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Pseudo-Wire Edge-to-Edge (PWE3) Working Group             Stewart Bryant
Internet Draft                                             Cisco Systems
Document: <draft-ietf-pwe3-arch-02.txt>
Expires: August 2003                                        Prayson Pate
                                                 Overture Networks, Inc.


                                                           February 2003

                           PWE3 Architecture

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
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   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


   This document describes an architecture for Pseudo Wire Emulation
   Edge-to-Edge (PWE3).  It discusses the emulation of services (such as
   Frame Relay, ATM, Ethernet, TDM and SONET/SDH) over packet switched
   networks (PSNs) using IP or MPLS.  It presents the architectural
   framework for pseudo wires (PWs), defines terminology, specifies the
   various protocol elements and their functions.

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   The following are co-authors of this document:

       Thomas K. Johnson   Litchfield Communications
       Kireeti Kompella    Juniper Networks, Inc.
       Andrew G. Malis     Vivace Networks
       Danny McPherson     TCB
       Thomas D. Nadeau    Cisco Systems
       Tricci So           Caspian Networks
       W. Mark Townsley    Cisco Systems
       Craig White         Level 3 Communications, LLC.
       Lloyd Wood          Cisco Systems
       XiPeng Xiao         Redback Networks

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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   1.  Introduction.............................................    5
      1.1  Pseudo Wire Definition...............................    5
      1.2  PW Service Functionality.............................    6
      1.3  Non-Goals of this document...........................    6
      1.4  Terminology..........................................    6

   2.  PWE3 Applicability.......................................    9

   3.  Protocol Layering Model..................................    9
      3.1  Protocol Layers......................................    9
      3.2  Domain of PWE3.......................................   11
      3.3  Payload Types........................................   11

   4.  Architecture of Pseudo-wires.............................   14
      4.1  Network Reference Model..............................   14
      4.2  PWE3 Pre-processing..................................   15
      4.3  Maintenance Reference Model..........................   19
      4.4  Protocol Stack Reference Model.......................   19
      4.5  Pre-processing Extension to Protocol Stack Reference.
           Model................................................   20

   5.  PW Encapsulation.........................................   21
      5.1  Payload Convergence Layer............................   22
      5.2  Payload-independent PW Encapsulation Layers..........   24
      5.3  Fragmentation........................................   27
      5.4  Instantiation of the Protocol Layers.................   27

   6.  PW Demultiplexer Layer and PSN Requirements..............   31
      6.1  Multiplexing.........................................   31
      6.2  Fragmentation........................................   31
      6.3  Length and Delivery..................................   31
      6.4  PW-PDU Validation....................................   31
      6.5  Congestion Considerations............................   32

   7.  Control Plane............................................   33
      7.1  Set-up or Teardown of Pseudo-Wires...................   33
      7.2  Status Monitoring....................................   33
      7.3  Notification of Pseudo-wire Status Changes...........   34
      7.4  Keep-alive...........................................   35
      7.5  Handling Control Messages of the Native Services.....   35

   8. Management and Monitoring.................................   36
      8.1  Status and Statistics................................   36
      8.2  PW SNMP MIB Architecture.............................   36
      8.3 Connection Verification and Traceroute................   40

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   9.  IANA considerations......................................   40

   10.  Security Considerations.................................   40

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1.  Introduction

   This document describes an architecture for Pseudo Wire Emulation
   Edge-to-Edge (PWE3) in support of [XIAO].  It discusses the emulation
   of services (such as Frame Relay, ATM, Ethernet, TDM and SONET/SDH)
   over packet switched networks (PSNs) using IP or MPLS.  It presents
   the architectural framework for pseudo wires (PWs), defines
   terminology, specifies the various protocol elements and their

1.1  Pseudo Wire Definition

   PWE3 is a mechanism that emulates the essential attributes of a
   service (such as a T1 leased line or Frame Relay) over a PSN. PWE3 is
   intended to provide only the minimum necessary functionality to
   emulate the wire with the required degree of faithfulness for the
   given service definition. Any required switching functionality is the
   responsibility of a forwarder function (FWRD).  Any translation or
   other operation needing knowledge of the payload semantics is carried
   out by native service processing (NSP) elements.  The functional
   definition of any FWRD or NSP elements is outside the scope of PWE3.

   The required functions of PWs include encapsulating service-specific
   bit-streams, cells or PDUs arriving at an ingress port, and carrying
   them across a IP path or MPLS tunnel. In some cases it is necessary
   to perform other operation such as managing their timing and order,
   to emulate the behavior and characteristics of the service to the
   required degree of faithfulness.

   From the perspective of a Customer Edge Equipment (CE), the PW is
   characterised as an unshared link or circuit of the chosen service.
   In some cases, there may be deficiencies in the PW emulation that
   impact the traffic carried over a PW, and hence limit the
   applicability of this technology.  These limitations must be fully
   described in the appropriate service-specific documentation.

   For each service type, there will be one default mode of operation
   that all PEs offering that service type MUST support.  However,
   OPTIONAL modes MAY be defined to improve the faithfulness of the
   emulated service, if it can be clearly demonstrated that the
   additional complexity associated with the OPTIONAL mode is offset by
   the value it offers to PW users.

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1.2  PW Service Functionality

   PWs provide the following functions in order to emulate the behavior
   and characteristics of the native service.
       o Encapsulation of service-specific PDUs or circuit data arriving
         at the PE-bound port (logical or physical).
       o Carriage of the encapsulated data across a PSN tunnel.
       o Establishment of the PW including the exchange and/or
         distribution of the PW identifiers used by the PSN
         tunnel endpoints.
       o Managing the signaling, timing, order or other aspects of the
         service at the boundaries of the PW.
       o Service-specific status and alarm management.

1.3  Non-Goals of this document

   The following are non-goals for this document:

       o The on-the-wire specification of PW encapsulations.
       o The detailed definition of the protocols involved in PW
         set-up and maintenance.

   The following are outside the scope of PWE3:
      o Any multicast service not native to the emulated medium.
        Thus, Ethernet transmission to a "multicast" IEEE-48 address
        is in scope, while multicast services like MARS [RFC2022] that
        are implemented on top of the medium are out of scope.
      o Methods to signal or control the underlying PSN.

1.4  Terminology

   This document uses the following definitions of terms.  These terms
   are illustrated in context in Figure 2.

   Attachment Circuit   The circuit or virtual circuit attaching
   (AC)                 a CE to a PE.

   CE-bound             The traffic direction where PW-PDUs are
                        received on a PW via the PSN, processed
                        and then sent to the destination CE.

   CE Signaling         Messages sent and received by the CEs
                        control plane.  It may be desirable or
                        even necessary for the PE to participate
                        in or monitor this signaling in order
                        to effectively emulate the service.

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   Control Word (CW)    A four octet header used in some encapsulations
                        to carry per packet information when the PSN
                        is MPLS.

   Customer Edge (CE)   A device where one end of a service
                        originates and/or terminates.  The CE is not
                        aware that it is using an emulated service
                        rather than a native service.

   Forwarder (FWRD)     A PE subsystem that selects the PW to use to
                        transmit a payload received on an AC.

   Fragmentation        The action of dividing a single PDU into
                        multiple PDUs before transmission with the
                        intent of the original PDU being reassembled
                        elsewhere in the network. Fragmentation MAY be
                        performed in order to allow sending of packets
                        of a larger size than the network MTU which
                        they will traverse.

   Maximum transmission The packet size (excluding data link header)
   unit (MTU)           that an interface can transmit without
                        needing to fragment.

   Native Service       Processing of the data received by the PE
   Processing (NSP)     from the CE before presentation to the PW
                        for transmission across the core, or
                        processing of the data received from a PW
                        by a PE before it is output on the AC.
                        NSP functionality is defined by standards
                        bodies other than the IETF, such as ITU-T,
                        ANSI, ATMF, etc.)

   Packet Switched      Within the context of PWE3, this is a
   Network (PSN)        network using IP or MPLS as the mechanism
                        for packet forwarding.

   Protocol Data        The unit of data output to, or received
   Unit (PDU)           from, the network by a protocol layer.

   Provider Edge (PE)   A device that provides PWE3 to a CE.

   PE-bound             The traffic direction where information
                        from a CE is adapted to a PW, and PW-PDUs
                        are sent into the PSN.

   PE/PW Maintenance    Used by the PEs to set up, maintain and
                        tear down the PW.  It may be coupled with

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                        CE Signaling in order to effectively manage
                        the PW.

   Pseudo Wire (PW)     A mechanism that carries the essential
                        elements of an emulated service from one PE
                        to one or more other PEs over a PSN.

