PIM WG IJ. Wijnands
Internet-Draft A. Boers
Intended status: Informational E. Rosen
Expires: August 25, 2008 Cisco Systems, Inc.
February 22, 2008
The RPF Vector TLV
draft-ietf-pim-rpf-vector-06
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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
2.3.4. Vector Attribute and Join suppression . . . . . . . . 6
3. Vector Attribute TLV Format . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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[RFC4601], PIM-SSM[RFC4607] or PIM-
BIDIR[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 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-D.ietf-pim-join-attributes]. It consists of a single
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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 routers 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 BSR IP address
is not routable in the core a solution needs to be developed for 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 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.
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 an assert winner and different from
the original upstream neighbor (the upstream neighbor for the
multicast route not influenced by the assert). 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 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 the 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.
2.3.4. 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 depending 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.
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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 | 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.
4. IANA Considerations
An attribute type needs to be assigned. For now we propose the value
0.
5. 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.
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6. 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 draft.
7. References
7.1. Normative References
[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.
[I-D.ietf-pim-join-attributes]
Boers, A., "Format for using TLVs in PIM messages",
draft-ietf-pim-join-attributes-03 (work in progress), I-D
Status iesg, IETF Datatracker State Publication Requested,
Intended Status Proposed Standard, Responsible AD David
Ward, May 2007.
7.2. Informative References
Authors' Addresses
IJsbrand Wijnands
Cisco Systems, Inc.
De kleetlaan 6a
Diegem 1831
Belgium
Email: ice@cisco.com
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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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