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RFC 7708 - Using a Generic Associated Channel Label as a Virtual

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Internet Engineering Task Force (IETF)                         T. Nadeau
Request for Comments: 7708                                       Brocade
Category: Standards Track                                     L. Martini
ISSN: 2070-1721                                                S. Bryant
                                                           Cisco Systems
                                                           November 2015

                Using a Generic Associated Channel Label
    as a Virtual Circuit Connectivity Verification Channel Indicator


   The Virtual Circuit Connectivity Verification (VCCV) protocol
   specified in RFC 5085 provides a control channel (CC) that is
   associated with a pseudowire (PW).  This document specifies an
   additional VCCV control channel type to be used with pseudowires that
   do not use the PW Control Word and that are carried over an MPLS
   network.  This new VCCV CC type uses the Generic Associated Channel
   Label defined in RFC 5586 to distinguish VCCV packets from packets
   carrying user data.  This new VCCV CC type introduces compatibility
   with the method of MPLS Label Switched Path Operations,
   Administration, and Maintenance (OAM) identification, particularly in
   MPLS Transport Profile (MPLS-TP) networks (RFC 5921).

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2015 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
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   publication of this document.  Please review these documents
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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.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Type 4 MPLS VCCV Control Channel Type . . . . . . . . . . . .   3
   4.  FAT PWs . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Multi-Segment Pseudowires . . . . . . . . . . . . . . . . . .   5
   6.  VCCV Capability Advertisement . . . . . . . . . . . . . . . .   5
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  MPLS VCCV Control Channel (CC) Type 4 . . . . . . . . . .   7
     9.2.  LDP Status Code . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Virtual Circuit Connectivity Verification (VCCV) protocol is
   specified in RFC 5085 [RFC5085].  This document specifies a new VCCV
   control channel (VCCV CC) type to be used with pseudowires (PWs)
   carried over an MPLS network that do not use the PW Control Word (CW)
   [RFC4385].  This new VCCV CC type uses the Generic Associated Channel
   Label (GAL) [RFC5586] to distinguish VCCV packets from packets
   carrying user data.  This new VCCV CC type provides compatibility
   with the method of MPLS Label Switched Path (LSP) Operations,
   Administration, and Maintenance (OAM) message identification, as used
   in MPLS Transport Profile (MPLS-TP) networks [RFC5921].

   VCCV currently specifies three CC types.  VCCV CC Type 1 uses the PW
   Control Word (CW) to distinguish VCCV packets from packets carrying
   user data.  VCCV CC Types 2 and 3 require IP encapsulation for OAM
   packets.  This was not an issue when [RFC5085] was designed, but it
   is in conflict with the design goals of MPLS-TP [RFC5921], which do
   not otherwise require the availability of IP.  VCCV CC Type 2 is not
   applicable to Multi-Segment PWs (MS-PWs) [RFC6073].  A MS-PW
   operating without the CW therefore has to use VCCV CC Type 3, which
   identifies VCCV packets on the basis of Time to Live (TTL) expiry.
   Whilst less of an issue with a single segment PW (SS-PW), on an MS-PW
   this requires accurately setting the TTL for expiry at the egress
   Terminating Provider Edge (T-PE) [RFC6073].  In the event of an error
   in the setting of the PW Label Stack Entry (LSE) TTL, VCCV packets
   will not be received by the Terminating Provider Edge (T-PE) and may
   leak into the attachment circuit [RFC6073].  The new VCCV CC type
   defined in this specification addresses these problems for PWs that
   do not use the CW.

   Note that mandating that PWs use the PW CW is not a viable way to
   address this issue.  This is because:

   o  PWs without the CW are already widely deployed.

   o  There is a significant deployment of existing hardware that does
      not support usage of the PW CW for some PW types.

   o  Some operators are concerned that the inclusion of the PW CW will
      increase the PW packet size.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

3.  Type 4 MPLS VCCV Control Channel Type

   When the PW CW is not used, the Type 4 MPLS VCCV Control Channel (CC)
   type defined in this section MAY be used.  This is referred to as
   VCCV CC Type 4 throughout the rest of this of this document.  VCCV CC
   Type 4 uses the encapsulation shown in Figure 1 in which the presence
   of a GAL at the end of the MPLS label stack indicates that the packet
   carries a VCCV message.

