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RFC 5254

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Requirements for Multi-Segment Pseudowire Emulation Edge-to-Edge (PWE3)


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Network Working Group                                      N. Bitar, Ed.
Request for Comments: 5254                                       Verizon
Category: Informational                                    M. Bocci, Ed.
                                                         L. Martini, Ed.
                                                     Cisco Systems, Inc.
                                                            October 2008

Requirements for Multi-Segment Pseudowire 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.


   This document describes the necessary requirements to allow a service
   provider to extend the reach of pseudowires across multiple domains.
   These domains can be autonomous systems under one provider
   administrative control, IGP areas in one autonomous system, different
   autonomous systems under the administrative control of two or more
   service providers, or administratively established pseudowire

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Table of Contents

   1. Introduction ....................................................3
      1.1. Scope ......................................................3
      1.2. Architecture ...............................................3
   2. Terminology .....................................................6
      2.1. Specification of Requirements ..............................6
   3. Use Cases .......................................................7
      3.1. Multi-Segment Pseudowire Setup Mechanisms ..................9
   4. Multi-Segment Pseudowire Requirements ..........................10
      4.1. All Mechanisms ............................................10
           4.1.1. Architecture .......................................10
           4.1.2. Resiliency .........................................11
           4.1.3. Quality of Service .................................11
           4.1.4. Congestion Control .................................12
           4.1.5  Generic Requirements for MS-PW Setup Mechanisms ....13
           4.1.6. Routing ............................................14
      4.2. Statically Configured MS-PWs ..............................15
           4.2.1. Architecture .......................................15
           4.2.2. MPLS-PWs ...........................................15
           4.2.3. Resiliency .........................................15
           4.2.4. Quality of Service .................................16
      4.3. Signaled PW Segments ......................................16
           4.3.1. Architecture .......................................16
           4.3.2. Resiliency .........................................16
           4.3.3. Quality of Service .................................17
           4.3.4. Routing ............................................17
           4.3.5. Additional Requirements on Signaled MS-PW Setup
                  Mechanisms .........................................17
      4.4. Signaled PW / Dynamic Route ...............................18
           4.4.1. Architecture .......................................18
           4.4.2. Resiliency .........................................18
           4.4.3. Quality of Service .................................18
           4.4.4. Routing ............................................18
   5. Operations and Maintenance (OAM) ...............................19
   6. Management of Multi-Segment Pseudowires ........................20
      6.1. MIB Requirements ..........................................20
      6.2. Management Interface Requirements .........................21
   7. Security Considerations ........................................21
      7.1. Inter-Provider MS-PWs .....................................21
           7.1.1. Data-Plane Security Requirements ...................21
           7.1.2. Control-Plane Security Requirements ................23
      7.2. Intra-Provider MS-PWs .....................................25
   8. Acknowledgments ................................................25
   9. References .....................................................25
      9.1. Normative References ......................................25
      9.2. Informative References ....................................25

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1.  Introduction

1.1.  Scope

   This document specifies requirements for extending pseudowires across
   more than one packet switched network (PSN) domain and/or more than
   one PSN tunnel.  These pseudowires are called multi-segment
   pseudowires (MS-PWs).  Requirements for single-segment pseudowires
   (SS-PWs) that extend edge to edge across only one PSN domain are
   specified in [RFC3916].  This document is not intended to invalidate
   any part of [RFC3985].

   This document specifies additional requirements that apply to MS-PWs.
   These requirements do not apply to PSNs that only support SS-PWs.

1.2.  Architecture

   The following three figures describe the reference models that are
   derived from [RFC3985] to support PW emulated services.

         |<-------------- Emulated Service ---------------->|
         |                                                  |
         |          |<------- Pseudowire ------->|          |
         |          |                            |          |
         |          |    |<-- PSN Tunnel -->|    |          |
         | PW End   V    V                  V    V  PW End  |
         V Service  +----+                  +----+  Service V
   +-----+    |     | PE1|==================| PE2|     |    +-----+
   |     |----------|............PW1.............|----------|     |
   | CE1 |    |     |    |                  |    |     |    | CE2 |
   |     |----------|............PW2.............|----------|     |
   +-----+  ^ |     |    |==================|    |     | ^  +-----+
         ^  |       +----+                  +----+     | |  ^
         |  |   Provider Edge 1         Provider Edge 2  |  |
         |  |                                            |  |
   Customer |                                            | Customer
   Edge 1   |                                            | Edge 2
            |                                            |
            |                                            |
    Attachment Circuit (AC)                    Attachment Circuit (AC)
      Native service                              Native service

                Figure 1: PWE3 Reference Configuration

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   Figure 1 shows the PWE3 reference architecture [RFC3985].  This
   architecture applies to the case where a PSN tunnel extends between
   two edges of a single PSN domain to transport a PW with endpoints at
   these edges.

         Native  |<--------Multi-Segment Pseudowire----->|  Native
         Service |         PSN              PSN          |  Service
          (AC)   |     |<-Tunnel->|     |<-Tunnel->|     |  (AC)
           |     V     V     1    V     V     2    V     V   |
           |     +-----+          +-----+          +---- +   |
   +---+   |     |T-PE1|==========|S-PE1|==========|T-PE2|   |    +---+
   |   |---------|........PW1.......... |...PW3..........|---|----|   |
   |CE1|   |     |     |          |     |          |     |   |    |CE2|
   |   |---------|........PW2...........|...PW4..........|--------|   |
   +---+   |     |     |==========|     |==========|     |   |    +---+
       ^         +-----+          +-----+          +-----+        ^
       |     Provider Edge 1         ^        Provider Edge 3     |
       |                             |                            |
       |                             |                            |
       |                     PW switching point                   |
       |                                                          |
       |                                                          |
       |<------------------- Emulated Service ------------------->|

                Figure 2: PW Switching Reference Model

   Figure 2 extends this architecture to show a multi-segment case.
   Terminating PE1 (T-PE1) and Terminating PE3 (T-PE3) provide PWE3
   service to CE1 and CE2.  These PEs terminate different PSN tunnels,
   PSN Tunnel 1 and PSN Tunnel 2, and may reside in different PSN or
   pseudowire domains.  One PSN tunnel extends from T-PE1 to S-PE1
   across PSN1, and a second PSN tunnel extends from S-PE1 to T-PE2
   across PSN2.

