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Internet Engineering Task Force (IETF)                       V. GovindanRequest for Comments: 7726                                  K. RajaramanUpdates:5884                                              Cisco SystemsCategory: Standards Track                                      G. MirskyISSN: 2070-1721                                                 Ericsson                                                                N. Akiya                                                     Big Switch Networks                                                               S. Aldrin                                                                  Google                                                            January 2016Clarifying Procedures for Establishing BFD Sessions forMPLS 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 inRFC5884.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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7726.Govindan, et al.             Standards Track                    [Page 1]

RFC 7726               Clarifications toRFC 5884           January 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Requirements Language . . . . . . . . . . . . . . . . . .32.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .32.1.  Procedures for Establishment of Multiple BFD Sessions . .32.2.  Procedures for Maintenance of Multiple BFD Sessions . . .4     2.3.  Procedures for Removing BFD Sessions at the Egress LSR  .   42.4.  Changing Discriminators for a BFD Session . . . . . . . .53.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .54.  Security Considerations . . . . . . . . . . . . . . . . . . .55.  References  . . . . . . . . . . . . . . . . . . . . . . . . .65.1.  Normative References  . . . . . . . . . . . . . . . . . .65.2.  Informative References  . . . . . . . . . . . . . . . . .6   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .71.  Background   [RFC5884] defines the procedures to bootstrap and maintain BFD   sessions for an <MPLS LSP, FEC> using a Label Switched Path (LSP)   ping.  WhileSection 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 toRFC 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 Operation2.1.  Procedures for Establishment of Multiple BFD SessionsSection 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 inSection 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 beforeGovindan, et al.             Standards Track                    [Page 3]

RFC 7726               Clarifications toRFC 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      Sections4-6 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 toRFC 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 toRFC 5884           January 20165.  References5.1.  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 toRFC 5884           January 2016Authors' 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.comGovindan, et al.             Standards Track                    [Page 7]