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Internet Engineering Task Force                                   PIM WG
INTERNET-DRAFT                                         Mark Handley/ICIR
draft-ietf-pim-bidir-04.txt                        Isidor Kouvelas/Cisco
                                                     Tony Speakman/Cisco
                                                  Lorenzo Vicisano/Cisco
                                                            26 June 2002
                                                  Expires: December 2002


       Bi-directional Protocol Independent Multicast (BIDIR-PIM)


Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This document is a product of the IETF PIM WG.  Comments should be
addressed to the authors, or the WG's mailing list at
pim@catarina.usc.edu.

                                Abstract


     This document discusses Bi-directional PIM, a variant of PIM
     Sparse-Mode [9] that builds bi-directional shared trees
     connecting multicast sources and receivers. Bi-directional
     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



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     natively forwarded from sources to the Rendezvous-Point and
     hence along the shared tree to receivers without requiring
     source-specific state.  The DF election takes place at RP
     discovery time and provides a default route to the RP thus
     eliminating the requirement for data-driven protocol events.

Note on BIDIR-PIM status

The differences between this version of the BIDIR-PIM specification and
draft-ietf-pim-bidir-new-00.txt are mostly in the format of the
information presented. As BIDIR-PIM has many similarities in operation
to Sparse-Mode PIM, the earlier version of this spec relied heavily on
the now obsolete PIM-SM [11] specification. This revision removes this
dependency and instead references the new Sparse-Mode documentation [9]
where necessary. In addition the method in which the protocol
specification is presented has been updated to follow the format of [9].



































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                           Table of Contents


1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . .   5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
 2.1. Definitions. . . . . . . . . . . . . . . . . . . . . . . . . .   6
 2.2. Pseudocode Notation. . . . . . . . . . . . . . . . . . . . . .   7
3. Protocol Specification. . . . . . . . . . . . . . . . . . . . . .   8
 3.1. BIDIR-PIM Protocol State . . . . . . . . . . . . . . . . . . .   8
  3.1.1. General Purpose State . . . . . . . . . . . . . . . . . . .   9
  3.1.2. RP State. . . . . . . . . . . . . . . . . . . . . . . . . .   9
  3.1.3. Group State . . . . . . . . . . . . . . . . . . . . . . . .  10
  3.1.4. State Summarization Macros. . . . . . . . . . . . . . . . .  11
 3.2. PIM Neighbor Discovery . . . . . . . . . . . . . . . . . . . .  12
 3.3. Data Packet Forwarding Rules . . . . . . . . . . . . . . . . .  12
  3.3.1. Source-Only Branches. . . . . . . . . . . . . . . . . . . .  13
 3.4. PIM Join/Prune Messages. . . . . . . . . . . . . . . . . . . .  13
  3.4.1. Receiving (*,G) Join/Prune Messages . . . . . . . . . . . .  14
  3.4.2. Sending Join/Prune Messages . . . . . . . . . . . . . . . .  16
 3.5. Designated Forwarder (DF) Election . . . . . . . . . . . . . .  19
  3.5.1. DF Requirements . . . . . . . . . . . . . . . . . . . . . .  19
  3.5.2. DF Election description . . . . . . . . . . . . . . . . . .  20
   3.5.2.1. Bootstrap Election . . . . . . . . . . . . . . . . . . .  20
   3.5.2.2. Loser Metric Changes . . . . . . . . . . . . . . . . . .  21
   3.5.2.3. Winner Metric Changes. . . . . . . . . . . . . . . . . .  22
   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 . . . . . . . . . . . . . .  23
   3.5.3.1. Election State . . . . . . . . . . . . . . . . . . . . .  23
   3.5.3.2. Election Messages. . . . . . . . . . . . . . . . . . . .  24
   3.5.3.3. Election Events. . . . . . . . . . . . . . . . . . . . .  24
   3.5.3.4. Election Notation. . . . . . . . . . . . . . . . . . . .  25
   3.5.3.5. Election State Transitions . . . . . . . . . . . . . . .  25
 3.6. Timers and Constants . . . . . . . . . . . . . . . . . . . . .  28
 3.7. BIDIR PIM Packet Formats . . . . . . . . . . . . . . . . . . .  32
  3.7.1. DF Election Packet Formats. . . . . . . . . . . . . . . . .  32
  3.7.2. Backoff Message . . . . . . . . . . . . . . . . . . . . . .  33
  3.7.3. Pass Message. . . . . . . . . . . . . . . . . . . . . . . .  34
  3.7.4. Bidir Capable PIM-Hello Option. . . . . . . . . . . . . . .  34
4. RP Discovery. . . . . . . . . . . . . . . . . . . . . . . . . . .  35
5. Security Considerations . . . . . . . . . . . . . . . . . . . . .  35
 5.1. Appendix A: Election Reliability
 Enhancements. . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
  5.1.1. A.1 Missing Pass. . . . . . . . . . . . . . . . . . . . . .  36
  5.1.2. A.2 Periodic Winner Announcement. . . . . . . . . . . . . .  36
 5.2. Appendix B: Interoperability with legacy
 code. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36



