RFC 6420

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


   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

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.

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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
         Conflict Resolution Rules for
                           Upstream Routers ..........................10
         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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   -  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",
   "OPTIONAL" in this document are to be interpreted as described in

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

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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RFC 6420                PIM MT-ID Join Attribute           November 2011

                              +++ 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

   -  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

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RFC 6420                PIM MT-ID Join Attribute           November 2011

   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

   -  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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RFC 6420                PIM MT-ID Join Attribute           November 2011

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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   -  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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   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

   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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   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.  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.  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

      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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RFC 6420                PIM MT-ID Join Attribute           November 2011

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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RFC 6420                PIM MT-ID Join Attribute           November 2011

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

   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

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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RFC 6420                PIM MT-ID Join Attribute           November 2011

   [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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