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rfc:rfc3985

Network Working Group S. Bryant, Ed. Request for Comments: 3985 Cisco Systems Category: Informational P. Pate, Ed.

                                               Overture Networks, Inc.
                                                            March 2005
       Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

Abstract

 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, and specifies
 the various protocol elements and their functions.

Table of Contents

 1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
      1.1.  Pseudo Wire Definition. . . . . . . . . . . . . . . . .  2
      1.2.  PW Service Functionality. . . . . . . . . . . . . . . .  3
      1.3.  Non-Goals of This Document. . . . . . . . . . . . . . .  4
      1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . .  4
 2.   PWE3 Applicability. . . . . . . . . . . . . . . . . . . . . .  6
 3.   Protocol Layering Model . . . . . . . . . . . . . . . . . . .  6
      3.1.  Protocol Layers . . . . . . . . . . . . . . . . . . . .  7
      3.2.  Domain of PWE3. . . . . . . . . . . . . . . . . . . . .  8
      3.3.  Payload Types . . . . . . . . . . . . . . . . . . . . .  8
 4.   Architecture of Pseudo Wires. . . . . . . . . . . . . . . . . 11
      4.1.  Network Reference Model . . . . . . . . . . . . . . . . 12
      4.2.  PWE3 Pre-processing . . . . . . . . . . . . . . . . . . 12
      4.3.  Maintenance Reference Model . . . . . . . . . . . . . . 16
      4.4.  Protocol Stack Reference Model. . . . . . . . . . . . . 17
      4.5.  Pre-processing Extension to Protocol Stack Reference
            Model . . . . . . . . . . . . . . . . . . . . . . . . . 17
 5.   PW Encapsulation. . . . . . . . . . . . . . . . . . . . . . . 18

Bryant & Pate Standards Track [Page 1] RFC 3985 PWE3 Architecture March 2005

      5.1.  Payload Convergence Layer . . . . . . . . . . . . . . . 19
      5.2.  Payload-independent PW Encapsulation Layers . . . . . . 21
      5.3.  Fragmentation . . . . . . . . . . . . . . . . . . . . . 24
      5.4.  Instantiation of the Protocol Layers. . . . . . . . . . 24
 6.   PW Demultiplexer Layer and PSN Requirements . . . . . . . . . 27
      6.1.  Multiplexing. . . . . . . . . . . . . . . . . . . . . . 27
      6.2.  Fragmentation . . . . . . . . . . . . . . . . . . . . . 28
      6.3.  Length and Delivery . . . . . . . . . . . . . . . . . . 28
      6.4.  PW-PDU Validation . . . . . . . . . . . . . . . . . . . 28
      6.5.  Congestion Considerations . . . . . . . . . . . . . . . 28
 7.   Control Plane . . . . . . . . . . . . . . . . . . . . . . . . 29
      7.1.  Set-up or Teardown of Pseudo Wires. . . . . . . . . . . 29
      7.2.  Status Monitoring . . . . . . . . . . . . . . . . . . . 30
      7.3.  Notification of Pseudo Wire Status Changes. . . . . . . 30
      7.4.  Keep-alive. . . . . . . . . . . . . . . . . . . . . . . 31
      7.5.  Handling Control Messages of the Native Services. . . . 32
 8.   Management and Monitoring . . . . . . . . . . . . . . . . . . 32
      8.1.  Status and Statistics . . . . . . . . . . . . . . . . . 32
      8.2.  PW SNMP MIB Architecture. . . . . . . . . . . . . . . . 33
      8.3.  Connection Verification and Traceroute. . . . . . . . . 36
 9.   IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
 10.  Security Considerations . . . . . . . . . . . . . . . . . . . 37
 11.  Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 38
 12.  References. . . . . . . . . . . . . . . . . . . . . . . . . . 38
      12.1.  Normative References . . . . . . . . . . . . . . . . . 38
      12.2.  Informative References . . . . . . . . . . . . . . . . 39
 13.  Co-Authors. . . . . . . . . . . . . . . . . . . . . . . . . . 40
 14.  Editors' Addresses. . . . . . . . . . . . . . . . . . . . . . 41
      Full Copyright Statement. . . . . . . . . . . . . . . . . . . 42

1. Introduction

 This document describes an architecture for Pseudo Wire Emulation
 Edge-to-Edge (PWE3) in support of [RFC3916].  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, and specifies the various protocol elements and their
 functions.

1.1. Pseudo Wire Definition

 PWE3 is a mechanism that emulates the essential attributes of a
 telecommunications 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

Bryant & Pate Standards Track [Page 2] RFC 3985 PWE3 Architecture March 2005

 (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 an IP path or MPLS tunnel.  In some cases it is necessary
 to perform other operations 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 Customer Edge Equipment (CE), the PW is
 characterized 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 therefore 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.

