Clarifying Use of LSP Ping to Bootstrap BFD over MPLS LSP

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Table of Contents

1. Introduction

[RFC5884] defines how LSP Ping [RFC8029] 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 [RFC5884] in use of Return mode field in MPLS LSP echo request message and use of BFD Discriminator TLV in MPLS LSP echo reply.¶

3. Use of Return Mode Field

[RFC5884] does not define the value to be used for the Return mode field [RFC8029] 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 positive acknowledgment. But when an LSP echo request is being used to bootstrap BFD session, then the positive acknowledgment, according to [RFC5884] 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) [RFC8029] when LSP echo request used to bootstrap BFD session.¶

4. Use of BFD Discriminator TLV in LSP Echo Reply

[RFC5884] 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 [RFC5884] does not define how the ingress LSR may use the returned value. From a practical point, as discussed in Section 3, 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 an echo reply message. Secondly, identifying the corresponding BFD session at ingress without returning its discriminator presents an unnecessary challenge for the implementation. Thus the egress LSR SHOULD NOT include BFD Discriminator TLV if sending echo reply to BFD bootstrapping echo request.¶

5. IANA Considerations

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

6. Security Considerations

This document does not introduce new security aspects but inherits all security considerations from [RFC5880], [RFC5884], [RFC8029].¶

7. Acknowledgements

TBA¶

Authors' Addresses