Internet Draft                                     Prayson Pate, Editor
Document: draft-pate-pwe3-framework-03.txt      Overture Networks, Inc.
                                                            XiPeng Xiao
                                                          Photuris Inc.
Craig White                                                   Tricci So
Level 3 Communications, LLC.                           Caspian Networks
Kireeti Kompella                                        Andrew G. Malis
Juniper Networks, Inc.                                  Vivace Networks
Thomas K. Johnson                                      Thomas D. Nadeau
Litchfield Communications                                Stewart Bryant
                                                          Cisco Systems

        Framework for Pseudo Wire Emulation Edge-to-Edge (PWE3)

                          Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
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   This document describes a framework for Pseudo Wire Emulation
   Edge-to-Edge (PWE3).  It discusses the emulation of circuits (such as
   T1, E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame
   Relay) over packet switched networks (PSNs) using IP or MPLS.  It
   presents an architectural framework for pseudo wires (PWs), defines
   terminology, specifies the various protocol elements and their
   functions, overviews some of the services that will be supported and
   discusses how PWs fit into the broader context of protocols.

Copyright Notice

   Copyright (C) The Internet Society (2002). All Rights Reserved.

Internet Draft        draft-pate-pwe3-framework-03         January, 2002

                             Table of Contents
   1 Introduction .................................................    3
   1.1 What Are Pseudo Wires?  ....................................    3
   1.2 Goals of This Document .....................................    4
   1.3 Non-Goals ..................................................    4
   1.4 Terminology ................................................    4
   2 Background and Motivation ....................................    7
   2.1 Current Network Architecture ...............................    7
   2.2 PWE3 as a Path to Convergence ..............................    9
   2.3 Suitable Applications for PWE3 .............................   10
   2.4 Summary ....................................................   10
   3 Architecture of Pseudo Wires .................................   10
   3.1 Network Reference Model ....................................   11
   3.2 Maintenance Reference Model ................................   11
   3.3 Maintenance Architecture ...................................   12
   3.4 Protocol Stack Reference Model .............................   12
   3.5 Logical Protocol Layering Model ............................   13
   3.6 Architecture Assumptions ...................................   16
   4 Design Considerations ........................................   16
   4.1 PW-PDU Validation ..........................................   16
   4.2 PW-PDU Sequencing ..........................................   17
   4.3 Session Multiplexing .......................................   17
   4.4 Security ...................................................   17
   4.5 Encapsulation Control ......................................   17
   4.6 Statistics .................................................   18
   4.7 Traceroute .................................................   18
   4.8 Congestion Considerations ..................................   19
   4.9 PW SNMP MIB Architecture ...................................   19
   5 Acknowledgments ..............................................   21
   6 References ...................................................   21
   7 Security Considerations ......................................   23
   8 IANA Considerations ..........................................   23
   9 Authors' Addresses ...........................................   23
   10 Full Copyright Section ......................................   24

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1.  Introduction

   This document describes a framework for Pseudo Wire Emulation
   Edge-to-Edge (PWE3).  It discusses the emulation of circuits (such as
   T1, E1, T3, E3 and SONET/SDH) and services (such as ATM and Frame
   Relay) over packet switched networks (PSNs) using IP or MPLS.  It
   presents an architectural framework for pseudo wires (PWs), defines
   terminology, specifies the various protocol elements and their
   functions, overviews the services supported and discusses how PWs fit
   into the broader context of protocols.  See [XIAO] for the
   requirements for PWs.

1.1.  What Are Pseudo Wires?

1.1.1.  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.  The
   required functions of PWs include encapsulating service-specific
   bit-streams or PDUs arriving at an ingress port, and carrying them
   across a path or tunnel, managing their timing and order, and any
   other operations required to emulate the behavior and characteristics
   of the service as faithfully as possible.

   From the customer perspective, the PW is perceived as an unshared
   link or circuit of the chosen service.  However, there may be
   deficiencies that impede some applications from being carried on a
   PW.  These limitations should be fully described in the appropriate
   service-specific Encapsulation, Emulation and Maintenance Documents
   (EEMDs) and Applicability Statements (ASes).

1.1.2.  Functions

   PWs provide the following functions in order to emulate the behavior
   and characteristics of the desired service.

   - Encapsulation of service-specific PDUs or circuit data arriving at
     an ingress port (logical or physical).

   - Carrying the encapsulated data across a tunnel.

   - Managing the signaling, timing, order or other aspects of the
     service at the boundaries of the PW.

