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Versions: 00 01 02 03                                                   
Network Working Group                                     Dino Farinacci
Internet Draft                                    Procket Networks, Inc.
Expiration Date: December 2000
                                                           Yakov Rekhter
                                                           Eric C. Rosen
                                                                Ted Qian
                                                     Cisco Systems, Inc.

                                                               June 2000


        Using PIM to Distribute MPLS Labels for Multicast Routes


                 draft-farinacci-mpls-multicast-02.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 document specifies a method of distributing MPLS labels [MPLS-
   ARCH, MPLS-MUL-FR] for multicast routes.

   The labels are distributed in the same PIM messages that are used to
   create the corresponding routes.  The method is media-type
   independent, and therefore works for multi-access/multicast capable
   LANs, point-to-point links, and NBMA networks.








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Table of Contents

    1          Overview  ...........................................   2
    2          Label Distribution for PIM-SM  ......................   3
    2.1        Piggybacking  .......................................   3
    2.2        LANs with Multiple Downstream Nodes  ................   5
    2.2.1      Partitioning the Label Space  .......................   5
    2.2.1.1    Partitioning Procedure  .............................   5
    2.2.1.2    Effect of Partition in Layer 2 Topology  ............   6
    2.2.1.3    Effect of Exceeding the Label Range  ................   7
    2.2.2      Assigning the Labels  ...............................   7
    2.3        Labels for Point-to-Point Links  ....................   8
    2.4        Labels for NBMA Networks  ...........................   8
    2.5        When NOT to Send a Labelled Multicast Packet  .......   9
    2.6        No Conflict between Unicast and Multicast Labels  ...   9
    2.7        Supporting Bidirectional PIM  .......................   9
    3          Modifications to PIMv2  .............................  10
    3.1        Join/Prune Packets  .................................  10
    3.2        Hello Packets  ......................................  11
    4          Label Distribution for PIM-DM  ......................  13
    5          Security Considerations  ............................  13
    6          Acknowledgments  ....................................  14
    7          References  .........................................  15




1. Overview

   PIM [PIMv1, PIMv2] is used to combine MPLS label distribution with
   the distribution of (*,G) join state, (S,G) join state, or (S,G)RPT-
   bit prune state. Labels and multicast routes are sent together in one
   message.

   This design has the following goals:

     o If an interface attaches to a network with data-link broadcast
       capability, an LSR should never have to send more than one copy
       of a given multicast data packet out that interface.  However, it
       is NOT a goal for that LSR to be able to send the same packet,
       with the same label, out multiple interfaces.

     o When an interface supports data link multicasting, it must be
       possible for the receiver of a labeled packet to interpret the
       label without knowing who the transmitter is.




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     o When a LAN contains multiple label distribution peers, it should
       be possible to use data link multicast to distribute the label
       distribution control packets themselves.  Other aspects of label
       distribution methodology should remain as consistent with unicast
       label distribution as possible.

     o Multicast label distribution procedures should not depend on the
       media type.

     o Once the label for a particular multicast tree on a given LAN has
       been assigned, unicast routing changes should not cause
       redistribution or reassignment of the label for that group on
       that LAN.

     o When a multicast routing table change requires a label
       distribution change, the latency between the two should be
       minimized, both to improve performance and to minimize the
       possibility of race conditions.

     o The procedures should work with either dense-mode or sparse mode
       operation.


2. Label Distribution for PIM-SM

2.1. Piggybacking

   An LSR that supports multicast sends PIM Join/Prune messages on
   behalf of hosts that join groups. It sends Join/Prune messages to
   upstream neighboring LSRs toward the RP for the shared-tree (*,G) or
   toward a source for a source-tree (S,G).  Labels are distributed by
   being associated with addresses in the join list or the prune list.
   In particular:

      1. If an LSR, Rd, joins the shared tree for a group, the
         Join/Prune message it sends upstream will contain the group
         address followed by a join-list.  The join-list will contain an
         element which contains the address of the RP.  This element
         will also contain a a label, and this label can be used by the
         upstream LSR, Ru, when it sends multicast data down the shared
         tree.  Intuitively, this label represents the route downstream
         from the current node along the shared tree.

