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rfc:rfc5015

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.

Handley, et al. Standards Track [Page 1] RFC 5015 Bidirectional PIM October 2007

Table of Contents

 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

Handley, et al. Standards Track [Page 2] RFC 5015 Bidirectional PIM October 2007

         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.

Handley, et al. Standards Track [Page 3] RFC 5015 Bidirectional PIM October 2007

 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

Handley, et al. Standards Track [Page 4] RFC 5015 Bidirectional PIM October 2007

    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

Handley, et al. Standards Track [Page 5] RFC 5015 Bidirectional PIM October 2007

    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.

Handley, et al. Standards Track [Page 6] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 7] RFC 5015 Bidirectional PIM October 2007

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

Handley, et al. Standards Track [Page 8] RFC 5015 Bidirectional PIM October 2007

              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.

Handley, et al. Standards Track [Page 9] RFC 5015 Bidirectional PIM October 2007

 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).

Handley, et al. Standards Track [Page 10] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 11] RFC 5015 Bidirectional PIM October 2007

 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

Handley, et al. Standards Track [Page 12] RFC 5015 Bidirectional PIM October 2007

 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

Handley, et al. Standards Track [Page 13] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 14] RFC 5015 Bidirectional PIM October 2007

 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).

Handley, et al. Standards Track [Page 15] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 16] RFC 5015 Bidirectional PIM October 2007

 The upstream (*,G) state machine only contains two states:
    Not Joined
       The downstream state machines indicate that the router does not
       need to join the RPA tree for this group.
    Joined
       The downstream state machines indicate that the router would
       like to join the RPA tree for this group.
 In addition, one timer JT(G) is kept, which is used to trigger the
 sending of a Join(*,G) to the upstream next hop towards the RPA (the
 DF on the RPF interface for RPA(G)).
        Figure 2: Upstream group state machine in tabular form
+---------------------+----------------------------------------------+
|                     |                    Event                     |
|  Prev State         +-----------------------+----------------------+
|                     |   JoinDesired(G)      |    JoinDesired(G)    |
|                     |   ->True              |    ->False           |
+---------------------+-----------------------+----------------------+
|                     |   -> J state          |    -                 |
|  NotJoined (NJ)     |   Send Join(*,G);     |                      |
|                     |   Set Timer to        |                      |
|                     |   t_periodic          |                      |
+---------------------+-----------------------+----------------------+
|  Joined (J)         |   -                   |    -> NJ state       |
|                     |                       |    Send Prune(*,G)   |
+---------------------+-----------------------+----------------------+
In addition, we have the following transitions that occur within the
Joined state:
+--------------------------------------------------------------------+
|                        In Joined (J) State                         |
+----------------+----------------+-----------------+----------------+
|Timer Expires   | See Join(*,G)  | See Prune(*,G)  | RPF_DF(RPA(G)) |
|                | to             | to              | GenID changes  |
|                | RPF_DF(RPA(G)) | RPF_DF(RPA(G))  |                |
+----------------+----------------+-----------------+----------------+
|Send            | Increase Timer | Decrease Timer  | Decrease Timer |
|Join(*,G); Set  | to             | to t_override   | to t_override  |
|Timer to        | t_suppressed   |                 |                |
|t_periodic      |                |                 |                |
+----------------+----------------+-----------------+----------------+

Handley, et al. Standards Track [Page 17] RFC 5015 Bidirectional PIM October 2007

+--------------------------------------------------------------------+
|                        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

Handley, et al. Standards Track [Page 18] RFC 5015 Bidirectional PIM October 2007

 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

Handley, et al. Standards Track [Page 19] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 20] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 21] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 22] RFC 5015 Bidirectional PIM October 2007

 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:

Handley, et al. Standards Track [Page 23] RFC 5015 Bidirectional PIM October 2007

    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.

Handley, et al. Standards Track [Page 24] RFC 5015 Bidirectional PIM October 2007

    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.

Handley, et al. Standards Track [Page 25] RFC 5015 Bidirectional PIM October 2007

 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).

