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Explicit Reverse Path Forwarding (RPF) Vector
RFC 7891

Document Type RFC - Proposed Standard (June 2016)
Authors Javed Asghar , IJsbrand Wijnands , Sowmya Krishnaswamy , Apoorva Karan , Vishal Arya
Last updated 2018-12-20
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Alvaro Retana
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RFC 7891
Internet Engineering Task Force (IETF)                         J. Asghar
Request for Comments: 7891                             IJ. Wijnands, Ed.
Category: Standards Track                                S. Krishnaswamy
ISSN: 2070-1721                                                 A. Karan
                                                           Cisco Systems
                                                                 V. Arya
                                                            DIRECTV Inc.
                                                               June 2016

             Explicit Reverse Path Forwarding (RPF) Vector


   The PIM Reverse Path Forwarding (RPF) Vector TLV defined in RFC 5496
   can be included in a PIM Join Attribute such that the RPF neighbor is
   selected based on the unicast reachability of the RPF Vector instead
   of the source or Rendezvous Point associated with the multicast tree.

   This document defines a new RPF Vector Attribute type such that an
   explicit RPF neighbor list can be encoded in the PIM Join Attribute,
   thus bypassing the unicast route lookup.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 7891      Explicit Reverse Path Forwarding (RPF) Vector    June 2016

Copyright Notice

   Copyright (c) 2016 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
   ( 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Specification of Requirements . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Use of the PIM Explicit RPF Vector  . . . . . . . . . . . . .   4
   5.  Explicit RPF Vector Attribute TLV Format  . . . . . . . . . .   5
   6.  Mixed Vector Processing . . . . . . . . . . . . . . . . . . .   5
   7.  Conflicting RPF Vectors . . . . . . . . . . . . . . . . . . .   5
   8.  PIM Asserts . . . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Join Suppression  . . . . . . . . . . . . . . . . . . . . . .   6
   10. Unsupported Explicit Vector Handling  . . . . . . . . . . . .   7
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   12. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     13.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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RFC 7891      Explicit Reverse Path Forwarding (RPF) Vector    June 2016

1.  Introduction

   The procedures in [RFC5496] define how an RPF Vector can be used to
   influence the path selection in the absence of a route to the source.
   The same procedures can be used to override a route to the source
   when it exists.  It is possible to include multiple RPF Vectors in
   the list where each router along the path will perform a unicast
   route lookup on the first Vector in the attribute list.  Once the
   router owning the address of the RPF Vector is reached, following the
   procedures in [RFC5496], the RPF Vector will be removed from the
   attribute list.  This will result in a 'loosely' routed path that
   still depends on unicast reachability to the RPF Vector(s).

   In some scenarios, the network administrators don't want to rely on
   the unicast reachability to the RPF Vector address and want to build
   a path strictly based on the RPF Vectors.  In that case, the RPF
   Vectors represent a list of directly connected PIM neighbors along
   the path.  For these Vectors, the router would not do a route lookup
   in the unicast routing table.  These Vectors are referred to as
   'Explicit' RPF Vector addresses.  If a router receiving an Explicit
   RPF Vector does not have a PIM neighbor matching the Explicit RPF
   Vector address, it does not fall back to loosely routing the Join.
   Instead, it could process the packet and store the RPF Vector list so
   that the PIM Join can be sent out as soon as the neighbor comes up.
   Since the behavior of the Explicit RPF Vector differs from the
   'loose' RPF Vector as defined in [RFC5496], a new attribute called
   the Explicit RPF Vector is defined.

   This document defines a new TLV in the PIM Join Attribute message
   [RFC5384] for specifying the explicit path.

2.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Motivation

   Some broadcast video transport networks use a multicast PIM Live-Live
   resiliency model for video delivery based on PIM Source-Specific
   Multicast (PIM-SSM) or PIM Any-Source Multicast (PIM-ASM).  Live-Live
   implies using two active, spatially diverse multicast trees to
   transport video flows from root to leaf multicast routers.  The leaf
   multicast router receives two copies from the PIM multicast core and
   will replicate one copy towards the receivers [RFC7431].

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   One of the requirements of the PIM Live-Live resiliency model is to
   ensure path diversity of the two active PIM trees in the core such
   that they do not intersect to avoid a single point of failure.  IGP-
   routed RPF paths of two PIM trees could be routed over the same
   transit router and create a single point of failure.  It is useful to
   have a way to specify the explicit path along which the PIM Join is

   How the Explicit RPF Vector list is determined is outside the scope
   of this document.  For example, it may either be manually configured
   by the network operator or procedures may be implemented on the
   egress router to dynamically calculate the Vector list based on a
   link-state database protocol, like OSPF or IS-IS.