   - Service-specific status and alarm management.

   EEMDs and/or ASes for each service will describe any shortfalls of
   the emulation's faithfulness.

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1.2.  Goals of This Document

   - Description of the motivation for creating PWs, and some background
     on how they may be deployed.

   - Description of an architecture and terminology for PWs.

   - Description of the statistics and other network management
     information needed for tunnel operation and management.

   - Whenever possible, relevant requirements from existing IETF
     documents and other sources will be incorporated by reference.

1.3.  Non-Goals

   The following are non-goals for this document:

   - The detailed specification of the bits and bytes of the
     encapsulations of the various services.  This description is
     contained in an EEMD and/or ES.

   - The detailed definition of the protocols involved in PW setup and

   The following are outside the scope of PWE3:

   - Discussion of the protection of the encapsulated content of the PW.

   - 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 that are implemented on top of
     the medium are out of scope.

   - Methods to signal or control the underlying PSN.

1.4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT",
   document are to be interpreted as described in RFC 2119.  Below are
   the definitions for the terms used throughout the document.

   Packet Switched Network
                    A Packet Switched Network (PSN) is a network using
                    IP or MPLS as the unit of switching.

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

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

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

   Pseudo Wire      A Pseudo Wire (PW) is a connection between two PEs
                    carried over a PSN.  The PE provides the adaptation
                    between the CE and the PW.

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

   Pseudo Wire PDU  A Pseudo Wire PDU is a PDU sent on the PW that
                    contains all of the data and control information
                    necessary to provide the desired service.

   PSN Tunnel       A PSN Tunnel is a tunnel inside which multiple PWs
                    can be nested so that they are transparent to core
                    network devices.

   PW Domain        A PW Domain (PWD) is a collection of instances of
                    PWs that are within the scope of a single
                    homogeneous administrative domain (e.g. PW over MPLS
                    network or PW over IP network etc.).

   Path-oriented PW A Path-oriented PW is a PW for which the network
                    devices of the underlying PSN must maintain path
                    state information.

   Non-path-oriented PW
                    A Non-path-oriented PW is a PW for which the network
                    devices of the underlying PSN need not maintain path
                    state information.

   Interworking     Interworking is used to express interactions between
                    networks, between end systems, or between parts
                    thereof, with the aim of providing a functional
                    entity capable of supporting an end-to-end
                    communication.  The interactions required to provide
                    a functional entity rely on functions and on the
                    means to select these functions.

   Interworking Function
                    An Interworking Function (IWF) is a functional
                    entity that facilitates interworking between two
                    dissimilar networks (e.g., ATM & MPLS, ATM & L2TP,
                    etc.).  A PE performs the IWF function.

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   Service Interworking
                    In Service Interworking, the IWF (Interworking
                    Function) between two dissimilar protocols (e.g.,
                    ATM & MPLS, Frame Relay & ATM, ATM & IP, ATM & L2TP,
                    etc.)  terminates the protocol used in one network
                    and translates (i.e. maps) its Protocol Control
                    Information (PCI) to the PCI of the protocol used in
                    other network for User, Control and Management Plane
                    functions to the extent possible.  In general, since
                    not all functions may be supported in one or other
                    of the networks, the translation of PCI may be
                    partial or non-existent.  However, this should not
                    result in any loss of user data since the payload is
                    not affected by PCI conversion at the service
                    interworking IWF.

   Network Interworking
                    In Network Interworking, the PCI (Protocol Control
                    Information) of the protocol and the payload
                    information used in two similar networks are
                    transferred transparently by an IWF of the PE across
                    the PSN.  Typically the IWF of the PE encapsulates
                    the information which is transmitted by means of an
                    adaptation function and transfers it transparently
                    to the other network.

   Applicability Statement
                    Each PW service will have an Applicability Statement
                    (AS) that describes the applicability of PWs for
                    that service.

   Encapsulation, Emulation and Maintenance Documents
                    Each PW service will have an Encapsulation,
                    Emulation and Maintenance Document (EEMDs) that
                    described the particulars of PWs for that service,
                    as well as the degree of faithfulness to that

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

   Outbound         The traffic direction where PW-PDUs are received on
                    a PW from the PSN, re-converted back in the emulated
                    service, and sent out to a CE.

