PIM WG                                                      IJ. Wijnands
Internet-Draft                                                  A. Boers
Intended status: Informational                                  E. Rosen
Expires: August 25, 2008                             Cisco Systems, Inc.
                                                       February 22, 2008

                           The RPF Vector TLV

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This document describes a use of the PIM Join Attribute as defined in
   draft-ietf-pim-join-attributes [I-D.ietf-pim-join-attributes] which
   enables PIM to build multicast trees through an MPLS-enabled network,
   even if that network's IGP does not have a route to the source of the

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Use of the RPF Vector TLV  . . . . . . . . . . . . . . . . . .  4
     2.1.  Attribute and shared tree joins  . . . . . . . . . . . . .  4
     2.2.  Attribute and Bootstrap messages . . . . . . . . . . . . .  5
     2.3.  The Vector Attribute . . . . . . . . . . . . . . . . . . .  5
       2.3.1.  Inserting a Vector Attribute in a Join . . . . . . . .  5
       2.3.2.  Processing a Received Vector Attribute . . . . . . . .  5
       2.3.3.  Vector Attribute and Asserts . . . . . . . . . . . . .  5
       2.3.4.  Vector Attribute and Join suppression  . . . . . . . .  6
   3.  Vector Attribute TLV Format  . . . . . . . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   It is sometimes convenient to distinguish the routers of a particular
   network into two categories: "edge routers" and "core routers".  The
   edge routers attach directly to users or to other networks, but the
   core routers attach only to other routers of the same network.  If
   the network is MPLS-enabled, then any unicast packet which needs to
   travel outside the network can be "tunneled" via MPLS from one edge
   router to another.  To handle a unicast packet which must travel
   outside the network, an edge router needs to know which of the other
   edge routers is the best exit point from the network for that
   packet's destination IP address.  The core routers, however, do not
   need to have any knowledge of routes which lead outside the network;
   as they handle only tunneled packets, they only need to know how to
   reach the edge routers and the other core routers.

   Consider, for example, the case where the network is an Autonomous
   System (AS), the edge routers are EBGP speakers, the core routers may
   be said to constitute a "BGP-free core".  The edge routers must
   distribute BGP routes to each other, but not to the core routers.

   However, when multicast packets are considered, the strategy of
   keeping the core routers free of "external" routes is more
   problematic.  When using PIM-SM[RFC4601], PIM-SSM[RFC4607] or PIM-
   BIDIR[RFC5015] to create a multicast distribution tree for a
   particular multicast group, one wants the core routers to be full
   participants in the PIM protocol, so that multicasting can be done
   efficiently in the core.  This means that the core routers must be
   able to correctly process PIM Join messages for the group, which in
   turn means that the core routes must be able to send the Join
   messages towards the root of the distribution tree.  If the root of
   the tree lies outside the network's borders (e.g., is in a different
   AS), and the core routers do not maintain routes to external
   destinations, then the PIM Join messages cannot be processed, and the
   multicast distribution tree cannot be created.

   In order to allow PIM to work properly in an environment where the
   core routers do not maintain external routes, a PIM extension is
   needed.  When an edge router sends a PIM Join message into the core,
   it must include in that message a "Vector" which specifies the IP
   address of the next edge router along the path to the root of the
   multicast distribution tree.  The core routers can then process the
   Join message by sending it towards the specified edge router (i.e.,
   toward the Vector).  In effect, the Vector serves as an attribute,
   within a particular network, for the root of the tree.

   This document defines a new TLV in the PIM Join Attribute
   message[I-D.ietf-pim-join-attributes].  It consists of a single

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   Vector which identifies the exit point of the network.

2.  Use of the RPF Vector TLV

   Before we can start forwarding multicast packets we need to build a
   forwarding tree by sending PIM Joins hop by hop.  Each router in the
   path creates a forwarding state and propagates the Join towards the
   root of the forwarding tree.  The building of this tree is receiver
   driven.  See Figure 1.

               ------------------ BGP -----------------
              |                                        |
   [S]---( Edge 1)--(Core 1)---( Core )--(Core 2)---( Edge 2 )---[R]
                  <--- (S,G) Join

                   Figure 1

   In this example, the 2 edge routers are BGP speakers.  The core
   routers are not BGP speakers and do not have any BGP distributed
   routes.  The route to S is a BGP distributed route, hence is known to
   the edge but not to the core.  The Edge 2 router determines the
   interface leading to S, and sends a PIM Join to the upstream router.
   In this example, though, the upstream router is a core router, with
   no route to S. Without the PIM extensions specified in this document,
   the core router cannot determine where the send the Join, so the tree
   cannot be constructed.

   To allow the core router to participate in the construction of the
   tree, the Edge 2 router will include an attribute field in the PIM
   Join.  In this example, the Attribute field will contain the IP
   address of Edge 1.  Edge 2 then forwards the PIM Join towards Edge 1.
   The intermediate core routers do their RPF check on the Attribute (IP
   address of Edge 1) rather than the Source, this allows the tree to be

2.1.  Attribute and shared tree joins

   In the example above we build a source tree to illustrate the
   attribute behavior.  The attribute is however not restricted to
   source tree only.  The tree may also be constructed towards a
   Rendezvous Point (RP) IP address.  The RP IP address is used in a
   similar way as the Source in the example above.  PIM Attribute
   procedures defined for sources are equally applicable to (*,G) and
   (*,*,RP) joins unless otherwise noted.

