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 (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
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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 ....................................................22. Specification of Requirements ...................................33. Use of the RPF Vector TLV .......................................33.1. Attribute and Shared Tree Joins ............................43.2. Attribute and Bootstrap Messages ...........................43.3. The Vector Attribute .......................................43.3.1. Inserting a Vector Attribute in a Join ..............43.3.2. Processing a Received Vector Attribute ..............53.3.3. Vector Attribute and Asserts ........................53.3.4. Vector Attribute and Join Suppression ...............64. Vector Attribute TLV Format .....................................65. IANA Considerations .............................................76. Security Considerations .........................................77. Acknowledgments .................................................78. Normative References ............................................71. 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",
"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
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
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.
Bottom of Stack. If this bit is set, then this is the last TLV in
The Vector Attribute type is 0.
Length depending on Address Family of 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.
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
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.