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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: February 2010                        Cisco Systems Inc.
Intended status: Standards Track
                                                        Thomas D. Nadeau
Matthew Bocci                                                         BT
Florin Balus
Mustapha Aissaoui                                           Mike Duckett
Alcatel-Lucent                                                 Bellsouth

                                                         August 14, 2009


                          Segmented Pseudowire


                  draft-ietf-pwe3-segmented-pw-13.txt

Status of this Memo

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   This Internet-Draft will expire on Expiration Date: February 2010

Abstract

   This document describes how to connect pseudowires (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        MPLS-PW to MPLS-PW Switching  .........................  10
    7.1      Static Control plane switching  .......................  11
    7.2      Two LDP control planes using the same FEC type  .......  11
    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 Switching Point PE 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        MPLS-PW to L2TPv3-PW Control Plane Switching  .........  16
    8.1      Static MPLS and L2TPv3 PWs  ...........................  17
    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  ................................  18
    8.4.2    Adaptation of PW Status message  ......................  18
    8.4.3    Session Tear Down  ....................................  19
    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    Mapping the MPLS Control Word to L2TP  ................  21
    9        Operation And Management  .............................  22
    9.1      Extensions to VCCV to Support MS-PWs  .................  22
    9.2      MPLS-PW to MPLS-PW OAM Data Plane Indication  .........  22
    9.3      Signaling OAM Capabilities for Switched Pseudowires  ..  23
    9.4      OAM Capability for MS-PWs Demultiplexed using MPLS  ...  23
    9.4.1    MS-PW and VCCV CC Type 1  .............................  24
    9.4.2    MS-PW and VCCV CC type 2  .............................  24
    9.4.3    MS-PW and VCCV CC type 3  .............................  24
    9.5      MS-PW VCCV Operations  ................................  24
    9.5.1    VCCV Echo Message Processing  .........................  25
    9.5.1.1  Sending a VCCV Echo Request  ..........................  26
    9.5.1.2  Receiving a VCCV Echo Request  ........................  26
    9.5.1.3  Receiving a VCCV Echo Reply  ..........................  27
    9.5.2    Detailed VCCV procedures  .............................  27
    9.5.2.1  End to End Connectivity Verification Between T-PEs  ...  27
    9.5.2.2  Partial Connectivity Verification from T-PE  ..........  28



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    9.5.2.3  Partial connectivity verification between S-PEs  ......  28
    9.5.2.4  MS-PW Path Verification  ..............................  29
    9.5.2.5  MS-PW Path Trace  .....................................  30
   10        Mapping Switched Pseudowire Status  ...................  31
   10.1      S-PE initiated PW status messages  ....................  32
   10.1.1    Local PW2 transmit direction fault  ...................  33
   10.1.2    Local PW1 transmit direction fault  ...................  34
   10.1.3    Local PW2 receive direction fault  ....................  34
   10.1.4    Local PW1 receive direction fault  ....................  34
   10.1.5    Clearing Faults  ......................................  34
   10.2      PW status messages and S-PE TLV processing  ...........  35
   10.3      T-PE processing of PW status messages  ................  35
   10.4      Pseudowire Status Negotiation Procedures  .............  35
   10.5      Status Dampening  .....................................  35
   11        Peering Between Autonomous Systems  ...................  35
   12        Security Considerations  ..............................  36
   12.1      Data Plane Security  ..................................  36
   12.1.1    VCCV Security considerations  .........................  36
   12.2      Control Protocol Security  ............................  36
   13        IANA Considerations  ..................................  37
   13.1      L2TPv3 AVP  ...........................................  37
   13.2      LDP TLV TYPE  .........................................  38
   13.3      LDP Status Codes  .....................................  38
   13.4      L2TPv3 Result Codes  ..................................  38
   13.5      New IANA Registries  ..................................  38
   14        Normative References  .................................  39
   15        Informative References  ...............................  40
   16        Author's Addresses  ...................................  41





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 Pseudowire (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 Pseudowire (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
       traverses one PSN tunnel in each direction 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.


