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An Architecture for Multi-Segment Pseudowire Emulation Edge-to-Edge
RFC 5659

Document Type RFC - Informational (October 2009) Errata
Authors Stewart Bryant , Matthew Bocci
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Ralph Droms
Send notices to (None)
RFC 5659
Network Working Group                                           M. Bocci
Request for Comments: 5659                                Alcatel-Lucent
Category: Informational                                        S. Bryant
                                                           Cisco Systems
                                                            October 2009

 An Architecture for Multi-Segment Pseudowire Emulation Edge-to-Edge


   This document describes an architecture for extending pseudowire
   emulation across multiple packet switched network (PSN) segments.
   Scenarios are discussed where each segment of a given edge-to-edge
   emulated service spans a different provider's PSN, as are other
   scenarios where the emulated service originates and terminates on the
   same provider's 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.

Status of This Memo

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

Copyright and License Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Motivation and Context .....................................3
      1.2. Non-Goals of This Document .................................6
      1.3. Terminology ................................................6
   2. Applicability ...................................................8
   3. Protocol Layering Model .........................................8
      3.1. Domain of MS-PW Solutions ..................................9
      3.2. Payload Types ..............................................9
   4. Multi-Segment Pseudowire Reference Model ........................9
      4.1. Intra-Provider Connectivity Architecture ..................11
           4.1.1. Intra-Provider Switching Using ACs .................11
           4.1.2. Intra-Provider Switching Using PWs .................11
      4.2. Inter-Provider Connectivity Architecture ..................11
           4.2.1. Inter-Provider Switching Using ACs .................12
           4.2.2. Inter-Provider Switching Using PWs .................12
   5. PE Reference Model .............................................13
      5.1. Pseudowire Pre-Processing .................................13
           5.1.1. Forwarding .........................................13
           5.1.2. Native Service Processing ..........................14
   6. Protocol Stack Reference Model .................................14
   7. Maintenance Reference Model ....................................15
   8. PW Demultiplexer Layer and PSN Requirements ....................16
      8.1. Multiplexing ..............................................16
      8.2. Fragmentation .............................................17
   9. Control Plane ..................................................17
      9.1. Setup and Placement of MS-PWs .............................17
      9.2. Pseudowire Up/Down Notification ...........................18
      9.3. Misconnection and Payload Type Mismatch ...................18
   10. Management and Monitoring .....................................18
   11. Congestion Considerations .....................................19
   12. Security Considerations .......................................20
   13. Acknowledgments ...............................................23
   14. References ....................................................23
      14.1. Normative References .....................................23
      14.2. Informative References ...................................23

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1.  Introduction

   RFC 3985 [1] 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 label is unchanged
   between the originating and terminating provider edges (PEs).  This
   is now known as a single-segment pseudowire (SS-PW).

   This document extends the architecture in RFC 3985 to enable point-
   to-point pseudowires to be extended through multiple PSN tunnels.
   These are known as multi-segment pseudowires (MS-PWs).  Use cases for
   multi-segment pseudowires (MS-PWs), and the consequent requirements,
   are defined in RFC 5254 [5].

1.1.  Motivation and Context

   RFC 3985 addresses the case where a PW spans a single segment between
   two PEs.  Such PWs are termed single-segment pseudowires (SS-PWs) and
   provide point-to-point connectivity between two edges of a provider
   network.  However, there is now a requirement to be able to construct
   multi-segment pseudowires.  These requirements are specified in RFC
   5254 [5] and address three main problems:

   i.   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.

   ii.  How to provide PWs across multiple PSN routing domains or areas
        in the same provider.

   iii. How to provide PWs across multiple provider domains and
        different PSN types.

