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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 6073

Network Working Group                                       Luca Martini
Internet Draft                                                Chris Metz
Expiration Date: August 2008                          Cisco Systems Inc.
Intended status: Standards Track
                                                        Thomas D. Nadeau
Matthew Bocci                                                         BT
Florin Balus
Mustapha Aissaoui                                           Mike Duckett
Alcatel-Lucent                                                 Bellsouth

                                                           February 2008


                         Segmented Pseudo Wire


                  draft-ietf-pwe3-segmented-pw-07.txt

Status of this Memo

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Abstract

   This document describes how to connect pseudo wires (PW) between two
   distinct PW control planes or PSN domains. The PW control planes may
   belong to independent autonomous systems, or the PSN technology is
   heterogeneous, or a PW might need to be aggregated at a specific PSN
   point. The PW packet data units are simply switched from one PW to



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   another without changing the PW payload.


















































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Table of Contents

    1        Specification of Requirements  ........................   4
    2        Terminology  ..........................................   5
    3        Introduction  .........................................   5
    4        General Description  ..................................   7
    5        PW Switching and Attachment Circuit Type  .............  10
    6        Applicability  ........................................  10
    7        PW-MPLS to PW-MPLS Control Plane Switching  ...........  11
    7.1      Static Control plane switching  .......................  11
    7.2      Two LDP control planes using the same FEC type  .......  12
    7.2.1    FEC 129 Active/Passive T-PE Election Procedure  .......  12
    7.3      LDP FEC 128 to LDP using the generalized FEC 129  .....  12
    7.4      LDP PW switching point TLV  ...........................  13
    7.4.1    PW Switching Point Sub-TLVs  ..........................  14
    7.4.2    Adaptation of Interface Parameters  ...................  15
    7.5      Group ID  .............................................  16
    7.6      PW Loop Detection  ....................................  16
    8        PW-MPLS to PW-L2TPv3 Control Plane Switching  .........  16
    8.1      Static MPLS and L2TPv3 PWs  ...........................  16
    8.2      Static MPLS PW and Dynamic L2TPv3 PW  .................  17
    8.3      Static L2TPv3 PW and Dynamic LDP/MPLS PW  .............  17
    8.4      Dynamic LDP/MPLS and L2TPv3 PWs  ......................  17
    8.4.1    Session Establishment  ................................  17
    8.4.2    Adaptation of PW Status message  ......................  18
    8.4.3    Session Tear Down  ....................................  18
    8.5      Adaptation of L2TPv3 AVPs to Interface Parameters  ....  19
    8.6      Switching Point TLV in L2TPv3  ........................  20
    8.7      L2TPv3 and MPLS PW Data Plane  ........................  20
    8.7.1    PWE3 Payload Convergence and Sequencing  ..............  20
    8.7.2    Mapping  ..............................................  21
    9        Operation And Management  .............................  22
    9.1      Extensions to VCCV to Support Switched PWs  ...........  22
    9.2      PW-MPLS to PW-MPLS OAM Data Plane Indication  .........  22
    9.2.1    Decreasing the PW Label TTL  ..........................  22
    9.3      Signaling OAM Capabilities for Switched Pseudo Wires  .  23
    9.3.1    OAM Capability for MH PWs Demultiplexed using MPLS  ...  23
    9.4      Detailed MH-VCCV Procedures  ..........................  24
    9.4.1    PW Label TTL  .........................................  24
    9.4.2    Partial tracing from T-PE  ............................  25
    9.4.3    Partial Tracing between S-PEs  ........................  25
    9.4.4    Automated VCCV Trace from T-PE  .......................  26
    9.5      Processing of an VCCV Echo Message in a MS-PW  ........  26
    9.5.1    Sending a VCCV Echo Request  ..........................  26
    9.5.2    Receiving an VCCV Echo Request  .......................  27
    9.5.3    Receiving an VCCV Echo Reply  .........................  27



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    9.6      VCCV Trace Operations  ................................  27
    9.7      Tracing Switched PW Switch Points Using MH-VCCV  ......  28
    9.8      Mapping Switched Pseudo Wire Status  ..................  29
    9.8.1    S-PE initiated PW status messages  ....................  31
    9.8.1.1  Local PW2 reverse direction fault  ....................  32
    9.8.1.2  Local PW1 reverse direction fault  ....................  32
    9.8.1.3  Local PW2 forward direction fault  ....................  32
    9.8.1.4  Local PW1 forward direction fault  ....................  33
    9.8.1.5  Clearing Faults  ......................................  33
    9.8.2    PW status messages and S-PE TLV processing  ...........  33
    9.8.3    T-PE processing of PW status messages  ................  33
    9.9      Pseudowire Status Negotiation Procedures  .............  34
    9.10     Status Dampening  .....................................  34
   10        Peering Between Autonomous Systems  ...................  34
   11        Security Considerations  ..............................  34
   11.1      Data Plane Security  ..................................  34
   11.2      Control Protocol Security  ............................  35
   12        IANA Considerations  ..................................  36
   12.1      Channel Type  .........................................  36
   12.2      L2TPv3 AVP  ...........................................  36
   12.3      LDP TLV TYPE  .........................................  36
   12.4      LDP Status Codes  .....................................  36
   12.5      L2TPv3 Result Codes  ..................................  37
   12.6      New IANA Registries  ..................................  37
   13        Intellectual Property Statement  ......................  37
   14        Full Copyright Statement  .............................  38
   15        Acknowledgments  ......................................  38
   16        Normative References  .................................  38
   17        Informative References  ...............................  39
   18        Author Information  ...................................  40






1. Specification of Requirements

   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 [RFC2119].










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2. Terminology

     - PW Terminating Provider Edge (T-PE). A PE where the customer-
       facing attachment circuits (ACs) are bound to a PW forwarder. A
       Terminating PE is present in the first and last segments of a
       MS-PW. This incorporates the functionality of a PE as defined in
       [RFC3985].

     - Single-Segment Pseudo Wire (SS-PW). A PW setup directly between
       two T-PE devices. Each PW in one direction of a SS-PW traverses
       one PSN tunnel that connects the two T-PEs.

     - Multi-Segment Pseudo Wire (MS-PW).  A static or dynamically
       configured set of two or more contiguous PW segments that behave
       and function as a single point-to-point PW. Each end of a MS-PW
       by definition MUST terminate on a T-PE.

     - PW Segment. A part of a single-segment or multi-segment PW, which
       is set up between two PE devices, T-PEs and/or S-PEs.

     - PW Switching Provider Edge (S-PE).  A PE capable of switching the
       control and data planes of the preceding and succeeding PW
       segments in a MS-PW. The S-PE terminates the PSN tunnels of the
       preceding and succeeding segments of the MS-PW. It is therefore a

     - PW switching point for a MS-PW. A PW Switching Point is never the
       S-PE and the T-PE for the same MS-PW. A PW switching point runs
       necessary protocols to setup and manage PW segments with other PW
       switching points and terminating PEs.


3. Introduction

   PWE3 defines the signaling and encapsulation techniques for
   establishing SS-PWs between a pair of ultimate PEs and in the vast
   majority of cases this will be sufficient. MS-PWs come into play in
   two general cases:

        -i. When it is not possible, desirable or feasible to establish
            a PW control channel between the ultimate source and
            destination PEs. At a minimum PW control channel
            establishment requires knowledge of and reachability to the
            remote (ultimate) PE IP address. The local (ultimate) PE may
            not have access to this information related to topology,
            operational or security constraints.

            An example is the inter-AS L2VPN scenario where the ultimate
            PEs reside in different provider networks (ASes) and it is



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            the practice to MD5-key all control traffic exchanged
            between two networks. Technically a SS-PW could be used but
            this would require MD5-keying on ALL ultimate source and
            destination PE nodes. An MS-PW allows the providers to
            confine MD5 key administration to just the PW switching
            points connecting the two domains.

            A second example might involve a single AS where the PW
            setup path between the ultimate PEs is computed by an
            external entity (i.e. client-layer routing protocol). Assume
            a full mesh of PWE3 control channels established between
            PE-A, PE-B and PE-C. A client-layer L2 connection tunneled
            through a PW is required between ultimate PE-A and PE-C. The
            external entity computes a PW setup path that passes through
            PE-B. This results in two discrete PW segments being built:
            one between PE-A and PE-B and one between PE-B and PE-C. The
            successful client-layer L2 connection between ultimate PE-A
            and ultimate PE-C requires that PE-B performs the PWE3
            switching process.

