





Internet Engineering Task Force (IETF)                       V. Govindan
Request for Comments: 7885                                  C. Pignataro
Updates: 5885                                                      Cisco
Category: Standards Track                                      July 2016
ISSN: 2070-1721


          Seamless Bidirectional Forwarding Detection (S-BFD)
          for Virtual Circuit Connectivity Verification (VCCV)

Abstract

   This document defines Seamless BFD (S-BFD) for VCCV by extending the
   procedures and Connectivity Verification (CV) types already defined
   for Bidirectional Forwarding Detection (BFD) for Virtual Circuit
   Connectivity Verification (VCCV).

   This document updates RFC 5885 by extending the CV Type values and
   the capability selection.

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/rfc7885.

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. Background ......................................................3
   2. S-BFD Connectivity Verification .................................3
      2.1. Co-existence of S-BFD and BFD Capabilities .................4
      2.2. S-BFD CV Operation .........................................4
           2.2.1. S-BFD Initiator Operation ...........................4
           2.2.2. S-BFD Reflector Operation ...........................5
                  2.2.2.1. Demultiplexing .............................5
                  2.2.2.2. Transmission of Control Packets ............5
                  2.2.2.3. Advertisement of Target
                           Discriminators Using LDP ...................5
                  2.2.2.4. Advertisement of Target
                           Discriminators Using L2TP ..................6
                  2.2.2.5. Provisioning of Target Discriminators ......6
      2.3. S-BFD Encapsulation ........................................6
   3. Capability Selection ............................................7
   4. Security Considerations .........................................7
   5. IANA Considerations .............................................8
      5.1. MPLS CV Types for the VCCV Interface Parameters Sub-TLV ....8
      5.2. L2TPv3 CV Types for the VCCV Capability AVP ................8
      5.3. PW Associated Channel Type .................................9
   6. References ......................................................9
      6.1. Normative References .......................................9
      6.2. Informative References ....................................10
   Acknowledgements ..................................................11
   Contributors ......................................................11
   Authors' Addresses ................................................11























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1.  Background

   Bidirectional Forwarding Detection (BFD) for Virtual Circuit
   Connectivity Verification (VCCV) [RFC5885] defines the CV Types for
   BFD using VCCV, protocol operation, and the required packet
   encapsulation formats.  This document extends those procedures and
   CV Type values to enable Seamless BFD (S-BFD) [RFC7880] operation
   for VCCV.

   The new S-BFD CV Types are Pseudowire (PW) demultiplexer agnostic and
   hence are applicable for both MPLS and Layer Two Tunneling Protocol
   version 3 (L2TPv3) PW demultiplexers.  This document concerns itself
   with the S-BFD VCCV operation over Single-Segment PWs (SS-PWs).  The
   scope of this document is as follows:

   o  This specification describes procedures for S-BFD asynchronous
      mode only.

   o  S-BFD Echo mode is outside the scope of this specification.

   o  S-BFD operation for fault detection and status signaling is
      outside the scope of this specification.

   This document specifies the use of a single S-BFD Discriminator per
   PW.  There are cases where multiple S-BFD Discriminators per PW can
   be useful.  One such case involves using different S-BFD
   Discriminators per Flow within a Flow-Aware Transport (FAT) PW
   [RFC6391]; however, the mapping between Flows and discriminators is a
   prerequisite.  FAT PWs can be supported as described in Section 7 of
   [RFC6391], which details Operations, Administration, and Maintenance
   (OAM) considerations for FAT PWs.

   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.  S-BFD Connectivity Verification

   The S-BFD protocol provides continuity check services by monitoring
   the S-BFD Control packets sent and received over the VCCV channel of
   the PW.  The term "Connectivity Verification" (CV) is used throughout
   this document to be consistent with [RFC5885].

   This section defines the CV Types to be used for S-BFD.  It also
   defines the procedures for the S-BFD reflector and S-BFD initiator
   operation.




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   Two CV Types are defined for S-BFD.  Table 1 summarizes the S-BFD
   CV Types, grouping them by encapsulation (i.e., with IP/UDP headers,
   without IP/UDP headers) for fault detection only.  S-BFD for fault
   detection and status signaling is outside the scope of this
   specification.