   CE Signaling     CE (end-to-end) Signaling refers to messages sent
                    and received by the CEs.  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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   PE/PW Maintenance
                    PE/PW Maintenance is used by the PEs to set up,
                    maintain and tear down the PW.  It may be coupled
                    with CE signaling in order to effectively manage the

   PSN Tunnel Signaling
                    PSN Tunnel Signaling is used to set up, maintain and
                    remove the underlying PSN tunnel.  An example would
                    be LDP in MPLS for maintaining LSPs.

2.  Background and Motivation

   Why is anyone interested in PWs?  This section gives some background
   on where networks are today and why they are changing.  It also talks
   about the motivation to provide converged networks while continuing
   to support existing services.  Finally it discusses how PWs can be a
   solution for this dilemma.

2.1.  Current Network Architecture

2.1.1.  Multiple Networks

   For any given service provider delivering multiple services, the
   current "network" usually consists of parallel or "overlay" networks.
   Each of these networks implements a specific service, such as voice,
   Frame Relay, Internet access, etc.  This is quite expensive, both in
   terms of capital expense as well as in operational costs.
   Furthermore, the presence of multiple networks complicates planning.
   Service providers wind up asking themselves these questions:

   - Which of my networks do I build out?

   - How many fibers do I need for each network?

   - How do I efficiently manage multiple networks?

   A converged network helps service providers answer these questions in
   a consistent and economical fashion.

2.1.2.  Convergence Today

   There are some examples of convergence in today's network:

   - Frame Relay is frequently carried over ATM networks using [FRF.5]

   - T1, E1 and T3 circuits are sometimes carried over ATM networks
     using [ATMCES].

   - Voice is carried over ATM (using AAL2), Frame Relay (using FRF.11
     VoFR), IP (using VoIP) and MPLS (using VoMPLS) networks.

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   Deployment of these examples range from limited (ATM CES) to fairly
   common (FRF.5 interworking) to rapidly growing (VoIP).

2.1.3.  The Emerging Converged Network

   Many service providers are finding that the new IP-based and
   MPLS-based switching systems are much less costly to acquire, deploy
   and maintain than the systems that they replace.  The new systems
   take advantage of advances in technology in these ways:

   - The newer systems leverage mass production of ASICs and optical
     interfaces to reduce capital expense.

   - The bulk of the traffic in the network today originates from packet
     sources.  Packet switches can economically switch and deliver this
     traffic natively.

   - Variable-length switches have lower system costs than ATM due to
     simpler switching mechanisms as well as elimination of segmentation
     and reassembly (SAR) at the edges of the network.

   - Deployment of services is simpler due to the connectionless nature
     of IP services or the rapid provisioning of MPLS applications.

2.1.4.  Transition to a Packet-Optimized Converged Network

   The greatest assets for many service providers are the physical
   communications links that they own.  The time and costs associated
   with acquiring the necessary rights of way, getting the required
   governmental approvals, and physically installing the cabling over a
   variety of terrains and obstacles represents a significant asset that
   is difficult to replace.  Their greatest on-going costs are the
   operational expenses associated with maintaining and operating their
   networks.  In order to maximize the return on their assets and
   minimize their operating costs, service providers often look to
   consolidate the delivery of multiple service types onto a single
   networking technology.

   The first generation converged network was based on TDM
   (time-division multiplexing) technology.  Voice, video, and data
   traffic have been carried successfully across TDM/DACS-based networks
   for decades.  TDM technology has some significant drawbacks as a
   converged networking technology.  Operational costs for TDM networks
   remain relatively high because the provisioning of end-to-end TDM
   circuits is typically a tedious and labor-intensive task.  In
   addition, TDM switching does not make the best use of the
   communications links.  This is because fixed assignment of timeslots
   does not allow for the statistical multiplexing of bursty data
   traffic (i.e.  temporarily unused bandwidth on one timeslot cannot be
   dynamically re-allocated to another service).

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   The second generation of converged network was based on ATM
   technology.  Today many service providers convert voice, video, and
   data traffic into fixed-length cells for carriage across ATM-based
   networks.  ATM improves upon TDM technology by providing the ability
   to statistically multiplex different types of traffic onto
   communications links.  In addition, ATM SPVC technology is often used
   to automatically provision end-to-end services, providing an
   additional advantage over traditional TDM networks.  However, ATM has
   several significant drawbacks.  One of the most frequently cited
   problems with ATM is the so-called cell-tax, which refers to the 5
   bytes out of 53 used as an ATM cell header.  Another significant
   problem with ATM is the AAL5 SAR, which becomes extremely difficult
   to implement above 1 Gbps.  There are also issues with the long-term
   scalability of ATM, especially as a switching layer beneath IP.