3. Introduction

   The PWE3 Architecture [RFC3985], defines the signaling and
   encapsulation techniques for establishing SS-PWs between a pair of
   terminating PEs and in the vast majority of cases this will be
   sufficient. MS-PWs are most useful in two general cases:

        -i. When it is not possible, desirable or feasible to establish
            a PW control channel between the terminating source and
            destination PEs. At a minimum PW control channel
            establishment requires knowledge of and reachability to the
            remote (terminating) PE IP address. The local (terminating)
            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
            terminating PEs reside in different provider networks (ASes)
            and it is the practice to cryptogtaphiclly sign all control
            traffic exchanged between two networks.  Technically a SS-PW
            could be used but this would require tp cryptogtaphiclly
            sign on ALL terminating source and destination PE nodes. An



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            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 terminating 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 terminating 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
            terminating PE-A and terminating 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 terminating 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 terminating
            PEs in this scenario.

   A more detailed discussion of the requirements pertining to MS-PWs
   can be found in [RFC5254].

   There are four different signaling protocols that are defined to
   signal PWs:
        -i. Static configuration of the PW (MPLS or L2TPv3).
       -ii. LDP using FEC 128






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      -iii. LDP using the generalized FEC 129
       -iv. L2TPv3


4. General Description

   A pseudowire (PW) is a mechanism that carries the essential elements
   of an emulated service from one PE to one or more other PEs over a
   PSN as described in Figure 1 and in [RFC3985]. Many providers have
   deployed PWs as a means of migrating existing (or building new) L2VPN
   services (e.g.. Frame Relay, ATM, or Ethernet) on to a PSN.

   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 [RFC4364]. 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 ---------------->|
         |                                                  |
         |          |<-------- Pseudowire ------>|          |
         |          |                            |          |
         |          |    |<-- 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
   terminate on different PEs.



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                |<-------Multi-Segment Pseudowire------->|
                |      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, pseudowires 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  |<------Multi-Segment Pseudowire------>|  Native
       Service |         PSN              PSN         |  Service
        (AC)   |     |<-Tunnel->|     |<-Tunnel->|    |   (AC)
          |    V     V     1    V     V    2     V    V     |
          |    +----+           +-----+          +----+     |
   +----+ |    |TPE1|===========|SPE1 |==========|TPE2|     | +----+
   |    |------|..... PW.Seg't1....X....PW.Seg't3.....|-------|    |
   | CE1| |    |    |           |     |          |    |     | |CE2 |
   |    |------|..... PW.Seg't2....X....PW.Seg't4.....|-------|    |
   +----+ |    |    |===========|     |==========|    |     | +----+
        ^      +----+           +-----+          +----+       ^
        |   Provider Edge 1        ^        Provider Edge 2   |
        |                          |                          |
        |                          |                          |
        |                  PW switching point                 |
        |                                                     |
        |<------------------ Emulated Service --------------->|

         Figure 3: PW Control Plane Switching Reference Model

   In Figure 3 SPE1 runs two separate control planes: one toward TPE1,



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   and one Toward TPE2. The PW switching point (S-PE) is configured to
   connect PW Segment 1 and PW Segement 3 together to complete the
   multi-hop PW between TPE1 and TPE2. PW Segment 1 and PW Segment 3
   MUST be of the same PW type, but PSN Tunnel 1 and PSN Tunnel 2 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 PSN Tunnel
   1 and PSN Tunnel 2 are the same technology the PW PDU does not need
   to be modified, and PDUs are then switched between the pseudowires 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 S-PE, 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 PE1.
   Other documents may build on this base specification to automate the
   configuration and selection of S-PE1. 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 S-PEs
   the PW has traversed (though this may be reported for troubleshooting
   purposes).

   The general approach taken for MS-PWs is to connect the individual
   control planes by passing along any signaling information immediately
   upon reception. First the S-PE is configured to switch a PW segment
   from a specific peer to another PW segment destined for a different
   peer. No control messages are exchanged yet as the S-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 MAY be 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 by
   the S-PE, and immediately used to form the PW setup messages for the
   next SS-PW of the MS-PW. If one of the S-PEs doesn't accept an LDP
   Label Mapping message then a Label Release message may be sent back
   to the originator T-PE depending on the cause of the error. LDP
   liberal label retention mode still applies, hence if a PE is simply
   not configured yet , the label mapping is stored for future use. A
   MS-PW is declared UP only 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 PW domain. For example, in Figure 2 for
   the 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 pseudowire, 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

   The general applicability of MS-PWs and their relationship to L2VPNs
   is described in [MS-PW-ARCH]. The applicability of a PW type, as
   specified in the relevant RFC for that encapsulation (e.g. [RFC4717]
   for ATM), applies to each segment. This section describes further
   applicability considerations.

