PIM WG Yiqun Cai
Internet Draft Heidi Ou
Intended Status: Proposed Standard
Expires: July 7, 2009 Cisco Systems, Inc.
January 7, 2009
PIM Multi-Topology ID (MT-ID) Join-Attribute
draft-ietf-pim-mtid-00.txt
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Internet Draft draft-ietf-pim-mtid-00.txt January 2009
Abstract
This document introduces a new type of PIM Join Attribute that
extends PIM signaling to identify a topology that should be used when
constructing a particular multicast distribution tree.
Table of Contents
1 Specification of Requirements ...................... 2
2 Introduction ....................................... 3
3 Functional Overview ................................ 3
3.1 PIM RPF Topology ................................... 3
3.2 PIM MT-ID .......................................... 4
3.3 Applicability ...................................... 4
4 Protocol Specification of PIM MT-ID ................ 5
4.1 Sending PIM MT-ID Join Attribute ................... 5
4.2 Receiving PIM MT-ID Join Attribute ................. 5
4.3 Validating PIM MT-ID Join Attribute ................ 6
4.4 Conflict Resolution ................................ 6
4.4.1 Upstream Routers ................................... 6
4.4.2 Downstream Routers ................................. 7
5 PIM MT-ID Join Attribute TLV Format ................ 7
6 IANA Considerations ................................ 8
7 Security Considerations ............................ 8
8 Acknowledgments .................................... 8
9 Authors' Addresses ................................. 8
10 Normative References ............................... 9
11 Informative References ............................. 9
1. 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].
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2. Introduction
Some unicast protocols, such as OSPF and IS-IS, allow a single
network to be viewed as multiple topologies [RFC4915, RFC5120]. This
enables PIM to construct multicast distribution trees using separate
network paths even when the roots of the trees are the same.
This capability can be used to improve the resilience of multicast
applications. For instance, a multicast stream can be duplicated and
transported using different network layer addresses simultaneously.
Assuming that two source trees, (S1, G1) and (S1, G2), are used for
the stream. By using MT capable unicast routing protocols and
procedures described in this document, it is possible to construct
two source trees for (S1, G1) and (S1, G2) in such a way that they do
not share any transit network segment. As a result, a single network
failure will not cause any loss to the stream.
This draft introduces a new type of PIM Join Attribute used to encode
the identity of the topology PIM uses for RPF. It is based on
[RFC5384], and specifies additional procedures and rules to process
the attribute and resolve conflict.
3. Functional Overview
3.1. PIM RPF Topology
PIM RPF topology is a collection of routes used by PIM to perform RPF
operation when building shared or source trees. In the rest of the
document, PIM RPF topology may be simply referred to as "topology"
when there is no ambiguity.
In a multi-topology environment, multiple RPF topologies can be
created in the same network. A particular source may be reachable in
only one of the topologies, or in several of them via different
paths.
To select the RPF topology for a particular multicast distribution
tree, one or more of the following can be done.
1. configure a policy that maps a group range to a topology. When
RPF information needs to be resolved for the RP or the sources
for a group within the range, the RPF lookup takes place in the
specified topology. This can be used for PIM-SM/SSM/Bidir.
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2. configure a policy that maps a source prefix range to a
topology. This can be used for PIM-SM and PIM-SSM.
3. use the topology identified by the Join Attribute encoding in
the received PIM packets.
The details of the first two methods are implementation specific and
are not discussed in this document. The specification to support the
third method is included in this document.
3.2. PIM MT-ID
For each PIM RPF topology created, a unique numerical ID is assigned.
This ID is called PIM MT-ID. PIM MT-ID has the following property,
- this value is not required to be the same as the MT-ID used by
the unicast routing protocols that contribute routes to the
topology. Although in practice, when only one unicast routing
protocol (such as OSPF or IS-IS) is used, PIM MT-ID is typically
assigned the same value as the IGP topology identifier.
- this value must be unique and consistent within the network
domain for the same topology
- 0 is reserved as the default, and MUST NOT be included in the
join attribute encoding.
- how to assign a PIM MT-ID to a topology is decided by the network
administrator and is outside the scope of this document
3.3. Applicability
The PIM MT-ID join attribute described in this draft applies to PIM
Join/Assert packets used by PIM SM/SSM/Bidir. It is not used in any
other PIM packets, such as Prune, Register, Register-Stop, Graft,
Graft-ack, DF Election, Candidate-RP, and Bootstrap. As such, it can
only be used to build shared or source trees for PIM SM/SSM and PIM-
bidir downstream.
When this attribute is used in combination with RPF vectors defined
in [ID.ietf-pim-rpf-vector] [ID.ietf-l3vpn-2547bis-mcast], they are
processed against the topology identified by the PIM MT-ID attribute.
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4. Protocol Specification of PIM MT-ID
4.1. Sending PIM MT-ID Join Attribute
When a PIM router originates a PIM Join/Assert packet, it may choose
to encode PIM MT-ID of the topology in which RPF lookup takes place
for the corresponding (*,G) or (S,G) entry. The chosen PIM MT-ID MUST
be the one decided by local topology selection configuration if it
exists, or the one received from downstream routers after conflict
resolution procedures are applied.
The following are the exceptions,
- a router MUST NOT attach the attribute if PIM MT-ID is 0. The
value of 0 is ignored on reception.
