Network Working Group                            Thomas D. Nadeau
Internet Draft                                   Cisco Systems, Inc.
Expires: January 2004
                                                 Rahul Aggarwal
                                                 Juniper Networks

                                                 July  2003

    Pseudo Wire (PW) Virtual Circuit Connection Verification

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   Distribution of this document is unlimited.  Please send comments to
   the Multiprotocol Label Switching (mpls) Working Group,


   This document describes Virtual Circuit Connection Verification
   (VCCV) procedures for use with psuedowire connections. VCCV
   supports connection verification applications for pseudowire
   VCs regardless of the underlying MPLS or IP tunnel technology.
   VCCV makes use of IP based protocols such as Ping and MPLS-Ping
   to perform operations and maintenance functions.  A network
   operator may use the VCCV procedures to test the network's
   forwarding plane liveliness.


1. Contributors.............................................1
2. Introduction.............................................2
3  MPLS as PSN..............................................3
4. IP Probe Traffic.........................................5
5. OAM Capability Indication................................6
6. L2TPv3/IP as PSN.........................................8
7. Acknowledgments.........................................11
8. References..............................................11
9. Security Considerations.................................12
10. Intellectual Property Rights Notices...................12
11. Full Copyright Statement...............................13

1. Contributors

  Thomas D. Nadeau                   Rahul Aggarwal
  Cisco Systems, Inc.                Juniper Networks
  250 Apollo Drive                   1194 North Mathilda Ave.
  Chelmsford, MA 01824               Sunnyvale, CA 94089
  Email:           Email:

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  George Swallow                     Monique Morrow
  Cisco Systems, Inc.                Cisco Systems, Inc.
  250 Apollo Drive                   Glatt-com
  Chelmsford, MA 01824               CH-8301 Glattzentrum
  Email:           Switzerland

  Yuichi Ikejiri                     Kenji Kumaki
  NTT Communications Corporation     KDDI Corporation
  1-1-6, Uchisaiwai-cho, Chiyoda-ku  KDDI Bldg. 2-3-2,
  Tokyo 100-8019                     Nishishinjuku, Shinjuku-ku,
  JAPAN                              Tokyo 163-8003,
  Email:           JAPAN
                                     E-mail :

2. Introduction

   As network operators deploy pseudowire services, fault
   detection and diagnostic mechanisms particularly for the PSN
   portion of the network are pivotal. Specifically, the ability
   to provide end-to-end fault detection and diagnostics for an
   emulated pseudowire service is critical for the network
   operator. Operators have indicated in [MPLSOAMREQS] that such
   a tool is required for pseudowire deployments. This document
   describes procedures for PSN-agnostic fault detection and
   diagnostics called virtual circuit connection verification

                 |<------- pseudowire ------>|
                 |    |<-- PSN Tunnel -->|    |
          PW     V    V                  V    V     PW
     End Service +----+                  +----+ End Service
+-----+     |    | PE1|==================| PE2|       |    +-----+
|     |----------|............PW1.............|------------|     |
| CE1 |     |    |    |                  |    |       |    | CE2 |
|     |----------|............PW2.............|------------|     |
+-----+     |    |    |==================|    |       |    +-----+
Customer    |    +----+                  +----+       |    Customer
 Edge 1     |  Provider Edge 1        Provider Edge 2 |     Edge 2
            |<----------- Emulated Service ---------->|
                 |<---------- VCCV ---------->|

            Figure 1: PWE3 VCCV Operation Reference Model

   Figure 1 depicts the basic functionality of VCCV. VCCV provides
   several means of creating a control channel between PEs that

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   attaches the VC under test.

   +-------------+                                +-------------+
   |  Layer2     |                                |  Layer2     |
   |  Emulated   |                                |  Emulated   |
   |  Services   |         Emulated Service       |  Services   |
   |             |<==============================>|             |
   +-------------+        VCCV/pseudowire         +-------------+
   +-------------+                                +-------------+
   |    PSN      |            PSN Tunnel          |    PSN      |
   |   MPLS      |<==============================>|   MPLS      |
   +-------------+                                +-------------+
   |  Physical   |                                |  Physical   |
   +-----+-------+                                +-----+-------+
         |                                              |
         |             ____     ___       ____          |
         |           _/    \___/   \    _/    \__       |
         |          /               \__/         \_     |
         |         /                               \    |
         +========/      MPLS or IP Network         |===+
                  \                                /
                   \   ___      ___     __      _/
                    \_/   \____/   \___/  \____/

      Figure 2: PWE3 Protocol Stack Reference Model
                including the VCCV control channel.