   As with SS-PWs, the performance of any segment will be limited by the
   performance of the underlying PSN. The performance may be further
   degraded by the emulation process, and performance degradation may be
   further degraded by traversing multiple PW segments. Furthermore, the
   overall performance of an MS-PW is no better than the worst
   performing segment of that MS-PW.

   Since different PSN types may be able to achieve different maximum
   performance objectives, it is necessary to carefully consider which
   PSN types are used along the path of a MS-PW.


7. MPLS-PW to MPLS-PW Switching

   Referencing Figure 3, T-PE1 set up PW segment 1 using the LDP
   targeted session as described in [RFC4447], at the same time a
   separate pseudowire PW segment 3 is setup to T-PE2. Each PW is
   configured independently on the PEs, but on S-PE1 pseudowire PW
   segment 1 is connected to pseudowire PW segment 3. PDUs are then
   switched between the pseudowires at the PW label level.  Hence the
   data plane does not need any special knowledge of the specific
   pseudowire type. A simple standard MPLS label swap operation is
   sufficient to connect the two PWs, and in this case the PW adaptation
   function cannot be used. However when pushing a new PSN label the TTL
   SHOULD be set to 255, or some other locally configured fixed value.

   This process can be repeated as many times as necessary, the only
   limitation to the number of S-PEs traversed is imposed by the TTL
   field of the PW MPLS Label. The setting of the TTL of the PW MPLS



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   label is a matter of local policy on the originating PE, but SHOULD
   be set to 255. However if the PW PDU contains an OAM packet then the
   TTL can be set to the required value as explained later in this
   document.

   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 S-PE 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 appropriate PW status if it detects a failure in
   sending or receiving packets over the static PW segment.  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
   S-PE PE behaves in a very similar manner to a T-PE, assuming an
   active signaling role. This means that the S-PE will immediately send
   the LDP Label Mapping message if the static PW is deemed to be UP.


7.2. Two LDP control planes using the same FEC type

   The S-PE SHOULD assume an initial passive role. This means that when
   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 Pseudowire Identifier (PWID) , as defined in [RFC4447] 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



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   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 S-
   PEs. 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
   larger, 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. In this case one of the T-
   PEs MUST be set to active role, and the other one MUST be set to
   passive role.


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 S-PE
   PE, will always remain passive until a PWID FEC 128 LDP message is
   received, which will cause the corresponding 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 S-PE configuration, or by
   some other means, such as some form of auto discovery. Such other
   means are outside the scope of this document.





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7.4. LDP Switching Point PE 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.

   Sending the PW switching Point TLV (S-PE TLV) is OPTIONAL, however
   the PE or S-PE MUST process the TLV upon reception. The "U" bit MUST
   be set for backward compatibility with T-PEs that do not support the
   MS-PW extensions described in the document. The S-PE TLV MAY appear
   only once for each switching point traversed. The S-PE TLV is
   appended to the PW FEC at each switching point, and the order of the
   S-PE TLVs in the LDP message MUST be preserved. The S-PE TLV MUST be
   sent if VCCV operation is required beyond the first MS-PW segment
   from a T-PE.

   The S-PE TLV 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|     S-PE TLV   (0x096D)   |     S-PE TLV   Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Variable Length Value                   |
   |                           "      "      "                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [note] LDP TLV type is pending IANA approval.

     - S-PE TLV Length

       Specifies the total length of all the following S-PE TLV fields
       in octets

     - Type

       Encodes how the Value field is to be interpreted.

     - Length

       Specifies the length of the Value field in octets.






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     - 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
   process defined in the "IANA Allocations" section below.

   For local policy reasons, a particular S-PE can filter out all S-PE
   TLVs in a label mapping message that traverses it and not include
   it's own S-PE TLV.  In this case, from any upstream PE, it will
   appear as if this particular S-PE is the T-PE. This might be
   necessary , depending on local policy if the S-PE is at the service
   provider administrative boundary. It should also be noted that
   because there are no S-PE TLVs describing the path beyond the S-PE
   that removed them, VCCV will only work as far as that S-PE .


7.4.1. PW Switching Point Sub-TLVs

   The S-PE TLV contains sub-TLVs that describe various characteristics
   of the S-PE traversed. Below are the definitions of PW Switching
   Point Sub-TLVs defined in this document:

     - PW ID of last PW segment traversed.

       This is only applicable if the last PW segment traversed used LDP
       FEC 128 to signal the PW. This sub-TLV type contains a PW ID in
       the format of the PWID described in [RFC4447]. This is just a 32
       bit unsigned integer number.

