place when information on a new
   RPA initially becomes available.  The result can be re-used as new

   bidir groups that map to the same RPA are encountered.  However,
   there are some conditions under which an update to the election is
   required:

      o There is a change in unicast metric to reach the RPA for any of
        the routers on the link.

      o The interface on which the RPA is reachable (RPF Interface)
        changes to an interface for which the router was previously the
        DF.

      o A new PIM neighbor starts up on a link that must participate in
        the elections and be informed of the current outcome.

      o The elected DF fails (detected through neighbor information
        timeout or MRIB RPF change at downstream router).

   The election process has to be robust enough to ensure with very high
   probability that all routers on the link have a consistent view of
   the DF.  Given the forwarding rules described in Section 3.3, loops
   may result if multiple routers end-up thinking that they should be
   responsible for forwarding.  To minimize the possibility of this
   occurrence, the election algorithm has been biased towards discarding
   DF information and suspending forwarding during periods of ambiguity.

3.5.2. DF Election Description

   This section gives an outline of the DF election process.  It does
   not provide the definitive specification for the DF election.  If any
   discrepancy exists between Section 3.5.3 and this section, the
   specification in Section 3.5.3 is to be assumed correct.

   To perform the election of the DF for a particular RPA, routers on a
   link need to exchange their unicast routing metric information for
   reaching the RPA.  Routers advertise their own metrics in Offer,
   Winner, Backoff, and Pass messages.  The advertised metric is
   calculated using the RPF Interface and metric to reach the RPA
   available through the MRIB.  When a router is participating in a DF
   election for an RPA on the interface that its MRIB indicates as the
   RPF Interface, then that router MUST always advertise an infinite
   metric in its election messages.  When a router is participating in a
   DF election on an interface other than the MRIB-indicated RPF
   Interface then it MUST advertise the MRIB-provided metrics in its
   election messages.

   In the election protocol described below, many message exchanges are
   repeated Election_Robustness times for reliability.  In all those
   cases, the message retransmissions are spaced in time by a small

random interval. All of the following description is specific to the
election on a single link for a single RPA.

3.5.2.1. Bootstrap Election

   Initially, when no DF has been elected, routers finding out about a
   new RPA start participating in the election by sending Offer
   messages.  Offer messages include the router's metric to reach the
   RPA.  Offers are periodically retransmitted with a period of
   Offer_Interval.

   If a router hears a better offer than its own from a neighbor, it
   stops participating in the election for a period of
   Election_Robustness * Offer_Interval, thus giving a chance to the
   neighbor with the better metric to be elected DF.  If during this
   period no winner is elected, the router restarts the election from
   the beginning.  If at any point during the initial election a router
   receives an out of order offer with worse metrics than its own, then
   it restarts the election from the beginning.

   The result should be that all routers except the best candidate stop
   advertising their offers.

   A router assumes the role of the DF after having advertised its
   metrics Election_Robustness times without receiving any offer from
   any other neighbor.  At that point, it transmits a Winner message
   that declares to every other router on the link the identity of the
   winner and the metrics it is using.

   Routers receiving a Winner message stop participating in the election
   and record the identity and metrics of the winner.  If the local
   metrics are better than those of the winner, then the router records
   the identity of the winner (accepting it as the acting DF) but re-
   initiates the election to try and take over.

3.5.2.2. Loser Metric Changes

   Whenever the unicast metric to an RPA changes at a non-DF router to a
   value that is better than that previously advertised by the acting
   DF, the router with the new better metric should take action to
   eventually assume forwarding responsibility.  When the metric change
   is detected, the non-DF router with the now better metric restarts
   the DF election process by sending Offer messages with this new
   metric.  Note that at any point during an election if no response is
   received after Election_Robustness retransmissions of an offer, a
   router assumes the role of the DF following the usual Winner
   announcement procedure.

   Upon receipt of an offer that is worse than its current metric, the
   DF will respond with a Winner message declaring its status and
   advertising its better metric.  Upon receiving the Winner message,
   the originator of the Offer records the identity of the DF and aborts
   the election.

