





Internet Engineering Task Force (IETF)                   L. Martini, Ed.
Request for Comments: 8077                                 G. Heron, Ed.
STD: 84                                                            Cisco
Obsoletes: 4447, 6723                                      February 2017
Category: Standards Track
ISSN: 2070-1721


                    Pseudowire Setup and Maintenance
              Using the Label Distribution Protocol (LDP)

Abstract

   Layer 2 services (such as Frame Relay, Asynchronous Transfer Mode,
   and Ethernet) can be emulated over an MPLS backbone by encapsulating
   the Layer 2 Protocol Data Units (PDUs) and then transmitting them
   over pseudowires (PWs).  It is also possible to use pseudowires to
   provide low-rate Time-Division Multiplexed and Synchronous Optical
   NETworking circuit emulation over an MPLS-enabled network.  This
   document specifies a protocol for establishing and maintaining the
   pseudowires, using extensions to the Label Distribution Protocol
   (LDP).  Procedures for encapsulating Layer 2 PDUs are specified in
   other documents.

   This document is a rewrite of RFC 4447 for publication as an Internet
   Standard.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8077.











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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

























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Table of Contents

   1. Introduction ....................................................4
   2. Changes from RFC 4447 ...........................................6
   3. Specification of Requirements ...................................6
   4. The Pseudowire Label ............................................7
   5. Details Specific to Particular Emulated Services ................9
      5.1. IP Layer 2 Transport .......................................9
   6. LDP .............................................................9
      6.1. The PWid FEC Element .......................................9
      6.2. The Generalized PWid FEC Element ..........................11
           6.2.1. Attachment Identifiers .............................12
           6.2.2. Encoding the Generalized PWid FEC Element ..........14
                  6.2.2.1. PW Interface Parameters TLV ...............15
                  6.2.2.2. PW Group ID TLV ...........................15
           6.2.3. Signaling Procedures ...............................16
      6.3. Signaling of Pseudowire Status ............................17
           6.3.1. Use of Label Mapping Messages ......................17
           6.3.2. Signaling PW Status ................................18
           6.3.3. Pseudowire Status Negotiation Procedures ...........19
      6.4. Interface Parameter Sub-TLV ...............................20
      6.5. LDP Label Withdrawal Procedures ...........................21
   7. Control Word ...................................................22
      7.1. PW Types for Which the Control Word Is REQUIRED ...........22
      7.2. PW Types for Which the Control Word Is NOT Mandatory ......22
      7.3. Control-Word Renegotiation by Label Request Message .......24
      7.4. Sequencing Considerations .................................25
           7.4.1. Label Advertisements ...............................25
           7.4.2. Label Release ......................................25
   8. IANA Considerations ............................................26
      8.1. LDP TLV TYPE ..............................................26
      8.2. LDP Status Codes ..........................................26
      8.3. FEC Type Name Space .......................................26
   9. Security Considerations ........................................26
      9.1. Data-Plane Security .......................................27
      9.2. Control-Plane Security ....................................28
   10. Interoperability and Deployment ...............................29
   11. References ....................................................29
      11.1. Normative References .....................................29
      11.2. Informative References ...................................30
   Acknowledgments ...................................................31
   Contributors ......................................................32
   Authors' Addresses ................................................35








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1.  Introduction

   [RFC4619], [RFC4717], [RFC4618], and [RFC4448] explain how to
   encapsulate a Layer 2 Protocol Data Unit (PDU) for transmission over
   an MPLS-enabled network.  Those documents specify that a "pseudowire
   header", consisting of a demultiplexer field, will be prepended to
   the encapsulated PDU.  The pseudowire demultiplexer field is
   prepended before transmitting a packet on a pseudowire.  When the
   packet arrives at the remote endpoint of the pseudowire, the
   demultiplexer is what enables the receiver to identify the particular
   pseudowire on which the packet has arrived.  To transmit the packet
   from one pseudowire endpoint to another, the packet may need to
   travel through a "Packet Switched Network (PSN) tunnel"; this will
   require that an additional header be prepended to the packet.

   [RFC4842] and [RFC4553] specify two methods for transporting time-
   division multiplexing (TDM) digital signals (TDM circuit emulation)
   over a packet-oriented MPLS-enabled network.  The transmission system
   for circuit-oriented TDM signals is the Synchronous Optical Network
   (SONET) [ANSI] / 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.  A pseudowire
   demultiplexer and a PSN tunnel header are prepended to this
   encapsulation.

   [RFC4553] describes methods for transporting low-rate time-division
   multiplexing (TDM) digital signals (TDM circuit emulation) over PSNs,
   while [RFC4842] 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.  [RFC4553] does this by encapsulating an arbitrary but
   constant amount of the TDM data in each packet, and the other methods
   encapsulate TDM structures.