     - PW Switching Point description string.

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

     - Local IP address of PW Switching Point.

       The Local IP V4 or V6 address of the PW Switching Point. This is
       an OPTIONAL Sub-TLV. In most cases this will be the local LDP
       session IP address of the S-PE.

     - Remote IP address of the last PW Switching Point traversed or of
       the T-PE

       The IP V4 or V6 address of the last PW Switching Point traversed
       or of the T-PE. This is an OPTIONAL Sub-TLV. In most cases this
       will be the remote IP address of the LDP session. This Sub-TLV
       SHOULD only be included if there are no other S-PE TLV present



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       from other S-PEs, or if the remote ip address of the LDP session
       does not correspond to the "Local IP address of PW Switching
       Point" TLV value contained in the last S-PE TLV.

     - The FEC element of last PW segment traversed.

       This is only applicable if the last PW segment traversed used LDP
       FEC 129 to signal the PW.

       The FEC element of the last PW segment traversed. This is encoded
       in the following format:

        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 Section 3.2 of [RFC5003]. This includes the
   AII type field, and length, as well as the L2 PW address with the AC
   ID field set to zero.


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 S-PE. However the
   following interface parameters MAY be modified as follows:




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     - 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 [RFC4623].

     - 0x0C VCCV parameter This Parameter contains the CC type , and CV
       type bit fields. The CV type bit field MUST be reset to reflect
       the CV type supported by the S-PE. CC type bit field MUST have
       bit 1 "Type 2: MPLS Router Alert Label " set to 0.  The other bit
       fields MUST be reset to reflect the CC type supported by the S-
       PE.


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 S-PE PE.


7.6. PW Loop Detection

   A switching point PE SHOULD inspect the PW switching Point TLV, to
   verify that it's  own IP address does not appears in it. If the PE'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. MPLS-PW to L2TPv3-PW 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.






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       -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 S-PE 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 PW 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.


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 appropriate 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 S-PE 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 appropriate 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 S-PE 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



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   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 S-PE 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 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].





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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.

   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 [RFC4667]

     * 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
       [RFC4454].




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     * 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].

     * 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 [RFC4667] defines how to encode the SAI and TAI
       parameters. These can be mapped directly.

   Other interface parameter mappings 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 S-PE, however this is
   outside the scope of this document.

   Section 5.4 of [RFC3985] discusses the purpose of the various shim



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   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.

   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.


8.7.1. Mapping the MPLS Control Word to L2TP

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

   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|  Reserved |   Length  |     Sequence Number           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


        -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 [RFC4717]. These map directly to the bits defined
            in [RFC4454]. 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.



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       -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 appropriate 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
            IANA Assignment).


9. Operation And Management

9.1. Extensions to VCCV to Support MS-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 interface parameter sub-
   TLV 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. MPLS-PW to MPLS-PW OAM Data Plane Indication

   As stated above the S-PE MUST perform a standard MPLS label swap
   operation on the MPLS PW label. 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 controlling 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 Pseudowires

   Similarly to 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].

   In Figure 3 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
   supports CC Types 1,2,3 and the Then the S-PE removes the Router
   Alert CC Type=2, leaving the rest of the TLV 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 only
   the CC types that are supported by the initial T-PE (T-PE1) and all
   S-PEs along the PW path.


9.4. OAM Capability for MS-PWs Demultiplexed using MPLS

   The 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:

      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).



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9.4.1. MS-PW and VCCV CC Type 1

   VCCV CC type 1 can be used for MS-PWs. However, if the CW is enabled
   on user packets, VCCV CC type 1 MUST be used according to the rules
   in [RFC5085].  When using CC type 1 for MS-PWs the PE transmitting
   the VCCV packet MUST set the TTL to the appropriate value to reach
   the destination S-PE. However if the packet is destined for the T-PE,
   the TTL can be set to any value that is sufficient for the packet to
   reach the T-PE.


9.4.2. MS-PW and VCCV CC type 2

   VCCV CC type 2 is not supported for MS-PWs and MUST be removed from a
   VCCV parameter field by the S-PE.


9.4.3. MS-PW and VCCV CC type 3

   VCCV CC type 3 can be used for MS-PWs, however if the CW is enabled
   VCCV type 1 is preferred according to the rules in [RFC5085]. Note
   that for using the VCCV type 3, TTL method, the PE will set the PW
   label TTL to the appropriate value necessary to reach the target PE,
   otherwise the VCCV packet might be forwarded over the AC to the CPE.