   Upon receipt of an offer that is better than its current metric, the
   DF records the identity and metrics of the offering router and
   responds with a Backoff message.  This instructs the offering router
   to hold off for a short period of time while the unicast routing
   stabilizes and other routers get a chance to put in their offers.
   The Backoff message includes the offering router's new metric and
   address.  All routers on the link that have pending offers with
   metrics worse than those in the Backoff message (including the
   original offering router) will hold further offers for a period of
   time defined in the Backoff message.

   If a third router sends a better offer during the Backoff_Period, the
   Backoff message is repeated for the new offer and the Backoff_Period
   is restarted.

   Before the Backoff_Period expires, the acting DF nominates the router
   having made the best offer as the new DF using a Pass message.  This
   message includes the IDs and metrics of both the old and new DFs.
   The old DF stops performing its tasks at the time the Pass message
   transmission is made.  The new DF assumes the role of the DF as soon
   as it receives the Pass message.  All other routers on the link take
   note of the new DF and its metric.  Note that this event constitutes
   an RPF Neighbor change, which may trigger Join messages to the new DF
   (see Section 3.4).

3.5.2.3. Winner Metric Changes

   If the DF's routing metric to reach the RPA changes to a worse value,
   it sends a set of Election_Robustness randomly spaced Winner messages
   on the link, advertising the new metric.  Routers that receive this
   announcement but have a better metric may respond with an Offer
   message that results in the same handoff procedure described above.
   All routers assume the DF has not changed until they see a Pass or
   Winner message indicating the change.

   There is no pressure to make this handoff quickly if the acting DF
   still has a path to the RPL.  The old path may now be suboptimal, but
   it will still work while the re-election is in progress.

3.5.2.4. Winner Loses Path

   If a router's RPF Interface to the RPA switches to be on a link for
   which it is acting as the DF, then it can no longer provide
   forwarding services for that link.  It therefore immediately stops
   being the DF and restarts the election.  As its path to the RPA is
   through the link, an infinite metric is used in the Offer message it
   sends.

3.5.2.5. Late Router Starting Up

   A late router starting up after the DF election process has completed
   will have no immediate knowledge of the election outcome.  As a
   result, it will start advertising its metric in Offer messages.  As
   soon as this happens, the currently elected DF will respond with a
   Winner message if its metric is better than the metric in the Offer
   message, or with a Backoff message if its metric is worse than the
   metric in the Offer message.

3.5.2.6. Winner Dies

   Whenever the DF dies, a new DF has to be elected.  The speed at which
   this can be achieved depends on whether there are any downstream
   routers on the link.

   If there are downstream routers, typically their MRIB reported next-
   hop before the DF dies will be the DF itself.  They will therefore
   notice either a change in the metric for the route to the RPA or a
   change in next-hop away from the DF and can restart the election by
   transmitting Offer messages.  If according to the MRIB the RPA is now
   reachable through the same link via another upstream router, an
   infinite metric will be used in the Offer.

   If no downstream routers are present, the only way for other upstream
   routers to detect a DF failure is by the timeout of the PIM neighbor
   information, which will take significantly longer.

3.5.3. Election Protocol Specification

   This section provides the definitive specification for the DF
   election process.  If any discrepancy exists between Section 3.5.2
   and this section, the specification in this section is to be assumed
   correct.

3.5.3.1. Election State

The DF election state is maintained per RPA for each multicast
enabled interface I on the router as introduced in Section 3.1.

   The state machine has the following four states:

      Offer
         Initial election state.  When in the Offer state, a router
         thinks it can eventually become the winner and periodically
         generates Offer messages.

      Lose
         In this state, the router knows that there either is a
         different election winner or that no router on the link has a
         path to the RPA.

      Win
         The router is the acting DF without any contest.

      Backoff
         The router is the acting DF but another router has made a bid
         to take over.

   In the state machine, a router is considered to be an acting DF if it
   is in the Win or Backoff states.

