| RFC 9780 | Multipoint BFD over P2MP MPLS LSP | May 2025 |
| Mirsky, et al. | Standards Track | [Page] |
This document describes procedures for using Bidirectional Forwarding Detection (BFD) for multipoint networks to detect data plane failures in MPLS point-to-multipoint Label Switched Paths (LSPs) and Segment Routing (SR) point-to-multipoint policies with an SR over MPLS (SR-MPLS) data plane.¶
Furthermore, this document updates RFC 8562 by recommending the use of an IPv6 address from the Dummy IPv6 Prefix range 100:0:0:1::/64 and discouraging the use of an IPv4 loopback address mapped to IPv6.¶
In addition, this document describes the applicability of LSP Ping, as in-band, and the control plane, as out-of-band, solutions to bootstrap a BFD session. The document also describes the behavior of the active tail for head notification.¶
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/rfc9780.¶
Copyright (c) 2025 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.¶
[RFC8562] defines a method of using Bidirectional Forwarding Detection (BFD) [RFC5880] to monitor and detect failures between the sender (head) and one or more receivers (tails) in multipoint or multicast networks.¶
[RFC8562] added two BFD session types: MultipointHead and MultipointTail. Throughout this document, MultipointHead and MultipointTail refer to the value to which the bfd.SessionType is set on a BFD endpoint.¶
This document describes procedures for using such modes of the BFD protocol to detect data plane failures in MPLS point-to-multipoint (P2MP) Label Switched Paths (LSPs) and Segment Routing (SR) point-to-multipoint policies with an SR over MPLS (SR-MPLS) data plane.¶
The document also describes the applicability of out-of-band solutions to bootstrap a BFD session in this environment.¶
Historically, an IPv6-mapped IPv4 loopback range address::ffff:127.0.0.1/128 was mandated, although functionally, an IPv6 address from that range is not analogous to its IPv4 counterpart. Furthermore, using the loopback address as the destination address, even for an inner IP encapsulation of a tunneled packet, violates Section 2.5.3 of [RFC4291]. Hence, IANA has allocated 100:0:0:1::/64 as a new Dummy IPv6 Prefix (Section 7.1) to select destination IPv6 addresses for IP/UDP encapsulation of management, control, and Operations, Administration, and Maintenance (OAM) packets. A source-only IPv6 dummy address is used as the destination to generate an exception and a reply message to the request message received. This document starts the transition to using the IPv6 addresses from the Dummy IPv6 Prefix range 100:0:0:1::/64 as the IPv6 destination address in the IP/UDP encapsulation of active OAM over the MPLS data plane. Thus, this document updates [RFC8562] by recommending the use of an IPv6 address from the Dummy IPv6 Prefix range 100:0:0:1::/64 (Section 7.1) while acknowledging that an address from the ::ffff:127.0.0.1/128 range might be used by existing implementations. This document discourages the use of an address in the IPv6-mapped IPv4 loopback range.¶
This document also describes the behavior of the active tail for head notification.¶
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.¶
[RFC8562] uses BFD in Demand mode from the very start of a point-to-multipoint (P2MP) BFD session. Because the head doesn't receive any BFD Control packets from a tail, the head of the P2MP BFD session transmits all BFD Control packets with the value of the Your Discriminator field set to zero. As a result, a tail cannot demultiplex BFD sessions using Your Discriminator, as defined in [RFC5880]. To demultiplex BFD sessions, [RFC8562] requires that the tail use the source IP address, My Discriminator, and the identity of the multipoint tree from which the BFD Control packet was received. If the BFD Control packet is encapsulated in IP/UDP, then the source IP address MUST be used to demultiplex the received BFD Control packet as described in Section 3.1. The non-IP encapsulation case is described in Section 3.2.¶
[RFC8562] defines IP/UDP encapsulation for multipoint BFD over P2MP MPLS LSP. This document updates Section 5.8 of [RFC8562] regarding the selection of the IPv6 destination address as follows:¶
Section 1.2 of [RFC6790] lists several advantages of generating the entropy value by an ingress Label Switching Router (LSR) compared to when a transit LSR infers entropy using the information in the MPLS label stack or payload. This specification further clarifies that:¶
In some environments, the overhead of extra IP/UDP encapsulations may be considered burdensome, which makes the use of more compact Generic Associated Channel (G-ACh) [RFC5586] encapsulation attractive. Also, the validation of the IP/UDP encapsulation of a BFD Control packet in a P2MP BFD session may fail because of a problem related to neither the MPLS label stack nor BFD. Avoiding unnecessary encapsulation of P2MP BFD over an MPLS LSP improves the accuracy of the correlation of the detected failure and defect in MPLS LSP.¶
If a BFD Control packet in PW-ACH encapsulation (without IP/UDP Headers) is to be used in ACH, an implementation would not be able to verify the identity of the MultipointHead and, as a result, will not properly demultiplex BFD packets. Hence, a new channel type value is needed.¶
Non-IP encapsulation for multipoint BFD over P2MP MPLS LSP (shown in Figure 1) MUST use the G-ACh Label (GAL) [RFC5586] at the bottom of the label stack followed by an Associated Channel Header (ACH). If a BFD Control packet in PW-ACH encapsulation (without IP/UDP Headers) is to be used in ACH, an implementation would not be able to verify the identity of the MultipointHead and, as a result, will not properly demultiplex BFD packets. Hence, a new channel type value is needed. The Channel Type field in ACH MUST be set to Multipoint BFD Session (0x0013) (see Section 7.2). To provide the identity of the MultipointHead for the particular multipoint BFD session, a Source Address TLV, as defined in Section 4.1 of [RFC7212], MUST immediately follow a BFD Control message. The use of other TLVs is outside the scope of this document.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LSP Label | TC |S| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | GAL | TC |1| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1|Version| Flags | Channel Type = 0x0013 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ BFD Control Message ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type=0 | Reserved | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Address Family | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Address ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LSP Ping is the part of the on-demand OAM toolset used to detect and localize defects in the data plane and verify the control plane against the data plane by ensuring that the LSP is mapped to the same FEC at both egress and ingress endpoints.¶