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 5.3. Appendix C: Comparison with PIM-SM . . . . . . . . . . . . . .  37
6. Todo list.... . . . . . . . . . . . . . . . . . . . . . . . . . .  38
7. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . .  38
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  38
9. References. . . . . . . . . . . . . . . . . . . . . . . . . . . .  39
10. Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  40













































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1.  Introduction

This document specifies Bi-directional PIM, a variant of PIM Sparse-Mode
(PIM-SM) [9] that builds bi-directional shared trees connecting
multicast sources and receivers.

PIM-SM constructs uni-directional 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 group ranges 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)
  which 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 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.

Bi-directional 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. For a complete discussion of the pros and cons of Bi-
directional PIM consult appendix C.


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 and indicate
requirement levels for compliant PIM-SM implementations.






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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:

MRIB  Multicast Routing Information Base.  This is the multicast
      topology table, which is typically derived from the unicast
      routing table, or routing protocols such as MBGP 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 (RP):
      An RP is a router that has been configured to be used as the root
      of the distribution tree for a range of multicast groups. Join
      messages from receivers for a group are sent towards the RP.

Upstream
      Towards the root (Rendezvous-Point) of the tree. The direction
      used by packets traveling from sources to the RP.

Downstream
      Away from the root of the tree. The direction on which packets
      travel from the RP 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 RP on every link within a BIDIR-PIM domain (this includes
      both multi-access and point-to-point links). The DF is the router
      on the link with the best unicast route to the RP.  A DF for a
      given RP is in charge of forwarding downstream traffic onto the
      link, and forwarding upstream traffic from the link towards the
      RP.  It does this for all the bi-directional groups served by the
      RP. The DF on a link is also responsible for interpreting IGMP
      information from local receivers and processing Join messages from
      other routers on the link.

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 forward packets to that address.  In
      the case of a BIDIR-PIM multicast group, the RPF interface is the
      interface that would be used to send packets to the RP for the
      group.



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RPF Neighbor
      The RPF Neighbor of a router with respect to an address is the
      neighbor that the MRIB indicates should be used to forward packets
      to that address. 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
      directed to the DF on the RPF interface for the group).

TIB   Tree Information Base.  This is the collection of state at a PIM
      router that has been created by receiving PIM Join/Prune messages,
      PIM DF election messages and IGMP information from local hosts.
      It essentially stores the state of all multicast distribution
      trees at that router.

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


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.





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3.  Protocol Specification

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

o Section 3.1 details the protocol state stored.

o Section 3.3 specifies the data packet forwarding rules.

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

o Designated Forwarder (DF) election is specified in Section 3.5.

o PIM packet formats are specified in Section 3.7.

o A summary of BIDIR-PIM timers and their default values is given in
  Section 3.6.


