Subject: [Fwd: draft-ietf-pwe3-control-protocol-02.txt]

From: Luca Martini <[email-masked]>
Date: Wed, 19 Mar 2003 11:34:54 -0700

Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: August 2003                Level 3 Communications, LLC.

Toby Smith                                                 Eric C. Rosen
Laurel Networks, Inc.                                Cisco Systems, Inc.
Giles Heron
PacketExchange Ltd.

                                                             February 2003

                 Pseudowire Setup and Maintenance using LDP

                  draft-ietf-pwe3-control-protocol-02.txt

Status of this Memo

     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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     Internet-Drafts are draft documents valid for a maximum of six months
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     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt.

     The list of Internet-Draft Shadow Directories can be accessed at
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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 SONET circuit 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  ..........................   3
      2      Introduction  ...........................................   3
      3      The Pseudowire Label  ...................................   5
      4      Details Specific to Particular Emulated Services  .......   6
      4.1    Frame Relay  ............................................   6
      4.2    ATM  ....................................................   6
      4.2.1  ATM AAL5 SDU VCC Transport  .............................   6
      4.2.2  ATM Transparent Cell Transport  .........................   7
      4.2.3  ATM n-to-one VCC and VPC Cell Transport  ................   7
      4.2.4  OAM Cell Support  .......................................   7
      4.2.5  ILMI Support  ...........................................   8
      4.2.6  IP Layer2 Transport  ....................................   8
      4.2.7  ATM AAL5 PDU VCC Transport  .............................   8
      4.2.8  ATM one-to-one VCC and VPC Cell Transport  ..............   9
      4.3    Ethernet VLAN  ..........................................   9
      4.4    Ethernet  ...............................................   9
      4.5    HDLC and PPP  ...........................................   9
      5      LDP  ....................................................  10
      5.1    The PWid FEC Element  ...................................  10
      5.2    The Generalized ID FEC Element  .........................  12
      5.2.1  Attachment Identifiers  .................................  12
      5.2.2  Encoding the Generalized ID FEC Element  ................  14
      5.2.3  Procedures  .............................................  15
      5.3    Interface Parameters Field  .............................  16
      5.3.1  PW types for which the control word is REQUIRED  ........  18
      5.3.2  PW types for which the control word is NOT mandatory  ...  18
      5.3.3  Status codes  ...........................................  19
      5.4    LDP label Withdrawal procedures  ........................  20
      5.5    Sequencing Considerations  ..............................  20
      5.5.1  Label Mapping Advertisements  ...........................  20
      5.5.2  Label Mapping Release  ..................................  21
      6      IANA Considerations  ....................................  21
      7      Security Considerations  ................................  21
      8      References  .............................................  22
      9      Author Information  .....................................  23
     10      Additional Contributing Authors  ........................  24
     11      Appendix A - C-bit Handling Procedures Diagram  .........  27

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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. Introduction

     In [7], [10], and [12] it is explained how to encapsulate a layer 2
     Protocol Data Unit (PDU) for transmission over an IP and/or MPLS
     network.  Those specifications 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 header"; this will require an additional header to be
     prepended to the packet.

     An accompanying document [8] also describes a method 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)[5]/Synchronous Digital Hierarchy (SDH) [6]. 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.

     In this document, we specify the use of the MPLS Label Distribution
     Protocol, LDP [RFC3036], as a protocol 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
     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

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     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, but it could equally well
     be an IP tunnel, a GRE tunnel, an IPsec tunnel, 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 [PWE3-FRAME] to support the Ethernet PW emulated
     services.

             Native    |<----- Pseudo Wire ---->|    Native
             Layer2    |                        |    Layer2
            Service    |  |<-- PSN Tunnel -->|  |    Service
               |       V  V                  V  V       |
               |     +----+                  +----+     |
     +----+    |     | PE1|==================| PE2|     |    +----+
     |    |----------|............PW1.............|----------|    |
     | CE1|    |     |    |                  |    |     |    |CE2 |
     |    |----------|............PW2.............|----------|    |
     +----+    |     |    |==================|    |     |    +----+
          ^          +----+                  +----+     |    ^
          |      Provider Edge 1         Provider Edge 2     |
          |                                                  |
          |<-------------- Emulated Service ---------------->|

        Figure 1: PWE3 Reference Model

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

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        Figure 2: Ethernet 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.