   In this document, we specify the use of the MPLS Label Distribution
   Protocol (LDP) [RFC5036] as a protocol for setting up and maintaining
   the pseudowires.  In particular, we define new TLVs, Forwarding
   Equivalence Class (FEC) elements, parameters, and codes 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 demultiplexer field value to a pseudowire and how it
   informs the remote endpoint of the binding.  We also specify
   procedures for reporting pseudowire status changes, for passing
   additional information about the pseudowire as needed, and for
   releasing the bindings.  These procedures are intended to be
   independent of the underlying version of IP used for LDP signaling.



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   In the protocol specified herein, the pseudowire demultiplexer field
   is an MPLS label.  Thus, the packets that are transmitted from one
   end of the pseudowire to the other are MPLS packets, which must be
   transmitted through an MPLS tunnel.  However, if the pseudowire
   endpoints are immediately adjacent and penultimate hop popping
   behavior is in use, the MPLS tunnel may not be necessary.  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 MPLS LSP, or any
   other sort of tunnel that can carry MPLS packets.  Procedures for
   setting up and maintaining the MPLS 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 that are
   derived from [RFC3985] to support the PW emulated services.

         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudowire ------->|          |
         |          |                            |          |
         |Attachment|    |<-- PSN Tunnel -->|    |Attachment|
         |  Circuit V    V                  V    V  Circuit |
         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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    +-----------------+                           +-----------------+
    |Emulated Service |                           |Emulated Service |
    |(e.g., TDM, ATM) |<==== Emulated Service ===>|(e.g., TDM, ATM) |
    +-----------------+                           +-----------------+
    |    Payload      |                           |    Payload      |
    |  Encapsulation  |<====== Pseudowire =======>|  Encapsulation  |
    +-----------------+                           +-----------------+
    |PW Demultiplexer |                           |PW Demultiplexer |
    |   PSN Tunnel,   |<======= PSN Tunnel ======>|  PSN Tunnel,    |
    | PSN & Physical  |                           | PSN & Physical  |
    |     Layers      |                           |    Layers       |
    +-------+---------+        ___________        +---------+-------+
            |                /             \                 |
            +===============/     PSN       \================+
                            \               /
                             \_____________/

              Figure 2: PWE3 Protocol Stack Reference Model

   For the purpose of this document, PE1 (Provider Edge 1) 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 and
   decapsulated at PE2, and transmitted out of PE2.

2.  Changes from RFC 4447

   The changes in this document are mostly minor fixes to spelling and
   grammar, or clarifications to the text, which were either noted as
   errata to [RFC4447] or found by the editors.

   Additionally, Section 7.3 ("Control-Word Renegotiation by Label
   Request Message") has been added, obsoleting [RFC6723].  The diagram
   of C-bit handling procedures has also been removed.  A note has been
   added in Section 6.3.2 to clarify that the C-bit is part of the FEC.

   A reference has also been added to [RFC7358] to indicate the use of
   downstream unsolicited mode to distribute PW FEC label bindings,
   independent of the negotiated label advertisement mode of the LDP
   session.

3.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].






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4.  The Pseudowire Label

   Suppose that it is desired to transport Layer 2 PDUs from ingress LSR
   PE1 to egress LSR PE2, across an intervening MPLS-enabled network.
   We assume that there is an MPLS 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 an "MPLS tunnel header" and sending the
   result to one of its adjacencies.  The MPLS tunnel is an MPLS Label
   Switched Path (LSP); thus, putting on an MPLS tunnel encapsulation is
   a matter of pushing on an MPLS label.

   We presuppose that a large number of pseudowires can be carried
   through a single MPLS tunnel.  Thus, it is never necessary to
   maintain state in the network core for individual pseudowires.  We do
   not presuppose that the MPLS tunnels are point to point; although the
   pseudowires are point to point, the MPLS tunnels may be multipoint to
   point.  We do not presuppose that PE2 will even be able to determine
   the MPLS tunnel through which a received packet was transmitted.
   (For example, if the MPLS 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 that
   serves as the pseudowire demultiplexer field shown in Figure 2.  Call
   this label the "PW label".

   When PE1 sends a Layer 2 PDU to PE2, it creates an MPLS packet by
   adding the PW label to the packet, thus creating the first entry of
   the label stack.  If the PSN tunnel is an MPLS LSP, the PE1 pushes
   another label (the tunnel label) onto the packet as the second entry
   of the label stack.  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.