            A third example involves the use of PWs in hierarchical
            IP/MPLS networks.  Access networks connected to a backbone
            use PWs to transport customer payloads between customer
            sites serviced by the same access network and up to the edge
            of the backbone where they can be terminated or switched
            onto a succeeding PW segment crossing the backbone. The use
            of PWE3 switching between the access and backbone networks
            can potentially reduce the PWE3 control channels and routing
            information processed by the access network T-PEs.

            It should be noted that PWE3 switching does not help in any
            way to reduce the amount of PW state supported by each
            access network T-PE.

       -ii. PWE3 signaling and encapsulation protocols are different.
            The ultimate PEs are connected to networks employing
            different PW signaling and encapsulation protocols. In this
            case it is not possible to use a SS-PW. A MS-PW with the
            appropriate interworking performed at the PW switching
            points can enable PW connectivity between the ultimate PEs
            in this scenario.


   There are four different signaling protocols that are defined to
   signal PWs:






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        -i. Static configuration of the PW (MPLS or L2TPv3).
       -ii. LDP using FEC 128
      -iii. LDP using the generalized FEC 129
       -iv. L2TPv3


4. General Description

   A pseudo-wire (PW) is a tunnel established between two provider edge
   (PE) nodes to transport L2 PDUs across a packet switched network
   (PSN) as described in Figure 1 and in [PWE3-ARCH]. Many providers are
   looking at PWs as a means of migrating existing (or building new)
   L2VPN services (i.e.  Frame-Relay, ATM, Ethernet) on top of a PSN by
   using PWs. PWs may span multiple autonomous systems of the same or
   different provider networks. In these scenarios PW control channels
   (i.e. targeted LDP, L2TPv3) and PWs will cross AS boundaries.

   Inter-AS L2VPN functionality is currently supported and several
   techniques employing MPLS encapsulation and LDP signaling have been
   documented [2547BIS]. It is also straightforward to support the same
   inter-AS L2VPN functionality employing L2TPv3. In this document we
   define methodology to switch a PW between two PW control planes.

         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudo Wire ------>|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnel -->|    |          |
         |          V    V                  V    V          |
         V    AC    +----+                  +----+     AC   V
   +-----+    |     | PE1|==================| PE2|     |    +-----+
   |     |----------|............PW1.............|----------|     |
   | CE1 |    |     |    |                  |    |     |    | CE2 |
   |     |----------|............PW2.............|----------|     |
   +-----+  ^ |     |    |==================|    |     | ^  +-----+
         ^  |       +----+                  +----+     | |  ^
         |  |   Provider Edge 1         Provider Edge 2  |  |
         |  |                                            |  |
   Customer |                                            | Customer
   Edge 1   |                                            | Edge 2
            |                                            |
      native service                               native service

                     Figure 1: PWE3 Reference Model


   There are two methods for switching a PW between two PW control
   planes. In the first method (Figure 2), the two control planes



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   terminate on different PEs.

                |<------------Emulated Service---------->|
                |      PSN                      PSN      |
            AC  |    |<-1->|                  |<-2->|    |  AC
            |   V    V     V                  V     V    V  |
            |   +----+     +-----+       +----+     +----+  |
   +----+   |   |    |=====|     |       |    |=====|    |  |    +----+
   |    |-------|......PW1.......|--AC1--|......PW2......|-------|    |
   | CE1|   |   |    |     |     |       |    |     |    |  |    |CE2 |
   |    |-------|......PW3.......|--AC2--|......PW4......|-------|    |
   +----+   |   |    |=====|     |       |    |=====|    |  |    +----+
        ^       +----+     +-----+       +----+     +----+       ^
        |         PE1        PE2          PE3         PE4        |
        |                     ^            ^                     |
        |                     |            |                     |
        |                  PW stitching points                   |
        |                                                        |
        |                                                        |
        |<-------------------- Emulated Service ---------------->|

            Figure 2: PW Switching using ACs Reference Model

   In Figure 2, pseudo wires in two separate PSNs are stitched together
   using native service attachment circuits. PE2 and PE3 only run the
   control plane for the PSN to which they are directly attached. At PE2
   and PE3, PW1 and PW2 are connected using attachment circuit AC1,
   while PW3 and PW4 are connected using attachment circuit AC2.

           Native  |<-----------Pseudo Wire----------->|  Native
           Layer2  |                                   |  Layer2
          Service  |    |<-PSN1-->|     |<--PSN2->|    |  Service
           (AC)    V    V         V     V         V    V   (AC)
             |     +----+         +-----+         +----+     |
   +----+    |     | PE1|=========| PE2 |=========| PE3|     |    +----+
   |    |----------|........PW1.........|...PW3........|----------|    |
   | CE1|    |     |    |         |     |         |    |     |    |CE2 |
   |    |----------|........PW2.........|...PW4........|----------|    |
   +----+    |     |    |=========|     |=========|    |     |    +----+
        ^          +----+         +-----+         +----+     |    ^
        |      Provider Edge 1       ^        Provider Edge 3     |
        |      (Terminating PE)      |        (Terminating PE)    |
        |                            |                            |
        |                    PW switching point                   |
        |            (Optional PW adaptation function)            |
        |                                                         |
        |<------------------- Emulated Service ------------------>|




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                 Figure 3: PW Control Plane Switching Reference Model

   In Figure 3 PE2 runs two separate control planes: one toward PE1, and
   one Toward PE3. The PW switching point is at PE2 which is configured
   to connect PW1 and PW3 together to complete the multi-hop PW between
   PE1 and PE3.  PW1 and PW3 MUST be of the same PW type, but PSN1 and
   PSN2 need not be the same technology. In the latter case, if the PW
   is switched to a different technology, the PEs must adapt the PDU
   encapsulation between the different PSN technologies. In the case
   where PSN1 and PSN2 are the same technology the PW PDU does not need
   to be modified, and PDUs are then switched between the pseudo-wires
   at the PW label level.

   It should be noted that it is possible to adapt one PSN technology to
   a different one, for example MPLS over an IP or GRE [RFC4023]
   encapsulation, but this is outside the scope of this document.
   Further, one could perform an interworking function on the PWs
   themselves at the PW switching point, allowing conversion from one PW
   type to another, but this is also outside the scope of this document.

   This document describes procedures for building multi-segment
   pseudowires using manual configuration of the switching point PE2.
   Other documents may build on this base specification to automate the
   configuration and selection of PE2. It should also be noted that a PW
   can traverse multiple PW switching points along it's path, and the
   edge PEs will not require any specific knowledge of how many PW
   switching points the PW has traversed (though this may be reported
   for troubleshooting purposes).

   In general the approach taken is to connect the individual control
   planes by passing along any signaling information immediately upon
   reception. First the PW switching point is configured to switch a
   SS-PW from a specific peer to another SS-PW destined for a different
   peer. No control messages are exchanged yet as the PW switching point
   PE does not have enough information to actually initiate the PW setup
   messages. However, if a session does not already exist, a control
   protocol (LDP/L2TP) session is setup. In this model the MS-PW setup
   is starting from the T-PE devices. Next once the T-PE is configured
   it sends the PW control setup messages. These messages are received,
   and immediately used to form the PW setup messages for the next SS-PW
   of the MS-PW. If one of the Switching PEs doesn't accept an LDP Label
   Mapping message then a Label Release message is sent back to the
   originator T-PE. A MS-PW is declared UP when all the constituent SS-
   PWs are UP.







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5. PW Switching and Attachment Circuit Type

   The PWs in each PSN are established independently, with each PSN
   being treated as a separate PWE3 domain. For example, in Figure 2 for
   case of MPLS PSNs, PW1 is setup between PE1 and PE2 using the LDP
   targeted session as described in [RFC4447], and at the same time a
   separate pseudo wire, PW2, is setup between PE3 and PE4. The ACs for
   PW1 and PW2 at PE2 and PE3 MUST be configured such that they are the
   same PW type e.g. ATM VCC, Ethernet VLAN, etc.


6. Applicability

   When using a PSN to transport a PW, the performance of the PW is
   equal to the performance of the PSN plus any impairments introduced
   by the PW layer itself. Therefore it is not possible for the PW to
   provide better performance than the PSN over which it is transported.

   Therefore, it is necessary to carefully consider the order in which
   different layer networks are stacked upon each other within a
   'network stack' in order to provide the topmost service with the
   performance that it requires. This performance inheritance within a
   PW/PSN relationship is vertical because the PW is vertically stacked
   upon its PSN.