LSP Ping, as defined in [RFC6425], MAY be used to bootstrap MultipointTail. If LSP Ping is used, it MUST include the Target FEC TLV and the BFD Discriminator TLV defined in [RFC5884]. For the case of P2MP MPLS LSP, the Target FEC TLV MUST use sub-TLVs defined in Section 3.1 of [RFC6425]. For the case of P2MP SR policy with an SR-MPLS data plane, an implementation of this specification MUST follow the procedures defined in [RFC8287]. Setting the value of the Reply Mode field to "Do not reply" [RFC8029] for the LSP Ping to bootstrap the MultipointTail of the P2MP BFD session is RECOMMENDED. Indeed, because BFD over a multipoint network uses BFD Demand mode, the MPLS echo reply from a tail has no useful information to convey to the head, unlike in the case of BFD over a P2P MPLS LSP [RFC5884]. A MultipointTail that receives an LSP Ping that includes the BFD Discriminator TLV:¶
Besides bootstrapping a BFD session over a P2MP LSP, LSP Ping SHOULD be used to verify the control plane against the data plane periodically by checking that the P2MP LSP is mapped to the same FEC at the MultipointHead and all active MultipointTails. The rate of generation of these LSP Ping Echo request messages SHOULD be significantly less than the rate of generation of the BFD Control packets because LSP Ping requires more processing to validate the consistency between the data plane and the control plane. An implementation MAY provide configuration options to control the rate of generation of the periodic LSP Ping Echo request messages.¶
The BFD Discriminator attribute MAY be used to bootstrap a multipoint BFD session on a tail, following the format and procedures given in Section 3.1.6 of [RFC9026].¶
[RFC8562] defines how BFD Demand mode can be used in multipoint networks. When applied in MPLS, the procedures specified in [RFC8562] allow an egress LSR to detect a failure in the part of the MPLS P2MP LSP from the ingress LSR to that egress LSR. The ingress LSR is not aware of the state of the P2MP LSP. [RFC8563], using mechanisms defined in [RFC8562], defines the behavior of an active tail. An active tail might notify the head of the detected failure and respond to a poll sequence initiated by the head. The first method, referred to as "Head Notification without Polling", is mentioned in Section 5.2.1 of [RFC8563]) and is the simplest of the methods described in [RFC8563]. The use of this method in BFD over MPLS P2MP LSP is discussed in this document. Analysis of other methods for a head to learn of the state of an MPLS P2MP LSP is outside the scope of this document.¶
As specified in [RFC8563], BFD variables MUST be as follows for the active tail mode:¶
Section 5.2.1 of [RFC8563] notes that "the tail sends unsolicited BFD packets in response to the detection of a multipoint path failure" but does not provide specifics about the information in the packets or the frequency of transmissions. The procedure for an active tail with unsolicited notifications for P2MP MPLS LSP is defined below.¶
Upon detecting the failure of the P2MP MPLS LSP, an egress LSR sends a BFD Control packet with the following settings:¶
As described above, an ingress LSR that has received the BFD Control packet sends the unicast IP/UDP encapsulated BFD Control packet with the Final (F) bit set to the egress LSR. In some scenarios (e.g., when a P2MP LSP is broken close to its root and the number of egress LSRs is significantly large), the root might receive a large number of notifications. The notifications from leaves to the root will not use resources allocated for the monitored multicast flow and, as a result, will not congest that particular flow, although they may negatively affect other flows. However, the control plane of the ingress LSR might be congested by the BFD Control packets transmitted by egress LSRs and the process of generating unicast BFD Control packets, as noted above. To mitigate that, a BFD implementation that supports this specification is RECOMMENDED to use a rate limiter of received BFD Control packets passed to the ingress LSR's control plane for processing.¶
This document does not introduce new security considerations but inherits all security considerations from [RFC5880], [RFC5884], [RFC7726], [RFC8562], [RFC8029], and [RFC6425].¶
Also, BFD for P2MP MPLS LSPs MUST follow the requirements listed in Section 4.1 of [RFC4687] to avoid congestion in the control plane or the data plane caused by the rate of generating BFD Control packets. An operator SHOULD consider the amount of extra traffic generated by P2MP BFD when selecting the interval at which the MultipointHead will transmit BFD Control packets. The operator MAY consider the size of the packet the MultipointHead transmits periodically as using IP/UDP encapsulation, which adds up to 28 octets (more than 50% of the BFD Control packet length) compared to G-ACh encapsulation.¶
IANA has allocated the following in the "IANA IPv6 Special-Purpose Address Registry" [IANA-IPv6-REG]:¶
IANA has allocated the following value in the "MPLS Generalized Associated Channel (G-ACh) Types" registry.¶
| Value | Description | Reference |
|---|---|---|
| 0x0013 | Multipoint BFD Session | RFC 9780 |
The authors sincerely appreciate the comments received from Andrew Malis, Italo Busi, and Shraddha Hegde. The authors also appreciate the thought-stimulating questions from Carlos Pignataro.¶