PIM WG Yiqun Cai
Internet Draft Heidi Ou
Intended Status: Proposed Standard
Expires: March 22, 2011 Cisco Systems, Inc.
September 22, 2010
PIM Multi-Topology ID (MT-ID) Join-Attribute
draft-ietf-pim-mtid-05.txt
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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 ...................... 3
2 Introduction ....................................... 3
3 Functional Overview ................................ 3
3.1 PIM RPF Topology ................................... 3
3.2 PIM MT-ID .......................................... 4
3.3 Applicability ...................................... 5
4 Protocol Specification of PIM MT-ID ................ 5
4.1 PIM MT-ID Hello Option ............................. 5
4.2 PIM MT-ID Join Attribute ........................... 5
4.2.1 Sending PIM MT-ID Join Attribute ................... 5
4.2.2 Receiving PIM MT-ID Join Attribute ................. 6
4.2.3 Validating PIM MT-ID Join Attribute ................ 6
4.2.4 Conflict Resolution ................................ 7
4.2.4.1 Conflict Resolution Rules For Upstream Routers ..... 7
4.2.4.2 Conflict Resolution Rules For Downstream Routers ... 8
5 Packet Format ...................................... 8
5.1 PIM MT-ID Hello Option ............................. 8
5.2 PIM MT-ID Join Attribute TLV Format ................ 9
6 IANA Considerations ................................ 9
7 Security Considerations ............................ 9
8 Acknowledgments .................................... 10
9 Authors' Addresses ................................. 10
10 Normative References ............................... 10
11 Informative References ............................. 11
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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].
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. The draft does not introduce any
change to the RPF check procedure used to verify the incoming
interface when a packet is forwarded.
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.
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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.
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
per PIM domain. 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. In practice, when only one unicast routing protocol
(such as OSPF or IS-IS) is used, PIM MT-ID is recommended to be
assigned using the same value as the IGP topology identifier.
This is for the purpose of reducing management overhead and
simplifying troubleshooting.
- this value must be unique and consistent within the network
domain for the same topology. For actual deployment, one should
have a means to detect inconsistency of the MT-ID configuration,
but the detail of such mechanism is beyond the scope of this
document.
- 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
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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 [RFC5496] [ID.ietf-l3vpn-2547bis-mcast], they are processed
against the topology identified by the PIM MT-ID attribute.
4. Protocol Specification of PIM MT-ID
The change to the PIM protocol includes two pieces, PIM MT-ID Hello
Option and PIM MT-ID Join Attribute.
4.1. PIM MT-ID Hello Option
A router MUST include both PIM MT-ID and PIM Join Attribute Hello
Option in its PIM Hello packets if it supports functionality
described by this document.
4.2. PIM MT-ID Join Attribute
4.2.1. 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 any of the
routers on the LAN does not include "PIM Join Attribute" or "PIM
MT-ID" option in its Hello packets.
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- a router SHOULD NOT encode PIM MT-ID for pruned sources. If
encoded, the value is ignored.
4.2.2. 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.
4.2.3. Validating PIM MT-ID Join Attribute
An upstream router must be known to support this draft in order for a
downstream router to include the PIM MT-ID attribute in its Join
packets. But an upstream router doesn't need to know if a downstream
router supports this draft or not when deciding whether to accept the
attribute. Hence, if the Join packet sender doesn't include "PIM Join
Attribute" or "PIM MT-ID" options in its Hello packets, the PIM MT-ID
attribute in the Join may still be considered valid. This is also in
accordance with the "Robustness Principle" outlined in [RFC761].
The following text specifies the detail of the validity check.
- there is at most 1 PIM MT-ID attribute encoded. If there are
multiple PIM MT-ID Join Attributes included, only the last one is
accepted for this particular source. Processing of the rest of
the Join message continues.
- the length field must be 2. If the length field is not 2, the
rest of the Join message, including the current (S,G) or (*,G)
entry, MUST be ignored. The group, source and the RP in the Join
message that have already been processed SHOULD still be
considered valid.
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- the value MUST not be 0. If it is 0, the PIM MT-ID attribute is
ignored. Processing of the rest of the Join message, including
the current (S,G) or (*,G) entry, continues as if the particular
PIM MT-ID attribute weren't present in the packet.
4.2.4. Conflict Resolution
The definition of "PIM MT-ID conflict" varies depending on whether it
is on an upstream or a downstream router.
On an upstream router, a conflict occurs when the 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. In another word,
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 does not apply the conflict resolution procedures.
On the other hand,when a downstream router sees a different PIM MT-ID
attribute from other routers on the LAN it applies rules to resolve
the conflicts regardless of whether the router has local topology
selection policy or not.
It MUST be noted that the MT-ID value being considered for comparison
does not include the four reserved bits. That is, only the lower
order 12 bits are used in resolving conflicting attributes.
4.2.4.1. Conflict Resolution Rules For Upstream Routers
- 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.
In order to minimize the chances of potential transient
forwarding loops, an upstream router MAY choose to ignore the
incoming PIM Join/Prune packets all together if it sees a
conflict in PIM MT-ID attributes. This action may also be taken
by an upstream router which has locally configured topology
selection policy, as an exception to the rules described above.
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- 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.
4.2.4.2. Conflict Resolution Rules For Downstream Routers
- 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. Packet Format
5.1. PIM MT-ID Hello Option
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionType | OptionLength |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OptionValue |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- OptionType: 30.
- OptionLength: 8.
- OptionValue: There is none specified at this moment.
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5.2. 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 |R R R R| Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- F bit: 0 Non-transitive Attribute.
- E bit: As specified by [RFC5384].
- Attr Type: 3.
- Length: 2.
- R: Reserved bits, 4 in total.
- Value: PIM MT-ID, 1 to 4095. Range 2048 to 4095 are for
experimental and proprietary use.
6. IANA Considerations
A new PIM Hello Option type, 30, has been assigned for PIM MT-ID
Hello Option. The detail is in [HELLO].
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.
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8. Acknowledgments
The authors would like to thank Eric Rosen, Ice Wijnands, Dino
Farinacci, Colby Barth, Les Ginsberg, Dimitri Papadimitriou and
Thomas Morin 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
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
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11. Informative References
[RFC761] ISI, "Transmission Control Protocol", RFC 761, January 1980.
[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.
[RFC5496] I. Wijnands, A. Boers, E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496, March 2009.
[HELLO] IANA, "PIM-Hello Options",
http://www.iana.org/assignments/pim-parameters/pim-
parameters.xhtml#pim-parameters-1
[ID.ietf-l3vpn-2547bis-mcast] E. Rosen,R Aggarwal, "Multicast in
MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast-10, January 2010
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