BIER BFD

Introduction Bit Index Explicit Replication (BIER) provides efficient multicast forwarding without requiring per‑flow state in the network. A BIER Forwarding Router (BFR) delivers packets to a set of receivers identified by the bitstring in the BIER header, enabling scalable multicast distribution across a domain. As BIER deployments grow, operators require mechanisms to verify continuity, detect failures, and assess reachability of multiple receivers in a BIER sub‑domain. Bidirectional Forwarding Detection (BFD) is widely used to provide rapid failure detection for unicast paths. Point‑to‑multipoint (P2MP) BFD, specified in , extends BFD to multipoint trees and allows a single head to monitor the liveness of multiple tails. In , the head transmits BFD Control packets to all tails, and each tail monitors the continuity of the path from the head. does not define any mechanism for the head to learn about failures affecting individual tails; it only enables tails to detect loss of continuity from the head. builds on by defining several mechanisms that allow a tail to notify the head about failures affecting the path between the head and that specific tail. These mechanisms include unsolicited tail‑to‑head notifications and procedures for "active tails", enabling the head to correlate failures with individual receivers. Together, and provide a complete framework for multipoint continuity checking and failure reporting. This document specifies the procedures for transmitting P2MP BFD over BIER. It defines how a BIER head encapsulates BFD Control packets, how tails demultiplex and process them, and how unsolicited notifications defined in may be sent from tails to the head. The document also describes bootstrapping mechanisms that allow tails to discover the BFD session parameters associated with a given BIER sub‑domain and bitstring. The goal of this specification is to provide a lightweight and scalable continuity‑checking mechanism for BIER paths, enabling operators to detect failures affecting any subset of receivers without introducing per‑flow state in the network. The procedures defined here apply to any BIER deployment and do not require modifications to the BIER forwarding architecture.

Conventions used in this document

Terminology This document uses the acronyms defined in along with the following: BFD: Bidirectional Forwarding Detection. OAM: Operations, Administration, and Maintenance. P2MP: Point-to-Multipoint.

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 when, and only when, they appear in all capitals, as shown here.

BIER BFD Encapsulation shows the encapsulation of a P2MP BFD Control packet in a BIER packet.

BIER BFD Encapsulation BIER BFD encapsulation uses the BIER OAM packet format defined in . The BitString identifies the set of tails that are expected to receive and process the BFD Control packet. The value of the Message Type field MUST be set to BIER BFD (TBD1 will be assigned by IANA ). The BFD Control packet, defined in Section 4 , immediately follows the BIER OAM header. The operation of Multipoint BFD with the BFD Control packet is described in .

BIER BFD Session Bootstrapping As defined in , a BIER BFD session MAY be established to monitor the state of the multipoint path. The BIER BFD session could be created for each multipoint path and the set of BFERs over which the BFIR is requested to run BIER BFD. The BFIR, according to Section 5.7 of , MAY bootstrap the BFD session using a BIER OAM message () or the control plane (). Either method MUST refer to the root of the multipoint path and the value of My Discriminator associated with the path to the set of BFERs. The BIER BFD bootstrapping MUST be repeated when the value of My Discriminator is changed. Also, a P2MP BFD session can be statically configured.

Bootstrapping a BIER BFD Session Using BIER Ping The BIER OAM can be used to bootstrap the BIER BFD session. The BFIR sends the BIER OAM Echo request message carrying a BFD discriminator TLV which immediately follows the Target SI-Bitstring TLV (Section 3.3.2 of ). The Target SI-Bitstring TLV MUST be used to carry the set of BFER information (including Sub-domain-id, Set ID, BS Len, Bitstring) for the purpose of the session establishment. displays the format of the BFD Discriminator for a P2MP BFD Session with Active Tails Using Unsolicited Notifications TLV.

BFD Discriminator for a P2MP BFD Session with Active Tails Using Unsolicited Notifications TLV where:

Type indicates BFD Discriminator for a P2MP BFD Session with Active Tails Using Unsolicited Notifications TLV. The value (TBD2) is to be allocated by IANA ().
Length MUST be set to 4.
My Discriminator - four-octet long field. The value is the local discriminator generated by BFIR for this session. This discriminator MUST be used as the My Discriminator field in the BIER BFD Control packets sent by the BFIR.

BGP Bootstrapping describes the applicability of in the detection of a Multicast Virtual Private Network. The new BGP Attribute, BFD Discriminator, can be used to bootstrap a P2MP BFD session according to . To bootstrap a P2MP BFD session with active tails using unsolicited notifications according to , IANA is requested to allocate a new value (TBD3) for the BFD Mode ().