9.5. MS-PW VCCV Operations

   This document specifies four VCCV operations:
        -i. End-to-end MS-PW connectivity verification. This operation
            enables the connectivity of the MS-PW to be tested from
            source T-PE to destination T-PE.  In order to do this, the
            sending T-PE must include the FEC used in the last segment
            of the MS-PW to the destination T-PE in the VCCV-Ping echo
            request.  This information is either configured at the
            sending T-PE or is obtained by processing the corresponding
            sub-TLVs of the optional S-PE TLV, as described below.
       -ii. Partial MS-PW connectivity verification. This operation
            enables the connectivity of any contiguous subset of the
            segments of a MS-PW to be tested from the source T-PE or a
            source S-PE to a destination S-PE or T-PE.  Again, the FEC
            used on the last segment to be tested must be included in
            the VCCV-Ping echo request message. This information is
            determined by the sending T-PE or S-PE as in (i) above.
      -iii. MS-PW path verification. This operation verifies the path of
            the MS-PW, as returned by the S-PE TLV, against the actual
            data path of the MS-PW. The sending T-PE or S-PE iteratively
            sends a VCCV echo request to each S-PE along the MS-PW path,



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            using the FEC for the corresponding MS-PW segment in the
            switching point TLV. If the S-PE TLV information is correct,
            then a VCCV echo reply showing that this is a valid router
            for the FEC will be received. However, if the switching
            point TLV information is incorrect, then this operation
            enables the first incorrect switching point to be
            determined, but not the actual path of the MS-PW beyond
            that. This operation cannot be used when the MS-PW is
            statically configured or when the S-PE TLV is not suported.
            The processing of the PW switching TLV used for this
            operation is described below. This operation is OPTIONAL.
       -iv. MS-PW path trace. This operation traces the data path of the
            MS-PW using FECs included in the Target FEC stack TLV
            [RFC4379] returned by S-PEs or T-PEs in an echo reply
            message. The sending T-PE or S-PE uses this information to
            recursively test each S-PE along the path of the MS-PW in a
            single operation in a similar manner to LSP trace. This
            operation is able to determine the actual data path of the
            MS-PW, and can be used for both statically configured and
            signaled MS-PWs. Support for this operation is OPTIONAL.

   Note that the above operations rely on intermediate S-PEs and/or the
   destination T-PE to include the switching point TLV as a part of the
   MS-PW setup process, or to include the Target FEC stack TLV in the
   VCCV echo reply message. For various reasons, e.g. privacy or
   security of the S-PE/T-PE, this information may not be available to
   the source T-PE. In these cases, manual configuration of the FEC MAY
   still be used.


9.5.1. VCCV Echo Message Processing

   The challenge for the control plane is to be able to build the VCCV
   echo request packet with the necessary information to reach the
   desired S-PE or T-PE, for example the target FEC 128 PW sub-TLV of
   the downstream PW segment that 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 3, T-PE1 has the FEC128 of the segment, PW
   segment 1, but it does not readily have the information required to
   compose the FEC128 of the following segment, PW segment 3, if a VCCV
   echo request to be sent to T-PE2. This can be achieved by the methods
   described in the following subsections.







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9.5.1.1. Sending a VCCV Echo Request

   When performing a partial or end-to-end connectivity or path
   verification, 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.

   The necessary S-PE sub-TLVs are :

   Type Description
   0x01 PW ID of last PW segment traversed
   0x03 Local IP address of PW Switching Point
   0x04 Remote IP address of last PW Switching Point traversed or
        of the T-PE


   When performing an OPTIONAL MS-PW path trace operation, the T-PE will
   automatically learn the target FEC by probing, one by one, the hops
   of the MS-PW path, using the FEC returned in the Target FEC stack of
   the previous VCCV echo reply.


9.5.1.2. Receiving a 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 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.

   S-PEs that wish to reveal their downstream next-hop in a trace
   operation should include the FEC of the downstream PW segment in the
   Target FEC stack (as per Sections 3.2 and 4.5 of [RFC4379]) of the
   echo reply message. FEC128 PWs MUST use the format shown in Section
   3.2.09 of [RFC4379] for the sub-TLV in the Target FEC stack, while
   FEC129 PWs MUST use the format shown in Section 3.2.10 of [RFC4379]
   for the sub-TLV in the Target FEC stack. Note that an S-PE MUST NOT
   include this FEC information in the reply if it has been configured
   not to do so for administrative reasons, or for reasons explained
   previously.