   The operation of the election protocol makes use of the variables and
   timers described below:

      Acting DF information
         Used to store the identity and advertised metrics of the
         election winner that is the currently acting DF.

      DF election-Timer (DFT)
         Used to schedule transmission of Offer, Winner, and Pass
         messages.

      Message-Count (MC)
         Used to maintain the number of times an Offer or Winner message
         has been transmitted.

      Best-Offer
         Used by the DF to record the identity and advertised metrics of
         the router that has made the last offer, for use when sending
         the Path message.

3.5.3.2. Election Messages

The election process uses the following PIM control messages. The
packet format is described in Section 3.7:

      Offer (OfferingID, Metric)
         Sent by routers that believe they have a better metric to the
         RPA than the metric that has been on offer so far.

      Winner (DF-ID, DF-Metric)
         Sent by a router when assuming the role of the DF or when re-
         asserting in response to worse offers.

      Backoff (DF-ID, DF-Metric, OfferingID, OfferMetric,
         BackoffInterval)
         Used by the DF to acknowledge better offers.  It instructs
         other routers with equal or worse offers to wait until the DF
         passes responsibility to the sender of the offer.

      Pass (Old-DF-ID, Old-DF-Metric, New-DF-ID, New-DF-Metric)
         Used by the old DF to pass forwarding responsibility to a
         router that has previously made an offer.  The Old-DF-Metric is
         the current metric of the DF at the time the pass is sent.

   Note that when a router is participating in a DF election for an RPA
   on the interface that its MRIB indicates as the RPF Interface, then
   that router MUST always advertise an infinite metric in its election
   messages.  When a router is participating in a DF election on an
   interface other than the MRIB-indicated RPF Interface, then it MUST
   advertise the MRIB-provided metrics in its election messages.

3.5.3.3. Election Events

   During protocol operation, the following events can take place:

      Control message reception
         Reception of one of the four control DF election messages
         (Offer, Winner, Backoff, and Pass).  When a control message is
         received and actions are specified on a condition that metrics
         are Better or Worse, the comparison must be performed as
         follows:

         o On receipt of an Offer or Winner message, compare the current
           metrics for the RPA with the metrics advertised for the
           sender of the message.

         o On receipt of a Backoff or Pass message, compare the current
           metrics for the RPA with the metrics advertised for the
           target of the message.

      Path to RPA lost
         Losing the path to the RPA can happen in two ways.  The first
         happens when the route learned through the MRIB is withdrawn
         and the MRIB no longer reports an available route to reach the
         RPA.  The second case happens when the next-hop information
         reported by the MRIB changes to indicate a next-hop that is
         reachable through the router interface under consideration.
         Clearly, as the router is using the interface as its RPF
         Interface, it cannot offer forwarding services towards the RPL
         to other routers on that link.

      Metric reported by the MRIB to reach the RPA changes
         This event is triggered when the MRIB supplied information for
         the RPA changes and the new information provides a path to the
         RPA.  If the new MRIB information either reports no route or
         reports a next-hop interface through the interface for which
         the DF election is taking place, then the "Path to RPA lost"
         event triggers instead.  In specific states, the event may be
         further filtered by specifying whether it is expected of the
         metric to become better or worse and which of the stored
         metrics the new MRIB information must be compared against.  The
         new information must be compared with either the router's old
         metric, the stored DF metric, or the stored Best Offer metric.

      Election-Timer (DFT) expiration
         Expiration of the DFT election timer can cause message
         transmission and state transitions.  The event might be further
         qualified by specifying the value of the Message Count (MC) as
         well as the current existence of a path to the RPA (as defined
         above).

      Detection of DF failure
         Detection of DF failure can occur through the timeout of PIM
         neighbor state.

3.5.3.4. Election Actions

   The DF election state machine action descriptions use the following
   notation in addition to the pseudocode notation described earlier in
   this specification:

      ?=  denotes the operation of lowering a timer to a new value.  If
          the timer is not running, then it is started using the new
          value.  If the timer is running with an expiration lower than
          the new value, then the timer is not altered.

