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Versions: 00 01 02 03 04 05 06 07 RFC 5659

Network Working Group                                         M Bocci
Internet Draft                                                Alcatel

                                                             S.Bryant
                                                         Cisco Systems

Expires: December 2007                                    June 6, 2007


    An Architecture for Multi-Segment Pseudo Wire Emulation Edge-to-Edge


                     draft-ietf-pwe3-ms-pw-arch-03.txt




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   This Internet-Draft will expire on December 6, 2007.







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Copyright Notice

   Copyright (C) The IETF Trust (2007).  All Rights Reserved.

Abstract

   This document describes an architecture for extending pseudowire
   emulation across multiple packet switched network segments. Scenarios
   are discussed where each segment of a given edge-to-edge emulated
   service spans a different provider's PSN, and where the emulated
   service originates and terminates on the same providers PSN, but may
   pass through several PSN tunnel segments in that PSN. It presents an
   architectural framework for such multi-segment pseudowires, defines
   terminology, and specifies the various protocol elements and their
   functions.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [1].

Table of Contents


   1. Introduction................................................3
      1.1. Motivation.............................................3
      1.2. Non-Goals of this Document..............................6
      1.3. Terminology............................................6
   2. Applicability...............................................7
   3. Protocol Layering model......................................7
      3.1. Domain of Multi-Segment PWE3............................8
      3.2. Payload Types..........................................8
   4. Multi-Segment PWE3 Reference Model...........................8
      4.1. Intra-Provider Architecture............................10
         4.1.1. Intra-Provider Switching Using ACs................10
         4.1.2. Intra-Provider Switching Using PWs................10
      4.2. Inter-Provider Architecture............................11
         4.2.1. Inter-Provider Switching Using ACs................11
         4.2.2. Inter-Provider Switching Using PWs................11
   5. PE Reference Model.........................................12
      5.1. PWE3 Pre-processing....................................12
         5.1.1. Forwarding........................................12
         5.1.2. Native Service Processing.........................12
   6. Protocol Stack reference Model..............................12
   7. Maintenance Reference Model.................................13
   8. PW Demultiplexer Layer and PSN Requirements.................14


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      8.1. Multiplexing..........................................14
      8.2. Fragmentation.........................................15
   9. Control Plane..............................................15
      9.1. Setup or Teardown of Pseudowires.......................15
      9.2. Pseudowire Up/Down Notification........................15
      9.3. Misconnection and Payload Type Mismatch................16
   10. Management and Monitoring..................................16
   11. Congestion Considerations..................................17
   12. IANA Considerations........................................17
   13. Security Considerations....................................17
   14. Acknowledgments...........................................18
   15. References................................................19
      15.1. Normative References..................................19
   Author's Addresses............................................19
   Intellectual Property Statement................................19
   Disclaimer of Validity........................................20
   Copyright Statement...........................................20
   Acknowledgment................................................20

1. Introduction

   RFC 3985 [2] defines the architecture for pseudowires, where a
   pseudowire (PW) both originates and terminates on the edge of the
   same packet switched network (PSN). The PW passes through a maximum
   of one PSN tunnel between the originating and terminating PEs.

   This document extends the architecture in RFC 3985 to enable
   pseudowires to be extended through multiple PSN tunnels. Use cases
   for multi-segment pseudowires, and the consequent requirements, are
   defined in [3].

1.1. Motivation

   Pseudowire Emulation Edge-to-Edge (PWE3) aims to provide point-to-
   point connectivity between two edges of a provider network.
   Requirements for Multi-Segment Pseudowires for this are specified in
   [3]. These requirements address three main problems:

   o How to constrain the density of the mesh of PSN tunnels when the
      number of PEs grows to many hundreds or thousands, while
      minimizing the complexity of the PEs and P routers.

   o How to provide PWE3 across multiple PSN routing domains or areas
      in the same provider.

   o How to provide PWE3 across multiple provider domains, and
      different PSN types.


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   Consider a single PWE3 domain, such as that shown in Figure 1. There
   are 4 PEs, and PWE3 must be provided from any PE to any other PE.
   Traditionally, this would be achieved by establishing a full mesh of
   PSN tunnels between the PEs. This would also require a full mesh of
   LDP signaling adjacencies between the PEs. Pseudowires could then be
   established between any PE and any other PE via a single, direct
   tunnel. PEs must terminate all pseudowires that are carried on PSN
   tunnels that terminate on that PE according to the architecture of
   RFC 3985. This solution is adequate for small numbers of PEs, but the
   number of PEs, PSN tunnels and signaling adjacencies will grow in
   proportion to the square of the number of PEs.

