





Network Working Group                                       X. Xiao, Ed.
Request for Comments: 3916                           Riverstone Networks
Category: Informational                                D. McPherson, Ed.
                                                          Arbor Networks
                                                            P. Pate, Ed.
                                                       Overture Networks
                                                          September 2004


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

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes base requirements for the Pseudo-Wire
   Emulation Edge to Edge Working Group (PWE3 WG).  It provides
   guidelines for other working group documents that will define
   mechanisms for providing pseudo-wire emulation of Ethernet, ATM, and
   Frame Relay.  Requirements for pseudo-wire emulation of TDM (i.e.,
   "synchronous bit streams at rates defined by ITU G.702") are defined
   in another document.  It should be noted that the PWE3 WG
   standardizes mechanisms that can be used to provide PWE3 services,
   but not the services themselves.

Table of Contents

   1.   Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  2
        1.1.  What Are Pseudo Wires?. . . . . . . . . . . . . . . . .  2
        1.2.  Current Network Architecture. . . . . . . . . . . . . .  3
        1.3.  PWE3 as a Path to Convergence . . . . . . . . . . . . .  4
        1.4.  Suitable Applications for PWE3. . . . . . . . . . . . .  4
        1.5.  Summary . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.   Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.   Reference Model of PWE3 . . . . . . . . . . . . . . . . . . .  6
   4.   Packet Processing . . . . . . . . . . . . . . . . . . . . . .  7
        4.1.  Encapsulation . . . . . . . . . . . . . . . . . . . . .  7
        4.2.  Frame Ordering. . . . . . . . . . . . . . . . . . . . .  8
        4.3.  Frame Duplication . . . . . . . . . . . . . . . . . . .  8
        4.4.  Fragmentation . . . . . . . . . . . . . . . . . . . . .  8



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        4.5.  Consideration of Per-PSN Packet Overhead. . . . . . . .  9
   5.   Maintenance of Emulated Services. . . . . . . . . . . . . . .  9
        5.1.  Setup and Teardown of Pseudo-Wires. . . . . . . . . . .  9
        5.2.  Handling Maintenance Message of the Native Services . . 10
        5.3.  PE-initiated Maintenance Messages . . . . . . . . . . . 10
   6.   Management of Emulated Services . . . . . . . . . . . . . . . 12
        6.1.  MIBs. . . . . . . . . . . . . . . . . . . . . . . . . . 12
        6.2.  General MIB Requirements. . . . . . . . . . . . . . . . 12
        6.3.  Configuration and Provisioning. . . . . . . . . . . . . 13
        6.4.  Performance Monitoring. . . . . . . . . . . . . . . . . 13
        6.5.  Fault Management and Notifications. . . . . . . . . . . 13
        6.6.  Pseudo-Wire Connection Verification and Traceroute. . . 13
   7.   Faithfulness of Emulated Services . . . . . . . . . . . . . . 13
        7.1.  Characteristics of an Emulated Service. . . . . . . . . 14
        7.2.  Service Quality of Emulated Services. . . . . . . . . . 14
   8.   Non-Requirements. . . . . . . . . . . . . . . . . . . . . . . 14
   9.   Quality of Service (QoS) Considerations . . . . . . . . . . . 15
   10.  Inter-domain Issues . . . . . . . . . . . . . . . . . . . . . 16
   11.  Security Considerations . . . . . . . . . . . . . . . . . . . 16
   12.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
   13.  References. . . . . . . . . . . . . . . . . . . . . . . . . . 17
        13.1. Normative References. . . . . . . . . . . . . . . . . . 17
        13.2. Informative References. . . . . . . . . . . . . . . . . 17
   14.  Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . 18
   15.  Full Copyright Statement. . . . . . . . . . . . . . . . . . . 19

1.  Introduction

1.1.  What Are Pseudo Wires?

   Pseudo Wire Emulation Edge-to-Edge (PWE3) is a mechanism that
   emulates the essential attributes of a service such as ATM, Frame
   Relay or Ethernet over a Packet Switched Network (PSN).  The required
   functions of PWs include encapsulating service-specific 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 documents and Applicability Statements.







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1.2.   Current Network Architecture

   The following sections give some background on where networks are
   today and why they are changing.  It also talks about the motivation
   to provide converged networks while continuing to support existing
   services.  Finally, it discusses how PWs can be a solution for this
   dilemma.