   Figure 2 depicts how the VCCV control channel is run along
   with the pseudowire to verify specific VCs. Ping and other
   IP messages are encapsulated using the PWE3 encapsulation
   as described below in sections 5 and 6. These messages, referred
   to as VCCV messages, are exchanged only after the desire to
   exchange such traffic has been negotiated between the PEs
   (see section 8).

3. MPLS as PSN

   In order to apply IP monitoring tools to PWE3 circuits, VCCV
   creates a control channel between PWE3 PEs[PWEARCH].  Packets
   sent across this channel are IP packets, allowing maximum

   Ideally such a control channel would be completely in band.
   When a control word is present on virtual circuit, it is
   possible to indicate the control channel by setting a bit in
   the control header.  This method is described in section 7.1
   and is referred to as inband MPLS VCCV.

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   However in order to address the case when the control header
   is not in use as well as to deal with a number of existent
   hardware devices, use of the MPLS Router Alert Label to indicate
   the IP control channel is also proposed.  This is described in
   section 7.2.

   The actual channel type is agreed through signaling as
   described in section 8.

3.1.  Inband MPLS VCCV

   The PW set-up protocol determines whether a PW uses a control word.
   When a control word is used, it SHOULD have the following preferred

       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
      |0 0 0 0| Flags |FRG|  Length   | Sequence Number               |

   for the purpose of indicating VCCV control channel messages.

   Note that for data, one uses the control word defined just
   above the MPLS payload [PWEARCH] .

   The MPLS payload type is defined 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

|0 0 0 1|  reserved             | PPP DLL Protocol Number       |
|           As defined by PPP DLL protocol definition           |
|                                                               |

   The first nibble 0000 indicates data.  When the first nibble is
   0001, the protocol of the frame is indicated by the Protocol
   Number.  IP OAM flows are identified by either an IPv4 or IPv6

3.2.  Router Alert Label Approach

   When the control word is not used, or the receiving hardware

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   cannot divert control traffic, an IP control channel can be
   created by including the MPLS router alert label immediately
   above the VC label.  If the control word is in use on this VC
   it is also included in the IP control flow.

   0x1  OAM Flag in PWE header
   0x2  Include the control channel label in stack above VC

4. IP Probe Traffic

   For connectivity verification, both ICMP Ping and LSP-Ping
   packets may be used on the control channel.  The type of
   packets used is agreed in signaling as described in section 9.

4.1.  ICMP Ping

   When ICMP packets are used, the source address should be set
   to the source address of the LDP session and the destination
   address to the destination of the LDP session.  The Identifier
   and Sequence Number fields of the ICMP Echo Request / Echo
   Reply messages are used to track what VCs are being tested.

   These fields are only interpreted by the sending PE.  Specific
   use of these fields is an implementation matter.

4.2.  MPLS Ping Packet

   The LSP Ping header must be used as described [LSP-PING] and
   must also contain the sub-TLV of 8 for PW circuits.  This
   sub-TLV must be sent containing the circuit to be verified as
   the "VC ID" field:

4.2.1 L2 Circuit ID TLV for MPLS LSP Ping

   The value field consists of a remote PE address (the address
   of the targeted LDP session), the source address of the PE
   that originated this request, a VC ID and an encapsulation
   type, 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
    |                      Remote PE Address                        |
    |                      Source PE Address                        |

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    |PWID Type      |PWID Length  | PWID                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                 +
    |                                                               |
    |                          Parameters                           |
    |                              "                                |
    |                              "                                |

   Two PWID types are defined:

   1. A FEC 128 VCID as defined in [MARTINISIG].
   2. A FEC 129 Attachment Identifier, as defined [L2SIG].

   The PWID length field contains the length of the
   PWID field in bytes. Zero to three bytes of padding will follow
   the PWID field, so that the parameters field starts on a 64-bit

   Parameters are:

   - Interface parameters, as defined in [MARTINISIG].