   As packet traffic takes up a larger and larger portion of the
   available network bandwidth, it becomes increasingly useful to
   optimize public networks for the Internet Protocol.  However, many
   service providers are confronting several obstacles in engineering
   packet-optimized networks.  Although Internet traffic is the fastest
   growing traffic segment, it does not generate the highest revenue per
   bit.  For example, Frame Relay traffic currently generates a higher
   revenue per bit than do native IP services.  Private line TDM
   services still generate even more revenue per bit than does Frame
   Relay.  In addition, there is a tremendous amount of legacy equipment
   deployed within public networks that does not communicate using the
   Internet Protocol.  Service providers continue to utilize non-IP
   equipment to deploy a variety of services, and see a need to
   interconnect this legacy equipment over their IP-optimized core

2.2.  PWE3 as a Path to Convergence

   How do service providers realize the capital and operational benefits
   of a new packet-based infrastructure, while leveraging the existing
   base of SONET (Synchronous Optical Network) gear, and while also
   protecting the large revenue stream associated with this equipment?
   How do they move from mature Frame Relay or ATM networks, while still
   being able to provide these lucrative services?

   One possibility is the emulation of circuits or services via PWs.
   Circuit emulation over ATM and interworking of Frame Relay and ATM
   have already been standardized.  Emulation allows existing circuits
   and/or services to be carried across the new infrastructure, and thus
   enables the interworking of disparate networks.  [ATMCES] provides
   some insight into the requirements for such a service:

        There is a user demand for carrying certain types of
        constant bit rate (CBR) or "circuit" traffic over
        Asynchronous Transfer Mode (ATM) networks.  As ATM is
        essentially a packet- rather than circuit-oriented
        transmission technology, it must emulate circuit

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        characteristics in order to provide good support for CBR

        A critical attribute of a Circuit Emulation Service (CES)
        is that the performance realized over ATM should be
        comparable to that experienced with the current PDH/SDH

   Implemented correctly, PWE3 can provide a means for supporting
   today's services over a new network.

2.3.  Suitable Applications for PWE3

   What makes an application suitable (or not) for PWE3 emulation?  When
   considering PWs as a means of providing an application, the following
   questions must be considered:

   - Is the application sufficiently deployed to warrant emulation?

   - Is there interest on the part of service providers in providing an
     emulation for the given application?

   - Is there interest on the part of equipment manufacturers in
     providing products for the emulation of a given application?

   - Are the complexities and limitations of providing an emulation
     worth the savings in capital and operational expenses?

   If the answer to all four questions is "yes", then the application is
   likely to be a good candidate for PWE3.  Otherwise, there may not be
   sufficient overlap between the customers, service providers,
   equipment manufacturers and technology to warrant providing such an

2.4.  Summary

   To maximize the return on their assets and minimize their operational
   costs, many service providers are looking to consolidate the delivery
   of multiple service offerings and traffic types onto a single
   IP-optimized network.

   In order to create this next-generation converged network, standard
   methods must be developed to emulate existing telecommunications
   formats such as Ethernet, Frame Relay, ATM, and TDM over IP-optimized
   core networks.  This document describes a framework accomplishing
   this goal.

3.  Architecture of Pseudo Wires

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3.1.  Network Reference Model

   Figure 1 below shows the network reference model for PWs.  As shown,
   the PW provides an emulated service between the customer edges (CEs).

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

                  Figure 1: PWE3 Network Reference Model

   Any bits or packets presented at the PW End Service (PWES) are
   encapsulated in a PW-PDU and carried across the underlying network.
   The PEs perform the encapsulation, decapsulation, order management,
   timing and any other functions required by the service.  In some
   cases the PWES can be treated as a virtual interfaces into a further
   processing (like switching  or bridging) of the original service
   before the physical connection to the CE.  Examples include Ethernet
   bridging, SONET cross-connect, translation of locally-significant
   identifiers such as VCI/VPI, etc.  to other service type, etc.  The
   underlying PSN is not involved in any of these service-specific

3.2.  Maintenance Reference Model

   Figure 2 below shows 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 2: PWE3 Maintenance Reference Model

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   - The CE (end-to-end) signaling is between the CEs.  This signaling
     includes Frame Relay PVC status signaling, ATM SVC signaling, etc.

   - The PW/PE Maintenance is used between the PEs to set up, maintain
     and tear down PWs, including any required coordination of
     parameters between the two ends.

   - The PSN Tunnel signaling controls the underlying PSN.  An example
     would be LDP in MPLS for maintaining LSPs.  This type of signaling
     is not within the scope of PWE3.

