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Versions: 01

Network Working Group                                     Deborah Estrin
INTERNET-DRAFT                                                   USC/ISI
Expiration Date: November 1999                            Dino Farinacci
                                                           cisco Systems
                                                            May 17, 1999

                 Bi-Directional Shared Trees in PIM-SM
                   <draft-farinacci-bidir-pim-01.txt>

Status of this Memo

   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.

Abstract

   This proposal extends the PIM-SM [1]  mechanism for multicast
   datagram forwarding. PIM-SM constructs and maintains uni-directional
   shared trees and uni-directional source trees. We describe how we can
   extend the elements of operation of sparse-mode PIM to support bi-
   directional shared trees.

1. Introduction

   A uni-directional shared tree allows sources to send multicast
   datagrams to members of a multicast group. Members receive packets
   sent to the group by joining the shared tree, using a particular node
   in the network as the root of the shared tree. The root of the shared

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Internet Draft      PIM Bi-Directional Shared Trees             May 1999

   tree is called the Rendezvous Point (RP). When using undirectional
   shared trees, all sources' datagrams initially go to the root (RP) of
   the tree before being delivered down the distribution tree. As a
   result, there can be suboptimal delivery paths to the receivers close
   to the source.

   In PIM-SM, the RP typically joins back to the source to draw
   datagrams down directly and natively (no encapsulation) from the
   source to the RP. The RP then forwards the datagrams down the uni-
   directional shared tree to the receivers. Eventually, receivers may
   join to the source as well, thus drawing datagrams down a source
   specific, uni-directional, shortest path tree. Or the receivers may
   continue to receive datagrams on the shared tree.

   When using bi-directional shared trees, data can flow in either
   direction on a branch of the tree. This allows improved data delivery
   to receivers close to the source because the traffic traveling
   upstream to the root node is "turned around" and forwarded on
   downstream branches [2].

   The bi-directional shared trees described in this extension to PIM-SM
   are used both to distribute datagrams from sources to the RP, as well
   as to distribute datagrams directly to receivers. Moreover, the
   protocol does not build source-specific trees from sources to the RP,
   nor to receivers. Instead, source transmissions travel up the shared
   tree toward the RP providing coverage to receivers along the way.
   The RP only needs to forward datagrams downward on those branches of
   the shared tree not covered by the path from the source to the RP.

   However, bi-directional trees are incompatible with source specific
   uni-directional trees and so no switching to source-trees is allowed.
   Source-trees have the best delay characteristics so there is a
   tradeoff between uni-directional shared trees with source-trees and
   bi-directional shared trees. For large numbers of moderate to low
   rate sources, bi-directional PIM may offer significant advantages.

2. Pros and Cons of Bi-Directional Shared Trees

   There are 3 basic advantages of bi-directional shared trees:

   1. State is reduced compared to source trees. Each router in
      the multicast routing domain needs only keep state for the
      group and not each source sending to each group. [2]

   2. Datagrams from sources to topologically near-by receivers do not have
      to travel all the way to the root of the shared tree. These improved
      distribution paths also support better scoping semantics for

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      applications that might use TTL based expanding ring scope to look
      for nearby resources.

   3. Bursty sources can send with no or little state in routers.

   There are 3 basic disadvantages of bi-directional shared trees:

   1. Since all traffic eventually goes to the root of the tree, there is a
      traffic concentration point at the root node and links leading to
      it (pruning mechanisms could be added but at the cost of additional
      state and complexity). Traffic always flows to the root node even when
      it doesn't have to. That is, if the root node has a single sender
      branch, the root does not take part in forwarding traffic but it must
      receive the traffic because downstream nodes don't know the group
      membership tree near the root.

   2. The path taken between the source and receivers might not travel
      over the shortest path, although it is likely to be a shorter
      path than via a uni-directional shared tree.

   3. Bi-directional trees are incompatible with uni-directional
      source-trees. There is an increase in complexity when both are
      used for the same group.

   Compared to CBT, the bi-directional trees proposed in this
   specification differ in two respects:

   1. Non-member senders do not encapsulate their data to the root, the data
      is forwarded along the same path that it would take if the sender were
      also a member.
   2. The protocol reuses much of the existing PIM-SM implementation.