3. 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; if the PSN tunnel is, e.g., a GRE tunnel, then putting on
     the tunnel encapsulation requires prepending an IP header and a GRE
     header.

     We presuppose that an arbitrary 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 PE 2 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
     serves as the pseudowire demultiplexor field.  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.

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     Note that the PW label must always be at the bottom of the packet's
     label stack.

     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.

4. Details Specific to Particular Emulated Services

4.1. Frame Relay

     When emulating a frame relay service, the Frame Relay PDUs are
     encapsulated according to the procedures defined in [7]. 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 [2] Q.933 Annex A.5 sited in IA FRF1.1, PE MUST
     withdraw the PW label that corresponds to the frame relay DLCI. The
     Egress PE SHOULD generate the corresponding errors and alarms as
     defined in [2] on the egress Frame relay VC.

4.2. ATM

4.2.1. ATM AAL5 SDU VCC Transport

     ATM AAL5 CSPS-SDUs are encapsulated according to [10] 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.

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4.2.2. 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 [10], 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. [10] ATM cell n-to-one mode
     allows for grouping of multiple cells into a single MPLS frame.
     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, should only concatenate cells up to the "Maximum Number of
     concatenated ATM cells" parameter received as part of the FEC
     element.

4.2.3. 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 [10], ATM 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.

4.2.4. OAM Cell Support

     OAM cells MAY be transported on the VC 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 VC LSP
     that contain a VC label with the T bit set [10]. When operating in
     AAL5 SDU transport mode an PE that supports transport of OAM cells
     using the T bit defined in [7], or an PE operating in any of the
     three cell transport modes MUST follow the procedures outlined in [9]
     section 8 for mode 0 only, in addition to the applicable procedures
     specified in [6].

4.2.4.1. OAM Cell Emulation Mode

     AN PE that does not support transport of OAM cells across an LSP MAY
     provide OAM support on ATM PVCs using the following procedures:

     A pair of PEs MAY emulate a bi-directional ATM VC by two uni-
     directional LSPs.  If an F5 end-to-end OAM cell is received from a

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     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.

     The ingress PE, PE1, may also optionally be configured to
     periodically generate F5 end-to-end loopback OAM cells on a VC. If
     the PE fails to receive a response to an F5 end-to-end loopback OAM
     cell for a pre-defined period of time it MUST withdraw the label
     mapping for the VC.

     If an ingress PE, PE1, receives an AIS F5 OAM cell, fails to receive
     a pre-defined number of the End-to-End loop OAM cells, or a physical
     interface goes down, it MUST withdraw the label mappings for all VCs
     associated with the failure. When a PW label mapping is withdrawn,
     the egress PE, PE2, MUST generate AIS F5 OAM cells on the VC
     associated with the withdrawn label mapping. In this mode it is very
     useful to apply a unique group ID to each interface. In the case
     where a physical interface goes down, a wild card label withdraw can
     be sent to all LDP neighbors, greatly reducing the signaling response
     time.

4.2.5. 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 withdraw the label mappings for all VCs associated with the
     failure. When a PW label mapping is withdrawn, the egress PE SHOULD
     notify its client of this failure by deleting the VC using ILMI.

4.2.6. IP Layer2 Transport

     This mode switches IP packets into a Peudo-Wire. the encapsulation
     used is according to [3]. IP interworking is implementation specific,
     part of the NSP function [13], and is outside the scope of this
     document.

4.2.7. ATM AAL5 PDU VCC Transport

     ATM AAL5 CSPS-PDUs are encapsulated according to [10] 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

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     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.

4.2.8. 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 [10], 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.

4.3. Ethernet VLAN

     The Ethernet frame will be encapsulated according to the procedures
     in [12] 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 withdraw the label
     mappings for all PWs associated with the port.