   When an action of "set DF to Sender or Target" is encountered during
   receipt of a Winner, Pass, or Backoff message, it means the
   following:

      o On receipt of a Winner message, set the DF to be the originator
        of the message and record its metrics.

      o On receipt of a Pass message, set the DF to be the target of the
        message and record its metrics.

      o On receipt of a Backoff message, set the DF to be the originator
        of the message and record its metrics.

3.5.3.5. Election State Transitions

   When a Designated Forwarder election is initiated, the starting state
   is the Offer state, the message counter (MC) is set to zero, and the
   DF election Timer (DFT) is set to OPlow (see Section 3.6 for a
   definition of timer values).

3.5.4. Election Reliability Enhancements

   For the correct operation of BIDIR-PIM, it is very important to avoid
   situations where two routers consider themselves to be Designated
   Forwarders for the same link.  The two precautions below are not
   required for correct operation but can help diagnose and correct
   anomalies.

3.5.5. Missing Pass

   After a DF has been elected, a router whose metrics change to become
   better than the DF will attempt to take over.  If during the re-
   election the acting DF has a condition that causes it to lose all of
   the election messages (like a CPU overload), the new candidate will
   transmit three offers and assume the role of the forwarder resulting
   in two DFs on the link.  This situation is pathological and should be
   corrected by fixing the overloaded router.  It is desirable that such
   an event can be detected by a network administrator.

   When a router becomes the DF for a link without receiving a Pass
   message from the known old DF, the PIM neighbor information for the
   old DF can be marked to this effect.  Upon receiving the next PIM
   Hello message from the old DF, the router can retransmit Winner
   messages for all the RPAs for which it is acting as the DF.  The
   anomaly may also be logged by the router in a rate-limited manner to
   alert the operator.

3.5.6. Periodic Winner Announcement

   An additional degree of safety can be achieved by having the DF for
   each RPA periodically announce its status in a Winner message.
   Transmission of the periodic Winner message can be restricted to
   occur only for RPAs that have active groups, thus avoiding the
   periodic control traffic in areas of the network without senders or
   receivers for a particular RPA.

3.6. Timers, Counters, and Constants

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

   Per Rendezvous-Point Address (RPA):

      Per interface (I):

         DF Election Timer: DFT(RPA,I)

   Per Group (G):

      Upstream Join Timer: JT(G)

      Per interface (I):

         Join Expiry Timer: ET(G,I)

         PrunePendingTimer: PPT(G,I)

   When timers are started or restarted, they are set to default values.
   This section summarizes those default values.

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

  +---------------+---------------+------------------------------------+
  |Value Name     | Value         | Explanation                        |
  +---------------+---------------+------------------------------------+
  |J/P HoldTime   | from message  | Hold Time from Join/Prune Message  |
  +---------------+---------------+------------------------------------+

  Timer Names: PrunePendingTimer (PPT(G,I))

  +-------------------------+-------------------+----------------------+
  | Value Name              | Value             |  Explanation         |
  +-------------------------+-------------------+----------------------+
  | J/P Override Interval   | Default: 3 secs   |  Short period after  |
  |                         |                   |  a Join or Prune to  |
  |                         |                   |  allow other         |
  |                         |                   |  routers on the LAN  |
  |                         |                   |  to override the     |
  |                         |                   |  Join or Prune       |
  +-------------------------+-------------------+----------------------+

  Note that the value of the J/P Override Interval is interface specific
  and depends on both the Propagation_Delay and the Override_Interval
  values that may change when Hello messages are received [4].

  Timer Names: Upstream Join Timer (JT(G))

  +------------+-------------------+-----------------------------------+
  Value Name   |Value              Explanation                         |
  +------------+-------------------+-----------------------------------+
  t_periodic   |Default: 60 secs   Period between Join/Prune Messages  |
  +------------+-------------------+-----------------------------------+
  t_suppressed |rand(1.1 *         Suppression period when someone     |
  |            |t_periodic, 1.4 *  else sends a J/P message so we      |
  |            |t_periodic)        don't need to do so.                |
  +------------+-------------------+-----------------------------------+
  t_override   |rand(0, 0.9 * J/P  Randomized delay to prevent         |
  |            |Override Interval) response implosion when sending a   |
  |            |                   Join message to override someone    |
  |            |                   else's Prune message.               |
  +------------+-------------------+-----------------------------------+

  For more information about these values, refer to the PIM-SM [4]
  documentation.