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 PIM-SM needs to hold the state in this form.
This is actually an abstract state definition, which is needed in order
to specify the router's behavior.  A 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:

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

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



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

3.1.1.  General Purpose State

A router holds the following state that is not specific to a RP or
group:

     Neighbor State:

          For each neighbor:

               o Information from neighbor's Hello

               o Neighbor's Gen ID.

               o Neighbor liveness timer (NLT)


3.1.2.  RP State

A router maintains a multicast-group to RP 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 RP a
router holds the following state:

     o RP 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)





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                 o Offer-Count (OC)

                   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 Prune Pending Timer (PPT)

                    o Join/Prune Expiry Timer (ET)

          Not interface specific:

               o Upstream Join/Prune Timer (JT)

               o Last RP Used

Local membership is the result of the local membership mechanism (such
as IGMP) 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.



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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 RP used must be stored because if the RP Set changes [9] then
state must be torn down and rebuilt for groups whose RP changes.



3.1.4.  State Summarization Macros

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


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


RPF_interface(RP) is the interface the MRIB indicates would be used to
route packets to RP. 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(RP(G),I) AND  local_receiver_include(G,I) }


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

The clause "local_receiver_include(G,I)" is true if the IGMP 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(RP(G),I) AND
      DownstreamJPState(G,I) is either Joined or PrunePending }



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

RPF_DF(RP) is the neighbor that Join messages must be sent to in order
to reach the RP. This is the Designated-Forwarder on the
RPF_interface(RP).

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 received
Hello messages as well as maintaining Neighbor State are specified in
the PIM-SM [9] documentation.

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

3.3.  Data Packet Forwarding Rules

For groups mapping to a given RP, the following responsibilities are
uniquely assigned to the DF for that RP 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 RP.

Non-DF routers on a link, that use that link as their RPF interface to
reach the RP, 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).
     RP is the address of the Rendezvous Point for this group.




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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 RP (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 on a data to G on interface iif:

 if( iif == RPF_interface(RP) || I_am_DF(RP,I) ) {
    oiflist = olist(G) (-) iif
    forward packet on all interfaces in oiflist
 }



Note: 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. This removes the need for
performing a directly-connected-source check for data to groups that do
not have existing state.


3.3.1.  Source-Only Branches

Source-only branches of the distribution tree for a group G are branches
which do not lead to any receivers, but which are used to forward
packets traveling upstream from sources towards the RP.  Routers along
source-only branches only have the RPF_interface to the RP in their
olist for G and hence do not need to maintain any group specific state.
Upstream forwarding can be performed using RP state.  An implementation
may decide to maintain group state for source-only branches for
accounting or performance reasons.

3.4.  PIM Join/Prune Messages

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



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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 in the message matches RP(G) (the router's idea of
who the RP is). If the RP in the message does not match RP(G) the Join
or Prune MUST be silently dropped. In addition a router MUST NOT process
Join(*,G) messages targeted to itself if it is not the DF for RP(G) on
the interface on which the message was received.

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

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

     Join (J)
          The interface has (*,G) Join state which will cause us to
          forward packets destined for G from 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.








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                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

           Figure 1: Downstream group per-interface state-machine


In tabular form, the group per-interface state-machine is:

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

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 0.0.0.0 destination address are also
accepted.




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The transition event "Stop being DF" implies a DF re-election taking
place on this router interface 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 triggering Join/Prune message.

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 on a point-to-point
interface.


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 RP.  Such
Join(*,G) messages are sent on the RPF_interface towards the RP 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 [9]) 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 RP trigger a prune off
from the old next hop, and join towards the new next hop. Such a change
can be cause by the following two reasons:

     o The MRIB indicates that the RPF_interface towards the RP has
       changed.




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

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

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

Joined
     The downstream state-machines indicate that the router would like
     to join the RP 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 RP (the DF
on the RPF_interface for RP(G)).