   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).






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9.5.1.3. Receiving a VCCV Echo Reply

   The operation to be taken by the node receiving the echo reply in
   response to an echo request depends on the VCCV mode of operation
   described above.  See Section 9.5.2 for detailed procedures.


9.5.2. Detailed VCCV procedures

9.5.2.1. End to End Connectivity Verification Between T-PEs

   In Figure 3, if T-PE1, S-PE and T-PE2 support Control Word , the PW
   control plane will automatically negotiate the use of the CW. VCCV CC
   type 3 will function correctly whether the CW is enable or not on the
   PW. However VCCV type 1 for (which can be use for end to end
   verification only), is only supported if the CW is enabled.

   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 the exact value to allow
            the packet to reach the far end T-PE. ( the value is
            determined by counting the number of S-PEs from the control
            plane information ) Alternatively, if CC type 1 is supported
            the packet can be encapsulated according to CC type 1 in
            [RFC5085]

      -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 may performs an outer label pop, if PHP is disabled,
            and will perform 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.







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


9.5.2.2. Partial Connectivity Verification 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 either checking the control word (CW)
   , or if the CW is not in use by checking for a valid IP header with
   UDP destination port 3503.  The packet should then be diverted to
   VCCV processing.

   In Figure 3, 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 pseudowire.

   The VCCV packet is built according to [RFC4379] section 3.2.9 for FEC
   128, or 3.2.10 for a FEC 129 PW. All the information necessary to
   build the VCCV LSP ping packet is collected by inspecting the S-PE
   TLVs.

   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.


9.5.2.3. Partial connectivity verification between S-PEs

   Assuming that all nodes along an MS-PW support the Control Word CC
   Type 3, VCCV between S-PEs may be accomplished using the PW label TTL
   as described above. In Figure 3, 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.  All the information needed to build the proper VCCV echo
   request packet as described in [RFC4379] section 3.2.9 or 3.2.10 is
   obtained automatically from the LDP label mapping that contains S-PE
   TLVs.








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9.5.2.4. MS-PW Path Verification

   As an example, in Figure 3, 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 FEC
            containing the pseudowire 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 sub-TLV such as LDP Prefix and/or RSVP LSP
            dependent on the type of transport tunnel the segmented PW
            is riding on.

       -ii. S-PE validates the echo request with the FEC. Since it is a
            switching point between the first and second segment it
            builds an echo reply with a return code of 8 and sends the
            echo reply back to T-PE1.

      -iii. T-PE1 builds a second VCCV echo request based on the
            infomation obtained from the control plane (S-PE TLV). It
            then increments the TTL and sends it 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 FEC.
            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).

        -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 has a return code of is 3. The trace process is
            completed.

   If no echo reply is received, or an error code is received from a
   particular PE, the trace process MUST stop immediately, and packets
   MUST NOT be sent further along the MS-PW.

   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
   (PW) label TTL.









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9.5.2.5. MS-PW Path Trace

   As an example, in Figure 3, VCCV trace can be performed on the MS-PW
   originating from T-PE1 by a single operational command. The following
   OPTIONAL process ensues:

        -i. T-PE1 sends a VCCV echo request with TTL set to 1 and a FEC
            containing the pseudowire 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 sub-TLV such as LDP Prefix and/or RSVP LSP
            dependent on the type of transport tunnel the segmented PW
            is riding on.

       -ii. The S-PE validates the echo request with the FEC.

      -iii. The S-PE builds an echo reply with a return code of 8 and
            sends the echo reply back to T-PE1, appending the FEC128
            information for the next segment along the MS-PW to the VCCV
            echo reply packet using the Target FEC stack TLV (as per
            Sections 3.2 and 4.5 of [RFC4379]).

       -iv. T-PE1 builds a second VCCV echo request based on the
            infomation obtained from the FEC stack TLV received in the
            previous VCCV echo reply. It then increments the TTL and
            sends it 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.

        -v. T-PE2 receives and validates the echo request with the FEC.
            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).

       -vi. 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 has a return code of is 3. The trace process is
            completed.

   If no echo reply is received, or an error code is received from a
   particular PE, the trace process MUST stop immediately, and packets
   MUST NOT be sent further along the MS-PW.

   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
   (PW) label TTL.




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10. Mapping Switched Pseudowire Status

   In the PW switching with attachment circuits case (Figure 2), PW
   status messages indicating PW or attachment circuit faults MUST be
   mapped to fault indications or OAM messages on the connecting AC as
   defined in [PW-MSG-MAP].

