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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 4447

Network Working Group                                       Luca Martini
Internet Draft                                             Eric C. Rosen
Expiration Date: March 2005                          Cisco Systems, Inc.

Nasser El-Aawar                                               Toby Smith
Level 3 Communications, LLC.                       Laurel Networks, Inc.

Giles Heron
Tellabs
                                                          September 2004


               Pseudowire Setup and Maintenance using LDP


                draft-ietf-pwe3-control-protocol-09.txt

Status of this Memo

   By submitting this Internet-Draft, we certify that any applicable
   patent or other IPR claims of which we are aware have been disclosed,
   or will be disclosed, and any of which we become aware will be
   disclosed, in accordance with RFC 3668.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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Abstract

   Layer 2 services (such as Frame Relay, ATM, ethernet) can be
   "emulated" over an IP and/or MPLS backbone by encapsulating the layer
   2 PDUs and then transmitting them over "pseudowires". It is also
   possible to use pseudowires to provide low-rate TDM and SONET circuit



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   emulation over an IP and/or MPLS network. This document specifies a
   protocol for establishing and maintaining the pseudowires, using
   extensions to LDP. Procedures for encapsulating layer 2 PDUs are
   specified in a set of companion documents.















































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Table of Contents

    1      Specification of Requirements  ..........................   4
    2      Intellectual Property Statement  ........................   4
    3      Introduction  ...........................................   5
    4      The Pseudowire Label  ...................................   7
    5      Details Specific to Particular Emulated Services  .......   8
    5.1    Frame Relay  ............................................   8
    5.2    ATM  ....................................................   9
    5.2.1  ATM AAL5 SDU VCC Transport  .............................   9
    5.2.2  ATM Transparent Cell Transport  .........................   9
    5.2.3  ATM n-to-one VCC and VPC Cell Transport  ................   9
    5.2.4  OAM Cell Support  .......................................   9
    5.2.5  ILMI Support  ...........................................  11
    5.2.6  ATM AAL5 PDU VCC Transport  .............................  11
    5.2.7  ATM one-to-one VCC and VPC Cell Transport  ..............  11
    5.3    Ethernet Tagged Mode  ...................................  11
    5.4    Ethernet  ...............................................  12
    5.5    HDLC and PPP  ...........................................  12
    5.6    IP Layer2 Transport  ....................................  12
    6      LDP  ....................................................  12
    6.1    The PWid FEC Element  ...................................  13
    6.2    The Generalized PW ID FEC Element  ......................  14
    6.2.1  Attachment Identifiers  .................................  15
    6.2.2  Encoding the Generalized ID FEC Element  ................  16
    6.2.3  Signaling Procedures  ...................................  19
    6.3    Signaling of Pseudo Wire Status  ........................  20
    6.3.1  Use of Label Mappings.  .................................  20
    6.3.2  Signaling PW status.  ...................................  20
    6.3.3  Pseudowire Status Negotiation Procedures  ...............  22
    6.4    Interface Parameters Field  .............................  24
    7      Control Word  ...........................................  26
    7.1    PW types for which the control word is REQUIRED  ........  26
    7.2    PW types for which the control word is NOT mandatory  ...  26
    7.3    LDP label Withdrawal procedures  ........................  28
    7.4    Sequencing Considerations  ..............................  28
    7.4.1  Label Mapping Advertisements  ...........................  28
    7.4.2  Label Mapping Release  ..................................  29
    8      IANA Considerations  ....................................  29
    8.1    FEC Type Name Space  ....................................  29
    8.2    Pseudowire Type  ........................................  29
    8.3    Interface Parameters  ...................................  30
    8.4    Status codes  ...........................................  30
    8.5    Pseudowire Status Codes  ................................  30
    9      Security Considerations  ................................  31
    9.1    Data-plane Security  ....................................  31



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    9.2    Control Protocol Security  ..............................  32
   10      Acknowledgments  ........................................  33
   11      Normative References  ...................................  33
   12      Informative References  .................................  34
   13      Author Information  .....................................  34
   14      Additional Contributing Authors  ........................  35
   15      Full Copyright Statement  ...............................  38
   16      Appendix A - C-bit Handling Procedures Diagram  .........  38




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 RFC 2119.


2. 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
   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.









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3. Introduction

   In [FRAME], [ATM], and [ETH] it is explained how to encapsulate a
   layer 2 Protocol Data Unit (PDU) for transmission over an IP and/or
   MPLS network. Those documents specify that a "pseudowire header",
   consisting of a demultiplexor field, will be prepended to the
   encapsulated PDU. The pseudowire demultiplexor field is put on before
   transmitting a packet on a pseudowire.  When the packet arrives at
   the remote endpoint of the pseudowire, the demultiplexor is what
   enables the receiver to identify the particular pseudowire on which
   the packet has arrived. To actually transmit the packet from one
   pseudowire endpoint to another, the packet may need to travel through
   a "PSN tunnel"; this will require an additional header to be
   prepended to the packet.

   Accompanying documents [CEP, SAToP, CESoPSN] describe methods for
   transporting time division multiplexed (TDM) digital signals (TDM
   circuit emulation) over a packet-oriented MPLS network. The
   transmission system for circuit-oriented TDM signals is the
   Synchronous Optical Network (SONET)[SDH]/Synchronous Digital
   Hierarchy (SDH) [ITUG]. To support TDM traffic, which includes voice,
   data, and private leased line service, the pseudowires must emulate
   the circuit characteristics of SONET/SDH payloads. The TDM signals
   and payloads are encapsulated for transmission over pseudowires. To
   this encapsulation is prepended a pseudowire demultiplexor and a PSN
   tunnel header.

   [SAToP], and [CESoPSN] describe methods for transporting low-rate
   time division multiplexed (TDM) digital signals (TDM circuit
   emulation) over PSNs, while [CEP] similarly describes transport of
   high-rate TDM (SONET/SDH). To support TDM traffic the pseudowires
   must emulate the circuit characteristics of the original T1, E1, T3,
   E3, SONET or SDH signals.  [SAToP] does this by encapsulating an
   arbitrary but constant amount of the TDM data in each packet, while
   the other methods encapsulate TDM structures.

   In this document, we specify the use of the MPLS Label Distribution
   Protocol, LDP [RFC3036], as a protocol for setting up and maintaining
   the pseudowires. In particular, we define new TLVs for LDP, which
   enable LDP to identify pseudowires and to signal attributes of
   pseudowires. We specify how a pseudowire endpoint uses these TLVs in
   LDP to bind a demultiplexor field value to a pseudowire, and how it
   informs the remote endpoint of the binding. We also specify
   procedures for reporting pseudowire status changes, passing
   additional information about the pseudowire as needed, and for
   releasing the bindings.

   In the protocol specified herein, the pseudowire demultiplexor field



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   is an MPLS label. Thus the packets which are transmitted from one end
   of the pseudowire to the other are MPLS packets. Unless the
   pseudowire endpoints are immediately adjacent, these MPLS packets
   must be transmitted through a PSN tunnel. Any sort of PSN tunnel can
   be used, as long as it is possible to transmit MPLS packets through
   it. The PSN tunnel can itself be an LSP, or any other sort of tunnel
   which can carry MPLS packets. Procedures for setting up and
   maintaining the PSN tunnels are outside the scope of this document.

   This document deals only with the setup and maintenance of point-to-
   point pseudowires.   Neither point-to-multipoint nor multipoint-to-
   point pseudowires are discussed.