                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

                   Figure 2: Upstream group state-machine





























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In tabular form, the state machine is:

+----------------------+------------------------------------------------+
|                      |                     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 which occur within the
Joined state:

+-----------------------------------------------------------------------+
|                         In Joined (J) State                           |
+-----------------+-----------------+-----------------+-----------------+
|Timer Expires    | See Join(*,G)   | See Prune(*,G)  | RPF_DF(RP(G))   |
|                 | to              | to              | changes         |
|                 | RPF_DF(RP(G))   | RPF_DF(RP(G))   |                 |
+-----------------+-----------------+-----------------+-----------------+
|Send             | Increase Timer  | Decrease Timer  | Decrease Timer  |
|Join(*,G); Set   | to              | to t_override   | to t_override   |
|Timer to         | t_suppressed    |                 |                 |
|t_periodic       |                 |                 |                 |
+-----------------+-----------------+-----------------+-----------------+

+-----------------------------------------------------------------------+
|                         In Joined (J) State                           |
+-------------------------------------+---------------------------------+
|     Change of RPF_DF(RP(G))         |        RPF_DF(RP(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                      |                                 |
+-------------------------------------+---------------------------------+







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This state machine uses the following macro:

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


3.5.  Designated Forwarder (DF) Election

This section presents a fail-safe mechanism for electing a per-RP
designated router on each link in a BIDIR-PIM domain. We call this
router the Designated Forwarder (DF).


3.5.1.  DF Requirements

The DF election chooses the best router on a link to assume the
responsibility of forwarding traffic between the RP and the link for the
range of multicast groups served by the RP.  Different multicast groups
that share a common RP must use the same bi-directional tree for data
forwarding. Hence, the election of an upstream forwarder on each link
does not have to be a group specific decision but instead can be RP-
specific. As the number of RPs 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 to the RP (as reported by the MRIB). When
comparing metrics from different unicast routing protocols, we use the
same comparison rules used by the PIM-SM assert process [9].

The election process needs to take place when information on a new RP
initially becomes available, and can be re-used as new bidir groups for
the same RP are encountered. There are however some conditions where an
update to the election is required:

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

     o The interface on which the RP is reachable changes to an
       interface for which the router was previously the DF.

     o A new PIM neighbor starts up on a link.




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     o The elected DF dies.

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. This is because with the forwarding rules described in section 3.3
if multiple routers end-up thinking that they should be responsible for
forwarding, loops may result. To reduce the possibility of this
occurrence to a minimum, 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 does not provide the definitive specification for the DF
election process. 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 RP, routers on a link
need to exchange their unicast routing metric information (as reported
by the MRIB) for reaching the RP.

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.


3.5.2.1.  Bootstrap Election

Initially when no DF has been elected, routers finding out about a new
RP start participating in the election by sending Offer messages.  Offer
messages include the router's metric to reach the RP. 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. If during this period no winner is elected, then the
router restarts the election from the beginning. If a router receives an
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



Handley/Kouvelas/Speakman/Vicisano           Section 3.5.2.1.  [Page 20]

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neighbor. At that point it transmits a Winner message which declares to
every other router on the link the identity of the winner and the
metrics it is using.

Routers hearing 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 but reinitiates the election.


3.5.2.2.  Loser Metric Changes

Whenever the unicast metric to a RP changes for a non-DF router to a
value that is better than that previously advertised by the acting DF,
the router with the new metric should take action to eventually assume
forwarding responsibility. After 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. If no response is received
after Election_Robustness retransmissions, the 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 metric. Upon receiving this 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 stabilises. The
Backoff message includes the offering router's new metric and address.
All routers on the link who 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 during the Backoff_Period, a third router sends a new better offer,
the Backoff message is repeated for the new offer and the Backoff_Period
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 as soon as the 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.




Handley/Kouvelas/Speakman/Vicisano           Section 3.5.2.2.  [Page 21]

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3.5.2.3.  Winner Metric Changes

If the DF's routing metric to reach the RP changes to a worse value, it
sends a set of Election_Robustness randomly spaced Winner messages on
the link, advertising the new metric. Routers who receive this
announcement but have a better metric may respond with an Offer message
which 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 RP. The old path may now be suboptimal but it will
still work while the re-election is in progress.

If the routing metric at the DF changes to a better value, a single
Winner message is sent advertising the new metric.