   PWs are used to connect the Attachment circuits (ACs) attached to
   T-PE1 to the corresponding ACs attached to T-PE2.  Each PW on PSN
   tunnel 1 is switched to a PW in the tunnel across PSN2 at S-PE1 to
   complete the multi-segment PW (MS-PW) between T-PE1 and T-PE2.  S-PE1
   is therefore the PW switching point and will be referred to as the PW
   switching provider edge (S-PE).  PW1 and PW3 are segments of the same
   MS-PW while PW2 and PW4 are segments of another pseudowire.  PW
   segments of the same MS-PW (e.g., PW1 and PW3) MAY be of the same PW
   type or different types, and PSN tunnels (e.g., PSN Tunnel 1 and PSN
   Tunnel 2) can be the same or different technology.  This document
   requires support for MS-PWs with segments of the same PW type only.

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   An S-PE switches an MS-PW from one segment to another based on the PW
   identifiers (e.g., PW label in case of MPLS PWs).  In Figure 2, the
   domains that PSN Tunnel 1 and PSN Tunnel 2 traverse could be IGP
   areas in the same IGP network or simply PWE3 domains in a single flat
   IGP network, for instance.

                |<------Multi-Segment Pseudowire------>|
                |         AS                AS         |
            AC  |    |<----1---->|     |<----2--->|    |  AC
            |   V    V           V     V          V    V  |
            |   +----+     +-----+     +----+     +----+  |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
   |    |-------|.....PW1..........PW2.........PW3.....|-------|    |
   | CE1|   |   |    |     |     |     |    |     |    |  |    |CE2 |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
        ^       +----+     +-----+     +----+     +----+       ^
        |       T-PE1       S-PE2       S-PE3     T-PE4        |
        |                     ^          ^                     |
        |                     |          |                     |
        |                  PW switching points                 |
        |                                                      |
        |                                                      |
        |<------------------- Emulated Service --------------->|

         Figure 3: PW Switching Inter-Provider Reference Model

   Note that although Figure 2 only shows a single S-PE, a PW may
   transit more than one S-PEs along its path.  For instance, in the
   multi-AS case shown in Figure 3, there can be an S-PE (S-PE2) at the
   border of one AS (AS1) and another S-PE (S-PE3) at the border of the
   other AS (AS2).  An MS-PW that extends from the edge of one AS (T-
   PE1) to the edge of the other AS (T-PE4) is composed of three
   segments:  (1) PW1, a segment in AS1, (2) PW2, a segment between the
   two border routers (S-PE2 and S-PE3) that are switching PEs, and (3)
   PWE3, a segment in AS2.  AS1 and AS2 could belong to the same
   provider (e.g., AS1 could be an access network or metro transport
   network, and AS2 could be an MPLS core network) or to two different
   providers (e.g., AS1 for Provider 1 and AS2 for Provider 2).

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2.  Terminology

   RFC 3985 [RFC3985] provides terminology for PWE3.  The following
   additional terminology is defined for multi-segment pseudowires:

      -  PW Terminating Provider Edge (T-PE).  A PE where the
         customer-facing attachment circuits (ACs) are bound to a PW
         forwarder.  A Terminating PE is present in the first and last
         segments of an MS-PW.  This incorporates the functionality of a
         PE as defined in RFC 3985.

      -  Single-Segment Pseudowire (SS-PW).  A PW setup directly between
         two PE devices.  Each direction of an SS-PW traverses one PSN
         tunnel that connects the two PEs.

      -  Multi-Segment Pseudowire (MS-PW).  A static or dynamically
         configured set of two or more contiguous PW segments that
         behave and function as a single point-to-point PW.  Each end of
         an MS-PW by definition MUST terminate on a T-PE.

      -  PW Segment.  A single-segment or a part of a multi-segment PW,
         which is set up between two PE devices, T-PEs and/or S-PEs.

      -  PW Switching Provider Edge (S-PE).  A PE capable of switching
         the control and data planes of the preceding and succeeding PW
         segments in an MS-PW.  The S-PE terminates the PSN tunnels
         transporting the preceding and succeeding segments of the MS-
         PW.  It is therefore a PW switching point for an MS-PW.  A PW
         switching point is never the S-PE and the T-PE for the same
         MS-PW.  A PW switching point runs necessary protocols to set up
         and manage PW segments with other PW switching points and
         terminating PEs.

2.1.  Specification of Requirements

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

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3.  Use Cases

   PWE3 defines the signaling and encapsulation techniques for
   establishing SS-PWs between a pair of terminating PEs (T-PEs), and in
   the vast majority of cases, this will be sufficient.  MS-PWs may be
   useful in the following situations:

      -i. Inter-Provider PWs:  An Inter-Provider PW is a PW that extends
          from a T-PE in one provider domain to a T-PE in another
          provider domain.

     -ii. It may not be possible, desirable, or feasible to establish a
          direct PW control channel between the T-PEs, residing in
          different provider networks, to set up and maintain PWs.  At a
          minimum, a direct PW control channel establishment (e.g.,
          targeted LDP session) requires knowledge of and reachability
          to the remote T-PE IP address.  The local T-PE may not have
          access to this information due to operational or security
          constraints.  Moreover, an SS-PW would require the existence
          of a PSN tunnel between the local T-PE and the remote T-PE.
          It may not be feasible or desirable to extend single,
          contiguous PSN tunnels between T-PEs in one domain and T-PEs
          in another domain for security and/or scalability reasons or
          because the two domains may be using different PSN

    -iii. MS-PW setup, maintenance, and forwarding procedures must
          satisfy requirements placed by the constraints of a
          multi-provider environment.  An example is the inter-AS L2VPN
          scenario where the T-PEs reside in different provider networks
          (ASs) and it is the current practice to MD5-key all control
          traffic exchanged between two networks.  An MS-PW allows the
          providers to confine MD5 key administration for the LDP
          session to just the PW switching points connecting the two

     -iv. PSN Interworking: PWE3 signaling protocols and PSN types may
          differ in different provider networks.  The terminating PEs
          may be connected to networks employing different PW signaling
          and/or PSN protocols.  In this case, it is not possible to use
          an SS-PW.  An MS-PW with the appropriate interworking
          performed at the PW switching points can enable PW
          connectivity between the terminating PEs in this scenario.