   A more efficient solution for large numbers of PEs would be to
   support a partial mesh of PSN tunnels between the PEs, as shown in
   Figure 1. For example, consider a PWE3 service whose endpoints are
   PE1 and PE4. Pseudowires for this can take the path PE1->PE2->PE4,
   and rather than terminating at PE2, be switched between ingress and
   egress PSN tunnels on that PE. This requires a capability in PE2 that
   can concatenate PW segments PE1-PE2 to PW segments PE2-PE4. The end-
   to-end PW is known as a multi-segment PW.

                                ,,..--..,,_
                            .-``           `'.,
                    +-----+`                   '+-----+
                    | PE1 |---------------------| PE2 |
                    |     |---------------------|     |
                    +-----+      PSN Tunnel     +-----+
                    / ||                          || \
                   /  ||                          ||  \
                  |   ||                          ||   |
                  |   ||         PSN              ||   |
                  |   ||                          ||   |
                   \  ||                          ||  /
                    \ ||                          || /
                     \||                          ||/
                    +-----+                     +-----+
                    | PE3 |---------------------| PE4 |
                    |     |---------------------|     |
                    +-----+`'.,_           ,.'` +-----+
                                `'''---''``
         Figure 1 Single PSN PWE3 with Partial Mesh of PSN Tunnels

   Figure 1 shows a simple flat PSN topology. However, large provider
   networks are typically not flat, consisting of many domains that are
   connected together to provide edge-to-edge services. The elements in
   each domain are specialized for a particular role.



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   An example application is shown in Figure 2. Here, the provider's
   network is divided into three domains: Two access domains and the
   core domain. The access domains represent the edge of the provider's
   network at which services are delivered. In the access domain,
   simplicity is required in order to minimize the cost of the network.
   The core domain must support all of the aggregated services from the
   access domains, and the design requirements here are for scalability,
   performance, and information hiding (i.e. minimal state). The core
   must not be exposed to the state associated with large numbers of
   individual edge-to-edge flows. That is, the core must be simple and
   fast.

   In a traditional layer 2 network, the interconnection points between
   the domains are where services in the access domains are aggregated
   for transport across the core to other access domains. In an IP
   network, the interconnection points would also represent interworking
   points between different types of IP networks e.g. those with MPLS
   and those without, and also points where network policies can be
   applied.

         <-------- Edge to Edge Emulated Services ------->

             ,'    .      ,-`       `',       ,'    .
            /       \   .`             `,    /       \
           /        \  /                 ,  /        \
    AC  +----+     +----+               +----+       +----+    AC
     ---| PE |-----| PE |---------------| PE |-------| PE |---
        |  1 |     |  2 |               | 3  |       | 4  |
        +----+     +----+               +----+       +----+
           \        /  \                 /  \        /
            \       /  \      Core       `   \       /
             `,    `     .             ,`     `,    `
               '-'`       `.,       _.`         '-'`
            Access 1         `''-''`         Access 2

                    Figure 2 Multi-Domain Network Model

   This model can also be applied to inter-provider services, where they
   also rely on a number of separate provider networks to be connected
   together.

   Consider the application of this model to PWE3. PWE3 uses tunneling
   mechanisms such as MPLS to enable the underlying PSN to emulate
   characteristics of the native service. One solution to the multi-
   domain network model above is to extend PSN tunnels edge-to-edge
   between all of the PEs in access domain 1 and all of the PEs in
   access domain 2, but this requires a large number of PSN tunnels as


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   described above, and also exposes the access and the core of the
   network to undesirable complexity. An alternative is to constrain the
   complexity to the network domain interconnection points (PE2 and PE3
   in the example above). Pseudowires between PE1 and PE4 would then be
   switched between PSN tunnels at the interconnection points, enabling
   PWs from many PEs in the access domains to be aggregated across only
   a few PSN tunnels in the core of the network. PEs in the access
   domains would only need to maintain direct signaling sessions, and
   PSN tunnels, with other PEs in their own domain, thus minimizing
   complexity of the access domains.

1.2. Non-Goals of this Document

   The following are non-goals for this document:

   o The on-the-wire specification of PW encapsulations

   o The detailed specification of mechanisms for establishing and
      maintaining multi-segment pseudo-wires.