   In the PW control plane switching case (Figure 3), there is no
   attachment circuit at the S-PE, 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 fault status of
   one of the locally attached PW segments at a S-PE. 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:

    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|     S-PE TLV   (0x096D)   |     S-PE TLV   Length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   Only one S-PE TLV can be present in this message. This message is
   then relayed by each S-PE unchanged. The T-PE decodes the status
   message and the included S-PE TLV to detect exactly where the fault
   occurred. At the T-PE if there is no S-PE 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 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 PWid in the
            case of a FEC 128 TLV is set to the value of the PW segment
            of the next hop.

       -ii. When the local status for any of the PW segments is at
            fault, the S-PE always sends the local status bits
            regardless if the received status bits from the remote node
            indicated that an upstream fault has cleared. AC status bit
            are passed along unchanged.


10.1. S-PE initiated PW status messages

   The PW fault directions are defined as follows:

                         +-------+
      ---PW1 receive---->|       |-----PW2 Transmit---->
   S-PE1                 | S-PE2 |                   S-PE3
      <--PW1 Transmit----|       |<----PW2 Receive------
                         +-------+
                Figure 4. S-PE and PW tx/rx directions.


   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 PW
   segment: 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



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   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 PW segment.

   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 S-
   PE are signalled on both PW segments.

   A T-PE can receive multiple failures from S-PEs along the MS-PW,
   however only the failure from the remote closest S-PE will be stored
   (last pw status message received). 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 PW segments at a particular S-PE for a
   particular MS-PW, referring to figure 4, there are four possible
   failure cases as follows:

        -i. PW2 Transmit direction fault
       -ii. PW1 Transmit direction fault
      -iii. PW2 Receive direction fault
       -iv. PW1 Receive direction fault

   Once a PW status notification message is initiated at a S-PE for a
   particular 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 S-PE original status error state is
   cleared.

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


10.1.1. Local PW2 transmit 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 PW segment
       toward S-PE3.






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10.1.2. Local PW1 transmit 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 PW segment
       toward S-PE1.


10.1.3. Local PW2 receive 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"
     * 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 PW segment
       toward S-PE3.


10.1.4. Local PW1 receive 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 PW segment
       toward S-PE1.


10.1.5. 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.

   When a PW status fault clearing message is forwarded, the S-PE will
   always send the S-PE TLV associated with the PE which cleared the
   fault.




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10.2. 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. The S-PE TLV
   stored is always the S-PE TLV that is associated with the PE that set
   that particular last fault. 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.


10.3. 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 PW segment. However T-PE
   participating a MS-PW, SHOULD be able to process the S-PE TLV.
   Otherwise the processing of PW status messages , and other PW setup
   messages is exactly as described in [RFC4447].


10.4. Pseudowire Status Negotiation Procedures

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


10.5. 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.


11. 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.







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12. Security Considerations

   This document specifies the LDP, L2TPv3, and VCCV 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 discuss the security considerations for both the data plane and
   the control plane in the following sections.


12.1. Data Plane Security

   Data plane security consideration as discussed in [RFC4447],
   [L2TPv3], and [RFC3985] apply to this extension without any changes.


12.1.1. VCCV Security considerations

   The VCCV technology for MS-PW offers a method for the service
   provider to verify the data path of a specific PW. This involves
   sending a packet to a specific PE and receiving an answer which
   either confirms , or indicates that the information contained in the
   packet is incorrect. This is a very similar process to the commonly
   used IP ICMP ping , and TTL expired methods for IP networks. It
   should be noted that when using VCCV Type 3 for PW when the CW is not
   enabled, if a packet is crafted with a TTL greater then the number of
   hops along the MS-PW path, or an S-PE along the path mis-processes
   the TTL, the packet could mistakenly be forwarded out the attachment
   circuit as a native PW packet. This packet would most likely be
   treated as an error packet by the CE. However if this possibility is
   not acceptable, the CW should be enabled to guarantee that a VCCV
   packet will never be mistakenly forwarded to the AC.


12.2. 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



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   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. (Note that 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 authentication option, as
   described in section 2.9 of [RFC5036], 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 important.
   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
   eligible peers associated with the PE as a whole.