1.2.1.  Multiple Networks

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

   - Which of my networks do I build out?
   - How many fibers do I need for each network?
   - How do I efficiently manage multiple networks?

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

1.2.2.  Transition to a Packet-Optimized Converged Network

   In order to maximize 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.

   As packet traffic takes up a larger and larger portion of the
   available network bandwidth, it becomes increasingly useful to
   optimize public networks for the Internet Protocol.  However, many
   service providers are confronting several obstacles in engineering
   packet-optimized networks.  Although Internet traffic is the fastest
   growing traffic segment, it does not generate the highest revenue per
   bit.  For example, Frame Relay traffic currently generates higher
   revenue per bit than native IP services do.  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.




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1.3.  PWE3 as a Path to Convergence

   How do service providers realize the capital and operational benefits
   of a new packet-based infrastructure, while leveraging the existing
   equipment and 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 services
   to be carried across the new infrastructure, and thus enables the
   interworking of disparate networks.

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

1.4.  Suitable Applications for PWE3

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

   -  Is the application sufficiently deployed to warrant emulation?
   -  Is there interest on the part of service providers in providing an
      emulation for the given application?
   -  Is there interest on the part of equipment manufacturers in
      providing products for the emulation of a given application?
   -  Are the complexities and limitations of providing an emulation
      worth the savings in capital and operational expenses?

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

1.5.  Summary

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

   In order to create this next-generation converged network, standard
   methods must be developed to emulate existing telecommunications





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   formats such as Ethernet, Frame Relay, and ATM over IP-optimized core
   networks.  This document describes requirements for accomplishing
   this goal.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALLNOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

   Some terms used throughout this document are listed below.

   Attachment Circuit (AC)
                         The physical or virtual circuit attaching a CE
                         to a PE.  An AC can be a Frame Relay DLCI, an
                         ATM VPI/VCI, an Ethernet port, a VLAN, a HDLC
                         link, a PPP connection on a physical interface,
                         a PPP session from an L2TP tunnel, an MPLS LSP,
                         etc.

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

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

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

   Pseudo Wire (PW)      A mechanism that carries the essential elements
                         of an emulated circuit from one PE to another
                         PE over a PSN.

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

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

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



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3.  Reference Model of PWE3

   A pseudo-wire (PW) is a connection between two provider edge (PE)
   devices which connects two attachment circuits (ACs).  An AC can be a
   Frame Relay DLCI, an ATM VPI/VCI, an Ethernet port, a VLAN, a HDLC
   link, a PPP connection on a physical interface, a PPP session from an
   L2TP tunnel, an MPLS LSP, etc.

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

   Customer                                                 Customer
    Edge 1                                                   Edge 2

                     Figure 1: PWE3 Reference Model

   During the setup of a PW, the two PEs will be configured or will
   automatically exchange information about the service to be emulated
   so that later they know how to process packets coming from the other
   end.  After a PW is set up between two PEs, frames received by one PE
   from an AC are encapsulated and sent over the PW to the remote PE,
   where native frames are re-constructed and forwarded to the other CE.
   For a detailed PWE3 architecture overview, readers should refer to
   the PWE3 architecture document [PWE3_ARCH].

   This document does not assume that a particular type of PWs (e.g.,
   [L2TPv3] sessions or [MPLS] LSPs) or PSNs (e.g., IP or MPLS) is used.
   Instead, it describes generic requirements that apply to all PWs and
   PSNs, for all services including Ethernet, ATM, and Frame Relay, etc.








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4.  Packet Processing

   This section describes data plane requirements for PWE3.

4.1.  Encapsulation

   Every PE MUST provide an encapsulation mechanism for PDUs from an AC.
   It should be noted that the PDUs to be encapsulated may or may not
   contain L2 header information.  This is service specific.  Every PWE3
   service MUST specify what the PDU is.

   A PW header consists of all the header fields in a PW PDU that are
   used by the PW egress to determine how to process the PDU.  The PSN
   tunnel header is not considered as part of the PW header.

   Specific requirements on PDU encapsulation are listed below.

