Internet DRAFT - draft-wijnands-mpls-ldp-mcast-ext


Network Working Group                                  IJsbrand Wijnands
Internet Draft                                                Bob Thomas
Expiration Date: September 2005                      Cisco Systems, Inc. 
                                                             Yuji Kamite 
                                                          Hitoshi Fukuda 
                                                      NTT Communications

                                                              March 2005

                      Multicast Extensions for LDP


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   Forwarding multicast packets efficiently over an MPLS core requires
   Point-to-Multi-Point (P2MP) or Multi-Point-to-Multi-Point (MP2MP)
   LSP's between one or more Ingress routers and one or more Egress
   routers.  For efficient forwarding core LSRs need to replicate
   labeled multicast packets where the branches of the P2MP/MP2MP tree
   diverge. This draft specifies LDP extensions that enable it to build
   P2MP/MP2MP LSPs in a receiver initiated manner.

Table of Contents

    1          Specification of Requirments  .......................   2
    2          Terminology  ........................................   3
    3          Introduction  .......................................   3
    4          Label distribution  .................................   3
    5          Label allocation  ...................................   4
    6          MP-T FEC element  ...................................   4
    7          In-band signaling using LDP  ........................   5
    8          Out-of-band signaling with LDP  .....................   5
    9          Building a P2MP LSP tree  ...........................   6
    9.1        Label mapping  ......................................   6
    9.2        Label withdraw  .....................................   6
   10          Building a MP2MP tree  ..............................   7
   11          Assigning Labels for MP2MP upstream Traffic  ........   9
   12          Acknowledgments  ....................................  10
   13          References  .........................................  10
   13.1        Normative References  ...............................  10
   13.2        Informational References  ...........................  10
   14          Authors' Addresses  .................................  11
   15          Full Copyright Statement  ...........................  11
   16          Intellectual Property  ..............................  12

1. Specification of Requirments

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

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2. Terminology

   P2MP   - Point to Multipoint Label Switched Tree.
   MP2MP  - Multipoint to Multipoint Label Switched Tree.
   MP-T   - Multipoint Tree, either a P2MP or MP2MP Tree.
   Ingess - Router that is a sender on the MP-T.
   Egress - Router that is a receiver of a MP-T.
   Root   - Router that acts as the Rendezvous point of the MP-T.

3. Introduction

   Multicast trees are built using Protocol Indenpendent Multicast
   [PIMv2].  PIM supports three different modes of operation: PIM
   sparse-mode, PIM bidir [BIDIR] and PIM SSM.  This draft specifies
   extensions to LDP that enable it to build point-to-multipoint and
   multipoint-to-multipoint trees for MPLS packet forwarding.  These
   P2MP and MP2MP MPLS trees are comparable to multicast PIM SSM and PIM
   Bidir mode trees and may be used to support multicast over MPLS
   LSP's. This draft does not specify procedures analogous to PIM
   sparse-mode multicast.

   PIM is a receiver driven soft state periodic protocol that builds
   trees to a source or rendezvous point.  Its receiver driven nature
   allows it to scale well with dynamic multicast group membership and
   large receiver populations.  A router forwards tree membership
   upstream, but does not forward any information about downstream
   receivers.  LDP extensions in this draft support receiver driven
   construction of MP-T's.  An advantage that the LDP extensions for
   multicast have over PIM extensions for signaling labels is that LDP
   has built-in reliability and flow control.

4. Label distribution

   The labels which are mapped to MP-T LSPs are distributed by LDP in
   Downstream Unsolicited Ordered Mode and retained using the
   Conservative Label Retention mode. An LSR distributes the label for
   an MP-T LSP to an upstream peer only if the peer is the its next hop
   for the root of the MP-T.

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5. Label allocation

   Since the extensions for signaling MP-T's use downstream label
   allocation MP-T LSPs may share the platform wide label space with
   unicast LSPs.  The MPLS forwarding engine is reponsible for deciding
   to replicate packets using information supplied by LDP.  Since MP-T
   and unicast LSPs share the same label space there is no need for a
   separate LDP session for MP-T's.

