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RFC 5496 - The Reverse Path Forwarding (RPF) Vector TLV

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Network Working Group                                       IJ. Wijnands
Request for Comments: 5496                                      A. Boers
Category: Standards Track                                       E. Rosen
                                                     Cisco Systems, Inc.
                                                              March 2009

              The Reverse Path Forwarding (RPF) Vector TLV

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.


   This document describes a use of the Protocol Independent Multicast
   (PIM) Join Attribute as defined in RFC 5384, 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 tree.

Table of Contents

   1. Introduction ....................................................2
   2. Specification of Requirements ...................................3
   3. Use of the RPF Vector TLV .......................................3
      3.1. Attribute and Shared Tree Joins ............................4
      3.2. Attribute and Bootstrap Messages ...........................4
      3.3. The Vector Attribute .......................................4
           3.3.1. Inserting a Vector Attribute in a Join ..............4
           3.3.2. Processing a Received Vector Attribute ..............5
           3.3.3. Vector Attribute and Asserts ........................5
           3.3.4. Vector Attribute and Join Suppression ...............6
   4. Vector Attribute TLV Format .....................................6
   5. IANA Considerations .............................................7
   6. Security Considerations .........................................7
   7. Acknowledgments .................................................7
   8. Normative References ............................................7

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 that needs to
   travel outside the network can be "tunneled" via MPLS from one edge
   router to another.  To handle a unicast packet that 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 that 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 External Border Gateway Protocol
   (EBGP) speakers, the core routers may be said to constitute a "BGP-
   free core".  The edge routers 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 Sparse-Mode (PIM-SM) [RFC4601], PIM
   Source-Specific Mode (PIM-SSM) [RFC4607], or Bidirectional PIM
   (BIDIR-PIM) [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 routers 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" that 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
   [RFC5384].  It consists of a single Vector that identifies the exit
   point of the network.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Use of the RPF Vector TLV

   Before a router can start forwarding multicast packets, it is
   necessary 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 two 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, it 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 includes an "RPF (Reverse Path Forwarding)
   Vector" TLV in the PIM Join Attribute [RFC5384] of the PIM Join.  In
   this example, the RPF Vector TLV will contain the IP address of Edge
   1.  Edge 2 forwards the PIM Join towards Edge 1.  Each intermediate
   core router does its RPF check [RFC4601] on the address contained in
   the RPF Vector TLV (i.e., on the IP address of Edge 1), instead of
   doing the RPF check on the address S.  This allows the tree to be

3.1.  Attribute and Shared Tree Joins

   In the example above, we build a source tree to illustrate the
   attribute behavior.  Use of the attribute is, however, not restricted
   to the construction of source trees.  It may also be used to
   construct a shared tree.  In this case, the RPF Vector TLV contains
   the IP address of a Rendezvous Point (RP).  Procedures defined in
   this document for (S,G) Joins are equally applicable to (*,G) and
   (*,*,RP) Joins unless otherwise noted.

3.2.  Attribute and Bootstrap Messages

   There is no way to carry an RPF Vector TLV in a Bootstrap Router
   (BSR) bootstrap message.  The procedures in this document do not
   define a way for BSR messages to be forwarded across a core in which
   the BSP IP address is not routable.

3.3.  The Vector Attribute

3.3.1.  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 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.

3.3.2.  Processing a Received Vector Attribute

   When processing a received PIM Join that 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 that receives a Vector
   Attribute MUST use it, even if that router happens to have a route to
   the source.  A router that 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 that does not have external routes.  In
   the absence of periodic refreshment, Vectors expire along with the
   corresponding (S,G) state.

3.3.3.  Vector Attribute and Asserts

   A PIM Assert message includes 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, 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

   A router may change its upstream neighbor on a particular multicast
   tree as the result of receiving Assert messages.  However, a Vector
   Attribute MUST NOT be sent in a PIM Join to an upstream neighbor that
   is chosen as the result of Assert processing, if that neighbor is
   different than the original upstream neighbor.  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 the
   real preferred next-hop, it is possible that the Assert winner does
   not know 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
   therefore MUST NOT be 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 MUST be included in the Join message.

3.3.4.  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 dependent 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

4.  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.

5.  IANA Considerations

   IANA has assigned the value 0 to the RPF Vector in the "PIM Join
   Attribute Types" registry.

6.  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.

7.  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 document.

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [RFC5384]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
              Independent Multicast (PIM) Join Attribute Format", RFC
              5384, November 2008.

Authors' Addresses

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

   EMail: ice@cisco.com

   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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