   +-----------------------------------------+-----------+-------------+
   |                                         |   Fault   |    Fault    |
   |                                         | Detection |  Detection  |
   |                                         |    Only   |  and Status |
   |                                         |           |  Signaling  |
   +-----------------------------------------+-----------+-------------+
   | S-BFD IP/UDP encapsulation (with IP/UDP |    0x40   |     N/A     |
   |                                headers) |           |             |
   |                                         |           |             |
   |   S-BFD PW-ACH encapsulation when using |    0x80   |     N/A     |
   |   MPLS PW or S-BFD L2-Specific Sublayer |           |             |
   | (L2SS) encapsulation when using L2TP PW |           |             |
   |                (without IP/UDP headers) |           |             |
   +-----------------------------------------+-----------+-------------+

                Table 1: Bitmask Values for S-BFD CV Types

   IANA has assigned two new bits to indicate S-BFD operation.

2.1.  Co-existence of S-BFD and BFD Capabilities

   Since the CV Types for S-BFD and BFD are unique, BFD and S-BFD
   capabilities can be advertised concurrently.

2.2.  S-BFD CV Operation

2.2.1.  S-BFD Initiator Operation

   The S-BFD initiator SHOULD bootstrap S-BFD sessions after it learns
   the discriminator of the remote target identifier.  This can be
   achieved, for example, through one or more of the following methods.
   (This list is not exhaustive.)

   1.  Advertisements of S-BFD Discriminators made through a
       PW signaling protocol -- for example, AVPs/TLVs defined in
       L2TP/LDP.

   2.  Provisioning of S-BFD Discriminators by manual configuration of
       the Provider Edge (PE) or L2TP Control Connection Endpoints
       (LCCEs).





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   3.  Assignment of S-BFD Discriminators by a controller.

   4.  Probing remote S-BFD Discriminators through a mechanism such as
       S-BFD Alert Discriminators [SBFD-ALERT-DISCRIM].

   The S-BFD initiator operation MUST be done as specified in
   Section 7.3 of [RFC7880].

2.2.2.  S-BFD Reflector Operation

   When a PW signaling protocol such as LDP or L2TPv3 is in use, the
   S-BFD reflector can advertise its target discriminators using that
   signaling protocol.  When static PWs are in use, the target
   discriminator of S-BFD needs to be provisioned on the S-BFD
   initiator nodes.

   All point-to-point PWs are bidirectional; the S-BFD reflector
   therefore reflects the S-BFD packet back to the initiator using the
   VCCV channel of the reverse direction of the PW on which it was
   received.

   The reflector has enough information to reflect the S-BFD Async
   packet received by it back to the S-BFD initiator using the PW
   context (e.g., fields of the L2TPv3 headers).

   The S-BFD reflector operation for BFD protocol fields MUST be
   performed as specified in [RFC7880].

2.2.2.1.  Demultiplexing

   Demultiplexing of S-BFD is achieved using the PW context, following
   the procedures in Section 7.1 of [RFC7880].

2.2.2.2.  Transmission of Control Packets

   S-BFD reflector procedures as described in [RFC7880] apply for S-BFD
   using VCCV.

2.2.2.3.  Advertisement of Target Discriminators Using LDP

   The advertisement of the target discriminator using LDP is left for
   further study.  It should be noted that S-BFD can still be used with
   signaled PWs over an MPLS Packet Switched Network (PSN) by
   provisioning the S-BFD Discriminators or by learning the S-BFD
   Discriminators via some other means.






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2.2.2.4.  Advertisement of Target Discriminators Using L2TP

   The S-BFD reflector MUST use the AVP defined in [RFC7886] for
   advertising its target discriminators using L2TP.

2.2.2.5.  Provisioning of Target Discriminators

   S-BFD target discriminators MAY be provisioned when static PWs
   are used.

2.3.  S-BFD Encapsulation

   Unless specified differently below, the encapsulation of S-BFD
   packets is identical to the method specified in Section 3.2 of
   [RFC5885] and in [RFC5880] for the encapsulation of BFD packets.

   o  IP/UDP BFD encapsulation (BFD with IP/UDP headers):

      *  The destination UDP port for the IP-encapsulated S-BFD packet
         MUST be 7784 [RFC7881].

      *  The contents of the S-BFD Control packets MUST be set according
         to Section 7.3.2 of [RFC7880].

      *  The Time to Live (TTL) (IPv4) or Hop Limit (IPv6) is set
         to 255.

   o  PW-ACH/L2SS BFD encapsulation (BFD without IP/UDP headers):

      *  The encapsulation of S-BFD packets using this format MUST be
         performed according to Section 3.2 of [RFC5885], with the
         exception of the value for the PW-ACH/L2SS type.

