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rfc:rfc7726

Internet Engineering Task Force (IETF) V. Govindan Request for Comments: 7726 K. Rajaraman Updates: 5884 Cisco Systems Category: Standards Track G. Mirsky ISSN: 2070-1721 Ericsson

                                                              N. Akiya
                                                   Big Switch Networks
                                                             S. Aldrin
                                                                Google
                                                          January 2016
      Clarifying Procedures for Establishing BFD Sessions for
                  MPLS Label Switched Paths (LSPs)

Abstract

 This document clarifies the procedures for establishing, maintaining,
 and removing multiple, concurrent BFD (Bidirectional Forwarding
 Detection) sessions for a given <MPLS LSP, FEC> as described in RFC
 5884.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7726.

Govindan, et al. Standards Track [Page 1] RFC 7726 Clarifications to RFC 5884 January 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
 2.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   3
   2.1.  Procedures for Establishment of Multiple BFD Sessions . .   3
   2.2.  Procedures for Maintenance of Multiple BFD Sessions . . .   4
   2.3.  Procedures for Removing BFD Sessions at the Egress LSR  .   4
   2.4.  Changing Discriminators for a BFD Session . . . . . . . .   5
 3.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .   5
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
 5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
   5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1. Background

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

Govindan, et al. Standards Track [Page 2] RFC 7726 Clarifications to RFC 5884 January 2016

 The ability for an ingress LSR to establish multiple BFD sessions for
 an <MPLS LSP, FEC> tuple is useful in scenarios such as LSPs based on
 Segment Routing [SEG-ROUTING] 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 an <MPLS LSP, FEC> tuple and
 the mechanism used to construct 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].

2. Theory of Operation

2.1. Procedures for Establishment of Multiple BFD Sessions

 Section 4 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'
 reading of the referenced sections:
 At the ingress LSR:
    MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
    given <MPLS LSP, FEC>.  Each LSP ping MUST carry a different
    discriminator value in the BFD discriminator TLV [RFC5884].
 The egress LSR needs to perform the following:
    If the validation of the Forwarding Equivalence Class (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 (remote)
    discriminator received in the MPLS Echo request, a new session is
    bootstrapped and a local discriminator is allocated.  The
    validation of a FEC is a necessary condition to be satisfied to
    create a new BFD session at the egress LSR.  However, the policy
    or procedure, if any, to be applied by the egress LSR before

Govindan, et al. Standards Track [Page 3] RFC 7726 Clarifications to RFC 5884 January 2016

    allowing a new BFD session to be created is outside the scope of
    this document.  Such policies or procedures could consider
    availability of system resources before allowing a session to be
    created.  When the egress LSR disallows the creation of a BFD
    session due to policy, it MUST drop the MPLS Echo request message.
    Ensure the uniqueness of the <MPLS LSP, FEC, Remote Discriminator>
    tuple.
    Except for the clarification mentioned above, the remaining
    procedures of BFD session establishment are as specified in
    Sections 4-6 of [RFC5884].

2.2. Procedures for Maintenance of Multiple BFD Sessions

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

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 (e.g., memory), especially in scenarios
 involving thousands or more of such sessions.  A few observations are
 made here:
    The BFD session MAY be removed in the egress LSR if the BFD
    session transitions from UP to DOWN.  This can either be done
    immediately after the BFD session transitions from UP to DOWN or
    after the expiry of a configurable timer started after the BFD
    session state transitions from UP to DOWN at the egress LSR to
    reduce flapping by adding hysteresis.
    The BFD session on the egress LSR MAY be removed by the ingress
    LSR by using the BFD diagnostic code AdminDown(7) as specified in
    [RFC5880].  When the ingress LSR wants to remove a session without
    triggering any state change at the egress, it MAY transmit BFD
    packets indicating the State as Down(1), diagnostic code
    AdminDown(7) detectMultiplier number of times.  Upon receiving
    such a packet, the egress LSR MAY remove the BFD session, without
    triggering a change of state.
    The procedures to be followed at the egress LSR when BFD
    session(s) remain in the DOWN state for a significant amount of
    time is a local matter.  Such procedures are outside the scope of
    this document.

Govindan, et al. Standards Track [Page 4] RFC 7726 Clarifications to RFC 5884 January 2016

    All BFD sessions established with the FEC MUST be removed
    automatically if the FEC is removed.
    The egress MUST use the discriminators exchanged when the session
    was brought UP to indicate any session state change to the
    ingress.  The egress SHOULD reset this to zero after transmitting
    bfd.detectMult number of packets if the BFD session transitions to
    DOWN state.

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

4. Security Considerations

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

Govindan, et al. Standards Track [Page 5] RFC 7726 Clarifications to RFC 5884 January 2016

5. References

5.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
            Label Switched (MPLS) Data Plane Failures", RFC 4379,
            DOI 10.17487/RFC4379, February 2006,
            <http://www.rfc-editor.org/info/rfc4379>.
 [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
            (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
            <http://www.rfc-editor.org/info/rfc5880>.
 [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, <http://www.rfc-editor.org/info/rfc5884>.

5.2. Informative References

 [SEG-ROUTING]
            Filsfils, C., Ed., Previdi, S., Ed., Decraene, B.,
            Litkowski, S., and R. Shakir, "Segment Routing
            Architecture", Work in Progress, draft-ietf-spring-
            segment-routing-07, December 2015.

Acknowledgements

 The authors would like to thank Marc Binderberger for performing
 thorough reviews and providing valuable suggestions.
 The authors would like to thank Mudigonda Mallik, Rajaguru Veluchamy,
 and Carlos Pignataro of Cisco Systems for their review comments.
 The authors would like to thank Alvaro Retana and Scott Bradner for
 their review comments.

Govindan, et al. Standards Track [Page 6] RFC 7726 Clarifications to RFC 5884 January 2016

Authors' Addresses

 Vengada Prasad Govindan
 Cisco Systems
 Email: venggovi@cisco.com
 Kalyani Rajaraman
 Cisco Systems
 Email: kalyanir@cisco.com
 Gregory Mirsky
 Ericsson
 Email: gregory.mirsky@ericsson.com
 Nobo Akiya
 Big Switch Networks
 Email: nobo.akiya.dev@gmail.com
 Sam Aldrin
 Google
 Email: aldrin.ietf@gmail.com

Govindan, et al. Standards Track [Page 7]

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