   PW End Service       The interface between a PE and a CE.  This
   (PWES)               can be a physical interface like a T1 or
                        Ethernet, or a virtual interface like a VC
                        or VLAN.

   Pseudo Wire          A mechanism that emulates the essential
   Emulation Edge to    attributes of service (such as a T1 leased
   Edge (PWE3)          line or frame relay) over a PSN.

   Pseudo Wire PDU      A PDU sent on the PW that contains all of
   (PW-PDU)             the data and control information necessary
                        to emulate the desired service.

   PSN Tunnel           A tunnel across a PSN inside which one or
                        more PWs can be carried.

   PSN Tunnel           Used to set up, maintain and tear down the
   Signaling            underlying PSN tunnel.

   PW Demultiplexer     Data-plane method of identifying a PW
                        terminating at a PE.

   Time Domain          Time Division Multiplexing. Frequently used
   Multiplexing (TDM)   to refer to the synchronous bit-streams at
                        rates defined by G.702.

   Tunnel               A method of transparently carrying information
                        over a network.

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2.  PWE3 Applicability

   The PSN carrying a PW will subject payload packets to loss, delay,
   delay variation, and re-ordering. During a network transient there
   may be a sustained period of impaired service.  The applicability of
   PWE3 to a particular service depends on the sensitivity of that
   service (or the CE implementation) to these effects, and the ability
   of the adaptation layer to mask them.  Some services, such as IP over
   FR over PWE3, may prove quite resilient to IP and MPLS PSN
   characteristics. Other services, such as the interconnection of PBX
   systems via PWE3, will require more careful consideration of the PSN
   and adaptation layer characteristics.  In some instances, traffic
   engineering of the underlying PSN will be required, and in some
   cases, the constraints may be such that it is not possible to provide
   the required service guarantees.

3.  Protocol Layering Model

   The PWE3 protocol-layering model is intended to minimise the
   differences between PWs operating over different PSN types.  The
   design of the protocol-layering model has the goals of making each PW
   definition independent of the underlying PSN, and maximizing the
   reuse of IETF protocol definitions and their implementations.

3.1  Protocol Layers

   The logical protocol-layering model required to support a PW is shown
   in Figure 1.

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          |         Payload           |
          |      Encapsulation        | <==== MAY be null
          |     PW Demultiplexer      |
          |     PSN Convergence       | <==== MAY be null
          |           PSN             |
          |         Data-link         |
          |          Physical         |

     Figure 1: Logical Protocol Layering Model

   The payload is transported over the Encapsulation Layer.  The
   Encapsulation Layer carries any information, not already present
   within the payload itself, that is needed by the PW CE-bound PE
   interface to send the payload to the CE via the physical interface.
   If no information is needed beyond that in the payload itself, this
   layer is empty.

   This layer also provides support for real-time processing, and
   sequencing, if needed.

   The PW Demultiplexer Layer provides the ability to deliver multiple
   PWs over a single PSN tunnel. The PW demultiplexer value used to
   identify the PW in the data-plane may be unique per PE, but this is
   not a PWE3 requirement.  It MUST, however, be unique per tunnel
   endpoint.  If it is necessary to identify a particular tunnel, then
   that is the responsibility of the PSN layer.

   The PSN Convergence Layer provides the enhancements needed to make
   the PSN conform to the assumed PSN service requirement.  This layer
   therefore provides a consistent interface to the PW, making the PW
   independent of the PSN type.  If the PSN already meets the service
   requirements, this layer is empty.

   The PSN header, MAC/Data-link and Physical Layer definitions are
   outside the scope of this document. The PSN can be IPv4, IPv6 or

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3.2  Domain of PWE3

   PWE3 defines the Encapsulation Layer, the method of carrying various
   payload types, and the interface to the PW Demultiplexer Layer.  It
   is expected that the other layers will be provided by tunneling
   methods such as L2TP or MPLS over the PSN.

3.3  Payload Types

   The payload is classified into the following generic types of native
   data unit:

       o Packet
       o Cell
       o Bit-stream
       o Structured bit-stream

   Within these generic types there are specific service types.  For

       Generic Payload Type    PW Service
       --------------------    ----------
       Packet                  Ethernet (all types), HDLC framing,
                               frame-relay, ATM AAL5 PDU.

       Cell                    ATM.

       Bit-stream              Unstructured E1, T1, E3, T3.

       Structured bit-stream   SONET/SDH (e.g. SPE, VT, NxDS0).

3.3.1.  Packet Payload

   A packet payload is a variable-size data unit presented to the PE on
   the AC.  A packet payload may be large compared to the PSN MTU. The
   delineation of the packet boundaries is encapsulation-specific.  HDLC
   or Ethernet PDUs can be considered as examples of packet payloads.
   Typically a packet will be stripped of transmission overhead such as
   HDLC flags and stuffing bits before transmission over the PW.

   A packet payload would normally be relayed across the PW as a single
   unit.  However, there will be cases where the combined size of the
   packet payload and its associated PWE3 and PSN headers exceeds the
   PSN path MTU.  In these cases, some fragmentation methodology needs
   to be applied.  This may, for example, be the case when a user is
   providing the service and attaching to the service provider via
   Ethernet, or where nested pseudo-wires are involved. Fragmentation is

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   discussed in more detail in Section 5.3

   A packet payload may need sequencing and real-time support.

   In some situations, the packet payload MAY be selected from the
   packets presented on the emulated wire on the basis of some sub-
   multiplexing technique.  For example, one or more frame-relay PDUs
   may be selected for transport over a particular pseudo-wire based on
   the frame-relay Data-Link Connection Identifier (DLCI), or, in the
   case of Ethernet payloads, using a suitable MAC bridge filter.  This
   is an FWRD function, and this selection would therefore be made
   before the packet was presented to the PW Encapsulation Layer.

3.3.2.  Cell Payload

   A cell payload is created by capturing, transporting and replaying
   groups of octets presented on the wire in a fixed-size format.  The
   delineation of the group of bits that comprise the cell is specific
   to the encapsulation type.  Two common examples of cell payloads are
   ATM 53-octet cells, and the larger 188-octet MPEG Transport Stream
   packets [DVB].

   To reduce per-PSN packet overhead, multiple cells MAY be concatenated
   into a single payload.  The Encapsulation Layer MAY consider the
   payload complete on the expiry of a timer, after a fixed number of
   cells have been received or when a significant cell (e.g. an ATM OAM
   cell) has been received.  The benefit of concatenating multiple PDUs
   should be weighed against a possible increase in packet delay
   variation and the larger penalty incurred by packet loss.  In some
   cases, it may be appropriate for the Encapsulation Layer to perform
   some type of compression, such as silence suppression or voice

   The generic cell payload service will normally need sequence number
   support, and may also need real-time support.  The generic cell
   payload service would not normally require fragmentation.

   The Encapsulation Layer MAY apply some form of compression to some of
   these sub-types (e.g. idle cells MAY be suppressed).

   In some instances, the cells to be incorporated in the payload MAY be
   selected by filtering them from the stream of cells presented on the
   wire.  For example, an ATM PWE3 service may select cells based on
   their VCI or VPI fields. This is an FWRD function, and the selection
   would therefore be made before the packet was presented to the PW
   Encapsulation Layer.

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3.3.3.  Bit-stream

   A bit-stream payload is created by capturing, transporting and
   replaying the bit pattern on the emulated wire, without taking
   advantage of any structure that, on inspection, may be visible within
   the relayed traffic (i.e. the internal structure has no effect on the
   fragmentation into packets).

   In some instances it is possible to apply suppression to bit-streams.
   For example, E1 and T1 send "all-ones" to indicate failure. This
   condition can be detected without any knowledge of the structure of
   the bit-stream, and transmission of packetized data suppressed.

   This service will require sequencing and real-time support.

3.3.4.  Structured bit-stream

   A structured bit-stream payload is created by using some knowledge of
   the underlying structure of the bit-stream to capture, transport and
   replay the bit pattern on the emulated wire.

   Two important points distinguish structured and unstructured bit-

       o Some parts of the original bit-stream MAY be stripped in the
         PSN-bound direction by NSP block.  For example, in Structured
         SONET the section and line overhead (and, possibly more) may
         be stripped.  A framer is required to enable such stripping.
         It is also required for frame/payload alignment for
         fractional T1/E1 applications.

       o The PW MUST preserve the structure across the PSN so that
         the CE-bound NSP block can insert it correctly into the
         reconstructed unstructured bit-stream. The stripped
         information (such as SONET pointer justifications) may
         appear in the encapsulation layer to facilitate this

   As an option, the Encapsulation Layer MAY also perform silence/idle
   suppression or similar compression on a structured bit-stream.