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      -v. Traffic Engineered PSN Tunnels and bandwidth-managed PWs:
          There is a requirement to deploy PWs edge to edge in large
          service provider networks.  Such networks typically encompass
          hundreds or thousands of aggregation devices at the edge, each
          of which would be a PE.  Furthermore, there is a requirement
          that these PWs have explicit bandwidth guarantees.  To satisfy
          these requirements, the PWs will be tunneled over PSN
          TE-tunnels with bandwidth constraints.  A single-segment
          pseudowire architecture would require that a full mesh of PSN
          TE-tunnels be provisioned to allow PWs to be established
          between all PEs.  Inter-provider PWs riding traffic engineered
          tunnels further add to the number of tunnels that would have
          to be supported by the PEs and the core network as the total
          number of PEs increases.

          In this environment, there is a requirement either to support
          a sparse mesh of PSN TE-tunnels and PW signaling adjacencies,
          or to partition the network into a number of smaller PWE3
          domains.  In either case, a PW would have to pass through more
          than one PSN tunnel hop along its path.  An objective is to
          reduce the number of tunnels that must be supported, and thus
          the complexity and scalability problem that may arise.

     -vi. Pseudowires in access/metro networks: Service providers wish
          to extend PW technology to access and metro networks in order
          to reduce maintenance complexity and operational costs.
          Today's access and metro networks are either legacy (Time
          Division Multiplexed (TDM), Synchronous Optical
          Network/Synchronous Digital Hierarchy (SONET/SDH), or Frame
          Relay/Asynchronous Transfer Mode (ATM)), Ethernet, or IP

          Due to these architectures, circuits (e.g., Ethernet Virtual
          Circuits (EVCs), ATM VCs, TDM circuits) in the access/metro
          are traditionally handled as attachment circuits, in their
          native format, to the edge of the IP-MPLS network where the PW
          starts.  This combination requires multiple separate access
          networks and complicates end-to-end control, provisioning, and
          maintenance.  In addition, when a TDM or SONET/SDH access
          network is replaced with a packet-based infrastructure,
          expenses may be lowered due to moving statistical multiplexing
          closer to the end-user and converging multiple services onto a
          single access network.

          Access networks have a number of properties that impact the
          application of PWs.  For example, there exist access
          mechanisms where the PSN is not of an IETF specified type, but
          uses mechanisms compatible with those of PWE3 at the PW layer.

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          Here, use case (iv) may apply.  In addition, many networks
          consist of hundreds or thousands of access devices.  There is
          therefore a desire to support a sparse mesh of PW signaling
          adjacencies and PSN tunnels.  Use case (v) may therefore
          apply.  Finally, access networks also tend to differ from core
          networks in that the access PW setup and maintenance mechanism
          may only be a subset of that used in the core.

          Using the MS-PWs, access and metro network elements need only
          maintain PW signaling adjacencies with the PEs to which they
          directly connect.  They do not need PW signaling adjacencies
          with every other access and metro network device.  PEs in the
          PSN backbone, in turn, maintain PW signaling adjacencies among
          each other.  In addition, a PSN tunnel is set up between an
          access element and the PE to which it connects.  Another PSN
          tunnel needs to be established between every PE pair in the
          PSN backbone.  An MS-PW may be set up from one access network
          element to another access element with three segments: (1)
          access-element - PSN-PE, (2) PSN-PE to PSN-PE, and (3) PSN-PE
          to access element.  In this MS-PW setup, access elements are
          T-PEs while PSN-PEs are S-PEs.  It should be noted that the
          PSN backbone can be also segmented into PWE3 domains resulting
          in more segments per PW.

3.1.  Multi-Segment Pseudowire Setup Mechanisms

   This requirements document assumes that the above use cases are
   realized using one or more of the following mechanisms:

      -i. Static Configuration: The switching points (S-PEs), in
          addition to the T-PEs, are manually provisioned for each

     -ii. Pre-Determined Route: The PW is established along an
          administratively determined route using an end-to-end
          signaling protocol with automated stitching at the S-PEs.

    -iii. Signaled Dynamic Route: The PW is established along a
          dynamically determined route using an end-to-end signaling
          protocol with automated stitching at the S-PEs.  The route is
          selected with the aid of one or more dynamic routing

   Note that we define the PW route to be the set of S-PEs through which
   an MS-PW will pass between a given pair of T-PEs.  PSN tunnels along
   that route can be explicitly specified or locally selected at the
   S-PEs and T-PEs.  The routing of the PSN tunnels themselves is
   outside the scope of the requirements specified in this document.

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4.  Multi-Segment Pseudowire Requirements

   The following sections detail the requirements that the above use
   cases put on the MS-PW setup mechanisms.

4.1.  All Mechanisms

   The following generic requirements apply to the three MS-PW setup
   mechanisms defined in the previous section.

4.1.1.  Architecture

      -i. If MS-PWs are tunneled across a PSN that only supports SS-PWs,
          then only the requirements of [RFC3916] apply to that PSN.
          The fact that the overlay is carrying MS-PWs MUST be
          transparent to the routers in the PSN.

     -ii. The PWs MUST remain transparent to the P-routers.  A P-router
          is not an S-PE or an T-PE from the MS-PW architecture
          viewpoint.  P-routers provide transparent PSN transport for
          PWs and MUST not have any knowledge of the PWs traversing

    -iii. The MS-PWs MUST use the same encapsulation modes specified for

     -iv. The MS-PWs MUST be composed of SS-PWs.

      -v. An MS-PW MUST be able to pass across PSNs of all technologies
          supported by PWE3 for SS-PWs.  When crossing from one PSN
          technology to another, an S-PE must provide the necessary PSN
          interworking functions in that case.