   QoS related issues are not discussed in this document.

   The following two figures describe the reference models which are
   derived from [ARCH] to support the Ethernet PW emulated services.

         |<-------------- 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












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   +-------------+                                +-------------+
   |  Layer2     |                                |  Layer2     |
   |  Emulated   |                                |  Emulated   |
   |  Services   |         Emulated Service       |  Services   |
   |             |<==============================>|             |
   +-------------+           Pseudo Wire          +-------------+
   |Demultiplexor|<==============================>|Demultiplexor|
   +-------------+                                +-------------+
   |    PSN      |            PSN Tunnel          |    PSN      |
   |   MPLS      |<==============================>|   MPLS      |
   +-------------+                                +-------------+
   |  Physical   |                                |  Physical   |
   +-----+-------+                                +-----+-------+

             Figure 2: PWE3 Protocol Stack Reference Model

   For the purpose of this document, PE1 will be defined as the ingress
   router, and PE2 as the egress router. A layer 2 PDU will be received
   at PE1, encapsulated at PE1, transported, decapsulated at PE2, and
   transmitted out of PE2.


4. The Pseudowire Label

   Suppose it is desired to transport layer 2 PDUs from ingress LSR PE1
   to egress LSR PE2, across an intervening PSN. We assume that there is
   a PSN tunnel from PE1 to PE2. That is, we assume that PE1 can cause a
   packet to be delivered to PE2 by encapsulating the packet in a "PSN
   tunnel header" and sending the result to one of its adjacencies.  If
   the PSN tunnel is an MPLS Label Switched Path (LSP), then putting on
   a PSN tunnel encapsulation is a matter of pushing on an additional
   MPLS label.

   We presuppose that a large number of pseudowires can be carried
   through a single PSN tunnel.  Thus it is never necessary to maintain
   state in the network core for individual pseudowires.  We do not
   presuppose that the PSN tunnels are point-to-point; although the
   pseudowires are point-to-point, the PSN tunnels may be multipoint-
   to-point.  We do not presuppose that PE2 will even be able to
   determine the PSN tunnel through which a received packet was
   transmitted.  (E.g., if the PSN tunnel is an LSP, and penultimate hop
   popping is used, when the packet arrives at PE2 it will contain no
   information identifying the tunnel.)

   When PE2 receives a packet over a pseudowire, it must be able to
   determine that the packet was in fact received over a pseudowire, and
   it must be able to associate that packet with a particular
   pseudowire.  PE2 is able to do this by examining the MPLS label which



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   serves as the pseudowire demultiplexor field shown in Figure 2.  Call
   this label the "PW label".

   So when PE1 sends a layer 2 PDU to PE2, it first pushes a PW label on
   its label stack, thereby creating an MPLS packet.  It then (if PE1 is
   not adjacent to PE2) encapsulates that MPLS packet in a PSN tunnel
   header.  (If the PSN tunnel is an LSP, this is just a matter of
   pushing on a second label.)  The PW label is not visible again until
   the MPLS packet reaches PE2. PE2's disposition of the packet is based
   on the PW label.

   Note that the PW label must always be at the bottom of the packet's
   label stack and labels MUST be allocated from the per-platform label
   space.


   This document specifies a protocol for assigning and distributing the
   PW label.  This protocol is LDP, extended as specified in the
   remainder of this document.  An LDP session must be set up between
   the pseudowire endpoints.  LDP MUST be used in its "downstream
   unsolicited" mode.  LDP's "liberal label retention" mode SHOULD be
   used.

   In addition to the protocol specified herein, static assignment of PW
   labels MAY be used, and implementations of this protocol SHOULD
   provide support for static assignment.

   This document specifies all the procedures necessary to set up and
   maintain the pseudowires needed to support "unswitched" point-to-
   point services, where each endpoint of the pseudowire is provisioned
   with the identify of the other endpoint.  There are also  protocol
   mechanisms specified herein which can be used to support switched
   services, and which can be used to support other provisioning models.
   However, the use of the protocol mechanisms to support those other
   models and services is not described in this document.


5. Details Specific to Particular Emulated Services

5.1. Frame Relay

   When emulating a frame relay service, the Frame Relay PDUs are
   encapsulated according to the procedures defined in [FRAME]. The PE
   MUST provide Frame Relay PVC status signaling to the Frame Relay
   network. If the PE detects a service-affecting condition for a
   particular DLCI, as defined in [ITUQ] Q.933 Annex A.5 sited in IA
   FRF1.1, PE MUST communicate to the remote PE the status of the PW
   corresponds to the frame relay DLCI. The Egress PE SHOULD generate



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   the corresponding errors and alarms as defined in [ITUQ] on the
   egress Frame relay PVC.


5.2. ATM

5.2.1. ATM AAL5 SDU VCC Transport

   ATM AAL5 CSPS-SDUs are encapsulated according to [ATM] ATM AAL5
   CPCS-SDU mode. This mode allows the transport of ATM AAL5 CSPS-SDUs
   traveling on a particular ATM PVC across the network to another ATM
   PVC.


5.2.2. ATM Transparent Cell Transport

   This mode is similar to the Ethernet port mode. Every cell that is
   received at the ingress ATM port on the ingress PE, PE1, is
   encapsulated according to [ATM], ATM cell mode n-to-one, and sent
   across the PW to the egress PE, PE2.  This mode allows an ATM port to
   be connected to only one other ATM port.  [ATM] ATM cell n-to-one
   mode allows for concatenation ( grouping ) of multiple cells into a
   single MPLS frame. Concatenation of ATM cells is OPTIONAL for
   transmission at the ingress PE, PE1. If the Egress PE PE2 supports
   cell concatenation the ingress PE, PE1, should only concatenate cells
   up to the "Maximum Number of concatenated ATM cells" parameter
   received as part of the FEC element.


5.2.3. ATM n-to-one VCC and VPC Cell Transport

   This mode is similar to the ATM AAL5 VCC transport except that cells
   are transported. Every cell that is received on a pre-defined ATM
   PVC, or ATM PVP, at the ingress ATM port on the ingress PE, PE1, is
   encapsulated according to [ATM], ATM n-to-one cell mode, and sent
   across the LSP to the egress PE PE2. Grouping of ATM cells is
   OPTIONAL for transmission at the ingress PE, PE1. If the Egress PE
   PE2 supports cell concatenation the ingress PE, PE1, MUST only
   concatenate cells up to the "Maximum Number of concatenated ATM cells
   in a frame" parameter received as part of the FEC element.


5.2.4. OAM Cell Support

   OAM cells MAY be transported on the PW LSP. When the PE is operating
   in AAL5 CPCS-SDU transport mode if it does not support transport of
   ATM cells, the PE MUST discard incoming MPLS frames on an ATM PW LSP
   that contain a PW label with the T bit set [ATM]. When operating in



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   AAL5 SDU transport mode an PE that supports transport of OAM cells
   using the T bit defined in [ATM], or an PE operating in any of the
   cell transport modes MUST follow the procedures outlined in [ATM] in
   addition to the applicable procedures specified in [ITUG].


5.2.4.1. SDU/PDU OAM Cell Emulation Mode

   A PE operating in ATM SDU, or PDU transport mode, that does not
   support transport of OAM cells across an LSP MAY provide OAM support
   on ATM PVCs using the following procedures:

     - Loopback cells response

       If an F5 end-to-end OAM cell is received from a ATM VC, by either
       PE that is transporting this ATM VC, with a loopback indication
       value of 1, and the PE has a label mapping for the ATM VC, then
       the PE MUST decrement the loopback indication value and loop back
       the cell on the ATM VC. Otherwise the loopback cell MUST be
       discarded by the PE.