    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
   |                            PW LSE                             |
   |                           GAL LSE                             |
   |0 0 0 1|Version|   Reserved    |        Channel Type           |
   |                                                               |
   ~                        VCCV Message Body                      ~
   |                                                               |

                                 Figure 1

   The VCCV message body is preceded by a Generic Associated Channel
   Header, as defined in [RFC5586], in which the Channel Type identifies
   the type and format of the OAM message carried in the VCCV message

   The GAL LSE MUST contain the GAL reserved label as defined in

   The PW LSE is constructed according to the existing procedures that
   apply to the type of pseudowire that is in use.

   Where the LSP used by the PW is subject to Equal-Cost Multipath
   (ECMP) load balancing, a problem arises if any LSR on that LSP treats
   special-purpose labels as ordinary labels in its ECMP selection
   method.  In these circumstances, the inclusion of a GAL following the
   PW LSE can cause the OAM packet to take a different path through the
   network than the corresponding PW data packets.  If the LSP traverses
   such equipment and this behaviour is not acceptable, then an
   alternative VCCV type needs to be used.  The requirement to not
   include special-purpose labels in the load-balancing decision is
   described in "MPLS Forwarding Compliance and Performance
   Requirements" [RFC7325].  For equipment that conforms to this, the
   VCCV type 4 traffic will follow the corresponding PW data packets.

4.  FAT PWs

   [RFC6391] specifies that when the flow-aware transport (FAT) of
   pseudowires over an MPLS packet switched network has been signalled
   or configured, the Flow LSE MUST be present.  It further specifies
   that "the flow label MUST NOT be an MPLS reserved label (values in
   the range 0..15) [RFC3032]", and that "If a flow LSE is present, it
   MUST be checked to determine whether it carries a reserved label.  If

   it is a reserved label, the packet is processed according to the
   rules associated with that reserved label; otherwise, the LSE is

   This document specifies that if the flow-aware transport of
   pseudowires over an MPLS packet switched network has been signalled
   or configured, then the presence of VCCV message is indicated by the
   use of a GAL in place of the flow LSE.

   This is consistent with [RFC6391], and the packet structure is
   identical to that shown in Figure 1.

   Flow LSEs are inserted into a PW label stack in order to enable the
   distribution of the PW traffic among multiple equal-cost MPLS paths.
   The use of GAL in place of the flow label will cause all OAM packets
   to take exactly one of the possible paths through the network.  As
   noted in Section 3, the ECMP selection method may result in the path
   taken by the OAM packets being different from the path taken by any
   of the actual traffic flows.  If this is not acceptable, then an
   alternative VCCV type needs be used.

5.  Multi-Segment Pseudowires

   When using VCCV CC Type 4 for MS-PWs, a PE transmitting the VCCV
   packet to a Switching PE (S-PE) MUST set the TTL to the appropriate
   value to expire at that S-PE.  An S-PE that supports this
   specification MUST inspect PW packets that are received as a result
   of TTL expiry, and determine whether a GAL follows the PW LSE.  If a
   GAL is present, the S-PE then processes the VCCV packet.

   An S-PE that does not support this specification would be expected to
   reject as malformed a VCCV CC Type 4 packet that was received.  This
   is because the S-PE would expect the PW LSE to be the bottom of stack
   (the non-FAT case) and for the LSE at the bottom of stack not to be a
   reserved label (both the FAT and the non-FAT cases).  An S-PE that
   did not make this reserved label check would then find that the first
   nibble following the label stack was 0x1 and not the expected start
   of an IP packet.  Thus, it would be expected to also reject the
   packet.  This update to the behaviour of S-PEs is therefore backwards

6.  VCCV Capability Advertisement

   The VCCV capability advertisement MUST match the C-bit setting that
   is advertised in the PW FEC element [RFC4447].  If the C-bit is set,
   indicating the use of the PW CW, then VCCV CC Type 4 MUST NOT be
   advertised.  If the C-bit is not set, indicating that the PW CW is
   not in use, then equipment supporting this specification MUST

   advertise VCCV CC Type 4.  Advertisement of VCCV CC Type 1 and
   advertisement of VCCV CC Type 4 are therefore mutually exclusive.

