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Internet Engineering Task Force (IETF)                       V. GovindanRequest for Comments: 7886                                  C. PignataroCategory: Standards Track                                          CiscoISSN: 2070-1721                                                July 2016Advertising 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 inSection 2 of RFC 7841.   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/rfc7886.Copyright 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.Govindan & Pignataro         Standards Track                    [Page 1]

RFC 7886             S-BFD Discriminators in L2TPv3            July 2016Table of Contents1. Introduction ....................................................21.1. Terminology ................................................22. S-BFD Target Discriminator ID AVP ...............................22.1. Encoding Format ............................................33. IANA Considerations .............................................44. Security Considerations .........................................45. References ......................................................55.1. Normative References .......................................55.2. Informative References .....................................5   Acknowledgements ...................................................6   Contributors .......................................................6   Authors' Addresses .................................................61.  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 (seeSection 2 of [RFC7880]) and   L2TPv3 (seeSection 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 inRFC 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.Govindan & Pignataro         Standards Track                    [Page 2]

RFC 7886             S-BFD Discriminators in L2TPv3            July 20162.1.  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.Govindan & Pignataro         Standards Track                    [Page 3]

RFC 7886             S-BFD Discriminators in L2TPv3            July 2016   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 ID4.  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].Govindan & Pignataro         Standards Track                    [Page 4]

RFC 7886             S-BFD Discriminators in L2TPv3            July 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>.   [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>.Govindan & Pignataro         Standards Track                    [Page 5]

RFC 7886             S-BFD Discriminators in L2TPv3            July 2016Acknowledgements   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: mmudigon@cisco.comAuthors' Addresses   Vengada Prasad Govindan   Cisco Systems, Inc.   Email: venggovi@cisco.com   Carlos Pignataro   Cisco Systems, Inc.   Email: cpignata@cisco.comGovindan & Pignataro         Standards Track                    [Page 6]