| Internet-Draft | Multi-Point BFD over P2MP MPLS LSP | March 2021 |
| Mirsky, et al. | Expires 1 October 2021 | [Page] |
This document describes procedures for using Bidirectional Forwarding Detection (BFD) for multipoint networks to detect data plane failures in Multiprotocol Label Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths (LSPs) using active tails with unsolicited notifications mode. It also describes the applicability of LSP Ping, as in-band, and the control plane, as out-band, solutions to bootstrap a BFD session in this environment.¶
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[RFC8562] defines a method of using Bidirectional Detection (BFD) [RFC5880] to monitor and detect unicast 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 of the bfd.SessionType is set on a BFD endpoint. This document describes procedures for using such modes of BFD protocol to detect data plane failures in Multiprotocol Label Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths (LSPs). The document also describes the applicability of out-band solutions to bootstrap a BFD session in this environment.¶
MPLS: Multiprotocol Label Switching¶
LSP: Label Switched Path¶
BFD: Bidirectional Forwarding Detection¶
p2mp: Point-to-Multipoint¶
FEC: Forwarding Equivalence Class¶
G-ACh: Generic Associated Channel¶
ACH: Associated Channel Header¶
GAL: G-ACh Label¶
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 the Demand mode from the very start of a point-to-multipoint (p2mp) BFD session. Because the head doesn't receive any BFD Control packet from a tail, the head of the p2mp BFD session transmits all BFD Control packets with the value of Your Discriminator field set to zero. As a result, a tail cannot demultiplex BFD sessions using Your Discriminator, as defined in [RFC5880]. [RFC8562] requires that to demultiplex BFD sessions, the tail uses the source IP address, My Discriminator, and the identity of the multipoint tree from which the BFD Control packet was received. The p2mp MPLS LSP label MAY provide the identification of the multipoint tree in case of an inclusive p-tree or upstream-assigned label in case of aggregate p-tree. 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 specification further clarifies that:¶
In some environments, the overhead of extra IP/UDP encapsulations may be considered burdensome, making the use of more compact G-ACh 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 to 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. Non-IP encapsulation for multipoint BFD over p2mp MPLS LSP MUST use Generic Associated Channel (G-ACh) Label (GAL) (see [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 TBA1 value Section 6. To provide the identity of the MultipointHead for the particular multipoint BFD session, a Source Address TLV [RFC7212] MUST immediately follow a BFD Control message.¶
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 used, it MUST include the Target FEC TLV and the BFD Discriminator TLV defined in [RFC5884]. The Target FEC TLV MUST use sub-TLVs defined in Section 3.1 [RFC6425]. Setting the value of Reply Mode field to "Do not reply" [RFC8029] for the LSP Ping to bootstrap MultipointTail of the p2mp BFD session is RECOMMENDED. Indeed, because BFD over a multipoint network uses BFD Demand mode, the LSP echo reply from a tail has no useful information to convey to the head, unlike in the case of the 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.¶
[RFC8562] defined how the BFD Demand mode can be used in multipoint networks. When applied in MPLS, procedures specified in [RFC8562] allow an egress LSR to detect a failure of the part of the MPLS p2mp LSP from the ingress LSR. The ingress LSR is not aware of the state of the p2mp LSP. [RFC8563], using mechanisms defined in [RFC8562], defined an "active tail" behavior. An active tail might notify the head of the detected failure and responds 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 [RFC8563], is the simplest of all described in [RFC8563]. The use of this method in BFD over MPLS p2mp LSP is discussed in this document. Analysis of other methods of a head learning of the state of an MPLS p2mp LSP is outside the scope of this document.¶
As specified in [RFC8563] for the active tail mode, BFD variables MUST be as follows:¶
On an ingress LSR:¶
On an egress LSR:¶
In Section 5.2.1 [RFC8563] 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 below the procedure of an active tail with unsolicited notifications for p2mp MPLS LSP.¶
Upon detecting the failure of the p2mp MPLS LSP, an egress LSR sends BFD Control packet with the following settings:¶
An ingress LSR that has received the BFD Control packet, as described above, sends the unicast IP/UDP encapsulated BFD Control packet with the Final (F) bit set to the egress LSR.¶
The BGP-BFD Attribute [I-D.ietf-bess-mvpn-fast-failover] MAY be used to bootstrap multipoint BFD session on a tail.¶
This document does not introduce new security aspects but inherits all security considerations from [RFC5880], [RFC5884], [RFC7726], [RFC8562], [RFC8029], and [RFC6425].¶
Also, BFD for p2mp MPLS LSP MUST follow the requirements listed in section 4.1 [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, comparing to G-ACh encapsulation.¶
IANA is requested to allocate value (TBA1) from its MPLS Generalized Associated Channel (G-ACh) Types registry.¶
| Value | Description | Reference |
|---|---|---|
| TBA1 | Multipoint BFD Session | This document |
The author sincerely appreciates the comments received from Andrew Malis and thought stimulating questions from Carlos Pignataro.¶