   If the payload of the MPLS packet is, for example, an ATM Adaptation
   Layer 5 (AAL5) PDU, the PW label will generally correspond to a
   particular ATM Virtual Circuit (VC) at PE2.  That is, PE2 needs to be
   able to infer from the PW label the outgoing interface and the
   VPI/VCI (Virtual Path Identifier / Virtual Circuit Identifier) value
   for the AAL5 PDU.  If the payload is a Frame Relay PDU, then PE2
   needs to be able to infer from the PW label the outgoing interface
   and the Data Link Connection Identifier (DLCI) value.  If the payload
   is an Ethernet frame, then PE2 needs to be able to infer from the PW
   label the outgoing interface, and perhaps the VLAN identifier.  This
   process is unidirectional and will be repeated independently for



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   bidirectional operation.  When using the PWid FEC Element, it is
   REQUIRED that the same PW ID and PW type be assigned for a given
   circuit in both directions.  The Group ID (see below) MUST NOT be
   required to match in both directions.  The transported frame MAY be
   modified when it reaches the egress router.  If the header of the
   transported Layer 2 frame is modified, this MUST be done at the
   egress LSR only.  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 does not specify a method for distributing the MPLS
   tunnel label or any other labels that may appear above the PW label
   on the stack.  Any acceptable method of MPLS label distribution will
   do.  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 exchange PW FEC
   label bindings in downstream unsolicited mode, independent of the
   negotiated label advertisement mode of the LDP session according to
   the specifications in [RFC7358].  LDP's "liberal label retention"
   mode SHOULD be used.  However, all the LDP procedures that are
   specified in [RFC5036] and that are also applicable to this protocol
   specification MUST be implemented.

   This document requires that a receiving LSR MUST respond to a Label
   Request message with either a Label Mapping for the requested label
   or a Notification message that indicates why it cannot satisfy the
   request.  These procedures are specified in [RFC5036], Sections 3.5.7
   ("Label Mapping Message") and 3.5.8 ("Label Request Message").  Note
   that sending these responses is a stricter requirement than is
   specified in [RFC5036], but these response messages are REQUIRED to
   ensure correct operation of this protocol.

   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.  PW encapsulation is always
   symmetrical in both directions of traffic along a specific PW,
   whether or not the PW uses an LDP control plane.

   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 identity of the other endpoint.  There are also protocol
   mechanisms specified herein that can be used to support switched
   services and other provisioning models.  However, the use of the
   protocol mechanisms to support those other models and services is not
   described in this document.




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5.  Details Specific to Particular Emulated Services

5.1.  IP Layer 2 Transport

   This mode carries IP packets over a pseudowire.  The encapsulation
   used is according to [RFC3032].  The PW control word MAY be inserted
   between the MPLS label stack and the IP payload.  The encapsulation
   of the IP packets for forwarding on the Attachment Circuit is
   implementation specific, is part of the native service processing
   (NSP) function [RFC3985], and is outside the scope of this document.

6.  LDP

   The PW label bindings are distributed using the LDP downstream
   unsolicited mode described in [RFC5036].  The PEs will establish an
   LDP session using the Extended Discovery mechanism described in
   Sections 2.4.2 and 2.5 of [RFC5036].

   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.

   The LDP base specification has several kinds of label TLVs, including
   the Generic Label TLV, as specified in Section 3.4.2.1 of [RFC5036].
   For setting up and maintaining 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 0x80 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PWid (0x80)  |C|         PW type             |PW info length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Group ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           PW ID                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Interface Parameter Sub-TLV                    |
   |                              "                                |
   |                              "                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   -  Control word bit (C)

      The C-bit 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 Section 7 ("Control Word") for further explanation.

   -  PW type

      A 15-bit quantity containing a value that represents the type of
      PW.  Assigned Values are specified in "IANA Allocations for
      Pseudowire Edge to Edge Emulation (PWE3)" [RFC4446].

   -  PW info length

      Length of the PW ID field and the Interface Parameter Sub-TLV
      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
      are there any Interface Parameter Sub-TLVs.

   -  Group ID

      An arbitrary 32-bit value that 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 Group ID at ingress could be part
      of a Group ID assigned to the virtual tunnel for transport to the
      egress router.  The Group ID is very useful for sending wildcard
      label withdrawals or PW wildcard status Notification messages 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 Parameter Sub-TLV

      This variable length TLV is used to provide interface-specific
      parameters, such as Attachment Circuit MTU.

      Note that as the Interface Parameter Sub-TLV is 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, that 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 setup 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 PWid 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 PWid 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
   configured.

   The "Generalized PWid FEC Element" is FEC type 0x81.

   The Generalized PWid FEC Element does not contain anything
   corresponding to the Group ID of the PWid FEC Element.  The
   functionality of the Group ID is provided by a separate optional LDP
   TLV, the PW Group ID TLV, described in Section 6.2.2.2.  The
   interface parameters field of the PWid FEC Element is also absent;
   its functionality is replaced by the optional PW Interface Parameters
   TLV, described in Section 6.2.2.1.