     -vi. Both directions of a PW segment MUST terminate on the same

    -vii. S-PEs MAY only support switching PWs of the same PW type.  In
          this case, the PW type is transparent to the S-PE in the
          forwarding plane, except for functions needed to provide for
          interworking between different PSN technologies.

   -viii. Solutions MAY provide a way to prioritize the setup and
          maintenance process among PWs.

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4.1.2.  Resiliency

   Mechanisms to protect an MS-PW when an element on the existing path
   of an MS-PW fails MUST be provided.  These mechanisms will depend on
   the MS-PW setup.  The following are the generic resiliency
   requirements that apply to all MS-PW setup mechanisms:

      -i. Configuration and establishment of a backup PW to a primary PW
          SHOULD be supported.  Mechanisms to perform a switchover from
          a primary PW to a backup PW upon failure detection SHOULD be

     -ii. The ability to configure an end-to-end backup PW path for a
          primary PW path SHOULD be supported.  The primary and backup
          paths may be statically configured, statically specified for
          signaling, or dynamically selected via dynamic routing
          depending on the MS-PW establishment mechanism.  Backup and
          primary paths should have the ability to traverse separate
          S-PEs.  The backup path MAY be signaled at configuration time
          or after failure.

    -iii. The ability to configure a primary PW and a backup PW with a
          different T-PE from the primary SHOULD be supported.

     -iv. Automatic Mechanisms to perform a fast switchover from a
          primary PW to a backup PW upon failure detection SHOULD be

      -v. A mechanism to automatically revert to a primary PW from a
          backup PW MAY be provided.  When provided, it MUST be

4.1.3.  Quality of Service

   Pseudowires are intended to support emulated services (e.g., TDM and
   ATM) that may have strict per-connection quality-of-service (QoS)
   requirements.  This may include either absolute or relative
   guarantees on packet loss, delay, and jitter.  These guarantees are,
   in part, delivered by reserving sufficient network resources (e.g.,
   bandwidth), and by providing appropriate per-packet treatment (e.g.,
   scheduling priority and drop precedence) throughout the network.

   For SS-PWs, a traffic engineered PSN tunnel (i.e., MPLS-TE) may be
   used to ensure that sufficient resources are reserved in the
   P-routers to provide QoS to PWs on the tunnel.  In this case, T-PEs
   MUST have the ability to automatically request the PSN tunnel
   resources in the direction of traffic (e.g., admission control of PWs
   onto the PSN tunnel and accounting for reserved bandwidth and

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   available bandwidth on the tunnel).  In cases where the tunnel
   supports multiple classes of service (CoS) (e.g., E-LSP), bandwidth
   management is required per CoS.

   For MS-PWs, each S-PE maps a PW segment to a PSN tunnel.  Solutions
   MUST enable S-PEs and T-PEs to automatically bind a PW segment to a
   PSN tunnel based on CoS and bandwidth requirements when these
   attributes are specified for a PW.  Solutions SHOULD also provide the
   capability of binding a PW segment to a tunnel as a matter of policy
   configuration.  S-PEs and T-PEs must be capable of automatically
   requesting PSN tunnel resources per CoS.

   S-PEs and T-PEs MUST be able to associate a CoS marking (e.g., EXP
   field value for MPLS PWs) with PW PDUs.  CoS marking in the PW PDUs
   affects packet treatment.  The CoS marking depends on the PSN
   technology.  Thus, solutions must enable the configuration of
   necessary mapping for CoS marking when the MS-PW crosses from one PSN
   technology to another.  Similarly, different administrative domains
   may use different CoS values to imply the same CoS treatment.
   Solutions MUST provide the ability to define CoS marking maps on
   S-PEs at administrative domain boundaries to translate from one CoS
   value to another as a PW PDU crosses from one domain to the next.

   [RFC3985] requires PWs to respond to path congestion by reducing
   their transmission rate.  Alternatively, RFC 3985 permits PWs that do
   not have a congestion control mechanism to transmit using explicitly
   reserved capacity along a provisioned path.  Because MS-PWs are a
   type of PW, this requirement extends to them as well.  RFC 3985
   applied to MS-PWs consequently requires that MS-PWs employ a
   congestion control mechanism that is effective across an MS path, or
   requires an explicit provisioning action that reserves sufficient
   capacity in all domains along the MS path before the MS-PW begins
   transmission.  S-PEs are therefore REQUIRED to reject attempts to
   establish MS-PW segments for PW types that either do not utilize an
   appropriate congestion control scheme or when resources that are
   sufficient to support the transmission rate of the PW cannot be
   reserved along the path.

4.1.4.  Congestion Control

   [RFC3985] requires all PWs to respond to congestion, in order to
   conform to [RFC2914].  In the absence of a well-defined congestion
   control mechanism, [RFC3985] permits PWs to be carried across paths
   that have been provisioned such that the traffic caused by PWs has no
   harmful effect on concurrent traffic that shares the path, even under
   congestion.  These requirements extend to the MS-PWs defined in this

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   Path provisioning is frequently performed through QoS reservation
   protocols or network management protocols.  In the case of SS-PWs,
   which remain within a single administrative domain, a number of
   existing protocols can provide this provisioning functionality.  MS-
   PWs, however, may transmit across network domains that are under the
   control of multiple entities.  QoS provisioning across such paths is
   inherently more difficult, due to the required inter-domain
   interactions.  It is important to note that these difficulties do not
   invalidate the requirement to provision path capacity for MS-PW use.
   Each domain MUST individually implement a method to control
   congestion.  This can be by QoS reservation, or other congestion
   control method.  MS-PWs MUST NOT transmit across unprovisioned, best
   effort, paths in the absence of other congestion control schemes, as
   required by [RFC3985].

   Solutions MUST enable S-PEs and T-PEs on the path of an MS-PW to
   notify other S-PEs and T-PEs on that path of congestion, when it
   occurs.  Congestion may be indicated by queue length, packet loss
   rate, or bandwidth measurement (among others) crossing a respective
   threshold.  The action taken by a T-PE that receives a notification
   of congestion along the path of one of its PWs could be to re-route
   the MS-PW to an alternative path, including an alternative T-PE if
   available.  If a PE, or an S-PE has knowledge that a particular link
   or tunnel is experiencing congestion, it MUST not set up any new
   MS-PW that utilize that link or tunnel.  Some PW types, such as TDM
   PWs, are more sensitive to congestion than others.  The reaction to a
   congestion notification MAY vary per PW type.