   ***Note that we propose that this field be removed from the
      LSP Ping draft [LSPPING] and defined here instead.

5. OAM Capability Indication

   To permit negotiation of the use and type of OAM for
   Connectivity Verification, a VCCV parameter is defined below.
   When a PE signals a PWE3 VC and desires OAM for that VC, it
   MUST indicate this during VC establishment using the messages
   defined below.  Specifically for LDP it MUST include the VCCV
   parameter in the VC setup message.

   As the overall method of PWE3 signaling is
   downstream, unsolicited, this leaves the decision of the type
   of IP control channel completely to the receiving control
   entity.  OAM capability MUST be signaled BEFORE a PE may send
   OAM messages. If a PE receives OAM messages prior to sending
   a VCCV parameter, it MUST discard these messages and not reply
   to them. In this case, the LSR SHOULD increment an error counter
   and optionally issues a system and/or SNMP notification to indicate
   to the system administrator that a mis-configuration exists.

   The requesting PE indicates its desire for the remote PE to
   support OAM capability by including the VCCV parameter with
   appropriate options set to indicate which methods of OAM are

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   acceptable.  The requesting PE MAY indicate multiple IP control
   IP control channel options.  The absence of the VCCV FEC TLV
   indicates that no OAM functions are supported or desired by
   the requesting PE.  This last method MUST be supported by all
   PEs in order to handle backward-compatibility with older PEs.
   The receiving PE agrees to accept any of the indicated
   OAM types and options by virtue of establishing the VC. If
   it does not or cannot support at least one of the options
   specified, it MUST not establish the VC. If the requesting
   PE wishes to continue, it may choose different options and
   try to signal the PE again.

5.1.  Optional VCCV Parameter

   [PWE3CONTROL] defines a VC FEC TLV for LDP.  Parameters can be
   carried within that TLV to signal different capabilities for
   specific PWs. We propose an optional parameter to be used to
   indicate the desire to use a control channel for VCCV as

   The TLV field structure is defined in [PWE3CONTROL] as

   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
   |  Parameter ID |    Length     |    Variable Length Value      |
   |                         Variable Length Value                 |
   |                             "                                 |

   The VCCV parameter ID is defined as follows:

     Parameter ID   Length     Description
       0x06           4           VCCV

   The format of the VCCV parameter 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
   |      0x06     |       0x04    |   CC Type     |   CV Types    |

   The CC type field defines the type of IP control channel.
   The defined values are:

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    0x1  OAM Flag set in PWE header
    0x2  MPLS Router Alert Label

   The CV Types field defines the types of IP control packets
   that may be sent on the control channel.  The defined values

    0x1  ICMP Ping
    0x2  LSP Ping

6.  L2TPV3 as PSN

   When L2TPv3 is used as the underlying PSN, a VCCV mechanism is
   needed for the L2TPv3 session. The L2TPv3 control connection does
   employ a keepalive mechanism. However this mechanism isn't
   sufficent for fault detection and diagnostic of the L2TPv3 session
   i.e. data plane. In L2TPv3 a session is analogous to a PW. A L2TPv3
   VCCV mechanism is needed in particular for verifying the session
   forwarding state at the egress router.

   When a PE verifies the connection status of a L2TPv3 session it must
   transmit a L2TPv3 VCCV message encoded in the L2TPv3 session packet.

   The presence of a VCCV message in a L2TPv3 session packet can be
   indicated by reserving a bit in the default L2-specific sublayer

    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
    |P|S|V|x|x|x|x|x|              Sequence Number                  |

           Default L2-Specific Sublayer Format with V bit.

   The 'V' bit indicates that this is a VCCV session packet. If the PW
   has not been signaled to include a L2-specific sublayer format, other

   mechanisms are needed to indicate the VCCV message. Such mechanisms are
   for further study.