   Note: Due to this vertical performance inheritance and the different
   performance provided by, and the characteristics of, each networking
   mode it is generally advisable to stack modes that less efficiently
   provide dedicated bandwidth/performance on top of modes that more
   efficiently provide dedicated bandwidth/performance.

   When performing peer partition interworking the PW inherits the
   performance of the PSN partition that provides the worst performance
   of all the peered PSN partitions over which the PW is transported.
   Therefore it is not possible for the PW to receive (or provide)
   better performance than the worst performing of the peered PSN
   partitions over which it is transported.

   Therefore, it is necessary to carefully consider which PSN modes
   (and/or technologies) it is appropriate to peer with one another in
   order to provide the service with the performance that it requires.
   This is a horizontal performance relationship because the server
   layer partitions are peered with each other horizontally.








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7. PW-MPLS to PW-MPLS Control Plane Switching

   Referencing Figure 3, PE2 sets up a PW1 using the LDP targeted
   session as described in [RFC4447], at the same time a separate pseudo
   wire PW3 is setup to PE3. Each PW is configured independently on the
   PEs, but on PE2 pseudo wire PW1 is connected to pseudo wire PW3. PDUs
   are then switched between the pseudo-wires at the PW label level.
   Hence the data plane does not need any special knowledge of the
   specific pseudo wire type. A simple standard MPLS label swap
   operation is sufficient to connect the two PWs, and in this case the
   PW adaptation function is not used.

   This process can be repeated as many times as necessary, the only
   limitation to the number of PW switching points traversed is imposed
   by the TTL field of the PW MPLS Label. The setting of the TTL is a
   matter of local policy on the originating PE, but SHOULD be set to
   255.

   There are three MPLS to MPLS PW control planes:
        -i. Static configuration of the PW.
       -ii. LDP using FEC 128
      -iii. LDP using the generalized FEC 129
   This results in four distinct PW switching situations that are
   significantly different, and must be considered in detail:
        -i. PW Switching between two static control planes.
       -ii. Static Control plane switching to LDP dynamic control plane.
      -iii. Two LDP control planes using the same FEC type
       -iv. LDP using FEC 128, to LDP using the generalized FEC 129


7.1. Static Control plane switching

   In the case of two static control planes the PW switching point MUST
   be configured to direct the MPLS packets from one PW into the other.
   There is no control protocol involved in this case. When one of the
   control planes is a simple static PW configuration and the other
   control plane is a dynamic LDP FEC 128 or generalized PW FEC, then
   the static control plane should be considered identical to an
   attachment circuit (AC) in the reference model of Figure 1. The
   switching point PE SHOULD signal the proper PW status if it detects a
   failure in sending or receiving packets over the static PW.  Because
   the PW is statically configured, the status communicated to the
   dynamic LDP PW will be limited to local interface failures. In this
   case, the PW switching point PE behaves in a very similar manner to a
   T-PE, assuming an active role. This means that the S-PE will
   immediately send the LDP Label Mapping message if the static PW is
   deemed to be UP.




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7.2. Two LDP control planes using the same FEC type

   As stated in a section above, the PW switching point PE should assume
   an initial passive role. This means that once independent PWs are
   configured on the switching point, the LSR does not advertise the LDP
   PW FEC mapping until it has received at least one of the two PW LDP
   FECs from a remote PE. This is necessary because the switching point
   LSR does not know a priori what the interface parameter field in the
   initial FEC advertisement will contain.

   The PWID is a unique number between each pair of PEs. Hence Each SS-
   PW that forms an MS-PW may have a different PWID. In the case of The
   Generalized PW FEC, the AGI/SAI/TAI may have to also be different for
   some, or sometimes all, SS-PWs.


7.2.1. FEC 129 Active/Passive T-PE Election Procedure

   When a MS-PW is signaled using FEC 129, each T-PE might independently
   start signaling the MS-PW. If the MS-PW path is not statically
   configured, in certain cases the signaling procedure could result in
   an attempt to setup each direction of the MS-PW through different
   paths. To avoid this situation one of the T-PE MUST start the PW
   signaling (active role), while the other waits to receive the LDP
   label mapping before sending the respective PW LDP label mapping
   message. (passive role). When the MS-PW path not statically
   configured, the Active T-PE (the ST-PE) and the passive T-PE (the
   TT-PE) MUST be identified before signaling is initiated for a given
   MS-PW.

   The determination of which T-PE assume the active role SHOULD be done
   as follows:

   the SAII and TAII are compared as unsigned integers, if the SAII is
   bigger then the T-PE assumes the active role.

   The selection process to determine which T-PE assumes the active role
   MAY be superseded by manual provisioning.


7.3. LDP FEC 128 to LDP using the generalized FEC 129

   When a PE is using the generalized FEC 129, there are two distinct
   roles that a PE can assume: active and passive. A PE that assumes the
   active role will send the LDP PW setup message, while a passive role
   PE will simply reply to an incoming LDP PW setup message. The PW
   switching point PE, will always remain passive until a PWID FEC 128
   LDP message is received, which will cause the corresponding



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   generalized PW FEC LDP message to be formed and sent. If a
   generalized FEC PW LDP message is received while the switching point
   PE is in a passive role, the corresponding PW FEC 128 LDP message
   will be formed and sent.

   PW IDs need to be mapped to the corresponding AGI/TAI/SAI and vice
   versa.  This can be accomplished by local PW switching point
   configuration, or by some other means, such as some form of auto
   discovery. Such other means are outside the scope of this document.


7.4. LDP PW switching point TLV

   The edge to edge PW might traverse several switching points, in
   separate administrative domains. For management and troubleshooting
   reasons it is useful to record information about the switching points
   that the PW traverses. This is accomplished by using a PW switching
   point TLV.

   Note that sending the PW switching point TLV is OPTIONAL, however the
   PE or SPE MUST process the TLV upon reception. The PW switching point
   TLV is appended to the PW FEC at each switching point and is encoded
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|     PW sw TLV  (0x096D)   |     PW sw TLV  Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Variable Length Value                   |
   |                               "                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [note] LDP TLV type is pending IANA approval.

     - PW sw TLV  Length

       Specifies the total length of all the following PW switching
       point TLV fields in octets

     - Type

       Encodes how the Value field is to be interpreted.






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     - Length

       Specifies the length of the Value field in octets.

     - Value

       Octet string of Length octets that encodes information to be
       interpreted as specified by the Type field.

   PW Switching point TLV Types are assigned by IANA according the the
   process defined in the "IANA Allocations" section below.

   The PW switching Point TLV is an OPTIONAL TLV that can appear once
   for each switching point traversed.


7.4.1. PW Switching Point Sub-TLVs

   Below are details specific to PW Switching Point Sub-TLVs described
   in this document:

     - PW ID of last PW segment traversed.

       This sub-TLV type contains a PW ID in the format of the PWID
       described in [RFC4447]

     - PW Switching Point description string.

       An optional description string of text up to 80 characters long.

     - IP address of PW Switching Point.

       The IP V4 or V6 address of the PW Switching Point. This is an
       OPTIONAL Sub-TLV.
     - MH Virtual Ciscuit Connectivity Verification (VCCV) Capability
       Indication.

     - The FEC  of last PW segment traversed.

       The Attachment Identifier of the last PW segment traversed. This
       is coded in the following format:










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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AGI Type    |    Length     |      Value                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    AGI  Value (contd.)                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |    Length     |      Value                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                   SAII  Value (contd.)                        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   AII Type    |    Length     |      Value                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                   TAII Value (contd.)                         ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     - L2 PW address of PW Switching Point (recommended format)

   This sub-TLV type contains a L2 PW address of PW Switching Point in
   the format described in [RFC5003]. This includes the AII type field ,
   and length, as well as the L2 PW address without the AC ID portion
   (if applicable).


7.4.2. Adaptation of Interface Parameters

   [RFC4447] defines several interface parameters, which are used by the
   Network Service Processing (NSP) to adapt the PW to the Attachment
   Circuit (AC). The interface parameters are only used at the end
   points, and MUST be passed unchanged across the PW switching point.
   However the following interface parameters MAY be modified as
   follows:

     - 0x03 Optional Interface Description string This Interface
       parameter MAY be modified, or altogether removed from the FEC
       element depending on local configuration policies.

