Internet Draft                                              Prayson Pate
Document: draft-pate-pwe3-framework-01.txt             Overture Networks
Expires: January 13, 2002                                    XiPeng Xiao
                                                           Photuris Inc.
                                                               Tricci So
                                                        Caspian Networks
Craig White                                             Kireeti Kompella
Level 3 Communications, LLC.                      Juniper Networks, Inc.
Andrew G. Malis                                        Thomas K. Johnson
Vivace Networks                                Litchfield Communications

                             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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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at

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   This document describes a framework for Pseudo Wires 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, L2TP 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 (2001). All Rights Reserved.

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                             Table of Contents
   1 Introduction .................................................    3
   2 Background and Motivation ....................................    5
   3 Architecture of Pseudo Wires .................................    8
   4 Layer 1 (Circuit) Applications ...............................   15
   5 Layer 2 (Packet/Cell) Applications ...........................   25
   6 PW Maintenance ...............................................   36
   7 Packet Switched Networks .....................................   40
   8 Acknowledgments ..............................................   43
   9 References ...................................................   43
   10 Security Considerations .....................................   45
   11 Authors' Addresses ..........................................   46
   12 Full Copyright Section ......................................   47

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1.  Introduction

   This document describes a framework for Pseudo Wires 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, L2TP 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.

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 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 signaling and alarm management.

   ASes for each service will describe any shortfalls of the emulation's

1.2.  Goals of This Document

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

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   - Description of an architecture and terminology for PWs.

   - Description of the relevant services that will be supported by PWs,
     including any relevant service-specific considerations.

   - Description of methods to ensure in-order final PDU delivery,

   - Description of methods to perform clock recovery, as needed or

   - Description of methods to perform edge-to-edge/inband signaling
     functions across the PSN, as needed or appropriate.

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

   - Description of the security mechanisms to be used to protect the
     control of the PW technology.  The protection of the encapsulated
     content (e.g., payload encryption) of the PW is outside of scope.

   - Description of a mechanism to exchange encapsulation control
     information at an administrative boundary of the PSN, including
     security methods.

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

1.3.  Non-Goals

   The following are out of scope:

   - 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 the underlying PSN.

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2.  Background and Motivation

   Many of today's service providers are struggling with the dilemma of
   moving to an optical network based on IP and/or MPLS.  How do they
   realize the capital and operational benefits of a new packet-based
   optical 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
        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

   Section 4 of [ANAVI] gives more background on why such emulation is

        The simplicity of TDMoIP translates into initial
        expenditure and operational cost benefits. In addition, due
        to its transparency TDMoIP can support mixed voice, data
        and video services. It is transparent to both protocols and
        signaling, irrespective of whether they are standards based
        or proprietary with full timing support and the capability
        of maintaining the integrity of framed and unframed DS1

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.

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   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?

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.

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

2.2.  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.

2.3.  Transition to a IP-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

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   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 is based on TDM (time-division
   multiplexing) technology.  Voice, video, and data traffic has 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).

   The second generation of converged network is 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 IP 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 IP-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 networks.

   To maximize the return on their assets and minimize their operational
   costs, many service providers are looking to consolidate the delivery

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   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

3.1.  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, MPLS or L2TP as the unit of

   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.

   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"

   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

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

   Pseudo Wire Domain    A PW Domain (PWD) is a collection of instances
                         of PWs that are within the scope of a single
                         homogenous 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 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 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.

   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

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                         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 particulars
                         of PWs for that service, as well as the degree
                         of faithfulness to that service.

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

   Inbound               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 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.

   PE/PW Signaling       PE/PW Signaling is signaling used by the PEs to
                         set up and tear down the PW.  It may be coupled
                         with CE signaling in order to effectively
                         manage the PW.

   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.  This type of signaling is not within the
                         scope of PWE3.

   <Editor's Note: The following figure is temporary.  It is intended to
   facilitate discussion of the preceding set of terms versus those used
   in [MARTINI].>

                    [MARTINI]            This Draft
                    MPLS Network         PSN (includes MPLS)
                    Tunnel LSP           PSN Tunnel
                    VC LSP               PW
                    Edge LSR, R1, R2     PE

                       Figure 1: Comparison of Terms

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3.2.  Reference Models

3.2.1.  Network Reference Model

   Figure 2 below shows the network reference model for PWs.

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

   Customer                                                Customer
    Edge 1                                                   Edge 2

                  Figure 2: PWE3 Network Reference Model

   As shown, the PW provides an emulated service between the customer
   edges (CEs).  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,

   The underlying PSN is not involved in any of these service-specific

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3.2.2.  Signaling Reference Model

   Figure 3 below shows the signaling reference model for PWs.

         |<------- CE (end-to-end) Signaling ------>|
         |                                          |
         |                                          |
         |     |<----- PW/PE Signaling ------>|     |
         |     |                              |     |
         |     |     |<-- PSN Tunnel -->|     |     |
         |     |     |    Signaling     |     |     |
         |     V     V                  V     V     |
         v     +-----+                  +-----+     v
   +-----+     | PE1 |==================| PE2 |     +-----+
   |     |-----|.............PW1..............|-----|     |
   | CE1 |     |     |                  |     |     | CE2 |
   |     |-----|.............PW2..............|-----|     |
   +-----+     |     |==================|     |     +-----+
         ^     +-----+                  +-----+     ^
         |  Provider Edge 1         Provider Edge 2 |
         |                                          |
         |<----------- Emulated Service ----------->|

   Customer                                         Customer
    Edge 1                                           Edge 2

                 Figure 3: PWE3 Signaling Reference Model

   - 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 signaling is used between the PEs to set up 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.

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3.2.3.  Protocol Stack Reference Model

   Figure 4 below shows the protocol stack reference model for PWs.  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.

   +-------------+                                   +-------------+
   |  Emulated   |                                   |  Emulated   |
   |  Service    |                                   |  Service    |
   | (TDM, ATM,  |         Emulated Service          | (TDM, ATM,  |
   |  Ethernet,  |<=================================>|  Ethernet,  |
   |Frame Relay, |                                   |Frame Relay, |
   |    etc.     |                                   |    etc.     |
   +-------------+           Pseudo Wire             +-------------+
   +-------------+                                   +-------------+
   |    PSN      |            PSN Tunnel             |    PSN      |
   |IP/MPLS/L2TP |<=================================>|IP/MPLS/L2TP |
   +-------------+                                   +-------------+
   |  Physical   |                                   |  Physical   |
   +-----+-------+                                   +-----+-------+
         |                                                 |
         |                IP/MPLS/L2TP Network             |
         |              ____     ___       ____            |
         |            _/    \___/   \    _/    \__         |
         |           /               \__/         \_       |
         |          /                               \      |
         +=========/                                 |=====+
                   \                                 /
                    \                               /
                     \   ___      ___     __      _/
                      \_/   \____/   \___/  \____/

               Figure 4: PWE3 Protocol Stack Reference Model

3.3.  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 further study.

   2) The initial design of PWE3 is focused on a single homogenous
      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 AS shall be

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      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, L2TP and MPLS.  Other encapsulation approach is 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), Frame Relay, ATM, TDM (e.g. DS1, DS3, E1, SONET/SDH
      etc.) and MPLS.

   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.  No PW specific security mechanism will be

3.4.  Suitable Applications for PWE3

   <Editor's Note: This section will discuss the attributes that make an
   application suitable (or not) for PWE3 emulation.  This section is
   currently under revision. >

   When considering PWs as a means of providing a service, the following
   questions regarding the application must be considered.

   - Preservation of Order - Does the application require in-order
     delivery of data?  Emulation of an application that requires in-
     order delivery over a PSN that does not guarantee such delivery may
     be difficult.

   - Preservation of Timing - Does the application require fine-grain
     preservation of timing?  If so, the adaptation may be complicated
     by providing such timing where it is not normally available.

   - Natural Delineation - What is the "natural" boundary for
     delineation of data for encapsulation?  (Note: For bit/byte-
     oriented services, such as TDM emulation, this "natural"
     delineation may not necessarily be the overriding consideration for
     determining the best "chunk" for packetizing the service.)