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.

Bryant & Pate Standards Track [Page 3] RFC 3985 PWE3 Architecture March 2005

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 setup
       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, but multicast services such as MARS [RFC2022] that are
       implemented on top of the medium are not.
    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 physical or virtual circuit attaching
 (AC)                 a CE to a PE. An attachment Circuit may be, for
                      example, a Frame Relay DLCI, an ATM VPI/VCI, an
                      Ethernet port, a VLAN, a PPP connection on a
                      physical interface, a PPP session from an L2TP
                      tunnel, or an MPLS LSP.  If both physical and
                      virtual ACs are of the same technology (e.g.,
                      both ATM, both Ethernet, both Frame Relay), the
                      PW is said to provide "homogeneous transport";
                      otherwise, it is said to provide "heterogeneous
                      transport".
 CE-bound             The traffic direction in which 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 CE's control
                      plane.  It may be desirable or even necessary
                      for the PE to participate in or to monitor this
                      signaling in order to emulate the service
                      effectively.
 Control Word (CW)    A four-octet header used in some encapsulations
                      to carry per-packet information when the PSN is
                      MPLS.

Bryant & Pate Standards Track [Page 4] RFC 3985 PWE3 Architecture March 2005

 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 in
                      order 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.  Packets may undergo
                      fragmentation if they are larger than the MTU of
                      the network 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, or ATMF.)
 Packet Switched      Within the context of PWE3, this is a
 Network (PSN)        network using IP or MPLS as the mechanism for
                      packet forwarding.
 PE-Bound             The traffic direction in which 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 CE
                      Signaling in order to manage the PW effectively.
 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.
 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.

Bryant & Pate Standards Track [Page 5] RFC 3985 PWE3 Architecture March 2005

 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.

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 on 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 make the required service guarantees impossible
 to provide.

3. Protocol Layering Model

 The PWE3 protocol-layering model is intended to minimize 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 of maximizing the
 reuse of IETF protocol definitions and their implementations.

Bryant & Pate Standards Track [Page 6] RFC 3985 PWE3 Architecture March 2005

3.1. Protocol Layers

 The logical protocol-layering model required to support a PW is shown
 in Figure 1.
        +---------------------------+
        |         Payload           |
        +---------------------------+
        |      Encapsulation        | <==== may be empty
        +---------------------------+
        |     PW Demultiplexer      |
        +---------------------------+
        |     PSN Convergence       | <==== may be empty
        +---------------------------+
        |           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 further information is needed in the payload itself, this layer
 is empty.
 The Encapsulation Layer also provides support for real-time
 processing, and if needed for sequencing.
 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.  Therefore,
 this layer 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.

Bryant & Pate Standards Track [Page 7] RFC 3985 PWE3 Architecture March 2005

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

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 units:
     o Packet
     o Cell
     o Bit stream
     o Structured bit stream
 Within these generic types there are specific service types:
     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 delivered to the PE via
 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 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 has to
 be applied.  This may, for example, be the case when a user provides

Bryant & Pate Standards Track [Page 8] RFC 3985 PWE3 Architecture March 2005

 the service and attaches to the service provider via Ethernet, or
 when nested pseudo-wires are involved.  Fragmentation is 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, by using a suitable MAC bridge filter.
 This is a forwarder 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
 compression.
 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 a forwarder function, and the
 selection would therefore be made before the packet was presented to
 the PW Encapsulation Layer.

Bryant & Pate Standards Track [Page 9] RFC 3985 PWE3 Architecture March 2005

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 can be 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
 streams:
     o Some parts of the original bit stream may be stripped in the
       PSN-bound direction by an 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 reconstitution.
 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.

Bryant & Pate Standards Track [Page 10] RFC 3985 PWE3 Architecture March 2005

3.3.5. Principle of Minimum Intervention

 To minimize 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
 [RFC1958].
 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 that does intervene can be more wire efficient in some
 cases and may result in fewer translations at the NSP whereby 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 that the intermediate format carries and is
 undesirable.

4. Architecture of Pseudo Wires

 This section describes the PWE3 architectural model.