     - AIS Alarm.

       If an ingress PE, PE1, receives an AIS F4/F5 OAM cell, it MUST
       notify the remote PE of the failure. The remote PE , PE2, MUST in
       turn send F5 OAM AIS cells on the respective PVCs. Note that if
       the PE supports forwarding of OAM cells, then the received OAM
       AIS alarm cells MUST be forwarded along the PW as well.

     - Interface failure.

       If the PE detects a physical interface failure, or the interface
       is administratively disabled, the PE MUST notify the remote PE
       for all VCs associated with the failure.

     - PSN/PW failure detection.

       If the PE detects a failure in the PW, by receiving a label
       withdraw for a specific PW ID, or the targeted LDP session fails,
       or a PW status TLV notification is received, then a proper AIS F5
       OAM cell MUST be generated for all the affected atm PVCs. The AIS
       OAM alarm will be generated on the ATM output port of the PE that
       detected the failure.








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5.2.5. ILMI Support

   An MPLS edge PE MAY provide an ATM ILMI to the ATM edge switch. If an
   ingress PE receives an ILMI message indicating that the ATM edge
   switch has deleted a VC, or if the physical interface goes down, it
   MUST send a PW status notification message for all PWs associated
   with the failure. When a PW label mapping is withdrawn, or PW status
   notification message is received the egress PE SHOULD notify its
   client of this failure by deleting the VC using ILMI.


5.2.6. ATM AAL5 PDU VCC Transport

   ATM AAL5 CSPS-PDUs are encapsulated according to [ATM] ATM AAL5
   CPCS-PDU mode. This mode allows the transport of ATM AAL5 CSPS-PDUs
   traveling on a particular ATM PVC across the network to another ATM
   PVC. This mode supports fragmentation of the ATM AAL5 CPCS-PDU in
   order to maintain the position of the OAM cells with respect to the
   user cells. Fragmentation may also be performed to maintain the size
   of the packet carrying the AAL5 PDU within the MTU of the link.


5.2.7. ATM one-to-one VCC and VPC Cell Transport

   This mode is similar to the ATM AAL5 n-to-one cell transport except
   an encapsulation method that maps one ATM VCC or one ATM VPC to one
   Pseudo-Wire is used. Every cell that is received on a pre-defined ATM
   PVC, or ATM PVP, at the ingress ATM port on the ingress PE, PE1, is
   encapsulated according to [ATM], ATM one-to-one cell mode, and sent
   across the LSP to the egress PE PE2. Grouping of ATM cells is
   OPTIONAL for transmission at the ingress PE, PE1. If the Egress PE
   PE2 supports cell concatenation the ingress PE, PE1, MUST only
   concatenate cells up to the "Maximum Number of concatenated ATM cells
   in a frame" parameter received as part of the FEC element.


5.3. Ethernet Tagged Mode

   The Ethernet frame will be encapsulated according to the procedures
   in [ETH] tagged mode. It should be noted that if the VLAN identifier
   is modified by the egress PE, according to the procedures outlined
   above, the Ethernet spanning tree protocol might fail to work
   properly. If the PE detects a failure on the Ethernet physical port,
   or the port is administratively disabled, it MUST send PW status
   notification message for all PWs associated with the port. This mode
   uses service-delimiting tags to map input ethernet frames to
   respective PWs.




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5.4. Ethernet

   The Ethernet frame will be encapsulated according to the procedures
   in [ETH] "ethernet raw mode". If the PE detects a failure on the
   Ethernet input port, or the port is administratively disabled, the PE
   MUST send a corresponding PW status notification message.


5.5. HDLC and PPP

   HDLC and PPP frames are encapsulated according to the procedures in
   [PPPHDLC]. If the MPLS edge PE detects that the physical link has
   failed, or the port is administratively disabled, it MUST send a PW
   status notification message that corresponds to the HDLC or PPP PW.


5.6. IP Layer2 Transport

   This mode switches IP packets into a Peudo-Wire. the encapsulation
   used is according to [RFC3032]. IP interworking is implementation
   specific, part of the NSP function [ARCH], and is outside the scope
   of this document.  The control word MUST NOT be used.


6. LDP

   The PW label bindings are distributed using the LDP downstream
   unsolicited mode described in [LDP]. The PEs will establish an LDP
   session using the Extended Discovery mechanism described in [1,
   section 2.4.2 and 2.5].

   An LDP Label Mapping message contains a FEC TLV, a Label TLV, and
   zero or more optional parameter TLVs.

   The FEC TLV is used to indicate the meaning of the label.  In the
   current context, the FEC TLV would be used to identify the particular
   pseudowire that a particular label is bound to.  In this
   specification, we define two new FEC TLVs to be used for identifying
   pseudowires.  When setting up a particular pseudowire, only one of
   these FEC TLVs is used.  The one to be used will depend on the
   particular service being emulated and on the particular provisioning
   model being supported.

   LDP allows each FEC TLV to consist of a set of FEC elements.  For
   setting up and maintaining pseudowires, however, each FEC TLV MUST
   contain exactly one FEC element.

   LDP has several kinds of label TLVs.  For setting up and maintaining



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   pseudowires, the Generic Label TLV MUST be used.


6.1. The PWid FEC Element

   The PWid FEC element may be used whenever both pseudowire endpoints
   have been provisioned with the same 32-bit identifier for the
   pseudowire.

   For this purpose a new type of FEC element is defined. The FEC
   element type is 128 [note1], and is defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    PW tlv     |C|         PW type             |PW info Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Group ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           PW ID                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Interface parameters                    |
   |                              "                                |
   |                              "                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     - PW type

       A 15 bit quantity containing a value which represents the type of
       PW. Assigned Values are specified in "IANA Allocations for pseudo
       Wire Edge to Edge Emulation (PWE3)" [IANA].

     - Control word bit (C)

       The bit (C) is used to flag the presence of a control word as
       follows:

           C = 1 control word present on this PW.
           C = 0 no control word present on this PW.

       Please see the section "C-Bit Handling Procedures" for further
       explanation.

     - PW information length

       Length of the PW ID field and the interface parameters field in
       octets. If this value is 0, then it references all PWs using the



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       specified group ID and there is no PW ID present, nor any
       interface parameters.

     - Group ID

       An arbitrary 32 bit value which represents a group of PWs that is
       used to create groups in the PW space. The group ID is intended
       to be used as a port index, or a virtual tunnel index. To
       simplify configuration a particular PW ID at ingress could be
       part of the virtual tunnel for transport to the egress router.
       The Group ID is very useful to send wild card label withdrawals,
       or PW wild card status notification messages to remote PEs upon
       physical port failure.

     - PW ID

       A non-zero 32-bit connection ID that together with the PW type,
       identifies a particular PW.  Note that the PW ID and the PW type
       must be the same at both endpoints.

     - Interface parameters

       This variable length field is used to provide interface specific
       parameters, such as CE-facing interface MTU.

       Note that as the "interface parameters" are part of the FEC, the
       rules of LDP make it impossible to change the interface
       parameters once the pseudowire has been set up.  Thus the
       interface parameters field must not be used to pass information,
       such as status information, which may change during the life of
       the pseudowire.  Optional parameter TLVs should be used for that
       purpose.

