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Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                              C. Pignataro
Intended status: Standards Track                                N. Kumar
Expires: December 9, 2014                                  Cisco Systems
                                                            June 7, 2014


         Seamless Bidirectional Forwarding Detection (BFD) for
                          Segment Routing (SR)
                     draft-akiya-bfd-seamless-sr-02

Abstract

   Note: this document needs to be updated to align with changes in the
   S-BFD base document.

   This specification defines procedures to use Seamless Bidirectional
   Forwarding Detection (S-BFD) in a Segment Routing (SR) based
   environment.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 9, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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
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   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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BFD Target Identifier Types . . . . . . . . . . . . . . . . .   2
   3.  Reserved BFD Discriminators . . . . . . . . . . . . . . . . .   3
   4.  BFD Target Identifier Table . . . . . . . . . . . . . . . . .   3
   5.  Full Reachability Validations . . . . . . . . . . . . . . . .   3
     5.1.  Initiator Behavior  . . . . . . . . . . . . . . . . . . .   3
     5.2.  Responder Behavior  . . . . . . . . . . . . . . . . . . .   3
   6.  Partial Reachability Validations  . . . . . . . . . . . . . .   4
   7.  MPLS Label Verifications  . . . . . . . . . . . . . . . . . .   4
   8.  Provisioning Active BFD Sessions for SR Networks  . . . . . .   4
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   12. Contributing Authors  . . . . . . . . . . . . . . . . . . . .   6
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     13.2.  Informative References . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   One application for Seamless Bidirectional Forwarding Detection
   (S-BFD) [I-D.akiya-bfd-seamless-base] is to perform full reachability
   validations, partial reachability validations and adjacency segment
   ID verifications on a Segment Routing (SR) based environment.

   This specification defines procedures to use Seamless BFD in a SR
   based environment.

2.  BFD Target Identifier Types

   BFD target identifier type of value 2 is used for SR.  Note that BFD
   target identifier type of value 2, which specifies segment routing
   node segment ID, is not tied to a specific routing protocol.  If
   definitions and procedures need routing protocol specifics, then IGP
   specific SR types will be defined.



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3.  Reserved BFD Discriminators

   With SR technology, BFD target identifier type 2 is used.  Node
   segment IDs are used as BFD discriminators.  BFD discriminator values
   corresponding to all or subset of local node segment IDs are to be
   allocated from the discriminator pool for Seamless BFD.

   Example:

   o  BFD Target Identifier Type 2: Node segment ID 0x03E9A0FF maps to
      BFD discriminator 0x03E9A0FF.

4.  BFD Target Identifier Table

   With SR BFD target identifier type, only locally reserved BFD
   discriminators and corresponding information are to be in this table.
   No inter-node communications are needed to exchange BFD discriminator
   and BFD target identifier mappings.

5.  Full Reachability Validations

5.1.  Initiator Behavior

   Any SR network node can attempt to perform a full reachability
   validation to any BFD target identifier of type 2 (node segment ID)
   on other network nodes, as long as destination BFD target identifier
   is provisioned to use this mechanism.  Transmitted BFD control packet
   by the initiator is to have "your discriminator" corresponding to
   destination BFD target identifier of type 2.

   Initiator is to use following procedures to construct BFD control
   packets to perform SR full reachability validations:

   o  MUST set "your discriminator" to target node segment ID.
   o  MUST use explicit label switching packet format described in
      [I-D.akiya-bfd-seamless-base].

5.2.  Responder Behavior

   To respond to received BFD control packet which was targeted to local
   BFD target identifier of type 2 (Segment Routing Node Segment ID),
   response BFD control packet is targeted to IP address taken from
   received "source IP address".  Responder MUST validate obtained IP
   address is in valid format (ex: not Martian address).  Responder MUST
   consult local routing table to ensure obtained IP address is
   reachable.  Responder MAY impose node segment ID, corresponding to
   obtained IP address, on the response BFD control packet.




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6.  Partial Reachability Validations

   Procedures described in [I-D.akiya-bfd-seamless-base] applies.

7.  MPLS Label Verifications

   With target identifier type 2, SR based, when a network node wants to
   test an adjacency segment ID, then adjacency segment ID (label value
   + EXP) being tested is encoded as lower 23 bits of localhost IP
   destination address.  When passive BFD session receives a SR BFD
   control packet with lower 23 bits of IP destination address non-zero,
   then response will contain adjacency segment ID (label value + EXP)
   corresponding to incoming interface as lower 23 bits of localhost IP
   destination address.