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6.2.1.  Attachment Identifiers

   As discussed in [RFC3985], a pseudowire can be thought of as
   connecting two "forwarders".  The protocol used to set up 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 that 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 that
   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 IP address, 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 that is shared by all the Attachment
   PWs (or pools thereof) that 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 applications, 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 PW direction from PE1 to PE2
   can be identified as:

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

   and the PW direction from PE2 to PE1 can be identified by:

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






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   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 that uses the Generalized PWid
   FEC must specify the rules for performing this association.



























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6.2.2.  Encoding the Generalized PWid FEC Element

   FEC element type 0x81 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Gen PWid (0x81)|C|         PW Type             |PW info length |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   AGI Type    |    Length     |      Value                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                    AGI  Value (contd.)                        ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   AII Type    |    Length     |      Value                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                   SAII  Value (contd.)                        ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   AII Type    |    Length     |      Value                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                   TAII Value (contd.)                         ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This document does not specify the AII and AGI type field values;
   specification of the type field values to be used for a particular
   application is part of the specification of that application.  IANA
   has assigned these values using the method defined in [RFC4446].

   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, and AGI, combined in octets.  If this value is 0, then it
   references all PWs using the specific Group ID (specified in the PW
   Group ID TLV).  In this case, there are no other FEC element fields
   (AGI, SAII, etc.) present, nor any PW Interface Parameters TLVs.

   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




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   occurrences of an AGI (or SAII or TAII), the two occurrences are
   considered identical if the Type, Length, and Value fields of one are
   identical, respectively, to those of the other.

6.2.2.1.  PW Interface Parameters TLV

   This TLV MUST only be used when sending the Generalized PWid 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             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Sub-TLV Type  |    Length     |    Variable Length Value      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Variable Length Value                 |
    |                             "                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A more detailed description of this field can be found in Section 6.4
   ("Interface Parameter Sub-TLV").

6.2.2.2.  PW Group ID 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 Group ID TLV (0x096C)  |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Value                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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






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   The PW Group ID TLV is not part of the FEC and will not be advertised
   except in the PW FEC advertisement.  The advertising PE MAY use the
   wildcard withdraw semantics, but the remote PEs MUST implement
   support for wildcard messages.  This TLV MUST only be used when
   sending the Generalized PWid FEC.

   To issue a wildcard command (status or withdraw):

   -  Set the PW Info Length to 0 in the Generalized PWid FEC Element.

   -  Send only the PW Group 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 auto-
   discovery procedure.

   The egress PE (PE1), which has knowledge of the ingress PE, initiates
   the setup by sending a Label Mapping message to the ingress PE (PE2).
   The Label Mapping message contains the FEC TLV, carrying the
   Generalized PWid FEC Element (type 0x81).  The Generalized PWid FEC
   Element contains the AGI, SAII, and TAII information.

   Next, 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 Virtual Private Wire Service (VPWS) provisioning
   models, the TAI might, for example, be a string that identifies a
   particular Attachment Circuit, such as "ATM3VPI4VCI5", or it might,
   for example, be a string, such as "Fred", that is associated by
   configuration with a particular Attachment Circuit.  In Virtual
   Private LAN Service (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 of
   "Unassigned/Unrecognized TAI", and the processing of the Label
   Mapping message is complete.

   The FEC TLV sent in a Label Release message is the same as the FEC
   TLV received in the Label Mapping message being released (but without
   the interface parameter TLV).  More generally, the FEC TLV is the





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   same in all LDP messages relating to the same PW.  In a Label Release
   message, 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.  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 [RFC5036].

   If PE2 decides to accept the Label Mapping message, then it has to
   make sure that a PW LSP is set up in the opposite (PE1-->PE2)
   direction.  If it has already signaled for the corresponding PW LSP
   in that direction, nothing more needs to 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
   the SAI and TAI are reversed.

   Thus, a bidirectional PW consists of two LSPs, where the FEC of one
   has the SAII and TAII reversed with respect to the FEC of the other.

6.3.  Signaling of Pseudowire Status

6.3.1.  Use of Label Mapping Messages

   The PEs MUST send Label Mapping messages to their peers as soon as
   the PW is configured and administratively enabled, regardless of the
   Attachment Circuit state.  The PW label should not be withdrawn
   unless the operator administratively configures the pseudowire 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 legacy means of signaling PW status and AC status.  In
   any case, if the label-to-PW binding is not available, the PW MUST be
   considered in the down state.

