Internet Engineering Task Force (IETF)                       V. Govindan
Request for Comments: 7886                                  C. Pignataro
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                                July 2016


   Advertising Seamless Bidirectional Forwarding Detection (S-BFD)
Discriminators in the Layer Two Tunneling Protocol Version 3 (L2TPv3)

Abstract

  This document defines a new Attribute-Value Pair (AVP) that allows
  L2TP Control Connection Endpoints (LCCEs) to advertise one or more
  Seamless Bidirectional Forwarding Detection (S-BFD) Discriminator
  values using the Layer Two Tunneling Protocol version 3 (L2TPv3).

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/rfc7886.

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.







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Table of Contents

  1. Introduction ....................................................2
     1.1. Terminology ................................................2
  2. S-BFD Target Discriminator ID AVP ...............................2
     2.1. Encoding Format ............................................3
  3. IANA Considerations .............................................4
  4. Security Considerations .........................................4
  5. References ......................................................5
     5.1. Normative References .......................................5
     5.2. Informative References .....................................5
  Acknowledgements ...................................................6
  Contributors .......................................................6
  Authors' Addresses .................................................6

1.  Introduction

  [RFC7880] defines a simplified mechanism to use Bidirectional
  Forwarding Detection (BFD) [RFC5880], referred to as Seamless
  Bidirectional Forwarding Detection (S-BFD).  The S-BFD mechanism
  depends on network nodes knowing the BFD Discriminators that each
  node in the network has reserved for this purpose.  S-BFD requires
  the usage of unique discriminators within an administrative domain.
  The use of the Layer Two Tunneling Protocol version 3 (L2TPv3)
  [RFC3931] is one possible means of advertising these discriminators.

  This document specifies the encoding to be used when S-BFD
  Discriminators are advertised using L2TPv3.

1.1.  Terminology

  The reader is expected to be very familiar with the terminology and
  protocol constructs defined in S-BFD (see Section 2 of [RFC7880]) and
  L2TPv3 (see Section 1.3 of [RFC3931]).

  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].

2.  S-BFD Target Discriminator ID AVP

  The S-BFD Target Discriminator Identifier (ID) Attribute Value Pair
  (AVP) is exchanged using the ICRQ (Incoming-Call-Request), ICRP
  (Incoming-Call-Reply), OCRQ (Outgoing-Call-Request), and OCRP
  (Outgoing-Call-Reply) control messages during session negotiation.






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2.1.  Encoding Format

  The S-BFD Target Discriminator ID AVP, Attribute Type 102, is an
  identifier used to advertise the S-BFD target discriminator(s)
  supported by an L2TP Control Connection Endpoint (LCCE) for the S-BFD
  reflector operation.  This AVP indicates that the advertiser
  implements an S-BFD reflector supporting the specified target
  discriminator(s) and is ready for S-BFD reflector operation.  The
  receiving LCCE MAY use this AVP if it wants to monitor connectivity
  to the advertising LCCE using S-BFD.

  The Attribute Value field for this AVP has the following format:

  S-BFD Target Discriminator ID (ICRQ, ICRP, OCRQ, OCRP):

                                         No. of octets
     +-----------------------------+
     | Discriminator Value(s)      |     4/Discriminator
     :                             :
     +-----------------------------+

  An LCCE MAY include the S-BFD Target Discriminator ID AVP
  advertisement in an L2TP control message (ICRQ, ICRP, OCRQ, OCRP)
  [RFC3931].  If the other LCCE does not wish to monitor connectivity
  using S-BFD, it MAY safely discard this AVP without affecting the
  rest of session negotiation.  While [RFC7880] concerns itself with
  the advertisement of only one discriminator unless the mapping of
  discriminators to entities is specified, the AVP encoding allows the
  specification of an arbitrary number of S-BFD Discriminators (at
  least one) for extensibility.

  When an LCCE uses the S-BFD Target Discriminator ID AVP
  advertisement, multiple S-BFD Discriminators MAY be included, and at
  least one S-BFD Discriminator MUST be included.  When one S-BFD
  Discriminator is advertised, such an S-BFD Discriminator is
  associated with the L2TPv3 session.  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.













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  The S-BFD Target Discriminator ID AVP allows for advertising at least
  one S-BFD Discriminator value:

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Discriminator 1                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Discriminator 2 (Optional)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               ...                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Discriminator n (Optional)                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The M bit of the L2TP control message (ICRQ, ICRP, OCRQ, OCRP)
  [RFC3931] MUST NOT be set inside the S-BFD Target Discriminator
  ID AVP.

3.  IANA Considerations

  IANA maintains the "Control Message Attribute Value Pairs"
  sub-registry as per [RFC3438].  IANA has assigned the following value
  to the S-BFD Target Discriminator ID:

  Control Message Attribute Value Pairs

        Attribute
        Type          Description
        -----------   ------------------
        102           S-BFD Target Discriminator ID

4.  Security Considerations

  Security concerns for L2TP are addressed in [RFC3931].  The
  introduction of the S-BFD Target Discriminator ID AVP advertisement
  introduces no new security risks for L2TP.

  Advertising the S-BFD Discriminators makes it possible for attackers
  to initiate S-BFD sessions using the advertised information.  The
  vulnerabilities this poses and how to mitigate them are discussed in
  the Security Considerations section of [RFC7880].









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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>.

  [RFC3438]  Townsley, W., "Layer Two Tunneling Protocol (L2TP)
             Internet Assigned Numbers Authority (IANA) Considerations
             Update", BCP 68, RFC 3438, DOI 10.17487/RFC3438,
             December 2002, <http://www.rfc-editor.org/info/rfc3438>.

  [RFC3931]  Lau, J., Ed., Townsley, M., Ed., and I. Goyret, Ed.,
             "Layer Two Tunneling Protocol - Version 3 (L2TPv3)",
             RFC 3931, DOI 10.17487/RFC3931, March 2005,
             <http://www.rfc-editor.org/info/rfc3931>.

  [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>.

5.2.  Informative References

  [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>.






















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Acknowledgements

  The authors would like to thank Nobo Akiya, Stewart Bryant, and Pawel
  Sowinski for providing core inputs for the document, performing
  thorough reviews, and providing a number of comments.  The authors
  would also like to thank Nagendra Kumar for his reviews.

Contributors

  Mallik Mudigonda
  Cisco Systems, Inc.

  Email: [email protected]

Authors' Addresses

  Vengada Prasad Govindan
  Cisco Systems, Inc.

  Email: [email protected]


  Carlos Pignataro
  Cisco Systems, Inc.

  Email: [email protected]

























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