3. Modifications to PIM

   A strong goal of this proposal is to make as few changes as possible
   to PIM and multicast forwarding. We also wish to make the changes
   compatible, to enable a phased (incrementally deployed) transition to
   bi-directional shared tree PIM. Therefore, we use (*,G) state to
   describe bi-directional shared tree state (traditionally (*,G) has
   been used to describe uni-directional shared tree state).

   By definition a PIM bi-directional shared tree group may not have any
   (S,G) state stored for the group. There are exceptions when mixing
   non-bidir PIM routers with bidir PIM routers (see later in this
   specification).

   We assume that at the same time a router learns the RP for a group,

Estrin, Farinacci                                               [Page 3]

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   it will know if the group is to operate in bi-directional shared tree
   mode or uni-directional shared tree mode. This assumption greatly
   simplifies the deployment and operation of the protocol.

4. Modifications to Multicast Forwarding

   There will be modifications to multicast forwarding since bi-
   directional shared tree delivery requires traffic to flow upstream
   (towards the root). This is contrary to RPF forwarding rules used on
   uni-directional shared trees (datagrams can only be forwarded away
   from the root node, downstream towards receivers).

5. No Modifications to Multicast Capable Hosts

   This proposal does not require modifications to multicast capable
   hosts [3].  Hosts that receive multicast datagrams with the UMP
   option must ignore the option and accept the datagram [6].

6. How are hosts Joined to a Bi-directional Shared Tree

   No change is required in hosts. Receiving hosts use IGMP [3] (as they
   do today) to join multicast groups. The attached designated router
   (DR) will initiate joining of the shared tree.

   The attached routers perform the same actions as are done to graft
   branches on the uni-directional tree. That is, the designated router
   (DR), on the attached subnet with the receiver, will send a PIM
   Join/Prune message for (*,G) with the RP in the Join-List toward the
   RP for the group. Routers maintain (*,G) state as defined in the
   sparse-mode PIM specification. [1]

7. How are Source's Datagrams sent onto a Bi-directional Shared Tree

   No change is required to hosts. A source host will send a multicast
   datagram by transmitting on its attached interface (as it does today)
   [3]. The attached DR will initiate the delivery of the multicast
   datagram upstream towards the RP.

   When a datagram flows upstream, a receiving router must know that it
   can bypass the RPF check on the (*,G) entry. To accomplish this, we
   introduce a new IP option called the Upstream Multicast Packet (UMP).
   The UMP IP header option is encoded as follows:

Estrin, Farinacci                                               [Page 4]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Option Type  | Option Length |         Reserved              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Upstream Router's IP address                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Option Type value is 152 (0x98). That is, the 1-bit copied flag is
   set to 1, the 2-bit option class is set to 0, and the 5-bit option
   number is 24.

   When a router forwards a multicast datagram upstream, it identifies
   the upstream router in the option. Only the indicated upstream router
   is responsible  for forwarding the datagram upstream. When a router
   forwards a multicast datagram with the UMP option, it will multicast
   the datagram on the attached upstream subnet so other routers can
   forward datagrams down the shared tree if they have (*,G) state. Any
   directly attached members also receive the datagram.

   In most cases, only a single copy of the datagram is sent upstream,
   taking advantage of multi-access/multicast capable media whereever
   possible. However, on the first hop LAN, there may be two datagrams
   that traverse the LAN. See next section for details.

   It is important to note that symmetry between receivers and senders
   along the same branch must be maintained. That is, a router must join
   along the same path it would forward traffic upstream or loops could
   result. This specification forces symmetry because the same choice
   for forwarding and joining is achieved by using the RPF neighbor to
   the RP.

8. First-hop LAN

   When a source transmits a multicast datagram, there is one router on
   the attached LAN that will insert the UMP option. The PIM designated
   router (DR) will be responsible for this. The DR will insert the UMP
   option using the address of the next-hop router it knows to reach the
   RP for the (*,G) entry.