4.4. Ethernet

     The Ethernet frame will be encapsulated according to the procedures
     in [12].  If the PE detects a failure on the Ethernet physical port,
     or the port is administratively disabled, the corresponding PW label
     mapping MUST be withdrawn.

4.5. HDLC and PPP

     HDLC and PPP frames are encapsulated according to the procedures in
     [11]. If the MPLS edge PE detects that the physical link has failed,
     or the port is administratively disabled, it MUST withdraw the label
     mapping that corresponds to the HDLC or PPP link.

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5. LDP

     The PW label bindings are distributed using the LDP downstream
     unsolicited mode described in [1]. 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
     pseudowires, the Generic Label TLV MUST be used.

5.1. 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:

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    VC tlv     |C|         PW type             |VC 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)" [14].

       - Control word bit (C)

         The  highest  order bit (C) of the PW type is used to flag the
         presence  of  a  control  word  ( defined in [7] ) as follows:

                 bit 15 = 1 control word present on this VC.
                 bit 15 = 0 no control word present on this VC.

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

       - VC 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
         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 VC 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
         to remote PEs upon physical port failure.

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       - 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 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.

5.2. The Generalized ID FEC Element

     There are cases where the PWid FEC element cannot be used, because
     both endpoints have not been provisioned with a common 32-bit PWid.
     In such cases, the "Generalized ID FEC Element" is used instead.
     This is FEC type 129 (provisionally, subject to assignment by IANA).
     It differs from the PWid FEC element in that the PWid and the group
     id are eliminated, and their place is taken by a generalized
     identifier field as described below.  The Generalized ID FEC element
     includes a PW type field, a C bit, and an interface parameters field;
     these three fields are identical to those in the PWid FEC, and are
     used as discussed in the previous section.

5.2.1. Attachment Identifiers

     As discussed in [13], 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

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     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 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  VCs (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.

     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).

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       - 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.

5.2.2. Encoding the Generalized ID FEC Element

     FEC element type 129 is used.   The FEC element 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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     129       |C|         VC Type             |VC info Length |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                          Parameters                           |
        |                              "                                |
        |                              "                                |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     additional Parameters are:

       - SAII, encoded as a one byte length field followed by the SAI.

       - TAII, encoded as a one byte length field followed by the TAI.

       - AGI, encoded as a one byte length field followed by the AGI.

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

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         byte to 0.

5.2.3. 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 TAI would 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", and the processing of the Mapping message is complete.

     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.

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5.3. 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)" [14].

     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, and 7,
         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, and 0x0a, 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, 99 - 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 status code (see section "Status codes" below).

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5.3.1. 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.

5.3.2. 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.
        -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).

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     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. Note that receiving a Label Withdraw should not
     cause a corresponding Label Release to be sent.

     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.

     The diagram in Appendix A illustrates the above procedure.

5.3.3. 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)" [14].

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5.4. LDP label Withdrawal procedures

     As mentioned above the Group ID field can be used to withdraw all PW
     labels associated with a particular group ID. This procedure is
     OPTIONAL, and if it is implemented the LDP label withdraw message
     should be as follows: the PW information length field is set to 0,
     the PW ID field is not present, and the interface parameters field is
     not present. 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 a withdraw message, but are not required
     to send it.

     The interface parameters field 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.

5.5. 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

5.5.1. 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
     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

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     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.

5.5.2. 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.

6. IANA Considerations

     As specified in this document, a Virtual Circuit FEC element contains
     the PW Type field. PW type value 0 is reserved. PW type values 1
     through 10 are defined in this document. PW type values 11 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 [RFC
     2434]. PW type values 128 through 32767 are vendor-specific, and
     values in this range are not to be assigned by IANA.

     As specified in this document, a Pseudo Wire FEC element contains the
     Interface Parameters field, which is a list of one or more
     parameters, and each parameter is identified by the Parameter ID
     field. Parameter ID value 0 is reserved. Parameter ID values 1
     through 6 are defined in this document.  Parameter ID values 7
     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.

7. Security Considerations

     This document does not affect the underlying security issues of MPLS.