   - Packet Size - Are the encapsulated packets variable or fixed in

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   - Data Rate - Is the data rate presented at the interface fixed or

   Figure 5 below shows a summary of the applications relevant to PWs,
   along with a comparison of their attributes.

     |Attribute ->|Preserve|Preserve|  "Natural" | Packet |  Data  |
     |Application |  Order | Timing | Delineation|  Size  |  Rate  |
     |T1/E1/T3/E3 |  yes   |  yes   |125 us frame| fixed  | fixed  |
     |SONET/SDH   |  yes   |  yes   |125 us frame| fixed  | fixed  |
     |Frame Relay |  yes   |   no   |   frame    |variable|variable|
     |ATM AAL1    |  yes   |  yes   |    cell    | fixed  | fixed  |
     |ATM AAL2    |  yes   |  yes   |    cell    | fixed  |variable|
     |ATM AAL5    |  yes   |   no   |cell or PDU |variable|variable|
     |Ethernet    |  yes   |   no   |   frame    |variable|variable|

             Figure 5: Summary of Applications and Attributes

4.  Layer 1 (Circuit) Applications

   For circuit applications the entire bit stream (or at least the
   payload) needs to be recreated at the far end of the PW.  As with ATM
   CES, the physical layer coding is terminated and re-generated on the
   far end.  In addition, framing may be terminated and regenerated,
   depending on the application.

4.1.  Reference Model

   Figure 6 below shows a pair of T1s being carried over a TDM/SONET
   network.  The node marked "M" is an M13 multiplexer, while the nodes
   marked "S" are SONET Add-Drop Multiplexers (ADMs).  Note that the
   physical interface of the circuit may change without affecting the
   circuit.  For example, the T1s in Figure 6 below enter the network as
   physical T1s but exit the network as Virtual Tributaries (VTs) in a
   physical OC12.

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                           SONET/TDM Network
                           ____     ___       ____
                         _/    \___/   \    _/    \__
   +------+ Physical    /               \__/         \
   |Site A|    T1      /    +---+     DS3             \      Hub Site
   |T1 #1=|=================|\M/|-------------+-----+  \ OC12+------+
   |      |           \     |/ \|=============|\   /|   \----|      |
   +------+           /\    +---+-------------| \ / |========|=T1 #1|
                     /                        |  S  |  /     |      |
   +------+ Physical/       +---+-------------| / \ |========|=T1 #2|
   |Site B|    T1   \       |\S/|=============|/   \|   \----|      |
   |T1 #2=|=================|/ \|-------------+-----+   /    +------+
   |      |          \      +---+     OC3       __     /
   +------+           \                      __/  \   /
                       \   ___      ___     /      \_/
                        \_/   \____/   \___/

                    Figure 6: T1/SONET Example Diagram

   Figure 7 below shows the same pair of T1s being carried over a packet
   network.  Here the emulation is performed by the PEs marked "PE", and
   the routers marked "R" carry the resulting packets.  Note that the
   PE, routing and/or SONET functions could be combined in the same
                          SONET/TDM/Packet Network
                          ____     ___       ____
                        _/    \___/   \    _/    \__
   +------+ Physical   /+-+            \__/         \_
   |Site A|    T1     / | | +---+                     \      Hub Site
   |T1 #1=|=============|P|=| R |   +---+ +-+ +-----+  \ OC12+------+
   |      |           \ |E| |   |===|   | | |=|\   /|   \----|      |
   +------+           /\+-+ +---+   |   | | | | \ / |========|=T1 #1|
                     /              | R |=|P| |  S  |  /     |      |
   +------+ Physical/   +-+ +---+   |   | |E| | / \ |========|=T1 #2|
   |Site B|    T1   \   |P| | R |===|   | | |=|/   \|   \----|      |
   |T1 #2=|=============|E|=|   |   +---+ +-+ +-----+   /    +------+
   |      |          \  | | +---+               __     /
   +------+           \ +-+                  __/  \   /
                       \   ___      ___     /      \_/
                        \_/   \____/   \___/

                  Figure 7: T1 Emulation Example Diagram

4.2.  Operational Considerations

4.2.1.  Transparency

   Circuits such as T1/E1/T3/E3/SONET/SDH lines need a greater degree of
   transparency than Layer 2 services.  These circuits may be carrying
   the services described in the section on Layer 2 services, but in the
   Layer 1 scenario the higher layer application is irrelevant and is
   ignored.  In general, these are "bits in, bits out" applications.

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   In this application a circuit or bit stream is encapsulated in fixed-
   size frames that are sent at a fixed rate.  The emulated stream must
   be delivered in a reliable and predictable fashion to the far end.
   Absolute delay and delay variation (also called jitter or wander)
   must be minimized.  Excess delay and delay variation may cause
   problems with the application carried by the TDM/SONET CEs.

   This encapsulation of TDM data must be transparent.  The emulated
   circuit could be carrying one or more types of data (ATM, Frame
   Relay, TCP/IP, etc.), voice traffic, video or anything else.  The
   data is not interpreted; it is simply transported.

4.2.2.  Structured Versus Unstructured Mode For TDM Circuits

   As discussed in [ATMCES], emulation of a T1, E1 or other circuit can
   be done in a structured (framed) mode or in an unstructured
   (unframed) mode.  This same distinction can be applied to higher rate
   circuits such as DS3, E3, and SONET/SDH.

   Unstructured mode generally involves collecting all bits received
   from a physical port (including transport framing), and packing them
   into packets for transport through the PSN.  The fact that the
   received bit stream contains a framed signal is more or less
   irrelevant to the adaptation function.

   Structured mode requires the use of a framer to identify and
   terminate the incoming transport framing, and delineate logical TDM
   channels within the TDM bit stream for emulation.  In addition, TDM
   framers are generally needed to detect maintenance signals such as
   Alarm Indication Signal (AIS) and Remote Defect Indication (RDI).
   Framers are also used to measure various performance parameters such
   as Errored Seconds, Frame Errored Seconds, etc.  Lastly, a framer is
   needed to generate and terminate the Facility Data Link (FDL) as well
   as the SONET/SDH Data Communications Channels (DCCs).

   The capabilities described in the rest of this section (except for
   LOS) are predicated on the presence of a framer.

4.2.3.  Fractional T1/E1

   A fractional T1 or E1 is composed of a number of concatenated DS0s
   and is sometimes referred to as NxDS0.  It may be emulated by
   replicating the contents of the relevant DS0s at the other end of the
   tunnel.  The value of the other timeslots and/or framing are
   irrelevant and are not transported in leased line application.  Even
   though the framing is not transported, a framer is still needed to
   delineate the timeslots for encapsulation.

4.2.4.  STS-1/Nc

   The SONET/SDH equivalent to Structured T1/E1 services are STS-1/Nc
   and their SDH equivalents.  For STS-1/Nc services a single SONET

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   timeslot or a concatenation of multiple timeslots is used to carry a
   single logical circuit.  As with structured T1/E1 services, the
   transport framing (i.e.  SONET Section and Line Overhead) is
   terminated, and only the relevant SONET timeslots are carried through
   the packet network.  A single physical SONET interface can be the
   source of multiple STS-1/Nc services, each of which may be emulated
   as an independent PWE3 service.

4.2.5.  Loopbacks

   It could be useful for a PE to process loopback messages as defined
   in [T1.403].  This would allow for isolation of faults in a network.
   It also facilitates the certification of equipment for operation in a
   carrier's network.

   There are also inband loopback commands that are used for voice
   equipment.  These loopback commands are triggered by patterns carried
   in with the data itself.  Voice is limited in the patterns it can
   present, so it won't falsely mimic the inband loopback command.
   These inband commands are falling out of favor due to their
   incompatibility with data services.  The inband pattern for the
   loopback may inadvertently appear in a data stream due to its
   arbitrary nature.  A PE device that implements inband loopbacks must
   have the capability to disable them.

4.2.6.  Performance Processing

   [T1.403] defines a Network Performance Report Message (NPRM) that
   carry periodic reports on the performance of the link.  It would be
   useful for a PE to generate these messages, as they are frequently
   used for surveillance and trouble-shooting.