6. MP-T FEC element

   The MP-T FEC element identifes an MP-T by means of the tree's root
   address, the tree type and information that is opaque to core LSRs.

         MP-T type FEC Element encoding:

          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
         | MP-T Type(TBD)|    Address Family             | Address Length|
         |                    Root Address                               |
         | Tree Type(TBD)|  Opaque Len   |  Opaque value ...

         MP-T Type
            MP-T type FEC element, value to be assigned by IANA.

         Address family
            Two octet quantity containing a value from ADDRESS FAMILY
            NUMBERS in [RFC1700] that encodes the address family for the
            Root address field.

         Address Length
           Length of the Root address value in octets.

         Root Address
           The root address of the MP-T. Used by receiving LSR to
           determine the next-hop toward the MP-T root.

         Tree Type
            1 octet that identifies the tree type ie.
            - P2MP LSP.

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            - MP2MP downstream LSP.
            - MP2MP upstream LSP.

         Opaque Len
            Length of the opaque value in octets.

         Opaque value
            Variable length opaque value that uniquely identifies the MP-T.

   The triple <Root Address, Tree Type, Opaque Value> uniquely identifies
   the MP-T.  LDP uses the Root Address to determine the upstream LSR
   toward the MP-T; the Tree Type determines the nature of LDP
   protocol interactions required to establish the MP-T LSP; and, the
   Opaque Value carries information that may be meaningful to edge LSRs.

7. In-band signaling using LDP

   LDP is used to build Label Switched paths through a network. The
   packets that traverse the LSP are not of interest to LDP. Edge LSRs
   may use the Opaque field of the MP-T FEC element to encode multicast
   stream information.  Egress LSRs may encode the source and group for
   a multicast stream in the Opaque field.  Of course, different Egress
   LSRs which receive the same multicast stream must use the same
   source/group encoding.  Such an opaque value could be used to signal
   the Root LSR which multicast stream is to be forwared on the MP-T.
   Specification of such encodings is outside the scope of this draft.

   The multicast component that wishes to receive multicast packets over
   a LDP created MP-T creates the opaque FEC value. Depending on the
   different applications there will be different Opaque FEC encodings.
   Different FEC encodings are to be documented elsewhere.

8. Out-of-band signaling with LDP

   When an egress router wishes to receive a multicast stream over a
   MP-T it needs to know the identifier of that MP-T. Using in-band
   signaling the egress router can create the MP-T identifier (Opaque
   FEC) using a pre-defined algorithm, so there no need for other
   signaling. If the egress router is not able to use in-band signaling,
   for example when different multicast streams are aggregated over the
   same MP-T then an out-of-band form of signaling is required to learn
   the MP-T identifier. Such out-of-band signaling is beyond the scope
   of this document.

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9. Building a P2MP LSP tree

   In order for a set of LSRs to become egress LSRs of the same P2MP or
   MP2MP LSP, they must encode the same "root node" and "opaque
   identifier" values into the FEC element of the LDP Mapping messages
   that they send. How they  determine what  the proper root node and
   opaque identifier values are is outside the scope of this

9.1. Label mapping

   An LSR which is setting up a particular MP-T only sends a Label
   Mapping Message for a P2MP LSP to the LSR which is to be its
   "upstream neighbor" on that  LSP. The upstream neighbor is the one
   which is the next hop on the best path to the root. If there are
   multiple paths to the root which are equally good, one is chosen.
   However, once a label for a given P2MP LSP has been advertised to an
   upstream LSR, no further label mapping messages for that LSP are sent
   upstream, until such time as the label has been withdrawn, released,
   or the LDP connection has failed.

   When an LSR receives a Label Mapping it determines if it already has
   state for this MP-T.  If it does, it updates its MPLS forwarding
   engine to reflect the new MP-T branch.