         Note that if Rd joins the shared tree for group G, but Ru
         happens to have (S,G) state for some S, then Ru must merge its
         (*,G) output interface list into its (S,G) output interface
         list.  This is necessary in order to ensure that Rd will
         receive packets sent from S to G, even though Ru only gets



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         these packets on the source tree.  In this case, when Ru
         receives an (S,G) packet, it should forward it to Rd using the
         same label that Rd assigned for (*,G) packets, AS LONG AS THE
         Ru-Rd LINK IS NOT AN LC-ATM interface [MPLS-ATM].  The case
         where the link is an LC-ATM interface is not considered in the
         current version of this document, but will be described in a
         future version.  (The procedure described in this paragraph
         would create a mulitpoint-to-multipoint VC, which ATM-LSRs
         might not be able to support.)

      2. If an LSR, Rd, joins a source tree for a group, the Join/Prune
         message it sends upstream will contain the group address
         followed by a join-list.  The join-list will contain an element
         which contains the address of the source.  This element will
         also contain a label, and this label can be used by the
         upstream LSR, Ru, when it sends multicast data down the source
         tree.  Intuitively, this label represents the route downstream
         from the current node along the specified source tree.

      3. Suppose an LSR, Rd, has (S,G)RPT-bit state with a null output
         interface list.  This indicates that all of its downstream
         neighbors on the shared tree for G have pruned source S from
         the shared tree.  Rd sends a Join/Prune message upstream (on
         the shared tree), containing the group address followed by a
         prune-list.  The prune-list contains an element which contains
         the address of the source.  In this case, no label is included
         in the element.

         Similarly, if an LSR has (S,G) state, and also has (*,G) state
         with a non-null output interface list, it will send a
         Join/Prune message upstream on the shared tree, with S in the
         prune-list, and will not include a label.

      4. Suppose an LSR, Rd, as the result of receiving, from a
         downstream neighbor on the shared tree, a Join/Prune message
         such as described in 3, creates (S,G)RPT-bit state with a non-
         null output interface list.  In this case, it may send a
         Join/Prune message upstream on the shared tree, containing the
         group address followed by a prune-list.  An element of the
         prune list will contain the address S and a corresponding
         label.  However, a special bit (the "Label Only" bit, or "L-
         bit") in the element will be set indicating to the upstream LSR
         that the source S is not really to be pruned from the shared
         tree.  The result is that the upstream LSR, Ru, will still send
         packets from S to G to Rd, and will label those packets as
         specified.  When Rd receives such packets, it forwards them
         according to the output interface list of the (S,G)RPT-bit
         entry.  Intuitively, this label represents a route along the



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         shared tree, but only for packets from the specified source.

      5. An LSR which receives a Join/Prune message as described in 4
         may send a corresponding Join/Prune message (with the L-bit
         set) to its upstream LSR on the shared tree. Again, this label
         represents a route along the shared tree, but only for packets
         from the specified source.

   Rules 3-5 above ensure that if a source is pruned off the shared tree
   at some point, any packets from that source which is sent down the
   shared tree will have a label that implicitly identifies the source.
   Thus if those packets encounter a node with (S,G)RPT-bit state, they
   will be sent according to the output interface list of the (S,G)RPT-
   bit entry, NOT according to the output interface list of the (*,G)
   entry.


2.2. LANs with Multiple Downstream Nodes

2.2.1. Partitioning the Label Space

   Only one copy of a given multicast data packet is sent downstream.
   On a LAN, this packet will be received by all the LSRs on the LAN.
   The label it carries is used, by the receiving LSRs, to find the
   packet's multicast distribution tree.  The label it carries must have
   a unique association, on that LAN, with a multicast distribution
   tree.

   Therefore, once an LSR assigns a label to a particular multicast
   distribution tree on a particular LAN, all other LSRs on that LAN are
   prohibited from making any other use of that label.  The prohibition
   remains in effect as long as the distribution tree in question
   exists.