Handley, et al. Standards Track [Page 26] RFC 5015 Bidirectional PIM October 2007

 Figure 3: Designated Forwarder election state machine in tabular form
+-------------+------------------------------------------------------+
|             |                        Event                         |
| Prev State  +-----------------+------------------+-----------------+
|             | Recv better     |  Recv better     | Recv better     |
|             | Pass / Win      |  Backoff         | Offer           |
+-------------+-----------------+------------------+-----------------+
|             | -> Lose         |  -               | -               |
| Offer       | DF = Sender or  |  DFT = BOperiod  | DFT = OPhigh;   |
|             | Target; Stop    |  + OPlow; MC =   | MC = 0          |
|             | DFT             |  0               |                 |
+-------------+-----------------+------------------+-----------------+
|             | -               |  -               | -> Offer        |
| Lose        | DF = Sender or  |  DF = Sender     | DFT = OPhigh;   |
|             | Target          |                  | MC = 0          |
+-------------+-----------------+------------------+-----------------+
|             | -> Lose         |  -> Lose         | -> Backoff      |
|             | DF = Sender or  |  DF = Sender;    | Set Best to     |
| Win         | Target; Stop    |  Stop DFT        | Sender; Send    |
|             | DFT             |                  | Backoff; DFT =  |
|             |                 |                  | BOperiod        |
+-------------+-----------------+------------------+-----------------+
|             | -> Lose         |  -> Lose         | -               |
|             | DF = Sender or  |  DF = Sender;    | Set Best to     |
| Backoff     | Target; Stop    |  Stop DFT        | Sender; Send    |
|             | DFT             |                  | Backoff; DFT =  |
|             |                 |                  | BOperiod        |
+-------------+-----------------+------------------+-----------------+

Handley, et al. Standards Track [Page 27] RFC 5015 Bidirectional PIM October 2007

+-----------+-------------------------------------------------------+
|           |                         Event                         |
|           +-------------+-------------+--------------+------------+
|Prev State |Recv Backoff |Recv Pass    |Recv Worse    |Recv worse  |
|           |for us       |for us       |Pass / Win /  |Offer       |
|           |             |             |Backoff       |            |
+-----------+-------------+-------------+--------------+------------+
|           |-            |-> Win       |-             |-           |
|           |DFT =        |Stop DFT     |Set DF to     |DFT ?=      |
|Offer      |BOperiod +   |             |Sender or     |OPlow; MC = |
|           |OPlow; MC =  |             |Target; DFT   |0           |
|           |0            |             |?= OPlow; MC  |            |
|           |             |             |= 0           |            |
+-----------+-------------+-------------+--------------+------------+
|           |-> Offer     |-> Offer     |-> Offer      |-> Offer    |
|           |DF = Sender; |DF = Sender; |DF = Sender   |DFT = OPlow;|
|Lose       |DFT = OPlow; |DFT = OPlow; |or Target;    |MC = 0      |
|           |MC = 0       |MC = 0       |DFT = OPlow;  |            |
|           |             |             |MC = 0        |            |
+-----------+-------------+-------------+--------------+------------+
|           |-> Offer     |-> Offer     |-> Offer      |-           |
|           |DF = Sender; |DF = Sender; |DF = Sender   |Send Winner |
|Win        |DFT = OPlow; |DFT = OPlow; |or Target;    |            |
|           |MC = 0       |MC = 0       |DFT = OPlow;  |            |
|           |             |             |MC = 0        |            |
+-----------+-------------+-------------+--------------+------------+
|           |-> Offer     |-> Offer     |-> Offer      |-> Win      |
|           |DF = Sender; |DF = Sender; |DF = Sender   |Send Winner;|
|Backoff    |DFT = OPlow; |DFT = OPlow; |or Target;    |Stop DFT    |
|           |MC = 0       |MC = 0       |DFT = OPlow;  |            |
|           |             |             |MC = 0        |            |
+-----------+-------------+-------------+--------------+------------+

Handley, et al. Standards Track [Page 28] RFC 5015 Bidirectional PIM October 2007