Bryant & Pate Standards Track [Page 11] RFC 3985 PWE3 Architecture March 2005

4.1. Network Reference Model

 Figure 2 illustrates the network reference model for point-to-point
 PWs.
          |<-------------- Emulated Service ---------------->|
          |                                                  |
          |          |<------- Pseudo Wire ------>|          |
          |          |                            |          |
          |          |    |<-- PSN Tunnel -->|    |          |
          |          V    V                  V    V          |
          V    AC    +----+                  +----+     AC   V
    +-----+    |     | PE1|==================| PE2|     |    +-----+
    |     |----------|............PW1.............|----------|     |
    | CE1 |    |     |    |                  |    |     |    | CE2 |
    |     |----------|............PW2.............|----------|     |
    +-----+  ^ |     |    |==================|    |     | ^  +-----+
          ^  |       +----+                  +----+     | |  ^
          |  |   Provider Edge 1         Provider Edge 2  |  |
          |  |                                            |  |
    Customer |                                            | Customer
    Edge 1   |                                            | Edge 2
             |                                            |
             |                                            |
       Native service                               Native service
                 Figure 2.  PWE3 Network Reference Model
 The two PEs (PE1 and PE2) have 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) arrive via the AC, are encapsulated
 in a PW-PDU, and are carried across the underlying network via the
 PSN tunnel.  The PEs perform the necessary encapsulation and
 decapsulation of PW-PDUs and handle any other functions required by
 the PW service, such as sequencing or timing.

4.2. PWE3 Pre-processing

 Some applications have 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 could be sent to the PE

Bryant & Pate Standards Track [Page 12] RFC 3985 PWE3 Architecture March 2005

 over one or more physical interfaces.  In most cases, could be in
 undertaking these operations within the PE provides cost and
 operational benefits.  Processed data is then presented to the PW via
 a virtual interface within the PE.  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.
                     PW
                  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
                  service
     Figure 3.  Pre-processing within the PWE3 Network Reference Model
 Figure 3 shows the interworking of one PE with pre-processing (PREP),
 and a second without this functionality.  This reference point
 emphasizes 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.

Bryant & Pate Standards Track [Page 13] RFC 3985 PWE3 Architecture March 2005

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

4.2.1. Forwarders

 Some applications have to forward payload elements selectively from
 one or more ACs to one or more PWs.  In such cases, there will also be
 a need to perform the inverse function on PWE3-PDUs received by a PE
 from the PSN.  This is the function of the forwarder.
 The forwarder 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 |                 |                      |
      AC     |                 |        Single        | PW Instance
     <------>o   Forwarder     +      PW Instance     X<===========>
             |                 |                      |
             +----------------------------------------+
                 Figure 4a.  Simple Point-to-Point Service
             +----------------------------------------+
             |                PE Device               |
             +----------------------------------------+
     Multiple|                 |        Single        | PW Instance
     AC      |                 +      PW Instance     X<===========>
     <------>o                 |                      |
             |                 |----------------------|
     <------>o                 |        Single        | PW Instance
             |    Forwarder    +      PW Instance     X<===========>
     <------>o                 |                      |
             |                 |----------------------|
     <------>o                 |        Single        | PW Instance
             |                 +      PW Instance     X<===========>
     <------>o                 |                      |
             +----------------------------------------+
             Figure 4b.  Multiple AC to Multiple PW Forwarding
 Figure 4a shows a simple forwarder that performs some type of
 filtering operation.  Because the forwarder has a single input and a
 single output interface, filtering is the only type of forwarding

Bryant & Pate Standards Track [Page 14] RFC 3985 PWE3 Architecture March 2005

 operation that applies.  Figure 4b shows a more general forwarding
 situation where payloads are extracted from one or more ACs and
 directed to one or more PWs.  In this case filtering, direction, and
 combination operations may be performed on the payloads.  For
 example, if the AC were Frame Relay, the forwarder might perform
 Frame Relay switching and the PW instances might be the inter-switch
 links.

4.2.2. Native Service Processing

 Some applications required some form of data or address translation,
 or some other operation requiring knowledge of the semantics of the
 payload.  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
 PWE3.
              +----------------------------------------+
              |                PE Device               |
      Multiple+----------------------------------------+
      AC      |      |          |        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 AC to Multiple PW Forwarding PE
 Figure 5 illustrates the relationship between NSP, forwarder, 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
 forwarder.  These transformation operations will, of course, be
 limited to those that have been implemented in the data path, and
 that are enabled by the PE configuration.  A PE device may contain
 more than one forwarder.

Bryant & Pate Standards Track [Page 15] RFC 3985 PWE3 Architecture March 2005

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

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 signaled is within the scope of
       PWE3, but the signaling protocol itself is not.

Bryant & Pate Standards Track [Page 16] RFC 3985 PWE3 Architecture March 2005

4.4. Protocol Stack Reference Model

 Figure 7 illustrates the protocol stack reference model for PWs.
  +-----------------+                           +-----------------+
  |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.