   In order to meet this requirement, the LSRs on a LAN must divide up
   the label space, such that each LSR has a particular unique range of
   labels which it may distribute.


2.2.1.1. Partitioning Procedure

   Each multicast LSR on a LAN is configured with the total number of
   labels (N) that may be used to represent multicast distribution trees
   on the LAN.  It is also configured with an approximate count (R) of
   the routers on the LAN.  The router divides the multicast label space
   into a number of equal-sized ranges, where the size of a range is
   T/R.  Each router will randomly select one of these ranges.




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   When a multicast LSR boots up or enables the LAN interface to do
   multicast routing, it will advertise in PIM Hello messages the total
   number of multicast labels, the router count, the label range it
   randomly selects, and optionally its priority. The lower range label
   value and the higher range label value accompany the advertisement.
   If a LSR doesn't advertise its priority, it is treated as if the LSR
   would advertise the highest possible priority.

   If the total number of multicast labels for the LAN is not configured
   consistently on all LSRs connected to a LAN, the smallest number
   advertised by any LSR will be used.

   If the router count is not configured consistently on all LSRs
   connected to a LAN, the smallest router count value advertised by any
   LSR will be used.

   If there is another LSR that has selected the same range, then the
   following procedures are used to determine which of the two LSRs
   would be able to keep its range, and which would be required to
   allocate another label range.  If the two LSRs have different
   priority, then the one with the lower priority is required to
   allocate another label range. If the two LSRs have the same priority,
   then the one with the lower IP address on the LAN is required to
   allocate another label range. In both cases the label range is
   allocated randomly. If as a result of these procedures a LSR has to
   allocate another label range, then the LSR has to withdraw its label
   bindings from its currently allocated range, and then (after it
   allocates another range) reallocate its bindings.

   A LSR can be configured to use more than one label range if one
   believes it will be an upstream LSR for many flows. It just inserts
   additional advertisements in the same PIM Hello message. The label
   table size and router-count should be the same in all advertisements
   contained in a message.


2.2.1.2. Effect of Partition in Layer 2 Topology

   When a subnet partitions (due to, say, the failure of a layer 2
   switch) and new multicast LSRs come up, they will allocate label
   ranges that are unique to their partition. When the partition heals,
   there may be conflicts. Once the PIM Hellos messages are received by
   LSRs on the other side of the partition, they will determine there is
   a label range allocation conflict and immediately perform the tie
   breaking rules described above.






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2.2.1.3. Effect of Exceeding the Label Range

   When a LSR cannot allocate a label range because all ranges within
   the label table size have been allocated, it will not participate in
   binding labels to multicast routes. Packets for these routes will not
   be label switched.  However, the LSR is still capable of label
   switching a packet as either an upstream or downstream LSR on that
   LAN. This is the case when another router is binding labels for the
   multicast route and has an allocated a label range successfully.


2.2.2. Assigning the Labels

   Since PIM Join/Prune messages are multicast on a LAN, other
   downstream LSRs that are interested in the group will hear the
   message.  They must cache the binding of multicast routing table
   state and label state together. Since the upstream LSR is going to
   forward data packets using the advertised label, they must be ready
   to accept the data packet with that advertised label.

   The first downstream LSR that joins a group is the label assigner on
   that LAN for that multicast route. All other downstream LSRs that
   send PIM Join/Prune messages will use the same label that the
   assigner selected. A LSR that sends a PIM Join/Prune message with a
   label of 0 means that it doesn't know the label for the associated
   multicast routing table entry. When this occurs, the assigner can
   trigger a PIM Join/Prune message making the label known.

   When the label assigner leaves the group, the label that it assigned
   still remains active. The next highest IP addressed downstream LSR
   becomes the owner of that label and may change it if it sees fit.
   However, it is not required to change it. All downstream LSRs can
   continue to use the assignment in their Join messages.

   If two systems simultaneously join a distribution tree for the first
   time (they do not have state for that tree), and each chooses a
   different label value, the highest IP addressed downstream LSR's
   label will be used by the upstream LSR. The lower addressed LSR will
   hear the higher addressed LSR's Join too and will also use it's
   label.