     - 0x09 Fragmentation indicator This parameter MAY be inserted in
       the FEC by the switching point if it is capable of re-assembly of
       fragmented PW frames according to [PWE3-FRAG].







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     - 0x0C VCCV parameter The switching point MAY not be able to
       inspect the VCCV control channel. If the new MH-VCCV sub-TLV is
       present, the VCCV parameter MUST be ignored in order to avoid
       conflicts with the new TLV.


7.5. Group ID

   The Group ID (GR ID) is used to reduce the number of status messages
   that need to be sent by the PE advertising the PW FEC. The GR ID has
   local significance only, and therefore MUST be mapped to a unique GR
   ID allocated by the PW switching point PE.


7.6. PW Loop Detection

   A switching point PE SHOULD check the OPTIONAL PW switching Point
   TLV, to verify if it's own IP address appears in it. If it's IP
   address appears in a received PW switching Point TLV, the PE SHOULD
   break the loop, and send a label release message with the following
   error code:
      Assignment E Description
      0x0000003A 0 "PW Loop Detected"

   [ note: error code pending IANA allocation ]


8. PW-MPLS to PW-L2TPv3 Control Plane Switching

   Both MPLS and L2TPv3 PWs may be static or dynamic. This results in
   four possibilities when switching between L2TPv3 and MPLS.

        -i. Switching between MPLS and L2TPv3 static control planes.
       -ii. Switching between a static MPLS PW and a dynamic L2TPv3 PW.
      -iii. Switching between a static L2TPv3 PW and a dynamic MPLS PW.
       -iv. Switching between a dynamic MPLS PW and a dynamic L2TPv3 PW.


8.1. Static MPLS and L2TPv3 PWs

   In the case of two static control planes, the PW switching point MUST
   be configured to direct packets from one PW into the other. There is
   no control protocol involved in this case. The configuration MUST
   include which MPLS VC Label maps to which L2TPv3 Session ID (and
   associated Cookie, if present) as well as which MPLS Tunnel Label
   maps to which PE destination IP address.





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8.2. Static MPLS PW and Dynamic L2TPv3 PW

   When a statically configured MPLS PW is switched to a dynamic L2TPv3
   PW, the static control plane should be considered identical to an
   attachment circuit (AC) in the reference model of Figure 1. The
   switching point PE SHOULD signal the proper PW status if it detects a
   failure in

   sending or receiving packets over the static PW. Because the PW is
   statically configured, the status communicated to the dynamic L2TPv3
   PW will be limited to local interface failures. In this case, the PW
   switching point PE behaves in a very similar manner to a T-PE,
   assuming an active role.


8.3. Static L2TPv3 PW and Dynamic LDP/MPLS PW

   When a statically configured L2TPv3 PW is switched to a dynamic
   LDP/MPLS PW, then the static control plane should be considered
   identical to an attachment circuit (AC) in the reference model of
   Figure 1. The switching point PE SHOULD signal the proper PW status
   (via an L2TPv3 SLI message) if it detects a failure in sending or
   receiving packets over the static PW.  Because the PW is statically
   configured, the status communicated to the dynamic LDP/MPLS PW will
   be limited to local interface failures. In this case, the PW
   switching point PE behaves in a very similar manner to a T-PE,
   assuming an active role.


8.4. Dynamic LDP/MPLS and L2TPv3 PWs

   When switching between dynamic PWs, the switching point always
   assumes an initial passive role. Thus, it does not initiate an
   LDP/MPLS or L2TPv3 PW until it has received a connection request
   (Label Mapping or ICRQ) from one side of the node. Note that while
   MPLS PWs are made up of two unidirectional LSPs bonded together by
   FEC identifiers, L2TPv3 PWs are bidirectional in nature, setup via a
   3-message exchange (ICRQ, ICRP and ICCN). Details of Session
   Establishment, Tear Down, and PW Status signaling are detailed below.


8.4.1. Session Establishment

   When the PW switching point receives an L2TPv3 ICRQ message, the
   identifying AVPs included in the message are mapped to FEC
   identifiers and sent in an LDP label mapping message. Conversely, if
   an LDP Label Mapping message is received, it is either mapped to an
   ICRP message or causes an L2TPv3 session to be initiated by sending



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   an ICRQ.

   Following are two example exchanges of messages between LDP and
   L2TPv3. The first is a case where an L2TPv3 T-PE initiates an MS-PW,
   the second is a case where an MPLS T-PE initiates an MS-PW.

      PE 1 (L2TPv3)      PW Switching Node       PE3 (MPLS/LDP)

        AC "Up"
        L2TPv3 ICRQ --->
                         LDP Label Mapping  --->
                                                    AC "UP"
                                           <--- LDP Label Mapping
                   <--- L2TPv3 ICRP
        L2TPv3 ICCN  --->
      <-------------------- MH PW Established ------------------>


      PE 1 (MPLS/LDP)      PW Switching Node       PE3 (L2TPv3)

        AC "Up"
        LDP Label Mapping --->
                              L2TPv3 ICRQ  --->
                                              <--- L2TPv3 ICRP
                         <--- LDP Label Mapping
                              L2TPv3 ICCN --->
                                                   AC "Up"
      <-------------------- MH PW Established ------------------>


8.4.2. Adaptation of PW Status message

   L2TPv3 uses the SLI message to indicate a interface status change
   (such as the interface transitioning from "Up" or "Down"). MPLS/LDP
   PWs either signal this via an LDP Label Withdraw or the PW Status
   Notification message defined in section 4.4 of [RFC4447].


8.4.3. Session Tear Down

   L2TPv3 uses a single message, CDN, to tear down a pseudowire. The CDN
   message translates to a Label Withdraw message in LDP. Following are
   two example exchanges of messages between LDP and L2TPv3. The first
   is a case where an L2TPv3 T-PE initiates the termination of an MS-PW,
   the second is a case where an MPLS T-PE initiates the termination of
   an MS-PW.





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   PE 1 (L2TPv3)      PW Switching Node       PE3 (MPLS/LDP)

   AC "Down"
     L2TPv3 CDN --->
                      LDP Label Withdraw  --->
                                                 AC "Down"
                                      <-- LDP Label Release

   <--------------- MH PW Data Path Down ------------------>



   PE 1 (MPLS LDP)     PW Switching Node       PE3 (L2TPv3)

   AC "Down"
   LDP Label Withdraw  --->
                           L2TPv3 CDN -->
                       <-- LDP Label Release
                                                 AC "Down"

   <---------------- MH PW Data Path Down ------------------>


8.5. Adaptation of L2TPv3 AVPs to Interface Parameters

   [RFC4447] defines several interface parameters which MUST be mapped
   to the equivalent AVPs in L2TPv3 setup messages.

     * Interface MTU

       The Interface MTU parameter is mapped directly to the L2TP
       Interface MTU AVP defined in [L2TP-L2VPN]

     * Max Number of Concatenated ATM cells

       This interface parameter is mapped directly to the L2TP "ATM
       Maximum Concatenated Cells AVP" described in section 6 of [L2TP-
       ATM].

     * Optional Interface Description String

       This string may be carried as the "Call-Information AVP"
       described in section 2.2 of [L2TP-INFOMSG]

     * PW Type

       The PW Type defined in [RFC4446] is mapped to the L2TPv3 "PW
       Type" AVP defined in [L2TPv3].



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     * PW ID (FEC 128)

       For FEC 128, the PW ID is mapped directly to the L2TPv3 "Remote
       End ID" AVP defined in [L2TPv3].

     * Generalized FEC 129 SAI/TAI

       Section 4.3 of [L2TP-L2VPN] defines how to encode the SAI and TAI
       parameters. These can be mapped directly.

   Other interface parameter mappings will either be defined in a future
   version of this document, or are unsupported when switching between
   LDP/MPLS and L2TPv3 PWs.


8.6. Switching Point TLV in L2TPv3

   When translating between LDP and L2TPv3 control messages, the PW
   Switching Point TLV described earlier in this document is carried in
   a single variable length L2TP AVP present in the ICRQ, ICRP messages,
   and optionally in the ICCN message.

   The L2TP "Switching Point AVP" is Attribute Type TBA-L2TP-AVP-1. The
   AVP MAY be hidden (the L2TP AVP H-bit may be 0 or 1), the length of
   the AVP is 6 plus the length of the series of Switching Point sub-
   TLVs included in the AVP, and the AVP MUST NOT be marked Mandatory
   (the L2TP AVP M-bit MUST be 0).