      *  When VCCV carries PW-ACH/L2SS-encapsulated S-BFD (i.e., "raw"
         S-BFD), the Channel Type of PW-ACH (the PW Control Word (CW))
         or L2SS MUST be set to 0x0008 to indicate "S-BFD Control,
         PW-ACH/L2SS-encapsulated" (i.e., S-BFD without IP/UDP headers;
         see Section 5.3).  This is done to allow the identification of
         the encapsulated S-BFD payload when demultiplexing the VCCV
         control channel.











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3.  Capability Selection

   When multiple S-BFD CV Types are advertised, and after applying the
   rules in [RFC5885], the set that both ends of the PW have in common
   is determined.  If the two ends have more than one S-BFD CV Type in
   common, the following list of S-BFD CV Types is considered in order,
   from the lowest list number CV Type to the highest list number
   CV Type, and the CV Type with the lowest list number is used:

   1.  0x40 - S-BFD IP/UDP-encapsulated, for PW Fault Detection only.

   2.  0x80 - S-BFD PW-ACH/L2SS-encapsulated (without IP/UDP headers),
       for PW Fault Detection only.

   The order of capability selection between S-BFD and BFD is defined as
   follows:

   +---------------------------+---------+-----------+-----------------+
   |  Advertised capabilities  |   BFD   |   S-BFD   |  Both S-BFD and |
   |         of PE1/PE2        |   Only  |    Only   |       BFD       |
   +---------------------------+---------+-----------+-----------------+
   |          BFD Only         |   BFD   |    None   |     BFD Only    |
   |                           |         |           |                 |
   |         S-BFD Only        |   None  |   S-BFD   |    S-BFD Only   |
   |                           |         |           |                 |
   |     Both S-BFD and BFD    |   BFD   |   S-BFD   |  Both S-BFD and |
   |                           |   Only  |    Only   |       BFD       |
   +---------------------------+---------+-----------+-----------------+

          Table 2: Capability Selection Matrix for BFD and S-BFD

4.  Security Considerations

   Security considerations for VCCV are addressed in Section 10 of
   [RFC5085].  The introduction of the S-BFD CV Types does not present
   any new security risks for VCCV.  Implementations of the additional
   CV Types defined herein are subject to the same security
   considerations as those defined in [RFC5085] as well as [RFC7880].

   The IP/UDP encapsulation of S-BFD makes use of the TTL / Hop Limit
   procedures described in the Generalized TTL Security Mechanism (GTSM)
   specification [RFC5082] as a security mechanism.

   This specification does not raise any additional security issues
   beyond these.






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5.  IANA Considerations

5.1.  MPLS CV Types for the VCCV Interface Parameters Sub-TLV

   The VCCV Interface Parameters Sub-TLV codepoint is defined in
   [RFC4446], and the "MPLS VCCV Connectivity Verification (CV) Types"
   registry is defined in [RFC5085].

   This section lists the new S-BFD CV Types.

   IANA has augmented the "MPLS VCCV Connectivity Verification (CV)
   Types" registry in the "Pseudowire Name Spaces (PWE3)" registry
   [IANA-PWE3].  These are bitfield values.  CV Type values are
   specified in Section 2 of this document.

      MPLS VCCV Connectivity Verification (CV) Types:

      Bit (Value)  Description                       Reference
      ===========  ===========                       ==============
      6 (0x40)     S-BFD IP/UDP-encapsulated,        RFC 7885
                   for PW Fault Detection only

      7 (0x80)     S-BFD PW-ACH-encapsulated,        RFC 7885
                   for PW Fault Detection only

5.2.  L2TPv3 CV Types for the VCCV Capability AVP

   This section lists the new S-BFD "L2TPv3 Connectivity Verification
   (CV) Types" that have been added to the existing "VCCV Capability AVP
   (Attribute Type 96) Values" registry in the "Layer Two Tunneling
   Protocol 'L2TP'" registry [IANA-L2TP].  IANA has assigned the
   following L2TPv3 Connectivity Verification (CV) Types in the "VCCV
   Capability AVP (Attribute Type 96) Values" registry.