+--------------------------------------------------------------------+
|                          In Offer State                            |
+----------------------+----------------------+----------------------+
| DFT Expires and MC   | DFT Expires and MC   |  DFT Expires and MC  |
| is less than         | is equal to          |  is equal to         |
| Robustness           | Robustness and we    |  Robustness and      |
|                      | have path to RPA     |  there is no path    |
|                      |                      |  to RPA              |
+----------------------+----------------------+----------------------+
| -                    | -> Win               |  -> Lose             |
| Send Offer; DFT =    | Send Winner          |  Set DF to None      |
| OPlow; MC = MC + 1   |                      |                      |
+----------------------+----------------------+----------------------+
+--------------------------------------------------------------------+
|                          In Offer State                            |
+--------------------------------------------------------------------+
|                  Metric changes and is now worse                   |
+--------------------------------------------------------------------+
|                  DFT ?= OPlow                                      |
|                  MC = 0                                            |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
|                           In Lose State                            |
+------------------------------+-------------------------------------+
|     Detect DF Failure        |        Metric changes and now       |
|                              |        is better than DF            |
+------------------------------+-------------------------------------+
|     -> Offer                 |        -> Offer                     |
|     DF = None; DFT =         |        DFT = OPlow_int; MC = 0      |
|     OPlow_int; MC = 0        |                                     |
+------------------------------+-------------------------------------+
+--------------------------------------------------------------------+
|                           In Win State                             |
+----------------------+-----------------------+---------------------+
| Metric changes and   |  Timer Expires and    |  Path to RPA lost   |
| is now worse         |  MC is less than      |                     |
|                      |  Robustness           |                     |
+----------------------+-----------------------+---------------------+
| -                    |  -                    |  -> Offer           |
| DFT = OPlow; MC =    |  Send Winner; DFT =   |  Set DF to None;    |
| 0                    |  OPlow; MC = MC + 1   |  DFT = OPlow; MC =  |
|                      |                       |  0                  |
+----------------------+-----------------------+---------------------+

Handley, et al. Standards Track [Page 29] RFC 5015 Bidirectional PIM October 2007

+--------------------------------------------------------------------+
|                         In Backoff State                           |
+----------------------+-----------------------+---------------------+
| Metric changes and   |  Timer Expires        |  Path to RPA lost   |
| is now better than   |                       |                     |
| Best                 |                       |                     |
+----------------------+-----------------------+---------------------+
| -> Win               |  -> Lose              |  -> Offer           |
| Stop Timer           |  Send Pass; Set DF    |  Set DF to None;    |
|                      |  to stored Best       |  DFT = OPlow; MC =  |
|                      |                       |  0                  |
+----------------------+-----------------------+---------------------+

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.

Handley, et al. Standards Track [Page 30] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 31] RFC 5015 Bidirectional PIM October 2007

 Timer Name: DF Election Timer (DFT)
+-------------------+------------------------+-----------------------+
| Value Name        |  Value                 |   Explanation         |
+-------------------+------------------------+-----------------------+
| Offer_Period      |  100 ms                |   Interval to wait    |
|                   |                        |   between repeated    |
|                   |                        |   Offer and Winner    |
|                   |                        |   messages.           |
+-------------------+------------------------+-----------------------+
| Backoff_Period    |  1 sec                 |   Period that acting  |
|                   |                        |   DF waits between    |
|                   |                        |   receiving a better  |
|                   |                        |   Offer and sending   |
|                   |                        |   the Pass message    |
|                   |                        |   to transfer DF      |
|                   |                        |   responsibility.     |
+-------------------+------------------------+-----------------------+
| OPlow             |  rand(0.5, 1) *        |   Range of actual     |
|                   |  Offer_Period          |   randomised value    |
|                   |                        |   used between        |
|                   |                        |   repeated messages.  |
+-------------------+------------------------+-----------------------+
| OPhigh            |  Election_Robustness   |   Interval to wait    |
|                   |  * Offer_Period        |   in order to give a  |
|                   |                        |   chance to a router  |
|                   |                        |   with a better       |
|                   |                        |   Offer to become     |
|                   |                        |   the DF.             |
+-------------------+------------------------+-----------------------+

Handley, et al. Standards Track [Page 32] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 33] RFC 5015 Bidirectional PIM October 2007

Constant Name: DF Election Robustness
+-------------------------+------------------+-----------------------+
|  Constant Name          |   Value          |   Explanation         |
+-------------------------+------------------+-----------------------+
|  Election_Robustness    |   Default: 3     |   Minimum number of   |
|                         |                  |   election messages   |
|                         |                  |   that must be lost   |
|                         |                  |   in order for        |
|                         |                  |   election to fail.   |
+-------------------------+------------------+-----------------------+

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'.

Handley, et al. Standards Track [Page 34] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 35] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 36] RFC 5015 Bidirectional PIM October 2007

  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.

Handley, et al. Standards Track [Page 37] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 38] RFC 5015 Bidirectional PIM October 2007

 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.

Handley, et al. Standards Track [Page 39] RFC 5015 Bidirectional PIM October 2007

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.

Handley, et al. Standards Track [Page 40] RFC 5015 Bidirectional PIM October 2007

 [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

Handley, et al. Standards Track [Page 41] RFC 5015 Bidirectional PIM October 2007

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

Handley, et al. Standards Track [Page 42] RFC 5015 Bidirectional PIM October 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Handley, et al. Standards Track [Page 43]

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