Bryant & Pate Standards Track [Page 17] RFC 3985 PWE3 Architecture March 2005

   /======================================\
   H             Forwarder                H<----Pre-processing
   H----------------======================/
   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 used to carry the PW over the PSN.
 The PW Encapsulation Layer consists of three sub-layers:
     o Payload Convergence
     o Timing
     o Sequencing
 The PW Encapsulation sub-layering and its context with the protocol
 stack are shown in Figure 9.

Bryant & Pate Standards Track [Page 18] RFC 3985 PWE3 Architecture March 2005

        +---------------------------+
        |         Payload           |
        /===========================\ <------ Encapsulation
        H    Payload Convergence    H         Layer
        H---------------------------H
        H          Timing           H
        H---------------------------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.  However grouping a number of target payload types into
 a generic class, and then providing a single convergence sub-layer
 type common to the group, reduces 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 and Sequencing Layers provide generic services to the
 Payload Convergence Layer for all payload types that require them.

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 an 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 part of the PW header.

Bryant & Pate Standards Track [Page 19] RFC 3985 PWE3 Architecture March 2005

 Not all the additional information needed to replay the native data
 units have 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 (channel type 1 plus one or more of
 channel types 2 through 4):
 1. A reliable control channel for signaling line events, status
    indications, and, in exceptional cases, CE-CE events that must be
    translated and sent reliably between PEs.  PWE3 may need this type
    of control channel to provide faithful emulation of complex data-
    link protocols.
 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 unsequenced channel for data traffic insensitive to packet
    order.
 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 PSN.
 Where end-to-end connectivity may be disrupted by address translation
 [RFC3022], access-control lists, firewalls, etc., the control channel
 may be able to pass traffic and setup the PW, while the PW data
 traffic is blocked by one or more of these mechanisms.  In these
 cases unless the control channel is also 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

Bryant & Pate Standards Track [Page 20] RFC 3985 PWE3 Architecture March 2005

 with TDM circuits (e.g., the on-hook/off-hook events in PSTN switches
 might be carried over a reliable control channel whereas 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 a particular PW instance needs them.  If the
 service is not needed, the associated header may be omitted in order
 to conserve processing and network resources.
 Sometimes a specific payload type will require transport 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 delivery in order 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 this constraint to reduce per-packet processing cost.
 The guiding principle is that, when possible, an existing IETF
 protocol should be used to provide these services.  When 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 omitted if it is not needed.

Bryant & Pate Standards Track [Page 21] RFC 3985 PWE3 Architecture March 2005

 The size of the sequence-number space depends on the speed of the
 emulated service, and on 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 from wrapping during
 the transient.

5.2.1.1. 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) must be delivered in order.
 For these 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.
 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 [RFC3550] 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 (as delay must be introduced to reorder),
       and
     o the expected incidence of misordering.

5.2.1.2. 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 these 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 that used to ensure
 in-order frame delivery.

Bryant & Pate Standards Track [Page 22] RFC 3985 PWE3 Architecture March 2005

5.2.1.3. Frame Loss Detection

 A destination PE can determine whether a frame has been lost by
 tracking the sequence numbers of the PW PDUs received.
 In some instances, if a PW PDU fails to arrive within a certain time,
 a destination PE will have to presume that it is lost.  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 they were designed to travel over.
 The emulated service may have to duplicate these network
 characteristics as closely as possible: e.g., in delivering native
 traffic with bitrate, jitter, wander, and delay characteristics
 similar to those received at the sending PE.
 In such cases, the receiving PE has to play out the native traffic as
 it was received at the sending PE.  This relies on timing information
 either sent between the two PEs, or in some cases received from an
 external reference.
 Therefore, Timing Sub-layer must support two timing functions:  clock
 recovery and timed payload delivery.  A particular payload type may
 require either or both of these services.

5.2.2.1. Clock Recovery

 Clock recovery is the extraction of output transmission bit timing
 information from the delivered packet stream, and it requires a
 suitable mechanism.  A physical wire carries the timing information
 natively, but extracting timing from a highly jittered source, such
 as packet stream, is a relatively complex task.  Therefore, it is
 desirable that an existing real-time protocol such as [RFC3550] be
 used for this purpose, unless it can be shown that this is unsuitable
 or unnecessary for a particular payload type.

5.2.2.2. 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 to an external clock.

