Clarifying Use of LSP Ping to Bootstrap BFD over MPLS LSP

defines how LSP Ping uses BFD Discriminator TLV to bootstrap Bidirectional Forwarding Detection (BFD) session over MPLS Label Switch Path (LSP). Implementation and operational experiences suggest that two aspects of using LSP ping to bootstrap BFD session can benefit from clarification. This document updates in use of Return mode field in MPLS LSP echo request message and use of BFD Discriminator TLV in MPLS LSP echo reply.

MPLS: Multiprotocol Label Switching LSP: Label Switched Path BFD: Bidirectional Forwarding Detection

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.

does not define the value to be used for the Return mode field when LSP ping is used to bootstrap a BFD session of MPLS LSP. When LSP echo request is being used to detect defects in MPLS data plane and verify consistency between the control plane and the data plane echo reply is needed to confirm the correct state, provide the positive acknowledgment. But when LSP echo request is being used to bootstrap BFD session, then the positive acknowledgement, according to is provided by the egress transmitting BFD control message. Thus LSP echo reply is not required to bootstrap BFD session and hence the Return mode field in echo request message SHOULD be set to 1 (Do not reply) when LSP echo request used to bootstrap BFD session.

in section 6 defines that echo reply by the egress LSR to BFD bootstrapping echo request MAY include BFD Discriminator TLV with locally assigned discriminator value for the BFD session. But the does not define how the ingress LSR may use the returned value. From practical point, as discussed in , the returned value is not useful since the egress is required to send the BFD control message right after successfully validating the FEC and before sending echo reply message. Secondly, identifying the corresponding BFD session at ingress without returning its discriminator presents unnecessary challenge for the implementation. Thus the egress LSR SHOULD NOT include BFD Discriminator TLV if sending echo reply to BFD bootstrapping echo request.

This document does not require any action by IANA. This section may be removed.

This document does not introduce new security aspects but inherits all security considerations from , , .

TBA