1.3. Terminology

   The terminology specified in RFC 3985 applies. In addition, we define
   the following terms:

   o 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 a MS-
      PW. This incorporates the functionality of a PE as defined in RFC
      3985.

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

   o 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 a MS-PW by
      definition MUST terminate on a T-PE.

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







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   o PW Switching Provider Edge (S-PE).  A PE capable of switching the
      control and data planes of the preceding and succeeding PW
      segments in a MS-PW. The S-PE terminates the PSN tunnels of the
      preceding and succeeding segments of the MS-PW. It is therefore a
      PW switching point for a 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 setup and manage PW segments with other PW
      switching points and terminating PEs.

2. Applicability

   A MS-PW is a single PW that for technical or administrative reasons
   is segmented into a number of concatenated hops. From the
   perspective of a L2VPN, a MS-PW is indistinguishable from a SS-PW.
   Thus, the following are equivalent from the perspective of the T-PE

       +----+                                                  +----+
       |TPE1+--------------------------------------------------+TPE2|
       +----+                                                  +----+

       |<---------------------------PW----------------------------->|

       +----+              +---+           +---+               +----+
       |TPE1+--------------+SPE+-----------+SPE+---------------+TPE2|
       +----+              +---+           +---+               +----+


                      Figure 3     MS-PW Equivalence

   Although a MS-PW may require services such as node discovery and path
   signaling to construct the PW, it should not be confused with a L2VPN
   system, which also requires these services. A VPWS connects its
   endpoints via a set of PWs. MS-PW is a mechanism that abstracts the
   construction of complex PWs from the construction of a L2VPN. Thus a
   T-PE might be an edge device optimized for simplicity and an S-PE
   might be an aggregation device designed to absorb the complexity of
   continuing the PW across the core of one or more service provider
   networks to another T-PE located at the edge of the network.

3. Protocol Layering model

   The protocol-layering model specified in RFC 3985 applies to multi-
   segment PWE3 with the following clarification: the pseudowires may be
   considered to be a separate layer to the PSN tunnel. That is, they
   are independent of the PSN tunnel routing, operations, signaling and
   maintenance. The design of PW routing domains should not imply that
   the underlying PSN routing domains are the same. However, MS-PW will


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   reuse the protocols of the PSN and may use information that is
   extracted from the PSN e.g. reachability.

3.1. Domain of Multi-Segment PWE3

   PWE3 defines the Encapsulation Layer, i.e. the method of carrying
   various payload types, and the interface to the PW Demultiplexer
   Layer. It is expected that other layers will provide the following:

      . PSN tunnel setup, maintenance and routing

      . T-PE discovery

   It is assumed that any node that is reachable via a PSN tunnel from
   an S-PE or T-PE is a PE, a subset of which may be capable of behaving
   as an S-PE. The selection of which S-PEs to use to reach a T-PE is
   considered to be within the domain of PWE3.

3.2. Payload Types

   Multi-segment PWE3 is applicable to all PWE3 payload types.
   Encapsulations defined for SS-PWs are also used for MS-PW without
   change. If different segments run over different PSN types, the
   encapsulation may change but the S-PE must not need an NSP. It is
   recommended that a list of compatible PWE3 encapsulations that do not
   need an NSP be published. Translations between segments must not
   require processing of the pseudowire payload.

4. Multi-Segment PWE3 Reference Model

   The PWE3 reference architecture for the single segment case is shown
   in [2]. 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.














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       Native  |<------Multi-Segment Pseudowire------>|  Native
       Service |         PSN              PSN         |  Service
        (AC)   |     |<-Tunnel->|     |<-Tunnel->|    |   (AC)
          |    V     V     1    V     V    2     V    V     |
          |    +----+           +-----+          +----+     |
   +----+ |    |TPE1|===========|SPE1 |==========|TPE2|     | +----+
   |    |------|..... PW.Seg't1.........PW.Seg't3.....|-------|    |
   | CE1| |    |    |           |     |          |    |     | |CE2 |
   |    |------|..... PW.Seg't2.........PW.Seg't4.....|-------|    |
   +----+ |    |    |===========|     |==========|    |     | +----+
        ^      +----+           +-----+          +----+       ^
        |   Provider Edge 1        ^        Provider Edge 2   |
        |                          |                          |
        |                          |                          |
        |                    PW switching point               |
        |                                                     |
        |<------------------ Emulated Service --------------->|

