    Internet Engineering Task Force (IETF)                      C. Pignataro
    Request for Comments: 7884                                         Cisco
    Category: Standards Track                                      M. Bhatia
    ISSN: 2070-1721                                           Ionos Networks
                                                                   S. Aldrin
                                                         Huawei Technologies
                                                                T. Ranganath
                                                                       Nokia
                                                                   July 2016

    OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection
                         (S-BFD) Target Discriminators

    Abstract

       This document defines a new OSPF Router Information (RI) TLV that
       allows OSPF routers to flood the Seamless Bidirectional Forwarding
       Detection (S-BFD) Discriminator values associated with a target
       network identifier.  This mechanism is applicable to both OSPFv2 and
       OSPFv3.

    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 7841.

       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/rfc7884.

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    RFC 7884              S-BFD Discriminators in OSPF             July 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. Introduction ....................................................3
          1.1. Relationship between OSPF and S-BFD ........................3
       2. Implementation ..................................................3
          2.1. S-BFD Discriminator TLV ....................................4
          2.2. Flooding Scope .............................................4
       3. Backward Compatibility ..........................................5
       4. Security Considerations .........................................5
       5. IANA Considerations .............................................6
       6. References ......................................................6
          6.1. Normative References .......................................6
          6.2. Informative References .....................................6
       Acknowledgements ...................................................7
       Authors' Addresses .................................................7

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    RFC 7884              S-BFD Discriminators in OSPF             July 2016

    1.  Introduction

       Seamless Bidirectional Forwarding Detection (S-BFD), specified in
       [RFC7880], is a simplified mechanism for using BFD with many
       negotiations eliminated.  This is achieved by using 4-octet
       discriminators, unique within an administrative domain, to identify
       the network targets.  These S-BFD Discriminators can be advertised by
       the IGPs, and this document concerns itself with OSPF.  Specifically,
       this document defines a new TLV (named the S-BFD Discriminator TLV)
       to be carried within the OSPF Router Information (RI) Link State
       Advertisement (LSA) [RFC7770].

       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].

    1.1.  Relationship between OSPF and S-BFD

       This document implicitly defines a relationship between OSPF and
       S-BFD.  S-BFD assigns one or more discriminators to each S-BFD
       reflector node.  OSPF, in turn, learns about these from S-BFD and
       floods them in the newly defined TLV.  After this information is
       flooded, it is stored in all the OSPF nodes such that S-BFD
       initiators can map out target nodes to target discriminators and can
       therefore construct the S-BFD probe.

       When multiple S-BFD Discriminators are advertised, how a given
       discriminator is mapped to a specific use case is out of scope for
       this document.

    2.  Implementation

       This extension makes use of the Router Information (RI) Opaque LSA,
       defined in [RFC7770], for both OSPFv2 [RFC2328] and OSPFv3 [RFC5340]
       by defining a new OSPF Router Information (RI) TLV: the S-BFD
       Discriminator TLV.

       The S-BFD Discriminator TLV is OPTIONAL.  Upon receipt of the TLV, a
       router may decide to install the S-BFD Discriminator in the BFD
       target identifier table.

       In the presence of multiple instances of the OSPFv2/OSPFv3 Router
       Information LSA, the S-BFD Discriminators for an OSPF router are the
       union of all discriminators advertised in all instances of the S-BFD
       Discriminator TLV (see Section 2.1) in all advertised non-MaxAge OSPF
       Router Information LSAs.

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    RFC 7884              S-BFD Discriminators in OSPF             July 2016

    2.1.  S-BFD Discriminator TLV

       The format of the S-BFD Discriminator TLV is as follows:

          0                   1                   2                   3
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |              Type             |             Length            |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                         Discriminator 1                       |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    Discriminator 2 (Optional)                 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                               ...                             |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                    Discriminator n (Optional)                 |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Type - S-BFD Discriminator TLV Type (11)

       Length - This field represents the total length of the
          discriminator(s) that appears in the Value field, in octets.  Each
          discriminator is 4 octets, so the Length is four times the number
          of discriminators included in the TLV.  There is no optional
          padding for this field.

       Discriminator(s) - The Value field of the TLV includes the S-BFD
          network target Discriminator value or values.

       Routers that do not recognize the S-BFD Discriminator TLV Type will
       ignore the TLV [RFC7770] and therefore will not learn S-BFD
       Discriminators via OSPF.

    2.2.  Flooding Scope

       The S-BFD Discriminator TLV is advertised within OSPFv2 Router
       Information LSAs (Opaque type of 4 and Opaque ID of 0) or OSPFv3
       Router Information LSAs (function code of 12), which are defined in
       [RFC7770].  As such, elements of this procedure are inherited from
       those defined in [RFC7770].