3.5.2.4.  Winner Loses Path

If a router's RPF_interface to the RP 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 RP is through the link, an
infinite metric is used in the Offer message it sends.

Note: At this stage the old DF will have a new RPF neighbor on the link
(indicated by unicast routing) which it could use in a Pass message but
this adds unnecessary complication to the election process.


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 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.




Handley/Kouvelas/Speakman/Vicisano           Section 3.5.2.6.  [Page 22]

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If there are downstream routers, typically their RPF_neighbor as
reported by the MRIB before the DF dies will be the DF itself. They will
therefore notice either a change in the metric for the route to the RP
or a change in RPF_neighbor away from the DF and will restart the
election by transmitting Offer messages.  If according to the MRIB the
RP 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 RP for each multicast enabled
interface 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 RP.

     Winner
          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:



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     Acting DF information
          Used to store the election winner who is the currently acting
          DF.

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

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

     Best-Offer
          Used by the DF to record who has made the last offer for
          sending the Pass message.


3.5.3.2.  Election Messages

The election process uses the following PIM control messages the packet
format of which is described in section 3.7:

     Offer (OfferingID, Metric)
          Sent by routers that believe they have a better metric to the
          RP 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 till 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.


3.5.3.3.  Election Events

During protocol operation, in addition to the expiration of the
Election-Timer and the reception of the four control messages, the
following events can take place:




Handley/Kouvelas/Speakman/Vicisano           Section 3.5.3.3.  [Page 24]

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     o Discovery of new RP

     o Metric reported by the MRIB to reach the RP changes

     o DF loses path to RP

     o Detection of DF failure

3.5.3.4.  Election Notation

The DF election state machine description uses the following notation in
addition to the pseudocode notation described earlier in this spec.

     ?=  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 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 our current
       metrics for the DF with the metrics advertised for the sender of
       the message.

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

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.










Handley/Kouvelas/Speakman/Vicisano           Section 3.5.3.5.  [Page 25]

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3.5.3.5.  Election State Transitions


                    +-----------------------------------+
                    | Figures omitted from text version |
                    +-----------------------------------+

            Figure 3: Designated Forwarder election state-machine


In tabular form, the state machine is:

+-------------++--------------------------------------------------------+
|             ||                         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; OC =   |  OC = 0          |
|             || DFT              |  0               |                  |
+-------------++------------------+------------------+------------------+
|             || -                |  -               |  -> Offer        |
| Lose        || DF = Sender or   |  DF = Sender     |  DFT = OPhigh;   |
|             || Target           |                  |  OC = 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        |
+-------------++------------------+------------------+------------------+












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+-----------++----------------------------------------------------------+
|           ||                          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; OC =  |
|           ||OPlow; OC =  |              | Target; DFT  | 0            |
|           ||0            |              | ?= OPlow; OC |              |
|           ||             |              | = 0          |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -> Offer     |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | DFT = OPlow; |
|Lose       ||DFT = OPlow; | DFT = OPlow; | or Target;   | OC = 0       |
|           ||OC = 0       | OC = 0       | DFT = OPlow; |              |
|           ||             |              | OC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -            |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | Send Winner  |
|Win        ||DFT = OPlow; | DFT = OPlow; | or Target;   |              |
|           ||OC = 0       | OC = 0       | DFT = OPlow; |              |
|           ||             |              | OC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+
|           ||-> Offer     | -> Offer     | -> Offer     | -> Win       |
|           ||DF = Sender; | DF = Sender; | DF = Sender  | Send Winner; |
|Backoff    ||DFT = OPlow; | DFT = OPlow; | or Target;   | Stop DFT     |
|           ||OC = 0       | OC = 0       | DFT = OPlow; |              |
|           ||             |              | OC = 0       |              |
+-----------++-------------+--------------+--------------+--------------+