   If the receiving LSR does not have state and is not the the Root LSR
   for the MP-T it allocates a label, sends a Label Mapping for the MP-T
   toward the Root LSR, and installs the binding on its chosen upstream
   interface.  This process is repeated until a Label Mapping message
   for the MP-T reaches the Root LSR.

   If the receiving LSR is the Root, it simply informs any subscribing
   applicaton that the P2MP tree exists.  How this is done is beyond the
   scope of this document.

9.2. Label withdraw

   When an MP-T egress LSR is no longer interested in an MP-T it
   withdraws its label for the MP-T by means of a Label Withdraw message
   using the MP-T FEC Element to identify the MP-T.

   The LSR receiving a Label Withdraw message for an MP-T from a peer
   updates its MPLS forwarding engine by removing the output information
   for the MP-T that corresponds to the peer and sends a Label Release
   message in reply.  If no output information for the MP-T remains in
   its MPLS forwarding engine the LSR sends a Label Withdraw for the

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   MP-T upstream.

10. Building a MP2MP tree

   A MP2MP LSP is much like a P2MP LSP, is has a Root node and multiple
   LSP-egress routers. The big difference compared with a P2MP tree is
   that there will be multiple senders that can forward MPLS packets
   upstream in the direction of the tree root. Each LSP-egress can be a
   LSP-ingress router for the same tree. The root node for a P2MP tree
   is always the same as the LSP-ingress router. This is not the case
   for MP2MP trees, it can be an LSP-ingress router but can also be a
   P-router in the core.

   MPLS forwards packets based on the incoming label. The incomming
   label identifies the next-hop(s) to which the MPLS packet should be
   sent. For any MP-T tree there can be multiple next-hops.  For an P2MP
   tree there is only one interface that receives (i.e has forwarding
   state for) packets belonging to the tree.  For MP2MP there is some
   set of interfaces which have state for the tree. Packets received on
   any of these interfaces are replicated to each of the other members
   of this set.

   From the perspective of a given LSR, a MP2MP "tree" consists of n
   P2MP LSPs, where n is the number of interfaces (upstream  +
   downstream) on the tree. Thus the forwarding state if O(number of
   interfaces).  If you instead used one P2MP LSP for each member of the
   tree, your state would be O(number of members), which scales much
   more poorly.

           +---+      +---+       +---+      +---+    +---+
      Rcv  |PE1|------|P1 |-------| P | -----|P3 |----|PE2| Rcv
           +---+      +---+       +---+      +---+    +---+
                                  |P2 |
                               S2 |   | S3
           +---+      +---+       |   |     +---+    +---+
      Rcv  |PE3|------|P4 |-------     -----|P5 |----|PE4| Rcv
      Src  +---+      +---+                 +---+    +---+

                                                  Figure 1.

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   Router P4 sends a label mapping to P2 and tells P2 to use label L2 for
   downstream traffic. Router P5 does the same and makes P2 assigned L3
   for downstream traffic. If we look at P2 the downstream state is as

              Incoming     Outgoing
          |   L1, S1    |   L2, S2   |
          |             |   L3, S3   |
                                       Table 1.

   Based on the label mapping send from P4 router P2 will send a label
   mapping reply to P4 with a label that is required for upstream traffic
   to P2. Same will happen for the upstream state from P5. Router P2
   tells router P5 to use label L5 for upstream traffic.

   From P2 to the root of the tree (P) the same protocol operations
   occur. We send a label mapping including a downstream label L1 (see
   table 1) and we receive an upstream label L6 mapping back for the
   upstream state. The upstream label we received in the label mapping
   reply is shared between the 2 upstream states since they both need to
   go to the same root via the same path, so we merge them
   together. Router P2 has the following upstream states:

              Incoming     Outgoing
          |   L4, S2    |   L6, S1   |
                                       Table 2.

              Incoming     Outgoing
          |   L5, S3    |   L6, S1   |
                                       Table 3.