Bryant & Pate Standards Track [Page 23] RFC 3985 PWE3 Architecture March 2005

5.3. Fragmentation

 Ideally, a payload would 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 will exceed the PSN path MTU.
 When a packet size exceeds the MTU of a given network, fragmentation
 and reassembly have to be performed for the packet to be delivered.
 Since fragmentation and reassembly generally consume considerable
 network resources, as compared to simply switching a packet in its
 entirety, the need for fragmentation and reassembly throughout a
 network should be reduced or eliminated 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 sent over the PW will
 have adaptive measures in place (e.g., [RFC1191], [RFC1981]) that
 ensure that packets needing 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
 possible.  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 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 AC, 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.

Bryant & Pate Standards Track [Page 24] RFC 3985 PWE3 Architecture March 2005

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-----------+->|     Flags, seq #, etc.  |
     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.  However, in
 certain circumstances it may be justifiable to transmit the payload
 in some processed form.  The reasons for this must be documented in
 the Encapsulation Layer definition for that payload type.
 Where appropriate, explicit timing is provided by RTP [RFC3550],
 which, when used, also provides a sequencing service.  When the PSN
 is UDP/IP, the RTP header follows the UDP header and precedes the PW
 control field.  For all other cases the RTP header follows the PW
 control header.
 The encapsulation layer may additionally carry a sequence number.
 Sequencing is to be provided either by RTP or by the PW encapsulation
 layer, but not by both.

Bryant & Pate Standards Track [Page 25] RFC 3985 PWE3 Architecture March 2005

 PW Demultiplexing is provided by the PW label, which may take the
 form specified in a number of IETF protocols;  e.g., an MPLS label
 [MPLSIP], an L2TP session ID [RFC3931], or a UDP port number
 [RFC768].  When PWs are carried over IP, the PSN Convergence Layer
 will not be needed.
 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.

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--------->|              RTP               |
 H---------------------H  |       +--------------------------------+
 H     Sequencing      H--+------>| Flags, Frag, Len, Seq #, etc   |
 \=====================/  |       +--------------------------------+
 |  PW Demultiplexer   |--------->|           PW Label             |
 +---------------------+  |       +--------------------------------+
 |  PSN Convergence    |--+  +--->| Outer Label or MPLS-in-IP encap|
 +---------------------+     |    +--------------------------------+
 |        PSN          |-----+
 +---------------------+
 |      Data-Link      |
 +---------------------+
 |       Physical      |
 +---------------------+
        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 a PSN fragmentation service within the PSN
 Convergence Layer (supported by a fragmentation control method).
 Ethernet pads all frames to a minimum size of 64 bytes.  The MPLS
 header does not include a length indicator.  Therefore, to allow PWE3

Bryant & Pate Standards Track [Page 26] RFC 3985 PWE3 Architecture March 2005

 to be carried in MPLS to pass correctly 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 [RFC3550].
 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 an IP PSN (such as GRE [RFC2784], [RFC2890]) is applied to the
 resultant PW-PDU.

5.4.3. PW-IP Packet Discrimination

 For MPLS PSNs, there is an additional constraint on the PW packet
 format.  Some label switched routers detect IP packets based on the
 initial four bits of the packet content.  To facilitate proper
 functioning, these bits in PW packets must not be the same as an IP
 version number in current use.

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.
 When a MPLS label is used as a PW Demultiplexer, setting of the TTL
 value [RFC3032] in the PW label is application specific.

Bryant & Pate Standards Track [Page 27] RFC 3985 PWE3 Architecture March 2005

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 ensure end-to-end integrity of frames.
 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 recalculated for insertion in CE-bound data.
 The former approach saves work, whereas 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) only have local significance and are
 changed on each hop or span.  If the circuit identifier (and thus
 checksum) were going to change as part of the PW emulation, it would
 be more efficient to strip and recalculate 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.
 If 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.

Bryant & Pate Standards Track [Page 28] RFC 3985 PWE3 Architecture March 2005

 If the PW is operating over a PSN that provides enhanced delivery,
 the PEs should monitor packet loss to ensure that the requested
 service 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 traffic is not known to
 be TCP friendly, the PEs should monitor packet loss to ensure that
 the 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, not less than that
 which 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 from immediately returning to service and causing a series of
 congestion pulses.
 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 [RFC3448].

7. Control Plane

 This section describes PWE3 control plane services.

7.1. Setup 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.
 Setup or teardown of a PW can be triggered by an operator command,
 from the management plane of a PE, by signaling set-up or teardown of
 an AC (e.g., an ATM SVC), or by an auto-discovery mechanism.
 During the setup process, the PEs have to exchange information (e.g.,
 learn each other's capabilities).  The tunnel signaling protocol may
 be extended to provide mechanisms that enable the PEs to exchange all
 necessary information on behalf of the PW.