   Consider a single PW domain, such as that shown in Figure 1.  There
   are 4 PEs, and PWs must be provided from any PE to any other PE.
   PWs can be supported by establishing a full mesh of PSN tunnels
   between the PEs, requiring a full mesh of LDP signaling adjacencies
   between the PEs.  PWs can therefore be established between any PE and
   any other PE via a single, direct PSN tunnel that is switched only by
   intermediate P-routers (not shown in the figure).  In this case, each
   PW is an SS-PW.  A PE must terminate all the pseudowires that are
   carried on the 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

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   For reasons of economy, the edge PEs that terminate the attachment
   circuits (ACs) are often small devices built to very low cost with
   limited processing power.  Consider an example where a particular PE,
   residing at the edge of a provider network, terminates N PWs to/from
   N different remote PEs.  This needs N PW signaling adjacencies to be
   set up and maintained.  If the edge PE attaches to a single
   intermediate PE that is able to switch the PW, that edge PE only
   needs a single adjacency to signal and maintain all N PWs.  The
   intermediate switching PE (which is a larger device) needs M
   signaling adjacencies, but statistically this is less than tN, where
   t is the number of edge PEs that it is serving.  Similarly, if the
   PWs are running over TE PSN tunnels, there is a statistical reduction
   in the number of TE PSN tunnels that need to be set up and maintained
   between the various PEs.

   One possible solution that is more efficient for large numbers of
   PEs, in particular for the control plane, is therefore to support a
   partial mesh of PSN tunnels between the PEs, as shown in Figure 1.
   For example, consider a PW 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: PWs Spanning a Single PSN with Partial Mesh of PSN Tunnels

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   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, for example,
   supporting different PSN types or using different routing protocols.

   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

   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 could also represent interworking
   points between different types of IP networks, e.g., those with MPLS
   and those without, and 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

   A similar model can also be applied to inter-provider services, where
   a single PW spans a number of separate provider networks in order to
   connect ACs residing on PEs in disparate provider networks.  In this
   case, each provider will typically maintain their own PE at the
   border of their network in order to apply policies such as security

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   and Quality of Service (QoS) to PWs entering their network.  Thus,
   the connection between the domains will normally be a link between
   two PEs on the border of each provider's network.

   Consider the application of this model to PWs.  PWs use 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
   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 pseudowires.

1.3.  Terminology

   The terminology specified in RFC 3985 [1] and RFC 4026 [2] 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 an MS-
     PW.  This incorporates the functionality of a PE as defined in RFC

   o Single-Segment Pseudowire (SS-PW).  A PW set up directly between
     two T-PE devices.  The PW label is unchanged between the
     originating and terminating T-PEs.

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   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 an MS-PW,
     by definition, terminates on a T-PE.

   o PW Segment.  A part of a single-segment or multi-segment PW, which
     traverses one PSN tunnel in each direction between two PE devices,
     T-PEs, and/or S-PEs (switching PE).

   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 an MS-PW.  The S-PE terminates the PSN tunnels of the preceding
     and succeeding segments of the MS-PW.  It therefore includes 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.  An S-PE can exist anywhere a
     PW must be processed or policy applied.  It is therefore not
     limited to the edge of a provider network.

     Note that it was originally anticipated that S-PEs would only be
     deployed at the edge of a provider network where they would be used
     to switch the PWs of different service providers.  However, as the
     design of MS-PW progressed, other applications for MS-PW were
     recognized.  By this time S-PE had become the accepted term for the
     equipment, even though they were no longer universally deployed at
     the provider edge.

   o PW Switching.  The process of switching the control and data planes
     of the preceding and succeeding PW segments in a MS-PW.

   o PW Switching Point.  The reference point in an S-PE where the
     switching takes place, e.g., where PW label swap is executed.

   o Eligible S-PE or T-PE.  An eligible S-PE or T-PE is a PE that meets
     the security and privacy requirements of the MS-PW, according to
     the network operator's policy.

   o Trusted S-PE or T-PE.  A trusted S-PE or T-PE is a PE that is
     understood to be eligible by its next-hop S-PE or T-PE, while a
     trust relationship exists between two S-PEs or T-PEs if they
     mutually consider each other to be eligible.