   Using the PWid FEC, each of the two pseudowire endpoints
   independently initiates the set up of a unidirectional LSP.  An
   outgoing LSP and an incoming LSP are bound together into a single
   pseudowire if they have the same PW ID  and PW type.


6.2. The Generalized PW ID FEC Element

   The PWid FEC element can be used if a unique 32-bit value has been
   assigned to the PW, and if each endpoint has been provisioned with
   that value.  The Generalized ID FEC element requires that the PW
   endpoints be uniquely identified; the PW itself is identified as a
   pair of endpoints.  In addition the endpoint identifiers are
   structured to support applications where the identity of the remote
   endpoints needs to be auto-discovered rather than statically



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   configured.

   The "Generalized ID FEC Element" is FEC type 129 (provisionally,
   subject to assignment by IANA).

   The Generalized ID FEC Element not contain anything corresponding to
   the "group id" of the PWid FEC element.  The functionality of the
   latter is provided by a separate optional LDP TLV, the "PW Grouping
   TLV", described below. The Interface Parameters field of the PWid FEC
   element is also absent; its functionality is replaced by the optional
   Interface Parameters TLV, described below.


6.2.1. Attachment Identifiers

   As discussed in [ARCH], a pseudowire can be thought of as connecting
   two "forwarders".  The protocol used to setup a pseudowire must allow
   the forwarder at one end of a pseudowire to identify the forwarder at
   the other end.  We use the term "attachment identifier", or "AI", to
   refer to the field which the protocol uses to identify the
   forwarders.  In the PWid FEC, the PWid field serves as the AI.  In
   this section we specify a more general form of AI which is structured
   and of variable length.

   Every Forwarder in a PE must be associated with an Attachment
   Identifier (AI), either through configuration or through some
   algorithm.  The Attachment Identifier must be unique in the context
   of the PE router in which the Forwarder resides.  The combination <PE
   router, AI> must be globally unique.

   It is frequently convenient to regard a set of Forwarders as being
   members of a particular "group", where PWs may only be set up among
   members of a group.  In such cases, it is convenient to identify the
   Forwarders relative to the group, so that an Attachment Identifier
   would consist of an Attachment Group Identifier (AGI) plus an
   Attachment Individual Identifier (AII).

   An Attachment  Group Identifier  may be thought  of as  a VPN-id, or
   a VLAN identifier, some  attribute which  is shared by  all the
   Attachment PWs (or pools thereof) which are allowed to be connected.

   The details of how to construct the AGI and AII fields identifying
   the pseudowire endpoints are outside the scope of this specification.
   Different pseudowire application, and different provisioning models,
   will require different sorts of AGI and AII fields.  The
   specification of each such application and/or model must include the
   rules for constructing the AGI and AII fields.




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   As previously discussed, a (bidirectional) pseudowire consists of a
   pair of unidirectional LSPs, one in each direction. If a particular
   pseudowire connects PE1 with PE2, the LSP in the PE1-->PE2 direction
   can be identified as:

           <PE1, <AGI, AII1>, PE2, <AGI, AII2>>,

   and the LSP in the PE2--PE1 direction can be identified by:

           <PE2, <AGI, AII2>, PE1, <AGI, AII1>>.

   Note that the AGI must be the same at both endpoints, but the AII
   will in general be different at each endpoint.  Thus from the
   perspective of a particular PE, each pseudowire has a local or
   "Source AII", and a remote or "Target AII".  The pseudowire setup
   protocol can carry all three of these quantities:

     - Attachment Group Identifier (AGI).

     - Source Attachment Individual Identifier (SAII)

     - Target Attachment Individual Identifier (TAII)

   If the AGI is non-null, then the Source AI (SAI) consists of the AGI
   together with the SAII, and the Target AI (TAI) consists of the TAII
   together with the AGI.  If the AGI is null, then the SAII and TAII
   are the SAI and TAI respectively.

   The interpretation of the SAI and TAI is a local matter at the
   respective endpoint.

   The association of two unidirectional LSPs into a single
   bidirectional pseudowire depends on the SAI and the TAI.  Each
   application and/or provisioning model which uses the Generalized ID
   FEC element must specify the rules for performing this association.


6.2.2. Encoding the Generalized ID FEC Element

   FEC element type 129 [note1] is used.   The FEC element is encoded 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     129       |C|         PW Type             |PW info Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      AGI      |    Length     |      Value                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                      Value (contd.)                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SAII      |    Length     |      Value                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                      Value (contd.)                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     TAII      |    Length     |      Value                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                      Value (contd.)                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   This document does not specify the SAII,TAII,AGI type field values;
   specification of the type field values to use for a particular
   application is part of the specification of that application. IANA
   will assign these values based on IETF consensus.

   The SAII, TAII, and AGI are simply carried as octet strings. The
   length byte specifies the size of the Value field. The null string
   can be sent by setting the length byte to 0.

   If a particular application does not need all three of these sub-
   elements, it MUST send all the sub-elements, but set the length to 0
   for the unused sub-elements.

   The PW information length field, contains the length of the SAII,
   TAII, AGI combined, and the interface parameters field in octets. If
   this value is 0, then it references all PWs using the specified
   grouping ID. In this case there are no other FEC element fields
   (AGI,SAII, etc. ) present, nor any interface parameters.

   Note that the interpretation of a particular field as AGI, SAII, or
   TAII depends on the order of its occurrence.  The type field
   identifies the type of the AGI, SAII, or TAII.  When comparing two
   occurrences of an AGI (or SAII or TAII), the two occurrences are
   considered to be identical if the type, length, and value fields of
   one are identical, respectively, to those of the other.




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6.2.2.1. Interface Parameters TLV

   This TLV MUST only be used when sending the Generalized PW FEC.  It
   specifies interface specific parameters. Specific parameters, when
   applicable, MUST be used to validate that the PEs, and the ingress
   and egress ports at the edges of the circuit, have the necessary
   capabilities to interoperate with each other.

    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|  PW Intf P. TLV (0x096B)  |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Parameter ID |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Variable Length Value                 |
   |                             "                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [ note: TLV type 0x096B as defined in [IANA] pending IANA allocation
   ]

   A more detailed description of this field can be found in the section
   "Interface Parameters Field" below.


6.2.2.2. PW Grouping TLV

    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|PW Grouping ID TLV (0x096C)|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Value                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [ note: TLV type 0x096C as defined in [IANA] pending IANA allocation
   ]

   The PW Grouping ID is an arbitrary 32 bit value which represents an
   arbitrary group of PWs.  It is used create groups PWs; for example, a
   PW Grouping ID can be used as a port index, and assigned to all PWs
   that lead to that port.  Use of the PW Grouping ID enables one to
   send "wild card" label withdrawals, or "wild card" status
   notification messages to remote PEs upon physical port failure.

   Note Well: The PW Grouping ID is different than, and has no relation
   to, the Attachment Group Identifier.



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   The PW Grouping ID TLV is not part of the FEC, and will not be
   advertised except in the initial PW FEC advertisement. The
   advertising PE MAY use the wild card withdraw semantics, but the
   remote PEs MUST implement support for wildcard messages. If the PW
   Grouping ID is not going to be used for wild card messages, it MAY be
   omitted. This TLV MAY only be used when sending the Generalized PW ID
   FEC.

   To issue a wildcard command ( status or withdraw ):

        -i. Set the PW Info Length to 0 in the Generalized ID FEC
            Element.
       -ii. Send only the PW Grouping ID TLV with the FEC ( No
            AGI/SAII/TAII is sent ).