4.1.5.  Generic Requirements for MS-PW Setup Mechanisms

   The MS-PW setup mechanisms MUST accommodate the service provider's
   practices, especially in relation to security, confidentiality of SP
   information, and traffic engineering.  Security and confidentiality
   are especially important when the MS-PWs are set up across autonomous
   systems in different administrative domains.  The following are
   generic requirements that apply to the three MS-PW setup mechanisms
   defined earlier:

      -i. The ability to statically select S-PEs and PSN tunnels on a PW
          path MUST be provided.  Static selection of S-PEs is by
          definition a requirement for the static configuration and
          signaled/static route setup mechanisms.  This requirement
          satisfies the need for forcing an MS-PW to traverse specific
          S-PEs to enforce service provider security and administrative

     -ii. Solutions SHOULD minimize the amount of configuration needed
          to set up an MS-PW.

Top      ToC       Page 14 
    -iii. Solutions should support different PW setup mechanisms on the
          same T-PE, S-PE, and PSN network.

     -iv. Solutions MUST allow T-PEs to simultaneously support use of
          SS-PW signaling mechanisms as specified in [RFC4447], as well
          as MS-PW signaling mechanisms.

      -v. Solutions MUST ensure that an MS-PW will be set up when a path
          that satisfies the PW constraints for bandwidth, CoS, and
          other possible attributes does exist in the network.

     -vi. Solutions must clearly define the setup procedures for each
          mechanism so that an MS-PW setup on T-PEs can be interpreted
          as successful only when all PW segments are successfully set

    -vii. Admission control to the PSN tunnel needs to be performed
          against available resources, when applicable.  This process
          MUST be performed at each PW segment comprising the MS-PW.  PW
          admission control into a PSN tunnel MUST be configurable.

   -viii. In case the PSN tunnel lacks the resources necessary to
          accommodate the new PW, an attempt to signal a new PSN tunnel,
          or increase the capacity of the existing PSN tunnel MAY be
          made.  If the expanded PSN tunnel fails to set up, the PW MUST
          fail to set up.

     -ix. The setup mechanisms must allow the setup of a PW segment
          between two directly connected S-PEs without the existence of
          a PSN tunnel.  This requirement allows a PW segment to be set
          up between two (Autonomous System Border Routers (ASBRs) when
          the MS-PW crosses AS boundaries without the need for
          configuring and setting up a PSN tunnel.  In this case,
          admission control must be done, when enabled, on the link
          between the S-PEs.

4.1.6.  Routing

   An objective of MS-PWs is to provide support for the following

      -i. MS-PWs MUST be able to traverse multiple service provider
          administrative domains.

     -ii. MS-PWs MUST be able to traverse multiple autonomous systems
          within the same administrative domain.

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    -iii. MS-PWs MUST be able to traverse multiple autonomous systems
          belonging to different administrative domains.

     -iv. MS-PWs MUST be able to support any hybrid combination of the
          aforementioned connectivity scenarios, including both PW
          transit and termination in a domain.

4.2.  Statically Configured MS-PWs

   When the MS-PW segments are statically configured, the following
   requirements apply in addition to the generic requirements previously

4.2.1.  Architecture

   There are no additional requirements on the architecture.

4.2.2.  MPLS-PWs

   Solutions should allow for the static configuration of MPLS labels
   for MPLS-PW segments and the cross-connection of these labels to
   preceding and succeeding segments.  This is especially useful when an
   MS-PW crosses provider boundaries and two providers do not want to
   run any PW signaling protocol between them.  A T-PE or S-PE that
   allows the configuration of static labels for MS-PW segments should
   also simultaneously allow for dynamic label assignments for other
   MS-PW segments.  It should be noted that when two interconnected
   S-PEs do not have signaling peering for the purpose of setting up
   MS-PW segments, they should have in-band PW Operations and
   Maintenance (OAM) capabilities that relay PW or attachment circuit
   defect notifications between the adjacent S-PEs.

4.2.3.  Resiliency

   The solution should allow for the protection of a PW segment, a
   contiguous set of PW segments, as well as the end-to-end path.  The
   primary and protection segments must share the same segment
   endpoints.  Solutions should allow for having the backup paths set up
   prior to the failure or as a result of failure.  The choice should be
   made by configuration.  When resources are limited and cannot satisfy
   all PWs, the PWs with the higher setup priorities should be given
   preference when compared with the setup priorities of other PWs being
   set up or the holding priorities of existing PWs.

   Solutions should strive to minimize traffic loss between T-PEs.

Top      ToC       Page 16 
4.2.4.  Quality of Service

   The CoS and bandwidth of the MS-PW must be configurable at T-PEs and

4.3.  Signaled PW Segments

   When the MS-PW segments are dynamically signaled, the following
   requirements apply in addition to the generic requirements previously
   defined.  The signaled MS-PW segments can be on the path of a
   statically configured MS-PW, signaled/statically routed MS-PW, or
   signaled/dynamically routed MS-PW.

   There are four different mechanisms that are defined to setup SS-PWs:

      -i. Static set up of the SS-PW (MPLS or L2TPv3 forwarding)

     -ii. LDP using PWid Forwarding Equivalence Class (FEC) 128

    -iii. LDP using the generalized PW FEC 129

     -iv. L2TPv3

   The MS-PW setup mechanism MUST be able to support PW segments
   signaled with any of the above protocols; however, the specification
   of which combinations of SS-PW signaling protocols are supported by a
   specific implementation is outside the scope of this document.

   For the signaled/statically routed and signaled/dynamically routed
   MS-PW setup mechanisms, the following requirements apply in addition
   to the generic requirements previously defined.

4.3.1.  Architecture

   There are no additional requirements on the architecture.