3.3.  Maintenance Architecture

   <Editor's Note: Luca Martini has asked that a section be added
   describing the architecture of the maintenance protocol(s).  This
   section will contain that architecture.  Some possible topics are
   listed below.>

3.3.1.  PW Setup

   How are PWs set up?

3.3.2.  PW Integrity

   How do we ensure the sanity or connectivity of the PW over the PSN?

3.3.3.  Service Maintenance

   How does CE maintenance behavior e.g. FR LMI, ATM AIS/RDI, etc.
   affect the PW?

3.4.  Protocol Stack Reference Model

   Figure 3 below shows the protocol stack reference model for PWs.

   +----------------+                             +----------------+
   |Emulated Service|       Emulated Service      |Emulated Service|
   |(TDM, ATM, etc).|<===========================>|(TDM, ATM, etc.)|
   +----------------+         Pseudo Wire         +----------------+
   |  Encapsulation |<===========================>|  Encapsulation |
   +----------------+          PSN Tunnel         +----------------+
   |    IP/MPLS     |<===========================>|    IP/MPLS     |
   +----------------+                             +----------------+
   |    Physical    |                             |    Physical    |
   +-------+--------+        _____________        +--------+-------+
           |                /             \                |
           +===============/      PSN      \===============+
                           \               /

               Figure 3: PWE3 Protocol Stack Reference Model

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   The PW provides the CE with what appears to be a connection to its
   peer at the far end.  Bits or PDUs from the CE are passed through an
   encapsulation layer.

3.5.  Logical Protocol Layering Model

3.5.1.  Protocol Layers

   The logical protocol-layering model needed to support a PW is shown
   in Figure 4 below.

                     |         Payload           |
                     |    Encapsulation Layer    |
                     |     Multiplexing Layer    |
                     |        PSN Header         |
                     |       MAC/Datalink        |
                     |          Physical         |

                 Figure 4: Logical Protocol Layering Model

   The logical protocol-layering model shown in Figure 4 above minimizes
   the differences between the PWs operating over different PSN types.

   The payload is transported over the Encapsulation Layer that carries
   any information that is not available in the payload itself and which
   is needed by the PW outbound interface to send the reconstructed
   service to the CE.  If needed, this layer also provides support for
   real-time processing, sequencing and indication of length.

   The Multiplexing Layer provides the ability to deliver multiple PWs
   over a single PSN tunnel.

   The PSN header, MAC/datalink and physical layer definitions are
   outside the scope of this framework.

3.5.2.  Instantiation of the Protocol Layers

   The instantiation of the logical protocol-layering model of Figure 4
   is shown in Figure 5 below.

   Where possible, the components shown below use existing IETF
   standards and common work in progress.  Otherwise, the goal was to
   call for the design of components that have the wider application
   within the IETF.

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       +=========================================+ - - - - - - - - - -
       |      Payload (circuit/cell/packet)      | Payload
       +=========================================+ - - - - - - - - - -
       |   Encapsulation-specific information    |
       |           Sequencing (optional)         | Encapsulation
       |       Length (payload/PSN-specific)     | Layer
       |  Timing Information (payload-specific)  |
       +============+============================+ - - - - - - - - - -
       |  L2TPv3    |       MPLS shim            | Multiplexing Layer
       +------------+-------+--------------------+ - - - - - - - - - -
       |    IPv4/v6         |       MPLS         | PSN Header
       +====================+====================+ - - - - - - - - - -
       |              MAC/Data-link              |
       |                Physical                 |

                 Figure 5: Instantiated Protocol Layering  Payload

   The payload is generically classified into three types: circuit, cell
   and packet. Within these generic types there will be specific service
   types. For example, the generic packet type includes Frame Relay and
   Ethernet.  The design of the encapsulation layer, and the choice
   between transporting the payload in a native or intermediate format,
   will be defined in the service-specific EEMD and/or AS.  Circuit Payload

   A circuit payload is a sampled set of bits relayed across the PW.
   This service will need sequencing and real-time support.  Cell Payload

   A cell payload is one, or more, ATM cells relayed across the PW. This
   service will need sequence number support, and may also need
   real-time support. The payload may consist of a single cell or a
   cluster of cells. The cells to be incorporated in the payload may be
   selected by filtering on VCI/VPI. In the case of a trunked interface
   the payload may be considered complete on the expiry of a timer or
   when a fixed number of cells have been received or both.  Packet Payload

   A packet payload would normally be relayed across the PW as a single
   unit. A packet payload may need sequencing or sequencing and
   real-time support. A packet payload may additionally require

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   Support for sequencing depends on the payload type, and may be
   omitted if not needed.  For example, Frame Relay always requires the
   preservation of packet order.  However, where the Frame Relay service
   is only being used to carry IP, it may be desirable to relax that
   constraint in return for reduced per-packet processing cost.