8.7. L2TPv3 and MPLS PW Data Plane

   When switching between an MPLS and L2TP PW, packets are sent in their
   entirety from one PW to the other, replacing the MPLS label stack
   with the L2TPv3 and IP header or vice versa. There are some
   situations where an additional amount of interworking must be
   provided between the two data planes at the PW switching node.


8.7.1. PWE3 Payload Convergence and Sequencing

   Section 5.4 of [PWE3-ARCH] discusses the purpose of the various shim
   headers necessary for enabling a pseudowire over an IP or MPLS PSN.
   For L2TPv3, the Payload Convergence and Sequencing function is
   carried out via the Default L2-Specific Sublayer defined in [L2TPv3].
   For MPLS, these two functions (together with PSN Convergence) are
   carried out via the MPLS Control Word. Since these functions are
   different between MPLS and L2TPv3, interworking between the two may
   be necessary.



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   The L2TP L2-Specific Sublayer and MPLS Control Word are shim headers
   which in some cases are not necessary to be present at all. For
   example, an Ethernet PW with sequencing disabled will generally not
   require an MPLS Control Word or L2TP Default L2-Specific Sublayer to
   be present at all. In this case, Ethernet frames are simply sent from
   one PW to the other without any modification beyond the MPLS and
   L2TP/IP encapsulation and decapsulation.

   The following section offers guidelines for how to interwork between
   L2TP and MPLS for those cases where the Payload Convergence,
   Sequencing, or PSN Convergence functions are necessary on one or both
   sides of the switching node.


8.7.2. Mapping

   The MPLS Control Word consists of (from left to right):

        -i. These bits are always zero in MPLS are not necessary to be
            mapped to L2TP.

       -ii. These six bits may be used for Payload Convergence depending
            on the PW type. For ATM, the first four of these bits are
            defined in [PWE3-ATM]. These map directly to the bits
            defined in [L2TP-ATM]. For Frame Relay, these bits indicate
            how to set the bits in the Frame Relay header which must be
            regenerated for L2TP as it carries the Frame Relay header
            intact.

      -iii. L2TP determines its payload length from IP. Thus, this
            Length field need not be carried directly to L2TP. This
            Length field will have to be calculated and inserted for
            MPLS when necessary.

       -iv. The Default L2-Specific Sublayer has a sequence number with
            different semantics than that of the MPLS Control Word. This
            difference eliminates the possibility of supporting
            sequencing across the MS-PW by simply carrying the sequence
            number through the switching point transparently. As such,
            sequence numbers MAY be supported by checking the sequence
            numbers of packets arriving at the switching point and
            regenerating a new sequence number in the proper format for
            the PW on egress. If this type of sequence interworking at
            the switching node is not supported, and a T-PE requests
            sequencing of all packets via the L2TP control channel
            during session setup, the switching node SHOULD NOT allow
            the session to be established by sending a CDN message with
            Result Code set to 17 "sequencing not supported" (subject to



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            IANA Assignment).


9. Operation And Management

9.1. Extensions to VCCV to Support Switched PWs

   Single-hop pseudowires are signaled using the Virtual Circuit
   Connectivity Verification (VCCV) parameter included in the interface
   parameter field of the PW ID FEC TLV or the sub-TLV interface
   parameter of the Generalized PW ID FEC TLV as described in [RFC5085].
   When a switching point exist between PE nodes, it is required to be
   able to continue operating VCCV end-to-end across a switching point
   and to provide the ability to trace the path of the MS-PW over any
   number of segments.

   This document provides a method for achieving these two objectives.
   This method is based on re-using the existing VCCV CW and
   decrementing the TTL of the PW label at each hop in the path of the
   MS-PW.



9.2. PW-MPLS to PW-MPLS OAM Data Plane Indication

9.2.1. Decreasing the PW Label TTL

   This method reuses the SS-PW control word as described in [RFC5085].
   VCCV control packets are indicated using the following CW in the
   packet header:

    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|Version|  Reserved = 0 |         Channel Type          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Where:
      Version = 0
      Channel Type = IPv4,IPV6, or no IP header (0x21,0x57,0x07)


   By the rules defined in [RFC3032] the PW label TTL MUST be decreased
   at every S-PE. Once the PW label TTL reaches the value of 0 , the
   packet is sent to the control plane to be processed. Hence , by
   controling the PW TTL value of the PW label it is possible to select
   exactly which hop will respond to the VCCV packet.





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9.3. Signaling OAM Capabilities for Switched Pseudo Wires

   Like in SS-PW, MS-PW VCCV capabilities are signaled using the VCCV
   parameter included in the interface parameter field of the PW ID FEC
   TLV or the sub-TLV interface parameter of the Generalized PW ID FEC
   TLV as described in [RFC5085]. The new MH-VCCV CW is indicated using
   a new CC type in the VCCV capability parameter field.

   In Figure 2 T-PE1 uses the VCCV parameter included in the interface
   parameter field of the PW ID FEC TLV or the sub-TLV interface
   parameter of the Generalized PW ID FEC TLV to indicate to the far end
   T-PE2 what VCCV capabilities T-PE1 supports. This is the same VCCV
   parameter as would be used if T-PE1 and T-PE2 were connected
   directly. S-PE2, which is a PW switching point, as part of the
   adaptation function for interface parameters, processes locally the
   VCCV parameter then passes it to T-PE2. If there were multiple S-PEs
   on the path between T-PE1 and T-PE2, each would carry out the same
   processing, passing along the VCCV parameter. The local processing of
   the VCCV parameter removes CC Types specified by the originating T-PE
   that are not supported on the S-PE. For example, if T-PE1 indicates
   as supported CC Types both Control Word and Router Alert and the S-PE
   only supports Control Word CC type. Then the S-PE removes the Router
   Alert CC Type, leaving Control Word unchanged, and passes the
   modified VCCV parameter to the next S-PE along the path.

   The far end T-PE (T-PE2) receives the VCCV parameter indicating that
   one or both Control Word CC types only if they are supported by the
   initial T-PE (T-PE1) and all S-PEs along the PW path. If the VCCV
   parameter indicates both the CW CC type and the new MH-VCCV CW CC
   types are supported, then the T-PE1 is indicating it can receive both
   types. If T-PE2 also supports both types, T-PE2 uses the CW CC type
   in preference.



9.3.1. OAM Capability for MH PWs Demultiplexed using MPLS

   The MH-VCCV parameter ID is defined as follows in [RFC4446]:

        Parameter ID   Length     Description
          0x0c           4           VCCV


   The format of the VCCV parameter field is as follows:







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      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      0x0c     |       0x04    |   CC Types    |   CV Types    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          0x01 Type 1: PWE3 control word with 0001b as first nibble
                       as defined in [RFC4385].
          0x02 Type 2: MPLS Router Alert Label.
          0x04 Type 3: MPLS PW De-multiplexor Label TTL = 1 (Type 3).


   When using the PW label TTL method, the T-PE signals CC type 1. When
   using the MH-VCCV CW method, the T-PE signal CC type 4.


9.4. Detailed MH-VCCV Procedures

   In order to test the end-to-end connectivity of the multi-segment PW,
   a T-PE must include the FEC used in the last segment to the
   destination T-PE. This information is either configured at the
   sending T-PE or is obtained by processing the corresponding sub-TLV's
   of the PW switching point TLV.


9.4.1. PW Label TTL

   In Figure 2, if T-PE1, S-PE and T-PE2 support Control Word for VCCV,
   then as described in the previous section the control plane
   negotiates the common use of Control Word for VCCV end to end.

   At the S-PE the data path operations include an outer label pop,
   inner label swap and new outer label push. Note that there is no
   requirement for the S-PE to inspect the CW.

   Thus, the end-to-end connectivity of the multi-segment pseudowire can
   be verified by performing all of the following steps:

        -i. T-PE forms a VCCV-ping echo request message with the FEC
            matching that of the last segment PW to the destination T-
            PE.

       -ii. T-PE sets the inner PW label TTL to a large enough value to
            allow the packet to reach the far end T-PE.







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      -iii. T-PE sends a VCCV packet that will follow the exact same
            data path at each S-PE as that taken by data packets.

       -iv. S-PE performs an outer label pop, an inner label swap with
            TTL decrement, and new outer label push.

        -v. There is no requirement for the S-PE to inspect the CW.

       -vi. The VCCV packet is diverted to VCCV control processing at
            the destination T-PE.

      -vii. Destination T-PE replies using the specified reply mode,
            i.e., reverse PW path or IP path.