4.2.7.  LOS/LOF/AIS

   Figure 8 shows an example for the generation of AIS and RAI.

            <-- Upstream   Downstream -->

                  LOS +-----+ AIS
         ------X----->|\   /|--------->
                      | \ / |
                      | / \ |
         <------------|/   \|<---------
         RAI/RDI+Data +-----+ Data

                  Figure 8: Generation of AIS and RAI/RDI

   A TDM multiplexer, SONET ADM, switch or other line terminating
   equipment (LTE) must respond to an LOS (Loss of Signal), LOF (Loss of
   Frame) or AIS (Alarm Indication Signal) condition (traditionally
   known as "red alarms") by generating AIS in the "downstream"
   direction i.e. the same direction in which the LOS was detected.  AIS

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   is conveyed by sending a certain pattern in the data stream.  It may
   also send RAI (or RDI in SONET) in the "upstream" direction i.e. the
   opposite direction from that where the LOS was detected.  See section
   9 of [T1.403] for more information on T1 AIS and RAI.  See section
   section 6.2.1 in [GR253] for more information on the SONET AIS-L and
   RDI-L signals.

   Bandwidth can be saved by suppressing the AIS signal in the emulated
   stream and sending instead an indication in the control overhead.
   This also applies to a received AIS signal.  [MALIS] discusses the
   propagation of AIS using the pointer bits in the TDM control word.

   A device emulating TDM circuit must either replicate the AIS
   indication or indicate this condition in the control overhead.

4.2.8.  SONET/SDH STS Unequipped

   The "STS Unequipped" indication may be treated in a fashion similar
   to that for LOS/AIS.  As discussed in [MALIS], bandwidth can be saved
   by suppressing the payload in the emulated stream and sending instead
   an indication in the control overhead.

4.3.  Encapsulations

   Encapsulation options for TDM services may be compared on the
   following criteria.

   - Timing - TDM services are very sensitive to timing and timing
     variations ("jitter").  The encapsulation may need to provide
     additional information (such as [RTP] timestamps) to help convey
     timing across the PW.

   - Line Signals - The encapsulation should provide a means to convey
     signals such as AIS and line conditions such as LOF.

   - The encapsulation should minimize overhead.

4.4.  Timing

   In the recent Ken Burns Jazz television series, it was said of Louis
   Armstrong that he was very economical with his notes, but that the
   timing of those notes was everything.  The timing of the
   reconstructed bit stream is similarly important.  This section
   describes the various approaches to this problem.  A summary is also
   provided at the end of the section.

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4.4.1.  Reference Model

   Consider the example network shown in Figure 9.  In this network, CE1
   and CE2 are connected by a PW provided by PE1, S1, S2 and PE2.

           +---------------+               +---------------+
           |      PE1      |               |      PE2      |  G
           |               |               |               |  |
           |               |               |               |  v
    +---+  | +-+  +-+  +-+ |  +--+   +--+  | +-+  +-+  +-+ |  +---+
    |   |  | |P|  |D|  |P| |  |  |   |  |  | |P|  |E|  |P| |  |   |
    |   |<===|h|<:|e|<:|h|<:::|  |<::|  |<:::|h|<:|n|<=|h|<===|   |
    |   |  | |y|  |c|  |y| |  |  |   |  |  | |y|  |c|  |y| |  |   |
    | C |  | +-+  +-+  +-+ |  |  |   |  |  | +-+  +-+  +-+ |  | C |
    | E |  |               |  |S1|   |S2|  |               |  | E |
    | 1 |  | +-+  +-+  +-+ |  |  |   |  |  | +-+  +-+  +-+ |  | 2 |
    |   |  | |P|  |E|  |P| |  |  |   |  |  | |P|  |D|  |P| |  |   |
    |   |===>|h|=>|n|:>|h|:::>|  |::>|  |:::>|h|:>|e|=>|h|===>|   |
    |   |  | |y|  |c|  |y| |  |  |   |  |  | |y|  |c|  |y| |  |   |
    +---+  | +-+  +-+  +-+ |  +--+   +--+  | +-+  +-+  +-+ |  +---+
     ^  ^  |   |  ^ ^  ^   |               |   |  ^ ^  ^   |  ^  ^
     |  |  |   |B | |  |   |<------+------>|   |D | |  |   |  |  |
     |  A  |   +--+ +--+-C |       |       |   +--+ +--+-E |  F  |
     |     +---------------+      +-+      +---------------+     |
     |                            |I|                            |
     |                            +-+                            |
     |                             |                             |

          "CEn" is the TDM edge device
          "PEn" is the PE adaptation device
          "Sn" is a core switch
          "A" - "I" are clocks
          "=" is the T1 Bit Stream
          ":" is the switched connection
          "Phy" is a physical interface
          "Enc" is a PWE3 encapsulation device
          "Dec" is a PWE3 decapsulation device

                Figure 9: Timing Recovery Reference Diagram

   For this application to work, CE2 needs to be clocked (by clock E) at
   the same frequency as CE1 (which is being clocked by clock A).  A
   jitter correction buffer at PE2 can handle short-term differences
   between these two clocks, but over time any absolute difference is
   going to cause this buffer to overflow or underflow.

   Bits are clocked into an encapsulation function in PE1 according to
   clock B, which is recovered from the incoming data stream.  Clocks A
   and B will have the same frequency.

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   The bits are packed into frames and clocked out according to clock C.
   Clock C could be related to clock B, but in most cases these clocks
   are completely independent.

   The frames arrive at switch S1, and are clocked out according to the
   internal oscillator on the output interface of switch S1.  The frames
   will depart at the same average rate at which they arrived, but the
   instantaneous rate will be different on each side of S1.  Note that
   there could be short-term variations due to congestion, but S1 can't
   experience long-term congestion with respect to the frames carrying
   emulated data, or the service won't work.

   The frames travel on to switch S2, which again forwards them.  Note
   that switch S2 introduces yet another clock, which it uses to
   transmit the packets toward PE2.  Again the average rate is
   preserved, but the instantaneous rate will vary.

   The frames arrive at PE2 and are clocked into the decapsulation
   function when they arrive (using clock D).  Note that clock D will
   have the same average frequency as A and B, but will have short-term
   variations.  The bits are clocked out of the FIFO according to clock
   E.  Clock F at CE2 is recovered from the bit stream and therefore
   runs at the same frequency as clock E.

4.4.2.  Recreating the Timing

   The big question is: where does clock E in Figure 9 come from?  There
   are 5 possibilities, and these are detailed in the following

   1) Clock E is derived from an external source such as clock I or B
      (indirectly via A) at CE1 and G (indirectly via H) at CE2.  This
      method is described in the "External Timing" section below.

   2) Clock E could be derived from Clock I and the pointers.  This
      approach is described in the "SONET Pointer Justification" section

   3) Clock E is derived from the average rate of Clock D.  This is the
      "Adaptive Timing" scenario described in a subsequent section.

   4) Clock E is derived from a combination of the local oscillator at
      PE2 and received SRTS timestamps.  The "Differential (SRTS)"
      section below describes this approach.

   5) Clock E is derived from inband RTP timestamps.  This method is
      discussed in the "RTP" section below.  External Timing

   The simplest method for communicating timing from one end of a system
   to the other is an external timing source, such as clock I in Figure

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   9.  This external timing source is normally a T1 or E1, but it could
   be delivered via SONET or a GPS receiver.  Its 8 KHz frame rate is
   extracted and used to directly clock the reconstructed data streams,
   or as an input to a phase-locked loop to synthesize the desired
   clock.  The drawback to this method is that a common external clock
   is not commonly available in a data network or in a multi-carrier

   Note that clock I may actually be two separate clocks of a particular
   accuracy or stratum.  The difference in frequency will eventually
   cause the FIFO to slip, but if the clock is of a high enough accuracy
   then the slips will be very infrequent.  For example, a stratum 1
   clock is accurate to one part in 10**11 [G.811].  This gives a
   frequency slip rate of 15.4**-6 bit-slip/sec:

       slip rate = 1.544 Mbps * 10**-11
                 = 15.4**-6 bit-slip/sec

   Taking the reciprocal yields 18 hours/bit-slip:

       bit slip period = 1 / ( 15.4**-6 bit-slip/sec * 3600 s/h)
                       = 18 hours/bit-slip.