   [RTP] or encapsulation-specific sequence numbers may be used for
   sequencing.  Timing

   A suitable real-time protocol is RTP [RTP]. This is an extensible
   protocol designed to be extended to carry new payload types over a
   transport layer running over an IP network. It includes a control
   protocol for managing the timing service. RTP is not currently
   defined for operation over L2TP. A short document describing the
   correct method of multiplexing the RTP control and data over LT2Pv3
   is required.  Multiplexing Layer

   Suitable protocols for use as a multiplexing layer are L2TPv3 [LAU]
   and MPLS [MPLS].

    - The updated definition of L2TP in [LAU] is specified to operate
      directly over a PSN, and in the limiting case the L2TP data
      encapsulation reduces to a four-octet opaque data field. L2TPv3 is
      specified for operation in both manually configured and negotiated
      mode. The associated control protocol is extensible and may be
      used to signal PW-specific configuration or run-time information.

    - MPLS may also be used in the multiplexing layer.  In this case, an
      MPLS tag is used as a "shim" to identify the particular PW of
      interest.  PSN Layer

   The three PSN types within the scope of the IETF are IPv4, IPv6 and
   MPLS. IPv4 and IPv6 both provide the necessary switching, length and
   fragmentation services needed to support all IETF specified transport
   protocols. L2TPv3 is specified to run directly over IPv4 and IPv6.
   When the PSN is IPv4 or IPv6 no PSN convergence layer is needed.

   MPLS provides switching service, but no length or fragmentation
   service. When MPLS is used as the PSN, the encapsulation must provide
   length and fragmentation services, if needed.

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3.6.  Architecture Assumptions

   1) The current design is focused on a point-to-point and same-to-same
      service interface at both end of the PW.  Only network
      interworking will be performed at the edge or the PW.  Support for
      service interworking is for out of scope.

   2) The initial design of PWE3 is focused on a single homogeneous
      administrative PWD (e.g. PW over MPLS or PW over IP etc. ONLY).
      Interworking between different PW types and the support of
      inter-domain PWs are for further study.

   3) The design of PW will not perfectly emulate the characteristics of
      the native service.  It will be dependent on both the emulated
      service, as well as on the network implementation.  An EEMD and AS
      shall be created for each service to describe the degree of
      faithfulness of a PW to the native service.

   4) Only the permanent emulated circuit type (e.g. PVC/PVP) is
      considered initially.  The switched emulated circuit type (e.g.
      SVC/SVP) will be for further study.

   5) The creation and placement of the PSN tunnel to support the PW is
      not within the scope.

   6) The current PW encapsulation approach considerations are focused
      on IPv4, IPv6 and MPLS.  Other encapsulation approaches are for
      further study.

   7) Current PW service applications are focused on Ethernet (i.e.
      Ethernet II (DIX), 802.3 "raw", Ethernet 802.2, Ethernet SNAP,
      802.3ac VLAN, 802.1Q), Frame Relay, ATM and TDM (e.g. DS1, DS3,
      E1, SONET/SDH etc.).

   8) Within the single administrative PWD, the design of the PW assumes
      the inheritance of the security mechanism that has been applied to
      the emulated services.  The PW also inherits any security features
      from the PSN e.g. IPsec for an IP PSN.  No PW-specific security
      mechanism will be specified.

4.  Design Considerations

4.1.  PW-PDU Validation

   It is a common practice to use a checksum, CRC or FCS to assure
   end-to-end integrity of frames.  The PW service-specific mechanisms
   must define whether the packet's checksum shall be preserved across
   the PWD or be removed at the ingress PE and then be re-calculated at
   the egress PE.  The former approach saves work, while the later saves

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

   Other PDU headers (e.g. UDP in IP) do not change during transit.  It
   would make sense to preserve these types of checksums.

   The EEMD for each protocol must describe the validation scheme to be

4.2.  PW-PDU Sequencing

   One major consideration of PW design is how to ensure in-sequence
   delivery of packets, if needed.  The design of the PW for each
   protocol must consider the support of the PSN for in-order delivery
   as well as the requirements of the particular application.  for
   example, IP is connectionless and does not guarantee in-order
   delivery.  When using IP, a PW sequence number may be needed for some
   applications (such as TDM).