9.4.2. Partial tracing from T-PE

   In order to trace part of the multi-segment pseudowire, the TTL of
   the PW label may be used to force the VCCV message to 'pop out' at an
   intermediate node. When the TTL expires, the S-PE can determine that
   the packet is a VCCV packet by checking the control word. If the
   control word format matches that specified in [RFC5085], the packet
   should be diverted to VCCV processing.

   In Figure 2, if T-PE1 sends a VCCV message with the TTL of the PW
   label equal to 1, the TTL will expire at the S-PE. T-PE1 can thus
   verify the first segment of the pseudo wire.

   Note that this use of the TTL is subject to the caution expressed in
   [RFC5085].  If a penultimate LSR between S-PEs or between an S-PE and
   a T-PE manipulates the PW label TTL, the VCCV message may not emerge
   from the MS-PW at the correct S-PE.


9.4.3. Partial Tracing between S-PEs

   Assuming that all nodes along an MS-PW support the Control Word CC
   Type, VCCV between S-PEs may be accomplished using the PW label TTL
   as in section 3.3. In Figure-1, the S-PE may verify the path between
   it and T-PE2 by sending a VCCV message with the PW label TTL set to
   1. Given a more complex network with multiple S-PEs, an S-PE may
   verify the connectivity between it and an S-PE two segments away by
   sending a VCCV message with the PW label TTL set to 2. Thus, an S-PE
   can diagnose connectivity problems by successively increasing the
   TTL.






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9.4.4. Automated VCCV Trace from T-PE

   Although tracing of the MS-PW path is possible using the methods
   explained in sections 3.3. and 3.4. , these require multiple manual
   iterations and that the FEC of the last PW segment to the target T-
   PE/S-PE be known a priori at the node originating the echo request
   message for each iteration.  This mode of operation will be referred
   to as the "ping" mode.

   A full automated path tracing capability that iteratively probes the
   segments the MS-PW to learn the target FEC information is required.
   This will be referred to as the "trace" mode of operation. The
   details of this method are described in a later section.


9.5. Processing of an VCCV Echo Message in a MS-PW

   The challenge for the control plane is to be able to build the VCCV
   echo request packet with the necessary information such as the target
   FEC 128 PW sub-TLV (FEC128) of the downstream PW segment which the
   packet is destined for. This could be even more difficult in
   situations in which the MS-PW spans different providers and
   Autonomous Systems.

   For example, in Figure 2, T-PE1 has the required information to
   compose the FEC128 of the PW1 segment but it does not readily have
   the information required to compose the FEC128 of the PW3 segment if
   a VCCV echo request is supposed to be destined to T-PE2.


9.5.1. Sending a VCCV Echo Request

   When in the "ping" mode of operation, the sender of the echo request
   message requires the FEC of the last segment to the target S-PE/T-PE
   node. This information can either be configured manually or be
   obtained by inspecting the corresponding sub-TLV's of the PW
   switching point TLV. However, the PW switching point TLV is optional
   and there is no guarantee that all S-PE nodes will populate it with
   their system address and the PWid of the last PW segment traversed by
   the label mapping message. Thus a manual configuration is always
   preferred.

   When in the "trace" mode operation, the T-PE will automatically learn
   the target FEC by probing one by one the hops of the MS-PW path. Each
   S-PE node includes the FEC to the downstream node in the echo reply
   message in a similar way that LSP trace will have the probed node
   return the downstream interface and label stack in the echo reply
   message. The details of this method are described in the following



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   sections.


9.5.2. Receiving an VCCV Echo Request

   Upon receiving a VCCV echo request the control plane on S-PEs (or the
   target node of each segment of the MS-PW) validates the request and
   responds to the request with an echo reply consisting of the FEC128
   of the next downstream segment and a return code of 8 (label switched
   at stack-depth) indicating that it is an S-PE and not the egress
   router for the MS-PW.

   If the node is the T-PE or the egress node of the MS-PW, it responds
   to the echo request with an echo reply with a return code of 3
   (egress router) and no FEC128 is included.


9.5.3. Receiving an VCCV Echo Reply

   The operation to be taken by the node that receives the echo reply in
   response to its echo request depends on its current mode of operation
   such as "ping" or "trace".

   In "ping" mode, the node may choose to ignore the target FEC128 in
   the echo reply and report only the return code to the operator.

   However, in "trace" mode, the node builds and sends the subsequent
   VCCV echo request with a incrementing TTL and the information (such
   as the downstream FEC128) it received in the echo request to the next
   downstream PW segment.


9.6. VCCV Trace Operations

   As an example, in Figure 2, VCCV trace can be performed on the MS-PW
   originating from T-PE1 by a single operational command. The following
   process ensues:
        -i. T-PE1 sends a VCCV echo request with TTL set to 1 and a
            FEC128 containing the pseudo-wire information of the first
            segment (PW1 between T-PE1 and S-PE) to S-PE for validation.
            If FEC Stack Validation is enabled, the request may also
            include additional subTLV such as LDP Prefix and/or RSVP LSP
            dependent on the type of transport tunnel the segmented PW
            is riding on.







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       -ii. S-PE validates the echo request with the FEC128. Since it is
            a switching point between the first and second segment it
            builds an echo reply with a return code of 8 and includes
            the FEC128 of the second segment (PW3 between S-PE and T-
            PE2) and sends the echo reply back to T-PE1. If FEC stack
            validation is requested the S-PE validates the received FEC
            stack and builds the echo reply with the downstream target
            FEC stack which includes FEC128 subTLV and any addition
            target FEC stack subTLVs required for the next hop FEC stack
            validation.

      -iii. T-PE1 builds a second VCCV echo request based on the target
            FEC stack in the echo reply from the S-PE. It increments the
            TTL and sends the next echo request out to T-PE2. Note that
            the VCCV echo request packet is switched at the S-PE
            datapath and forwarded to the next downstream segment
            without any involvement from the control plane.

       -iv. T-PE2 receives and validates the echo request with the
            FEC128, or the target FEC stack if the FEC stack validation
            is required, of the PW3 from T-PE1.  Since T-PE2 is the
            destination node or the egress node of the MS-PW it replies
            to T-PE1 with an echo reply with a return code of 3 (Egress
            Router) and no FEC128 is included.

        -v. T-PE1 receives the echo reply from T-PE2. T-PE1 is made
            aware that T-PE2 is the destination of the MS-PW because the
            echo reply does not contain the FEC128 and because its
            return code is 3. The trace process is completed.

   Note that the above example assumes only FEC128 sub-TLV is exchanged
   but it is possible that the exchanged information could also involve
   other TLV or Target FEC sub-TLVs (such as FEC129, LDP Prefix or RSVP
   LSP). For more detail on the format of the VCCV echo packet, refer to
   [RFC5085] and [RFC4379].  The TTL here refers to that of the inner
   (VC) label TTL.


9.7. Tracing Switched PW Switch Points Using MH-VCCV

   Although the signaling of switched PWs includes functionality to
   record all switch points traversed by a particular switched
   pseudowire, this information is limited to the control plane.
   Specifically, this is the information which is then used to program
   the actual switching hardware. In an effort to provide explicit
   diagnostic capability of the data plane used by the switched
   pseudowire, it is necessary in some cases to compare the control and
   data planes used by a particular switched pseudowire. In these cases,



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   it is possible to trace the pseudowire switch points by sending
   single-hop VCCV messages with TTL described above in the MH VCCV
   header, and increasing TTL values. This algorithm can be used to
   "walk" across the network of switching points until the ultimate PE
   is reached.

   Details of the operation for both methods will be provided in a
   future version of the document


9.8. Mapping Switched Pseudo Wire Status

   In the PW switching with attachment circuits case (Figure 2), PW
   status messages indicating PW or attachment circuit faults SHOULD be
   mapped to fault indications or OAM messages on the connecting AC as
   defined in [PW-MSG-MAP]. If the AC connecting two PWs crosses an
   administrative boundary, then the manner in which those OAM messages
   are treated at the boundary is out of scope of this draft.