6.2.3. Signaling Procedures

   In order for PE1 to begin signaling PE2, PE1 must know the address of
   the remote PE2, and a TAI.  This information may have been configured
   at PE1, or it may have been learned dynamically via some
   autodiscovery procedure.

   To begin the signaling procedure, a PE (PE1) that has knowledge of
   the other endpoint (PE2) initiates the setup of the LSP in the
   incoming (PE2-->PE1) direction by sending a Label Mapping message
   containing the FEC type 129.  The FEC element includes the SAII, AGI,
   and TAII.

   What happens when PE2 receives such a Label Mapping message?

   PE2 interprets the message as a request to set up a PW whose endpoint
   (at PE2) is the Forwarder identified by the TAI.  From the
   perspective of the signaling protocol, exactly how PE2 maps AIs to
   Forwarders is a local matter.  In some VPWS provisioning models, the
   TAI might, e.g., be a string which identifies a particular Attachment
   Circuit, such as "ATM3VPI4VCI5", or it might, e.g., be a string such
   as "Fred" which is associated by configuration with a particular
   Attachment Circuit.  In VPLS, the AGI could be a VPN-id, identifying
   a particular VPLS instance.

   If PE2 cannot map the TAI to one of its Forwarders, then PE2 sends a
   Label Release message to PE1, with a Status Code meaning "invalid
   TAI" ,[ note:  Status Code 0x00000029 as defined in [IANA] pending
   IANA allocation ] and the processing of the Mapping message is
   complete.

   The FEC TLV sent in a Label Release is the same as the FEC TLV



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   received in the Label Mapping being released (but without the
   interface parameters).  More generally, the FEC TLV is the same in
   all LDP messages relating to the same LSP.  In a Label Release this
   means that the SAII is the remote peer's AII and the TAII is the
   sender's local AII.

   If the Label Mapping Message has a valid TAI, PE2 must decide whether
   to accept it or not. The procedures for so deciding will depend on
   the particular type of Forwarder identified by the TAI. Of course,
   the Label Mapping message may be rejected due to standard LDP error
   conditions as detailed in [LDP].

   If PE2 decides to accept the Label Mapping message, then it has to
   make sure that an LSP is set up in the opposite (PE1-->PE2)
   direction.  If it has already signaled for the corresponding LSP in
   that direction, nothing more need be done.  Otherwise, it must
   initiate such signaling by sending a Label Mapping message to PE1.
   This is very similar to the Label Mapping message PE2 received, but
   with the SAI and TAI reversed.

   Thus a bidirectional PW consists of two LSPs, where the FEC of one is
   the "reverse" of the FEC of the other.


6.3. Signaling of Pseudo Wire Status

6.3.1. Use of Label Mappings.

   The PEs MUST send PW label mapping messages to their peers as soon as
   the PW is configured and administratively enabled, regardless of the
   CE-facing interface state. The PW label should not be withdrawn
   unless the user administratively configures the CE-facing interface
   down (or the PW configuration is deleted entirely). Using the
   procedures outlined in this section a simple label withdraw method
   MAY also be supported as a primitive means of signaling PW status. It
   is strongly RECOMMENDED that the PW status signaling procedures below
   be fully implemented. In any case if the Label mapping is not
   available the PW MUST be considered in the down state.


6.3.2. Signaling PW status.

   The PE devices use an LDP TLV to indicate status to their remote
   peers. This PW Status TLV contains more information than the
   alternative simple Label Withdraw message.

   The format of the PW Status TLV is:




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|     PW Status (0x096A)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Status Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   [ note: TLV type 0x096A as defined in [IANA] pending IANA allocation
   ]

   Where the status code is a 4 octet bit field is specified in the PW
   IANA Allocations document [IANA]. The length specifies the length of
   the Status Code field in octets ( equal to 4 ).

   Each bit in the status code field can be set individually to indicate
   more then a single failure at once. Each fault can be cleared by
   sending an appropriate status message with the respective bit
   cleared. The presence of the lowest bit (PW Not Forwarding) acts only
   as a generic failure indication when there is a link-down event for
   which none of the other bits apply.

   The Status TLV is transported to the remote PW peer via the LDP
   notification message. The general format of the Notification Message
   is:

    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                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Status (TLV)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      PW Status TLV                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 PWId FEC or Generalized ID FEC                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The Status TLV status code is set to 0x00000028 "PW status", to
   indicate that PW status follows. Since this notification does not
   refer to any particular message the Message Id, and Message Type
   fields are set to 0.  [ note: Status Code 0x00000028 as defined in
   [IANA] pending IANA allocation ] A more detailed diagram is shown in
   Appendix B.




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   The PW FEC TLV SHOULD not include the interface parameters as they
   are ignored in the context of this message. When a PE's CE-facing
   interface encounters an error, use of the PW status message allows
   the PE to send a single "wild card" status message, using a PW FEC
   TLV with only the group ID set, to denote this change in status for
   all affected PW connections.  This status message contains either the
   PW FEC TLV with only the group ID set, or else it contains the
   Generalized FEC TLV and the PW Grouping ID TLV.

   As mentioned above the Group ID field of the PWid FEC element, or the
   PW Grouping ID TLV used with the Generalized ID FEC element, can be
   used to send a status notification for all arbitrary sets of PWs.
   This procedure is OPTIONAL, and if it is implemented the LDP
   Notification message should be as follows: If the PWid FEC element is
   used, the PW information length field is set to 0, the PW ID field is
   not present, and the interface parameters field is not present. If
   the Generalized FEC element is used, the AGI, SAII, and TAII are not
   present,the PW information length field is set to 0, the PW Grouping
   ID TLV is included, and the Interface Parameters TLV is omitted.  For
   the purpose of this document this is called the "wild card PW status
   notification procedure", and all PEs implementing this design are
   REQUIRED to accept such a notification message, but are not required
   to send it.


6.3.3. Pseudowire Status Negotiation Procedures

   When a PW is first set up the PEs MUST attempt to negotiate the usage
   of the PW status TLV. This is accomplished as follows:  A PE that
   supports the PW Status TLV MUST include it the initial label mapping
   signaling following label mapping TLV, the PW FEC, and the interface
   parameters field. The PW Status TLV will then be used for the
   lifetime of the Pseudowire. This is shown in the following diagram:


















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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                 PWId FEC or Generalized ID FEC                +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Interface parameters                    |
   |                              "                                |
   |                              "                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0| Generic Label (0x0200)    |      Length                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Label                                                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|     PW Status (0x0???)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Status Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   If PW status TLV is sent following the label mapping TLV in the
   initial PW FEC Message, but the remote PE corresponding FEC
   advertisement does not include a PW status TLV, or the remote PE does
   not support the PW Status TLV and the PW will revert to the label
   withdraw method to signal PW status.  Note that if the PW Status TLV
   is not supported, by the remote peer, it will automatically be
   ignored, since the LDP ignore bit is set. The PW Status TLV,
   therefore, will not be present in the corresponding FEC advertisement
   from the remote LDP peer resulting in exactly the above behavior.

   If the PW Status TLV is not present following the label mapping TLV
   in the initial PW FEC Message received by a PE, then the PW Status
   TLV will not be used and both PEs supporting the pseudowire will
   revert to the label withdraw procedure for signaling status changes.

