PIM WG                                                         Yiqun Cai
Internet Draft                                                  Heidi Ou
Intended Status: Proposed Standard
Expires: July 7, 2009 January 2, 2010                             Cisco Systems, Inc.

                                                         January 7,

                                                            July 2, 2009

              PIM Multi-Topology ID (MT-ID) Join-Attribute



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   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",
   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.

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

   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.

     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] [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

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.

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.
   silently ignored. 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. implementation.

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.

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

       The router MUST NOT use the rules specified in [RFC5384] to
       select a PIM MT-ID from Join packets sent by other downstream

     - 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        |        |R R R R| Value                 |

     - F bit: 1 Transitive 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 65535. 4095.

6. IANA Considerations

   A new PIM Join Attribute type needs to be assigned. 3 is proposed for

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

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.


   [RFC5496] I. Wijnands, A. Boers, E. Rosen, "The RPF Reverse Path
   Forwarding (RPF) Vector TLV", draft-ietf-pim-rpf-vector. RFC 5496, March 2009.

   [ID.ietf-l3vpn-2547bis-mcast] E. Rosen,R Aggarwal, "Multicast in
   MPLS/BGP IP VPNs", draft-ietf-l3vpn-2547bis-mcast