Internet Engineering Task Force (IETF) Y. Cai
Request for Comments: 6420 H. Ou
Category: Standards Track Cisco Systems, Inc.
ISSN: 2070-1721 November 2011
PIM Multi-Topology ID (MT-ID) Join Attribute
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6420.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Cai & Ou Standards Track [Page 1]RFC 6420 PIM MT-ID Join Attribute November 2011
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Functional Overview .............................................4
3.1. PIM RPF Topology ...........................................4
3.2. PIM MT-ID ..................................................6
3.3. Applicability ..............................................7
4. Protocol Specification of PIM MT-ID .............................7
4.1. PIM MT-ID Hello Option .....................................7
4.2. PIM MT-ID Join Attribute ...................................7
4.2.1. Sending PIM MT-ID Join Attribute ....................7
4.2.2. Receiving PIM MT-ID Join Attribute ..................8
4.2.3. Validating PIM MT-ID Join Attribute .................8
4.2.4. Conflict Resolution .................................9
4.2.4.1. Conflict Resolution Rules for
Upstream Routers ..........................10
4.2.4.2. Conflict Resolution Rules for
Downstream Routers ........................10
5. Packet Format ..................................................10
5.1. PIM MT-ID Hello Option ....................................11
5.2. PIM MT-ID Join Attribute TLV Format .......................11
6. IANA Considerations ............................................11
6.1. PIM MT-ID Hello Option ....................................11
6.2. PIM MT-ID Join Attribute Type .............................12
7. Security Considerations ........................................12
8. Acknowledgments ................................................12
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................13
1. Introduction
Some unicast protocols, such as OSPF and IS-IS, allow a single
network to be viewed as multiple topologies [RFC4915] [RFC5120].
Deploying multi-topology (MT) routing allows different paths through
the network to be selected to support different traffic or to offer
protection paths in the event of failures.
PIM [RFC4601] employs a technique known as Reverse Path Forwarding
(RPF) to construct forwarding trees between multicast sources and
receivers. The procedure of RPF uses topology information provided
by routing protocols, such as OSPF and IS-IS. Using the PIM MT-ID
Join Attribute specified in this document enables PIM to access the
multiple topologies created by the routing protocols and construct
multicast forwarding trees using separate network paths even when the
roots of the trees are the same.
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This capability would allow for an improvement to the resilience of
multicast applications. For instance, a multicast stream can be
duplicated and transported using two source trees, (S1, G1) and (S1,
G2), simultaneously. 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 document introduces a new type of PIM Join Attribute [RFC5384],
named "MT-ID Join Attribute". It is used to encode the numerical
identity of the topology PIM uses when performing RPF for the
forwarding tree that is being joined. This document also specifies
procedures and rules to process the attribute and resolve conflicts
arising from mismatches in capabilities to support the attribute or
the value of the attribute.
This document does not introduce any change to the RPF check
procedure used to verify the incoming interface when a packet is
forwarded as defined in [RFC4601]. For example, to use the
capability described by this document, an application can choose to
use group addresses, and/or source addresses, to identify a unique
multicast stream. It might further need to perform the functions of
splitting and merging. However, the detailed processing is beyond
the scope of the document.
In the rest of the document, the MT-ID Join Attribute will be
referred to as "MT-ID".
2. Terminology
The following acronyms are frequently used in the document.
- RPF: RPF stands for "Reverse Path Forwarding". A PIM router
performs RPF for two purposes. When building a forwarding tree, a
PIM router identifies an interface (the RPF interface) and an
upstream PIM neighbor (the RPF neighbor) to which to send PIM
Joins. Upon receiving a data packet, a PIM router verifies if the
packet arrives from the expected incoming interface (aka RPF
check) before deciding whether or not to replicate the packets.
- RPF Topology: An RPF topology is a collection of routes that a PIM
router uses for RPF. One or more RPF topologies may be created on
a PIM router.
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- MT: MT stands for "Multi-Topology" in this document. Sometimes it
is also referred to as "multi-topology routing". In the context
of PIM, MT refers to the capability of building and maintaining
multiple RPF topologies.
- PIM MT-ID: An MT-ID is a numerical identifier associated with an
RPF topology.
- PIM MT-ID Join Attribute: This is a new type of Join Attribute
that is introduced by this document in order to specify RPF
topology in the PIM Join messages.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Functional Overview
PIM relies on routes learned from routing protocols for the purpose
of RPF. These routes form one or more topologies. This section
describes the function of multi-topology routing for PIM and its
applicability.
3.1. PIM RPF Topology
PIM RPF topology is a collection of routes used by PIM to perform the
RPF operation when building shared or source trees. The routes in
the topology may be contributed by different protocols. 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 help explain the relationship between an MT-capable unicast
routing protocol and MT-capable RPF topologies, consider the
following example described by Figure 1.