   In the PW control plane switching case (Figure 3), there is no
   attachment circuit at the PW switching point, but the two PWs are
   connected together. Similarly, the status of the PWs are forwarded
   unchanged from one PW to the other by the control plane switching
   function. However, it may sometimes be necessary to communicate
   status of one of the locally attached SS-PW at a PW switching point.
   For LDP this can be accomplished by sending an LDP notification
   message containing the PW status TLV, as well as an OPTIONAL PW
   switching point TLV as follows:























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    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|   Notification   (0x0001)   |      Message Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Message ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1| Status (0x0300)           |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1|                 Status Code=0x00000028                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Message ID=0                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Message Type=0           |      PW Status TLV            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         PW Status TLV                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         PW Status TLV         |            PWId FEC           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                                                               |
   |                 PWId FEC or Generalized ID FEC                |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|     PW sw TLV  (0x096D)   |     PW sw TLV  Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Only one PW switching point TLV can be present in this message. This
   message is then relayed by each PW switching point unchanged. The T-
   PE decodes the status message and the included PW switching point TLV
   to detect exactly where the fault occurred. At the T-PE if there is
   no PW switching point TLV included in the LDP status notification
   then the status message can be assumed to have originated at the
   remote T-PE.

   The merging of the received T-LDP status and the local status for the
   PW segments at an S-PE can be summarized as follows:

        -i. When the local status for both PW segments is UP, the S-PE
            passes any received AC or PW status bits unchanged, i.e.,
            the status notification TLV is unchanged but the VCid in the
            case of a FEC 128 TLV is set to value of the PW segment to
            the next hop.




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       -ii. When the local status for any of the PW segments is Down,
            the S-PE always sends "PW Down" status bits regardless if
            the received status bits from the remote node indicated AC
            UP/Down or PW UP/Down."




9.8.1. S-PE initiated PW status messages

   The PW fault directions are defined as follows:

                         +-------+
      ---PW1 forward---->|       |-----PW2 reverse---->
   S-PE1                 | S-PE2 |                   S-PE3
      <--PW1 reverse-----|       |<----PW2 forward-----
                         +-------+


   When a local fault is detected by the S-PE, a PW status message is
   sent in both directions along the PW. Since there are no attachment
   circuits on an S-PE, only the following status messages are relevant:

      0x00000008 - Local PSN-facing PW (ingress) Receive Fault
      0x00000010 - Local PSN-facing PW (egress) Transmit Fault


   Each S-PE needs to store only two 32-bit PW status words for each
   SS-PW: One for local failures , and one for remote failures (normally
   received from another PE). The first failure will set the appropriate
   bit in the 32-bit status word, and each subsequent failure will be
   ORed to the appropriate PW status word. In the case of the PW status
   word storing remote failures, this rule has the effect of a logical
   OR operation with the first failure received on the particular SS-PW.

   It should be noted that remote failures received on an S-PE are just
   passed along the MS-PW unchanged while local failures detected an an
   S-PE are signalled on both SS-PWs.

   A T-PE can receive multiple failures from S-PEs along the MH-PW,
   however only the failure from the remote closest S-PE will be stored.
   The PW status word received are just ORed to any existing remote PW
   status already stored on the T-PE.

   Given that there are two SS-PW at a particular S-PE for a particular
   MH-PW, there are for possible failure cases as follows:





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        -i. PW2 reverse direction fault
       -ii. PW1 reverse direction fault
      -iii. PW2 forward direction fault
       -iv. PW1 forward direction fault

   It should also be noted that once a PW status notification message is
   initiated at a PW switching point for a partucular pw status bit any
   further status message, for the same status bit, received from an
   upstream neighbor is processed locally and not forwarded until the PW
   switching point original status error state is cleared.

   Each S-PE along the MS-PW MUST store any PW status messages
   transiting it.  If more then one status message with the same PW
   status bit set is received by a T-PE only the last PW status message
   is stored.


9.8.1.1. Local PW2 reverse direction fault

   When this failure occurs the S-PE will take the following actions:

     * Send a PW status message to S-PE3 containing "0x00000010 - Local
       PSN-facing PW (egress) Transmit Fault"
     * Send a PW status message to S-PE1 containing "0x00000008 - Local
       PSN-facing PW (ingress) Receive Fault"
     * Store 0x00000010 in the local PW status word for the SS-PW toward
       S-PE3.


9.8.1.2. Local PW1 reverse direction fault

   When this failure occurs the S-PE will take the following actions:

     * Send a PW status message to S-PE1 containing "0x00000010 - Local
       PSN-facing PW (egress) Transmit Fault"
     * Send a PW status message to S-PE3 containing "0x00000008 - Local
       PSN-facing PW (ingress) Receive Fault"
     * Store 0x00000010 in the local PW status word for the SS-PW toward
       S-PE1.


9.8.1.3. Local PW2 forward direction fault

   When this failure occurs the S-PE will take the following actions:
     * Send a PW status message to S-PE3 containing "0x00000008 - Local
       PSN-facing PW (ingress) Receive Fault"





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     * Send a PW status message to S-PE1 containing "0x00000010 - Local
       PSN-facing PW (egress) Transmit Fault"
     * Store 0x00000008 in the local PW status word for the SS-PW toward
       S-PE3.


9.8.1.4. Local PW1 forward direction fault

   When this failure occurs the S-PE will take the following actions:
     * Send a PW status message to S-PE1 containing "0x00000008 - Local
       PSN-facing PW (ingress) Receive Fault"
     * Send a PW status message to S-PE3 containing "0x00000010 - Local
       PSN-facing PW (egress) Transmit Fault"
     * Store 0x00000008 in the local PW status word for the SS-PW toward
       S-PE1.


9.8.1.5. Clearing Faults

   Remote PW status fault clearing messages received by an S-PE will
   only be forwarded if there are no corresponding local faults on the
   S-PE. ( local faults always supersede remote faults )

   Once the local fault has cleared, and there is no corresponding (
   same PW status bit set ) remote fault, a PW status messages is sent
   out to the adjacent PEs clearing the fault.


9.8.2. PW status messages and S-PE TLV processing

   When a PW status message is received that includes a S-PE TLV, the
   S-PE TLV information MAY be stored, along with the contents of the PW
   status Word according to the procedures described above. If
   subsequent PW status message for the same pw status bit are received
   the S-PE TLV will overwrite the previously stored S-PE TLV.



9.8.3. T-PE processing of PW status messages

   The PW switching architecture is based on the concept that the T-PE
   should process the PW LDP messages in the same manner as if it was
   participating in the setup of a SS-PW. However T-PE participating a
   MS-PW, SHOULD be able to process the PW switching point TLV.
   Otherwise the processing of PW status messages , and other PW setup
   messages is exactly as described in [RFC4447].





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9.9. Pseudowire Status Negotiation Procedures

   Pseudowire Status signaling methodology, defined in [RFC4447], SHOULD
   be transparent to the switching point.


9.10. Status Dampening

   When the PW control plane switching methodology is used to cross an
   administrative boundary it might be necessary to prevent excessive
   status signaling changes from being propagated across the
   administrative boundary.  This can be achieved by using a similar
   method as commonly employed for the BGP protocol route advertisement
   dampening. The details of this OPTIONAL algorithm are a matter of
   implementation, and are outside the scope of this document.


10. Peering Between Autonomous Systems

   The procedures outlined in this document can be employed to provision
   and manage MS-PWs crossing AS boundaries.

   The use of more advanced mechanisms involving auto-discovery and
   ordered PWE3 MS-PW signaling will be covered in a separate document.


11. Security Considerations

   This document specifies the LDP and L2TPv3 extensions that are needed
   for setting up and maintaining pseudowires. The purpose of setting up
   pseudowires is to enable layer 2 frames to be encapsulated and
   transmitted from one end of a pseudowire to the other. Therefore we
   treat the security considerations for both the data plane and the
   control plane.


11.1. Data Plane Security

   Data plane security consideration as discussed in [RFC4447],
   [L2TPv3], and [PWE3-ARCH] apply to this extension without any
   changes.










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11.2. Control Protocol Security

   General security considerations with regard to the use of LDP are
   specified in section 5 of RFC 3036. Security considerations with
   regard to the L2TPv3 control plane are specified in [L2TPv3]. These
   considerations apply as well to the case where LDP or L2TPv3 is used
   to set up PWs.

   A Pseudowire connects two attachment circuits. It is important to
   make sure that LDP connections are not arbitrarily accepted from
   anywhere, or else a local attachment circuit might get connected to
   an arbitrary remote attachment circuit. Therefore an incoming session
   request MUST NOT be accepted unless its IP source address is known to
   be the source of an "eligible" peer. The set of eligible peers could
   be pre-configured (either as a list of IP addresses, or as a list of
   address/mask combinations), or it could be discovered dynamically via
   an auto-discovery protocol which is itself trusted. (Obviously if the
   auto-discovery protocol were not trusted, the set of "eligible peers"
   it produces could not be trusted.)