+-----------------------------------------------------------------------+
|                            In Offer State                             |
+-----------------------+-----------------------+-----------------------+
| DFT Expires and OC    |  DFT Expires and OC   |   DFT Expires and OC  |
| is less than          |  is equal to          |   is equal to         |
| Robustness            |  Robustness and we    |   Robustness and      |
|                       |  have path to RP      |   there is no path    |
|                       |                       |   to RP               |
+-----------------------+-----------------------+-----------------------+
| -                     |  -> Win               |   -> Lose             |
| Send Offer; DFT =     |  Send Winner          |   Set DF to None      |
| OPlow; OC = OC + 1    |                       |                       |
+-----------------------+-----------------------+-----------------------+




Handley/Kouvelas/Speakman/Vicisano           Section 3.5.3.5.  [Page 27]

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+-----------------------------------------------------------------------+
|                            In Lose State                              |
+--------------------------------+--------------------------------------+
|     Detect DF Failure          |        Metric changes and now        |
|                                |        is better than DF             |
+--------------------------------+--------------------------------------+
|     -> Offer                   |        -> Offer                      |
|     DF = None; DFT =           |        DFT = OPlow_int; OC = 0       |
|     OPlow_int; OC = 0          |                                      |
+--------------------------------+--------------------------------------+


+-----------------------------------------------------------------------+
|                             In Win State                              |
+-----------------------+------------------------+----------------------+
| Metric changes and    |  Timer Expires and     |   No path to RP      |
| is now worse          |  Count is less than    |                      |
|                       |  Robustness            |                      |
+-----------------------+------------------------+----------------------+
| -                     |  -                     |   -> Offer           |
| DFT = OPlow; OC =     |  Send Winner; DFT =    |   Set DF to None;    |
| 0                     |  OPlow; OC = OC + 1    |   DFT = OPlow; OC =  |
|                       |                        |   0                  |
+-----------------------+------------------------+----------------------+


+-----------------------------------------------------------------------+
|                           In Backoff State                            |
+-----------------------------------+-----------------------------------+
|     Metric changes and is         |         Timer Expires             |
|     now better than Best          |                                   |
+-----------------------------------+-----------------------------------+
|     -> Win                        |         -> Lose                   |
|     Stop Timer                    |         Send Pass; Set DF to      |
|                                   |         stored Best               |
+-----------------------------------+-----------------------------------+

3.6.  Timers 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.





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Per Rendezvous-Point (RP):

     Per interface (I):

          DF Election Timer: DFT(RP,I)

Per Group (G):

     Upstream Join Timer: JT(G)

     Per interface (I):

          Join Expiry Timer: ET(G,I)

          PrunePending Timer: PPT(G,I)

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

































Handley/Kouvelas/Speakman/Vicisano               Section 3.6.  [Page 29]

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

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


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











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Timer Names: Prune Pending Timer (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       |
+--------------------------+--------------------+-----------------------+

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 [9]
documentation.

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




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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 [9] 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 Bidir_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 `224.0.0.13'.

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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Encoded-Unicast-RP-Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   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





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Rsvd Set to zero on transmission.  Ignored upon receipt.

Checksum
     The checksum is standard IP checksum, 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 address of the bidir RP for which the election is taking place
     (note that the length of this field is more than 32 bits).

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

Sender Metric
     The unicast routing table metric used by the message sender to
     reach the RP. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               Encoded-Unicast-Offering-Address                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Offering Metric Preference                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Offering Metric                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Interval           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Offering Address
     The address of the router that made the last (best) Offer (note
     that the length of this field is more than 32 bits).

Offering Metric Preference
     Preference value assigned to the unicast routing protocol that the



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     offering router used to obtain the route to RP-address.

Offering Metric
     The unicast routing table metric used by the offering router to
     reach the RP. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Encoded-Unicast-New-Winner-Address               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 New Winner Metric Preference                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      New Winner Metric                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


New Winner Address
     The address of the router that made the last (best) Offer (note
     that the length of this field is more than 32 bits).

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

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

3.7.4.  Bidir Capable PIM-Hello Option

BIDIR-PIM introduces one new PIM-Hello option.

o OptionType 22: Bidir Capable





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


4.  RP Discovery

Routers discover that a range of multicast group addresses operates in
bi-directional mode and the address of the Rendezvous-Point serving the
group range either through static configuration or using an automatic RP
discovery mechanism like the PIM Bootsrtap mechanism (BSR).  [12].