4.1.1.  Conveyance of Necessary L2 Header Information

   The egress of a PW needs some information, e.g., which native service
   the PW PDUs belong to, and possibly some L2 header information, in
   order to know how to process the PDUs received.  A PWE3 encapsulation
   approach MUST provide some mechanism for conveying such information
   from the PW ingress to the egress.  It should be noted that not all
   such information must be carried in the PW header of the PW PDUs.
   Some information (e.g., service type of a PW) can be stored as state
   information at the egress during PW setup.

4.1.2.  Support of Variable Length PDUs

   A PWE3 approach MUST accommodate variable length PDUs, if variable
   length PDUs are allowed by the native service.  For example, a PWE3
   approach for Frame Relay MUST accommodate variable length frames.

4.1.3.  Support of Multiplexing and Demultiplexing

   If a service in its native form is capable of grouping multiple
   circuits into a "trunk", e.g., multiple ATM VCCs in a VPC or multiple
   Ethernet 802.1Q interfaces in a port, some mechanism SHOULD be
   provided so that a single PW can be used to connect two end-trunks.
   From encapsulation perspective, sufficient information MUST be
   carried so that the egress of the PW can demultiplex individual
   circuits from the PW.








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4.1.4.  Validation of PW-PDU

   Most L2 frames have a checksum field to assure frame integrity.
   Every PWE3 service MUST specify whether the frame's checksum should
   be preserved across the PW, or should be removed at the ingress PE
   and then be re-calculated and inserted at the egress PE.  For
   protocols such as ATM and FR, the checksum covers link-local
   information such as the circuit identifiers (e.g., FR DLCI or ATM
   VPI/VCI).  Therefore, such checksum MUST be removed at the ingress PE
   and recalculated at the egress PE.

4.1.5.  Conveyance of Payload Type Information

   Under some circumstances, it is desirable to be able to distinguish
   PW traffic from other types of traffic such as IPv4 or IPv6 or OAM.
   For example, if Equal Cost Multi-Path (ECMP) is employed in a PSN,
   this additional distinguishability can be used to reduce the chance
   that PW packets get misordered by the load balancing mechanism.  Some
   mechanism SHOULD provide this distinguishability if needed.  Such
   mechanism MAY be defined in the PWE3 WG or other WGs.

4.2.  Frame Ordering

   When packets carrying the PW PDUs traverse a PW, they may arrive at
   the egress out of order.  For some services, the frames (either
   control frames only or both control and data frames) must be
   delivered in order.  For such services, some mechanism MUST be
   provided for ensuring in-order delivery.  Providing a sequence number
   in the PW header for each packet is one possible approach to detect
   out-of-order frames.  Mechanisms for re-ordering frames may be
   provided by Native Service Processing (NSP) [PWE3_ARCH] but are out
   of scope of PWE3.

4.3.  Frame Duplication

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

4.4.  Fragmentation

   If the combined size of the L2 payload and its associated PWE3 and
   PSN headers exceeds the PSN path MTU, the L2 payload may need to be
   fragmented (Alternatively the L2 frame may be dropped).  For certain
   native service, fragmentation may also be needed to maintain a
   control frame's relative position to the data frames (e.g., an ATM PM



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   cell's relative position).  In general, fragmentation has a
   performance impact.  It is therefore desirable to avoid fragmentation
   if possible.  However, for different services, the need for
   fragmentation can be different.  When there is potential need for
   fragmentation, each service-specific PWE3 document MUST specify
   whether to fragment the frame in question or to drop it.  If an
   emulated service chooses to drop the frame, the consequence MUST be
   specified in its applicability statement.

4.5.  Consideration of Per-PSN Packet Overhead

   When the L2 PDU size is small, in order to reduce PSN tunnel header
   overhead, multiple PDUs MAY be concatenated before a PSN tunnel
   header is added.  Each encapsulated PDU still carries its own PW
   header so that the egress PE knows how to process it.  However, the
   benefit of concatenating multiple PDUs for header efficiency should
   be weighed against the resulting increase in delay, jitter and the
   larger penalty incurred by packet loss.

5.  Maintenance of Emulated Services

   This section describes maintenance requirements for PWE3.

5.1.  Setup and Teardown of Pseudo-Wires

   A PW must be set up before an emulated circuit can be established,
   and must be torn down when an emulated circuit is no longer needed.
   Setup and teardown of a PW can be triggered by a command from the
   management plane of a PE, or by Setup/Teardown of an AC (e.g., an ATM
   SVC), or by an auto-discovery mechanism.