3.7. BIDIR-PIM Packet Formats

   This section describes the details of the packet formats for BIDIR-
   PIM control messages.  BIDIR-PIM shares a number of control messages
   in common with PIM-SM [4].  These include the Hello and Join/Prune
   messages as well as the format for the Encoded-Unicast address.  For
   details on the format of these packets, please refer to the PIM-SM
   documentation.  Here we will only define the additional packets that
   are introduced by BIDIR-PIM.  These are the packets used in the DF
   election process as well as the Bidirectional Capable PIM-Hello
   option.

3.7.1. DF Election Packet Formats

   All PIM control messages have IP protocol number 103.

   BIDIR-PIM messages are multicast with TTL 1 to the `ALL-PIM-ROUTERS'
   group.  The IPv4 `ALL-PIM-ROUTERS' group is `224.0.0.13'.  The IPv6
   `ALL-PIM-ROUTERS' group is `ff02::d'.

   All DF election BIDIR-PIM control messages share the common header
   below:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |Subtype| Rsvd  |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               RP Address (Encoded-Unicast format)           ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Sender Metric Preference                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sender Metric                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PIM Ver
      PIM Version number is 2.

   Type
      All DF-Election PIM control messages share the PIM message Type of
      10.

   Subtype
   Subtypes for DF election messages are:

              1 = Offer
              2 = Winner
              3 = Backoff
              4 = Pass

   Rsvd
      Set to zero on transmission.  Ignored on receipt.

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

   RP Address
      The bidirectional RPA for which the election is taking place.  The
      format is described in [4], Section 4.9.1.

   Sender Metric Preference
      Preference value assigned to the unicast routing protocol that the
      message sender used to obtain the route to the RPA.

   Sender Metric
      The unicast routing table metric used by the message sender to
      reach the RPA.  The metric is in units applicable to the unicast
      routing protocol used.

   In addition to the fields defined above, the Backoff and Pass
   messages have the extra fields described below.

3.7.2. Backoff Message

   The Backoff message uses the following fields in addition to the
   common election message format described above.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Offering Address (Encoded-Unicast format)      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Offering Metric Preference                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Offering Metric                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Interval           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Offering Address
      The address of the router that made the last (best) Offer.  The
      format is described in [4], Section 4.9.1.

   Offering Metric Preference
      Preference value assigned to the unicast routing protocol that the
      offering router used to obtain the route to the RPA.

   Offering Metric
      The unicast routing table metric used by the offering router to
      reach the RPA.  The metric is in units applicable to the unicast
      routing protocol used.

   Interval
      The backoff interval in milliseconds to be used by routers with
      worse metrics than the offering router.

3.7.3. Pass Message

The Pass message uses the following fields in addition to the common
election fields described above.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              New Winner Address (Encoded-Unicast format)    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 New Winner Metric Preference                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      New Winner Metric                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   New Winner Address
      The address of the router that made the last (best) Offer.  The
      format is described in [4], Section 4.9.1.

   New Winner Metric Preference
      Preference value assigned to the unicast routing protocol that the
      offering router used to obtain the route to the RPA.

   New Winner Metric
      The unicast routing table metric used by the offering router to
      reach the RPA.  The metric is in units applicable to the unicast
      routing protocol used.

3.7.4. Bidirectional Capable PIM-Hello Option

   BIDIR-PIM introduces one new PIM-Hello option.

   o OptionType 22: Bidirectional Capable

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Type = 22            |         Length = 0            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4. RP Discovery

   Routers discover that a range of multicast group addresses operates
   in bidirectional mode, and that the address of the Rendezvous-Point
   address (RPA) is serving the group range either through static
   configuration or using an automatic RP discovery mechanism like the
   PIM Bootstrap mechanism (BSR) [7] or Auto-RP.

