Internet Engineering Task Force (IETF)                         G. Mirsky
Request for Comments: 9186                                      Ericsson
Category: Standards Track                                          X. Ji
ISSN: 2070-1721                                          ZTE Corporation
                                                           January 2022


Fast Failover in Protocol Independent Multicast - Sparse Mode (PIM-SM)
Using Bidirectional Forwarding Detection (BFD) for Multipoint Networks

Abstract

  This document specifies how Bidirectional Forwarding Detection (BFD)
  for multipoint networks can provide sub-second failover for routers
  that participate in Protocol Independent Multicast - Sparse Mode
  (PIM-SM).  An extension to the PIM Hello message used to bootstrap a
  point-to-multipoint BFD session is also defined in this document.

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
  https://www.rfc-editor.org/info/rfc9186.

Copyright Notice

  Copyright (c) 2022 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
  (https://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 Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
    1.1.  Conventions Used in This Document
      1.1.1.  Terminology
      1.1.2.  Requirements Language
  2.  BFD Discriminator PIM Hello Option
    2.1.  Using P2MP BFD in PIM Router Monitoring
    2.2.  P2MP BFD in PIM DR Load Balancing
    2.3.  Multipoint BFD Encapsulation
  3.  IANA Considerations
  4.  Security Considerations
  5.  References
    5.1.  Normative References
    5.2.  Informative References
  Acknowledgments
  Authors' Addresses

1.  Introduction

  Faster convergence in the control plane minimizes the periods of
  traffic loss due to the use of stale routing information, transient
  routing loops, and other situations that may negatively affect
  service data flow.  Faster convergence in the control plane is
  beneficial to unicast and multicast routing protocols.

  [RFC7761] is the current specification of the Protocol Independent
  Multicast - Sparse Mode (PIM-SM) for IPv4 and IPv6 networks.  A
  conforming implementation of PIM-SM elects a Designated Router (DR)
  on each PIM-SM interface.  When a group of PIM-SM nodes is connected
  to a shared media segment, e.g., Ethernet, the node elected as the DR
  acts on behalf of directly connected hosts in the context of the PIM-
  SM protocol.  Failure of the DR impacts the quality of the multicast
  services it provides to directly connected hosts because the default
  failure detection interval for PIM-SM routers is 105 seconds.

  Bidirectional Forwarding Detection (BFD) [RFC5880] was originally
  defined to detect a failure of a point-to-point (P2P) path, single
  hop [RFC5881], or multihop [RFC5883].  In some PIM-SM deployments, a
  P2P BFD can be used to detect a failure and enable faster failover.
  [RFC8562] extends the BFD base specification [RFC5880] for multipoint
  and multicast networks, which matches the deployment scenarios for
  PIM-SM over a LAN segment.  A BFD system in a point-to-multipoint
  (P2MP) environment that transmits BFD Control messages using the BFD
  Demand mode [RFC5880] creates less BFD state than the Asynchronous
  mode.  P2MP BFD can enable faster detection of PIM-SM router failure
  compared to PIM-SM without BFD and thus minimizes multicast service
  disruption.  The monitored PIM-SM router acts as the head and other
  routers act as tails of a P2MP BFD session.  This document defines
  the monitoring of a PIM-SM router using P2MP BFD.  This document also
  defines the extension to PIM-SM [RFC7761] to bootstrap a PIM-SM
  router to join in the P2MP BFD session over a shared media segment.

1.1.  Conventions Used in This Document

1.1.1.  Terminology

  This document uses terminology defined in [RFC5880], [RFC8562], and
  [RFC7761].  Familiarity with these specifications and the terminology
  used is expected.

1.1.2.  Requirements Language

  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
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

2.  BFD Discriminator PIM Hello Option

  Figure 1 displays the new optional BFD Discriminator PIM Hello Option
  to bootstrap a tail of the P2MP BFD session:


       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          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       HeadDiscriminator                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: BFD Discriminator PIM Hello Option

  where new fields are interpreted as:

  OptionType:  39

  OptionLength:  MUST be set to 4.

  HeadDiscriminator:  the 4-octet field MUST be included in the BFD
     Discriminator PIM-SM Hello Option.  The value MUST NOT be zero.
     It equals the value of My Discriminator [RFC5880] allocated by the
     head.

  If the value of the OptionLength field is not equal to 4, the BFD
  Discriminator PIM Hello Option is considered malformed, and the
  receiver MUST stop processing PIM Hello Options.  If the value of the
  HeadDiscriminator field equals zero, then the BFD Discriminator PIM
  Hello Option MUST be considered invalid, and the receiver MUST ignore
  it.  The receiver SHOULD log a notification regarding the malformed
  or invalid BFD Discriminator Hello Option under the control of a
  throttling logging mechanism.

2.1.  Using P2MP BFD in PIM Router Monitoring

  If the head is no longer serving the function that prompted it to be
  monitored, then it MUST cease including the BFD Discriminator PIM
  Hello Option in its PIM Hello message, and it SHOULD shut down the
  BFD session following the procedures described in [RFC8562],
  Section 5.9.

  The head MUST create a BFD session of type MultipointHead [RFC8562].
  Note that any PIM-SM router, regardless of its role, MAY become a
  head of a P2MP BFD session.  To control the volume of BFD Control
  traffic on a shared media segment, an operator should carefully
  select PIM-SM routers configured as a head of a P2MP BFD session.
  The head MUST include the BFD Discriminator PIM Hello Option in its
  PIM Hello messages.