13. IANA Considerations

13.1. L2TPv3 AVP

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

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





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13.2. LDP TLV TYPE

   This document uses a new LDP TLV types, IANA already maintains a
   registry of name "TLV TYPE NAME SPACE" defined by RFC5036. The
   following value is suggested for assignment:

      TLV type  Description
       0x096D   Pseudowire Switching Point PE TLV


13.3. LDP Status Codes

   This document uses a 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"


13.4. L2TPv3 Result Codes

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

      Assignment  Description
          TBD     "sequencing not supported"


13.5. New IANA Registries

   IANA needs to set up a registry of "Pseudowire Switching Point PE TLV
   Type".  These are 8-bit values. Types value 1 through 6 are defined
   in this document. Type values 7 through 64 are to be assigned by IANA
   using the "Expert Review" policy defined in RFC5226. Type values 65
   through 127, 0 and 255 are to be allocated using the IETF consensus
   policy defined in [RFC5226]. 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 RFC5226.

   The Type Values are assigned as follows:









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   Type  Length   Description

   0x01     4     PW ID of last PW segment traversed
   0x02  variable PW Switching Point description string
   0x03    4/16   Local IP address of PW Switching Point
   0x04    4/16   Remote IP address of last PW Switching Point traversed
                  or of the T-PE
   0x05  variable FEC Element of last PW segment traversed
   0x06     10    L2 PW address of PW Switching Point



14. 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] "Pseudowire Setup and Maintenance Using the
        Label Distribution Protocol (LDP)", Martini, L.,
         et al., rfc4447 April 2006.

   [RFC4364] "BGP/MPLS IP Virtual Private Networks (VPNs)", Rosen, E,
        Rekhter, Y., RFC4364, February 2006
        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.

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
        IANA Considerations section in RFCs", BCP 26, RFC 5226, May 2008

   [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 Aggregation", RFC5003,
        September 2007.

   [RFC4379] K. Kompella, G. Swallow, "Detecting Multi-Protocol
        Label Switched (MPLS) Data Plane Failures", RFC4379,



Martini, et al.                                                [Page 39]

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        September 2007

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
        Specification", RFC 5036, October 2007.


15. Informative References

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

   [RFC3985] "Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture"
        Bryant, et al., RFC 3985, March 2005.

   [RFC4623] "Pseudowire Emulation Edge-to-Edge (PWE3) Fragmentation
        and Reassembly", A. Malis, W. M. Townsley, RFC 4623, August 2006

   [RFC4667] "Layer 2 Virtual Private Network (L2VPN)
        Extensions for Layer 2 Tunneling Protocol (L2TP)", Luo, Wei,
        RFC4667, W. Luo, September 2006

   [L2TP-INFOMSG] "L2TP Call Information Messages", Mistretta,
        Goyret, McGill, Townsley, draft-mistretta-l2tp-infomsg-01.txt,
        ( work in progress ), July 2004

   [RFC4454] "Asynchronous Transfer Mode (ATM) over Layer 2
        Tunneling Protocol Version 3 (L2TPv3)", Singh, Townsley,
        Pignataro, RFC4454, May 2006
        ( work in progress ), March 2004.

   [RFC4717] "Encapsulation Methods for Transport of (ATM)
        MPLS Networks", Martini et al., RFC4717, December 2006

   [RFC3438] W. M. Townsley, "Layer Two Tunneling Protocol
        (L2TP) Internet Assigned Numbers Authority (IANA)
        Considerations Update", December 2002, RFC3438

   [PW-MSG-MAP] "Pseudo Wire (PW) OAM Message Mapping", Nadeau et al,
        draft-ietf-pwe3-oam-msg-map-10.txt, ( work in progress ),
        April 2009

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

   [MS-PW-ARCH] "An Architecture for Multi-Segment Pseudo Wire Emulation
        Edge-to-Edge", Bocci et al, draft-ietf-pwe3-ms-pw-arch-06.txt
        February 2009




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Internet Draft    draft-ietf-pwe3-segmented-pw-13.txt    August 14, 2009


   [RFC5254] "Requirements for Multi-Segment Pseudowire Emulation
        Edge-to-Edge (PWE3)", N. Bitar, M. Bocci, L. Martini, RFC5254,
        October 2008


16. Author's Addresses


   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


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








Martini, et al.                                                [Page 41]

Internet Draft    draft-ietf-pwe3-segmented-pw-13.txt    August 14, 2009



   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,
   Kanata, ON, Canada
   e-mail: mustapha.aissaoui@alcatel-lucent.com



Full Copyright Statement

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   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008. The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


Acknowledgments

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

   Expiration Date: February 2010




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