   Structured bit-streams are distinguished from cells in that the
   structures may be too long to be carried in a single packet.  Note
   that "short" structures are indistinguishable from cells and may
   benefit from the use of methods described in section 3.3.2.

   This service REQUIRES sequencing and real-time support.

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3.3.5.  Principle of Minimum Intervention

   To minimise the scope of information, and to improve the efficiency
   of data flow through the Encapsulation Layer, the payload SHOULD be
   transported as received with as few modifications as possible

   This minimum intervention approach decouples payload development from
   PW development and requires fewer translations at the NSP in a system
   with similar CE interfaces at each end.  It also prevents unwanted
   side-effects due to subtle misrepresentation of the payload in the
   intermediate format.

   An approach which does intervene can be more wire-efficient in some
   cases and may result in fewer translations at the NSP where the CE
   interfaces are of different types. Any intermediate format
   effectively becomes a new framing type, requiring documentation and
   assured interoperability.  This increases the amount of work for
   handling the protocol the intermediate format carries, and is

4.  Architecture of Pseudo-wires

   This section describes the PWE3 architectural model.

4.1  Network Reference Model

   Figure 2 illustrates the network reference model for point-to-point

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

                   Figure 2: PWE3 Network Reference Model

   The two PEs (PE1 and PE2) need to provide one or more PWs on behalf
   of their client CEs (CE1 and CE2) to enable the client CEs to
   communicate over the PSN.  A PSN tunnel is established to provide a
   data path for the PW.  The PW traffic is invisible to the core
   network, and the core network is transparent to the CEs.  Native data
   units (bits, cells or packets) presented to the PW End Service (PWES)
   are encapsulated in a PW-PDU and carried across the underlying
   network via the PSN tunnel. The PEs perform the necessary
   encapsulation and decapsulation of PW-PDUs, as well as handling any
   other functions required by the PW service, such as sequencing or
   timing. A PE MAY provide multiple PWESs.

4.2  PWE3 Pre-processing

   In some applications, there is a need to perform operations on the
   native data units received from the CE (including both payload and
   signaling traffic) before they are transmitted across the PW by the
   PE. Examples include Ethernet bridging, SONET cross-connect,
   translation of locally-significant identifiers such as VCI/VPI, or
   translation to another service type.  These operations could be
   carried out in external equipment, and the processed data sent to the
   PE over one or more physical interfaces.  In most cases, there are
   cost and operational benefits in undertaking these operations within
   the PE.  This processed data is then presented to the PW via a
   virtual interface within the PE.

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   These pre-processing operations are included in the PWE3 reference
   model to provide a common reference point, but the detailed
   description of these operations is outside the scope of the PW
   definition given here.

                    End Service
                        |<------- Pseudo Wire ------>|
                        |                            |
                        |    |<-- PSN Tunnel -->|    |
                        V    V                  V    V     PW
                  +-----+----+                  +----+ End Service
       +-----+    |PREP | PE1|==================| PE2|     |    +-----+
       |     |    |     |............PW1.............|----------|     |
       | CE1 |----|     |    |                  |    |     |    | CE2 |
       |     | ^  |     |............PW2.............|----------|     |
       +-----+ |  |     |    |==================|    |     | ^  +-----+
               |  +-----+----+                  +----+     | |
               |        ^                                  | |
               |        |                                  | |
               |        |<------- Emulated Service ------->| |
               |        |                                    |
               | Virtual physical                            |
               |  termination                                |
               |        ^                                    |
          CE1 native    |                                CE2 native
           service      |                                service
                   CE2 native

      Figure 3: Pre-processing within the PWE3 Network Reference Model

   Figure 3 shows the inter-working of one PE with pre-processing
   (PREP), and a second without this functionality.  This is a useful
   reference point because it emphasises that the functional interface
   between PREP and the PW is that represented by a physical interface
   carrying the service.  This effectively defines the necessary inter-
   working specification.

   The operation of a system in which both PEs include PREP
   functionality is also supported.

   The required pre-processing can be divided into two components:
       o Forwarder (FWRD)

       o Native Service Processing (NSP)

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4.2.1. Forwarders

   In some applications there is the need to selectively forward payload
   elements from one of more ACs to one or more PWs. In such cases there
   will also be the need to perform the inverse function on PWE3-PDUs
   received by a PE from the PSN. This is the function of the FWRD.

   The FWRD selects the PW based on, for example: the incoming AC, the
   contents of the payload, or some statically and/or dynamically
   configured forwarding information.

                |                PE Device               |
         Single |                 |                      |
         PWES   |                 |        Single        | PW Instance
        <------>o   Forwarder     +      PW Instance     X<===========>
                |                 |                      |

                Figure 4a: Simple point-to-point service

                |                PE Device               |
        Multiple|                 |        Single        | PW Instance
        PWES    |                 +      PW Instance     X<===========>
        <------>o                 |                      |
                |                 |----------------------|
        <------>o                 |        Single        | PW Instance
                |    Forwarder    +      PW Instance     X<===========>
        <------>o                 |                      |
                |                 |----------------------|
        <------>o                 |        Single        | PW Instance
                |                 +      PW Instance     X<===========>
        <------>o                 |                      |

                Figure 4b: Multiple PWES to Multiple PW Forwarding

   Figure 4a shows a simple FWRD that performs some type of filtering
   operation. Because the FWRD has a single input and a single output
   interface, filtering is the only type of forwarding operation that
   applies. Figure 4b shows a more general forwarding situation where
   payloads are extracted from one or more PWESs and directed to one or
   more PWs, including, in this instance, a multipoint PW. In this case

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   both filtering and direction operations MAY be performed on the

4.2.2. Native Service Processing

   In some applications some form of data or address translation, or
   other operation requiring knowledge of the semantics of the payload,
   will be required. This is the function of the Native Service
   Processor (NSP).

   The use of the NSP approach simplifies the design of the PW by
   restricting a PW to homogeneous operation.  NSP is included in the
   reference model to provide a defined interface to this functionality.
   The specification of the various types of NSP is outside the scope of

                |                PE Device               |
        PWES    |      |          |        Single        | PW Instance
        <------>o  NSP #          +      PW Instance     X<===========>
                |      |          |                      |
                |------|          |----------------------|
                |      |          |        Single        | PW Instance
        <------>o  NSP #Forwarder +      PW Instance     X<===========>
                |      |          |                      |
                |------|          |----------------------|
                |      |          |        Single        | PW Instance
        <------>o  NSP #          +      PW Instance     X<===========>
                |      |          |                      |

                Figure 5: NSP in a Multiple PWEs to Multiple
                          PW Forwarding PE

   Figure 5 illustrates the relationship between NSP, FWRD and PWs in a
   PE.  The NSP function MAY apply any transformation operation
   (modification, injection, etc.) on the payloads as they pass between
   the physical interface to the CE and the virtual interface to the
   FWRD.  A PE device MAY contain more than one FWRD.

   This model also supports the operation of a system in which the NSP
   functionality includes terminating the data-link, and applying
   Network Layer processing to the payload is also supported.

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4.3  Maintenance Reference Model

   Figure 6 illustrates the maintenance reference model for PWs.

             |<------- CE (end-to-end) Signaling ------>|
             |     |<---- PW/PE Maintenance ----->|     |
             |     |     |<-- PSN Tunnel -->|     |     |
             |     |     |    Signaling     |     |     |
             |     V     V  (out of scope)  V     V     |
             v     +-----+                  +-----+     v
       +-----+     | PE1 |==================| PE2 |     +-----+
       |     |-----|.............PW1..............|-----|     |
       | CE1 |     |     |                  |     |     | CE2 |
       |     |-----|.............PW2..............|-----|     |
       +-----+     |     |==================|     |     +-----+
                   +-----+                  +-----+
       Customer   Provider                 Provider     Customer
        Edge 1     Edge 1                   Edge 2       Edge 2

            Figure 6: PWE3 Maintenance Reference Model

   The following signaling mechanisms are REQUIRED:

       o The CE (end-to-end) signaling is between the CEs.  This
         signaling could be frame relay PVC status signaling, ATM SVC
         signaling, TDM CAS signaling, etc.

       o The PW/PE Maintenance is used between the PEs (or NSPs) to set
         up, maintain and tear down PWs, including any required
         coordination of parameters.

       o The PSN Tunnel signaling controls the PW multiplexing and some
         elements of the underlying PSN.  Examples are L2TP control
         protocol, MPLS LDP and RSVP-TE.  The definition of the
         information that PWE3 needs to be signaled is within the scope
         of PWE3, but the signaling protocol itself is not.