                   Figure 4 PW switching Reference Model

   Figure 4 extends this architecture to show a multi-segment case. The
   PEs that provide PWE3 to CE1 and CE2 are Terminating-PE1 (T-PE1) and
   Terminating-PE2 (T-PE2) respectively. A PSN tunnel extends from T-PE1
   to switching-PE1 (S-PE1) across PSN1, and a second PSN tunnel extends
   from S-PE1 to S-PE2 across PSN2. PWs are used to connect the
   attachment circuits (ACs) attached to PE1 to the corresponding ACs
   attached to PE3.

   Each PW segment on the tunnel across PSN1 is switched to a PW segment
   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. PW segment 1 and PW segment 3 are segments of the same MS-PW
   while PW segment 2 and PW segment 4 are segments of another MS-PW. PW
   segments of the same MS-PW (e.g., PW1 and PW3) MAY be of the same PW
   type or different type, and PSN tunnels (e.g., PSN1 and PSN2) can be
   the same or different technology. This document requires support for
   MS-PWs with segments of the same type. 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).

   Note that although Figure 4 only shows a single S-PE, a PW may
   transit more one S-PE along its path. This architecture is applicable
   when the S-PEs are statically chosen, or when they are chosen using a
   dynamic path selection mechanism. Both directions of an MS-PW must
   traverse the same set of S-PEs. Note that although the S-PE path is


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   therefore reciprocal, the path taken by the PSN tunnels between the
   T-PEs and S-PEs may not be reciprocal due to choices made by the PSN
   routing protocol.

4.1. Intra-Provider Architecture

   There is a requirement to deploy PWs edge-to-edge in large service
   provider networks [3]. Such networks typically encompass hundreds or
   thousands of aggregation devices at the edge, each of which would be
   a PE. These networks may be partitioned into separate metro and core
   PWE3 domains, where the PEs are interconnected by a sparse mesh of
   tunnels.

   Whether or not the network is partitioned into separate PWE3 domains,
   there is also a requirement to support a partial mesh of traffic
   engineered PSN tunnels.

   The architecture shown in Figure 4 can be used to support such cases.
   PSN1 and PSN2 may be in different administrative domains or access,
   core or metro regions within the same providers network. PSN 1 and
   PSN 2 may also be of different types. For example, S-PEs may be used
   to connect PW segments traversing metro networks of one technology
   e.g. statically allocated labels, with segments traversing a MPLS
   core network.

   Alternatively, TPE1, SPE1 and TPE2 may reside at the edges of the
   same PSN.

4.1.1. Intra-Provider Switching Using ACs

   In this model, the PW reverts to the native service AC at the PE.
   This AC is then connected to a separate PW on the same PE. In this
   case, the reference models of RFC 3985 apply to each segment and to
   the PEs. The remaining PE architectural considerations in this
   document do not apply to this case.



4.1.2. Intra-Provider Switching Using PWs

   In this model, PW segments are switched between PSN tunnels that span
   portions of a provider's network, without reverting to the native
   service at the boundary. For example, in Figure 4, PSN 1 and PSN 2
   would be portions of the same provider's network.





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4.2. Inter-Provider Architecture

   Intra-provider PWs may need to be switched between PSN tunnels at the
   provider boundary in order to minimize the number of tunnels required
   to provide PWE3 services to CEs attached to each providers network.
   In addition, AAA and security mechanisms may need to be implemented
   on a per-PW basis at the provider boundary.



4.2.1. Inter-Provider Switching Using ACs.

   In this model, the PW reverts to the native service at the provider
   boundary PE. This AC is then connected to a separate PW at the peer
   provider boundary PE. In this case, the reference models of RFC 3985
   apply to each segment and to the PEs. The remaining PE architectural
   considerations in this document do not apply to this case.