   If the label assigner crashes, the highest IP addressed downstream
   LSR assigns a new label to the multicast routes, which were assigned
   by the crashing LSR, and triggers a Join message so all other LSRs on
   the LAN to use the new label.

   When a LAN partitions due to a layer-2 switch failure, it follows the
   same logic for the case when a LSR stops joining for a group. When



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   the partition heals, there may be an RPF neighbor change in one of
   the partitions.  When there is an RPF neighbor change and the
   downstream routers trigger joins to their new RPF neighbor with a
   different label assignment than the other partition is using, one of
   two resolutions occur:

      1) The LSR which is the allocator in the partition of the new RPF
         neighbor will trigger a join if it has a higher IP address than
         the allocator in the other region. The downstream routers in
         the other partition use the new label assignment immediately.

      2) If the LSR which is the allocator in the partition of the new
         RPF neighbor has a lower IP address, all downstream routers and
         the new RPF neighbor will switch to the label assigned by the
         allocator in the other partition.

   If an RPF change occurs (the topology changed so the upstream LSR is
   different), the PIM protocol spec indicates that a PIM Join may be
   triggered to get on the new distribution tree as soon as possible. In
   this case, if the label assigner becomes the upstream LSR, then the
   new highest IP addressed downstream LSR may become the label
   assigner. It may change the label if it sees fit. Otherwise, the same
   label is used.


2.3. Labels for Point-to-Point Links

   The procedure of section 2.2 works on point-to-point links because
   there is only one downstream LSR on the link which always becomes the
   label assigner.


2.4. Labels for NBMA Networks

   On NBMA networks, all PIM routers are known to each other through
   pseudo-broadcast mechanisms provided by the data-link layer. However,
   PIM Join messages are unicast to the upstream LSR. Therefore, other
   downstream LSRs will not hear the label assigner's advertisement.
   Therefore we treat an NBMA network with one upstream and n downstream
   LSRs as n point-to-point links, from the upstream LSR to each of the
   downstream LSRs.  Each downstream LSR then assigns its own label, and
   the upstream LSR must replicate the multicast data packets.
   Therefore the procedure of section 2.2 applies.

   Note that this is not incompatible with the use of native point-to-
   multipoint capabilities at the data link layer.





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2.5. When NOT to Send a Labelled Multicast Packet

   PIM Hello messages, sent periodically by all PIM-capable routers,
   will indicate if the router is MPLS-capable.  An upstream router on a
   LAN will therefore know if all routers on the same LAN are LSRs or
   not.  If there are ANY MPLS-incapable routers which are interested in
   a particular group, the upstream router will transmit to the LAN only
   unlabelled multicast data packets for that group.

   If there are any group members on a LAN, only unlabelled multicast
   data for that group will be transmitted onto that LAN.

   Routers that support non-PIM multicast are assumed, for the purposes
   of this procedure, to be MPLS-incapable.


2.6. No Conflict between Unicast and Multicast Labels

   MPLS uses different data-link layer code-points [MPLS-ENCAPS] to
   distinguish multicast labeled packets from unicast labeled packets.
   Therefore, the assignment of labels for unicast routes is completely
   independent from the assignment of labels for multicast routes.  For
   example, the same label value could be allocated for a unicast route
   and for a multicast route, without any possibility of ambiguity.


2.7. Supporting Bidirectional PIM

   We consider support of Bidirectional PIM [PIM-BIDIR] only in LSRs
   which are not ATM-LSRs.  In the absence of an ATM multipoint-to-
   multipoint capability, bidirectional PIM over ATM will not have the
   favorable scaling properties that make it interesting.

   On links which are not sender-only links, support for Bidirectional
   PIM is straightforward.  Labels are assigned in the usual manner by
   downstream LSRs.  However, a label can be used in either direction
   (i.e., can be carried by packets traveling either upstream or
   downstream).  On a given link, the label is bound to the same
   multicast route (*,G) or (S,G) in both directions.   As long as the
   procedures of section 2.2 are always used to partition the label
   space (even on point-to-point links), it is possible to use the same
   label in both directions.

