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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: gregimirsky@gmail.com
 Xiaoli Ji
 ZTE Corporation
 Yuhuatai District
 No. 50 Software Avenue
 Nanjing
 China
 Email: ji.xiaoli@zte.com.cn
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