Bryant & Pate Standards Track [Page 29] RFC 3985 PWE3 Architecture March 2005

 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 these services, the
 corresponding emulated services must specify how to perform status
 monitoring.

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 PW and PSN tunnels (if used), are functional in both directions.
 Because the two CEs of an emulated service are not adjacent, a
 failure may occur at a place so 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 these native services,
 the corresponding PWE3 service must provide a failure notification
 mechanism.
 Similarly, if a native service has notification mechanisms so that
 all the affected services will change status from "Down" to "Up" when
 a network failure is fixed, 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] [RFC3931] has this
 capability, and LDP has the ability to withdraw the corresponding
 MPLS label.

7.3.2. Misconnection and Payload Type Mismatch

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

Bryant & Pate Standards Track [Page 30] RFC 3985 PWE3 Architecture March 2005

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

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 affect the working condition
 of an emulated service.  For some payload types, packet loss,
 corruption, and out-of-order delivery can be mapped either to a bit
 error burst, or to 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
 notifications, if any are needed.

7.3.5. Collective Status Notification

 The 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 a group of emulated services 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
 services.
 A PWE3 approach may provide a 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,
 this 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 it across the
 PW.  Transparently transporting keep-alive messages over the PW would
 follow the principle of minimum intervention.  However, to reproduce

Bryant & Pate Standards Track [Page 31] RFC 3985 PWE3 Architecture March 2005

 the semantics of the native mechanism accurately, 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 out-of-band,
 (e.g., the signaling VC of an ATM VP, or TDM CCS signaling).
 Given 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 emulate the native service
 satisfactorily.
 If control messages are passed through, it may be desirable to send
 them by using either a higher priority or a reliable channel provided
 by the PW Demultiplexer layer.  See Section 5.1.2, PWE3 Channel
 Types.

8. Management and Monitoring

 This section describes the management and monitoring architecture for
 PWE3.

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 help
 measure service-level agreements (SLAs).  Typical counters include
 the following:
     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.

Bryant & Pate Standards Track [Page 32] RFC 3985 PWE3 Architecture March 2005

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 the pertinent MIBs fit together to
 form a cohesive management framework for deploying PWE3 services.
 Note that the names of MIB modules used below are suggestions and do
 not necessarily require that the actual modules used to realize the
 components in the architecture be named exactly so.

8.2.1. MIB Layering

 The SNMP MIBs created for PWE3 should fit the architecture shown in
 Figure 12.  The architecture provides a layered modular model into
 which any supported emulated service can be connected to any
 supported PSN type.  This model fosters reuse of as much
 functionality as possible.  For instance, the emulated service layer
 MIB modules do not redefine the existing emulated service MIB module;
 rather, they only associate it with the pseudo wires used to carry
 the emulated service over the configured PSN.  In this way, the PWE3
 MIB architecture follows the overall PWE3 architecture.
 The architecture does allow for the joining of unsupported emulated
 service or PSN types by simply defining additional MIB modules to
 associate new types with existing ones.  These new modules can
 subsequently be standardized.  Note that there is a separate MIB
 module for each emulated service, as well as one for each underlying
 PSN.  These MIB modules may be used in various combinations as
 needed.

Bryant & Pate Standards Track [Page 33] RFC 3985 PWE3 Architecture March 2005

     Native
  Service MIBs    ...           ...               ...
                   |             |                 |
             +-----------+ +-----------+     +-----------+
   Service   |    CEP    | | Ethernet  |     |    ATM    |
    Layer    |Service MIB| |Service MIB| ... |Service MIB|
             +-----------+ +-----------+     +-----------+
                     \           |             /
                       \         |           /
 - - - - - - - - - - - - \ - - - | - - - - / - - - - - - -
                           \     |       /
             +-------------------------------------------+
  Generic PW |            Generic PW MIBs                |
    Layer    +-------------------------------------------+
                          /             \
 - - - - - - - - - - - - / - - - - - - - - \ - - - - - - -
                       /                     \
                     /                         \
             +--------------+             +----------------+
   PSN VC    |L2TP VC MIB(s)|             | MPLS VC MIB(s) |
    Layer    +--------------+             +----------------+
                    |                              |
   Native     +-----------+                  +-----------+
    PSN       |L2TP MIB(s)|                  |MPLS MIB(s)|
    MIBs      +-----------+                  +-----------+
             Figure 12.  MIB Module Layering Relationship