4.3.2.  Resiliency

   Solutions should allow for the signaling of a protection path for a
   PW segment, sequence of segments, or end-to-end path.  The protection
   and primary paths for the protected segment(s) share the same
   respective segments endpoints.  When admission control is enabled,
   systems must be careful not to double account for bandwidth
   allocation at merged points (e.g., tunnels).  Solutions should allow
   for having the backup paths set up prior to the failure or as a
   result of failure.  The choice should be made by configuration at the
   endpoints of the protected path.  When resources are limited and
   cannot satisfy all PWs, the PWs with the higher setup priorities

Top      ToC       Page 17 
   should be given preference when compared with the setup priorities of
   other PWs being set up or the holding priorities of existing PWs.
   Procedures must allow for the primary and backup paths to be diverse.

4.3.3.  Quality of Service

   When the T-PE attempts to signal an MS-PW, the following capability
   is required:

      -i. Signaling must be able to identify the CoS associated with an

     -ii. Signaling must be able to carry the traffic parameters for an
          MS-PW per CoS.  Traffic parameters should be based on existing
          INTSERV definitions and must be used for admission control
          when admission control is enabled.

    -iii. The PW signaling MUST enable separate traffic parameter values
          to be specified for the forward and reverse directions of the

     -iv. PW traffic parameter representations MUST be the same for all
          types of MS-PWs.

      -v. The signaling protocol must be able to accommodate a method to
          prioritize the PW setup and maintenance operation among PWs.

4.3.4.  Routing

   See the requirements for "Resiliency" above.

4.3.5.  Additional Requirements on Signaled MS-PW Setup Mechanisms

   The following are further requirements on signaled MS-PW setup

      -i. The signaling procedures MUST be defined such that the setup
          of an MS-PW is considered successful if all segments of the
          MS-PW are successfully set up.

     -ii. The MS-PW path MUST have the ability to be dynamically set up
          between the T-PEs by provisioning only the T-PEs.

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    -iii. Dynamic MS-PW setup requires that a unique identifier be
          associated with a PW and be carried in the signaling message.
          That identifier must contain sufficient information to
          determine the path to the remote T-PE through intermediate

     -iv. In a single-provider domain, it is natural to have the T-PE
          identified by one of its IP addresses.  This may also apply
          when an MS-PW is set up across multiple domains operated by
          the same provider.  However, some service providers have
          security and confidentiality policies that prevent them from
          advertising reachability to routers in their networks to other
          providers (reachability to an ASBR is an exception).  Thus,
          procedures MUST be provided to allow dynamic set up of MS-PWs
          under these conditions.

4.4.  Signaled PW / Dynamic Route

   The following requirements apply, in addition to those in Sections
   4.1 and 4.3, when both dynamic signaling and dynamic routing are

4.4.1.  Architecture

   There are no additional architectural requirements.

4.4.2.  Resiliency

   The PW routing function MUST support dynamic re-routing around
   failure points when segments are set up using the dynamic setup

4.4.3.  Quality of Service

   There are no additional QoS requirements.

4.4.4.  Routing

   The following are requirements associated with dynamic route
   selection for an MS-PW:

      -i. Routing must enable S-PEs and T-PEs to discover S-PEs on the
          path to a destination T-PE.

     -ii. The MS-PW routing function MUST have the ability to
          automatically select the S-PEs along the MS-PW path.  Some of
          the S-PEs MAY be statically selected and carried in the
          signaling to constrain the route selection process.

Top      ToC       Page 19 
    -iii. The PW routing function MUST support re-routing around
          failures that occur between the statically configured segment
          endpoints.  This may be done by choosing another PSN tunnel
          between the two segment endpoints or setting up an alternative

     -iv. Routing protocols must be able to advertise reachability
          information of attachment circuit (AC) endpoints.  This
          reachability information must be consistent with the AC
          identifiers carried in signaling.

5.  Operations and Maintenance (OAM)

   OAM mechanisms for the attachment circuits are defined in the
   specifications for PW emulated specific technologies (e.g., ITU-T
   I.610 [i610] for ATM).  These mechanisms enable, among other things,
   defects in the network to be detected, localized, and diagnosed.
   They also enable communication of PW defect states on the PW
   attachment circuit.  Note that this document uses the term OAM as
   Operations and Management as per ITU-T I.610.

   The interworking of OAM mechanisms for SS-PWs between ACs and PWs is
   defined in [PWE3-OAM].  These enable defect states to be propagated
   across a PWE3 network following the failure and recovery from faults.

   OAM mechanisms for MS-PWs MUST provide at least the same capabilities
   as those for SS-PWs.  In addition, it should be possible to support
   both segment and end-to-end OAM mechanisms for both defect
   notifications and connectivity verification in order to allow defects
   to be localized in a multi-segment network.  That is, PW OAM segments
   can be T-PE to T-PE, T-PE to S-PE, or S-PE to S-PE.

   The following requirements apply to OAM for MS-PWs:

      -i. Mechanisms for PW segment failure detection and notification
          to other segments of an MS-PW MUST be provided.

     -ii. MS-PW OAM SHOULD be supported end-to-end across the network.

    -iii. Single ended monitoring SHOULD be supported for both
          directions of the MS-PW.

     -iv. SS-PW OAM mechanisms (e.g., [RFC5085]) SHOULD be extended to
          support MS-PWs on both an end-to-end basis and segment basis.

      -v. All PE routers along the MS-PW MUST agree on a common PW OAM
          mechanism to use for the MS-PW.

Top      ToC       Page 20 
     -vi. At the S-PE, defects on an PSN tunnel MUST be propagated to
          all PWs that utilize that particular PSN tunnel.

    -vii. The directionality of defect notifications MUST be maintained
          across the S-PE.

   -viii. The S-PE SHOULD be able to behave as a segment endpoint for PW
          OAM mechanisms.

     -ix. The S-PE MUST be able to pass T-PE to T-PE PW OAM messages

      -x. Performance OAM is required for both MS-PWs and SS-PWs to
          measure round-trip delay, one-way delay, jitter, and packet
          loss ratio.

6.  Management of Multi-Segment Pseudowires

   Each PWE3 approach that uses MS-PWs SHOULD provide some mechanisms
   for network operators to manage the emulated service.  Management
   mechanisms for MS-PWs MUST provide at least the same capabilities as
   those for SS-PWs, as defined in [RFC3916].