   Due to the fact that the leaf router receives two copies of the
   multicast stream via two diverse paths, there is no need for PIM to
   repair the broken path immediately.  It is up to the egress router to
   either wait for the broken path to be repaired or build a new
   explicit path using a new RPF Vector list.  Which method is applied
   depends very much on how the Vector list was determined initially.
   Double failures are not considered and are outside the scope of this

   This document describes the procedures to carry Explicit RPF Vectors
   in PIM.  It is up to the mechanism(s) that produce the Explicit RPF
   Vectors to ensure they are correct.  Existing mechanisms like
   [MTRACE-V2] may be used to verify how the PIM tree was built.

4.  Use of the PIM Explicit RPF Vector

   Figure 1 provides an example multicast join path
   R4->R3->R6->R5->R2->R1, where the multicast join is explicitly routed
   to the source hop by hop using the Explicit RPF Vector list.  When
   the R5-R6 link fails, the Join will NOT take an alternate path.

                         <---   |      |  ---
                            |   |      |  |
                            | (R5)---(R6) |
                            - (S,G) Join -
                                |      |
                                |      |

                                 Figure 1

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   In comparison, when the procedures specified in [RFC5496] are used,
   if the R5-R6 link fails, then the Join may be rerouted using the
   R6-R8-R7 path to reach R5.

5.  Explicit RPF Vector 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| Type      | Length        |        Value

                                 Figure 2

   F bit:  'Transitive Attribute' bit.  The F bit MUST be set to 0.
      Otherwise, there could be loops.

   E bit:  'End of Attributes' bit.  If the E bit is set, then this is
      the last TLV specified in the list.

   Type:  4 (Explicit RPF Vector)

   Length:  The length depending on the Address Family (IPv4 or IPv6) of
      the Encoded-Unicast address.

   Value:  Encoded-Unicast address.  This SHOULD be a valid IPv4 or IPv6
      address of an upstream router.

6.  Mixed Vector Processing

   The Explicit RPF Vector Attribute does not impact or restrict the
   functionality of other RPF Vector Attributes in a PIM Join.  It is
   possible to mix Vectors of different types such that some part of the
   tree is explicit and other parts are loosely routed.  RPF Vectors are
   processed in the order in which they are specified.

7.  Conflicting RPF Vectors

   It is possible that a PIM router has multiple downstream neighbors.
   If for the same multicast route there is an inconsistency between the
   Explicit RPF Vector lists provided by the downstream PIM neighbor,
   the procedures as documented in Section 3.3.3 of [RFC5384] apply.

   The conflict resolution procedures in Section 3.3.3 of [RFC5384] only
   apply to attributes of the same Join Attribute type.  Join Attributes
   that have a different type can't be compared because the content of
   the Join Attribute may have a totally different meaning and/or
   encoding.  This may cause a problem if a mix of Explicit RPF Vectors

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   (this document) and 'loose' RPF Vectors [RFC5496] is received from
   two or more downstream routers.  The order in which the RPF Vectors
   are encoded may be different, and/or the combination of RPF Vectors
   may be inconsistent.  The procedures in Section 3.3.3 of [RFC5384]
   would not resolve the conflict.  The following procedures MUST be
   applied to deal with this scenario.

   When a PIM Join with a Join Attribute list is received from a
   downstream neighbor, the router MUST verify that the order in which
   the RPF Vector types appear in the PIM Join Attribute list matches
   what is stored as the Join Attribute list for reaching the source or
   Rendezvous Point listed in the PIM Join.  Once it is determined that
   the RPF Vector types on the stack are equal, the content of the RPF
   Vectors MUST be compared ([RFC5384]).  If it is determined that there
   is either a conflict with RPF Vector types or the RPF Vector content,
   the router uses the RPF Vector stack from the PIM adjacency with the
   numerically smallest IP address.  In the case of IPv6, the link-local
   address will be used.  When two neighbors have the same IP address,
   either for IPv4 or IPv6, the interface index MUST be used as a tie
   breaker.  It's RECOMMENDED that the router doing the conflict
   resolution log a message.