4.4  Protocol Stack Reference Model

   Figure 7 illustrates the protocol stack reference model for PWs.

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

             Figure 7: PWE3 Protocol Stack Reference Model

   The PW provides the CE with an emulated physical or virtual
   connection to its peer at the far end.  Native service PDUs from the
   CE are passed through an Encapsulation Layer at the sending PE, and
   then sent over the PSN. The receiving PE removes the encapsulation
   and restores the payload to its native format for transmission to the
   destination CE.

4.5  Pre-processing Extension to Protocol Stack Reference Model

   Figure 8 illustrates how the protocol stack reference model is
   extended to include the provision of pre-processing (Forwarding and
   NSP).  This shows the placement of the physical interface relative to
   the CE.

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      H             Forwarder                H<----Pre-processing
      H Native Service H   |                 |
      H  Processing    H   |                 |
      \================/   |                 |
      |                |   | Emulated        |
      | Service        |   | Service         |
      | Interface      |   | (TDM, ATM,      |
      | (TDM, ATM,     |   | Ethernet,       |<== Emulated Service ==
      | Ethernet,      |   | frame relay,    |
      | frame relay,   |   | etc.)           |
      | etc.)          |   +-----------------+
      |                |   |    Payload      |
      |                |   | Encapsulation   |<=== Pseudo Wire ======
      |                |   +-----------------+
      |                |   |PW Demultiplexer |
      |                |   |  PSN Tunnel,    |
      |                |   | PSN & Physical  |<=== PSN Tunnel =======
      |                |   |    Headers      |
      +----------------+   +-----------------+
      |   Physical     |   |   Physical      |
      +-------+--------+   +-------+---------+
              |                    |
              |                    |
              |                    |
              |                    |
              |                    |
              |                    |
    To CE <---+                    +---> To PSN

        Figure 8: Protocol Stack Reference Model with Pre-processing

5.  PW Encapsulation

   The PW Encapsulation Layer provides the necessary infrastructure to
   adapt the specific payload type being transported over the PW to the
   PW Demultiplexer Layer that is used to carry the PW over the PSN.

   The PW Encapsulation Layer consists of three sub-layers:

       o Payload Convergence
       o Timing
       o Sequencing

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   The PW Encapsulation sub-layering and its context with the protocol
   stack are shown, in Figure 9.

          |         Payload           |
          /===========================\ <------ Encapsulation
          H    Payload Convergence    H         Layer
          H          Timing           H
          H        Sequencing         H
          |     PW Demultiplexer      |
          |     PSN Convergence       |
          |           PSN             |
          |         Data-link         |
          |          Physical         |

     Figure 9: PWE3 Encapsulation Layer in Context

   The Payload Convergence Sub-layer is highly tailored to the specific
   payload type, but, by grouping a number of target payload types into
   a generic class, and then providing a single convergence sub-layer
   type common to the group, we achieve a reduction in the number of
   payload convergence sub-layer types.  This decreases implementation
   complexity. The provision of per-packet signaling and other out-of-
   band information (other than sequencing or timing) is undertaken by
   this layer.

   The Timing Layer and the Sequencing Layer provide generic services to
   the Payload Convergence Layer for all payload types that require

5.1  Payload Convergence Layer

5.1.1.  Encapsulation

   The primary task of the Payload Convergence Layer is the
   encapsulation of the payload in PW-PDUs.  The native data units to be
   encapsulated MAY contain a L2 header or L1 overhead.  This is service
   specific.  The Payload Convergence header carries the additional
   information needed to replay the native data units at the CE-bound
   physical interface. The PW Demultiplexer header is not considered as

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   part of the PW header.

   Not all the additional information needed to replay the native data
   units need to be carried in the PW header of the PW PDUs.  Some
   information (e.g. service type of a PW) MAY be stored as state
   information at the destination PE during PW set-up.

5.1.2.  PWE3 Channel Types

   The PW Encapsulation Layer and its associated signaling require one
   or more of the following types of channels from its underlying PW
   Demultiplexer and PSN Layers:

   1. A reliable control channel for signaling line events, status
      indications, and, in some exceptional cases, CE-CE events
      that must be translated and sent reliably between PEs.

      For example, this capability is needed in [PPPoL2TP]
      (PPP negotiation has to be split between the two ends of the
      tunnel).  PWE3 may also need this type of control channel to
      provide faithful emulation of complex data-link protocols.

   plus one or more data channels with the following characteristics:

   2. A high-priority, unreliable, sequenced channel.  A typical use
      is for CE-to-CE signaling.  "High priority" may simply be
      indicated via the DSCP bits for IP or the EXP bits for MPLS,
      giving the packet priority during transit.  This channel type
      could also use a bit in the tunnel header itself to indicate
      that packets received at the PE SHOULD be processed with higher
      priority [RFC2474].

   3. A sequenced channel for data traffic that is sensitive to
      packet reordering (one classification for use could be for
      any non-IP traffic).

   4. An un-sequenced channel for data traffic insensitive to packet

   The data channels (2, 3 and 4 above) SHOULD be carried "in band" with
   one another to as much of a degree as is reasonably possible on a

   Where end-to-end connectivity may be disrupted by address translation
   [RFC3022], access-control lists, firewalls etc., there exists the
   possibility that the control channel may be able to pass traffic and
   set-up the PW, but the PW data traffic is blocked by one or more of
   these mechanisms.  In these cases unless the control channel is also

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   carried "in band" the signaling to set-up the PW will not confirm the
   existence of an end-to-end data path.

   In some cases there is a need to synchronize CE events with the data
   carried over a PW.  This is especially the case with TDM circuits
   (e.g., the on-hook/off-hook events in PSTN switches might be carried
   over a reliable control channel, whilst the associated bit-stream is
   carried over a sequenced data channel).

   PWE3 channel types that are not needed by the supported PWs need not
   be included in such an implementation.

5.1.3.  Quality of Service Considerations

   Where possible, it is desirable to employ mechanisms to provide PW
   Quality of Service (QoS) support over PSNs.

5.2  Payload-independent PW Encapsulation Layers

   Two PWE3 Encapsulation Sub-layers provide common services to all
   payload types: Sequencing and Timing.  These services are optional
   and are only used if needed by a particular PW instance.  If the
   service is not needed, the associated header MAY be omitted in order
   to conserve processing and network resources.

   There will be instances where a specific payload type will be
   required to be transported with or without sequence and/or real-time
   support.  For example, an invariant of frame relay transport is the
   preservation of packet order. Some frame-relay applications expect
   in-order delivery, and may not cope with reordering of the frames.
   However, where the frame relay service is itself only being used to
   carry IP, it may be desirable to relax that constraint in return for
   reduced per-packet processing cost.

   The guiding principle is that, where possible, an existing IETF
   protocol SHOULD be used to provide these services.  Where a suitable
   protocol is not available, the existing protocol should be extended
   or modified to meet the PWE3 requirements, thereby making that
   protocol available for other IETF uses. In the particular case of
   timing, more than one general method may be necessary to provide for
   the full scope of payload timing requirements.

5.2.1.  Sequencing

   The sequencing function provides three services: frame ordering,
   frame duplication detection and frame loss detection. These services
   allow the emulation of the invariant properties of a physical wire.
   Support for sequencing depends on the payload type, and MAY be

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   omitted if not needed.

   The size of the sequence-number space depends on the speed of the
   emulated service, and the maximum time of the transient conditions in
   the PSN.  A sequence number space greater than 2^16 may therefore be
   needed to prevent the sequence number space wrapping during the
   transient.  Frame Ordering

   When packets carrying the PW-PDUs traverse a PSN, they may arrive out
   of order at the destination PE.  For some services, the frames
   (control frames, data frames, or both control and data frames) MUST
   be delivered in order.  For such services, some mechanism MUST be
   provided for ensuring in-order delivery. Providing a sequence number
   in the sequence sub-layer header for each packet is one possible
   approach to out-of-sequence detection.  Alternatively it can be noted
   that sequencing is a subset of the problem of delivering timed
   packets, and that a single combined mechanism such as [RFC1889] MAY
   be employed.

   There are two possible misordering strategies:

       o Drop misordered PW PDUs.

       o Try to sort PW PDUs into the correct order.