   Sender-only links present a bit more of a difficulty since PIM
   Join/Prune messages are not generally sent on those links.  In order
   to assign labels to these links, a downstream node on a sender only
   link should send a PIM Join message, as if it were going to join the
   tree, but should set the newly defined "label only" bit (L-bit).  In



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   essence, these nodes will contain (*,G) state, and will associate the
   (*,G) state with a label that is distributed upstream.  However,
   there will be no output interface list associated with the (*,G)
   state, and packets will just be forwarded towards the RP.


3. Modifications to PIMv2

3.1. Join/Prune Packets

   PIMv2 has a packet format for each address type it may support when
   encoding both multicast and unicast addresses. We will define a new
   address type called "Label Address" for unicast address encoding.
   The label will accompany the source address in the Encoded Source
   Address format as specified in [PIMv2].  The label value will be in a
   32-bit quantity following the source address. We also take one bit
   from the PIMv2 reserved field to be the "label only" bit (shown below
   as the "L-bit").  So, for example, an IPv4 Label Address format would
   look like:


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Rsrvd |L|S|W|R|   Mask Len    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Source Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Label                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Current Multicast Route Timer                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Label
      If the high-order bit is clear, the low-order 20 bits are a label
      value (as described in [5]) assigned by the LSR sending the
      Join/Prune message.  All other bits should be set to 0 by the
      sender and should be ignored by the receiver.

      If the high-order bit is set, the low-order 28 bits are a label
      value in the VPI/VCI format of (as described in [MPLS-ATM])
      assigned by the LSR sending the Join/Prune message.  All other
      bits should be set to 0 by the sender and should be ignored by the
      receiver.

   Current Multicast Route Timer
      The sender of a Join/Prune message inserts the current time left



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      before expiration for the multicast route table entry described by
      the Source Address (either the (S,G) or (*,G) entry). This is
      needed so all routers on a common multi-access subnet can time-out
      the entry close to the same time without each other recreating the
      state when the source goes inactive.

   Refer to [PIMv2] for other field descriptions not specified here.




3.2. Hello Packets

   The PIM Hello message will carry 2 new OptionTypes (called "Label
   Parameters" and "VCI Capability") as specified in [PIMv2]. A router
   that sends a PIM Hello with the Label Parameters option is regarded
   as being label-capable. This Option can appear multiple times in a
   Hello packet if a LSR wants to allocate multiple ranges. When this
   option appears multiple times in the Hello message, the Label Table
   Size and Router Count must be the same for each Label Parameters
   Option supplied in the message.

   When sent on point-to-point links, this option should have Router
   Count, Lower Label Range, and Upper Label Range set to 0. These
   fields are ignored on receipt.

   Label Parameters TLV

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OptionType = 17          |      OptionLength = 16        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Total Number of Multicast Labels for this LAN              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router Count                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Lower Label Range                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Upper Label Range                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   OptionType "Label Parameters"
      Set to value 17 decimal.

   OptionLength
      The option is 16 bytes in length.




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   Total Number of Multicast Labels
      The total number of multicast labels the sending router can
      support on the interface the Hello is sent on.

   Router Count
      The approximate maximum number of routers that may be connected to
      the subnet the Hello is sent on.

   Lower Label Range
      The lower label value in the label range that was been randomly
      selected by the sending router. This value must be less than the
      Upper Label Range value.

   Upper Label Range
      The upper label value in the label range that was been randomly
      selected by the sending router. This value must be greater than
      the Lower Label Range value.


   VCI Capability TLV

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OptionType = 18          |      OptionLength = 1         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Direction   |
   +-+-+-+-+-+-+-+-+

   Direction
      When set to 0, VCI capability is bidirectional. When set to 1, VCI
      capability is unidirectional. Bidirectional capability indicates
      an ATM-LSR issuing this option can, within a single VPI, support
      binding  of the same VCI to different routes on the different
      directions of the link. Unidirectional capability indicates an
      ATM-LSR issuing this option can, within a single VPI, a single VCI
      may appear in one binding only. In such systems when a VCI has
      been bound in one direction on the link it may not be used in the
      other.