   A typical multiplexer has a buffer that is two frames deep.  Assuming
   that it starts out centered, the expected time for a slip would be
   almost 5 months:

       frame slip period = 18 hours/bit-slip * 193 bits/frame
                         = 3474 hours
                         = 145 days
                         = 4.8 months

   This slip rate could be higher or lower depending on the bit rate,
   clock accuracy and the depth of the FIFO.  SONET Pointer Justification

   SONET defines layers of pointers that allow for the multiplexing and
   transmission of asynchronous signals.  These pointers convey the
   timing of the carried signal with respect to the timing of the
   encapsulating signal.  Each SONET ADM must manipulate these pointers
   to preserve the timing.  This method has the advantage of being well-
   defined and understood.

   One way to apply this method to a packet-based network would be to
   ensure that all of the links on a given path are synchronous.  This
   would be difficult for Gigabit Ethernet or POS links.

   Another way would be for each router to update the pointers as the
   packet traversed the router.  This would be compute intensive.

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   The method defined in [MALIS] requires pointer manipulation only at
   the end points.  It does require an external clock as a reference for
   the pointer adjustments.  Adaptive Timing

   Adaptive timing is used when an external reference is not available.
   [ATMCES] describes adaptive timing as follows:

        The adaptive technique does not require a network-wide
        synchronization signal to regenerate the input Service
        clock at the output IWF.  A variety of techniques could be
        used to implement Adaptive clock recovery.  For example,
        the depth of the reassembly buffer in the output IWF could
        be monitored:

        1.  When the buffer depth is too great or tends to increase
            with time, the frequency of the Service clock could be
            increased to cause the buffer to drain more quickly.

        2.  When the buffer contains fewer than the configured
            number of bits, the Service clock could be slowed to
            cause the buffer to drain less quickly.  Wander may be
            introduced by the Adaptive clock recovery technique if
            there is a low-frequency component to the Cell Delay
            Variation inserted by the ATM network carrying cells
            from the input to output IWF.

        Careful design is required to make adaptive timing work
        well.  The instantaneous buffer depth must be filtered to
        extract the average frequency and to reject the jitter and

   Adaptive timing is ideal for many network applications where there is
   no external timing reference available (needed for SRTS), and where
   the packet rate is decoupled from the line rate (as in a routed
   network).  Adaptive timing may not meet the requirements of [G.823],
   [G.824] and other similar specifications.  Differential (SRTS)

   [ATMCES] describes the SRTS (Synchronous Residual Time Stamp) method:

        The SRTS technique measures the Service Interface input
        clock frequency against a network-wide synchronization
        signal that must be present in the IWF, and sends
        difference signals, called Residual Time Stamps, in the
        AAL1 header to the reassembly IWF. At the output IWF, the
        differences can be combined with the network-wide
        synchronization signal to re-create the input Service
        Interface clock.

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   The requirement for a network-wide signal is reasonable in a Telco or
   SONET environment, where such clocks are commonly available.  It may
   be problematic in a packet network.

   Here is the correspondence between the clocks in Figure 9

     Description       I.363.1     Figure 9
     Service Clock       Fs          A, B
     Network Clock       Fn           C
     Derived Reference   Fnx      Based on C  RTP

   [RTP] uses an absolute timestamp to play out the bits at the same
   rate that they were received and packetized.  RTCP (RTP Control
   Protocol) provides a means to synchronize the transmit and receive
   clocks.  RTP Timestamps

   Section 4 of [RTP] defines a timestamp that is either 32-bits or
   64-bits in size:

        Wallclock time (absolute time) is represented using the
        timestamp format of the Network Time Protocol (NTP), which
        is in seconds relative to 0h UTC on 1 January 1900 [NTP].
        The full resolution NTP timestamp is a 64-bit unsigned
        fixed-point number with the integer part in the first 32
        bits and the fractional part in the last 32 bits. In some
        fields where a more compact representation is appropriate,
        only the middle 32 bits are used; that is, the low 16 bits
        of the integer part and the high 16 bits of the fractional
        part.  The high 16 bits of the integer part must be
        determined independently.

   A 32-bit absolute time stamp with a 16-bit fractional part would give
   a 15 us granularity (= 1/65535), which is too coarse for circuit
   emulation.  This means that the 64-bit timestamp must be used, with a
   granularity of 23 ns.

   The transmit timestamps are created according to clocks B and C at
   PE1 and interpreted according to clocks D and E at PE2.  These two
   oscillators will vary by some amount, even if they are very accurate.
   This drift means that RTCP, NTP or some other means must be used to
   synchronize the clocks at each end.

4.4.3.  Summary of Timing Recovery Methods

   All of the previously described methods for timing recovery can be
   made to work for Layer 1 circuit services.  How then can we compare

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   them?  Here are some criteria:

   - Is an external timing source required?  This might be a direct
     timing source as described in "External Timing", or it could be an
     indirect source as with SRTS.

   - Must the PE synthesize clocks?  Synthesis of clocks generally
     requires a Phase-Locked Loop (PLL) to create one clock from

   - Is the method provably correct?  Some methods such as external
     timing and SRTS can be proven to meet specifications.  The
     performance of others, such as adaptive timing, is more dependent
     on particular implementations.

   Figure 10 below shows a summary of the methods for timing recovery.

     |                     Method->| Ext. |SONET|SRTS |RTP/|Adap-|
     |Parameter                    |Source| Ptr |     |RTCP|tive |
     |External timing required?    |  yes | yes | yes | no |  no |
     |Clock synthesis?             |  no  | yes | yes | yes| yes |
     |Provably correct?            |  yes | yes | yes | ?  |  ?  |

               Figure 10: Summary of Timing Recovery Methods

5.  Layer 2 (Packet/Cell) Applications

5.1.  Layer 2 PW Reference Model

   Figure 11 below shows the reference model for Layer 2 PWs.  The Layer
   2 emulated protocols/services include ATM VCC, ATM VPC, Frame Relay
   DLCI, IEEE 802.1Q VLAN, IEEE 802.3x link, etc.  The nodes marked "S"
   are protocol-specific switches e.g. Frame Relay switches.

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                           Carrier A           Carrier B
                         ____     ___
             Layer 2   _/    \___/   \          ___
   +-------+   Link   /               \____    /   \__ Layer 2
   |Site A |---------/     +---+           \  /       \  Link
   |  CE#1=|===============|\S/|            || +-----+ \   +------+
   |       |--------\      |/ \|===============|\   /|  \--|Site C|
   +-------+         \     +---+            || | \ / |=====|=CE#3 |
                     /                      || |  S  | /   |      |
   +-------+        /      +---+            || | / \ |=====|=CE#4 |
   |Site B |--------\      |\S/|===============|/   \|  \--|      |
   |  CE#2=|===============|/ \|            || +-----+ _/  +------+
   |       |---------\     +---+           / \__      /
   +-------+          \                   /     \   _/
                       \   ___     ___    \      \_/
                        \_/   \___/   \___/

              Figure 11: Layer 2 Interconnect Reference Model

   Figure 12 below shows the reference model for PW emulation of Layer 2

                            Carrier A           Carrier B
                        ____     ___
             Layer 2  _/    \___/   \            ___
   +-------+   Link  /+-+            \____      /   \__ Layer 2
   |Site A |--------/ |P| +---+  +---+    \    /       \  Link
   |  CE#1=|==========|W|=| R |  | R | +-+ |  | +-----+ \   +------+
   |       |-------\  |E| |   |==|   |=| |=|==|=|\   /| |   |Site C|
   +-------+        \ +-+ +---+  +---+ |P| |  | | \ / |=|===|=CE#3 |
                    /\                 |W| |  | |  F  | |   |      |
   +-------+       /  +-+ +---+  +---+ |E| |  | | / \ |=|===|=CE#4 |
   |Site B |-------\  |P|=| R |==| R |=| |=|==|=|/   \| |   |      |
   |  CE#2=|==========|W| |   |  |   | | | |  | +-----+ |   +------+
   |       |--------\ |E| +---+  +---+ +-+ |  |         |
   +-------+         \+-+                  /   \_______/
                      \                   /
                       \   _     __      /
                        \_/ \___/  \____/