Bryant & Pate Standards Track [Page 34] RFC 3985 PWE3 Architecture March 2005

 Figure 13 shows an example for a SONET PW carried over MPLS Traffic
 Engineering Tunnel and an LDP-signaled LSP.
                          +-----------------+
                          |    SONET MIB    |  RFC3592
                          +-----------------+
                                   |
                     +------------------------------+
          Service    | Circuit Emulation Service MIB|
           Layer     +------------------------------+
         - - - - - - - - - - - - - | - - - - - - - - - - - - -
                          +-----------------+
         Generic PW       | Generic PW MIB  |
           Layer          +-----------------+
         - - - - - - - - - - - - - | - - - - - - - - - - - - -
                          +-----------------+
           PSN VC         |   MPLS VC MIBs  |
           Layer          +-----------------+
                             |           |
                +-----------------+  +------------------+
                | MPLS-TE-STD-MIB |  | MPLS-LSR-STD-MIB |
                +-----------------+  +------------------+
          Figure 13.  SONET PW over MPLS PSN Service-Specific Example

8.2.2. Service Layer MIB Modules

 This conceptual layer in the model contains MIB modules used to
 represent the relationship between emulated PWE3 services such as
 Ethernet, ATM, or Frame Relay and the pseudo-wire used to carry that
 service across the PSN.  This layer contains corresponding MIB
 modules used to mate or adapt those emulated services to the generic
 pseudo-wire representation these are represented in the "Generic PW
 MIB" functional block in Figure 13 above.  This working group should
 not produce any MIB modules for managing the general service; rather,
 it should produce just those modules used to interface or adapt the
 emulated service onto the PWE3 management framework as shown above.
 For example, the standard SONET-MIB [RFC3592] is designed and
 maintained by another working group.  The SONET-MIB is designed to
 manage the native service without PW emulation.  However, the PWE3
 working group is chartered to produce standards that show how to
 emulate existing technologies such as SONET/SDH over pseudo-wires
 rather than reinvent those modules.

Bryant & Pate Standards Track [Page 35] RFC 3985 PWE3 Architecture March 2005

8.2.3. Generic PW MIB Modules

 The middle layer in the architecture is referred to as the Generic PW
 Layer.  MIBs in this layer are responsible for providing pseudo-wire
 specific counters and service models used for monitoring and
 configuration of PWE3 services over any supported PSN service.  That
 is, this layer provides a general model of PWE3 abstraction for
 management purposes.  This MIB is used to interconnect the MIB
 modules residing in the Service Layer to the PSN VC Layer MIBs (see
 section 8.2.4).

8.2.4. PSN VC Layer MIB Modules

 The third layer in the PWE3 management architecture is referred to as
 the PSN VC Layer.  It is composed of MIBs that are specifically
 designed to associate pseudo-wires onto those underlying PSN
 transport technologies that carry the pseudo-wire payloads across the
 PSN.  In general, this means that the MIB module provides a mapping
 between the emulated service that is mapped to the pseudo-wire via
 the Service Layer and the Generic PW MIB Layer onto the native PSN
 service.  For example, in the case of MPLS, for example, it is
 required that the general VC service be mapped into MPLS LSPs via the
 MPLS-LSR-STD-MIB [RFC3813] or Traffic-Engineered (TE) Tunnels via the
 MPLS-TE-STD-MIB [RFC3812].  In addition, the MPLS-LDP-STD-MIB
 [RFC3815] may be used to reveal the MPLS labels that are distributed
 over the MPLS PSN in order to maintain the PW service.  As with the
 native service MIB modules described earlier, the MIB modules used to
 manage the native PSN services 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 that the emulated PWE3 service will function correctly.

8.3. Connection Verification and Traceroute

 A connection verification mechanism should be supported by PWs.
 Connection verification and 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.

Bryant & Pate Standards Track [Page 36] RFC 3985 PWE3 Architecture March 2005

9. IANA Considerations

 IANA considerations will be identified in the PWE3 documents that
 define the PWE3 encapsulation, control, and management protocols.

10. Security Considerations

 PWE3 provides no means of protecting the integrity, confidentiality,
 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 with 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 [RFC3768] 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 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 provide authentication and data
 integrity of the data between the PW Demultiplexer End-points.
 IPSec may provide authentication, integrity, 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,

Bryant & Pate Standards Track [Page 37] RFC 3985 PWE3 Architecture March 2005

 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.
 The unlimited transformation capability of the NSP may be perceived
 as a security risk.  In practice the type of operation that the NSP
 may perform will be limited to those that have been implemented in
 the data path.  A PE designed and managed to best current practice
 will have controls in place that protect and validate its
 configuration, and these will be sufficient to ensure that the NSP
 behaves as expected.

11. Acknowledgements

 We thank Sasha Vainshtein for his work on Native Service Processing
 and advice on bit stream over PW services and 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, Danny McPherson, Eric Rosen, John
 Rutemiller, Scott Wainner, and David Zelig for their comments and
 contributions.