   The P (root) router will have the following upstream state.

              Incoming     Outgoing
          |   L6, S0    |            |
                                       Table 4.

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   The upstream state has been setup and packets can travel to the root
   of the tree. However, while forwarding on a MP2MP tree we want to send
   packets down the tree on intermediate nodes while packets travel
   upstream. That means packets received from PE3 and PE4 need to be sent
   to P5 via P2. We don't need to depend on the root to send the packets
   downstream. We do this by merging the downstream state and the
   upstream states. Each upstream state will copy the interfaces from the
   downstream state replication list, except if it's the same as its
   incoming interface. So the upstream state's on router P2 look like

              Incoming     Outgoing
          |   L4, S2    |   L6, S1   |
          |             |   L3, S3   |
                                       Table 5.

              Incoming     Outgoing
          |   L5, S3    |   L6, S1   |
          |             |   L2, S2   |
                                       Table 6.

   Using the technique of creating specific upstream states in
   combination with merging the downstream replication list we are able to build a
   full feature MP2MP LSP tree. The LSP tree does contain more then one
   LSP path which costs extra labels, but the advantage is that the
   forwarding logic does not need to deal with any specific forwarding
   exceptions like we have in PIM bidir trees (forwarding packet that are
   received on an Outgoing Interface List).

11. Assigning Labels for MP2MP upstream Traffic

   To support MP2MP (bidirectional tree) LSP's we need to setup both a
   downstream and a upstream LSP. The downstream path has the same
   protocol operation as is used for P2MP LSP's, the upstream path is
   different. The upstream path is setup in response to the downstream
   path that is build. For a label mapping we sent to an upstream
   router, the upstream router sends a label mapping in the opposite
   direction to assign us a label for upstream traffic for this MP2MP

   The label mapping for the upstream path has the same encoding as the
   downstream path. To be able to distinguish between the two we use the

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   Tree type encoded in the MP-T FEC.  If an LSR receives a label
   mapping of the type "upstream LSP" then this router will respond with
   a label mapping of type "downstream LSP". When the downstream router
   receives this label mapping it knows what this labbel mapping is for
   the upstream path.

12. Acknowledgments

   The authors would like to thank Arjen Boers, Eric Rosen, Nidhi
   Bhaskar, Toerless Eckert and George Swallow for their contribution.

13. References

13.1. Normative References

   [MPLS] "Multiprotocol Label Switching Architecture", Rosen, E.,
   Viswanathan, A. and R. Callon, RFC 3031, January 2001.

   [BIDIR] "Bi-directional Protocol Independent Multicast", Handley,
   Kouvelas, Speakman, Vicisano, June 2002, <draft-ietf-pim-bidir-

   [PIMv2]  "Protocol Independent Multicast - Sparse Mode (PIM-SM)",
   Fenner, Handley, Holbrook, Kouvelas, December 2002, draft-ietf-pim-

   [LDP] "LDP Specification", Andersson, Doolan, Feldman, Fredette,
   Thomas, January 2001, rfc3036.

13.2. Informational References

   [MPLS-PIM] "Using PIM to Distribute MPLS Labels for Multicast
   Routes", Farinacci, Rekhter, Rosen, Qian, November 2000, <draft-

   [RFC2547bis] "BGP/MPLS VPNs", Rosen, et. al., November 2002, draft-

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14. Authors' Addresses

   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a
   1831 Diegem

   Bob Thomas
   Cisco Systems, Inc.
   300 Beaver Brook Road
   Boxborough, MA, 01719

   Yuji Kamite
   NTT Communications Corporation
   Tokyo Opera City Tower
   3-20-2 Nishi Shinjuku, Shinjuku-ku,
   Tokyo 163-1421,

   Hitoshi Fukuda
   NTT Communications Corporation
   1-1-6, Uchisaiwai-cho, Chiyoda-ku
   Tokyo 100-8019,

15. Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights."

   "This document and the information contained herein are provided on

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16. Intellectual Property

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