   The choice of strategy will depend on:

       o How critical the loss of packets is to the operation of
         the PW (e.g. the acceptable bit error rate).

       o The speeds of the PW and PSN.

       o The acceptable delay (since delay must be introduced to

       o The incidence of expected misordering.  Frame Duplication Detection

   In rare cases, packets traversing a PW may be duplicated by the
   underlying PSN.  For some services frame duplication is not
   acceptable.  For such services, some mechanism MUST be provided to
   ensure that duplicated frames will not be delivered to the
   destination CE. The mechanism MAY be the same as the mechanism used
   to ensure in-order frame delivery.

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   A destination PE can determine whether a frame has been lost by
   tracking the sequence numbers of the received PW PDUs.

   In some instances, a destination PE will have to presume that a PW
   PDU is lost if it fails to arrive within a certain time.  If a PW-PDU
   that has been processed as lost subsequently arrives, the destination
   PE MUST discard it.

5.2.2.  Timing

   A number of native services have timing expectations based on the
   characteristics of the networks that they were designed to travel
   over, and it can be necessary for the emulated service to duplicate
   these network characteristics as closely as possible, e.g. in
   delivering native traffic with bit-rate, jitter, wander and delay
   characteristics similar to those received at the sending PE.

   In such cases, it is necessary for the receiving PE to play out the
   native traffic as it was received at the sending PE.  This relies on
   either timing information sent between the two PEs, or in some case
   timing information received from an external reference.

   The Timing Sub-layer must therefore support two timing functions:
   clock recovery and timed payload delivery.  A particular payload type
   may require either or both of these services.  Clock Recovery

   Clock recovery is the extraction of output transmission bit timing
   information from the delivered packet stream, and requires a suitable
   mechanism.  A physical wire carries the timing information natively,
   but it is a relatively complex task to extract timing from a highly
   jittered source such as packet stream.  It is therefore desirable
   that an existing real-time protocol such as [RFC1889] be used for
   this purpose, unless it can be shown that this is unsuitable or
   unnecessary for a particular payload type.  Timed delivery

   Timed delivery is the delivery of non-contiguous PW PDUs to the PW
   output interface with a constant phase relative to the input
   interface.  The timing of the delivery may be relative to a clock
   derived from the packet stream received over the PSN clock recovery,
   or with reference to an external clock.

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5.3  Fragmentation

   A payload would ideally be relayed across the PW as a single unit.
   However, there will be cases where the combined size of the payload
   and its associated PWE3 and PSN headers exceeds the PSN path MTU.
   When a packet size exceeds the MTU of a given network, fragmentation
   and reassembly have to be performed in order for the packet to be
   delivered.  Since fragmentation and reassembly generally consume a
   considerable network resources as compared to simply switching a
   packet in its entirety, efforts SHOULD be made to reduce or eliminate
   the need for fragmentation and reassembly throughout a network to the
   extent possible. Of particular concern for fragmentation and
   reassembly are aggregation points where large numbers of PWs are
   processed (e.g. at the PE).

   Ideally, the equipment originating the traffic being sent over the PW
   will be configured to have adaptive measures (e.g. [RFC1191],
   [RFC1981]) in place that ensure that packets that need to be
   fragmented are not sent.  When this fails, the point closest to the
   sending host with fragmentation and reassembly capabilities SHOULD
   attempt to reduce the size of packets to satisfy the PSN MTU.  Thus,
   in the reference model for PWE3 [Figure 3] fragmentation SHOULD first
   be performed at the CE if at all possible.  If and only if the CE
   cannot adhere to an acceptable MTU size for the PW should the PE
   attempt its own fragmentation method.

   In cases where MTU management fails to limit the payload to a size
   suitable for transmission of the PW, the PE MAY fall back to either a
   generic PW fragmentation method, or, if available the fragmentation
   service of the underlying PSN.

   It is acceptable for a PE implementation not to support
   fragmentation.  A PE that does not support fragmentation will drop
   packets that exceed the PSN MTU, and the management plane of the
   encapsulating PE MAY be notified.

   If the length of a L2/L1 frame, restored from a PW PDU, exceeds the
   MTU of the destination PWES, it MUST be dropped.  In this case, the
   management plane of the destination PE MAY be notified.

5.4  Instantiation of the Protocol Layers

   This document does not address the detailed mapping of the Protocol
   Layering model to existing or future IETF standards.  The
   instantiation of the logical Protocol Layering model is shown in
   Figure 9.

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5.4.1. PWE3 over an IP PSN

   The protocol definition of PWE3 over an IP PSN SHOULD employ existing
   IETF protocols where possible.

       +---------------------+              +-------------------------+
       |      Payload        |------------->| Raw payload if possible |
       /=====================\              +-------------------------+
       H Payload Convergence H------------->|        As Needed        |
       H---------------------H              +-------------------------+
       H       Timing        H----------+-->|            RTP          |
       H---------------------H         /    +-------------+           |
       H     Sequencing      H----one of    |             |
       \=====================/         \    |             +-----------+
       |  PW Demultiplexer   |----------+-->|     L2TP, MPLS etc.     |
       +---------------------+              +-------------------------+
       |  PSN Convergence    |------------->|       Not needed        |
       +---------------------+              +-------------------------+
       |        PSN          |------------->|            IP           |
       +---------------------+              +-------------------------+
       |      Data-link      |------------->|         Data-link       |
       +---------------------+              +-------------------------+
       |       Physical      |------------->|          Physical       |
       +---------------------+              +-------------------------+

   Figure 10: PWE3 over an IP PSN

   Figure 10 shows the protocol layering for PWE3 over an IP PSN. As a
   rule, the payload SHOULD be carried as received from the NSP, with
   the Payload Convergence Layer provided when needed.  (It is accepted
   that there MAY sometimes be good reason not to follow this rule, but
   the exceptional circumstances need to be documented in the
   Encapsulation Layer definition for that payload type).

   Where appropriate, timing is provided by RTP [RFC1889], which when
   used also provides a sequencing service.  PW Demultiplexing may be
   provided by a number of existing IETF tunnel protocols.  Some of
   these tunnel protocols provide an optional sequencing service.
   (Sequencing is provided either by RTP, or by the PW Demultiplexer
   Layer, but not both).  A PSN Convergence Layer is not needed, because
   all the tunnel protocols shown above are designed to operate directly
   over an IP PSN.

   As a special case, if the PW Demultiplexer is an MPLS label, the
   protocol architecture of section 5.4.2 can be used instead of the
   protocol architecture of this section.

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5.4.2. PWE3 over an MPLS PSN

   The MPLS ethos places importance on wire efficiency.  By using a
   control word, some components of the PWE3 protocol layers can be
   compressed to increase this efficiency.

   |      Payload        |
   H Payload Convergence H-----+
   H---------------------H     |
   H       Timing        H------------------------+
   H---------------------H     |                  |
   H     Sequencing      H-----|                  |
   \=====================/     |                  |
   |  PW Demultiplexer   |---+ |                  v
   +---------------------+   | |  +--------------------------------+
   |  PSN Convergence    |-----|  .                                .
   +---------------------+   | |  .              RTP               .
   |        PSN          |-+ | |  |                                |
   +---------------------+ | | |  +--------------------------------+
   |      Data-link      | | | +->| Flags, Frag, Len, Seq #, etc   |
   +---------------------+ | |    +--------------------------------+
   |       Physical      | | +--->|           PW Label             |
   +---------------------+ |      +--------------------------------+
                           +----->| Outer Label or MPLS-in-IP encap|

   Figure 11: PWE3 over an MPLS PSN using a control word

   Figure 11 shows the protocol layering for PWE3 over an MPLS PSN.  An
   inner MPLS label is used to provide the PW demultiplexing function.
   A control word is used to carry most of the information needed by the
   PWE3 Encapsulation Layer and the PSN Convergence Layer in a compact
   format.  The flags in the control word provide the necessary payload
   convergence.  A sequence field provides support for both in-order
   payload delivery and (supported by a fragmentation control method) a
   PSN fragmentation service within the PSN Convergence Layer.  Ethernet
   pads all frames to a minimum size of 64 bytes. The MPLS header does
   not include a length indicator. Therefore to allow PWE3 to be carried
   in MPLS to correctly pass over an Ethernet data-link, a length
   correction field is needed in the control word. As with an IP PSN,
   where appropriate, timing is provided by RTP [RFC1889].

   In some networks it may be necessary to carry PWE3 over MPLS over IP.
   In these circumstances, the PW is encapsulated for carriage over MPLS
   as described in this section, and then a method of carrying MPLS over

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   an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to the
   resultant PW-PDU.