   Once the PW status negotiation procedures are completed, if they
   result in the use of the label withdraw method for PW status
   communication, and this method is not supported by one of the PEs,
   then that PE must send a Label Release message to its peer with the
   following error:

   "Label Withdraw PW Status Method Not Supported"

   If the label withdraw method for PW status communication is selected
   for the PW, it will result in the Label Mapping message being
   advertised only if the Attachment Circuit is active.  The PW status
   signaling procedures described in this section MUST be fully
   implemented.



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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:

     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                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The status code is a 4-octet bit field as specified in "IANA
   Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [RFC4446].
   The Length field 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 than a single failure at once.  Each fault can be cleared by
   sending an appropriate Notification message in which the respective
   bit is 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 as described in [RFC5036].  The format of the
   Notification message for carrying the PW Status is as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|   Notification (0x0001)     |      Message Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Message ID                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Status (TLV)                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      PW Status TLV                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           PWid FEC TLV or Generalized ID FEC TLV              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                PW Group ID TLV (Optional)                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   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 field is set to 0.

   The PW FEC TLV SHOULD NOT include the Interface Parameter Sub-TLVs,
   as they are ignored in the context of this message.  However, the PW
   FEC TLV MUST include the C-bit, where applicable, as it is part of
   the FEC.  When a PE's Attachment Circuit encounters an error, use of
   the PW Notification message allows the PE to send a single "wildcard"
   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 with only the PW
   Group ID TLV.

   As mentioned above, the Group ID field of the PWid FEC Element, or
   the PW Group ID TLV used with the Generalized PWid 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 Parameter Sub-TLVs are 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
   Group ID TLV is included, and the PW Interface Parameters TLV is
   omitted.  For the purpose of this document, this is called the
   "wildcard 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 in the initial Label
   Mapping message following the PW FEC and the Interface Parameter Sub-
   TLVs.  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 PWid FEC              +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Interface Parameters                    |
    |                              "                                |
    |                              "                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|0| Generic Label (0x0200)    |      Length                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Label                                                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|0|     PW Status (0x096A)    |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Status Code                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If a PW Status TLV is included in the initial Label Mapping message
   for a PW, then if the Label Mapping message from the remote PE for
   that PW does not include a PW Status TLV, or if the remote PE does
   not support the PW Status TLV, the PW will revert to the label
   withdraw method of signaling PW status.  Note that if the PW Status
   TLV is not supported by the remote peer, the peer will automatically
   ignore it, since the I (ignore) bit is set in the TLV.  The PW Status
   TLV, therefore, will not be present in the corresponding FEC
   advertisement from the remote LDP peer, which results in exactly the
   above behavior.

   If the PW Status TLV is not present following the FEC TLV in the
   initial PW Label Mapping 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 message.  Subsequent updates
   of PW status are conveyed through the Notification message.

6.4.  Interface Parameter Sub-TLV

   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:



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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Sub-TLV Type  |    Length     |    Variable Length Value      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Variable Length Value                 |
    |                             "                                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Length field is defined as the length of the interface parameter
   including the Sub-TLV Type and Length field itself.  Processing of
   the interface parameters should continue when unknown interface
   parameters are encountered, and they MUST be silently ignored.

   The Interface Parameter Sub-TLV Type values are specified in "IANA
   Allocations for Pseudowire Edge to Edge Emulation (PWE3)" [RFC4446].

   -  Interface MTU sub-TLV type

      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-enabled network.  This parameter is
      applicable only to PWs transporting packets 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.

   -  Optional Interface Description string sub-TLV type

      This arbitrary, and OPTIONAL, interface description string is used
      to send a human-readable administrative string describing the
      interface to the remote PE.  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].

6.5.  LDP Label Withdrawal Procedures

   As mentioned above, the Group ID field of the PWid FEC Element, or
   the PW Group ID TLV used with the Generalized PWid 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, the Interface Parameter Sub-TLVs are not present, and
   the Label TLV is not present.  If the Generalized FEC Element is
   used, the AGI, SAII, and TAII are not present, the PW information



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   length field is set to 0, the PW Group ID TLV is included, the PW
   Interface Parameters TLV is not present, and the Label TLV is not
   present.  For the purpose of this document, this is called the
   "wildcard withdraw procedure", and all PEs implementing this design
   are REQUIRED to accept such withdraw messages but are not required to
   send it.  Note that the PW Group ID TLV only applies to PWs using the
   Generalized ID FEC Element, while the Group ID only applies to PWid
   FEC Element.

   The Interface Parameter Sub-TLVs, or TLV, MUST NOT be present in any
   LDP PW Label Withdraw or Label Release message.  A wildcard Label
   Release message MUST include only the Group ID or PW Group ID TLV.  A
   Label Release message initiated by a PE router must always include
   the PW ID.