5. Security Considerations

   The IPsec [5] authentication header MAY be used to provide data
   integrity protection and group-wise data origin authentication of
   BIDIR-PIM protocol messages.  Authentication of BIDIR-PIM messages
   can protect against unwanted behaviour caused by unauthorized or
   altered BIDIR-PIM messages.

5.1. Attacks Based on Forged Messages

   As in PIM Sparse-Mode, the extent of possible damage depends on the
   type of counterfeit messages accepted.  BIDIR-PIM only uses link-
   local multicast messages sent to the ALL_PIM_ROUTERS address, hence
   attacks can only be carried out by directly connected nodes, or with
   the complicity of directly connected routers.

   Some of the BIDIR-PIM protocol messages (Join/Prune and Hello) are
   identical, both in format and functionality, to the respective
   messages used in PIM-SM.  Security considerations for these messages
   are to be found in [4].  Other messages (DF-election messages) are
   specific to BIDIR-PIM and will be discussed in the following
   paragraphs.

   By forging DF-election messages, an attacker can disrupt the election
   of the Designated Forwarder on a link in two different ways:

5.1.1. Election of an Incorrect DF

   An attacker can force its election as DF by participating in a
   regular election and advertising the best metric to reach the RPA.
   An attacker can also try to force the election of another router as
   DF by sending an Offer, Winner, or Pass message and impersonating
   another router.  In some cases (e.g., the Offer), multiple messages
   might be needed to carry out an attack.

   In the case of Offer or Winner messages, the attacker will have to
   impersonate the node that it wants to have become the DF.  In the
   case of the Pass, it will have to impersonate the current DF.  This
   type of attack causes the wrong DF to be recorded in all nodes apart
   from the one that is being impersonated.  This node typically will be
   able to detect the anomaly and, possibly, restart a new election.

   A more sophisticated attacker might carry out a concurrent DoS attack
   on the node being impersonated, so that it will not be able to detect
   the forged packets and/or take countermeasures.

   All attacks based on impersonation can be detected by all routers and
   avoided if the source of DF-election messages can be authenticated.
   When authentication is available, spoofed messages MUST be discarded
   and a rate-limited warning message SHOULD be logged.

   A more subtle attacker could use MAC-level addresses to partition the
   set of recipients of DF-election messages and create an inconsistent
   DF view on the link.  For example, the attacker could use unicast MAC
   addresses for its forged DF-election messages.  To prevent this type
   of attack, BIDIR-PIM routers SHOULD check the destination MAC address
   of received DF-election messages.  This however is ineffective on
   links that do not support layer-2 multicast delivery.

   Source authentication is also sufficient to prevent this kind of
   attack.

5.1.2. Preventing Election Convergence

   By forging DF election messages, an attacker can prevent the election
   from converging, thus disrupting the establishment of multicast
   forwarding trees.  There are many ways to achieve this.  The simplest
   is by sending an infinite sequence of Offer messages (the metric used
   in the messages is not important).

5.2. Non-Cryptographic Authentication Mechanisms

   A BIDIR-PIM router SHOULD provide an option to limit the set of
   neighbors from which it will accept Join/Prune, Assert, and DF-
   election messages.  Either static configuration of IP addresses or an
   IPsec security association may be used.  Furthermore, a PIM router
   SHOULD NOT accept protocol messages from a router from which it has
   not yet received a valid Hello message.

5.2.1. Basic Access Control

   In a PIM-SM domain, when all routers are trusted, it is possible to
   implement a basic form of access control for both sources and
   receivers: Receivers can be validated by the last-hop DR and sources
   can be validated by the first-hop DR and/or the RP.

   In BIDIR-PIM, this is generally feasible only for receivers, as
   sources can send to the multicast group without the need for routers
   to detect their activity and create source-specific state.  However,
   it is possible to modify the standard BIDIR-PIM behaviour, in a
   backward compatible way, to allow per-source access control.  The
   tradeoff would be protocol simplicity, memory, and processing
   requirements.

