Clarifications to RFC 5884
draft-grmas-bfd-rfc5884-clarifications-00

Internet Engineering Task Force                              V. Govindan
Internet-Draft                                             Cisco Systems
Updates: 5884 (if approved)                                 K. Rajaraman
Intended status: Standards Track                               G. Mirsky
Expires: April 6, 2015                                          Ericsson
                                                                N. Akiya
                                                           Cisco Systems
                                                               S. Aldrin
                                                     Huawei Technologies
                                                         October 3, 2014


                       Clarifications to RFC 5884
               draft-grmas-bfd-rfc5884-clarifications-00

Abstract

   This document clarifies the procedures for establishing, maintaining
   and removing multiple, concurrent BFD sessions for a given <MPLS LSP,
   FEC> described in RFC5884.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 6, 2015.

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Table of Contents

   1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Procedures for establishment of multiple BFD sessions . .   3
     2.2.  Procedures for maintenance of multiple BFD sessions . . .   3
     2.3.  Procedures for removing BFD sessions at the egress LSR  .   4
     2.4.  Changing discriminators for a BFD session . . . . . . . .   4
   3.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .   4
   4.  Encapsulation . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Background

   [RFC5884] defines the procedures to bootstrap and maintain BFD
   sessions for a <MPLS FEC, LSP> using LSP ping.  While Section 4 of
   [RFC5884] specifies that multiple BFD sessions can be established for
   a <MPLS FEC, LSP> tuple, the procedures to bootstrap and maintain
   multiple BFD sessions concurrently over a <MPLS FEC, LSP> are not
   clearly specified.  Additionally, the procedures of removing BFD
   sessions bootstrapped on the egress LSR are unclear.  This document
   provides those clarifications without deviating from the principles
   outlined in [RFC5884].

   The ability for an ingress LSR to establish multiple BFD sessions for
   a <MPLS FEC, LSP> tuple is useful in scenarios such as Segment
   Routing based LSPs or LSPs having Equal-Cost Multipath (ECMP).  The
   process used by the ingress LSR to determine the number of BFD
   session(s) to be bootstrapped for a <MPLS FEC, LSP> tuple and the
   mechanism of constructing those session(s) are outside the scope of
   this document.

1.1.  Requirements Language

   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
   [RFC2119].



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2.  Theory of Operation

2.1.  Procedures for establishment of multiple BFD sessions

   Section 6 of [RFC5884] specifies the procedure for bootstrapping BFD
   sessions using LSP ping.  It further states that a BFD session SHOULD
   be established for each alternate path that is discovered.  This
   requirement has been the source of some ambiguity as the procedures
   of establishing concurrent, multiple sessions have not been
   explicitly specified.  This ambiguity can also be attributed in part
   to the text in Section 7 of [RFC5884] forbidding either end to change
   local discriminator values in BFD control packets after the session
   reaches the UP state.  The following procedures are described to
   clarify the ambiguity based on the interpretation of the authors's
   reading of the referenced sections:

   At the ingress LSR:

      MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
      given <MPLS FEC, LSP>.  Each LSP ping MUST carry a different
      discriminator value in the BFD discriminator TLV [RFC4379].

   The egress LSR needs to perform the following:

      If the validation of the FEC in the MPLS Echo request message
      succeeds, check the discriminator specified in the BFD
      discriminator TLV of the MPLS Echo request.  If there is no local
      session that corresponds to the discriminator (remote) received in
      the MPLS Echo request, a new session is bootstrapped and a local
      discriminator is allocated.  Since the BFD local discriminator of
      either ends cannot change as long as the session is in the UP
      state, a new discriminator received in the LSP ping unambiguously
      conveys the intent of the LSR ingress to bootstrap a new BFD
      session for the FEC specified in the LSP ping.

      Ensure the uniqueness of the <MPLS FEC, LSP, Remote
      Discriminiator> tuple.

      The remaining procedures of session establishment are as specified
      in [RFC5884].

2.2.  Procedures for maintenance of multiple BFD sessions

   Both the ingress LSR and egress LSR use the YourDiscriminator of the
   received BFD packet to demultiplex BFD sessions.






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2.3.  Procedures for removing BFD sessions at the egress LSR

   [RFC5884] does not specify an explicit procedure for deleting BFD
   sessions.  The procedure for removing a BFD session established by an
   out-of-band discriminator exchange using the MPLS LSP ping can
   improve resource management (like memory etc.) especially in
   scenarios involving thousands or more of such sessions.  A few
   options are possible here:

      The BFD session MAY be removed in the egress LSR if the BFD
      session transitions from UP to DOWN.  This can be done after the
      expiry of a configurable timer started after the BFD session state
      transitions from UP to DOWN at the egress LSR.

      The BFD session on the egress LSR MAY be gracefully removed by the
      ingress LSR by using the BFD diagnostic code AdminDown(7)
      specified in [RFC5880].  When the ingress LSR wants to gracefully
      remove a session, it MAY transmit BFD packets containing the
      diagnostic code AdminDown(7) detectMultiplier number of times.
      Upon receiving such a packet, the egress LSR MAY remove the BFD
      session gracefully, without triggering a change of state.

   Ed Note: The procedures to be followed at the egress LSR when the BFD
   session never transitions to UP from DOWN state are yet to be
   clarified

   Regardless of the option chosen to proceed, all BFD sessions
   established with the FEC MUST be removed automatically if the FEC is
   removed.

2.4.  Changing discriminators for a BFD session

   The discriminators of a BFD session established over an MPLS LSP
   cannot be changed when it is in UP state.  The BFD session could be
   removed after a graceful transition to AdminDown state using the BFD
   diagnostic code AdminDown.  A new session could be established with a
   different discriminator.  The initiation of the transition from the
   Up to Down state can be done either by the ingress LSR or the egress
   LSR.

3.  Backwards Compatibility

   The procedures clarified by this document are fully backward
   compatible with an existing implementation of [RFC5884].  While the
   capability to bootstrap and maintain multiple BFD sessions may not be
   present in current implementations, the procedures outlined by this
   document can be implemented as a software upgrade without affecting
   existing sessions.  In particular, the egress LSR needs to support



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   multiple BFD sessions per <MPLS FEC, LSP> before the ingress LSR is
   upgraded.

4.  Encapsulation

   The encapsulation of BFD packets are the same as specified by
   [RFC5884].

5.  Security Considerations

   This document clarifies the mechanism to bootstrap multiple BFD
   sessions per <MPLS FEC, LSP>.  BFD sessions, naturally, use system
   and network resources.  More BFD sessions means more resources will
   be used.  It is highly important to ensure only minimum number of BFD
   sessions are provisioned per FEC, and bootstrapped BFD sessions are
   properly deleted when no longer required.  Additionally security
   measures described in [RFC4379] and [RFC5884] are to be followed.

6.  IANA Considerations

   This document does not make any requests to IANA.

7.  Acknowledgements

   The authors would like to thank Mudigonda Mallik, Rajaguru Veluchamy
   and Carlos Pignataro of Cisco Systems for their review comments.

8.  Normative References

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

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

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

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

Authors' Addresses







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   Vengada Prasad Govindan
   Cisco Systems

   Email: venggovi@cisco.com


   Kalyani Rajaraman
   Ericsson

   Email: kalyani.rajaraman@ericsson.com


   Gregory Mirsky
   Ericsson

   Email: gregory.mirsky@ericsson.com


   Nobo Akiya
   Cisco Systems

   Email: nobo@cisco.com


   Sam Aldrin
   Huawei Technologies

   Email: aldrin.ietf@gmail.com























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