12. References

12.1. Normative References

 [RFC3931]   Lau, J., Townsley, M., and I. Goyret, "Layer Two
             Tunneling Protocol - Version 3 (L2TPv3), RFC 3931, March
             2005.
 [RFC768]    Postel, J., "User Datagram Protocol", STD 6, RFC 768,
             August 1980.
 [RFC2401]   Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.
 [RFC2474]   Nichols, K., Blake, S., Baker, F., and D. Black,
             "Definition of the Differentiated Services Field (DS
             Field) in the IPv4 and IPv6 Headers", RFC 2474, December
             1998.
 [RFC3592]   Tesink, K., "Definitions of Managed Objects for the
             Synchronous Optical Network/Synchronous Digital Hierarchy
             (SONET/SDH) Interface Type", RFC 3592, September 2003.

Bryant & Pate Standards Track [Page 38] RFC 3985 PWE3 Architecture March 2005

 [RFC2661]   Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
             G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
             RFC 2661, August 1999.
 [RFC2784]   Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
             Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
             March 2000.
 [RFC2890]   Dommety, G., "Key and Sequence Number Extensions to GRE",
             RFC 2890, September 2000.
 [RFC3032]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
             Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
             Encoding", RFC 3032, January 2001.
 [RFC3550]   Schulzrinne, H.,  Casner, S., Frederick, R., and V.
             Jacobson, "RTP: A Transport Protocol for Real-Time
             Applications", STD 64, RFC 3550, July 2003.

12.2. Informative References

 [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).
 [RFC3815]   Cucchiara, J., Sjostrand, H., and J. Luciani,
             "Definitions of Managed Objects for the Multiprotocol
             Label Switching (MPLS), Label Distribution Protocol
             (LDP)", RFC 3815, June 2004.
 [RFC3813]   Srinivasan, C., Viswanathan, A., and T. Nadeau,
             "Multiprotocol Label Switching (MPLS) Label Switching
             Router (LSR) Management Information Base (MIB)", RFC
             3813, June 2004.
 [MPLSIP]    Rosen et al, "Encapsulating MPLS in IP or Generic Routing
             Encapsulation (GRE)", Work in Progress, March 2004.
 [RFC1191]   Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
             November 1990.
 [RFC1958]   Carpenter, B., "Architectural Principles of the
             Internet", RFC 1958, June 1996.
 [RFC1981]   McCann, J., Deering, S., and J. Mogul, "Path MTU
             Discovery for IP version 6", RFC 1981, August 1996.

Bryant & Pate Standards Track [Page 39] RFC 3985 PWE3 Architecture March 2005

 [RFC2022]   Armitage, G., "Support for Multicast over UNI 3.0/3.1
             based ATM Networks", RFC 2022, November 1996.
 [RFC3768]   Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
             RFC 3768, April 2004.
 [RFC3022]   Srisuresh, P. and K. Egevang, "Traditional IP Network
             Address Translator (Traditional NAT)", RFC 3022, January
             2001.
 [RFC3448]   Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
             Friendly Rate Control (TFRC): Protocol Specification",
             RFC 3448, January 2003.
 [RFC3812]   Srinivasan, C., Viswanathan, A., and T. Nadeau,
             "Multiprotocol Label Switching (MPLS) Traffic Engineering
             (TE) Management Information Base (MIB)", RFC 3812, June
             2004.
 [RFC3916]   Xiao, X., McPherson, D., and P. Pate, Eds, "Requirements
             for Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
             September 2004.

13. Co-Authors

 The following are co-authors of this document:
 Thomas K. Johnson
 Litchfield Communications
 Kireeti Kompella
 Juniper Networks, Inc.
 Andrew G. Malis
 Tellabs
 Thomas D. Nadeau
 Cisco Systems
 Tricci So
 Caspian Networks
 W. Mark Townsley
 Cisco Systems

Bryant & Pate Standards Track [Page 40] RFC 3985 PWE3 Architecture March 2005

 Craig White
 Level 3 Communications, LLC.
 Lloyd Wood
 Cisco Systems

14. Editors' Addresses

 Stewart Bryant
 Cisco Systems
 250, Longwater
 Green Park
 Reading, RG2 6GB,
 United Kingdom
 EMail: stbryant@cisco.com
 Prayson Pate
 Overture Networks, Inc.
 507 Airport Boulevard
 Morrisville, NC, USA 27560
 EMail: prayson.pate@overturenetworks.com

Bryant & Pate Standards Track [Page 41] RFC 3985 PWE3 Architecture March 2005

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Bryant & Pate Standards Track [Page 42]

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