                      Figure 12: Layer 2 PW Emulation

5.2.  Ethernet

5.2.1.  Reference Model Scope

   PW carriage of Ethernet operates as point-to-point trunking in a non-
   shared medium.  The Ethernet interface can operate in a half-duplex
   or full-duplex mode.  Control functions such as IEEE 802.3 Carrier
   Sense Multiple Access with Collision Detection (CSMA/CD)[802.3] and
   IEEE 802.1D Spanning Tree [802.1D] are not applicable nor within the
   scope of PWs.  However, the PW shall conform to the service

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   definitions as defined in IEEE 802.1P,Q [802.1Q], as required.  Also,
   it shall support all Ethernet framing i.e. Ethernet Frame and IEEE
   802.3x including IEEE 802.3ac VLAN Tagging [802.3] as well as Jumbo

5.2.2.  Operational Considerations  Operational Modes

   The design of the Ethernet PW must consider the support of the two
   operational modes in this framework.  Both modes shall be supported
   for all Ethernet interfaces, i.e. from 10 Mbps to 10,000 Mbps, and
   the design of the Ethernet PW functions shall be agnostic to the
   Ethernet's link capacity.  Both modes shall transparently support the
   address resolution protocols, i.e. ARP, InverseARP, proxy ARP and
   Ethernet-based control protocol (e.g. Generic Attribute Registration
   Protocol (GARP), GARP VLAN Registration Protocol (GVRP) etc.).

   - Opaque Trunking - In this mode, the ingress PE shall relay all of
     the traffic from an Ethernet port into the PW.

   - Transparent Trunking - This mode is particularly designed for
     support of Virtual LANs (VLAN).  VLAN types include Port-based
     VLANs, MAC-Address-Based VLANs, IP-Based VLANs, 802.1Q Tag-based
     VLANs and 802.10 Security-based VLANs.

     The ingress PE may pay attention to the MAC header and other
     relevant VLAN classification information based on the configuration
     policy.  The Ethernet PW shall carry multiple VLANs traffic and can
     extend VLANs across the PWD.  In the case when 802.1Q Tag-based
     VLAN is configured, if the received frame is tagged with a NULL
     VLAN_ID, it will be associated with the VLAN equal to the Port's
     default VLAN.  At frame transmission, all frames that are
     associated with 802.1Q Tag-based VLAN shall be tagged except for
     those assigned for the default VLAN.

     The PE may provide translation of the VLAN_ID in order to
     facilitate deployment.  Note that this does not increase the
     VLAN_ID space, so it has no effect on scalability.  Quality Of Service Support Considerations

   The Ethernet AS shall describe the faithfulness of the PW with
   respect to these attributes described in IEEE 802.1p [802.1Q].

   - Service Availability - Service availability is measured as ratio
     between times when MAC service is unavailable and when it is

   - Frame Loss - The MAC service does not provide guaranteed delivery
     of service data units.  However, the Ethernet PW system should
     consider monitoring frame loss.

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   - Frame Misorder - The MAC service does not permit reordering frames
     within the same user-priority for a source and destination address

   - Frame Duplication - The MAC service does not permit duplicating

   - Transit Delay Performance - IEEE 802.1p [802.1Q] defines frame
     transit delay is the elapsed time between an MA_UNITDATA.request
     and corresponding MA_UNITDATA.indication on a successful transfer.

   - Undetected Frame Error Rate - By using the Frame Checksum (FCS)
     calculation for each frame, the undetected frame error rate should
     be low.

   - Maximum Service Data Unit Size - The maximum service data unit size
     is dependent on the access media used.  In general, it is the
     lowest common denominator of the two adjacent Ethernet interface.

   - Priority Tags and Traffic Classes - IEEE 802.1p defines eight
     traffic classes (priority).  The PRI bits on the VLAN fields should
     be carried transparently over the PW.  COS differentiation on the
     PW level based on the received 802.1p bits is possible but is out-

5.3.  Frame Relay

5.3.1.  Reference Model

   Frame Relay service offerings often have a different physical format
   and speed at each end of the link.  For example, a hub and spoke
   deployment of Frame Relay might provide fractional T1 access at the
   spokes and a clear channel T3 to the hub.  The Virtual Circuits (VCs)
   are aggregated by switches in the Frame Relay network.  This is shown
   in figure 13 below, where the Frame Relay switches are marked with an

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                               Wide Area Frame Relay Network

                            Carrier A           Carrier B
                          ____     ___
             Fractional _/    \___/   \          ___
   +------+     T1     /               \____    /   \__
   |Site A|-----------/     +---+           \  /       \     Hub Site
   |VC #1=|=================|\F/|            || +-----+ \ DS3+------+
   |      |----------\      |/ \|===============|\   /|  \---|      |
   +------+           \     +---+            || | \ / |======|=VC #1|
                      /\                     || |  F  | /    |      |
   +------+          /      +---+            || | / \ |======|=VC #2|
   |Site B|----------\      |\F/|===============|/   \|  \---|      |
   |VC #2=|=================|/ \|            || +-----+ _/   +------+
   |      |-----------\     +---+           / \__      /
   +------+  Trans-    \                   /     \   _/
           Atlantic E1  \   ___     ___    \      \_/
                         \_/   \___/   \___/

                   Figure 13: Frame Relay Example Model

   Figure 14 shows an emulated network, where Carrier "A" is providing a
   transparent Frame Relay emulation connection to Carrier "B".

                       Wide Area Frame Relay/Router Network

                             Carrier A           Carrier B
                           ____     ___
           Fractional    _/    \___/   \          ___
   +------+    T1       /+-+            \____    /   \__
   |Site A|------------/ | | +---+           \  /       \     Hub Site
   |VC #1=|==============|P|=| R |  +---+ +-+ || +-----+ \ DS3+------+
   |      |-----------\  |E| |   |==|   |=| |====|\   /|  \---|      |
   +------+            \ +-+ +---+  |   | | | || | \ / |======|=VC #1|
                      / \           | R | |P| || |  F  | /    |      |
   +------+          /   +-+ +---+  |   | |E| || | / \ |======|=VC #2|
   |Site B|----------\   |P| | R |==|   |=| |====|/   \|  \---|      |
   |VC #2=|==============|E|=|   |  +---+ +-+ || +-----+  /   +------+
   |      |-----------\  | | +---+           /  \_       /
   +------+  Trans-    \ +-+                /     \    _/
           Atlantic E1  \___   ___     __   \      \__/
                            \_/   \___/  \__/

   Figure 14: Frame Relay Emulation Example Diagram For Transparent

   Here the emulation is performed by the PEs marked "PE", and the
   resulting packets are carried by the routers marked "R".  In this
   case, the emulated VCs can transparently carry the PVC status
   signaling (if any) and need not perform any higher layer function.
   Also, note that the emulation and routing functions could be combined
   in the same device.

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   If the Frame Relay switches are to be completely eliminated (as shown
   in figure 15 below), then the emulation service must implement Frame
   Relay PVC status signaling and/or connection signaling for SVCs.  As
   previously noted, the PE and routing functions could be combined in
   the same device.