   There is one case where two datagrams traverse the first-hop LAN. The
   first datagram is transmitted as multicast by the source and the
   second is transmitted by the DR as MAC-level unicast to the next-hop
   upstream router. This only occurs when the DR uses the first-hop LAN
   as its RPF interface for (*,G). If the DR is an upstream router, the
   extra datagram is not sent because the RPF interface for (*,G) is not
   the first-hop LAN.

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   The DR is made responsible for selecting the upstream router in order
   to avoid inconsistent join and forwarding decisions if multiple
   downstream routers on the LAN receive joins or datagrams for the same
   group. If all routers on a LAN always ran a common link state
   protocol or always had some other means to guarantee consistent
   routing information, then this would not be necessary. However, in
   order to allow loop free operation in the widest range of
   environments, without making restrictive assumptions about unicast
   routing protocols, configurations and policies, we make use of the DR
   to enforce consistent decisions.

   A network administrator can control which router is the PIM DR [5] to
   avoid particular suboptimal cases.

9. Multicast Forwarding Rules

   The following steps describe the rules for bi-directional shared tree
   forwarding in PIM.  When a router receives a multicast datagram, it
   may arrive on the RPF interface for a (*,G) entry or another (the
   non-RPF) interface.

   A. When a multicast datagram arrives on the RPF interface toward the RP:

   A1. A multicast routing table lookup is performed. Only a (*,G) entry
       can be returned (based on our definition that PIM bi-directional
       shared trees groups will not have (S,G) route entries).

       If the entry is not found, the datagram is silently discarded.

   A2. If the entry is found, the datagram is sent out each outgoing
       interface that resides in the outgoing interface list for the (*,G)
       entry. In this situation, the router doesn't care if the (*,G) tree
       state is bi-directional or uni-directional.

   A3. Before replicating the datagram on each outgoing interface, a router
       checks to see if the UMP option is present. If so, it can either
       remove the option or replace the existing address with 0.0.0.0 in
       the Upstream Router IP Address field. This is to indicate to
       downstream routers the datagram is not traveling upstream.

   A4. If the UMP option isn't present and the router is DR on the
       interface the datagram was received on and the source is directly
       attached, the DR is responsible for inserting the UMP option. It
       includes in the UMP option address the next-hop IP address of its
       RPF neighbor for (*,G). The DR forwards the datagram using the
       MAC-level address (unicast address) of this RPF neighbor.

Estrin, Farinacci                                               [Page 6]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

   A5. If the UMP option isn't present and the router isn't the DR, or the
       source isn't directly attached, the datagram is silently discarded.

   B. When multicast datagram arrives on a non-RPF interface toward the RP:

   B1. A multicast routing table lookup is performed. Only a (*,G) entry
       can be returned (based on our definition of PIM bi-directional shared
       trees).

   B2. The router looks at the UMP option. If the option is present and
       the Upstream Router IP Address is not its own IP address on the
       received interface, the datagram is silently discarded.

   B3. If the UMP option is not present and the router is directly connected
       to the source of the multicast datagram and is the DR on the
       interface, the DR inserts the UMP option and follows the steps B4.1
       and B4.2.

   B4. If the UMP option is present and the Upstream Router IP Address field
       contains the IP address of the receiving router (on the received
       interface), it will forward the datagram as follows:

       B4.1 The datagram is sent out each outgoing interface that resides
            in the outgoing interface list for the (*,G) entry except for
            the interface on which the datagram was received on. Before
            sending out each interface, the router may remove the UMP
            option from the datagram or replace the existing address with
            0.0.0.0 in the Upstream Router IP Address field.

       B4.2 The datagram is forwarded on the RPF interface for (*,G) by
            replacing the Upstream Router IP Address field in the UMP
            option with the next-hop address of the router that is used
            to reach the RP.

10. Distinguishing Bi-Directional Shared Tree Groups from other Groups

   When routers discover the identity of the RP for a multicast group
   they can determine if the group will operate in bi-directional shared
   tree mode or uni-directional tree mode. We modify the Encoded-Group
   Address fields in PIM Bootstrap and Candidate-RP Advertisement
   messages to include the Bidir-bit (see bit B below):

Estrin, Farinacci                                               [Page 7]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

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

   When the Bidir-bit is set, all upgraded bi-directional PIM routers
   will follow the forwarding rules described in this specification.