6.1. L2TPv3 VCCV Message

   The VCCV message MUST contain a VCCV AVP. It does not contain a message
   header. A new AVP, called the VCCV AVP is defined. The usage of the
   L2TPv3 AVP format leaves room for adding further AVPs to this message

   in the future as needed.

6.1.1. L2TPv3 VCCV AVP

   This AVP encodes the LSP Ping header as defined in [LSP-PING]. M and H

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   bits must not be set. The attribute type is TBD. The LSP Ping header is
   not encapsulated in UDP. The modifications to the semantics of the
   fields of this header are specified here. Unless otherwise specified
   the semantics of the fields as explained in [LSP-PING] are to be
   followed. For reference the format of the LSP Ping header is shown

       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
      |         Version Number        |         Must Be Zero          |
      |  Message Type |   Reply mode  |  Return Code  | Return Subcode|
      |                        Sender's Handle                        |
      |                        Sequence Number                        |
      |                    TimeStamp Sent (seconds)                   |
      |                  TimeStamp Sent (microseconds)                |
      |                  TimeStamp Received (seconds)                 |
      |                TimeStamp Received (microseconds)              |
      |                            TLVs ...                           |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |

   The version number is currently 1. The message type is one of the

   1 - L2TPv3 VCCV Echo Request
   2 - L2TPv3 VCCV Echo Reply

   The Reply Mode is:

   1 - Do not reply
   2 - Reply using the L2TPv3 session

   As explained in [LSP-PING] a reply mode of "do not reply" can be used

   for one way connectivity tests. The VCCV message will normally contain
   a reply mode of "reply using the L2TPv3 session".

   The return code can be set to the following by the receiver:

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   1 - Malformed echo request received
   2 - One or more of the TLVs was not understood
   3 - Replying router has a session mapping for the verified pseudo wire
   4 - Replying router does not have a mapping for the verified pseudo

   The LSP Ping header must contain the L2 Circuit ID TLV as defined in
   section 8.2. This TLV identifies the pseudo wire associated with the
   session, that is being verified. For L2TPv3 the remote PE address is
   the address of the session's remote end. A new PWID type is defined
   for L2TPv3, in addition to the ones defined in section 8.2:

   3. L2TPv3 Remote End Identifier AVP

6.2. L2TPv3 VCCV Capability Negotiation

   A LCCE or a LAC should be able to indicate whether the session is
   capable of processing VCCV packets. This is done by including the
   optional VCCV capability AVP in an ICRQ, ICRP, OCRQ or OCRP.

6.2.1. L2TPv3 VCCV Capability AVP

   This AVP specifies the VCCV capability. Its attribute type
   is TBD. The value field has the following format:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      | Reserved                     |

6.3. L2TPv3 VCCV Operation

   A PE sends VCCV echo requests on a L2TPv3 signaled pseudo wire for
   fault detection and diagnostic of the L2TPv3 session. The destination

   IP address in the echo request is set to the remote PE's IP address,
   while the source IP address is set to the local PE's IP address. The
   egress of the L2TPv3 session verifies the signaling and forwarding state
   of the pseudo wire, on reception of the VCCV message. Any faults
   detected can be signaled in the VCCV echo response. Its to be noted
   that the VCCV mechanism for L2TPv3 is primarily targeted at verifying

   the pseudo wire forwarding and signaling state at the egress PE. It
   also helps when L2TPv3 control and session paths are not identical.

   A PE must send VCCV packets on a L2TPv3 session only if it has signaled
   VCCV capability to the remote end and received VCCV capability from the
   remote end. If a PE receives VCCV packets and its not VCCV capable or

   it has not received VCCV capability indication from the remote end, it
   must discard these messages. In addition if a PE receives VCCV messages
   and it has not received VCCV capability from the remote end, it should

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   increment an error counter. In this case the PE can optionally issue a
   system and/or SNMP notification.

7.   Acknowledgments

   The authors would like to thank Hari Rakotoranto, Michel
   Khouderchah, Bertrand Duvivier, Vanson Lim, Chris Metz, W.
   Mark Townsley, Eric Rosen, Dan Tappan, and
   Danny McPherson for their valuable comments and suggestions.