       The flooding scope is controlled by the Opaque LSA type (as defined
       in [RFC5250]) in OSPFv2 and by the S1/S2 bits (as defined in
       [RFC5340]) in OSPFv3.  If the flooding scope is area local, then the
       S-BFD Discriminator TLV MUST be carried within an OSPFv2 type 10
       Router Information LSA or an OSPFV3 Router Information LSA with the
       S1 bit set and the S2 bit clear.  If the flooding scope is the entire

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    RFC 7884              S-BFD Discriminators in OSPF             July 2016

       IGP domain, then the S-BFD Discriminator TLV MUST be carried within
       an OSPFv2 type 11 Router Information LSA or OSPFv3 Router Information
       LSA with the S1 bit clear and the S2 bit set.

       When the S-BFD reflector is deactivated, the OSPF speaker advertising
       a particular S-BFD Discriminator MUST originate a new Router
       Information LSA that no longer includes the corresponding S-BFD
       Discriminator TLV, provided there are other TLVs in the LSA.  If
       there are no other TLVs in the LSA, it MUST either send an empty
       Router Information LSA or purge it by prematurely aging it.

       For intra-area reachability, the S-BFD Discriminator TLV information
       regarding a specific target identifier is only considered current and
       usable when the router advertising that information is itself
       reachable via OSPF calculated paths in the same area of the LSA in
       which the S-BFD Discriminator TLV appears.  In the case of
       domain-wide flooding, i.e., where the originator is sitting in a
       remote area, the mechanism described in Section 5 of [RFC5250] should
       be used.

       Although the S-BFD Discriminators may change when enabling the S-BFD
       functionality or via an explicit configuration event, such changes
       are expected to occur very rarely.  Such changes in information will
       require that the S-BFD Discriminator TLV in OSPF be advertised.

       A change in information in the S-BFD Discriminator TLV MUST NOT
       trigger any SPF computations at a receiving router.

    3.  Backward Compatibility

       The S-BFD Discriminator TLV defined in this document does not
       introduce any interoperability issues.

       A router not supporting the S-BFD Discriminator TLV will just
       silently ignore the TLV, as specified in [RFC7770].

    4.  Security Considerations

       This document defines OSPF extensions to distribute the S-BFD
       Discriminator within an administrative domain.  Hence, the security
       of S-BFD Discriminator distribution relies on the security of OSPF.

       OSPF provides no encryption mechanism for protecting the privacy of
       LSAs and, in particular, the privacy of the S-BFD Discriminator
       advertisement information.  However, this is not a concern, as there
       isn't any need to hide the Discriminator value that can be used to
       reach the reflectors.

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    RFC 7884              S-BFD Discriminators in OSPF             July 2016

    5.  IANA Considerations

       IANA has defined a registry for TLVs carried in the Router
       Information LSA defined in [RFC7770].  IANA has assigned a new TLV
       codepoint (11) for the S-BFD Discriminator TLV in the "OSPF Router
       Information (RI) TLVs" registry.

            Value      TLV Name                 Reference
            -----      --------                 ----------
            11         S-BFD                    RFC 7884
                       Discriminator

    6.  References

    6.1.  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>.

       [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
                  DOI 10.17487/RFC2328, April 1998,
                  <http://www.rfc-editor.org/info/rfc2328>.

       [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
                  for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
                  <http://www.rfc-editor.org/info/rfc5340>.

       [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
                  S. Shaffer, "Extensions to OSPF for Advertising Optional
                  Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
                  February 2016, <http://www.rfc-editor.org/info/rfc7770>.

       [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
                  Pallagatti, "Seamless Bidirectional Forwarding Detection
                  (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
                  <http://www.rfc-editor.org/info/rfc7880>.

    6.2.  Informative References

       [RFC5250]  Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
                  OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250,
                  July 2008, <http://www.rfc-editor.org/info/rfc5250>.

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    RFC 7884              S-BFD Discriminators in OSPF             July 2016

    Acknowledgements

       The authors would like to thank Nobo Akiya, Les Ginsberg, Mach Chen,
       and Peter Psenak for insightful comments and useful suggestions.

    Authors' Addresses

       Carlos Pignataro
       Cisco Systems, Inc.

       Email: cpignata@cisco.com

       Manav Bhatia
       Ionos Networks

       Email: manav@ionosnetworks.com

       Sam Aldrin
       Huawei Technologies

       Email: aldrin.ietf@gmail.com

       Trilok Ranganath
       Nokia

       Email: trilok.ranganatha@nokia.com

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