PIM WG IJ. Wijnands
Internet-Draft A. Boers
Expires: August 5, 2005 E. Rosen
Cisco Systems, Inc.
february 2005
The RPF Vector Proxy
draft-ietf-pim-rpf-vector-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes a use of the PIM Proxy as defined in
draft-pim-proxy [Forthcoming] 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use of the RPF Vector TLV . . . . . . . . . . . . . . . . . . 4
2.1 Proxy and shared tree joins . . . . . . . . . . . . . . . 4
2.2 The Vector Proxy . . . . . . . . . . . . . . . . . . . . . 4
2.2.1 Inserting a Vector Proxy in a Join . . . . . . . . . . 5
2.2.2 Processing a Received Vector Proxy . . . . . . . . . . 5
2.2.3 Vector Proxy and Asserts . . . . . . . . . . . . . . . 5
3. Vector Proxy TLV Format . . . . . . . . . . . . . . . . . . . 6
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1 Normative References . . . . . . . . . . . . . . . . . . . 6
5.2 Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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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 [I-D.ietf-pim-sm-v2-new] 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 a proxy, within
a particular network, for the root of the tree.
This document defines a new TLV in the PIM Proxy
message[draft-pim-proxy]. It consists of a single Vector which
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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 a Proxy field in the PIM Join.
In this example, the Proxy field will contain the IP address of Edge
1. Edge 2 then forwards the PIM Join towards Edge 1. The
intermediate core router do their RPF check on the Proxy (IP address
of Edge 1) rather than the Source, this allows the tree to be
constructed.
2.1 Proxy and shared tree joins
In the example above we build a source tree to illustrate the proxy
behavior. The proxy 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 Proxy procedures defined for sources are
equally applicable to RPs unless otherwise noted.
2.2 The Vector Proxy
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2.2.1 Inserting a Vector Proxy 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 Proxy
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 a Proxy 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 a Proxy field) is
outside the scope of this document.
2.2.2 Processing a Received Vector Proxy
When processing a received PIM Join which contains a Vector Proxy, a
router must first check to see if the Vector IP address is one of its
own IP addresses. If so, the Vector Proxy is discarded, and not
passed further upstream. Otherwise, the Vector Proxy 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 Proxy must use
it, even if that router happens to have a route to the source. A
router which discards a Vector Proxy may of course insert a new
Vector Proxy. 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.
2.2.3 Vector Proxy 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,
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 a Proxy 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
Proxy should not be sent in a PIM Join to an upstream neighbor which
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is chosen as the result of processing the Assert messages.
Reachability of the Vector is only guaranteed by the router that
advertises reachability to the Vector in it's 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.
3. Vector Proxy 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| Type | Length | IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-.......
F bit
-----
Forward Unknown TLV. If this bit is set the TLV is forwarded
regardless if the router understands the Type.
Type
----
The Vector Proxy type is 0.
Length
------
Length in bytes is 4.
Value
-----
IPv4 address.
4. Acknowledgments
The authors would like to thank Yakov Rekhter and Dino Farinacci for
their initial ideas on this topic.
5. References
5.1 Normative References
[I-D.ietf-pim-sm-v2-new]
Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode PIM-SM):
Protocol Specification (Revised)",
Internet-Draft draft-ietf-pim-sm-v2-new-11, October 2004.
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5.2 Informative References
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
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