Clarifying Procedures for Establishing BFD Sessions for MPLS Label Switched Paths (LSPs)
updates
- rfc-5884 — Bidirectional Forwarding Detection (BFD) for MPLS Label Switched Paths (LSPs)
Extracted elements (15)
An explicit session removal procedure at the egress LSR (not specified in RFC 5884) is introduced to improve resource management (memory, etc.), which is especially important in deployments with thousands or more concurrent BFD sessions.
The ability to establish multiple BFD sessions for an <MPLS LSP, FEC> tuple is motivated by use cases such as Segment Routing LSPs and LSPs with Equal-Cost Multipath (ECMP), where different paths must each be independently monitored.
The ambiguity in RFC 5884 regarding multiple concurrent BFD sessions arose partly from Section 7 of RFC 5884 forbidding either end from changing local discriminator values after a session reaches UP state, which created unclear interaction with bootstrapping additional sessions.
The procedures in this document are fully backward compatible with RFC 5884 implementations. The egress LSR must be upgraded to support multiple BFD sessions per <MPLS LSP, FEC> before the ingress LSR is upgraded, to ensure correct operation during a phased rollout.
All BFD sessions established with a given FEC MUST be removed automatically if the FEC itself is removed.
Each MPLS LSP ping used to bootstrap a BFD session MUST carry a different discriminator value in the BFD discriminator TLV. This enables multiple concurrent BFD sessions for the same <MPLS LSP, FEC> tuple.
FEC validation in the MPLS Echo request is a necessary condition for creating a new BFD session at the egress LSR; a session MUST NOT be created without successful FEC validation.
The egress LSR MAY remove a BFD session when the session transitions from UP to DOWN, either immediately or after a configurable hysteresis timer to reduce flapping.
The egress LSR must ensure uniqueness of the <MPLS LSP, FEC, Remote Discriminator> tuple. A new BFD session is bootstrapped only when no local session corresponds to the remote discriminator received in the MPLS Echo request.
The egress LSR MUST use the discriminators exchanged when the session was brought UP to signal any session state change to the ingress, and SHOULD reset them to zero after transmitting bfd.detectMult packets once the session transitions to DOWN.
The ingress LSR MAY remove a BFD session at the egress by sending BFD packets with State Down(1) and diagnostic code AdminDown(7) for detectMultiplier iterations; upon receiving these, the egress MAY remove the session without triggering a state change.
When the egress LSR disallows creation of a BFD session due to policy (e.g., resource constraints), it MUST drop the MPLS Echo request message rather than sending an error response.
Both ingress and egress LSRs use the 'Your Discriminator' field of received BFD packets to demultiplex multiple concurrent BFD sessions established for the same <MPLS LSP, FEC>.
Multiple BFD sessions consume system and network resources. It is critical to provision only the minimum number of BFD sessions needed per FEC and to promptly delete sessions when no longer required. Security measures from RFC 4379 and RFC 5884 must also be followed.
BFD session discriminators for an MPLS LSP session cannot be changed in UP state. To change discriminators, the session must transition gracefully to AdminDown (via diagnostic code AdminDown), be removed, then a new session established with different discriminators. Either the ingress or egress LSR may initiate this transition.