7.  Control Word

7.1.  PW Types for Which the Control Word Is REQUIRED

   The Label Mapping messages that 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 message 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 PW types for which the control word is not mandatory, then it
   behaves 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 as follows:

        -i. If the received Label Mapping message has C=0, send a Label
            Mapping message with C=0; the control word is not used.






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       -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; 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.  (That is, set C=1
   if locally configured to prefer the control word, and 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 as follows:

        -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 is 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 message, but do not
            respond.  Continue to wait for the next control message for
            the PW.







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   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 messages 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 has no extra protocol to execute; it just waits for a Label
   Mapping message that has C=0.

7.3.  Control-Word Renegotiation by Label Request Message

   It is possible that after the PW C-bit negotiation procedure
   described above is complete, the local PE is re-provisioned with a
   different control word preference.  Therefore, once the control-word
   negotiation procedures are complete, the procedure can be restarted
   as follows:

        -i. If the local PE previously sent a Label Mapping message, it
            MUST send a Label Withdraw message to the remote PE and wait
            until it has received a Label Release message from the
            remote PE.

       -ii. The local PE MUST send a Label Release message to the remote
            PE for the specific label associated with the FEC that was
            advertised for this specific PW.  Note: The above-mentioned
            steps of the Label Release message and Label Withdraw
            message are not required to be executed in any specific
            sequence.

      -iii. The local PE MUST send a Label Request message to the peer
            PE and then MUST wait until it receives a Label Mapping
            message containing the remote PE's currently configured
            preference for use of the control word.

   Once the remote PE has successfully processed the Label Withdraw
   message and Label Release messages, it will reset the C-bit
   negotiation state machine and its use of the control word with the
   locally configured preference.

   From this point on, the local and remote PEs will follow the C-bit
   negotiation procedures defined in the previous section.

   The above C-bit renegotiation process SHOULD NOT be interrupted until
   it is completed, or unpredictable results might occur.



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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 details when
   advertising labels.

7.4.1.  Label Advertisements

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

   This precaution is required to prevent the imposition router from
   restarting packet forwarding with a sequence number of 1 when it
   receives a Label Mapping message that binds the same FEC to the same
   label if there are still older packets in the network with a 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 a sequence
   number of 1.  Receipt of a packet with a 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 are for
   the disposition router always to advertise a different PW label, or
   for the disposition router to wait for a sufficient time before
   attempting to re-advertise a recently released label.  This is only
   an issue when sequence number processing is enabled at the
   disposition router.

7.4.2.  Label Release

   In situations where the imposition router wants to restart forwarding
   of packets with sequence number 1, the router shall 1) send to the
   disposition router a Label Release message, and 2) send to the
   disposition router a Label Request message.  When sequencing is
   supported, advertisement of a PW label in response to a Label Request
   message MUST also consider the issues discussed in Section 7.4.1
   ("Label Advertisements").








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8.  IANA Considerations

8.1.  LDP TLV TYPE

   This document uses several new LDP TLV types; IANA already maintains
   a registry titled "TLV Type Name Space", defined by RFC 5036.  The
   following values have been assigned from said registry:

     TLV Type  Description
     =====================================
     0x096A    PW Status TLV
     0x096B    PW Interface Parameters TLV
     0x096C    PW Group ID TLV

8.2.  LDP Status Codes

   This document uses several new LDP status codes; IANA already
   maintains a registry titled "Status Code Name Space", defined by RFC
   5036.  The following values have been assigned:

     Range/Value     E     Description                       Reference
     ------------- -----   ----------------------            ---------
     0x00000024      0     Illegal C-Bit                     [RFC8077]
     0x00000025      0     Wrong C-Bit                       [RFC8077]
     0x00000026      0     Incompatible bit-rate             [RFC8077]
     0x00000027      0     CEP-TDM mis-configuration         [RFC8077]
     0x00000028      0     PW Status                         [RFC8077]
     0x00000029      0     Unassigned/Unrecognized TAI       [RFC8077]
     0x0000002A      0     Generic Misconfiguration Error    [RFC8077]
     0x0000002B      0     Label Withdraw PW Status          [RFC8077]
                           Method Not Supported

8.3.  FEC Type Name Space

   This document uses two new FEC element types, 0x80 and 0x81, from the
   registry "Forwarding Equivalence Class (FEC) Type Name Space" for the
   Label Distribution Protocol (LDP) [RFC5036].

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
   and transmitted from one end of a pseudowire to the other.
   Therefore, we address the security considerations for both the data
   plane and the control plane.





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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 an MPLS PSN is used to provide pseudowire service, there is a
   perception that security must be at least equal to the currently
   deployed Layer 2 native protocol networks that the MPLS/PW network
   combination is emulating.  This means that the MPLS-enabled network
   SHOULD be isolated from outside packet insertion in such a way that
   it SHOULD NOT be possible to insert an MPLS packet into the network
   directly.  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.