By default the BSR protocol advertises RPs that operate the PIM-SM
protocol. In order to identify a RP as operating in BIDIR mode, the
Encoded-Group Address field in Bootstrap and Candidate-RP Advertisement
messages has been extended by adding the BIDIR bit (B-bit) as specified
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family   | Encoding Type |B|   Reserved  |  Mask Len     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Group Multicast Address                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


B-bit
     When the Bidir-bit is set, all BIDIR capable PIM routers will
     operate the protocol described in this document for the specified
     group range.

5.  Security Considerations

All PIM control messages MAY use IPsec [6] to address security concerns.
The authentication methods are addressed in a companion document [7].
Keys may be distributed as described in [8].

5.1.  Appendix A: Election Reliability Enhancements

For the correct operation of bi-directional 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 anomalies and correct them.





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5.1.1.  A.1 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 RPs
for which it acting as the DF. The anomaly may also be logged by the
router to alert the operator.


5.1.2.  A.2 Periodic Winner Announcement

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


5.2.  Appendix B: Interoperability with legacy code

The rules provided in [10] for interoperating between legacy PIM-SM
routers and new bi-directional capable routers change only slightly to
support this new proposal. The only difference is in the definition of a
boundary between a bi-directional capable area and a legacy area of the
network.  In [10], a bidir capable router forwarding upstream, register
encapsulates the data packet to the RP if its RPF neighbor is not bidir
capable.

In our proposal, since all the routers on a link need to co-operate to
elect the Designated Forwarder, if even one of the routers on the link
is a legacy router, the election cannot take place. As a result register
encapsulation is necessary if one or more routers on the RPF interface
are not bi-directional capable.

As in [10], a Hello option must be used to differentiate between bi-
directional capable and legacy routers, and (S,G) state must be created
on the router doing the register encapsulation to prevent loops.




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5.3.  Appendix C: Comparison with PIM-SM

This section describes the main differences between Bidir PIM and
sparse-mode PIM:

     o Bidir PIM uses a single shared tree for distributing the data for
       all the sources of a multicast group. The use of a single tree
       significantly reduces state requirements on a router. The
       drawback is that it may produce suboptimal paths from sources to
       receivers possibly resulting in higher network latency and less
       efficient bandwidth utilisation.

     o In Bidir PIM, packets traveling from a source to the RP, are
       natively forwarded on the shared tree. In contrast sparse-mode
       PIM uses unicast encapsulation or source-specific state.

     o In Bidir PIM, sender-only branches do not need to keep group
       state. Data from the source can be natively forwarded towards the
       RP using RP-specific forwarding state.

     o The Bidir Designated Forwarder (DF) assumes all the
       responsibilities of the sparse-mode DR. In a multi-access link,
       the DF responds to IGMP notifications. Downstream routers on the
       link use the DF as their upstream neighbor and direct all
       Join/Prune messages towards it.

     o To enforce a single forwarder on multi-access links, sparse-mode
       PIM uses the Assert mechanism which requires data-packets to
       trigger protocol events. In Bidir PIM, data-driven events are
       completely eliminated as a correct route is always available at
       packet forwarding time.

     The DF election problem is easier than the assert problem because
     there is a small number of RPs and the per RP DF election can be
     done in advance. With the assert mechanism, in addition to each RP,
     a forwarder has to be elected for each possible source to a group.
     This can not be done before data is available.

     o With sparse-mode PIM, when forwarding packets using shared-tree
       (*,G) state, a directly-connected-source check has to be made on
       every packet.  This is done to determine if the packet was
       originated by a source which is directly connected to the router.
       For a connected source, source-specific state has to be created
       to register packets to the RP and prune the source off the shared
       tree.

     With Bidir PIM directly connected sources do not need any special
     handling. The DF for the RP of the group the source is sending to,



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     seamlessly picks-up and forwards upstream traveling packets.