4.2.2. Inter-Provider Switching Using PWs.

   In this model, PW segments are switched between PSN tunnels in each
   provider's network, without reverting to the native service at the
   boundary. This architecture is shown in Figure 5. Here, S-PE1 and S-
   PE2 are provider border routers. PW segment 1 is switched to PW
   segment 2 at S-PE1. PW segment 2 is then carried across an inter-
   provider PSN tunnel to S-PE2, where it is switched to PW segment 3 in
   PSN 2.

                |<------Multi-Segment Pseudowire------>|
                |       Provider         Provider      |
           AC   |    |<----1---->|     |<----2--->|    |  AC
            |   V    V           V     V          V    V  |
            |   +----+     +-----+     +----+     +----+  |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
   |    |-------|......PW..........PW.........PW.......|-------|    |
   | CE1|   |   |    |Seg 1|     |Seg 2|    |Seg 3|    |  |    |CE2 |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
        ^       +----+     +-----+     +----+     +----+       ^
        |       T-PE1       S-PE1       S-PE2     T-PE2        |
        |                     ^          ^                     |
        |                     |          |                     |
        |                  PW switching points                 |
        |                                                      |
        |                                                      |
        |<------------------- Emulated Service --------------->|

                  Figure 5 Inter-Provider Reference Model


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5. PE Reference Model

5.1. PWE3 Pre-processing

   PWE3 preprocessing is applied in the T-PEs as specified in RFC 3985.
   Processing at the S-PEs is specified in the following sections.

5.1.1. Forwarding

   Each forwarder in the S-PE forwards packets from one PW segment on
   the ingress PSN facing interface of the S-PE to one PW segment on the
   egress PSN facing interface of the S-PE.

   The forwarder selects the egress segment PW based on the ingress PW
   label. The mapping of ingress to egress PW label may be statically or
   dynamically configured. Figure 6 shows how a single forwarder is
   associated with each PW segment at the S-PE.

               +------------------------------------------+
               |                S-PE Device               |
               +------------------------------------------+
     Ingress   |             |             |              |   Egress
   PW instance |   Single    |             |    Single    | PW Instance
   <==========>X PW Instance +  Forwarder  + PW Instance  X<==========>
               |             |             |              |
               +------------------------------------------+

                      Figure 6 Point-to-Point Service

   Other mappings of PW to forwarder are for further study.

5.1.2. Native Service Processing

   There is no native service processing in the S-PEs.

6. Protocol Stack reference Model

   Figure 7 illustrates the protocol stack reference model for multi-
   segment PWs.









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+----------------+                                  +----------------+
|Emulated Service|                                  |Emulated Service|
|(e.g., TDM, ATM)|<======= Emulated Service =======>|(e.g., TDM, ATM)|
+----------------+                                  +----------------+
|    Payload     |                                  |    Payload     |
|  Encapsulation |<=== Multi-segment Pseudowire ===>|  Encapsulation |
+----------------+            +--------+            +----------------+
|PW Demultiplexer|<PW Segment>|PW Demux|<PW Segment>|PW Demultiplexer|
+----------------+            +--------+            +----------------+
|   PSN Tunnel,  |<PSN Tunnel>|  PSN   |<PSN Tunnel>|  PSN Tunnel,   |
| PSN & Physical |            |Physical|            | PSN & Physical |
|     Layers     |            | Layers |            |    Layers      |
+-------+--------+            +--------+            +----------------+
        |            ..........   |   ..........            |
        |           /          \  |  /          \           |
        +==========/    PSN     \===/    PSN     \==========+
                   \  domain 1  /   \  domain 2  /
                    \__________/     \__________/
                     ``````````       ``````````

                 Figure 7 Multi-Segment PW Protocol Stack

   The MS-PW provides the CE with an emulated physical or virtual
   connection to its peer at the far end. Native service PDUs from the
   CE are passed through an Encapsulation Layer and a PW demultiplexer
   is added at the sending T-PE. The PDU is sent over PSN domain 1. The
   receiving S-PE removes the existing PW demultiplexer, adds a new
   demultiplexer, and then sends the PDU over PSN2. Policies may also be
   applied to the PW at this point. Examples of such policies include:
   admission control, rate control, QoS mappings, and security. The
   receiving T-PE removes the PW demultiplexer and restores the payload
   to its native format for transmission to the destination CE.

   Where the encapsulation format is different e.g. MPLS and L2TPv3, the
   payload encapsulation may be transparently translated at the S-PE.

7. Maintenance Reference Model

   Figure 8 shows the maintenance reference model for multi-segment
   PWE3.