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2.2.  Attribute and Bootstrap messages

   The RPF vector does not apply to BSR bootstrap messages.  To allow
   BSR messages to be forwarded across a core where the BSR IP address
   is not routable in the core a solution needs to be developed for BSR.

2.3.  The Vector Attribute

2.3.1.  Inserting a Vector Attribute in a Join

   In the example of Figure 1, when the Edge 2 router looks up the route
   to the source of the multicast distribution tree, it will find a BGP-
   distributed route whose "BGP next-hop" is Edge 1.  Edge 2 then looks
   up the route to Edge 1 to find interface and PIM adjacency which is
   the next hop to the source, namely Core 2.

   When Edge 2 sends a PIM Join to Core 2, it includes a Vector
   Attribute specifying the address of Edge 1.  Core 2, and subsequent
   core routers, will forwarding the Join along the Vector (i.e, towards
   Edge 1) instead of trying to forward it towards S.

   Whether an attribute is actually needed depends on whether the Core
   routers have a route to the source of the multicast tree.  How the
   Edge router knows whether or not this is the case (and thus how the
   Edge router determines whether or not to insert an attribute field)
   is outside the scope of this document.

2.3.2.  Processing a Received Vector Attribute

   When processing a received PIM Join which contains a Vector
   Attribute, a router must first check to see if the Vector IP address
   is one of its own IP addresses.  If so, the Vector Attribute is
   discarded, and not passed further upstream.  Otherwise, the Vector
   Attribute is used to find the route to the source, and is passed
   along when a PIM Join is sent upstream.  Note that a router which
   receives a Vector Attribute must use it, even if that router happens
   to have a route to the source.  A router which discards a Vector
   Attribute may of course insert a new Vector Attribute.  This would
   typically happen if a PIM Join needed to pass through a sequence of
   Edge routers, each pair of which is separated by a core which does
   not have external routes.  In the absence of periodic refreshment,
   Vectors expire along with the corresponding (S,G) state.

2.3.3.  Vector Attribute and Asserts

   In a PIM Assert message we include the routing protocol's "metric" to
   the source of the tree.  This information is used in the selection of
   the assert winner.  If a PIM Join is being sent towards a Vector,

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   rather than towards the source, the Assert message must have the
   metric to the Vector instead of the metric to the source.  The Assert
   message however does not have an attribute field and does not mention
   the Vector.

   A router may change its upstream neighbor on a particular multicast
   tree as the result of receiving Assert messages.  However a Vector
   Attribute should not be sent in a PIM Join to an upstream neighbor
   which is chosen as the result of an assert winner and different from
   the original upstream neighbor (the upstream neighbor for the
   multicast route not influenced by the assert).  Reachability of the
   Vector is only guaranteed by the router that advertises reachability
   to the Vector in its IGP.  If the assert winner upstream is not our
   real preferred next-hop, we can't be sure this router knows the path
   to the Vector.  In the worst case the assert winner has a route to
   the Vector that is on the same interface where the assert was won.
   That will point the RPF interface to that interface and will result
   in the O-list being NULL.  The Vector attribute is not inserted if
   the RPF neighbor was chosen via an assert process and the RPF
   neighbor is different from the RPF neighbor that would have been
   selected via the local routing table.  In all other cases the Vector
   has to be included in the Join message.

2.3.4.  Vector Attribute and Join suppression

   If a router receives a PIM join on the upstream LAN interface for a
   particular multicast state, join suppression may be applied if that
   PIM join is targeted to the same upstream neighbor.  Which router(s)
   will suppress their PIM join is depending on timing and is
   unpredictable.  Downstream routers on a LAN may include different RPF
   vectors in the PIM joins.  Therefore an upstream router on that LAN
   may receive and use different RPF vectors over time to reach the
   destination (depending on which downstream router(s) suppressed their
   Join).  To make the upstream router behavior more predictable the RPF
   vector address MUST be used as additional condition to the join
   suppression logic.  Only if the RPF vector in the PIM join matches
   the RPF vector in the multicast state, the suppression logic is
   applied.  It is also possible to disable join suppression on that

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3.  Vector Attribute TLV Format

   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
   |F|S| Type      | Length        |        Value

   F bit
   Forward Unknown TLV. If this bit is set the TLV is forwarded
   regardless of whether the router understands the Type. If the TLV is
   known the F bit is ignored.

   S bit
   Bottom of Stack. If this bit is set then this is the last
   TLV in the stack.

   The Vector Attribute type is 0.

   Length depending on Address Family of Encoded-Unicast address.

   Encoded-Unicast address.

4.  IANA Considerations

   An attribute type needs to be assigned.  For now we propose the value

5.  Security Considerations

   Security of the RPF Vector Attribute is only guaranteed by the
   security of the PIM packet, so the security considerations for PIM
   join packets as described in PIM-SM [RFC4601] apply here.

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6.  Acknowledgments

   The authors would like to thank Yakov Rekhter and Dino Farinacci for
   their initial ideas on this topic and Su Haiyang for the comments on
   the draft.

7.  References

7.1.  Normative References

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, October 2007.

              Boers, A., "Format for using TLVs in PIM messages",
              draft-ietf-pim-join-attributes-03 (work in progress), I-D
              Status iesg, IETF Datatracker State Publication Requested,
              Intended Status Proposed Standard, Responsible AD David
              Ward, May 2007.

7.2.  Informative References

Authors' Addresses

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

   Email: ice@cisco.com

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   Arjen Boers
   Cisco Systems, Inc.
   Avda. Diagonal, 682
   Barcelona  08034

   Email: aboers@cisco.com

   Eric Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, Ma  01719

   Email: erosen@cisco.com

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