5.4.3. PW over MPLS Generic Control Word

   The PW set-up protocol determines whether a particular PW uses a
   control word.  When a control word is used, it MUST have the
   following form:

       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
      |  PID  | Flags |FRG|  Length   | Sequence Number               |

        Figure 12 - MPLS Generic Control Word

   The meaning of the fields of the MPLS Generic Control Word (Figure
   12) is as follows:

       PID (bits 0 to 3):
           In an environment in which all PWs use the control word,
           the payload type of an MPLS packet can be determined by
           inspecting the first four bits of the longword, which
           follows the bottom of the label stack.  A value of 0
           indicates "pseudowire", 4 indicates IPv4, 6 indicates IPv6.

       Flags (bits 4 to 7):
           These bits are available for per payload signaling. Their
           definition is encapsulation specific.

       FRG (bits 8 and 9):
           These bits are used when fragmenting a PW payload. Their use
           is defined in [FRAG].

       Length (bits 10 to 15):
           The length field is used to determine the size of a PW
           payload that might have been padded to the minimum Ethernet
           MAC frame size during its transit across the PSN.  If the
           MPLS payload (defined as the CW + the PW payload + any
           additional PW headers is less than 46 bytes, the length MUST
           be set to the length of the MPLS payload.  If the MPLS
           payload is between 46 bytes and 63 bytes the implementation
           MAY either set to the length to the length of the MPLS
           payload, or it MAY set it to 0.  If the length of the MPLS
           payload is greater than 63 bytes the length MUST be set to 0.

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       Sequence number (Bit 16 to 31):
           If the sequence number is not used, it is set to zero by
           the sender and ignored by the receiver.  Otherwise it
           specifies the sequence number of a packet. A circular list
           of sequence numbers is used. A sequence number takes a value
           from 1 to 65535 (2**16-1).

6.  PW Demultiplexer Layer and PSN Requirements

   PWE3 places three service requirements on the protocol layers used to
   carry it across the PSN:

       o Multiplexing
       o Fragmentation
       o Length and Delivery

6.1  Multiplexing

   The purpose of the PW Demultiplexer Layer is to allow multiple PWs to
   be carried in a single tunnel.  This minimizes complexity and
   conserves resources.

   Some types of native service are capable of grouping multiple
   circuits into a "trunk", e.g. multiple ATM VCs in a VP, multiple
   Ethernet VLANs on a physical media, or multiple DS0 services within a
   T1 or E1.  A PW MAY interconnect two end-trunks.  That trunk would
   have a single multiplexing identifier.

6.2  Fragmentation

   If the PSN provides a fragmentation and reassembly service of
   adequate performance, it MAY be used to obtain an effective MTU that
   is large enough to transport the PW PDUs.  See Section 5.3 for a full
   discussion of the PW fragmentation issues.

6.3  Length and Delivery

   PDU delivery to the egress PE is the function of the PSN Layer.

   If the underlying PSN does not provide all the information necessary
   to determine the length of a PW-PDU, the Encapsulation Layer MUST
   provide it.

6.4  PW-PDU Validation

   It is a common practice to use an error detection mechanism such as a
   CRC or similar mechanism to assure end-to-end integrity of frames.

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   The PW service-specific mechanisms MUST define whether the packet's
   checksum shall be preserved across the PW, or be removed from PE-
   bound PDUs and then be re-calculated for insertion in CE-bound data.

   The former approach saves work, while the latter saves bandwidth. For
   a given implementation the choice may be dictated by hardware
   restrictions, which may not allow the preservation of the checksum.

   For protocols such as ATM and FR, the scope of the checksum is
   restricted to a single link.  This is because the circuit identifiers
   (e.g. FR DLCI or ATM VPI/VCI) have only local significance and are
   changed on each hop or span.  If the circuit identifier (and thus
   checksum) were going to change as a part of the PW emulation, it
   would be more efficient to strip and re-calculate the checksum.

   The service specific document for each protocol MUST describe the
   validation scheme to be used.

6.5  Congestion Considerations

   The PSN carrying the PW may be subject to congestion. The congestion
   characteristics will vary with the PSN type, the network architecture
   and configuration, and the loading of the PSN.

   Where the traffic carried over the PW is known to be TCP friendly
   (by, for example, packet inspection), packet discard in the PSN will
   trigger the necessary reduction in offered load, and no additional
   congestion avoidance action is necessary.

   If the PW is operating over a PSN that provides enhanced delivery,
   the PEs SHOULD monitor packet loss to ensure that the service that
   was requested is actually being delivered. If it is not, then the PE
   SHOULD assume that the PSN is providing a best-effort service, and
   SHOULD use the best-effort service congestion avoidance measures
   described below.

   If best-effort service is being used and the trafic is not known to
   be TCP friendly, the PEs SHOULD monitor packet loss to ensure that
   the packet loss rate is within acceptable parameters. Packet loss is
   considered acceptable if a TCP flow across the same network path and
   experiencing the same network conditions would achieve an average
   throughput, measured on a reasonable timescale, that is not less than
   the PW flow is achieving. This condition can be satisfied by
   implementing a rate-limiting measure in the NSP, or by shutting down
   one or more PWs.  The choice of which approach to use depends upon
   the type of traffic being carried.  Where congestion is avoided by
   shutting down a PW, a suitable mechanism MUST be provided to prevent
   it immediately returning to service, causing a series of congestion

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   The comparison to TCP cannot be specified exactly, but is intended as
   an "order-of-magnitude" comparison in timescale and throughput. The
   timescale on which TCP throughput is measured is the round-trip time
   of the connection. In essence, this requirement states that it is not
   acceptable to deploy an application (using PWE3 or any other
   transport protocol) on the best-effort Internet which consumes
   bandwidth arbitrarily and does not compete fairly with TCP within an
   order of magnitude.  One method of determining an acceptable PW
   bandwidth is described in [TFRC].

7.  Control Plane

   This section describes PWE3 control plane services.

7.1  Set-up or Teardown of Pseudo-Wires

   A PW MUST be set up before an emulated service can be established,
   and MUST be torn down when an emulated service is no longer needed.

   Set up or teardown of a PW can be triggered by an operator command,
   from the management plane of a PE, by signaling (i.e., set-up or
   teardown) of a PWES, e.g., an ATM SVC, or by an auto-discovery

   During the set-up process, the PEs need to exchange some information
   (e.g. learn each other's capabilities).  The tunnel signaling
   protocol MAY be extended to provide mechanisms to enable the PEs to
   exchange all necessary information on behalf of the PW.

   Manual configuration of PWs can be considered a special kind of
   signaling, and is allowed.

7.2  Status Monitoring

   Some native services have mechanisms for status monitoring. For
   example, ATM supports OAM for this purpose.  For such services, the
   corresponding emulated services MUST specify how to perform status

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7.3  Notification of Pseudo-wire Status Changes

7.3.1.  Pseudo-wire Up/Down Notification

   If a native service REQUIRES bi-directional connectivity, the
   corresponding emulated service can only be signaled as being up when
   the associated PWs, and PSN tunnels if any, are functional in both

   Because the two CEs of an emulated service are not adjacent, a
   failure may occur at a place such that one or both physical links
   between the CEs and PEs remain up.  For example, in Figure 2, if the
   physical link between CE1 and PE1 fails, the physical link between
   CE2 and PE2 will not be affected and will remain up.  Unless CE2 is
   notified about the remote failure, it will continue to send traffic
   over the emulated service to CE1.  Such traffic will be discarded at
   PE1.  Some native services have failure notification so that when the
   services fail, both CEs will be notified.  For such native services,
   the corresponding PWE3 service MUST provide a failure notification

   Similarly, if a native service has notification mechanisms so that
   when a network failure is fixed, all the affected services will
   change status from "Down" to "Up", the corresponding emulated service
   MUST provide a similar mechanism for doing so.

   These mechanisms may already be built into the tunneling protocol.
   For example, the L2TP control protocol [RFC2661] [L2TPv3] has this
   capability and LDP has the ability to withdraw the corresponding MPLS

7.3.2.  Misconnection and Payload Type Mismatch

   With PWE3, misconnection and payload type mismatch can occur.  If a
   misconnection occurs it can breach the integrity of the system.  If a
   payload mismatch occurs it can disrupt the customer network.  In both
   instances, there are security and operational concerns.

   The services of the underlying tunneling mechanism, and its
   associated control protocol, can be used to mitigate this.  As part
   of the PW set-up a PW-TYPE identifier is exchanged. This is then used
   by the FWRD and NSP to verify the compatibility of the PWESs.

7.3.3.  Packet Loss, Corruption, and Out-of-order Delivery

   A PW can incur packet loss, corruption, and out-of-order delivery on
   the PSN path between the PEs.  This can impact the working condition
   of an emulated service. For some payload types, packet loss,

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   corruption, and out-of-order delivery can be mapped to either a bit
   error burst, or loss of carrier on the PW.  If a native service has
   some mechanism to deal with bit error, the corresponding PWE3 service
   should provide a similar mechanism.