   It SHOULD also be possible to manage the additional attributes for
   MS-PWs.  Since the operator that initiates the establishment of an
   MS-PW may reside in a different PSN domain from the S-PEs and one of
   the T-PEs along the path of the MS-PW, mechanisms for the remote
   management of the MS-PW SHOULD be provided.

   The following additional requirements apply:

6.1.  MIB Requirements

      -i. MIB Tables MUST be designed to facilitate configuration and
          provisioning of the MS-PW at the S-PEs and T-PEs.

     -ii. The MIB(s) MUST facilitate inter-PSN configuration and
          monitoring of the ACs.

Top      ToC       Page 21 
6.2.  Management Interface Requirements

      -i. Mechanisms MUST be provided to enable remote management of an
          MS-PW at an S-PE or T-PE.  It SHOULD be possible for these
          mechanisms to operate across PSN domains.  An example of a
          commonly available mechanism is the command line interface
          (CLI) over a telnet session.

     -ii. For security or other reasons, it SHOULD be possible to
          disable the remote management of an MS-PW.

7.  Security Considerations

   This document specifies the requirements both for MS-PWs that can be
   set up across domain boundaries administered by one or more service
   providers (inter-provider MS-PWs), and for MS-PWs that are only set
   up across one provider (intra-provider MS-PWs).

7.1.  Inter-Provider MS-PWs

   The security requirements for MS-PW setup across domains administered
   by one service provider are the same as those described under
   security considerations in [RFC4447] and [RFC3916].  These
   requirements also apply to inter-provider MS-PWs.

   In addition, [RFC4111] identifies user and provider requirements for
   L2 VPNs that apply to MS-PWs described in this document.  In this
   section, the focus is on the additional security requirements for
   inter-provider operation of MS-PWs in both the control plane and data
   plane, and some of these requirements may overlap with those in

7.1.1.  Data-Plane Security Requirements

   By security in the "data plane", we mean protection against the
   following possibilities:

      -i. Packets from within an MS-PW traveling to a PE or an AC to
          which the PW is not intended to be connected, other than in a
          manner consistent with the policies of the MS-PW.

     -ii. Packets from outside an MS-PW entering the MS-PW, other than
          in a manner consistent with the policies of the MS-PW.

   MS-PWs that cross service provider (SP) domain boundaries may connect
   one T-PE in a SP domain to a T-PE in another provider domain.  They
   may also transit other provider domains even if the two T-PEs are
   under the control of one SP.  Under these scenarios, there is a

Top      ToC       Page 22 
   chance that one or more PDUs could be falsely inserted into an MS-PW
   at any of the originating, terminating, or transit domains.  Such
   false injection can be the result of a malicious attack or fault in
   the S-PE.  Solutions MAY provide mechanisms for ensuring the
   end-to-end authenticity of MS-PW PDUs.

   The data plane security requirements at a service provider border for
   MS-PWs are similar to those for inter-provider BGP/MPLS IP Virtual
   Private Networks [RFC4364].  In particular, an S-PE or T-PE SHOULD
   discard a packet received from a particular neighbor over the service
   provider border unless one of the following two conditions holds:

      -i. Any MPLS label processed at the receiving S-PE or T-PE, such
          the PSN tunnel label or the PW label has a label value that
          the receiving system has distributed to that neighbor; or

     -ii. Any MPLS label processed at the receiving S-PE or T-PE, such
          as the PSN tunnel label or the PW label has a label value that
          the receiving S-PE or T-PE has previously distributed to the
          peer S-PE or T-PE beyond that neighbor (i.e., when it is known
          that the path from the system to which the label was
          distributed to the receiving system is via that neighbor).

   One of the domains crossed by an MS-PW may decide to selectively
   mirror the PDUs of an MS-PW for eavesdropping purposes.  It may also
   decide to selectively hijack the PDUs of an MS-PW by directing the
   PDUs away from their destination.  In either case, the privacy of an
   MS-PW can be violated.

   Some types of PWs make assumptions about the security of the
   underlying PSN.  The minimal security provided by an MPLS PSN might
   not be sufficient to meet the security requirements expected by the
   applications using the MS-PW.  This document does not place any
   requirements on protecting the privacy of an MS-PW PDU via
   encryption.  However, encryption may be required at a higher layer in
   the protocol stack, based on the application or network requirements.

   The data plane of an S-PE at a domain boundary MUST be able to police
   incoming MS-PW traffic to the MS-PW traffic parameters or to an
   administratively configured profile.  The option to enable/disable
   policing MUST be provided to the network administrator.  This is to
   ensure that an MS-PW or a group of MS-PWs do not grab more resources
   than they are allocated.  In addition, the data plane of an S-PE MUST
   be able to police OAM messages to a pre-configured traffic profile or
   to filter out these messages upon administrative configuration.

Top      ToC       Page 23 
   An ingress S-PE MUST ensure that an MS-PW receives the CoS treatment
   configured or signaled for that MS-PW at the S-PE.  Specifically, an
   S-PE MUST guard against packets marked in the exp bits or IP-header
   Differentiated Services (DS) field (depending on the PSN) for a
   better CoS than they should receive.

   An ingress S-PE MUST be able to define per-interface or
   interface-group (a group may correspond to interfaces to a peer-
   provider) label space for MPLS-PWs.  An S-PE MUST be configurable not
   to accept labeled packets from another provider unless the bottom
   label is a PW-label assigned by the S-PE on the interface on which
   the packet arrived.

   Data plane security considerations for SS-PWs specified in [RFC3985]
   also apply to MS-PWs.

7.1.2.  Control-Plane Security Requirements

   An MS-PW connects two attachment circuits.  It is important to make
   sure that PW connections are not arbitrarily accepted from anywhere,
   or else a local attachment circuit might get connected to an
   arbitrary remote attachment circuit.  The fault in the connection can
   start at a remote T-PE or an S-PE.

