PIM WG IJ. Wijnands
Internet-Draft A. Boers
Expires: April 4, 2006 E. Rosen
Cisco Systems, Inc.
october 2005
The RPF Vector TLV
draft-ietf-pim-rpf-vector-01
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Copyright (C) The Internet Society (2005).
Abstract
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
tree.
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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
3. Vector Attribute TLV Format . . . . . . . . . . . . . . . . . 7
4. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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[I-D.ietf-pim-sm-v2-new], PIM-SSM[I-
D.ietf-ssm-arch] or PIM-BIDIR[I-D.ietf-pim-bidir] 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-
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D.ietf-pim-join-attributes]. It consists of a single 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 router do their RPF check on the Attribute (IP
address of Edge 1) rather than the Source, this allows the tree to be
constructed.
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 RP IP address is
not routable in the core a solution has the developed in 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 ttribute 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 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. 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 a 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.
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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 | IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-.......
F bit
-----
Forward Unknown TLV. If this bit is set the TLV is forwarded
regardless if the router understands the Type.
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 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)",
draft-ietf-pim-sm-v2-new-11 (work in progress),
October 2004.
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[I-D.ietf-pim-bidir]
Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bi-directional Protocol Independent Multicast (BIDIR-
PIM)", draft-ietf-pim-bidir-07 (work in progress),
March 2005.
[I-D.ietf-pim-join-attributes]
Boers, A., "Format for using TLVs in PIM messages",
draft-ietf-pim-join-attributes-00 (work in progress),
October 2005.
[I-D.ietf-ssm-arch]
Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", draft-ietf-ssm-arch-06 (work in progress),
September 2004.
5.2. Informative References
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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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