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+++ A +++ B +++
+ +
S -- R1 R2 -- receivers
* *
*** C *** D ***
Figure 1. A simple topology for multicast
- The traffic source is S. S is announced by R1 using Multiprotocol
BGP (MBGP) to every router. This route is installed in every
topology.
- Two topologies are created in the unicast IGP, let us call them
OSPF 1000 and OSPF 2000. OSPF 1000 includes A, B, and interfaces
in R1 and R2 that are configured to be part of OSPF 1000. OSPF
2000 includes C, D, and interfaces on R1 and R2 that are
configured to be part of OSPF 2000.
- Two PIM RPF topologies are created, let us call them PIM 500 and
PIM 600.
PIM 500 comprises the following routes: S announced by MBGP and those
learned via OSPF 1000.
PIM 600 comprises the following routes: S announced by MBGP and those
learned via OSPF 2000
The above example illustrates that the naming spaces of MT-ID are not
required to be the same between PIM and IGPs. Furthermore, a unicast
IGP topology and the PIM RPF topology to which the IGP topology
contributes routes are not required to have the same set of routes.
In the above example, the prefix covering S does not exist in either
OSPF 1000 or OSPF 2000, but since it exists in PIM 500 and PIM 600,
R2 is able to join to it via either path.
There are two methods to select the RPF topology for a particular
multicast distribution tree, via configuration or via PIM.
When it is done via configuration, a network administrator configures
a policy that maps a group range to a topology and/or maps a source
prefix range to a topology. Using the same example, the policy can
say that to build a forwarding tree for G1 only routes in PIM 500 are
to be used, and to build a forward tree for G2 only routes in PIM 600
are used. The result is that packets for (S, G1) will follow the
path of S-R1-A-B-R2 and packets for (S, G2) will follow the path of
S-R1-C-D-R2.
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An alternative to static configuration is to include the RPF topology
information as a new PIM Join Attribute in the PIM Join packets sent
by downstream routers.
Both methods can be used at the same time. The details of the first
method are implementation specific and are not discussed in this
document. The specification to support the second 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 the PIM MT-ID. The PIM MT-ID has
the following properties.
- It is the path identifier that is used by the PIM control plane,
but it does not function in the forwarding state for a specific
topology. The differentiation for topologies on the forwarding
plane is made by different group addresses and/or source addresses
instead.
- As shown earlier, 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, the PIM MT-ID is
RECOMMENDED to be assigned using the same value as the IGP
topology identifier. Using the same example presented earlier, if
every route in PIM 500 is contributed by OSPF 1000, it is
RECOMMENDED to name this RPF topology as PIM 1000 instead of PIM
500. This is for the purpose of reducing management overhead and
simplifying troubleshooting.
- This value MUST be unique and consistent within the network for
the same topology. For example, PIM 500 MUST refer to the same
topology on routers R1, C, D, and R2. For actual deployment, one
should have a means to detect inconsistency of the PIM MT-ID
configuration, but the detail of such mechanism is beyond the
scope of this document.
- 0 is reserved as the default, and it 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 document applies to
PIM Join/Assert packets used by PIM SM/SSM/Bidir (Sparse Mode/Source-
Specific Mode/Bidirectional). It is not used in any other PIM
packets. 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] and [MVPN], the vectors 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: the PIM MT-ID
Hello Option and the PIM MT-ID Join Attribute.
4.1. PIM MT-ID Hello Option
The PIM MT-ID Hello Option is used by a router to indicate if it
supports the functionality described by this document. If it does,
it MUST include the PIM Hello Option in its PIM Hello packets and
MUST include both the Join Attribute Option [RFC5384] and the new PIM
MT-ID Option (see Section 5.1 of this document for packet format).
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 the PIM MT-ID of the topology in which RPF lookup is to
take place for the corresponding (*,G) or (S,G) entry. The PIM MT-ID
identifies the topology chosen by local policy/configuration or is
the value received from downstream routers after MT-ID conflict
resolution procedures have been applied (See Section 4.2.4 for
further detail).
The following are the exceptions:
- A router SHOULD NOT include the attribute if PIM MT-ID is 0. The
value of 0 is ignored on reception.
- A router SHOULD NOT include the PIM MT-ID in its Join/Assert
packets if the upstream router, or any of the routers on the LAN,
does not include the "PIM Join Attribute" or "PIM MT-ID" option in
its Hello packets.
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- A router SHOULD NOT attach PIM MT-ID for pruned sources. PIM
MT-ID MUST be ignored for a pruned source by a router processing
the Prune message.
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.
While it is an exception case, it is worthwhile to describe what will
happen if a router receives PIM MT-ID Join Attribute but doesn't
support the functionality described in [RFC5384] or this document.