   As mentioned above, an MPLS-enabled network 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 Service Provider (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 problems faced when using an MPLS-enabled
   network to transport PWs are spoofing, alteration, and inspection.
   First, there is a possibility that the PE receiving PW PDUs will get
   a PDU that appears to be from the PE transmitting the PW into the PSN
   but that was not actually transmitted by the PE originating the PW.
   (That is, the specified encapsulations do not by themselves enable
   the decapsulator to authenticate the encapsulator.)  A second problem
   is the possibility that the PW PDU will be altered between the time
   it enters the 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 the PSN itself is.




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9.2.  Control-Plane Security

   General security considerations with regard to the use of LDP are
   specified in Section 5 of [RFC5036].  Those considerations also apply
   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 preconfigured (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 that 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.  Therefore, 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.

   The LDP MD5 authentication key option, as described in Section 2.9 of
   [RFC5036], MUST be implemented, and for a greater degree of security,
   it must 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
   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 the set of eligible neighbors is
   determined by an auto-configuration protocol.

   When the Generalized PWid 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 PWid 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 have just a single
   set of eligible peers associated with the PE as a whole.




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10.  Interoperability and Deployment

   Section 2.2 of [RFC6410] specifies four requirements that an Internet
   Standard must meet.  This section documents how this document meets
   those requirements.

   The pseudowire technology was first deployed in 2001 and has been
   widely deployed by many carriers.  [RFC7079] documents the results of
   a survey of PW implementations with specific emphasis on control-word
   usage.  [EANTC] documents a public multi-vendor interoperability test
   of MPLS and Carrier Ethernet equipment, which included testing of
   Ethernet, ATM, and TDM pseudowires.

   The errata against [RFC4447] are generally editorial in nature and
   have been addressed in this document.

   All features in this specification have been implemented by multiple
   vendors.

   No IPR disclosures have been made to the IETF related to this
   document, to RFCs 4447 or 6723, or to the Internet-Drafts that
   resulted in RFCs 4447 and 6723.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI
              10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <http://www.rfc-editor.org/info/rfc5036>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <http://www.rfc-editor.org/info/rfc3032>.

   [RFC4446]  Martini, L., "IANA Allocations for Pseudowire Edge to Edge
              Emulation (PWE3)", BCP 116, RFC 4446, DOI
              10.17487/RFC4446, April 2006,
              <http://www.rfc-editor.org/info/rfc4446>.






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   [RFC7358]  Raza, K., Boutros, S., Martini, L., and N. Leymann, "Label
              Advertisement Discipline for LDP Forwarding Equivalence
              Classes (FECs)", RFC 7358, DOI 10.17487/RFC7358, October
              2014, <http://www.rfc-editor.org/info/rfc7358>.

11.2.  Informative References

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <http://www.rfc-editor.org/info/rfc2277>.

   [RFC3985]  Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985, DOI
              10.17487/RFC3985, March 2005,
              <http://www.rfc-editor.org/info/rfc3985>.

   [RFC4842]  Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
              "Synchronous Optical Network/Synchronous Digital Hierarchy
              (SONET/SDH) Circuit Emulation over Packet (CEP)", RFC
              4842, DOI 10.17487/RFC4842, April 2007,
              <http://www.rfc-editor.org/info/rfc4842>.

   [RFC4553]  Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
              Agnostic Time Division Multiplexing (TDM) over Packet
              (SAToP)", RFC 4553, DOI 10.17487/RFC4553, June 2006,
              <http://www.rfc-editor.org/info/rfc4553>.

   [RFC4619]  Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
              "Encapsulation Methods for Transport of Frame Relay over
              Multiprotocol Label Switching (MPLS) Networks", RFC 4619,
              DOI 10.17487/RFC4619, September 2006,
              <http://www.rfc-editor.org/info/rfc4619>.

   [RFC4717]  Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
              Brayley, J., and G. Koleyni, "Encapsulation Methods for
              Transport of Asynchronous Transfer Mode (ATM) over MPLS
              Networks", RFC 4717, DOI 10.17487/RFC4717, December 2006,
              <http://www.rfc-editor.org/info/rfc4717>.

   [RFC4618]  Martini, L., Rosen, E., Heron, G., and A. Malis,
              "Encapsulation Methods for Transport of PPP/High-Level
              Data Link Control (HDLC) over MPLS Networks", RFC 4618,
              DOI 10.17487/RFC4618, September 2006,
              <http://www.rfc-editor.org/info/rfc4618>.