7.3.4.  Other Status Notification

   A PWE3 approach MAY provide a mechanism for other status
   notification, if any are needed.

7.3.5.  Collective Status Notification

   Status of a group of emulated services may be affected identically by
   a single network incident.  For example, when the physical link (or
   sub-network) between a CE and a PE fails, all the emulated services
   that go through that link (or sub-network) will fail.  It is likely
   that there exists a group of emulated services that all terminate at
   a remote CE. There may also be multiple such CEs affected by the
   failure. Therefore, it is desirable that a single notification
   message be used to notify failure of the whole group of emulated

   A PWE3 approach MAY provide some mechanism for notifying status
   changes of a group of emulated circuits.  One possible method is to
   associate each emulated service with a group ID when the PW for that
   emulated service is set up.  Multiple emulated services can then be
   grouped by associating them with the same group ID. In status
   notification, that group ID can be used to refer all the emulated
   services in that group.  The group ID mechanism should be a mechanism
   provided by the underlying tunnel signaling protocol.

7.4  Keep-alive

   If a native service has a keep-alive mechanism, the corresponding
   emulated service MUST provide a mechanism to propagate this across
   the PW.  An approach following the principle of minimum intervention
   would be to transparently transport keep-alive messages over the PW.
   However, to accurately reproduce the semantics of the native
   mechanism, some PWs MAY REQUIRE an alternative approach, such as
   piggy-backing on the PW signaling mechanism.

7.5  Handling Control Messages of the Native Services

   Some native services use control messages for circuit maintenance.
   These control messages MAY be in-band, e.g. Ethernet flow control,
   ATM performance management, or TDM tone signaling, or they MAY be
   out-of-band, e.g. the signaling VC of an ATM VP, or TDM CCS

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   From the principle of minimum intervention, it is desirable that the
   PEs participate as little as possible in the signaling and
   maintenance of the native services.  This principle SHOULD NOT,
   however, override the need to satisfactorily emulate the native

   If control messages are passed through, it may be desirable to send
   them using either a higher priority or a reliable channel provided by
   the PW Demultiplexer layer.  See PWE3 Channel Types.

8. Management and Monitoring

   This section describes the management and monitoring architecture for

8.1  Status and Statistics

   The PE should report the status of the interface and tabulate
   statistics that help monitor the state of the network, and to help
   with measurement of service level agreements (SLAs). Typical counters

       o Counts of PW-PDUs sent and received, with and without errors.
       o Counts of sequenced PW-PDUs lost.
       o Counts of service PDUs sent and received over the PSN, with
         and without errors (non-TDM).
       o Service-specific interface counts.
       o One way delay and delay variation.

   These counters would be contained in a PW-specific MIB, and they
   should not replicate existing MIB counters.

8.2  PW SNMP MIB Architecture

   This section describes the general architecture for SNMP MIBs used to
   manage PW services and the underlying PSN.  The intent here is to
   provide a clear picture of how all of the pertinent MIBs fit together
   to form a cohesive management framework for deploying PWE3 services.

8.2.1.  MIB Layering

   The SNMP MIBs created for PWE3 should fit the architecture shown in
   Figure 13.

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                  +-----------+ +-----------+     +-----------+
        Service   |    CEM    | | Ethernet  |     |    ATM    |
         Layer    |Service MIB| |Service MIB| ... |Service MIB|
                  +-----------+ +-----------+     +-----------+
                          \           |             /
                            \         |           /
      - - - - - - - - - - - - \ - - - | - - - - / - - - - - - -
                                \     |       /
       Generic PW |            Generic PW MIBs                |
         Layer    +-------------------------------------------+
                                /             \
      - - - - - - - - - - - - / - - - - - - - - \ - - - - - - -
                            /                     \
                          /                         \
                  +-----------+                   +-----------+
        PSN VC    |L2TP VC MIB|                   |MPLS VC MIB|
         Layer    +-----------+                   +-----------+
                        |                               |
      - - - - - - - - - | - - - - - - - - - - - - - - - | - - -
                        |                               |
                  +-----------+                   +-----------+
          PSN     |L2TP MIB(s)|                   |MPLS MIB(s)|
         Layer    +-----------+                   +-----------+

                       Figure 13: Relationship of SNMP MIBs

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   Figure 14 shows an example for a SONET PW carried over MPLS.

                     |    SONET MIB    | RFC2558
           Service   |SONET Service MIB| pw-cem-mib
            Layer    +-----------------+
          - - - - - - - - - - | - - - - - - - - - - - - - - -
          Generic PW | Generic PW MIBS | pw-tc-mib
            Layer    +-----------------+ pw-mib
          - - - - - - - - - - | - - - - - - - - - - - - - - -
            PSN VC   |   MPLS VC MIBS  | pw-mpls-mib
            Layer    +-----------------+
          - - - - - - - - - - | - - - - - - - - - - - - - - -
             PSN     |    MPLS MIBs    |   mpls-te-mib
            Layer    +-----------------+   mpls-lsr-mib

               Figure 14: Service-specific Example for MIBs

   Note that there is a separate MIB for each emulated service as well
   as one for each underlying PSN.  These MIBs MAY be used in various
   combinations as needed.

8.2.2.  Service Layer MIBs

   The first layer is referred to as the Service Layer.  It contains
   MIBs for PWE3 services such as Ethernet, ATM, circuits and Frame
   Relay. This layer contains those corresponding MIBs used to mate or
   adapt those emulated services to the underlying services.  This
   working group should not produce any MIBs for managing the general
   service; rather, it should produce just those MIBs that are used to
   interface or adapt the emulated service onto the PWE3 management
   framework.  For example, the standard SONET MIB [SONETMIB] is
   designed and maintained by another working group. Also, the SONET MIB
   is designed to manage the native service without PW emulation.  Since
   the PWE3 working group is chartered to produce the corresponding
   adaptation MIB, in this case, it would produce the PW-CEM-MIB
   [PWMPLSMIB] that would be used to adapt SONET services to the
   underlying PSN that carries the PWE3 service.

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8.2.3.  Generic PW MIBs

   The second layer is referred to as the Generic PW Layer.  This layer
   is composed of two MIBs: the PWE-TC-MIB [PWTCMIB] and the PWE-MIB
   [PWMIB]. These MIBs are responsible for providing general PWE3
   counters and service models used for monitoring and configuration of
   PWE3 services over any supported PSN service.  That is, this MIB
   provides a general model of PWE3 abstraction for management purposes.
   This MIB is used to interconnect the Service Layer MIBs to the PSN VC
   Layer MIBs. The latter will be described in the next section.  This
   layer also provides the PW-TC-MIB [PWTCMIB].  This MIB contains
   common SMI textual conventions [RFC1902] that MAY be used by any PW

8.2.4.  PSN VC Layer MIBs

   The third layer in the PWE3 management architecture is referred to as
   the PSN VC layer.  This layer is comprised of MIBs that are
   specifically designed to interface general PWE3 services (VCs) onto
   those underlying PSN services.  In general this means that the MIB
   provides a means with which an operator can map the PW service onto
   the native PSN service. For example, in the case of MPLS, it is
   required that the general VC service be layered onto MPLS LSPs or
   Traffic Engineered (TE) Tunnels [RFC3031].  In this case, the PW-
   MPLS-MIB [PWMPLSMIB] was created to adapt the general PWE3 circuit
   services onto MPLS.  Like the Service Layer described above the PWE3
   working group should produce these MIBs.

8.2.5.  PSN Layer MIBs

   The fourth and final layer in the PWE3 management architecture is
   referred to as the PSN layer.  This layer is comprised of those MIBs
   that control the PSN service-specific services.  For example, in the
   case of the MPLS [RFC3031] PSN service, the MPLS-LSR-MIB [LSRMIB] and
   the MPLS-TE-MIB [TEMIB] are used to interface the general PWE3 VC
   services onto native MPLS LSPs and/or TE tunnels to carry the
   emulated services.  In addition, the MPLS-LDP-MIB [LDPMIB] MAY be
   used to reveal the MPLS labels that are distributed over the MPLS PSN
   in order to maintain the PW service. The MIBs in this layer are
   produced by other working groups that design and specify the native
   PSN services.  These MIBs should contain the appropriate mechanisms
   for monitoring and configuring the PSN service such that the emulated
   PWE3 service will function correctly.

