Internet-Draft BFD P2MP Use in PIM-SM August 2021
Mirsky & Xiaoli Expires 5 February 2022 [Page]
PIM Working Group
Intended Status:
Standards Track
G. Mirsky
ZTE Corp.
J. Xiaoli
ZTE Corporation

The Use of Bidirectional Forwarding Detection (BFD) for Multipoint Networks in Protocol Independent Multicast - Sparse Mode (PIM-SM)


This document specifies the use of Bidirectional Forwarding Detection (BFD) for multipoint networks to provide nodes that participate in Protocol Independent Multicast - Sparse Mode (PIM-SM) with sub-second convergence. An extension to the PIM Hello message used to bootstrap point-to-multipoint BFD sessions is also defined in this document.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 5 February 2022.

Table of Contents

1. Introduction

Faster convergence in the control plane, in general, is beneficial and allows minimizing periods of traffic blackholing, transient routing loops, and other scenarios that may negatively affect service data flow. That equally applies 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 DR is to act 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. Introduction of Backup DR (BDR) ([I-D.ietf-pim-dr-improvement]) improves convergence time in PIM-SM over shared-media segment but still depends on a long failure detection interval.

Bidirectional Forwarding Detection (BFD) [RFC5880] had been originally defined to detect failure of point-to-point (p2p) paths - single-hop [RFC5881], multihop [RFC5883]. In some PIM-SM deployments, a p2p BFD can be used to detect a failure and enable faster conversion. [RFC8562] extends the BFD base specification [RFC5880] for multipoint and multicast networks 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 comparing to 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 PIM-SM DR and BDR using p2mp BFD. Other cases of using p2mp BFD in PIM-SM are outside the scope of this specification. 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 link.

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          |
    |                       My  Discriminator                       |
Figure 1: BFD Discriminator PIM Hello Option

where new fields are interpreted as:

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 My Discriminator field equals zero, then the BFD Discriminator PIM Hello option MUST be considered invalid, and the receiver MUST ignore it. The receiver SHOULD log the 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 DR/BDR Monitoring

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. The head MUST include the BFD Discriminator option in its Hello messages.

If a PIM-SM router is configured to monitor the head by using p2mp BFD, referred to through this document as 'tail', receives PIM-Hello packet with 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 values of the BFD packets MUST be the same as those used 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. If the failed head was the DR (or BDR), the DR (or BDR) election mechanism in [RFC7761] or [I-D.ietf-pim-dr-improvement] is followed.

If the head ceases to include the BFD Discriminator PIM Hello option in its PIM-Hello message, tails MUST close the corresponding MultipointTail BFD session. Thus the tail stops using BFD to monitor the head and reverts to the procedures defined in [RFC7761] and [I-D.ietf-pim-dr-improvement].

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 packet:

  • 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:

Table 1: BFD Discriminator option type
Value Name  Length Number  Name Protocol  Reference 
TBA 4 BFD Discriminator Option This document

4. Security Considerations

The security considerations discussed in [RFC7761], [RFC5880], [RFC8562], [RFC8775], and [I-D.ietf-pim-dr-improvement] apply to this document.

An implementation that supports this specification SHOULD use a mechanism to control the maximum number of BFD sessions that can be active at the same time.

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

Zhang, Z., Hu, F., Xu, B., and M. Mishra, "Protocol Independent Multicast - Sparse Mode (PIM-SM) Designated Router (DR) Improvement", Work in Progress, Internet-Draft, draft-ietf-pim-dr-improvement-11, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky, Ed., "Bidirectional Forwarding Detection (BFD) for Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562, , <>.
Cai, Y., Ou, H., Vallepalli, S., Mishra, M., Venaas, S., and A. Green, "PIM Designated Router Load Balancing", RFC 8775, DOI 10.17487/RFC8775, , <>.

6.2. Informative References

Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI 10.17487/RFC5881, , <>.
Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883, , <>.

Authors' Addresses

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