   If the negotiation process results in the usage of the PW status TLV,
   then the actual PW status is determined by the PW status TLV that was
   sent within the initial PW label mapping. Subsequent updates of PW
   status are conveyed through the notification message











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6.4. Interface Parameters Field

   This field specifies interface specific parameters. When applicable,
   it MUST be used to validate that the PEs, and the ingress and egress
   ports at the edges of the circuit, have the necessary capabilities to
   interoperate with each other. The field structure is defined as
   follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Parameter ID |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Variable Length Value                 |
   |                             "                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The parameter ID Values are specified in "IANA Allocations for pseudo
   Wire Edge to Edge Emulation (PWE3)" [IANA].

   The Length field is defined as the length of the interface parameter
   including the parameter id and length field itself. Processing of the
   interface parameters should continue when encountering unknown
   interface parameters and they MUST be silently ignored.

     - Interface MTU

       A 2 octet value indicating the MTU in octets. This is the Maximum
       Transmission Unit, excluding encapsulation overhead, of the
       egress packet interface that will be transmitting the
       decapsulated PDU that is received from the MPLS network. This
       parameter is applicable only to PW types 1, 2, 4, 5, 6, 7,14, and
       15 and is REQUIRED for these PW types. If this parameter does not
       match in both directions of a specific PW, that PW MUST NOT be
       enabled.

     - Maximum Number of concatenated ATM cells

       A 2 octet value specifying the maximum number of concatenated ATM
       cells that can be processed as a single PDU by the egress PE. An
       ingress PE transmitting concatenated cells on this PW can
       concatenate a number of cells up to the value of this parameter,
       but MUST NOT exceed it. This parameter is applicable only to PW
       types 3, 9, 0x0a, 0xc, and 0xd and is REQUIRED for these PWC
       types. This parameter does not need to match in both directions
       of a specific PW.




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     - Optional Interface Description string

       This arbitrary, OPTIONAL, interface description string is used to
       send a human-readable administrative string describing the
       interface to the remote. This parameter is OPTIONAL, and is
       applicable to all PW types.  The interface description parameter
       string length is variable, and can be from 0 to 80 octets.
       Human-readable text MUST be provided in the UTF-8 charset using
       the Default Language [RFC2277].

     - Payload Bytes

       A 2 octet value indicating the number of TDM payload octets
       contained in all packets on the CEM stream, from 48 to 1,023
       octets. All of the packets in a given CEM stream have the same
       number of payload bytes. Note that there is a possibility that
       the packet size may exceed the SPE size in the case of an STS-1
       SPE, which could cause two pointers to be needed in the CEM
       header, since the payload may contain two J1 bytes for
       consecutive SPEs. For this reason, the number of payload bytes
       must be less than or equal to 783 for STS-1 SPEs.

     - CEP Options.

       An optional 16 Bit value of CEM Flags. See [8] for the definition
       of the bit values.

     - Requested VLAN ID.

       An Optional 16 bit value indicating the requested VLAN ID. This
       parameter MAY be used by an PE that is incapable of rewriting the
       802.1Q ethernet VLAN tag on output. If the ingress PE receives
       this request it MAY rewrite the VLAN ID tag in input to match the
       requested VLAN ID. If this is not possible, and the VLAN ID does
       not already match configured ingress VLAN ID the PW should not be
       enabled.This parameter is applicable only to PW type 4.

     - CEP/TDM bit rate.

       This 32-bit integer is mandatory for CEP. For other PWs carrying
       TDM traffic it is mandatory if the bit-rate cannot be directly
       inferred from the service type. If present, it expresses the bit
       rate of the attachment circuit as known to the advertizing PE in
       "units" of 64 kbit/s. I.e., the value 26 must be used for CEP
       carrying VT1.5 SPE, 35 - for CEP carrying a VT2 SPE, 107 - for
       VT6 SPE, 783 - for STS-1 SPE and n*783 - for STS-nc, n = 3, 12,
       48, 192.  Attempts to establish a PWC between a pair of TDM ports
       with different bit-rates MUST be rejected with the appropriate



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       status code (see section "Status codes" in [IANA]).

     - Frame-Relay DLCI length.

       An optional 16 bit value indicating the lenght of the frame-relay
       DLCI field. This OPTIONAL interface paremeter can have value of 2
       , or 4, with the default being equal to 2. If this interface
       parameter is not present the default value of 2 is assumed.


7. Control Word

7.1. PW types for which the control word is REQUIRED

   The Label Mapping messages which are sent in order to set up these
   PWs MUST have c=1. When a Label Mapping message for a PW of one of
   these types is received, and c=0, a Label Release MUST be sent, with
   an "Illegal C-bit" status code. In this case, the PW will not be
   enabled.


7.2. PW types for which the control word is NOT mandatory

   If a system is capable of sending and receiving the control word on
   PW types for which the control word is not mandatory, then each such
   PW endpoint MUST be configurable with a parameter that specifies
   whether the use of the control word is PREFERRED or NOT PREFERRED.
   For each PW, there MUST be a default value of this parameter. This
   specification does NOT state what the default value should be.

   If a system is NOT capable of sending and receiving the control word
   on PWC types for which the control word is not mandatory, then it
   behaves as exactly as if it were configured for the use of the
   control word to be NOT PREFERRED.

   If a Label Mapping message for the PW has already been received, but
   no Label Mapping message for the PW has yet been sent, then the
   procedure is the following:

        -i. If the received Label Mapping message has c=0, send a Label
            Mapping message with c=0, and the control word is not used.
       -ii. If the received Label Mapping message has c=1, and the PW is
            locally configured such that the use of the control word is
            preferred, then send a Label Mapping message with c=1, and
            the control word is used.






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      -iii. If the received Label Mapping message has c=1, and the PW is
            locally configured such that the use of the control word is
            not preferred or the control word is not supported, then act
            as if no Label Mapping message for the PW had been received
            (i.e., proceed to the next paragraph).

   If a Label Mapping message for the PW has not already been received
   (or if the received Label Mapping message had c=1 and either local
   configuration says that the use of the control word is not preferred
   or the control word is not supported), then send a Label Mapping
   message in which the c bit is set to correspond to the locally
   configured preference for use of the control word.  (I.e., set c=1 if
   locally configured to prefer the control word, set c=0 if locally
   configured to prefer not to use the control word or if the control
   word is not supported).

   The next action depends on what control message is next received for
   that PW.  The possibilities are:

        -i. A Label Mapping message with the same c bit value as
            specified in the Label Mapping message that was sent. PW
            setup is now complete, and the control word is used if c=1
            but not used if c=0.
       -ii. A Label Mapping message with c=1, but the Label Mapping
            message that was sent has c=0. In this case, ignore the
            received Label Mapping message, and continue to wait for the
            next control message for the PW.
      -iii. A Label Mapping message with c=0, but the Label Mapping
            message that was sent has c=1. In this case, send a Label
            Withdraw message with a "Wrong c-bit" status code, followed
            by a Label Mapping message that has c=0. PW setup is now
            complete, and the control word is not used.
       -iv. A Label Withdraw message with the "Wrong c-bit" status code.
            Treat as a normal Label Withdraw, but do not respond.
            Continue to wait for the next control message for the PW.

   If at any time after a Label Mapping message has been received, a
   corresponding Label Withdraw or Release is received, the action taken
   is the same as for any Label Withdraw or Release that might be
   received at any time.

