BFD Working Group                                              G. Mirsky
Internet-Draft                                                 ZTE Corp.
Intended status: Standards Track                           June 28, 2017
Expires: December 30, 2017

            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 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) is 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.  Once the BFD session is in Up state the ingress LER
   that supports this specification MUST switch to the Demand mode by
   setting Demand (D) bit in its Control packet and initiating a Poll
   Sequence.  If the egress LER supports this specification it MUST
   respond with the Final (F) bit set in its BFD Control packet sent
   over IP network to the egress LER and ceases 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 MAY check liveness of the egress LER by
   setting Poll flag.  The egress LER will respond by transmitting BFD
   control packet with the Final flag set.  If the ingress LER doesn't
   receive BFD packet with the Final flag from its peer after
   predetermined period of time, default wait time recommended 1 second,
   the ingress MAY transmit another packet with the Poll flag set.  If
   ingress doesn't receive BFD control packet with the Final flag set in
   response to three consecutive packets with Poll flag, it MAY declare
   the BFD peer non-responsive and change state of the BFD session to
   Down state.

   If the Detection timer at the egress LER expires it MUST send BFD
   Control packet to the ingress LER over the IP network with the Poll
   (P) bit set, Status (Sta) field set to Down value, and the Diagnostic
   (Diag) field set to Control Detection Time Expired value.  The egress
   LER sends these Control packets to the ingress LER at the rate of one
   per second 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.

   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.  Once the ingress LER receives such BFD Control packet

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   the ingress LER transmits the BFD Control packet to the egress LER
   over the IP network with:

   o  Final (F) flag set;

   o  Demand (D) flag 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,

   [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, <>.

   [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,

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   [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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