4.3.  Session Multiplexing

   One way to facilitate scaling is to increase the number of PWs per
   underlying tunnel.  There are two ways to achieve this:

   - For a service like Relay or ATM, all of the VCs on a given port
     could be lumped together.  VCs would not be distinguishable within
     the PWD.

   - Service SDUs could be distinguished within a PW-PDU by port,
     channel or VC identifiers.  This approach would allow for switching
     or grooming in the PWD.

4.4.  Security

   Each EEMD must specify a means to protect the control of the PWE and
   the PE/PW signaling.  The security-related protection of the
   encapsulated content of the PW is outside of scope.

4.5.  Encapsulation Control

4.5.1.  Scalability

   Different service types may be required between CEs.  Support of
   multiple services implies that a range of PWD label spaces may be
   needed.  If the PWD spans a PSN supporting traffic engineering, then
   the ability to supporting label stacking would be desirable.

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4.5.2.  Service Integration

   It may be desirable to design a PW to transport a variety of services
   which have different transport characteristics.  To achieve this
   integration it may be useful to allow the service requirements to be
   mapped to the tunneling label in such a way that the PWD can apply
   the appropriate service and transport management to the PW.

4.6.  Statistics

   The PE can tabulate statistics that help monitor the state of the
   network, and to help with measurement of SLAs.  Typical counters

   - Counts of PW-PDUs sent and received, with and without errors.

   - Counts of PW-PDUs lost (TDM only).

   - Counts of service PDUs sent and received, with and without errors

   - Service-specific interface counts.

   These counters would be contained in a MIB, they should not replicate
   existing MIB counters.

4.7.  Traceroute

   Tracing functionality is desirable for emulated circuits and
   services, because it allows verification and remediation of the
   operation and configuration of the forwarding plane.  [BONICA]
   describes the requirements for a generic route tracing application.
   Applicability of these requirements to PWE3 is an interesting
   problem, as many of the emulated services have no equivalent
   function.  In general, there is not a way to trace the forwarding
   plane of an TDM or Frame Relay PVC.  ATM does provide an option in
   the loopback OAM cell to return each intermediate hop (see [I.610]).

   There needs to be a mechanism through which upper layers can ask
   emulated services to reveal their internal forwarding details.  A
   common mechanism for all emulated services over a particular PSN may
   be possible.  For example, if MPLS is the PSN, the path for a VC LSP
   could be revealed via the signaling from the underlying TE tunnel
   LSP, or perhaps via the proposed MPLS OAM.  However, when we are
   trying to trace the entire emulated service, starting from the CE
   (e.g. an ATM VCC), then a uniform approach probably will not work and
   different approaches would be required for different emulated

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4.8.  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.

   Each PW EEMD and/or AS will have to specify whether it needs an
   appropriate mechanism for operating in the presence of this
   congestion, including methods of mapping between its native
   congestion reporting and avoidance mechanisms, and those provided by
   the PW.

4.9.  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
   provides a clear picture of how all of the pertinent MIBs fit
   together to form a cohesive management framework for deploying PWE3

4.9.1.  MIB Layering

   The SNMP MIBs created for PWE3 should fit the architecture shown in
   Figure 6.

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

                    Figure 6: Relationship of SNMP MIBs

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   Figure 7 shows an example for a TDM 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 7: 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.

4.9.2.  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.

4.9.3.  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

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   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 [SMIv2] that may be used by any PW MIB.

4.9.4.  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 [MPLS]. 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.

4.9.5.  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 [MPLS] 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.

5.  Acknowledgments

   This document was created by the PWE3 Framework design team.
   Valuable input was also provided by John Rutemiller, David Zelig,
   Durai Chinnaiah, Jayakumar Jayakumar, Ron Bonica, Ghassem Koleyni,
   and Eric Rosen.

6.  References

[L2TP]      W.M. Townsley, A. Valencia, A. Rubens, G. Singh Pall, G.
            Zorn, B. Palter, "Layer Two Tunneling Protocol (L2TP)", RFC
            2661, August 1999.

[RTP]       H. Schulzrinne et al, "RTP: A Transport Protocol for
            Real-Time Applications", RFC1889, January 1996.

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

[IP]        DARPA, "Internet Protocol", RFC791, September 1981.

[CONGEST]   S. Floyd, "Congestion Control Principles", RFC2914,
            September 2000.