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8.3 Connection Verification and Traceroute

   A connection verification mechanism should be supported by PWs.
   Connection verification as well as other alarm mechanisms can alert
   the operator that a PW has lost its remote connection.  The opaque
   nature of a PW means that it is not possible to specify a generic
   connection verification or traceroute mechanism that passes this
   status to the CEs over the PW.  If connection verification status of
   the PW is needed by the CE, it MUST be mapped to the native
   connection status method.

   For troubleshooting purposes, it is sometimes desirable to know the
   exact functional path of a PW between PEs. This is provided by the
   traceroute service of the underlying PSN.  The opaque nature of the
   PW means that this traceroute information is only available within
   the provider network, e.g., at the PEs.

9.  IANA considerations

   The control word PID bits need to be assigned by IANA.

10.  Security Considerations

   PWE3 provides no means of protecting the contents or delivery of the
   native data units. The use of PWE3 can therefore expose a particular
   environment to additional security threats. Assumptions that might be
   appropriate when all communicating systems are interconnected via a
   point to point or circuit-switched network may no longer hold when
   they are interconnected using an emulated wire carried over some
   types of PSN.  It is outside the scope of this specification, to
   fully analyze and review the risks of PWE3, particularly as these
   risks will depend on the PSN. An example should make the concern
   clear.  A number of IETF standards employ relatively weak security
   mechanisms when communicating nodes are expected to be connected to
   the same local area network.  The Virtual Router Redundancy Protocol
   [RFC2338] is one instance.  The relatively weak security mechanisms
   represent a greater vulnerability in an emulated Ethernet connected
   via a PW.

   Exploitation of vulnerabilities from within the PSN may be directed
   to the PW Tunnel end-point so that PW Demultiplexer and PSN tunnel
   services are disrupted.  Controlling PSN access to the PW Tunnel

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   end-point is one way to protect against this. By restricting PW
   Tunnel end-point access to legitimate remote PE sources of traffic,
   the PE may reject traffic that would interfere with the PW
   Demultiplexing and PSN tunnel services.

   Protection mechanisms MUST also address the spoofing of tunneled PW
   data. The validation of traffic addressed to the PW Demultiplexer
   end-point is paramount in ensuring integrity of PW encapsulation.
   Security protocols such as IPSec [RFC2401] MAY be used by the PW
   Demultiplexer Layer in order to maintain the integrity of the PW by
   authenticating data between the PW Demultiplexer End-points.  IPSec
   MAY provide authentication, integrity, non-repudiation, and
   confidentiality of data transferred between two PEs. It cannot
   provide the equivalent services to the native service.

   Based on the type of data being transferred, the PW MAY indicate to
   the PW Demultiplexer Layer that enhanced security services are
   required.  The PW Demultiplexer Layer MAY define multiple protection
   profiles based on the requirements of the PW emulated service.  CE-
   to-CE signaling and control events emulated by the PW and some data
   types may require additional protection mechanisms.  Alternatively,
   the PW Demultiplexer Layer may use peer authentication for every PSN
   packet to prevent spoofed native data units from being sent to the
   destination CE.


   We thank:  Sasha Vainshtein for his work on Native Service Processing
   and advice on bit-stream over PW services. Thomas K. Johnson for his
   work on the background and motivation for PWs.

   We also thank: Ron Bonica, Stephen Casner, Durai Chinnaiah, Jayakumar
   Jayakumar, Ghassem Koleyni, Eric Rosen, John Rutemiller, Scott
   Wainner and David Zelig for their comments and contributions.


   Internet-drafts are works in progress available from

   [DVB]       EN 300 744 Digital Video Broadcasting (DVB); Framing
               structure, channel coding and modulation for digital
               terrestrial television (DVB-T), European
               Telecommunications Standards Institute (ETSI)

   [FRAG]      Malis and Townsley, "PWE3 Fragmentation and

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               Reassembly", <draft-ietf-pwe3-fragmentation-00.txt>,
               work in progress, October 2002.

   [LDP-MIB]   Cucchiara, J., Sjostrand, H., and Luciani, J.,
               "Definitions of Managed Objects for the Multiprotocol
               Label Switching, Label Distribution Protocol (LDP)",
               <draft-ietf-mpls-ldp-mib-09.txt>, work in progress,
               October 2002.

   [LSRMIB]    Srinivasan et al, "MPLS Label Switch Router Management
               Information Base Using SMIv2",
               <draft-ietf-mpls-lsr-mib-09.txt>, work in progress,
               October 2002.

   [L2TPv3]    Layer Two Tunneling Protocol (Version 3)'L2TPv3', J Lau,
               et. al. <draft-ietf-l2tpext-l2tp-base-05.txt>, work
               in progress, January 2003.

   [PPPoL2TP]  PPP Tunneling Using Layer Two Tunneling Protocol,
               J Lau et al. <draft-ietf-l2tpext-l2tp-ppp-02.txt>,
               work in progress, June 2002.

   [PWMIB]     Zelig et al, "Pseudo Wire (PW) Management Information
               Base Using SMIv2", <draft-ietf-pwe3-pw-mib-00.txt>,
               work in progress, June 2002.

   [PWTCMIB]   Nadeau et al, "Definitions for Textual Conventions and
               OBJECT-IDENTITIES for Pseudo-Wires Management"
               <draft-ietf-pwe3-pw-tc-mib-00.txt>, work in progress,
               June 2002.

   [PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service
               Over MPLS (CEM) Management Information Base Using
               SMIv2", <draft-ietf-pwe3-cep-mib-01.txt>, work in
               progress, October 2002.

   [RFC1191]   RFC-1191: Path MTU discovery. J.C. Mogul, S.E. Deering.

   [RFC1889]   RFC-1889: RTP: A Transport Protocol for Real-Time
               Applications.  H. Schulzrinne et. al.

   [RFC1902]   RFC-1902: Structure of Management Information for
               Version 2 of the Simple Network Management Protocol
               (SNMPv2), Case et al, January 1996.

   [RFC1958]   RFC-1958: Architectural Principles of the Internet,
               B. Carpenter et al.

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   [RFC1981]   RFC-1981: Path MTU Discovery for IP version 6. J. McCann,
               S. Deering, J. Mogul.

   [RFC2022]   RFC-2022: Support for Multicast over UNI 3.0/3.1 based
               ATM Networks, G. Armitage.

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

   [RFC2338]   RFC-2338: Virtual Router Redundancy Protocol,
               S. Knight, M. Shand et. al.

   [RFC2401]   RFC-2401: Security Architecture for the Internet
               Protocol. S. Kent, R. Atkinson.

   [RFC2474]   RFC-2474: Definition of the Differentiated Services
               Field (DS Field) in the IPv4 and IPv6 Headers,
               K. Nichols, et. al.

   [RFC2661]   RFC-2661: Layer Two Tunneling Protocol "L2TP".
               W. Townsley, et. al.

   [RFC2784]   RFC-2784: Generic Routing Encapsulation (GRE).
               D. Farinacci et al.

   [RFC2890]   RFC-2890: Key and Sequence Number Extensions to GRE.
               G. Dommety.

   [RFC3022]   RFC-3022: Traditional IP Network Address Translator
               (Traditional NAT). P Srisuresh et al.

   [RFC3031]   RFC3031: Multiprotocol Label Switching Architecture,
               E. Rosen, January 2001.

   [SONETMIB]  K. Tesink, "Definitions of Managed Objects for the
               SONET/SDH Interface Type", RFC2558, March 1999.

   [TEMIB]     Srinivasan et al, "Traffic Engineering Management
               Information Base Using SMIv2",
               <draft-ietf-mpls-te-mib-09.txt>, work in progress,
               November 2002.

   [TFRC]      M. Handley et al, "TCP Friendly Rate Control (TFRC):
               Protocol Specification" <draft-ietf-tsvwg-tfrc-05.txt>,
               work in progress, October 2002.

   [VPLS]      M. Lasserre, "Virtual Private LAN Services over MPLS",
               <draft-lasserre-vkompella-ppvpn-vpls-03.txt>, work in

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               progress, January 2003.

   [XIAO]      Xiao et al, "Requirements for Pseudo-Wire Emulation
               Edge-to-Edge  (PWE3)",
               (draft-ietf-pwe3-requirements-04.txt), X Xiao et al.
               work in progress, December 2002.

Editors' Addresses

   Stewart Bryant
   Cisco Systems,
   4, The Square,
   Stockley Park,
   Uxbridge UB11 1BL,
   United Kingdom.             Email: stbryant@cisco.com

   Prayson Pate
   Overture Networks, Inc.
   Airport Boulevard
   Morrisville, NC, USA 27560  Email: prayson.pate@overturenetworks.com

   Full copyright statement

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