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   [RFC4448]  Martini, L., Ed., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, DOI 10.17487/RFC4448, April 2006,
              <http://www.rfc-editor.org/info/rfc4448>.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447, DOI
              10.17487/RFC4447, April 2006,
              <http://www.rfc-editor.org/info/rfc4447>.

   [RFC6410]  Housley, R., Crocker, D., and E. Burger, "Reducing the
              Standards Track to Two Maturity Levels", BCP 9, RFC 6410,
              DOI 10.17487/RFC6410, October 2011,
              <http://www.rfc-editor.org/info/rfc6410>.

   [RFC6723]  Jin, L., Ed., Key, R., Ed., Delord, S., Nadeau, T., and S.
              Boutros, "Update of the Pseudowire Control-Word
              Negotiation Mechanism", RFC 6723, DOI 10.17487/RFC6723,
              September 2012, <http://www.rfc-editor.org/info/rfc6723>.

   [RFC7079]  Del Regno, N., Ed., and A. Malis, Ed., "The Pseudowire
              (PW) and Virtual Circuit Connectivity Verification (VCCV)
              Implementation Survey Results", RFC 7079, DOI
              10.17487/RFC7079, November 2013,
              <http://www.rfc-editor.org/info/rfc7079>.

   [ANSI]     American National Standards Institute, "Telecommunications
              - Synchronous Optical Network (SONET) - Basic Description
              Including Multiplex Structures, Rates, and Formats", ANSI
              T1.105, October 1995.

   [ITUG]     International Telecommunications Union, "Network node
              interface for the synchronous digital hierarchy (SDH)",
              ITU-T Recommendation G.707, May 1996.

   [EANTC]    European Advanced Networking Test Center, "MPLS and
              Carrier Ethernet: Service - Connect - Transport. Public
              Multi-Vendor Interoperability Test", February 2009.

Acknowledgments

   The authors wish to acknowledge the contributions of Vach Kompella,
   Vanson Lim, Wei Luo, Himanshu Shah, and Nick Weeds.  The authors wish
   to also acknowledge the contribution of the authors of RFC 6723,
   whose work has been incorporated in this document: Lizhong Jin,
   Raymond Key, Simon Delord, Tom Nadeau, and Sami Boutros.




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Contributors

   The following individuals were either authors or contributing authors
   for RFC 4447.  They are listed here in recognition of their work on
   that document.

   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO 80021
   United States of America

   Email: nna@level3.net


   Eric C.  Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   United States of America

   Email: erosen@cisco.com


   Dan Tappan
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA 01719
   United States of America

   Email: tappan@cisco.com


   Toby Smith
   Google
   6425 Penn Ave. #700
   Pittsburgh, PA 15206
   United States of America

   Email: tob@google.com


   Dimitri Vlachos
   Riverbed Technology

   Email: dimitri@riverbed.com





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   Jayakumar Jayakumar
   Cisco Systems Inc.
   3800 Zanker Road, MS-SJ02/2
   San Jose, CA 95134
   United States of America

   Email: jjayakum@cisco.com


   Alex Hamilton,
   Cisco Systems Inc.
   485 East Tasman Drive, MS-SJC07/3
   San Jose, CA 95134
   United States of America

   Email: tahamilt@cisco.com


   Steve Vogelsang
   ECI Telecom
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   United States of America

   Email: stephen.vogelsang@ecitele.com


   John Shirron
   ECI Telecom
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   United States of America

   Email: john.shirron@ecitele.com


   Andrew G. Malis
   Verizon
   60 Sylvan Rd.
   Waltham, MA 02451
   United States of America

   Email: andrew.g.malis@verizon.com






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RFC 8077                     PWE3 Using LDP                February 2017


   Vinai Sirkay
   Reliance Infocomm
   Dhirubai Ambani Knowledge City
   Navi Mumbai 400 709
   India

   Email: vinai@sirkay.com


   Vasile Radoaca
   Nortel Networks
   600  Technology Park
   Billerica MA 01821
   United States of America

   Email: vasile@nortelnetworks.com


   Chris Liljenstolpe
   149 Santa Monica Way
   San Francisco, CA 94127
   United States of America

   Email: ietf@cdl.asgaard.org


   Dave Cooper
   Global Crossing
   960 Hamlin Court
   Sunnyvale, CA 94089
   United States of America

   Email: dcooper@gblx.net


   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   United States of America

   Email: kireeti@juniper.net









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Authors' Addresses

   Luca Martini (editor)
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO 80202
   United States of America

   Email: lmartini@monoski.com


   Giles Heron (editor)
   Cisco Systems
   10 New Square
   Bedfont Lakes
   Feltham
   Middlesex
   TW14 8HA
   United Kingdom

   Email: giheron@cisco.com






























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