   Every PWE3 approach MUST define some setup mechanism for establishing
   the PWs.  During the setup process, the PEs need to exchange some
   information (e.g., to learn each other's capability).  The setup
   mechanism MUST enable the PEs to exchange all necessary information.
   For example, both endpoints must agree on methods for encapsulating
   PDUs and handling frame ordering.  Which signaling protocol to use
   and what information to exchange are service specific.  Every PWE3
   approach MUST specify them.  Manual configuration of PWs can be
   considered as a special kind of signaling and is allowed.

   If a native circuit is bi-directional, the corresponding emulated
   circuit can be signaled "Up" only when the associated PW and PSN
   tunnels in both directions are functional.







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5.2.  Handling Maintenance Message of the Native Services

   Some native services have mechanisms for maintenance purpose, e.g.,
   ATM OAM and FR LMI.  Such maintenance messages can be in-band (i.e.,
   mixed with data messages in the same AC) or out-of-band (i.e., sent
   in a dedicated control circuit).  For such services, all in-band
   maintenance messages related to a circuit SHOULD be transported in-
   band just like data messages through the corresponding PW to the
   remote CE.  In other words, no translation is needed at the PEs for
   in-band maintenance messages.  In addition, it MAY be desirable to
   provide higher reliability for maintenance messages.  The mechanisms
   for providing high reliability do not have to be defined in the PWE3
   WG.

   Out-of-band maintenance messages between a CE and a PE may relate to
   multiple ACs between the CE and the PE.  They need to be processed at
   the local PE and possibly at the remote PE as well.  If a native
   service has some out-of-band maintenance messages, the corresponding
   emulated service MUST specify how to process such messages at the
   PEs.  In general, an out-of-band maintenance message is either
   translated into an in-band maintenance message of the native service
   or a PWE-specific maintenance message for every AC related to that
   out-of-band message.  As an example, assume the ACs between a CE and
   a PE are some ATM VCCs inside a VPC.  When a F4 AIS [UNI3.0] from the
   CE is received by the PE, the PE should translate that F4 AIS into a
   F5 AIS and send it to the remote CE for every VCC.  Alternatively,
   the PE should generate a PWE-specific maintenance message (e.g.,
   label withdrawal) to the remote PE for every VCC.  When the remote PE
   receives such a PWE-specific maintenance message, it may need to
   generate a maintenance message of the native service and send it to
   the attached CE.

5.3.  PE-initiated Maintenance Messages

   A PE needs to initiate some maintenance messages under some
   circumstances without being triggered by any native maintenance
   messages from the CE.  These circumstances are usually caused by
   fault, e.g., a PW failure in the PSN or a link failure between the CE
   and the PE.

   The reason the PEs need to initiate some maintenance messages under a
   fault condition is because the existence of a PW between two CEs
   would otherwise reduce the CEs' maintenance capability.  This is
   illustrated in the following example.  If two CEs are directly
   connected by a physical wire, a native service (e.g., ATM) can use
   notifications from the lower layer (e.g., the physical link layer) to





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   assist its maintenance.  For example, an ATM PVC can be signaled
   "Down" if the physical wire fails.  However, consider the following
   scenario.

   +-----+ Phy-link +----+              +----+ Phy-link +-----+
   | CE1 |----------| PE1|......PW......|PE2 |----------| CE2 |
   +-----+          +----+              +----+          +-----+

   If the PW between PE1 and PE2 fails, CE1 and CE2 will not receive
   physical link failure notification.  As a result, they cannot declare
   failure of the emulated circuit in a timely fashion, which will in
   turn affect higher layer applications.  Therefore, when the PW fails,
   PE1 and PE2 need to initiate some maintenance messages to notify the
   client layer on CE1 and CE2 that use the PW as a server layer.  (In
   this case, the client layer is the emulated service).  Similarly, if
   the physical link between PE1-CE1 fails, PE1 needs to initiate some
   maintenance message(s) so that the client layer at CE2 will be
   notified.  PE2 may need to be involved in this process.

   In the rare case when a physical wire between two CEs incurs many bit
   errors, the physical link can be declared "Down" and the client layer
   at the CEs be notified.  Similarly, a PW can incur packet loss,
   corruption, and out-of-order delivery.  These can be considered as
   "generalized bit error".  Upon detection of excessive "generalized
   bit error", a PW can be declared "Down" and the detecting PE needs to
   initiate a maintenance message so that the client layer at the CE is
   notified.