6.  Todo list...

o Update legacy interoperability section to remove dependency on 10.

o Incorporate BSR mods into BSR spec.

o In the state machine, perhaps add an arc from NI to NI, labelled
  "(*,G) Join but not DF" just to make it really clear.


7.  Authors' Addresses

     Mark Handley
     ICIR/ICSI
     1947 Center St, Suite 600
     Berkeley, CA 94708
     mjh@icir.org


     Isidor Kouvelas
     Cisco Systems
     kouvelas@cisco.com


     Tony Speakman
     Cisco Systems
     speakman@cisco.com


     Lorenzo Vicisano
     Cisco Systems
     lorenzo@cisco.com


8.  Acknowledgments

The bidir proposal in this draft is heavily based on the ideas and text
presented by Estrin and Farinacci in [10]. The main difference between
the two proposals is in the method chosen for upstream forwarding.

We would also like to thank Deborah Estrin at ISI/USC as well as Nidhi
Bhaskar, Yiqun Cai, Apoorva Karan, Rajitha Sumanasekera and Beau
Williamson at cisco for their contributions and comments to this draft.




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9.  References

[1] T. Bates , R. Chandra , D. Katz , Y. Rekhter, "Multiprotocol
     Extensions for BGP-4", RFC 2283

[2] S.E. Deering, "Host extensions for IP multicasting", RFC 1112, Aug
     1989.

[3] W. Fenner, "Internet Group Management Protocol, Version 2", RFC
     2236.

[4] IANA, "Address Family Numbers", linked from
     http://www.iana.org/numbers.html

[5] T. Narten , H. Alvestrand, "Guidelines for Writing an IANA
     Considerations Section in RFCs", RFC 2434.

[6] S. Kent, R. Atkinson, "Security Architecture for the Internet
     Protocol.", RFC 2401.

[7] L. Wei, "Authenticating PIM version 2 messages", draft-ietf-pim-
     v2-auth-01.txt, work in progress.

[8] T. Hardjono, B. Cain, "Simple Key Management Protocol for PIM",
     draft-ietf-pim-simplekmp-01.txt, work in progress.

[9] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas "Protocol
     Independent Multicast - Sparse Mode (PIM-SM):  Protocol
     Specification (Revised)", Work In Progress, <draft-ietf-pim-sm-
     v2-new-05.txt>, 2002.

[10] D. Estrin, D. Farinacci, "Bi-directional Shared Trees in PIM-SM",
     Work In Progress, <draft-farinacci-bidir-pim-01.txt>, May 1999.

[11] D. Estrin et al, "Protocol Independent Multicast-Sparse Mode (PIM-
     SM): Protocol Specification", RFC 2362, Nov 1999.

[12] W. Fenner, M. Handley, R. Kermode and D. Thaler, "Bootstrap Router
     (BSR) Mechanism for PIM Sparse Mode", draft-ietf-pim-sm-bsr-00.txt,
     work in progress.











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10.  Index
DownstreamJPState(G,I) . . . . . . . . . . . . . . . . . . . . . . .  11
ET(G,I). . . . . . . . . . . . . . . . . . . . . . . . . . . . .10,14,30
ET(RP,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
I_am_DF(RP,I). . . . . . . . . . . . . . . . . . . . . . . . . .11,13,16
J/P_HoldTime . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
J/P_Override_Interval. . . . . . . . . . . . . . . . . . . . . . . 16,31
JoinDesired(G) . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
joins(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
JT(*,G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
JT(G). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10,31
local_receiver_include(G,I). . . . . . . . . . . . . . . . . . . . .  11
NLT(N,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
Offer_Period . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
olist(G) . . . . . . . . . . . . . . . . . . . . . . . . . . . .11,13,18
OT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30
pim_include(G) . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
PPT(G,I) . . . . . . . . . . . . . . . . . . . . . . . . . . . .10,14,31
RPF_interface(RP). . . . . . . . . . . . . . . . . . . . . . . . . 11,13
t_override . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18,31
t_periodic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18,31
t_suppressed . . . . . . . . . . . . . . . . . . . . . . . . . . . 18,31





























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