8.  PIM Asserts

   Section 3.3.3 of [RFC5496] specifies the procedures for how to deal
   with PIM Asserts when RPF Vectors are used.  The same procedures
   apply to the Explicit RPF Vector.  There is a minor behavioral
   difference: the route 'metric' that is included in the PIM Assert
   should be the route metric of the first Explicit RPF Vector address
   in the list.  However, the first Explicit Vector should always be
   directly connected, so the metric may likely be zero.  The metric
   will therefore not be a tie breaker in the PIM Assert selection

9.  Join Suppression

   Section 3.3.4 of [RFC5496] specifies the procedures for how to apply
   Join Suppression when an RPF Vector Attribute is included in the PIM
   Join.  The same procedure applies to the Explicit RPF Vector
   Attribute.  The procedure MUST match against all the Explicit RPF
   Vectors in the PIM Join before a PIM Join can be suppressed.

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10.  Unsupported Explicit Vector Handling

   The F bit MUST be set to 0 in all Explicit RPF Vectors in case the
   upstream router receiving the Join does not support the TLV.  As
   described in Section 3.3.2 of [RFC5384], routers that do not
   understand the type of a particular attribute that has the F bit
   clear will discard it and continue to process the Join.

   This processing is particularly important when the routers that do
   not support the Explicit RPF TLV are identified as hops in the
   Explicit RPF list because failing to remove the RPF Vectors could
   cause upstream routers to send the Join back toward these routers
   causing loops.

   As the administrator is manually specifying the path that the Joins
   need to be sent on, it is recommended that the administrator computes
   the path to include routers that support the Explicit Vector and
   check that the state is created correctly on each router along the
   path.  Tools like mtrace can be used for debugging and to ensure that
   the Join state is setup correctly.

11.  IANA Considerations

   In the "PIM Join Attribute Types" registry, IANA has assigned the
   value 4 to the Explicit RPF Vector Attribute.

12.  Security Considerations

   Security of the Explicit RPF Vector Attribute is only guaranteed by
   the security of the PIM packet, so the security considerations for
   PIM Join packets as described in PIM-SM [RFC7761] apply here.  A
   malicious downstream node can attempt a denial-of-service attack by
   sending PIM Join packets with invalid addresses listed in the RPF
   Vector stack with an intent to stop the propagation of the Joins to
   the correct upstream node.  Another denial-of-service attack would be
   a malicious downstream node targeting all Joins to a specific node
   with an intent to overload the bandwidth on that node by making it
   responsible for forwarding multicast traffic for more streams that it
   can handle.  In order to minimize the risk of a denial-of-service
   attack from forged PIM Join packets with Explicit RPF Vector stack,
   it should be used within a single trusted management domain.

   If a router finds that it cannot use the Vector list due to the next
   hop router not being a PIM neighbor, it may log an error.  Also, if a
   router is receiving two conflicting Vectors, it may log an error.  It
   is up to the mechanisms that produced the Explicit RPF Vector to
   ensure that the PIM tree is built correctly and to monitor any error

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13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5384]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
              Independent Multicast (PIM) Join Attribute Format",
              RFC 5384, DOI 10.17487/RFC5384, November 2008,

   [RFC5496]  Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
              Forwarding (RPF) Vector TLV", RFC 5496,
              DOI 10.17487/RFC5496, March 2009,

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <>.

13.2.  Informative References

              Asaeda, H., Meyer, K., and W. Lee, "Mtrace Version 2:
              Traceroute Facility for IP Multicast", Work in Progress,
              draft-ietf-mboned-mtrace-v2-13, June 2016.

   [RFC7431]  Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
              Decraene, "Multicast-Only Fast Reroute", RFC 7431,
              DOI 10.17487/RFC7431, August 2015,


   The authors would like to thank Vatsa Kumar, Nagendra Kumar, and
   Bharat Joshi for their comments on the document.

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Authors' Addresses

   Javed Asghar
   Cisco Systems
   725, Alder Drive
   Milpitas, CA  95035
   United States


   IJsbrand Wijnands (editor)
   Cisco Systems
   De Kleetlaan 6a
   Diegem  1831


   Sowmya Krishnaswamy
   Cisco Systems
   3750 Cisco Way
   San Jose, CA  95134
   United States


   Apoorva Karan
   Cisco Systems
   3750 Cisco Way
   San Jose, CA  95134
   United States


   Vishal Arya
   2230 E Imperial Hwy
   El Segundo, CA  90245
   United States


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