   In general, every emulated service MUST specify:
     * Under what circumstances PE-initiated maintenance messages are
       needed,
     * Format of the maintenance messages, and
     * How to process the maintenance messages at the remote PE.

   Some monitoring mechanisms are needed for detecting such
   circumstances, e.g., a PW failure.  Such mechanisms can be defined in
   the PWE3 WG or elsewhere.

   Status of a group of emulated circuits may be affected identically by
   a single network incidence.  For example, when the physical link
   between a CE and a PE fails, all the emulated circuits that go
   through that link will fail.  It is desirable that a single
   maintenance message be used to notify failure of the whole group of
   emulated circuits connected to the same remote PE.  A PWE3 approach
   MAY provide some mechanism for notifying status changes of a group of
   emulated circuits.  One possible approach is to associate each





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   emulated circuit with a group ID while setting up the PW for that
   emulated circuit.  In a maintenance message, that group ID can be
   used to refer to all the emulated circuits in that group.

   If a PE needs to generate and send a maintenance message to a CE, the
   PE MUST use a maintenance message of the native service.  This is
   essential in keeping the emulated service transparent to the CEs.

   The requirements stated in this section are aligned with the ITU-T
   maintenance philosophy for telecommunications networks [G805] (i.e.,
   client layer/server layer concept).

6.  Management of Emulated Services

   Each PWE3 approach SHOULD provide some mechanisms for network
   operators to manage the emulated service.  These mechanisms can be in
   the forms described below.

6.1.  MIBs

   SNMP MIBs [SMIV2] MUST be provided for managing each emulated circuit
   as well as pseudo-wire in general.  These MIBs SHOULD be created with
   the following requirements.

6.2.  General MIB Requirements

   New MIBs MUST augment or extend where appropriate, existing tables as
   defined in other existing service-specific MIBs for existing services
   such as MPLS or L2TP.  For example, the ifTable as defined in the
   Interface MIB [IFMIB] MUST be augmented to provide counts of out-of-
   order packets.  A second example is the extension of the MPLS-TE-MIB
   [TEMIB] when emulating circuit services over MPLS.  Rather than
   redefining the tunnelTable so that PWE can utilize MPLS tunnels, for
   example, entries in this table MUST instead be extended to add
   additional PWE-specific objects.  A final example might be to extend
   the IP Tunnel MIB [IPTUNMIB] in such a way as to provide PWE3-
   specific semantics when tunnels other than MPLS are used as PSN
   transport.  Doing so facilitates a natural extension of those objects
   defined in the existing MIBs in terms of management, as well as
   leveraging existing agent implementations.

   An AC MUST appear as an interface in the ifTable.









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6.3.  Configuration and Provisioning

   MIB Tables MUST be designed to facilitate configuration and
   provisioning of the AC.

   The MIB(s) MUST facilitate intra-PSN configuration and monitoring of
   ACs.

6.4.  Performance Monitoring

   MIBs MUST collect statistics for performance and fault management.

   MIBs MUST provide a description of how existing counters are used for
   PW emulation and SHOULD not replicate existing MIB counters.

6.5.  Fault Management and Notifications

   Notifications SHOULD be defined where appropriate to notify the
   network operators of any interesting situations, including faults
   detected in the AC.

   Objects defined to augment existing protocol-specific notifications
   in order to add PWE functionality MUST explain how these
   notifications are to be emitted.

6.6.   Pseudo-Wire Connection Verification and Traceroute

   For network management purpose, a connection verification mechanism
   SHOULD be supported by PWs.  Connection verification as well as other
   alarming mechanisms can alert network operators that a PW has lost
   its remote connection.  It is sometimes desirable to know the exact
   functional path of a PW for troubleshooting purpose, thus a
   traceroute function capable of reporting the path taken by data
   packets over the PW SHOULD be provided.

7.  Faithfulness of Emulated Services

   An emulated service SHOULD be as similar to the native service as
   possible, but NOT REQUIRED to be identical.  The applicability
   statement of a PWE3 service MUST report limitations of the emulated
   service.

   Some basic requirements on faithfulness of an emulated service are
   described below.