   If both endpoints prefer the use of the control word, this procedure
   will cause it to be used. If either endpoint prefers not to use the
   control word, or does not support the control word, this procedure
   will cause it not to be used. If one endpoint prefers to use the
   control word but the other does not, the one that prefers not to use
   it is has no extra protocol to execute, it just waits for a Label
   Mapping message that has c=0.



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   The diagram in Appendix A illustrates the above procedure.


7.3. LDP label Withdrawal procedures

   As mentioned above the Group ID field of the PWid FEC element, or the
   PW Grouping ID TLV used with the Generalized ID FEC element, can be
   used to withdraw all PW labels associated with a particular PW group.
   This procedure is OPTIONAL, and if it is implemented the LDP label
   withdraw message should be as follows: If the PWid FEC element is
   used, the PW information length field is set to 0, the PW ID field is
   not present, and the interface parameters field is not present. If
   the Generalized FEC element is used, the AGI, SAII, and TAII are not
   present,the PW information length field is set to 0, the PW Grouping
   ID TLV is included, and the Interface Parameters TLV is omitted. For
   the purpose of this document this is called the "wild card withdraw
   procedure", and all PEs implementing this design are REQUIRED to
   accept such withdrawn message, but are not required to send it.

   The interface parameters field, or TLV, MUST NOT be present in any
   LDP PW label withdrawal message or release message. A wildcard
   release message MUST include only the group ID. A Label Release
   message initiated from the imposition router must always include the
   PW ID.


7.4. Sequencing Considerations

   In the case where the router considers the sequence number field in
   the control word, it is important to note the following when
   advertising labels


7.4.1. Label Mapping Advertisements

   After a label has been withdrawn by the disposition router and/or
   released by the imposition router, care must be taken to not re-
   advertise (re-use) the released label until the disposition router
   can be reasonably certain that old packets containing the released
   label no longer persist in the MPLS network.

   This precaution is required to prevent the imposition router from
   restarting packet forwarding with sequence number of 1 when it
   receives the same label mapping if there are still older packets
   persisting in the network with sequence number between 1 and 32768.
   For example, if there is a packet with sequence number=n where n is
   in the interval[1,32768] traveling through the network, it would be
   possible for the disposition router to receive that packet after it



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   re-advertises the label. Since the label has been released by the
   imposition router, the disposition router SHOULD be expecting the
   next packet to arrive with sequence number to be 1. Receipt of a
   packet with sequence number equal to n will result in n packets
   potentially being rejected by the disposition router until the
   imposition router imposes a sequence number of n+1 into a packet.
   Possible methods to avoid this is for the disposition router to
   always advertise a different PW label, or for the disposition router
   to wait for a sufficient time before attempting to re-advertised a
   recently released label. This is only an issue when sequence number
   processing at the disposition router is enabled.


7.4.2. Label Mapping Release

   In situations where the imposition router wants to restart forwarding
   of packets with sequence number 1, the router shall 1) Send to
   disposition router a label mapping release, and 2) Send to
   disposition router a label mapping request. When sequencing is
   supported, advertisement of a PW label in response to a label mapping
   request MUST also consider the issues discussed in the section on
   Label Mapping Advertisements.


8. IANA Considerations

8.1. FEC Type Name Space

   This document uses two new FEC element types, 128 and 129.  IANA
   already maintains a registry of values of the "FEC Type Name Space"
   for the Label Distribution Protocol (LDP).  In that registry, values
   in the range 128-191 are assignable on a First Come, First Served
   basis.  IANA should assign values 128 and 129 from that space as
   specified in this document.


8.2. Pseudowire Type

   IANA needs to set up a registry of "Pseudowire Types".  These are
   15-bit values.  PW Type values 1 through 63 are to be assigned by
   IANA using the "IETF Consensus" policy defined in RFC2434. PW Type
   values 64 through 127 are to be assigned by IANA, using the "First
   Come First Served" policy defined in RFC2434. VC Type values 128
   through 32767 are vendor-specific, and values in this range are not
   to be assigned by IANA.

   Initial PW type value allocations are specified in "IANA Allocations
   for pseudo Wire Edge to Edge Emulation (PWE3)" [IANA], and should be



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   incorporated by IANA into the registry.


8.3. Interface Parameters

   IANA needs to set up a registry of "Pseudowire Interface Parameter
   Identifiers".  Parameter ID values 1 through 63 are to be assigned by
   IANA using the "IETF Consensus" policy defined in RFC2434. Parameter
   ID values 64 through 127 are to be assigned by IANA, using the "First
   Come First Served" policy defined in RFC2434. Parameter ID values 128
   through 255 are vendor- specific, and values in this range are not to
   be assigned by IANA.

   Initial Pseudowire Interface Parameter Identifier value allocations
   are specified in "IANA Allocations for pseudo Wire Edge to Edge
   Emulation (PWE3)" [IANA], and should be incorporated by IANA into the
   registry.


8.4. Status codes

   RFC 3036 has a range of Status Code values which are assigned by IANA
   on a First Come, First Served basis. These additional status codes,
   and  assigned Values are specified in "IANA Allocations for pseudo
   Wire Edge to Edge Emulation (PWE3)" [IANA].


8.5. Pseudowire Status Codes

   IANA needs to set up a registry of "Pseudowire Status Codes". These
   are bitstrings of length 32. Status bits 1-15 are to be assigned by
   IANA using the "IETF Consensus" policy defined in RFC2434. Initial
   bit allocations are specified in "IANA Allocations for pseudo Wire
   Edge to Edge Emulation (PWE3)" [IANA], and should be incorporated by
   IANA into the registry. PW Status Bits 16 through 23 are to be
   assigned by IANA, using the "First Come First Served" policy defined
   in RFC2434. PW Status Bits 24 through 31 are vendor-specific, and
   values in this range are not to be assigned by IANA.













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9. Security Considerations

   This document specifies the LDP 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 in MPLS
   [8,9,10,11] 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.


9.1. Data-plane Security

   With regard to the security of the data plane, the following areas
   must be considered:

     - MPLS PDU inspection.
     - MPLS PDU spoofing.
     - MPLS PDU alteration.
     - MPLS PSN protocol security.
     - Access Circuit security.
     - Denial of service prevention on the PE routers.

       When a MPLS PSN is used to provide pseudowire service, there is a
       perception that security MUST be at least equal to the currently
       deployed layer2 native protocol networks that the MPLS/PW network
       combination is emulating. This means that the MPLS network SHOULD
       be isolated from outside packet insertion in such a way that it
       SHOULD not be possible to directly insert an MPLS packet into the
       network. To prevent unwanted packet insertion, it is also
       important to prevent unauthorized physical access to the PSN as
       well as unauthorized administrative access to individual network
       elements. The PSN transporting the PWs is an  MPLS backbone, it
       should not accept MPLS packets from its external interfaces (i.e.
       interfaces to CE devices or to other providers' networks) unless
       the top label of the packet was legitimately distributed to the
       system from which the packet is being received. If the packet's
       incoming interface leads to a different SP (rather than to a
       customer), an appropriate trust relationship must also be
       present, including the trust that the other SP also provides
       appropriate security measures.