   Priority TLV

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      OptionType = 19          |      OptionLength = 4         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   |                           Priority                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Priority
      When several LSRs on a LAN allocate the same label range, this
      field is used to determine which one of those LSRs may keep the
      allocation.  The Priority field is treated as a 32-bits unsigned
      integer. Higher value is associated with higher Priority.



4. Label Distribution for PIM-DM

   In dense-mode PIM, there is no downstream Join message traveling
   upstream to perform the binding of multicast routes with labels.
   However, since we don't want a separate algorithm for dense-mode
   groups, we extend this basic design for dense-mode PIM.

   When a downstream LSR creates (S,G) state from the receipt of 1)
   data, or 2) Join/Prune or Graft messages, it will start a periodic
   timer to send Join messages with label assignment information
   present. The messages look no different and are treated on receipt no
   differently than in the sparse-mode case.

   The periodic Join message will be multicast on the LAN with an
   upstream target address of 0.0.0.0. All multicast LSRs on the LAN
   must know the group operates in dense-mode. This is accomplished
   using standard PIM mechanisms.


5. Security Considerations

   The security considerations for MPLS in general and label
   distribution in particular are discussed in [MPLS-ARCH] and [LDP]
   respectively.  Security considerations for PIM are discussed in
   [PIMv2].

   The use of IPSEC for securing the PIM messages, as suggested in
   [PIMv2], provides adequate security for this application.












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6. Acknowledgments

   The authors would like to thank Fred Baker for his comments.  We also
   thank the authors of [MPLS-MUL-FR] for their critique of an earlier
   version.

   9.0 Author's Addresses


      Dino Farinacci
      Procket Networks, Inc.
      3850 North First Street
      San Jose, CA 95134
      Email: dino@procket.com



      Yakov Rekhter
      Cisco Systems, Inc.
      170 Tasman Drive
      San Jose, CA, 95134
      Email: yakov@cisco.com



      Eric C. Rosen
      Cisco Systems, Inc.
      250 Apollo Drive
      Chelmsford, MA, 01824
      Email: erosen@cisco.com



      Ted Qian
      Cisco Systems, Inc.
      250 Apollo Drive
      Chelmsford, MA, 01824
      Email: tqian@cisco.com













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7. References

   [LDP] "LDP Specification", draft-ietf-mpls-ldp-7.txt, Andersson,
   Doolan, Feldman, Fredette, Thomas, June 2000.

   [MPLS-ARCH] "Multiprotocol Label Switching Architecture", draft-
   ietf-mpls-arch-06.txt, Rosen, Viswanathan, Callon, August 1999.

   [PIMv1] "Protocol Independent Multicast-Sparse Mode (PIM-SM):
   Protocol" Specification", RFC 2362, Estrin, Farinacci, Helmy, Thaler,
   Deering, Handley, Jacobson, Liu, Sharma, Wei, June 1998.

   [PIMv2] "Protocol Independent Multicast-Sparse Mode (PIM-SM):
   Protocol Specification", draft-ietf-pim-v2-sm-01.txt, Wei, et. al.,
   November, 1999.

   [PIM-BIDIR] "Bi-directional Protocol Independent Multicast", <draft-
   ietf-pim-bidir-00.txt>, Handley, Kouvelas, and Vicisano, March 2000.

   [MPLS-ENCAPS] "MPLS Label Stack Encoding", <draft-ietf-mpls-label-
   encaps-07.txt>, Rosen, Rekhter, Farinacci, Tappan, Fedorkow, Li,
   Conta, September 1999.

   [MPLS-MUL-FR] "Framework for IP Multicast in MPLS", <draft-ietf-
   mpls-multicast-01.txt>, Ooms, Sales, Livens, Acharya, Griffoul,
   Ansari, May 2000.

   [MPLS-ATM] "MPLS using LDP and ATM VC Switching", <draft-ietf-mpls-
   atm-02.txt>, Davie, Lawrence, McCloghrie, Rekhter, Rosen, Swallow,
   Doolan, April 1999.





















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