   A PE supporting VCCV CC Type 4 MAY advertise other VCCV CC types as
   defined in [RFC5085] .

   If the remote PE supports VCCV CC Type 4, and the PW CW is not in
   use, then for cases where multiple CC Types are advertised, the
   following precedence rules apply when choosing which CC Type to use:

   1.  Type 4: GAL VCCV Control Channel.

   2.  Type 2: MPLS Router Alert Label.

   3.  Type 3: MPLS PW Label with TTL == 1.

   If the remote PE finds that VCCV CC Types 1 and 4 are both
   advertised, or that C-bit is set and VCCV CC Type 4 is advertised,
   then it should report the error to the operator through the
   management interface in use, and send a Label Release Message with a
   status code "VCCV Type Error".

7.  Manageability Considerations

   Whilst the introduction of this additional VCCV CC type increases the
   number of VCCV CC types that the operator needs to manage, it
   addresses the issues with VCCV CC Types 2 and 3 described in
   Section 1.

   In the event of a misconfiguration of this VCCV CC type, the PW is
   taken out of service and the operator advised as described in
   Section 6.

   Attention is drawn to the possible absence of fate sharing between PW
   data packets and VCCV CC Type 4 packets described in Section 3 and
   Section 4.

8.  Security Considerations

   This document does not by itself raise any new security
   considerations beyond those described in [RFC5085] and [RFC6073].
   [RFC6073] provides detailed operational procedures that can be used
   to verify the MS-PW connectivity.  In addition, the procedure
   specified in this document eliminates the possibility of packet
   leaking that can occur with VCCV Type 3.

9.  IANA Considerations

9.1.  MPLS VCCV Control Channel (CC) Type 4

   IANA has assigned a new bit from the MPLS VCCV Control Channel (CC)
   Types registry in the "Pseudowire Name Spaces (PWE3)" registry in
   order to identify VCCV type 4.

   MPLS VCCV Control Channel (CC) Types

         Bit (Value)    Description   Reference
         ============   ===========   ==================
         Bit 3 (0x08)   Type 4: GAL   RFC 7708

9.2.  LDP Status Code

   IANA has assigned a new Status Code from the "Label Distribution
   Protocol (LDP) Parameters" registry:

   Status Code Name Space
         Range/Value  E  Description      Reference
         ===========  =  ===============  =========
         0x00000035   0  VCCV Type Error  RFC 7708

10.  References

10.1.  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,

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, DOI 10.17487/RFC4385,
              February 2006, <http://www.rfc-editor.org/info/rfc4385>.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447,
              DOI 10.17487/RFC4447, April 2006,

   [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,

   [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
              Aissaoui, "Segmented Pseudowire", RFC 6073,
              DOI 10.17487/RFC6073, January 2011,

   [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,

10.2.  Informative References

   [RFC5921]  Bocci, M., Ed., Bryant, S., Ed., Frost, D., Ed., Levrau,
              L., and L. Berger, "A Framework for MPLS in Transport
              Networks", RFC 5921, DOI 10.17487/RFC5921, July 2010,

   [RFC7325]  Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
              and C. Pignataro, "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
              August 2014, <http://www.rfc-editor.org/info/rfc7325>.


   The authors wish to thank Alexander (Sasha) Vainshtein for his
   proposal to make the GAL and Flow labels mutually exclusive.  This
   proposal led to a significant simplification of this design.  The
   authors also thank Sasha, Matthew Bocci, Loa Andersson, and Deborah
   Brungard for their review comments.

Authors' Addresses

   Thomas D. Nadeau

   Email: tnadeau@lucidvision.com

   Luca Martini
   Cisco Systems

   Email: lmartini@cisco.com

   Stewart Bryant
   Cisco Systems

   Email: stewart.bryant@gmail.com


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