BFD Working Group                                              G. Mirsky
Internet-Draft                                                 ZTE Corp.
Intended status: Standards Track                       September 9, 2020
Expires: March 13, 2021

            BFD in Demand Mode over Point-to-Point MPLS LSP


   This document describes procedures for using Bidirectional Forwarding
   Detection (BFD) in Demand mode to detect data plane failures in
   Multiprotocol Label Switching (MPLS) point-to-point Label Switched

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   2
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   2
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   3.  Use of the BFD Demand Mode  . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC5884] defined use of the Asynchronous method of Bidirectional
   Detection (BFD) [RFC5880] to monitor and detect failures in the data
   path of a Multiprotocol Label Switching (MPLS) Label Switched Path
   (LSP).  Use of the Demand mode, also specified in [RFC5880], has not
   been defined so far.  This document describes procedures for using
   the Demand mode of BFD protocol to detect data plane failures in MPLS
   point-to-point (p2p) LSPs.

2.  Conventions used in this document

2.1.  Terminology

   MPLS: Multiprotocol Label Switching

   LSP: Label Switched Path

   LER: Label switching Edge Router

   BFD: Bidirectional Forwarding Detection

   p2p: Point-to-Point

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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3.  Use of the BFD Demand Mode

   [RFC5880] defines that the Demand mode MAY be:

   o  asymmetric, i.e. used in one direction of a BFD session;

   o  switched to and from without bringing BFD session to Down state
      through using a Poll Sequence.

   For the case of BFD over MPLS LSP, ingress Label switching Edge
   Router (LER) usually acts as Active BFD peer and egress LER acts as
   Passive BFD peer.  The Active peer bootstraps the BFD session by
   using LSP ping.  If the BFD session is configured to use the Demand
   mode, once the BFD session is in Up state the ingress LER MUST switch
   to the Demand mode as defined in Section 6.6 [RFC5880].  The egress
   LER also follows procedures defined in Section 6.6 [RFC5880] and
   ceases further transmission of periodic BFD control packets to the
   ingress LER.

   In this state BFD peers MAY remain as long as the egress LER is in Up
   state.  The ingress LER SHOULD periodically check continuity of a
   bidirectional path between the ingress and egress LERs by using the
   Poll Sequence, as described in Section 6.6 [RFC5880].  An
   implementation that supports using the Poll Sequence as the mechanism
   for bidirectional path continuity check MUST be able to control the
   interval between consecutive Poll Sequences.  The RECOMMENDED default
   value is 1 second.

   If the Detection timer at the egress LER expires it MUST send BFD
   Control packet to the ingress LER with the Poll (P) bit set, Status
   (Sta) field set to the Down value, and the Diagnostic (Diag) field
   set to Control Detection Time Expired value.  The egress LER
   periodically transmits these Control packets to the ingress LER until
   either it receives the valid for this BFD session control packet with
   the Final (F) bit set from the ingress LER or the defect condition
   clears and the BFD session state reaches Up state at the egress LER.
   An implementation that supports this specification MUST provide
   control of the interval between consecutive Poll messages signaling
   the expiration of the Detection timer.  The RECOMMENDED default value
   of the interval is 1 second.

   The ingress LER transmits BFD Control packets over the MPLS LSP with
   the Demand (D) flag set at negotiated interval per [RFC5880], the
   greater of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval,
   until it receives the valid BFD packet from the egress LER with the
   Poll (P) bit and the Diagnostic (Diag) field value Control Detection
   Time Expired.  Reception of such BFD control packet by the ingress
   LER indicates that the monitored LSP has a failure and sending BFD

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   control packet with the Final flag set to acknowledge failure
   indication is likely to fail.  Instead, the ingress LER transmits the
   BFD Control packet to the egress LER over the IP network with:

   o  destination IP address MUST be set to the destination IP address
      of the LSP Ping Echo request message [RFC8029];

   o  destination UDP port set to 4784 [RFC5883];

   o  Final (F) flag in BFD control packet MUST be set;

   o  Demand (D) flag in BFD control packet MUST be cleared.

   The ingress LER changes the state of the BFD session to Down and
   changes rate of BFD Control packets transmission to one packet per
   second.  The ingress LER in Down mode changes to Asynchronous mode
   until the BFD session comes to Up state once again.  Then the ingress
   LER switches to the Demand mode.

4.  IANA Considerations


5.  Security Considerations

   This document does not introduce new security aspects but inherits
   all security considerations from [RFC5880], [RFC5884], [RFC7726],
   [RFC8029], and [RFC6425].

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <>.

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   [RFC6425]  Saxena, S., Ed., Swallow, G., Ali, Z., Farrel, A.,
              Yasukawa, S., and T. Nadeau, "Detecting Data-Plane
              Failures in Point-to-Multipoint MPLS - Extensions to LSP
              Ping", RFC 6425, DOI 10.17487/RFC6425, November 2011,

   [RFC7726]  Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S.
              Aldrin, "Clarifying Procedures for Establishing BFD
              Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726,
              DOI 10.17487/RFC7726, January 2016,

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

Appendix A.  Acknowledgements


Author's Address

   Greg Mirsky
   ZTE Corp.


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