If the router supports [RFC5384] but not this document, it is able to
skip the PIM MT-ID Join Attribute and move on to the next Join
Attribute, if one is present. The RPF decision will not be altered
because the router doesn't understand the meaning of the PIM MT-ID
Join Attribute. The router will use the procedures described by
[RFC5384] to perform conflict resolution.
If a router doesn't support [RFC5384], it will ignore the Join/Assert
message because it is not able to parse the encoded sources.
If a router does support both [RFC5384] and this document, but
chooses not to send either the PIM MT-ID or the PIM Join Attribute
Option in its Hello packets (likely due to administrative reasons),
it SHOULD ignore the Join/Assert message when it receives a PIM
Join/Assert packet with the PIM MT-ID Join Attribute.
4.2.3. Validating PIM MT-ID Join Attribute
An upstream router MUST be known to support this document in order
for a downstream router to include the PIM MT-ID attribute in its
Join packets. However, an upstream router doesn't need to know
whether or not a downstream router supports this document when
deciding whether to accept the attribute. Hence, if the Join packet
sender doesn't include the "PIM Join Attribute" or "PIM MT-ID"
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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 [RFC793].
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 (possibly due to an
error in the implementation), 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 Rendezvous Point
(RP) in the Join message that have already been processed SHOULD
still be considered valid.
- 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.
PIM MT-ID conflicts arises on an upstream router when the router
doesn't have a local topology selection policy and receives Join
packets from downstream routers and/or Assert packets from other
forwarding routers on the LAN and those packets contain different PIM
MT-IDs.
However, if an upstream router has a local configuration that
specifies PIM MT-IDs to identify RPF topologies, and those MT-IDs do
not match the MT-ID on a received Join or Assert packet, this is not
considered to be a conflict and the resolution procedures are not
applied. This includes the case where there are local PIM MT-IDs,
but there is no PIM MT-ID encoded in the incoming packet.
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 or not the router has
local topology selection policy.
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When two PIM MT-IDs are compared, only the 12-bit Value field (see
Section 5.2) is compared. Other fields of the PIM MT-ID Join
Attribute TLV Format (including the four reserved bits) MUST NOT be
used in the comparison.
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 packets altogether if it sees a conflict in PIM MT-ID
attributes. This action may also be taken by an upstream router
that has locally configured topology selection policy, as an
exception to the rules described above.
- If an upstream router receives a different PIM MT-ID attribute in
an Assert packet, it MUST use the tiebreaker 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
This section describes the format of new PIM messages introduced by
this document. The messages follow the same transmission order as
the messages defined in [RFC4601].
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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 = 30 | OptionLength = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
- OptionType: 30.
- OptionLength: 0.
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: 2
- Length: 2.
- R: Reserved bits, 4 in total. Set to zero on transmission.
Ignored upon receipt.
- Value: PIM MT-ID, 1 to 4095.
6. IANA Considerations
6.1. PIM MT-ID Hello Option
IANA maintains a registry of "Protocol Independent Multicast (PIM)
Parameters" with a sub-registry called "PIM-Hello Options".
The IANA has assigned the PIM Hello Option type value 30 for the PIM
MT-ID Hello Option according to the First Come First Served
allocation policy.
The IANA has assigned a Length value of 0.
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6.2. PIM MT-ID Join Attribute Type
The IANA maintains a registry of "Protocol Independent Multicast
(PIM) Parameters" with a sub-registry called "PIM Join Attribute
Types".
The IANA has assigned a value of 2 for the PIM MT-ID Join Attribute
defined in Section 5.2 of this document.
7. Security Considerations
As described in [RFC5384], the security of the Join Attribute is only
guaranteed by the security of the PIM packet that carries it.
Similarly, the security of the Hello Option is only guaranteed by
securing the whole Hello Packet.
In view of the fact that malicious alteration of the PIM MT-ID Hello
Option or the PIM MT-ID carried in a packet might cause the PIM
resiliency goals to be violated, the security considerations of
[RFC4601] apply to the extensions described in this document.
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, Les Ginsberg, Dimitri Papadimitriou, Thomas
Morin, and Hui Liu for their input.
The authors would also like to thank Adrian Farrel for his detailed
and constructive comments during the AD review.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, 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.
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[RFC5384] Boers, A., Wijnands, I., and E. Rosen, "The Protocol
Independent Multicast (PIM) Join Attribute Format", RFC
5384, November 2008.
9.2. Informative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC5496] Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496, March 2009.
[MVPN] Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
VPNs", Work in Progress, January 2010.
Authors' Addresses
Yiqun Cai
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: ycai@cisco.com
Heidi Ou
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: hou@cisco.com
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