Discriminators and Packet Demultiplexing P2MP BFD modifies the demultiplexing rules defined in . In , a received BFD packet is demultiplexed using the Your Discriminator field, which uniquely identifies the session at the receiver. In P2MP BFD (), tails do not allocate discriminators and therefore cannot rely on Your Discriminator for demultiplexing. Instead, defines that tails demultiplex packets using the My Discriminator field, which is assigned by the MultipointHead. Because the MultipointHead assigns the My Discriminator, it is not globally unique across all tails. Therefore, the discriminator alone is insufficient to identify a P2MP BFD session at a tail uniquely. requires that tails select the correct session using the discriminator in combination with other context, and explicitly leaves the exact method of selection to the application:

“the session MUST be selected based on some combination of other fields, possibly including source addressing information, the My Discriminator field, and the interface over which the packet was received. The exact method of selection is application specific and is thus outside the scope of this specification.”

In BIER, the value of the BFIR-id field () in the BIER header provides the additional context required by RFC 8562 for demultiplexing P2MP BFD sessions at the tail. Because the MultipointHead assigns the My Discriminator and uniqueness is guaranteed only per head, the discriminator alone does not uniquely identify a session across all tails. The BFIR-id uniquely identifies the originating head within the BIER domain. Therefore, the tuple (BFIR-id, My Discriminator) uniquely identifies the P2MP BFD session at every tail. Tails MUST use this tuple as the demultiplexing key, consistent with the requirement in that session selection use a combination of fields beyond the My Discriminator alone.

Active Tail Behavior in BIER BFD Active Tail behavior for P2MP BFD over BIER, as specified in this document, is based on the unsolicited notification mechanisms defined in , not on the solicited notification methods defined in . In , the MultipointHead learns the state of the multicast distribution tree by explicitly querying tails; the tails respond to these queries, and the resulting messages are therefore solicited notifications. , in contrast, defines mechanisms by which an Active tail can independently detect failures and send unsolicited notifications to the head, without being queried. This document specifies the applicability of the Active Tail mechanisms to P2MP BFD sessions transported over BIER. When P2MP BFD is transported over BIER, an Active tail:

Monitors continuity from the head using procedures.
Identifies the session using the tuple (BFIR-id, My Discriminator).
Upon detecting a failure, sends an unsolicited notification toward the head using the mechanisms defined in . Note that a unicast message must be sent over a path which is disjoint from the multicast distribution tree

Unsolicited Head Notification Mode provides detailed information on using the unsolicited notification method for P2MP MPLS LSP, which is also applicable to BIER. In Section 5.2.1 it is noted that "the tail sends unsolicited BFD packets in response to the detection of a multipoint path failure" but without the specifics on the information in the packet and frequency of transmissions. This document defines the procedure of the active tail with unsolicited notifications for BIER as specified below. Upon detecting the failure, a BFER sends a BFD Control packet with the following settings:

the Poll (P) bit MUST be set;
the Status (Sta) field MUST be set to the Down value;
the Diagnostic (Diag) field MUST be set to Control Detection Time Expired value;
the value of the Your Discriminator field MUST be set to the My Discriminator value associated with the failed P2MP BFD session;
BFD Control packet MUST use IP/UDP encapsulation with the destination IP address of the BFIR and the UDP destination port number set to 4784 per
the BFD Control packets MUST be transmitted at the rate of one per second until either the BFER receives a valid for this BFD session control packet with the Final (F) bit is set from the BFIR or the defect condition clears.

To improve the likelihood of notifying the BFIR of the failure, the BFER SHOULD transmit three BFD Control packets defined above in pseudo-random intervals between packets within a one-second interval. A BFIR that has received the BFD Control packet demultiplexes BFD sessions as defined in , i.e., based on the value in Your Discriminator field. After the BFIR matches the received BFD Control Packet to the BFD session, it sends the unicast IP/UDP encapsulated BFD Control packet with the Final (F) bit set to the BFER.

Operational Considerations Operational considerations formulated in Section 6 of , Section 8 of , and in apply to BIER BFD.

Security Considerations This document inherits all security considerations from , , , and . A single failure could affect a significant number of BFERs, thus causing a spike in the number of BFD Control packets with notifications, as defined in . To mitigate the overloading of the control plane, an implementation MUST control the number of BFD Control packets passed to the control plane for processing.

Acknowledgments The authors would like to thank the comments and suggestions from Sandy Zhang, Jeffrey (Zhaohui) Zhang, Donald Eastlake 3rd, Reshad Rahman, and Les Ginsberg.

IANA Considerations

BIER OAM Message Type IANA is requested to assign a new type from the BIER OAM Message Type registry in the BIER OAM registry group as follows:

Value	Description	Reference
TBD1	BIER BFD	[this document]

BFD Discriminator for a P2MP BFD Session with Active Tails Using Unsolicited Notification TLV IANA is requested to assign a new type from the TLVs registry in the BIER OAM registry group as follows:

Value	Description	Reference
TBD2	BFD Discriminator for a P2MP BFD Session with Active Tails Using Unsolicited Notification TLV	[this document]

BGP's BFD Discriminator Attribute Mode of P2MP BFD Session with Active Tails Using Unsolicited Notifications IANA is requested to assign a new value from "BFD Mode" subregistry (defined in ) in the Border Gateway Protocol (BGP) Parameters registry according to :

Value	Description	Reference
TBD3	P2MP BFD Session with Active Tails Using Unsolicited Notifications	[this document]