   Simple ASCII art is provided to illustrate the MPLS label
   verification concept on a SR network.

               md=50/yd=R3/DIP=127...R2R3
   Active  [1] - - - - - - - - - - - -- - - >  Passive
   BFD     < - - - - - - - - - - - - - - [2]   BFD
   Session     md=R3/yd=50/DIP=127...R3R2      Session

                               (adj SID R2R3)->
     R1 ------------------ R2 ------------------ R3
                               <-(adj SID R3R2)

   If a response BFD control packet is received, then initiator can
   conclude that a packet has reached intended node correctly.  With
   information embedded in last 23 bits of response BFD control packet
   from responder, initiator has the ability to perform further
   verifications on how responded node received BFD control packet.

8.  Provisioning Active BFD Sessions for SR Networks

   Many factors will influence how to provision active BFD sessions on
   which network nodes.  This section provides some provisioning
   suggestions of active BFD sessions on SR networks.  However, they are
   only suggestions.  Less provisioning of active BFD sessions may be
   required in some cases, or further active BFD sessions may be
   required in other cases.

   Traffic engineered segment routing

   o  Segment routing eliminates hop-by-hop signaling to create traffic
      engineered paths, as described in
      [I-D.previdi-filsfils-isis-segment-routing].  When traffic
      engineered segment routing path is instantiated on an ingress



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      node, with stack of segment IDs, absence of hop-by-hop signaling
      results in less confidence in reachability to egress as well as
      traversal of strictly routed segments.  S-BFD can perform rapid
      verification of both prior to allowing service over instantiated
      traffic engineered segment routing paths.  In addition, S-BFD can
      provide continuity check on both aspects, as detection time and
      coverage of S-BFD is much superior than IGP failure detection and
      convergence time.

   Single node segment ID data forwarding

   o  In order to protect all data passing through local network using
      single node segment ID, active BFD sessions can be instantiated on
      each network node to verify full reachability to all node segment
      IDs.

   Centralized controller initiated S-BFD

   o  Centralized controller based segment routing network monitoring
      techniques, such as the one described in
      [I-D.geib-spring-oam-usecase], are powerful.  One aspect that is
      lacking from such techniques is the guarantee that monitor packet
      did indeed reach certain network node (i.e. u-turned at expected
      network node).  Related aspect is the lack of guarantee that
      monitor packet over adjacency segment ID did indeed result in
      traversal of expected adjacency.  Since S-BFD can fill in the
      missing holes, also running S-BFD in parallel from the central
      controller device will even strengthen the technique.

9.  Security Considerations

   Security considerations for BFD are discussed in [RFC5880] and
   security considerations for S-BFD are discussed in
   [I-D.akiya-bfd-seamless-base].

10.  IANA Considerations

   None

11.  Acknowledgements

   Authors would like to thank Marc Binderberger from Cisco Systems for
   providing valuable comments.








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12.  Contributing Authors

   Dave Ward
   Cisco Systems
   Email: wardd@cisco.com

   Tarek Saad
   Cisco Systems
   Email: tsaad@cisco.com

   Siva Sivabalan
   Cisco Systems
   Email: msiva@cisco.com

13.  References

13.1.  Normative References

   [I-D.akiya-bfd-seamless-base]
              Akiya, N., Pignataro, C., Ward, D., Bhatia, M., and J.
              Networks, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", draft-akiya-bfd-seamless-base-03 (work in
              progress), April 2014.

   [I-D.previdi-filsfils-isis-segment-routing]
              Previdi, S., Filsfils, C., Bashandy, A., Horneffer, M.,
              Decraene, B., Litkowski, S., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., and J. Tantsura, "Segment
              Routing with IS-IS Routing Protocol", draft-previdi-
              filsfils-isis-segment-routing-02 (work in progress), March
              2013.

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

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

13.2.  Informative References

   [I-D.geib-spring-oam-usecase]
              Geib, R. and C. Filsfils, "Use case for a scalable and
              topology aware MPLS data plane monitoring system", draft-
              geib-spring-oam-usecase-01 (work in progress), February
              2014.






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Authors' Addresses

   Nobo Akiya
   Cisco Systems

   Email: nobo@cisco.com


   Carlos Pignataro
   Cisco Systems

   Email: cpignata@cisco.com


   Nagendra Kumar
   Cisco Systems

   Email: naikumar@cisco.com

































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