11. Mixing Bi-Directional Capable with Uni-Directional-Only Routers

   It will take time to upgrade all PIM routers in a domain to be bi-
   directional shared tree capable. However, enabling bi-directional
   shared tree routers in an existing network can be easy and simple.
   First, no special attention at the protocol level needs to be taken
   if the network is engineered where you can place bidir PIM routers
   strategically near sources. That is, if sources are located on
   sender-only branches (no Joins have traveled up that branch) of the
   bi-directional shared tree, only that branch needs to be upgraded
   with bi-directional shared tree capable routers. All other routers on
   receiver branches forward based on (*,G) uni-directional shared tree
   forwarding rules.

   When the network cannot be engineered to locate bi-directional shared
   tree capable routers on sender-only branches, the following
   transition support can be implemented:

   o A router will detect if its upstream neighboring router toward the
     RP is bi-directional shared tree capable or not. We will use the
     Bidir-Capable PIM Hello Option to convey this information.

   o A router that is one-hop downstream (of the RP) from a non-bidir
     capable router will maintain (S,G) state and will be responsible for
     forwarding multicast traffic as to the RP by Registering to the RP as
     it would if it was a DR (or MBR) in uni-directional mode. When the
     data arrives at the RP, it can be delivered on the uni-directional
     shared-tree (or any source trees that overlap with the shared tree).

Estrin, Farinacci                                               [Page 8]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

   We define the following PIM Hello Option:

   Bidir-Capable Option

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |PIM Ver| Type  | Reserved      |           Checksum            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       OptionType              |         OptionLength          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          OptionValue                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               .                               |
       |                               .                               |
       |                               .                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       OptionType              |         OptionLength          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          OptionValue                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       OptionType = 19 (decimal)

       OptionLength
           The length of the option payload in bytes. This is set to 0.

   Therefore, the following encoding is inserted in the PIM  Hello  mes-
   sage:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             19                |              0                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   By definition, in a pure bi-directional router environment, a bidir
   capable PIM router will not create (S,G) state when it either 1)
   receives a datagram or 2) receives any PIM control message.  However,
   there is one exception. When a router receives a datagram that is
   traveling upstream (the UMP option is present or the router is the DR
   directly attached to the source) and the upstream neighbor toward the
   RP is not bidir capable, it will create (S,G) state and set the
   necessary flags indicating datagrams that match the route entry will
   be Register encapsulated to the RP. In this case, the router still
   doesn't accept join messages (and therefore doesn't populate the
   (S,G) olist) if there are routers upstream that are sending (S,G)
   Joins or Prunes. A router that does this transition logic is called,

Estrin, Farinacci                                               [Page 9]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

   a bidir border router.

   If a bidir router creates (S,G) state for a bi-directional group, it
   will not send Join/Prune messages for the entry. If a bidir router
   changes its RPF neighbor toward the RP and the RPF neighbor is bidir
   capable, it will delete its (S,G) entries.

   A bidir router must do longest match lookups for a group that is in
   bi-directional tree mode.  This handles the case where the RP
   forwards datagrams down a branch that has a both a sender and a
   member on it and avoids datagrams returning to the sender. In this
   case, a bidir border router should RPF fail for such datagrams since
   it will use the (S,G) entry rather than the (*,G) entry for the
   forwarding decision.

   If the RP is a bidir capable router and it receives a Register
   message, it will not create (S,G) state. It will forward the data
   encapsulated in the Register message down the shared tree. The RP
   will only send a Register-Stop if there are no members for the group
   (the (*,G) outgoing interface list is empty). An RP will receive a
   Register message in two cases, 1) the DR is a non-bidir capable
   router, or 2) it was sent by a bidir border router.

   If the RP is not bidir capable and it receives a Register from either
   a non-bidir capable DR or a bidir border router, it may trigger a
   Join toward the source. If there are any bidir capable routers on the
   path, they will not create (S,G) state. In this case, the RP will
   never get data natively and therefore never send Register-Stop
   messages. The data will continue to be delivered via Register
   encapsulation.