      VCCV Capability AVP (Attribute Type 96) Values
      ----------------------------------------------

      L2TPv3 Connectivity Verification (CV) Types:

      Bit (Value)  Description                  Reference
      ===========  ===========                  ==============
      6 (0x40)     S-BFD IP/UDP-encapsulated,   RFC 7885
                   for PW Fault Detection only

      7 (0x80)     S-BFD L2SS-encapsulated,     RFC 7885
                   for PW Fault Detection only





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5.3.  PW Associated Channel Type

   As per the IANA considerations in [RFC5586], IANA has allocated a
   Channel Type in the "MPLS Generalized Associated Channel (G-ACh)
   Types (including Pseudowire Associated Channel Types)" registry
   [IANA-G-ACh].

   IANA has assigned a new Pseudowire Associated Channel Type value, as
   follows:

    Value   Description                          Reference
    ------  ----------------------------------   ---------------
    0x0008  S-BFD Control, PW-ACH/L2SS           RFC 7885
            encapsulation
            (without IP/UDP Headers)

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>.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446,
              DOI 10.17487/RFC4446, April 2006,
              <http://www.rfc-editor.org/info/rfc4446>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <http://www.rfc-editor.org/info/rfc5082>.

   [RFC5085]  Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
              Virtual Circuit Connectivity Verification (VCCV): A
              Control Channel for Pseudowires", RFC 5085,
              DOI 10.17487/RFC5085, December 2007,
              <http://www.rfc-editor.org/info/rfc5085>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <http://www.rfc-editor.org/info/rfc5586>.






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   [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>.

   [RFC5885]  Nadeau, T., Ed., and C. Pignataro, Ed., "Bidirectional
              Forwarding Detection (BFD) for the Pseudowire Virtual
              Circuit Connectivity Verification (VCCV)", RFC 5885,
              DOI 10.17487/RFC5885, June 2010,
              <http://www.rfc-editor.org/info/rfc5885>.

   [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>.

   [RFC7881]  Pignataro, C., Ward, D., and N. Akiya, "Seamless
              Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6,
              and MPLS", RFC 7881, DOI 10.17487/RFC7881, July 2016,
              <http://www.rfc-editor.org/info/rfc7881>.

   [RFC7886]  Govindan, V. and C. Pignataro, "Advertising Seamless
              Bidirectional Forwarding Detection (S-BFD) Discriminators
              in the Layer Two Tunneling Protocol Version 3 (L2TPv3)",
              RFC 7886, DOI 10.17487/RFC7886, July 2016,
              <http://www.rfc-editor.org/info/rfc7886>.

6.2.  Informative References

   [IANA-G-ACh]
              Internet Assigned Numbers Authority, "MPLS Generalized
              Associated Channel (G-ACh) Types (including Pseudowire
              Associated Channel Types)",
              <http://www.iana.org/assignments/g-ach-parameters>.

   [IANA-L2TP]
              Internet Assigned Numbers Authority, "Layer Two Tunneling
              Protocol 'L2TP'",
              <http://www.iana.org/assignments/l2tp-parameters>.

   [IANA-PWE3]
              Internet Assigned Numbers Authority, "Pseudowire Name
              Spaces (PWE3)",
              <http://www.iana.org/assignments/pwe3-parameters>.








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   [RFC6391]  Bryant, S., Ed., Filsfils, C., Drafz, U., Kompella, V.,
              Regan, J., and S. Amante, "Flow-Aware Transport of
              Pseudowires over an MPLS Packet Switched Network",
              RFC 6391, DOI 10.17487/RFC6391, November 2011,
              <http://www.rfc-editor.org/info/rfc6391>.

   [SBFD-ALERT-DISCRIM]
              Akiya, N., Pignataro, C., and D. Ward, "Seamless
              Bidirectional Forwarding Detection (S-BFD) Alert
              Discriminator", Work in Progress,
              draft-akiya-bfd-seamless-alert-discrim-03, October 2014.

Acknowledgements

   The authors would like to thank Nobo Akiya, Stewart Bryant, Greg
   Mirsky, Pawel Sowinski, Yuanlong Jiang, Andrew Malis, and Alexander
   Vainshtein for providing input to this document, performing thorough
   reviews, and providing useful comments.

Contributors

   Mallik Mudigonda
   Cisco Systems, Inc.

   Email: mmudigon@cisco.com

Authors' Addresses

   Vengada Prasad Govindan
   Cisco Systems, Inc.

   Email: venggovi@cisco.com


   Carlos Pignataro
   Cisco Systems, Inc.

   Email: cpignata@cisco.com













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