                       Wide Area Frame Relay/Router Network

                           Carrier A           Carrier B
                         ____     ___
           Fractional  _/    \___/   \          _____
   +------+    T1     /+-+            \____    /     \__
   |Site A|----------/ | | +---+           \  /         \      Hub Site
   |VC #1=|============|P|=| R |  +---+ +-+ ||           \ DS3 +------+
   |      |---------\  |E| |   |==|   |=| | ||            \----|      |
   +------+          \ +-+ +---+  |   | | |====================|=VC #1|
                     /\           | R | |P| ||           /     |      |
   +------+         /  +-+ +---+  |   | |E|====================|=VC #2|
   |Site B|---------\  |P| | R |==|   |=| | ||            \----|      |
   |VC #2=|============|E|=|   |  +---+ +-+ ||           _/    +------+
   |      |----------\ | | +---+           /  \__       /
   +------+ Trans-    \+-+                /      \    _/
          Atlantic E1  \___   ___    _    \       \__/
                           \_/   \__/ \___/

   Figure 15: Frame Relay Emulation Example Diagram For Non-Transparent

5.3.2.  Operational Considerations

   Frame Relay provides a connection-oriented circuit-based carriage of
   variable-sized frames.  There are two types of virtual circuits
   supported in Frame Relay: Permanent Virtual Circuits (PVCs) and
   Switched Virtual Circuits (SVCs).  The following sections describe
   the considerations to support the operation of Frame Relay over the
   PW.  Frame Sequence

   The PW must deliver frames in the proper sequence.  Frame Size

   In general, the maximum frame size for Frame Relay is 1600 bytes per
   [FRF.1.2].  This can be made larger in some implementations.  If the
   MTU of the PW is less than (1600 bytes - size of PW headers), a
   fragmentation and reassembly mechanism may be needed.  End-to-End Characteristics

   [FRF.5] and [FRF.13] define a set of traffic parameters to support
   Service Level Agreements (SLAs).  The design of the PW may be

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   required to preserve these end-to-end transport characteristics.  Connection Management and Congestion Control

   Each Frame Relay header contains some connection management
   information, including

   - a command/response (CR) bit

   - a discard eligibility (DE) bit

   - a connection ID (DLCI)

   - an address extension indicator (EA)

   - Forward/Backward Congestion Notification (FECN/BECN).  Figure 16
     shows an example of how BECN and FECN are sent.

                                  +-----+ Data+FECN
                     ------------>|\   /|--------->
                                  | \ / |
                                  | / \ |
                     <------------|/   \|<---------
                        BECN+Data +-----+ Data

                   Figure 16: Generation of BECN and FECN

   All of this information is vital to the integrity of the Frame Relay
   circuit.  The Frame Relay PW AS must define a means to preserve such
   information across the PWD.  Link Management Support

   Frame Delay defines a set of link management functions for PVC Status
   Management as specified in [T1.617D] and [Q.933A].  Link Management
   runs on a dedicated PVC; therefore, its operation does not impact
   actual user data.  The management functions include:

   - a heartbeat exchange that verifies that the link is operational

   - a report regarding the status of one or more individual DLCIs

   For some networks, such as the one shown in Figure 15, this link
   management is the only means to verify the end-to-end integrity of
   the Frame Relay virtual circuit.  The PE may required to emulate such
   functions.  These functions will be transparent to the underlying

   Another important consideration is that there should be some
   coordination between the PW's link status and the associated Frame
   Relay VCs.  For example, it might be necessary to tear down the VCs

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   in the presence of a network fault.  DLCI Association

   There are two scopes of DLCI addressing that have been defined by
   ANSI and ITU-T: Local and Global DLCIs.

   - Local DLCI addressing means that DLCI numbers are only significant
     at one end of a Frame Relay virtual circuit.

   - Global DLCI addressing is an extension to PVC status management
     that allows a DLCI number to have universal significance.  A global
     DLCI identifies the same VC at both ends of the network.

   In the case when the DLCI is locally significant, the management of
   the PWD must provide a mechanism to coordinate the DLCIs at the two
   ends of the PW.  The association can be done via signaling or
   configuration.  Multiplexing VCs over PWs

   To preserve PW tunnel space and to enhance scalability of PWs, it
   would be very valuable to allow one or more VCs to be multiplexed
   onto the same PW.  One scenario might be to associate an entire Frame
   Relay logical interface to a PW.  Another possibility is that the
   assignment of VCs to the PW could be done via signaling or
   management.  In either of these cases, the DLCI for each frame would
   need to be preserved across the PW.

   If such multiplexing approach is used, the earlier discussion related
   to the packet sequencing, end-to-end characteristics, SLA
   preservation and link status management, shall be addressed with the
   same considerations.  Signaling Transparency

   Since Frame Relay supports SVCs, the PE may need to support signaling
   interworking at the PWES.  InverseARP frames should be passed on
   without interpretation.  In either case, these frames shall be
   transparent to the underlying PSN.  Soft PVC Support

   One type of connection service that is provided by Frame Relay
   networks is called a Soft PVC (SPVC).  A SPVC may be considered to be
   composed of three parts: two peer-to-peer PVCs at each side of the
   core, and a SVC between them.

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   Figure 17 shows the SPVC interconnection example.

       +---+        +---+       +---+       +---+      +---+
       | E |========| C |:::::::| C |:::::::| C |======| E |
       +---+        +---+       +---+       +---+      +---+

          "E" is the edge switch
          "C" is the core switch
          "=" is the permanent connection
          ":" is the switched connection

         Figure 17: Example of an SPVC Over a Frame Relay Network

   The creation of the SPVC within the core is triggered by detecting
   the existence of the PVCs at the edges.  This detection is done
   either by network management or by some proprietary signaling.

   Now consider the case where the core switches are replaced by PEs as
   shown in Figure 14.  The SVC connection within the core is replaced
   by the PW.  The PVCs configuration which are maintained by the
   ingress and the egress CEs of the PWD should remain the same.  The
   ingress and egress PEs shall maintain the SPVC behavior such that it
   is transparent to the CEs.

5.4.  ATM

5.4.1.  Reference Model

   As far as PWs are concerned, ATM is very similar to Frame Relay.  We
   will use the same reference network (Figure 13) for ATM as we did for
   Frame Relay.  Of course, the Frame Relay switches would be ATM
   switches instead.  Likewise, the PE networks shown in Figures 14 and
   15 are applicable to ATM.

5.4.2.  Operational Considerations

   Like Frame Relay, ATM provides connection-oriented circuit-based
   carriage of fixed-size cells.  There are two types of virtual circuit
   supported in ATM: PVC and SVC.  In addition to virtual circuit
   connections (VCCs), ATM also supports virtual path connections
   (VPCs).  There are also permanent virtual paths (PVPs) and switched
   virtual paths (SVPs).

   ATM carries data in fixed size (53 byte) frames called "cells".
   Higher layer frames are adapted to these fixed size cells via ATM
   Adaptation Layers (e.g. AAL1, AAL2 and AAL5) and SAR.  Different
   types of AALs have different cell header formats, and the cells may
   contain signaling information.

   The following sections describe the set of considerations for PW
   support of ATM.

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Internet Draft        draft-pate-pwe3-framework-01         July 13, 2001  Cell Sequence

   The PW must deliver cells in the proper sequence.  End-to-end Transport Characteristics

   The ITU-T [I.356] and the ATM Forum [TM4.0] each define a set of
   traffic and QoS parameters.  The AS for ATM PWs should specify how
   the PW will maintain the end-to-end characteristics for such VCs.  ATM SLA

   The ITU-T [I.371] defines performance targets for managing ATM
   traffic and congestion control.  These targets may be used by some
   service providers to define their ATM SLAs.  The AS for ATM PWs
   should specify how SLA transparency will be achieved.  Connection Management and Congestion Control

   The ATM header contains some connection management and congestion
   control information, as defined in [I.371]:

   - Cell Loss priority (CLP)

   - Connection Identifier (VPI/VCI)

   - Payload Type Identifier (PTI) - distinguishes between an OAM
     (Operations, Administration and Management) cell and a user cell

   - Explicit Forward Congestion Indication (EFCI)

   All of this information is vital to the integrity of the ATM circuit.
   The ATM PW AS must define a means to preserve such information across
   the PWD.  OAM Support

   ATM OAM functions are defined in the ITU-T standard [I.610].  OAM
   cells are used to provide functions like fault management,
   performance management and continuity checks.

   OAM is implemented differently in VCCs and VPCs.  In the case of a
   VCC, the OAM cell is sent along the same VC as the user cells.  For a
   VPC, the OAM cell is sent over a dedicated VC within the VPC.  OAM
   flows are also classified as end-to-end flows (covering the entire
   virtual connection) or as segment flows (covering only parts of the
   virtual connection).