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         |<------------- CE (end-to-end) Signaling ------------>|
         |                                                      |
         |       |<-------- MS-PW/T-PE Maintenance ----->|      |
         |       |  |<---PW Seg't-->| |<--PW Seg't--->|  |      |
         |       |  |   Maintenance | | Maintenance   |  |      |
         |       |  |               | |               |  |      |
         |       |  |     PSN       | |     PSN       |  |      |
         |       |  | |<-Tunnel1->| | | |<-Tunnel2->| |  |      |
         |       V  V V Signaling V V V V Signaling V V  V      |
         V       +----+           +-----+           +----+      V
    +----+       |TPE1|===========|SPE1 |===========|TPE2|      +----+
    |    |-------|......PW.Seg't1.........PW Seg't3......|------|    |
    | CE1|       |    |           |     |           |    |      |CE2 |
    |    |-------|......PW.Seg't2.........PW Seg't4......|------|    |
    +----+       |    |===========|     |===========|    |      +----+
      ^          +----+           +-----+           +----+         ^
      |        Terminating           ^            Terminating      |
      |      Provider Edge 1         |          Provider Edge 2    |
      |                              |                             |
      |                      PW switching point                    |
      |                                                            |
      |<--------------------- Emulated Service ------------------->|

               Figure 8 MS-PWE3 Maintenance Reference Model

   RFC 3985 specifies the use of CE (end-to-end) and PSN tunnel
   signaling, and PW/PE maintenance. CE and PSN tunnel signaling is as
   specified in RFC 3985. However, in the case of MS-PWE3, signaling
   between the PEs now has both an edge-to-edge and a hop-by-hop
   context. That is, signaling and maintenance between T-PEs and S-PEs
   and between adjacent S-PEs is used to set up, maintain, and tear down
   the MS-PW segments, which include the coordination of parameters
   related to each switching point, as well as the MS-PW end points.

8. PW Demultiplexer Layer and PSN Requirements

8.1. Multiplexing

   The purpose of the PW demultiplexer layer at the S-PE is to
   demultiplex PWs from ingress PSN tunnels and to multiplex them into
   egress PSN tunnels. Although each PW may contain multiple native
   service circuits, e.g. multiple ATM VCs, the S-PEs do not have
   visibility of, and hence do not change, this level of multiplexing
   because they contain no NSP.





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8.2. Fragmentation

   An S-PE is not required to make any attempt to reassemble a
   fragmented PW payload. An S-PE may fragment a PW payload.



9. Control Plane

9.1. Setup or Teardown of Pseudowires

   For multi-segment pseudowires, the intermediate PW switching points
   may be statically provisioned, or they may be dynamically signaled.

   For the static case, there are two options for exchanging the PW
   labels:

   o By configuration at the T-PEs or S-PEs

   o By signaling across each segment using a dynamic maintenance
      protocol.

   A multi-segment pseudowire may thus consist of segments where the
   labels are statically configured, and segments where the labels are
   signaled.

   For the dynamic case, there are two options for selecting the path of
   the PW:

   o T-PEs determine the full path of the PW through intermediate
      switching points. This may be either static or based on a dynamic
      PW path selection mechanism.

   o Each segment of the PW path is determined locally by each T-PE or
      S-PE, either through static configuration or based on a dynamic PW
      path selection mechanism.

9.2. Pseudowire Up/Down Notification

   Since a multi-segment PW consists of a number of concatenated PW
   segments, the emulated service can only be considered as being up
   when all of the PW segments and PSN tunnels (if used) are functional
   along the entire path of the MS-PW.

   If a native service requires bi-directional connectivity, the
   corresponding emulated service can only be signaled as being up when



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   the PW segments and PSN tunnels (if used), are functional in both
   directions.

   RFC 3985 describes the need for failure and other status notification
   mechanisms for PWs. These considerations also apply to multi-segment
   pseudowires. In addition, if a failure notification mechanism is
   provided for consecutive segments of the same PW, the S-PE must be
   able to propagate such notifications between the consecutive
   concatenated segments.

9.3. Misconnection and Payload Type Mismatch

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

   The services of the underlying tunneling mechanism and its associated
   control protocol can be used to ensure that the identity of the PW
   next hop is as expected. As part of the PW setup, a PW-TYPE
   identifier is exchanged. This is then used by the forwarder and the
   NSP of the T-PEs to verify the compatibility of the ACs. This can
   also be used by S-PEs to ensure that concatenated segments of a given
   MS-PW are compatible, or that a MS-PW is not misconnected into a
   local AC. In addition, it is advisable to do an end-to-end connection
   verification to check the integrity of the PW and to verify the
   identity of the T-PE.