       The three main security problem faced when using an MPLS network
       to transport PWs are spoofing, alteration, and inspection. First
       there is a possibility that the PW receive endpoint will get a
       PDU which appears to be from the PE encapsulating the PW into the
       PSN, but which was not actually transmitted by the PE originating
       the PW. (I.e., the specified encapsulations do not by themselves
       enable the decapsulator to authenticate the encapsulator.) A



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       second problem is the possibility that the PW PDU will be altered
       between the time it enters PSN and the time it leaves the PSN.
       (I.e., the specified encapsulations do not by themselves assure
       the decapsulator of the packet's integrity.) A third problem is
       the possibility that the PDU's contents will be seen while the
       PDU is in transit through the PSN. (I.e., the specification
       encapsulations do not ensure privacy.) How significant these
       issues are in practice depends on the security requirements of
       the applications whose traffic is being sent through the tunnel,
       and how secure is the PSN itself. E.g. if the PSN is an MPLS
       backbone, with no external MPLS interfaces, then it might be
       secured enought to transport PW PDUs.


9.2. Control Protocol Security

   General security considerations with regard to the use of LDP are
   specified in section 5 of RFC 3036.  Those considerations apply as
   well to the case where LDP is used to set up Pseudowires.

   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 LDP
   session request MUST NOT be accepted unless its IP source address is
   known to be the source of an "eligible" LDP 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 an LDP 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, MAY be used.  This
   provides integrity and authentication for the LDP messages, and
   eliminates the possibility of source address spoofing.  Use of the
   MD5 option does not provide privacy, but privacy of the LDP control
   messages is not usually considered to be important.  As the MD5
   option relies on the configuration of pre-shared keys, it does not
   provide much protection against replay attacks.  In addition, its
   reliance on pre-shared keys may make it very difficult to deploy when



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   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 LDP peer may be one of the eligible LDP 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.


10. Acknowledgments

   The authors wish to acknowledge the contributions of Vach Kompella,
   Vanson Lim, Wei Luo, Himanshu Shah, and Nick Weeds.


11. Normative References

   [LDP] "LDP Specification." L. Andersson, P. Doolan, N. Feldman, A.
        Fredette, B. Thomas. January 2001. RFC3036

   [CEP] "SONET/SDH Circuit Emulation Service Over Packet (CEP)",
        draft-ietf-pwe3-sonet-09.txt (work in progress)

   [SAToP] "Structure-Agnostic TDM over Packet (SAToP)",
        draft-ietf-pwe3-satop-01.txt (work in progress)

   [FRAME] "Frame Relay over Pseudo-Wires", draft-ietf-pwe3-frame-relay-02.txt
        (work in progress )

   [ATM] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP and
        MPLS Networks", draft-ietf-pwe3-atm-encap-05.txt (work in progress)

   [PPPHDLC] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP and
        MPLS Networks", draft-ietf-pwe3-hdlc-ppp-encap-03.txt (work in progress)

   [ETH] "Encapsulation Methods for Transport of Ethernet Frames Over IP/MPLS
        Networks", draft-ietf-pwe3-ethernet-encap-06.txt. (work in progress)

   [RFC3032] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
        Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032




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   [IANA] "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)"
        Martini, Townsley, draft-ietf-pwe3-iana-allocation-04.txt (work in
        progress), April 2004


12. Informative References

   [ITUQ] ITU-T Recommendation Q.933, and Q.922 Specification for Frame Mode Basic
        call control, ITU Geneva 1995

   [802.3] "IEEE 802.3ac-1998" IEEE standard specification.

   [SDH] American National Standards Institute, "Synchronous Optical Network
        Formats," ANSI T1.105-1995.

   [ITUG] ITU Recommendation G.707, "Network Node Interface For The Synchronous
        Digital Hierarchy", 1996.

   [ARCH] "PWE3 Architecture" Bryant, et al., draft-ietf-pwe3-arch-07.txt (work
        in progress), March 2004

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

   [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
        Languages", BCP 18, RFC 2277, January 1998.

[CESoPSN] "Structure-aware TDM Circuit Emulation Service over Packet
   Switched", draft-ietf-pwe3-cesopsn-01.txt (work in progress)

   [note1] FEC element type 128,129 is pending IANA approval.


13. Author Information


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


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net



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   Giles Heron
   Tellabs
   Abbey Place
   24-28 Easton Street
   High Wycombe
   Bucks
   HP11 1NT
   UK
   e-mail: giles.heron@tellabs.com


   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   e-mail: erosen@cisco.com


   Dan Tappan
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   e-mail: tappan@cisco.com


   Toby Smith
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   Laurel Networks, Inc.
   e-mail: tob@laurelnetworks.com


14. Additional Contributing Authors


   Dimitri Stratton Vlachos
   Mazu Networks, Inc.
   125 Cambridgepark Drive
   Cambridge, MA 02140
   e-mail: d@mazunetworks.com









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   Jayakumar Jayakumar,
   Cisco Systems Inc.
   225, E.Tasman, MS-SJ3/3,
   San Jose, CA, 95134
   e-mail: jjayakum@cisco.com


   Alex Hamilton,
   Cisco Systems Inc.
   285 W. Tasman, MS-SJCI/3/4,
   San Jose, CA, 95134
   e-mail: tahamilt@cisco.com


   Steve Vogelsang
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   e-mail: sjv@laurelnetworks.com


   John Shirron
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   Laurel Networks, Inc.
   e-mail: jshirron@laurelnetworks.com


   Andrew G. Malis
   Tellabs
   90 Rio Robles Dr.
   San Jose, CA 95134
   e-mail: Andy.Malis@tellabs.com


   Vinai Sirkay
   Reliance Infocomm
   Dhirubai Ambani Knowledge City
   Navi Mumbai 400 709
   e-mail: vinai@sirkay.com








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   Vasile Radoaca
   Nortel Networks
   600  Technology Park
   Billerica MA 01821
   e-mail: vasile@nortelnetworks.com


   Chris Liljenstolpe
   Cable & Wireless
   11700 Plaza America Drive
   Reston, VA 20190
   e-mail: chris@cw.net


   Dave Cooper
   Global Crossing
   960 Hamlin Court
   Sunnyvale, CA 94089
   e-mail: dcooper@gblx.net


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   e-mail: kireeti@juniper.net
























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15. Full Copyright Statement

   Copyright (C) The Internet Society (2004).  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 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.


16. Appendix A - C-bit Handling Procedures Diagram

                   ------------------
               Y   | Received Label |       N
            -------|  Mapping Msg?  |--------------
            |      ------------------             |
        --------------                            |
        |            |                            |
     -------      -------                         |
     | C=0 |      | C=1 |                         |
     -------      -------                         |
        |            |                            |
        |    ----------------                     |
        |    | Control Word |     N               |
        |    |    Capable?  |-----------          |
        |    ----------------          |          |
        |          Y |                 |          |
        |            |                 |          |
        |   ----------------           |          |
        |   | Control Word |  N        |          |
        |   |  Preferred?  |----       |          |
        |   ----------------   |       |          |
        |          Y |         |       |          |
        |            |         |       |   ----------------
        |            |         |       |   | Control Word |
        |            |         |       |   |  Preferred?  |
        |            |         |       |   ----------------
        |            |         |       |     N |     Y |
        |            |         |       |       |       |
      Send         Send      Send    Send    Send    Send
       C=0          C=1       C=0     C=0     C=0     C=1
                               |       |       |       |
                            ----------------------------------



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                            | If receive the same as sent,   |
                            | PW setup is complete. If not:  |
                            ----------------------------------
                               |       |       |       |
                              ------------------- -----------
                              |     Receive     | | Receive |
                              |       C=1       | |   C=0   |
                              ------------------- -----------
                                       |               |
                                 Wait for the        Send
                                 next message     Wrong C-Bit
                                                       |
                                                  Send Label
                                               Mapping Message





































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