Estrin, Farinacci                                              [Page 10]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

   The following shows all possible cases mixing non-bidir (old) and bidir
   capable (new) routers. Each column shows the capability of 1) the RP, 2)
   an router between the DR and the RP, and 3) the DR. The following
notation
   is used:

      "new-rp" - RP is bidir capable
      "old-rp" - RP is non-bidir capable
      "old"    - router is non-bidir capable
      "new"    - router is bidir capaple
      "new-dr" - DR is bidir capable
      "old-dr" - DR is non-bidir capable

    (1)      (2)      (3)      (4)      (5)      (6)      (7)      (8)
   old-rp   old-rp   old-rp   old-rp   new-rp   new-rp   new-rp   new-rp
   old      old      new      new      new      new      old      old
   old-dr   new-dr   old-dr   new-dr   new-dr   old-dr   new-dr   old-dr

   (1) This case is uni-directional mode.

   (2) The bidir DR sends Registers and does not forward datagrams with
       the UMP option. It does this because it detects the upstream router
       is not bidir-capable. The RP joins back to the source through the
       intermediate router. The intermediate router's join is ignored by
       the bidir DR. Datagrams get to receivers via Register encapsulation
       only from the DR.

   (3) The non-bidir DR sends Registers. The non-bidir RP may send joins
       but the bidir intermediate router will ignore them. Datagrams get
       to receivers via Register encapsulation only from the DR.

   (4) The bidir DR forwards multicast datagram with the UMP option
       upstream. It does this because it detects the upstream router is
       bidir-capable. The bidir intermediate router (acting as a bidir
       border router) sends Registers to the non-bidir RP. Datagrams get
       to receivers via Register encapsulation only from the bidir border
       router.

   (5) This case is bi-directional shared tree mode.

   (6) The non-bidir DR will Register to the bidir RP. The bidir RP will
       not send Joins back to the source. It only Register-Stops if there
       are no members. The bidir intermediate router is not involved in
       forwarding multicast datagrams. Datagrams get to receivers via
       Register encapsulation only from the DR.

   (7) The bidir DR will Register to the bidir RP. It does this because it
       detects the upstream router is non-bidir capable. It is performing
       as a bidir border router. The bidir RP will not send Joins back to

Estrin, Farinacci                                              [Page 11]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

       the source. It only Register-Stops if there are no members. The
       non-bidir intermediate router is not involved in forwarding
       multicast datagrams. Datagrams get to receivers via Register
       encapsulation only from the DR.

   (8) The non-bidir DR will Register to the bidir RP. The bidir RP will
       not send Joins back to the source. It only Register-Stops if there
       are no members. The non-bidir intermediate router is not involved
       in forwarding multicast datagrams. Datagrams get to receivers via
       Register encapsulation only from the DR.

12. Security Considerations

   When IPsec [4] is used on a multicast datagram, the UMP IP option
   will not be part of the encrypted payload. This is justified by
   allowing routers to be performant when forwarding datagrams upstream.

Estrin, Farinacci                                              [Page 12]

Internet Draft      PIM Bi-Directional Shared Trees             May 1999

13. Acknowledgments

   The authors would like to acknowledge Rusty Eddy (USC), Radia Perlman
   (Sun), Tony Speakman (cisco), and Liming Wei (cisco) for their
   comments and contributions to this specification.

14. Author Information

   Deborah Estrin
   ISI/USC
   estrin@usc.edu

   Dino Farinacci
   cisco Systems, Inc.
   dino@cisco.com

15. References

   [1] Estrin, et al., "Protocol Independent Multicast-Sparse Mode (PIM-
       SM): Protocol Specification", RFC 2362, June 1998.

   [2] A.J. Ballardie, P.F. Francis, and J.Crowcroft. Core based trees.
       In Proceedings of the ACM SIGCOMM, San Francisco, 1993.

   [3] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
       1112, August 1989.

   [4] Atkinson, R., "Security Architecture for the Internet Protocol",
       RFC 1825, August 1995.

   [5] Wei, L., Farinacci, D., "PIM Version 2 DR Election Priority Option",
       INTERNET-DRAFT, March 1998.

   [6] ISI/USC, "Internet Protocol", RFC 791, September 1981.

Estrin, Farinacci                                              [Page 13]


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