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7.1.  Characteristics of an Emulated Service

   From the perspective of a CE, an emulated circuit is characterized as
   an unshared link or circuit of the chosen service, although service
   quality of the emulated service may be different from that of a
   native one.  Specifically, the following requirements MUST be met:

   1) It MUST be possible to define type (e.g., Ethernet, which is
      inherited from the native service), speed (e.g., 100Mbps), and MTU
      size for an emulated circuit, if it is possible to do so for a
      native circuit.

   2) If the two endpoints CE1 and CE2 of emulated circuit #1 are
      connected to PE1 and PE2, respectively, and CE3 and CE4 of
      emulated circuit #2 are also connected to PE1 and PE2, then the
      PWs of these two emulated circuits may share the same physical
      paths between PE1 and PE2.  But from each CE's perspective, its
      emulated circuit MUST appear as unshared.  For example, CE1/CE2
      MUST NOT be aware of existence of emulated circuit #2 or CE3/CE4.

   3) If an emulated circuit fails (either at one of the ACs or in the
      middle of the PW), both CEs MUST be notified in a timely manner,
      if they will be notified in the native service (see Section 5.3
      for more information).  The definition of "timeliness" is
      service-dependent.

   4) If a routing protocol (e.g., IGP) adjacency can be established
      over a native circuit, it MUST be possible to be established over
      an emulated circuit as well.

7.2.  Service Quality of Emulated Services

   It is NOT REQUIRED that an emulated service provide the same service
   quality as the native service.  The PWE3 WG only defines mechanisms
   for providing PW emulation, not the services themselves.  What
   quality to provide for a specific emulated service is a matter
   between a service provider (SP) and its customers, and is outside
   scope of the PWE3 WG.

8.  Non-Requirements

   Some non-requirements are mentioned in various sections of this
   document.  Those work items are outside scope of the PWE3 WG.  They
   are summarized below:







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   -  Service interworking;

      In Service Interworking, the IWF (Interworking Function) between
      two dissimilar protocols (e.g., ATM & MPLS, Frame Relay & ATM, ATM
      & IP, ATM & L2TP, etc.) terminates the protocol used in one
      network and translates (i.e., maps) its Protocol Control
      Information (PCI) to the PCI of the protocol used in other network
      for User, Control and Management Plane functions to the extent
      possible.

   -  Selection of a particular type of PWs;

   -  To make the emulated services perfectly match their native
      services;

   -  Defining mechanisms for signaling the PSN tunnels;

   -  Defining how to perform traffic management on packets that carry
      PW PDUs;

   -  Providing any multicast service that is not native to the emulated
      medium.

      To illustrate this point, Ethernet transmission to a multicast
      IEEE-48 address is considered in scope, while multicast services
      like [MARS] that are implemented on top of the medium are out of
      scope;

9.  Quality of Service (QoS) Considerations

   Some native services such as ATM can offer higher service quality
   than best effort Internet service.  QoS is therefore essential for
   ensuring that emulated services are compatible (but not necessarily
   identical) to their native forms.  It is up to network operators to
   decide how to provide QoS - They can choose to rely on over-
   provisioning and/or deploy some QoS mechanisms.

   In order to take advantage of QoS mechanisms defined in other working
   groups, e.g., the traffic management schemes defined in DiffServ WG,
   it is desirable that some mechanisms exists for differentiating the
   packets resulted from PDU encapsulation.  These mechanisms do not
   have to be defined in the PWE3 approaches themselves.  For example,
   if the resulted packets are MPLS or IP packets, their EXP or DSCP
   field can be used for marking and differentiating.  A PWE3 approach
   MAY provide guidelines for marking and differentiating.






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   The applicability of PWE3 to a particular service depends on the
   sensitivity of that service (or the CE implementation) to
   delay/jitter etc and the ability of the application layer to mask
   them.  PWE3 may not be applicable to services that have severe
   constraints in this respect.

10.  Inter-domain Issues

   PWE is a matter between the PW end-points and is transparent to the
   network devices between the PW end-points.  Therefore, inter-domain
   PWE is fundamentally similar to intra-domain PWE.  As long as PW
   end-points use the same PWE approach, they can communicate
   effectively, regardless of whether they are in the same domain.
   Security may become more important in the inter-domain case and some
   security measure such as end-point authentication MAY be applied.
   QoS may become more difficult to deliver too, as one service provider
   has no control over another service provider's provisioning and
   traffic management policy.  To solve the inter-domain QoS problem,
   service providers have to cooperate.  Once they agree at a
   contractual level to provider high quality of service to certain
   traffic (e.g., PWE traffic), the mechanisms defined in other working
   groups, e.g., Diffserv WG, can be used.

