PIM Working Group                                              G. Mirsky
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               J. Xiaoli
Expires: 28 March 10 April 2022                                   ZTE Corporation
                                                       24 September
                                                          7 October 2021

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


   This document specifies how Bidirectional Forwarding Detection 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

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   This Internet-Draft will expire on 28 March 10 April 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  BFD Discriminator PIM Hello Option  . . . . . . . . . . . . .   3
     2.1.  Using P2MP BFD in PIM Router Monitoring . . . . . . . . .   4
     2.2.  P2MP BFD in PIM DR Load Balancing . . . . . . . . . . . .   5
     2.3.  Multipoint BFD Encapsulation  . . . . . . . . . . . . . .   5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Faster convergence in the control plane minimizes the periods of
   traffic blackholing, 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

   [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 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] had been
   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, and it precisely
   characterizes deployment scenarios for PIM-SM over a LAN segment.
   Among specific characteristics of p2mp BFD that particularly benefit
   PIM-SM over a LAN segment is a faster transition to the Up state of
   the p2mp BFD session due to avoidance of the three-way handshake
   required in p2p BFD [RFC5880].  Also, because the router that
   transmits BFD Control messages uses the BFD Demand mode [RFC5880], it
   maintains less BFD state than the Asynchronous mode.  Point-to-
   multipoint (p2mp) BFD can enable faster detection of PIM-SM router
   failure and thus minimize multicast service disruption.  The
   monitored PIM-SM router acts as the head and other routers as tails
   of a p2mp BFD session.  This document defines the monitoring of a
   PIM-SM router using p2mp BFD.  The document also defines the
   extension to PIM-SM [RFC7761] to bootstrap a PIM-SM router to join in
   p2mp BFD session over 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",
   "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: TBA.

      OptionLength: MUST be set to 4.

      HeadDiscriminator: 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 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 option in its Hello
   messages, and it MUST include a 4-byte HeadDiscriminator with a value
   other than zero.

   If a PIM-SM router is configured to monitor the head by using p2mp
   BFD, referred to through this document as '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

   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 values of the BFD packets MUST be the same as those of
   the HeadDiscriminator in 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].

   If the head ceases to include the BFD Discriminator PIM Hello option
   in its PIM-Hello message, tails MUST 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 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
   Section 5.9 [RFC8562].  For each GDR Candidate that includes BFD
   Discriminator option in its PIM Hello, the PIM DR creates 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 TTL or Hop Limit value to 255 (Section 5 [RFC5881]);

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

3.  IANA Considerations

   IANA is requested to allocate a new OptionType value from PIM-Hello
   Options registry according to:

       | Value | Length | Name                     | Reference     |
       | TBA   | 4      | BFD Discriminator Option | This document |

                   Table 1: BFD Discriminator option type

4.  Security Considerations

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

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

5.  Acknowledgments

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

6.  References

6.1.  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,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

   [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,

6.2.  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>.

Authors' Addresses

   Greg Mirsky

   Email: gregimirsky@gmail.com

   Ji Xiaoli
   ZTE Corporation
   No.50 Software Avenue, Yuhuatai District

   Email: ji.xiaoli@zte.com.cn