   Even if a connection request appears to come from an eligible peer,
   its source address may have been spoofed.  So some means of
   preventing source address spoofing must be in place.  For example, if
   all the eligible peers are in the same network, source address
   filtering at the border routers of that network could eliminate the
   possibility of source address spoofing.

   For a greater degree of security, the LDP MD5 authentication key
   option, as described in section 2.9 of RFC 3036, or the Control
   Message Authentication option of [L2TPv3] MAY be used.  This provides
   integrity and authentication for the control messages, and eliminates
   the possibility of source address spoofing.  Use of the message
   authentication option does not provide privacy, but privacy of
   control messages are not usually considered to be highly urgent.
   Both the LDP and L2TPv3 message authentication options rely on the
   configuration of pre-shared keys, making it difficult to deploy when
   the set of eligible neighbors is determined by an auto-configuration
   protocol.

   When the Generalized ID FEC Element is used, it is possible that a
   particular peer may be one of the eligible peers, but may not be the
   right one to connect to the particular attachment circuit identified
   by the particular instance of the Generalized ID FEC element.
   However, given that the peer is known to be one of the eligible peers
   (as discussed above), this would be the result of a configuration
   error, rather than a security problem.  Nevertheless, it may be
   advisable for a PE to associate each of its local attachment circuits
   with a set of eligible peers, rather than having just a single set of



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   eligible peers associated with the PE as a whole.


12. IANA Considerations

12.1. Channel Type

   The Channel Type code point is defined in [RFC4385], and an IANA
   registry was requested in [RFC5085]. This draft further requests the
   following code point to be assigned to that registry. 0x01 OAM
   Indication For Multi Segment Pseudowires (MH-VCCV)


12.2. L2TPv3 AVP

   This document uses a ne L2TP parameter, IANA already maintains a
   registry of name "Control Message Attribute Value Pair" defined by
   [RFC3438]. The following new values are required:

   TBA-L2TP-AVP-1 - PW Switching Point AVP


12.3. LDP TLV TYPE

   This document uses several new LDP TLV types, IANA already maintains
   a registry of name "TLV TYPE NAME SPACE" defined by RFC3036. The
   following value is suggested for assignment:

      TLV type  Description
       0x096D   Pseudo Wire Switching TLV


12.4. LDP Status Codes

   This document uses several new LDP status codes, IANA already
   maintains a registry of name "STATUS CODE NAME SPACE" defined by
   RFC3036. The following value is suggested for assignment:

      Assignment E Description
      0x0000003A 0 "PW Loop Detected"











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12.5. L2TPv3 Result Codes

   This document uses several new L2TPv3 status codes, IANA already
   maintains a registry of name "L2TPv3 Result Codes" defined by
   RFCxxxx. The following value is suggested for assignment:

      Assignment  Description
          17      "sequencing not supported"


12.6. New IANA Registries

   IANA needs to set up a registry of "PW Switching Point TLV Type".
   These are 8-bit values. Types value 1 through 3 are defined in this
   document. Type values 4 through 64 are to be assigned by IANA using
   the "Expert Review" policy defined in RFC2434. Type values 65 through
   127, 0 and 255 are to be allocated using the IETF consensus policy
   defined in [RFC2434]. Types values 128 through 254 are reserved for
   vendor proprietary extensions and are to be assigned by IANA, using
   the "First Come First Served" policy defined in RFC2434.

   The Type Values are assigned as follows:
   Type  Length    Description

   0x01     4       PW ID of last PW segment traversed
   0x02  variable   PW Switching Point description string
   0x03    4/16     IP address of PW Switching Point
   0x04  variable   MH VCCV Capability Indication
   0x05  variable   AI of last PW segment traversed
   0x06  variable   L2 PW address of PW Switching Point



13. Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this



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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.


14. Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


15. Acknowledgments

   The authors wish to acknowledge the contributions of Wei Luo, Skip
   Booth, Neil Hart, Michael Hua, and Tiberiu Grigoriu.


16. Normative References

   [RFC4385] " Pseudowire Emulation Edge-to-Edge (PWE3)
        Control Word for Use over an MPLS PSN", S. Bryant, et al.,
        RFC4385, February 2006.

   [RFC4446] "IANA Allocations for Pseudowire Edge to Edge
        mulation (PWE3)", L. Martini, RFC4446,  April 2006.

   [RFC4447] "Transport of Layer 2 Frames Over MPLS", Martini, L.,
         et al., rfc4447 April 2006.

   [RFC3985] Stewart Bryant, et al., PWE3 Architecture,
        RFC3985




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   [2547BIS] "BGP/MPLS IP VPNs", Rosen, E, Rekhter, Y.
        draft-ietf-l3vpn-rfc2547bis-03.txt ( work in progress ),
        October 2004.

   [L2TPv3] "Layer Two Tunneling Protocol (Version 3)", J. Lau,
        M. Townsley, I. Goyret, RFC3931

   [RFC5085] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
        Verification (VCCV),   A Control Channel for Pseudowires",
        RFC5085 December 2007.

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
        IANA Considerations section in RFCs", BCP 26, RFC 2434, October
        1998.

   [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5003] C. Metz, L. Martini, F. Balus, J. Sugimoto, "Attachment
        Individual Identifier (AII) Types for Aggregati", RFC5003,
        September 2007.


17. Informative References

   [RFC4023] "Encapsulating MPLS in IP or Generic
        Routing Encapsulation (GRE)", Rosen, E, Rekhter, Y.
        RFC4023, March 2005.

   [PWE3-ARCH] "PWE3 Architecture" Bryant, et al.,
        draft-ietf-pwe3-arch-07.txt ( work in progress ), June 2003.

   [PWE3-FRAG] "PWE3 Fragmentation and Reassembly", A. Malis,
        W. M. Townsley, draft-ietf-pwe3-fragmentation-05.txt
        ( work in progress ) February 2004

   [L2TP-L2VPN] "L2VPN Extensions for L2TP", Luo, Wei,
        draft-ietf-l2tpext-l2vpn-00.txt, ( work in progress ), Jan 2004

   [L2TP-INFOMSG] "L2TP Call Information Messages", Mistretta,
        Goyret, McGill, Townsley, draft-mistretta-l2tp-infomsg-02.txt,
        ( work in progress ), July 2004

   [L2TP-ATM] "ATM Pseudo-Wire Extensions for L2TP", Singh,
        Townsley, Lau, draft-ietf-l2tpext-pwe3-atm-00.txt,
        ( work in progress ), March 2004.





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   [PWE3-ATM] "Encapsulation Methods for Transport of ATM
        Over IP and MPLS Networks", Martini, Rosen, Bocci,
        "draft-ietf-pwe3-atm-encap-05.txt", ( work in progress ),
        April 2004.

   [RFC3438]  W. M. Townsley, "Layer Two Tunneling Protocol
        (L2TP) Internet"

   [PW-MSG-MAP] "Pseudo Wire (PW) OAM Message Mapping", Nadeau et al,
        draft-ietf-pwe3-oam-msg-map-02.txt, ( work in progress ),
        February 2005

   [RFC4379] "Detecting Multi-Protocol Label Switched (MPLS) Data
        Plane Failures", RFC4379, February 2006.

   [RFC3032] "MPLS Label Stack Encoding", RFC3032, January 2001

18. Author Information


   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112
   e-mail: lmartini@cisco.com


   Thomas D. Nadeau
   BT
   BT Centre
   81 Newgate Street
   London,   EC1A 7AJ
   United Kingdom
   e-mail: tom.nadeau@bt.com


   Chris Metz
   Cisco Systems, Inc.
   e-mail: chmetz@cisco.com


   Mike Duckett
   Bellsouth
   Lindbergh Center
   D481
   575 Morosgo Dr
   Atlanta, GA  30324
   e-mail: mduckett@bellsouth.net



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   Matthew Bocci
   Alcatel-Lucent
   Grove House, Waltham Road Rd
   White Waltham, Berks, UK. SL6 3TN
   e-mail: matthew.bocci@alcatel-lucent.co.uk


   Florin Balus
   Alcatel-Lucent
   701 East Middlefield Rd.
   Mountain View, CA 94043
   e-mail: florin.balus@alcatel-lucent.com


   Mustapha Aissaoui
   Alcatel-Lucent
   600, March Road,
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Martini, et al.                                                [Page 41]


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