   The PE may emulate the end-to-end OAM flows by encapsulating the OAM
    cells in a PW-PDU.  A PE that supports the OAM function should
   support coordination between the OAM behavior between the PE peers.
   For example, an OAM AIS cell at one end can result in PW signaling

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   that causes the PW link to go down at the far end.  If the PE does
   support OAM, the emulation of the OAM function shall be transparent
   to the underlying network.  ILMI Support

   The Integrated Local Management Interface [ILMI] protocol facilitates
   network deployment and management in several ways:

   - ILMI allows an ATM node to determine the various features supported
     by its neighbors (such as type of signaling, size of connection
     space etc).

   - ILMI allows for smoother administration of ATM addresses through
     address registration.

   - ILMI facilitates automatic configuration of private interfaces.

   - ILMI supports procedures for detecting loss of connectivity through
     periodic polling.

   For some networks, such as the one shown in Figure 15, ILMI is the
   only means to verify the end-to-end integrity of the ATM virtual
   circuit.  It may be desirable for the PE to emulate such functions.
   If supported, these functions must be transparent to the underlying
   network.  VC Associations

   Each ATM connection identifier has local significance only.  Local
   significance means that each ATM connection identifier (VPI and/or
   VCI) is only significant at a local ATM interface, and is independent
   from the identifier at the other end of the link.  The management of
   the PWD must provide a mechanism to coordinate the identifiers at the
   two ends of the PW.  The association can be done via signaling or
   configuration.  Multiplexing ATM VCs over PWs

   See the discussion in the "Multiplexing VCs over PWs" sub-section of
   the previous "Frame Relay" section of this document.  ATM Signaling Transparency

   See the discussion in the "Signaling Transparency" sub-section of the
   previous "Frame Relay" section of this document.  Soft PVC Support

   See the discussion and figures in the "Soft PVC Support" sub-section
   of the previous "Frame Relay" section of this document.

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Internet Draft        draft-pate-pwe3-framework-01         July 13, 2001  Segmentation and Reassembly (SAR)

   The bandwidth overhead of the ATM cell is about 10% (= 5 overhead
   bytes out of 53 bytes total).  For AAL5 traffic [I.363.5], it may be
   more efficient in terms of bandwidth to carry the re-assembled AAL5
   frames instead of the individual ATM cells.  This would involve some
   cost in terms of a SAR operation at each end of the PW.  In some
   cases, especially if OAM is required to be supported over the PW, the
   PW may have no choice but to transport the ATM traffic in cell

   Whether the ATM traffic is transported in a frame or cell format, it
   is the responsibility of the PE to emulate the OAM functions to the
   adjacent ATM interface at each end.

6.  PW Maintenance

6.1.  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

   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 AS for each protocol must describe the validation scheme to be

6.2.  PW-PDU Sequencing

   One major consideration of PW design is 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, MPLS supports connection-oriented transport with a guarantee
   of in-order delivery.  A sequence number in the PW layer is not
   needed when used with MPLS.  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).

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6.3.  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.

6.4.  Security

   Each AS 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.

6.5.  Encapsulation Control

6.5.1.  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.

6.5.2.  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.

6.6.  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

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   - Service-specific interface counts.

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

6.7.  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

6.8.  Congestion Considerations

   [RFC2914] describes how devices connected to the Internet should
   handle congestion.  The discussion of congestion with respect to PWE3
   will be broken into two sections: CBR applications and VBR

6.8.1.  VBR Applications

   VBR applications include Ethernet, Frame Relay, and ATM (other than
   AAL CES).  During periods of congestion the PE may be able to take
   action to communicate to the CE the need to slow down.  Frame Relay

   In the presence of congestion, the PE could perform several actions.
   These are the same actions that a Frame Relay switch might take.  If
   available, a measure of the degree of congestion would be useful.

   - While a service provide may define an SLA for a Frame Relay
     Service, Frame Relay itself does not have a guarantee of delivery.
     Given this fact, the PE could do nothing in the face of congestion.
     The Frame Relay application at the CE would then have to detect
     congestion and act appropriately.

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   - Frame Relay defines BECN as an indication to a Frame Relay device
     that traffic that it sent is experiencing congestion.  See Figure
     16 for an example of how BECN is sent.  For mild congestion, the PE
     could send BECN back to the CE.  The CE could then reduce the
     amount of traffic being sent.  It is worth nothing that many Frame
     Relay devices ignore BECN.

   - The CE could also send FECN in the direction in which congestion is
     occurring.  See Figure 16 for an example of how FECN is sent.

   - During congestion, the PE could discard all frames with DE set

   - If the PE was aware of the CIR for the VCs, it could drop any
     traffic in excess of CIR.

   - For severe congestion the PE could take the interface down.  If the
     PE was generating the PVC status signaling then these messages
     could be used to convey the problem to the CE.  This approach is
     not elegant and may not work well with existing Frame Relay
     applications.  ATM

   ATM has a forward notification of congestion (EFCI), but unlike Frame
   Relay there is no backwards notification.  This leaves the following
   choices of action.  These are the same actions that an ATM switch
   might take.

   - Do nothing and let the ATM application at the CE handle the
     problem.  This may work for some applications, but it will make it
     difficult for service providers to guarantee a high QoS on the VC.

   - If the PE was aware of the traffic parameters for the VCs, it could
     drop any traffic that was out of profile.

   - For severe congestion the PE could take the interface down.  This
     may be worse than doing nothing.  Ethernet

   A PE providing a PW to an Ethernet CE could react to congestion in
   one of the following ways.

   - A PE could use Ethernet flow control during congestion by sending a
     PAUSE frame as described in Annex 31B of [802.3].

   - A PE could do nothing and let the Ethernet application at the CE
     handle the problem.

   - For severe congestion the PE could take the interface down.  This
     may be worse than doing nothing.

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6.8.2.  CBR Applications

   CBR applications include layer 1 applications such as emulated
   TDM/SONET streams, as well as layer 2 applications such as ATM AAL1
   CES.  These applications present a constant load on the network at
   all times.  They cannot slow down; they are either running at full
   speed, or they are impaired.  If congestion causes an excessive
   number of packets to be lost, the PE could take down the interface
   and send AIS to the CE.  There is probably not much point in doing
   this if the PE is operating in an unstructured mode, as the framer in
   the CE will probably declare a LOS condition anyway.  A PE operating
   in a structured (framed) mode would always send a clean frame pattern
   to the CE, so it might be desirable to send AIS to notify the CE that
   there are problems with the PW.

7.  Packet Switched Networks

   This section discusses various types of PSNs for providing the PW
   transport.  The areas of considerations are:

   - Explicit Multi-protocol Encapsulation Identifier

   - Transport Integrity

   - Traffic Engineering Ability

   - Session Multiplexing

   - Flow and Congestion Control

   - Packet Ordering

   - Tunnel Maintenance

   - Scalability

   - Overhead

   - QoS and Traffic Management

7.1.  IP

   Below is a description of the aspects of the Internet Protocol [IP].

   Explicit MP Encap ID     No support for a full range of multi-service
                            protocols e.g. there is no protocol type
                            assigned for ATM or MPLS.

   Transport Integrity      IP has a checksum over the header but not
                            over the payload.

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   Traffic Engineering      The TOS bits may be used as DiffServ code

   Session Multiplexing     No support for session multiplexing.

   Packet Order             No support for preservation of order.

   Tunnel Maintenance       Protocols such as L2TP may be used to
                            establish tunnels using IP packets.

   Flow/Congestion Control  No built-in flow control to manage
                            congestion.  IP relies on the upper layer
                            protocol, e.g. TCP, to perform the
                            congestion management.

   Scalability              It would be hard to imagine a protocol more
                            scalable than IP.

   Transport Overhead       Minimum of 20-byte header.

   QoS/Traffic Management   No built-in QoS and traffic management.
                            However, one can apply DiffServ to select a
                            per-hop behavior for a class of traffic.