   Inter-domain PSN tunnels are generally more difficult to set up, tear
   down and maintain than intra-domain ones.  But that is an issue for
   PSN tunneling protocols such as MPLS and L2TPv3 and is outside the
   scope of PWE3.

11.  Security Considerations

   The PW end-point, PW demultiplexing mechanism, and the payloads of
   the native service can all be vulnerable to attack.  PWE3 should
   leverage security mechanisms provided by the PW Demultiplexer or PSN
   Layers.  Such mechanisms SHOULD protect PW end-point and PW
   Demultiplexer mechanism from denial-of-service (DoS) attacks and
   spoofing of the native data units.  Preventing unauthorized access to
   PW end-points and other network devices is generally effective
   against DoS attacks and spoofing, and can be part of protection
   mechanism.  Protection mechanisms SHOULD also address the spoofing of
   tunneled PW data.  The validation of traffic addressed to the PW
   Demultiplexer end-point is paramount in ensuring integrity of PW
   encapsulation.  Security protocols such as IPsec [RFC2401] can be
   used.








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12.  Acknowledgments

   The authors would like to acknowledge input from M. Aissaoui, M.
   Bocci, S. Bryant, R. Cohen, N. Harrison, G. Heron, T. Johnson, A.
   Malis, L. Martini, E. Rosen, J. Rutemiller, T. So, Y. Stein, and S.
   Vainshtein.

13.  References

13.1.  Normative References

   [IFMIB]     McCloghrie, K. and F. Kastenholz, "The Interfaces Group
               MIB", RFC 2863, June 2000.

   [SMIV2]     McCloghrie, K., Perkins, D., and J. Schoenwaelder,
               "Structure of Management Information Version 2 (SMIv2)",
               STD 58, RFC 2578, April 1999.

13.2.  Informative References

   [G805]      "Generic Functional Architecture of Transport Networks",
               ITU-T Recommendation G.805, 2000.

   [IPTUNMIB]  Thaler, D., "IP Tunnel MIB", RFC 2667, August 1999.

   [L2TPv3]    Lau, J., Townsley, M., and I. Goyret, et al., "Layer Two
               Tunneling Protocol (Version 3)", Work in Progress, June
               2004.

   [MARS]      Armitage, G., "Support for Multicast over UNI 3.0/3.1
               based ATM Networks", RFC 2022, November 1996.

   [MPLS]      Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
               Label Switching Architecture", RFC 3031, January 2001.

   [PWE3_ARCH] S. Bryant and P. Pate, et. al., "PWE3 Architecture", Work
               in Progress, March 2004.

   [RFC2401]   Kent, S. and R. Atkinson, "Security Architecture for the
               Internet Protocol", RFC 2401, November 1998.

   [TEMIB]     Srinivasan, C., Viswanathan, A., and T. Nadeau,
               "Multiprotocol Label Switching (MPLS) Traffic Engineering
               (TE) Management Information Base (MIB)", RFC 3812, June
               2004.

   [UNI3.0]    ATM Forum, "ATM User-Network Interface Specification
               Version 3.0", Sept. 1993.



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14.  Authors' Addresses

   XiPeng Xiao  (Editor)
   Riverstone Networks
   5200 Great America Parkway
   Santa Clara, CA 95054

   EMail: xxiao@riverstonenet.com

   Danny McPherson (Editor)
   Arbor Networks

   EMail: danny@arbor.net

   Prayson Pate (Editor)
   Overture Networks
   507 Airport Boulevard, Suite 111
   Morrisville, NC, USA 27560

   EMail: prayson.pate@overturenetworks.com

   Vijay Gill
   AOL Time Warner

   EMail: vijaygill9@aol.com

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

   EMail: kireeti@juniper.net

   Thomas D. Nadeau
   Cisco Systems, Inc.
   300 Beaver Brook Drive
   Boxborough, MA 01719
   EMail: tnadeau@cisco.com

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

   EMail: Craig.White@Level3.com






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15.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

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







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