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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.

Abstract

 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.

Wijnands, et al. Standards Track [Page 1] RFC 5496 The RPF Vector TLV March 2009

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

Wijnands, et al. Standards Track [Page 2] RFC 5496 The RPF Vector TLV March 2009

 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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

Wijnands, et al. Standards Track [Page 3] RFC 5496 The RPF Vector TLV March 2009

 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
 constructed.

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.

Wijnands, et al. Standards Track [Page 4] RFC 5496 The RPF Vector TLV March 2009

 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
 Vector.

 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.

Wijnands, et al. Standards Track [Page 5] RFC 5496 The RPF Vector TLV March 2009

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
 LAN.

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.

 Type
    The Vector Attribute type is 0.

 Length
    Length depending on Address Family of Encoded-Unicast address.

 Value
    Encoded-Unicast address.

5. IANA Considerations

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

Wijnands, et al. Standards Track [Page 6] RFC 5496 The RPF Vector TLV March 2009

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.

Wijnands, et al. Standards Track [Page 7] RFC 5496 The RPF Vector TLV March 2009

Authors' Addresses

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

 EMail: ice@cisco.com

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

 EMail: aboers@cisco.com

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

 EMail: erosen@cisco.com

Wijnands, et al. Standards Track [Page 8]