7.2.  L2TP

   Layer 2 Tunneling (L2TP) [L2TP] provides a virtual extension of PPP
   across an IP PSN.

   Explicit MP Encap ID     Supports any routed protocol, e.g. IP, IPX
                            and AppleTalk that is supported by PPP.

   Transport Integrity      Support a checksum for the entire
                            encapsulated frame.

   Traffic Engineering      No companion traffic engineering mechanism
                            to support L2TP tunnel establishment.

   Session Multiplexing     Supports two levels of session multiplexing
                            via the use of the "tunnel-id" and "session-
                            id" fields.

   Packet Order             By supporting the optional sequence number,
                            packet re-ordering can be done at the PWE

   Tunnel Maintenance       L2TP uses control messages to establish,
                            terminate and monitor the status of the
                            logical PPP sessions.  These are independent
                            of the data messages.  L2TP also provides an
                            optional keep-alive mechanism to detect non-
                            operational tunnel.

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   Flow/Congestion Control  L2TP defines flow and congestion control
                            mechanisms for the control traffic only; no
                            control for the data traffic.  Even so, the
                            PE could apply value-added functions such as
                            admission control, policing and shaping of
                            the L2TP tunnel at the aggregate flows
                            level, e.g. DiffServ-TE.

   Scalability              Lack of label stacking ability.

   Transport Overhead       Minimum overhead is 44-byte (20-byte IP
                            header + 12-byte UDP header + 8-byte minimum
                            L2TP header + 4-byte PPP header) to support
                            L2TP encapsulation

   QoS/Traffic Management   No built-in QoS and traffic management.
                            However, one can apply IntServ or DiffServ
                            to select the preferred transport behavior
                            for the entire tunnel, i.e. one traffic
                            class per L2TP tunnel.

7.3.  MPLS

   Multi-protocol Label Switching [MPLS] is designed to combine Layer 2
   switching and Layer 3 routing technology to provide efficient packet
   processing and forwarding over a variety of link layer and transport
   technologies e.g. ATM, Frame Relay and SONET.

   Explicit MP Encap ID     No defined standard to identify the
                            encapsulated multi-protocol PDU.

   Transport Integrity      No checksum support.

   Traffic Engineering      Designed with many signaling, routing and
                            traffic management extensions to support
                            traffic engineering.

   Session Multiplexing     Supports session multiplexing via the MPLS
                            label and the EXP field.

   Packet Order             Connection-oriented transport with
                            guaranteed in-sequence delivery.

   Tunnel Maintenance       MPLS signaling provides the ability to
                            establish, terminate and monitor the status
                            of the LSP.

   Flow and Congestion Control
                            MPLS-TE assumes external admission control,
                            policy and shaping mechanism to provide flow
                            and congestion control at the aggregate
                            traffic level.

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   Scalability              Label stacking facilitates scalability.

   Transport Overhead       Minimum overhead is 4-byte + any MPLS
                            extension to support multi-protocol
                            encapsulation and transport.

   QoS/Traffic Management   MPLS-TE supports QoS and traffic management.

8.  Acknowledgments

   This document was created by the PWE3 Framework design team.

9.  References

9.1.  IETF RFCs

[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.

[NTP]       D. Mills, "Network Time Protocol Version 3", RFC1305, March

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

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

9.2.  IETF Drafts

[ANAVI]     Anavi et al, "TDM over IP" draft-anavi-tdmoip-01.txt, work
            in progress, February 2001.

[MALIS]     Malis et al, "SONET/SDH Circuit Emulation Service Over MPLS
            (CEM) Encapsulation" (draft-malis-sonet-ces-mpls-03.txt),
            work in progress, February 2001.

[XIAO]      Xiao et al, "Requirements for Pseudo Wire Emulation Edge-to-
            Edge (PWE3)" (draft-pwe3-requirements-01.txt), work in
            progress, July 2001.

[MARTINI]   Martini et al, "Transport of Layer 2 Frames Over MPLS"
            (draft-martini-l2circuit-trans-mpls-06.txt), work in
            progress, May 2001.

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

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[CALLON]    Callon et al, "A Framework for Provider Provisioned Virtual
            Private Networks" (draft-ietf-ppvpn-framework-00.txt), work
            in progress, February 2001.

9.3.  ATM Forum

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

[TM4.0]     ATM Forum, "Traffic Management Specification Version 4.0",
            (af-tm-0056.000), April, 1996.

[ILMI]      ATM Forum, "Integrated Local Management Interface (ILMI)
            Specification Version 4.0", (af-ilmi-0065.000), September,

9.4.  Frame Relay Forum

[FRF.1.2]   D. Sinicrope, "PVC User-to-Network Interface (UNI)
            Implementation Agreement", Frame Relay Forum FRF.1.2, July

[FRF.5]     O'Leary et al, "Frame Relay/ATM PVC Network Interworking
            Implementation Agreement", Frame Relay Forum FRF.5, December
            20, 1994.

[FRF.13]    K. Rehbehn, "Service Level Definitions Implementation
            Agreement", Frame Relay Forum FRF.13, August 4, 1998.

9.5.  ITU

[Q.933A]    ITU, "ISDN Signaling Specifications for Frame Mode Switched
            and Permanent Virtual Connections Control and Status
            Monitoring" ITU Recommendation Q.933, Annex A, Geneva, 1995.

[I.356]     ITU, "B-ISDN ATM Layer Cell Transfer Performance", ITU
            Recommendation I.356, To Be Published.

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

[I.363.2]   ITU, "B-ISDN ATM Adaptation Layer (AAL) type 2
            specification", Recommendation I.363.2, To Be Published.

[I.363.5]   ITU, "B-ISDN ATM Adaptation Layer specification: Type 5
            AAL", Recommendation I.363.5, August, 1996.

[I.371]     ITU, "Traffic Control and Congestion Control in B-ISDN" ITU
            Recommendation I.371, To Be Published.

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

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[G.811]     ITU, "Timing Characteristics of Primary Reference Clocks",
            ITU Recommendation G.811, September 1997.

[G.823]     "The Control of Jitter and Wander Within Digital Networks
            Which Are Based on the 2048 kbit/s Hierarchy", ITU
            Recommendation G.823, March 2000.

[G.824]     "The Control of Jitter and Wander Within Digital Networks
            Which Are Based on the 1544 kbit/s Hierarchy", ITU
            Recommendation G.824, March 2000.

9.6.  IEEE

[802.1D]    IEEE, "ISO/IEC 15802-3:1998,(802.1D, 1998 Edition),
            Information technology --Telecommunications and information
            exchange between systems --IEEE standard for local and
            metropolitan area networks --Common specifications--Media
            access control (MAC) Bridges", June, 1998.

[802.1Q]    ANSI/IEEE Standard 802.1Q, "IEEE Standards for Local and
            Metropolitan Area Networks: Virtual Bridged Local Area
            Networks", 1998 .

[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.

9.7.  ANSI

[T1.403]    ANSI, "Network and Customer Installation Interfaces - DS1
            Electrical Interfaces", T1.403-1999, May 24, 1999.

[T1.617D]   ANSI, "Digital Subscriber System No. 1 DSS1 Signaling
            Specification for Frame Relay Bearer Service", ANSI
            T1.617-1991 (R1997), Annex D.

9.8.  Telcordia

[GR253]     Telcordia, "Synchronous Optical Network (SONET) Transport
            Systems: Common Generic Criteria" (GR253CORE), Issue 3,
            September 2000.

10.  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.

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11.  Authors' Addresses

   Prayson Pate
   Overture Networks
   P. O. Box 14864
   RTP, NC, USA 27709

   XiPeng Xiao
   Photuris, Inc.
   2025 Stierlin Court
   Mountain View, CA 94043

   Tricci So
   Caspian Networks
   170 Baytech Dr.
   San Jose, CA 95134

   Craig White
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021

   Kireeti Kompella
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA 94089

   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134

   Thomas K. Johnson
   Litchfield Communications
   76 Westbury Park Rd.
   Watertown, CT 06795

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12.  Full Copyright Section

Copyright (C) The Internet Society (2000). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   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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