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8. References

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

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

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

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

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

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

     [7] "Encapsulation Methods for Transport of Frame-Relay Over IP and
     MPLS Networks", draft-ietf-pwe3-frame-encap-01.txt. ( work in
     progress )

     [8] "SONET/SDH Circuit Emulation Service Over Packet (CEP)",
        draft-ietf-pwe3-sonet-01.txt ( Work in progress )

     [9] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM
     over SONET/SDH Transport (FAST)

     [10] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
     and MPLS Networks", draft-ietf-pwe3-atm-encap-00.txt ( work in
     progress )

     [11] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP
     and MPLS Networks", draft-martini-ppp-hdlc-encap-mpls-00.txt. ( work
     in progress )

     [12] "Encapsulation Methods for Transport of Ethernet Frames Over
     IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-01.txt. ( work in
     progress )

     [13] "Protocol Layering in PWE3" Bryant, et al., draft-ietf-pwe3-
     protocol-layer-00.txt ( work in progress ), November 2002.

     [14] "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)"
     Martini, Townsley, draft-ietf-pwe3-iana-allocation-00.txt ( work in
     progress ), February 2003

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     [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.

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

     [note2] Status codes assigment is pending IANA approval.

9. Author Information

     Luca Martini
     Level 3 Communications, LLC.
     1025 Eldorado Blvd.
     Broomfield, CO, 80021
     e-mail: [email-masked]

     Nasser El-Aawar
     Level 3 Communications, LLC.
     1025 Eldorado Blvd.
     Broomfield, CO, 80021
     e-mail: [email-masked]

     Giles Heron
     PacketExchange Ltd.
     The Truman Brewery
     91 Brick Lane
     LONDON E1 6QL
     United Kingdom
     e-mail: [email-masked]

     Eric Rosen
     Cisco Systems, Inc.
     250 Apollo Drive
     Chelmsford, MA, 01824
     e-mail: [email-masked]

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     Dan Tappan
     Cisco Systems, Inc.
     250 Apollo Drive
     Chelmsford, MA, 01824
     e-mail: [email-masked]

10. Additional Contributing Authors

     Dimitri Stratton Vlachos
     Mazu Networks, Inc.
     125 Cambridgepark Drive
     Cambridge, MA 02140
     e-mail: [email-masked]

     Jayakumar Jayakumar,
     Cisco Systems Inc.
     225, E.Tasman, MS-SJ3/3,
     San Jose, CA, 95134
     e-mail: [email-masked]

     Alex Hamilton,
     Cisco Systems Inc.
     285 W. Tasman, MS-SJCI/3/4,
     San Jose, CA, 95134
     e-mail: [email-masked]

     Steve Vogelsang
     Laurel Networks, Inc.
     Omega Corporate Center
     1300 Omega Drive
     Pittsburgh, PA 15205
     e-mail: [email-masked]

     John Shirron
     Omega Corporate Center
     1300 Omega Drive
     Pittsburgh, PA 15205
     Laurel Networks, Inc.
     e-mail: [email-masked]

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     Toby Smith
     Omega Corporate Center
     1300 Omega Drive
     Pittsburgh, PA 15205
     Laurel Networks, Inc.
     e-mail: [email-masked]

     Andrew G. Malis
     Vivace Networks, Inc.
     2730 Orchard Parkway
     San Jose, CA 95134
     Phone: +1 408 383 7223
     Email: [email-masked]

     Vinai Sirkay
     Vivace Networks, Inc.
     2730 Orchard Parkway
     San Jose, CA 95134
     e-mail: [email-masked]

     Vasile Radoaca
     Nortel Networks
     600  Technology Park
     Billerica MA 01821
     e-mail: [email-masked]

     Chris Liljenstolpe
     Cable & Wireless
     11700 Plaza America Drive
     Reston, VA 20190
     e-mail: [email-masked]

     Dave Cooper
     Global Crossing
     960 Hamlin Court
     Sunnyvale, CA 94089
     e-mail: [email-masked]

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     Kireeti Kompella
     Juniper Networks
     1194 N. Mathilda Ave
     Sunnyvale, CA 94089
     e-mail: [email-masked]

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11. 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
                                 |       |       |       |
                              ----------------------------------
                              | 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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