  A PIM-SM router that is configured to monitor the head by using P2MP
  BFD is referred to throughout this document as a "tail".  When such a
  tail receives a PIM Hello packet with the BFD Discriminator PIM Hello
  Option, the tail MAY create a P2MP BFD session of type
  MultipointTail, as defined in [RFC8562].

  The node that includes the BFD Discriminator PIM Hello Option
  transmits BFD Control packets periodically.  For the tail to
  correctly demultiplex BFD [RFC8562], the source address and My
  Discriminator of the BFD packets MUST be the same as the source
  address and the HeadDiscriminator, respectively, of the PIM Hello
  message.  If that is not the case, the tail BFD node would not be
  able to monitor the state of the PIM-SM node -- that is, the head of
  the P2MP BFD session -- though the regular PIM-SM mechanisms remain
  fully operational.

  If the tail detects a MultipointHead failure [RFC8562], it MUST
  delete the corresponding neighbor state and follow procedures defined
  in [RFC7761] for the DR and additional neighbor state deletion after
  the neighbor timeout expires.

  If the head ceases to include the BFD Discriminator PIM Hello Option
  in its PIM Hello message, the tail SHOULD close the corresponding
  MultipointTail BFD session without affecting the PIM state in any
  way.  Thus, the tail stops using BFD to monitor the head and reverts
  to the procedures defined in [RFC7761].

2.2.  P2MP BFD in PIM DR Load Balancing

  [RFC8775] specifies the PIM Designated Router Load-Balancing (DRLB)
  functionality.  Any PIM router that advertises the DR Load-Balancing
  Capability (DRLB-Cap) Hello Option can become the head of a P2MP BFD
  session, as specified in Section 2.1.  The head router
  administratively sets the bfd.SessionState to Up in the
  MultipointHead session [RFC8562] only if it is a Group Designated
  Router (GDR) Candidate, as specified in Sections 5.5 and 5.6 of
  [RFC8775].  If the router is no longer the GDR, then it MUST shut
  down following the procedures described in [RFC8562], Section 5.9.
  For each GDR Candidate that includes the BFD Discriminator Option in
  its PIM Hello, the PIM DR MUST create a MultipointTail session
  [RFC8562].  PIM DR demultiplexes BFD sessions based on the value of
  the My Discriminator field and the source IP address.  If PIM DR
  detects a failure of one of the sessions, it MUST remove that router
  from the GDR Candidate list and immediately transmit a new DRLB-List
  option.

2.3.  Multipoint BFD Encapsulation

  The MultipointHead of a P2MP BFD session when transmitting BFD
  Control packets:

  *  MUST set the TTL or Hop Limit value to 255 ([RFC5881], Section 5).
     Similarly, all received BFD Control packets that are demultiplexed
     to the session MUST be discarded if the received TTL or Hop Limit
     is not equal to 255, and

  *  MUST use the group address ALL-PIM-ROUTERS ("224.0.0.13" for IPv4
     and "ff02::d" for IPv6) as the destination IP address.

3.  IANA Considerations

  IANA has allocated a new OptionType value in the "PIM-Hello Options"
  registry according to Table 1:

        +=======+========+==========================+===========+
        | Value | Length | Name                     | Reference |
        +=======+========+==========================+===========+
        | 39    | 4      | BFD Discriminator Option | RFC 9186  |
        +-------+--------+--------------------------+-----------+

                  Table 1: BFD Discriminator Option Type

4.  Security Considerations

  This document defines a way to accelerate detection of a failure that
  affects PIM functionality by using BFD.  The operation of either
  protocol is not changed.

  The security considerations discussed in [RFC5880], [RFC5881],
  [RFC7761], [RFC8562], and [RFC8775] apply to this document.

5.  References

5.1.  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,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
             (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
             <https://www.rfc-editor.org/info/rfc5880>.

  [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
             (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
             DOI 10.17487/RFC5881, June 2010,
             <https://www.rfc-editor.org/info/rfc5881>.

  [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, <https://www.rfc-editor.org/info/rfc7761>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [RFC8562]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
             Ed., "Bidirectional Forwarding Detection (BFD) for
             Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
             April 2019, <https://www.rfc-editor.org/info/rfc8562>.

  [RFC8775]  Cai, Y., Ou, H., Vallepalli, S., Mishra, M., Venaas, S.,
             and A. Green, "PIM Designated Router Load Balancing",
             RFC 8775, DOI 10.17487/RFC8775, April 2020,
             <https://www.rfc-editor.org/info/rfc8775>.

5.2.  Informative References

  [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
             (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
             June 2010, <https://www.rfc-editor.org/info/rfc5883>.

Acknowledgments

  The authors cannot say enough to express their appreciation of the
  comments and suggestions that were received from Stig Venaas.  The
  authors also greatly appreciate the comments and suggestions by
  Alvaro Retana that improved the clarity of this document.

Authors' Addresses

  Greg Mirsky
  Ericsson

  Email: [email protected]


  Xiaoli Ji
  ZTE Corporation
  Yuhuatai District
  No. 50 Software Avenue
  Nanjing
  China

  Email: [email protected]

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