- a router SHOULD NOT do so if the upstream router, or one of the
routers on the LAN does not include "PIM Join Attribute" option
in its Hello packets.
- a router SHOULD NOT encode PIM MT-ID for pruned sources. If
encoded, the value is ignored.
4.2. Receiving PIM MT-ID Join Attribute
When a PIM router receives a PIM MT-ID join attribute in a
Join/Assert packet, it MUST perform the following,
- validate the attribute encoding. The detail is described in the
next section.
- if the join attribute is valid, use the rules described in the
section "Conflict Resolution" to determine a PIM MT-ID to use.
- use the topology identified by the selected PIM MT-ID to perform
RPF lookup for the (*,G)/(S,G) entry unless a different topology
is specified by a local configuration. The local configuration
always takes precedence.
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4.3. Validating PIM MT-ID Join Attribute
An encoded PIM MT-ID join attribute is valid if all of the following
conditions are satisfied,
- there is at most 1 PIM MT-ID attribute encoded.
- the length field must be 2 and the value must not be 0.
If an encoded PIM MT-ID join attribute is deemed invalid, it is
ignored and not forwarded further. The packet is processed as if the
attribute were not present.
It is important to note that, if the sender is not a PIM neighbor
that has included "PIM Join Attribute" option in its Hello packets,
or if the "F" bit in the encoding is reset, the encoding may still be
considered valid by an implementation and is allowed to be forwarded.
4.4. Conflict Resolution
Depending on whether a PIM router is an upstream or a downstream
router, the action it takes to resolve conflicting PIM MT-ID
attributes differs. The detail is described below.
4.4.1. Upstream Routers
If an upstream router has a local configuration that specifies a
different topology than that from an incoming Join/Assert packet,
including the case PIM MT-ID is not encoded in the incoming packet,
it is not considered a conflict.
A conflict occurs when a router doesn't have local topology selection
policy and it has received different PIM MT-ID from Join packets sent
by its downstream routers or Assert packets from another forwarding
router on the LAN.
- if an upstream router receives different PIM MT-ID attributes
from PIM Join packets, it MUST follow the rules specified in
[RFC5384] to select one. The PIM MT-ID chosen will be the one
encoded for its upstream neighbor.
- if an upstream router receives a different PIM MT-ID attribute in
an ASSERT packet, it MUST use the tie-breaker rules as specified
in [RFC4601] to determine an ASSERT winner. PIM MT-ID is not
considered in deciding a winner from Assert process.
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4.4.2. Downstream Routers
A conflict is detected by a downstream router when it sees a
different PIM MT-ID attribute from other routers on the LAN,
regardless of whether the router has local topology selection policy
or not.
- if a downstream router sees different PIM MT-ID attributes from
PIM Join packets, it MUST follow the specification of [RFC4601]
as if the attribute did not exist. For example, the router
suppresses its own Join packet if a Join for the same (S,G) is
seen.
The router MUST NOT use the rules specified in [RFC5384] to
select a PIM MT-ID from Join packets sent by other downstream
routers.
- if a downstream router sees its preferred upstream router loses
in the ASSERT process, and the ASSERT winner uses a different PIM
MT-ID, the downstream router SHOULD still choose the ASSERT
winner as the RPF neighbour but it MUST NOT encode PIM MT-ID when
sending Join packets to it.
5. PIM MT-ID Join 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|E| Attr Type | Length | Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- F bit: 1 Transitive Attribute.
- E bit: As specified by [RFC5384]
- Attr Type: 3.
- Length: 2.
- Value: PIM MT-ID, 1 to 65535.
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6. IANA Considerations
A new PIM Join Attribute type needs to be assigned. 3 is proposed for
now.
7. Security Considerations
As a type of PIM Join Attribute, the security considerations
described in [RFC5384] apply here. Specifically, malicious alteration
of PIM MT-ID may cause the resiliency goals to be violated.
8. Acknowledgments
The authors would like to thank Eric Rosen, Ice Wijnands, Dino
Farinacci, Colby Barth and Les Ginsberg for their input.
9. Authors' Addresses
Yiqun Cai
Cisco Systems, Inc
170 West Tasman Drive
San Jose, CA 95134
E-mail: ycai@cisco.com
Heidi Ou
Cisco Systems, Inc
170 West Tasman Drive
San Jose, CA 95134
E-mail: hou@cisco.com
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10. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", 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.
[RFC5384] A. Boers, I. Wijnands, E. Rosen, "The Protocol Independent
Multicast (PIM) Join Attribute Format", RFC 5384, November 2008
11. Informative References
[RFC4915] P. Psenak, S. Mirtorabi, A. Roy, L. Nguyen, P. Pillay-
Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, June 2007.
[RFC5120] T. Przygienda, N. Shen, N. Sheth, "M-ISIS: Multi Topology
(MT) Routing in Intermediate System to Intermediate Systems (IS-
ISs)", RFC 5120, February 2008.
[ID.ietf-pim-rpf-vector] I. Wijnands, A. Boers, E. Rosen, "The RPF
Vector TLV", draft-ietf-pim-rpf-vector.
[ID.ietf-l3vpn-2547bis-mcast] E. Rosen,R Aggarwal, "Multicast in
MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast
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