5.3. Authentication Using IPsec

   Just as with PIM-SM, the IPsec [5] transport mode using the
   Authentication Header (AH) is the recommended method to prevent the
   above attacks against BIDIR-PIM.

   It is recommended that IPsec authentication be applied to all BIDIR-
   PIM protocol messages.  The specification on how this is done is
   found in [4].  Specifically, the authentication of PIM-SM link-local
   messages, described in [4], applies to all BIDIR-PIM messages as
   well.

5.4. Denial-of-Service Attacks

The denial-of-service attack based on forged Join messages, described
in [4], also applies to BIDIR-PIM.

6. IANA Considerations

IANA has assigned OptionType 22 to the "Bidirectional Capable"
option.

7. Acknowledgments

   The bidirectional proposal in this document is heavily based on the
   ideas and text presented by Estrin and Farinacci in [6].  The main
   difference between the two proposals is in the method chosen for
   upstream forwarding.

   We would also like to thank John Zwiebel at Cisco, Deborah Estrin at
   ISI/USC, Bill Fenner at AT&T Research, as well as Nidhi Bhaskar,
   Yiqun Cai, Toerless Eckert, Apoorva Karan, Rajitha Sumanasekera, and
   Beau Williamson at Cisco for their contributions and comments to this
   document.

8. Normative References

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

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

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

   [4] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol
       Independent Multicast - Sparse Mode (PIM-SM): Protocol
       Specification (Revised)", RFC 4601, August 2006.

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

9. Informative References

   [6] Estrin, D. and D. Farinacci, "Bi-directional Shared Trees in
       PIM-SM", Work in Progress, May 1999.

   [7] Bhaskar, N., Gall, A., Lingard, J., and S. Venaas, "Bootstrap
       Router (BSR) Mechanism for PIM", Work in Progress, February 2007.

   [8] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
       Extensions for BGP-4", RFC 4760, January 2007.

Index

   DF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5,18
   Downstream. . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   DownstreamJPState(G,I). . . . . . . . . . . . . . . . . . . . . .  10
   ET(G,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . 9,14,33
   ET(RPA,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   I_am_DF(RPA,I). . . . . . . . . . . . . . . . . . . . . . . .10,12,14
   J/P_HoldTime. . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   J/P_Override_Interval . . . . . . . . . . . . . . . . . . . . . 16,33
   JoinDesired(G). . . . . . . . . . . . . . . . . . . . . . . . . .  18
   joins(G). . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   JT(*,G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   JT(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9,33
   local_receiver_include(G,I) . . . . . . . . . . . . . . . . . . .  10
   MFIB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   NLT(N,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Offer_Period. . . . . . . . . . . . . . . . . . . . . . . . . . .  32
   olist(G). . . . . . . . . . . . . . . . . . . . . . . . . . .10,12,18
   Bidirectional Capable OptionType  . . . . . . . . . . . . . . . .  37
   pim_include(G). . . . . . . . . . . . . . . . . . . . . . . . . .  10
   PPT(G,I). . . . . . . . . . . . . . . . . . . . . . . . . . . 9,14,33
   RPA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   RPF_interface(RPA). . . . . . . . . . . . . . . . . . . . . . . 10,12
   RPL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   TIB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   t_override. . . . . . . . . . . . . . . . . . . . . . . . . . . 17,33
   t_periodic. . . . . . . . . . . . . . . . . . . . . . . . . . . 17,33
   t_suppressed. . . . . . . . . . . . . . . . . . . . . . . . . . 17,33
   Upstream. . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5

Authors' Addresses

   Mark Handley
   Computer Science Department
   University College London
   EMail: M.Handley@cs.ucl.ac.uk

   Isidor Kouvelas
   Cisco Systems
   EMail: kouvelas@cisco.com

   Tony Speakman
   Cisco Systems
   EMail: speakman@cisco.com

   Lorenzo Vicisano
   Digital Fountain
   EMail: lorenzo@digitalfountain.com

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