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

[SMIv2]     Case et al, "Structure of Management Information for Version
            2 of the Simple Network Management Protocol (SNMPv2)",
            RFC1902, January 1996.

[MALIS]     Malis et al, "SONET/SDH Circuit Emulation over Packet (CEP)"
            (draft-malis-pwe3-sonet-01.txt), work in progress, November

[XIAO]      Xiao et al, "Requirements for Pseudo Wire Emulation
            Edge-to-Edge (PWE3)" (draft-pwe3-requirements-02.txt), work
            in progress, November 2001.

[BONICA]    Bonica et al, "Tracing Requirements for Generic Tunnels"
            (draft-bonica-tunneltrace-01.txt), work in progress,
            February 2001.

[LAU]       J Lau et al, Layer Two Tunneling Protocol "L2TP",
            (draft-ietf-l2tpext-l2tp-base-01.txt) work in progress,
            October 2001.

[PWMIB]     Zelig et al, "Pseudo Wire (PW) Management Information Base
            Using SMIv2", (draft-zelig-pw-mib-00.txt), work in progress,
            July 2001.

[PWTCMIB]   Nadeau et al, "Definitions for Textual Conventions and
            OBJECT-IDENTITIES for Pseudo-Wires Management"
            (draft-nadeau-pw-tc-mib-00.txt), work in progress, July

[PWMPLSMIB] Danenberg et al, "SONET/SDH Circuit Emulation Service Over
            MPLS (CEM) Management Information Base Using SMIv2",
            (draft-danenberg-pw-cem-mib-00.txt), work in progress, July

[LSRMIB]    Srinivasan et al, "MPLS Label Switch Router Management
            Information Base Using SMIv2",
            draft-ietf-mpls-lsr-mib-07.txt, work in progress, January

[TEMIB]     Srinivasan et al, "Traffic Engineering Management
            Information Base Using SMIv2",

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            <draft-ietf-mpls-te-mib-05.txt>, work in progress, November

[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-08.txt>, work in progress, August

[ATMCES]    ATM Forum, "Circuit Emulation Service Interoperability
            Specification Version 2.0" (af-vtoa-0078-000), January 1997.

[FRF.5]     O'Leary et al, "Frame Relay/ATM PVC Network Interworking
            Implementation Agreement", Frame Relay Forum FRF.5, December
            20, 1994.  ITU Recommendation Q.933, Annex A, Geneva, 1995.

[I.363.1]   ITU, "B-ISDN ATM Adaptation Layer specification: Type 1
            AAL", Recommendation I.363.1, August, 1996.

[I.610]     ITU, "B-ISDN Operation and Maintenance Principles and
            Functions", ITU Recommendation I.610, February, 1999.

[802.3]     IEEE, "ISO/IEC 8802-3: 2000 (E), Information
            technology--Telecommunications and information exchange
            between systems --Local and metropolitan area networks
            --Specific requirements --Part 3: Carrier Sense Multiple
            Access with Collision Detection (CSMA/CD) Access Method and
            Physical Layer Specifications", 2000.

7.  Security Considerations

   It may be desirable to define methods for ensuring security during
   exchange of encapsulation control information at an administrative
   boundary of the PSN.

8.  IANA Considerations

   There are no IANA considerations for this document.

9.  Authors' Addresses

   Prayson Pate                        XiPeng Xiao
   Overture Networks, Inc.             Photuris, Inc.
   P. O. Box 14864                     2025 Stierlin Court
   RTP, NC, USA 27709                  Mountain View, CA 94043

   Tricci So                           Craig White
   Caspian Networks                    Level 3 Communications, LLC.
   170 Baytech Dr.                     1025 Eldorado Blvd.
   San Jose, CA 95134                  Broomfield, CO, 80021   

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   Kireeti Kompella                    Andrew G. Malis
   Juniper Networks, Inc.              Vivace Networks, Inc.
   1194 N. Mathilda Ave.               2730 Orchard Parkway
   Sunnyvale, CA 94089                 San Jose, CA 95134       

   Thomas K. Johnson                   Thomas D. Nadeau
   Litchfield Communications           Cisco Systems, Inc.
   76 Westbury Park Rd.                250 Apollo Drive
   Watertown, CT 06795                 Chelmsford, MA 01824

   Stewart Bryant
   Cisco Systems Ltd
   4, The Square
   Stockley Park
   Uxbridge UB11 1BL  UK

10.  Full Copyright Section

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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

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