   Where a PW segment crosses a border between one provider and another
   provider, the PW segment endpoints (S-PEs) SHOULD be on ASBRs
   interconnecting the two providers.  Directly interconnecting the
   S-PEs using a physically secure link, and enabling signaling and
   routing authentication between the S-PEs, eliminates the possibility
   of receiving an MS-PW signaling message or packet from an untrusted
   peer.  Other configurations are possible.  For example, P routers for
   the PSN tunnel between the adjacent S-PEs/T-PEs may reside on the
   ASBRs.  In which case, the S-PEs/T-PEs MUST satisfy themselves of the
   security and privacy of the path.

   The configuration and maintenance protocol MUST provide a strong
   authentication and control protocol data protection mechanism.  This
   option MUST be implemented, but it should be deployed according to
   the specific PSN environment requirements.  Furthermore,
   authentication using a signature for each individual MS-PW setup
   message MUST be available, in addition to an overall control protocol
   session authentication and message validation.

   Since S-PEs in different provider networks SHOULD reside at each end
   of a physically secure link, or be interconnected by a limited number
   of trusted PSN tunnels, each S-PE will have a trust relationship with
   only a limited number of S-PEs in other ASs.  Thus, it is expected
   that current security mechanisms based on manual key management will

Top      ToC       Page 24 
   be sufficient.  If deployment situations arise that require large
   scale connection to S-PEs in other ASs, then a mechanism based on RFC
   4107 [RFC4107] MUST be developed.

   Peer authentication protects against IP address spoofing but does not
   prevent one peer (S-PE or T-PE) from connecting to the wrong
   attachment circuit.  Under a single administrative authority, this
   may be the result of a misconfiguration.  When the MS-PW crosses
   multiple provider domains, this may be the result of a malicious act
   by a service provider or a security hole in that provider network.
   Static manual configuration of MS-PWs at S-PEs and T-PEs provides a
   greater degree of security.  If an identification of both ends of an
   MS-PW is configured and carried in the signaling message, an S-PE can
   verify the signaling message against the configuration.  To support
   dynamic signaling of MS-PWs, whereby only endpoints are provisioned
   and S-PEs are dynamically discovered, mechanisms SHOULD be provided
   to configure such information on a server and to use that information
   during a connection attempt for validation.

   An incoming MS-PW request/reply MUST NOT be accepted unless its IP
   source address is known to be the source of an "eligible" peer.  An
   eligible peer is an S-PE or a T-PE with which the originating S-PE or
   T-PE has a trust relationship.  The number of such trusted T-PEs or
   S-PEs is bounded and not anticipated to create a scaling issue for
   the control plane authentication mechanisms.

   If a peering adjacency has to be established prior to exchanging
   setup requests/responses, peering MUST only be done with eligible
   peers.  The set of eligible peers could be pre-configured (either as
   a list of IP addresses, or as a list of address/mask combinations) or
   automatically generated from the local PW configuration information.

   Furthermore, the restriction of peering sessions to specific
   interfaces MUST also be provided.  The S-PE and T-PE MUST drop the
   unaccepted signaling messages in the data path to avoid a
   Denial-of-Service (DoS) attack on the control plane.

   Even if a connection request appears to come from an eligible peer,
   its source address may have been spoofed.  Thus, means of preventing
   source address spoofing must be in place.  For example, if eligible
   peers are in the same network, source address filtering at the border
   routers of that network could eliminate the possibility of source
   address spoofing.

Top      ToC       Page 25 
   S-PEs that connect one provider domain to another provider domain
   MUST have the capability to rate-limit signaling traffic in order to
   prevent DoS attacks on the control plane.  Furthermore, detection and
   disposition of malformed packets and defense against various forms of
   attacks that can be protocol-specific MUST be provided.

7.2.  Intra-Provider MS-PWs

   Security requirements for pseudowires are provided in [RFC3916].
   These requirements also apply to MS-PWs.

   MS-PWs are intended to enable many more PEs to provide PWE3 services
   in a given service provider network than traditional SS-PWs,
   particularly in access and metro environments where the PE may be
   situated closer to the ultimate endpoint of the service.  In order to
   limit the impact of a compromise of one T-PE in a service provider
   network, the data path security requirements for inter-provider
   MS-PWs also apply to intra-provider MS-PWs in such cases.

8.  Acknowledgments

   The editors gratefully acknowledge the following contributors:
   Dimitri Papadimitriou (Alcatel-Lucent), Peter Busschbach
   (Alcatel-Lucent), Sasha Vainshtein (Axerra), Richard Spencer (British
   Telecom), Simon Delord (France Telecom), Deborah Brungard (AT&T),
   David McDysan (Verizon), Rahul Aggarwal (Juniper), Du Ke (ZTE),
   Cagatay Buyukkoc (ZTE), and Stewart Bryant (Cisco).

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3916]  Xiao, X., Ed., McPherson, D., Ed., and P. Pate, Ed.,
              "Requirements for Pseudo-Wire Emulation Edge-to-Edge
              (PWE3)", RFC 3916, September 2004.

   [RFC3985]  Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.

9.2.  Informative References

   [i610]     Recommendation I.610 "B-ISDN operation and maintenance
              principles and functions", February 1999.

Top      ToC       Page 26 
   [RFC5085]  Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
              Virtual Circuit Connectivity Verification (VCCV): A
              Control Channel for Pseudowires", RFC 5085, December 2007.

   [RFC4447]  Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
              G. Heron, "Pseudowire Setup and Maintenance Using the
              Label Distribution Protocol (LDP)", RFC 4447, April 2006.

   [RFC4111]  Fang, L., Ed., "Security Framework for Provider-
              Provisioned Virtual Private Networks (PPVPNs)", RFC 4111,
              July 2005.

   [PWE3-OAM] Nadeau, T., Ed., Morrow, M., Ed., Busschbach, P., Ed.,
              Alissaoui, M.,Ed., D. Allen, Ed., "Pseudo Wire (PW) OAM
              Message Mapping", Work in Progress, March 2005.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41, RFC
              2914, September 2000.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

Authors' Addresses

   Nabil Bitar
   117 West Street
   Waltham, MA 02145

   Matthew Bocci
   Alcatel-Lucent Telecom Ltd,
   Voyager Place
   Shoppenhangers Road
   Berks, UK

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO, 80112

Top      ToC       Page 27 
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