8.   References

[PWREQ]    Xiao, X., McPherson, D., Pate, P., Gill, V.,
           Kompella, K., Nadeau, T., White, C., "Requirements
           for Pseudo Wire Emulation Edge-to-Edge (PWE3)",
           <draft-ietf-pwe3-requirements-02.txt>, November 2001.
[PWE3FW]   Prayson Pate, et al., Internet draft, Framework for
           Pseudo Wire Emulation Edge-to-Edge (PWE3), draft-
           ietf-pwe3-framework-01.txt, work in progress.
[PWEARCH]  Bryant, S., Pate, P., Johnson, T., Kompella, K.,
           Malis, A., McPherson, D., Nadeau, T., So, T., Townsley,
           W., Systems, White., C., Wood, L., Xiao, X., Internet
           draft, Framework for Pseudo Wire Emulation Edge-to-Edge
           (PWE3), draft-ietf-pwe3-framework-01.txt, work in
[L2SIG]    Rosen, E., LDP-based Signaling for L2VPNs,
           Internet Draft <draft-rosen-ppvpn-l2-signaling-02.txt>,
           September 2002.
[LSPPING]  Kompella, K., Pan, P., Sheth, N., Cooper, D.,
           Swallow, G., Wadhwa, S., Bonica, R., " Detecting
           Data Plane Liveliness in MPLS", Internet Draft
           <draft-ietf-mpls-lsp-ping-01.txt>, April 2003.
[MARTINISIG] "Transport of Layer 2 Frames Over MPLS", Martini et.
             al., draft-martini-l2circuit-trans-mpls-10.txt,
             August 2002
[GTTP]     Bonica, R., Kompella, K., Meyer, D., "Generic
           Tunnel Tracing Protocol (GTTP) Specification", Internet
           Draft <draft-bonica-tunproto-01.txt>, April, 2003
[FRF 8.1]  Frame Relay Forum, Frame Relay / ATM PVC Service
           Interworking Implementation Agreement, February 2000
[ITU-T]    "Draft Recommendation Y.17fw" (MPLS Management
           Framework), July 2002.
[ITU-T]    "Frame Relay Bearer Service Interworking," I.555,
           September 2997.
[ITU-T],   "Frame Relay Operations Principles and Functions",
           I.620, October, 1996.
[ITU-T]    Q.933, ISDN Digital Subscriber Signalling System
           No. 1 (DSS 1) - Signalling specification for frame

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           mode basic call control, November 1995.
[ICMP]     Postel, J. "Internet Control Message Protocol, "
           RFC 792
[PWEATM]   Martini, L., et al., "Encapsulation Methods for
           Transport of ATM Cells/Frame Over IP and MPLS
           Networks", Internet Draft <draft-ietf-pwe3-atm-
           encap-00.txt>, October 2002
[MPLSOAMREQS] Nadeau, T., et al,"OAM Requirements for MPLS
              Networks, Internet Draft <draft-ietf-oam-
              requirements-01.txt>, June 2003.
[OAMMsgMap] Nadeau, T., et al, " Pseudo Wire (PW) OAM Message
            Mapping, Internet Draft < draft-nadeau-pwe3-OAMMap.txt>,
            December, 2002.
[PWE3CONTROL] L.Martini et al., "Transport of Layer 2 Frames
              over MPLS, Internet Draft, <draft-ietf-pwe3-control-
              protocol-01.txt>, May 2003
[PPVPNFW]     Callon, R., Suzuki, M., Gleeson, B., Malis, A.,
              Muthukrishnan, K., Rosen, E., Sargor, C., and J. Yu,
              "A Framework for Provider Provisioned Virtual
              Private Networks", Internet Draft <draft-ietf-
              ppvpn-framework-01.txt>, July 2001.
[RFC3031]  Rosen, E., Viswanathan, A., and R. Callon,
           "Multiprotocol Label Switching Architecture", RFC
           3031, January 2001.

9.   Security Considerations


10.   Intellectual Property Rights Notices

   The IETF takes no position regarding the validity or scope of any
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   pertain to the implementation or use of the technology described in
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   can be obtained from the IETF Secretariat.
   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice

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   this standard.  Please address the information to the IETF
   Executive Director.

11.   Full Copyright Statement

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