10. Management and Monitoring

   The management and monitoring as described in RFC 3985 apply here.

   The MS-PW architecture introduced two additional considerations
   related to management and monitoring.

   The first is that each S-PE is a new point at which defects may occur
   along the path of the PW. In order to troubleshoot MS-PWs, management
   and monitoring should be able to operate on a subset of the segments
   of an MS-PW, as well as edge-to-edge. That is, connectivity
   verification mechanisms should be able to troubleshoot the
   connectivity between T-PEs and intermediate S-PEs, as well as T-PE to
   T-PE.

   The second is that the set of S-PEs used by an MS-PW to reach a T-PE
   may not coincide with that which would be determined by the routing
   and path selection mechanisms in the underlying PSN. While the path
   taken between consecutive T/S-PEs on a given MS-PW will be determined


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   by the path of the PSN tunnel, the set of T/S-PEs that are used may
   be chosen by configuration or by a dynamic MS-PW path selection
   mechanism that operates independently of the underlying PSN.
   Troubleshooting mechanisms should therefore be provided to verify the
   set of S-PEs that are traversed by a MS-PW to reach a T-PE.

11. Congestion Considerations

   The following congestion considerations apply to MS-PWs. These are in
   addition to the considerations for PWs described in RFC 3985 [2] and
   in the respective RFCs specifying each PW type.

   The control plane and the data plane fate-share in traditional IP
   networks. The implication of this is that congestion in the data
   plane can cause degradation of the operation of the control plane.
   Under quiescent operating conditions it is expected that the network
   will be designed to avoid such problems. However, MS-PW mechanisms
   should also consider what happens when congestion does occur, when
   the network is stretched beyond its design limits, for example during
   unexpected network failure conditions.

   In addition to protecting the operation of the underlying PSN,
   consistent QoS and traffic engineering mechanisms should be used on
   each segment of a MS-PW to support the requirements of the emulated
   service. The QoS treatment given to a PW packet at an S-PE may be
   derived from context information of the PW (e.g. traffic or QoS
   parameters signaled to the S-PE by an MS-PW control protocol), or
   from PSN-specific QoS flags in the PSN tunnel label or PW
   demultiplexer e.g. EXP bits for an MPLS PSN or the DS field of the
   outer IP header for L2TPv3.

12. IANA Considerations

   This document does not contain any IANA actions.

13. Security Considerations

   The security considerations described in RFC-3985 apply here.

   Additional consideration needs to be given to the security of the S-
   PEs, particularly when these are dynamically selected and/or when the
   MS-PW transits the networks of multiple operators.

   When the MS-PW is dynamically created by the use of a signaling
   protocol, an S-PE SHOULD determine the authenticity of the peer
   entity from which it receives the request, and its compliance with
   policy.


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   Particular consideration needs to be given to Quality of Service
   requests because the inappropriate use of priority may impact other
   service guarantees. Consideration also needs to be given to the
   avoidance of spoofing the PW demultiplexer.

   Where an S-PE provides interconnection between different providers,
   similar considerations to those applied to ASBRs apply. In particular
   peer entity authentication SHOULD be used.

   Where an S-PE also supports T-PE functionality, mechanisms should be
   provided to ensure that MS-PWs to switched correctly to the
   appropriate outgoing PW segment, rather than a local AC. Other
   mechanisms for PW end point verification may also be used to confirm
   the correct PW connection prior to enabling the attachment circuits.

14. Acknowledgments

   The authors gratefully acknowledge the input of Mustapha Aissaoui,
   Dimitri Papadimitrou, Sasha Vainshtein, and Luca Martini.





























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15. References

15.1. Normative References

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

   [2]  Bryant, S. and Pate, P. (Editors), ''Pseudo Wire Emulation Edge-
         to-Edge (PWE3) Architecture'', RFC 3985, March 2005

   [3]  Martini, L. Bitar, N. and Bocci, M (Editors), ''Requirements for
         inter domain Pseudo-Wires'', draft-ietf-pwe3-ms-pw-requirements-
         05.txt, Internet Draft, March 2007



Author's Addresses

   Matthew Bocci
   Alcatel
   Voyager Place,
   Shoppenhangers Rd,
   Maidenhead, Berks, UK    Email: matthew.bocci@alcatel.co.uk


   Stewart Bryant
   Cisco Systems,
   250, Longwater,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: stbryant@cisco.com


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