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2.  Applicability

   An 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 Layer 2 Virtual Private Network (L2VPN), an MS-PW is
   indistinguishable from an SS-PW.  Thus, the following are equivalent
   from the perspective of the T-PE:

    +----+                                                  +----+
    +----+                                                  +----+


    +----+              +---+           +---+               +----+
    +----+              +---+           +---+               +----+

                       Figure 3: MS-PW Equivalence

   Although an MS-PW may require services such as node discovery and
   path signaling to construct the PW, it should not be confused with an
   L2VPN system, which also requires these services.  A Virtual Private
   Wire Service (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.

   As well as supporting traditional L2VPNs, an MS-PW is applicable to
   providing connectivity across a transport network based on packet
   switching technology, e.g., the MPLS Transport Profile (MPLS-TP) [6],
   [8].  Such a network uses pseudowires to support the transport and
   aggregation of all services.  This application requires deterministic
   characteristics and behavior from the network.  The operational
   requirements of such networks may need pseudowire segments that can
   be established and maintained in the absence of a control plane, and
   may also need the operational independence of PW maintenance from the
   underlying PSN.

3.  Protocol Layering Model

   The protocol layering model specified in RFC 3985 applies to MS-PWs
   with the following clarification: the pseudowires may be considered
   to be a separate layer to the PSN tunnel.  That is, although a PW
   segment will follow the path of the PSN tunnel between S-PEs, the

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   MS-PW is 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-PWs will reuse the protocols of the PSN and may, if
   applicable, use information that is extracted from the PSN, e.g.,

3.1.  Domain of MS-PW Solutions

   PWs provide the Encapsulation Layer, i.e., the method of carrying
   various payload types, and the interface to the PW Demultiplexer
   Layer.  Other layers provide the following:

      o PSN tunnel setup, maintenance, and routing

      o T-PE discovery

   Not all PEs may be capable of providing S-PE functionality.
   Connectivity to the next-hop S-PE or T-PE must be provided by a PSN
   tunnel, according to [1].  The selection of which set of S-PEs to use
   to reach a given T-PE is considered to be within the scope of MS-PW

3.2.  Payload Types

   MS-PWs are applicable to all PW payload types.  Encapsulations
   defined for SS-PWs are also used for MS-PW without change.  Where the
   PSN types for each segment of an MS-PW are identical, the PW types of
   each segment must also be identical.  However, if different segments
   run over different PSN types, the encapsulation may change but the PW
   segments must be of an equivalent PW type, i.e., the S-PE must not
   need to process the PW payload to provide translation.

4.  Multi-Segment Pseudowire Reference Model

   The pseudowire emulation edge-to-edge (PWE3) reference architecture
   for the single-segment case is shown in [1].  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....X....PW.Seg't3.....|-------|    |
   | CE1| |    |    |           |     |          |    |     | |CE2 |
   |    |------|..... PW.Seg't2....X....PW.Seg't4.....|-------|    |
   +----+ |    |    |===========|     |==========|    |     | +----+
        ^      +----+           +-----+          +----+       ^
        |   Provider Edge 1        ^        Provider Edge 2   |
        |                          |                          |
        |                          |                          |
        |                  PW switching point                 |
        |                                                     |
        |<------------------ Emulated Service --------------->|

                     Figure 4: MS-PW Reference Model

   Figure 4 extends this architecture to show a multi-segment case.  The
   PEs that provide services 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 T-PE2 across PSN2.  PWs are used to connect the
   attachment circuits (ACs) attached to PE1 to the corresponding ACs
   attached to T-PE2.

   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., PW segment 1 and PW segment 3)
   must be of equivalent PW types, as described in Section 3.2, while
   PSN tunnels (e.g., PSN1 and PSN2) may be of the same or different PSN
   types.  An S-PE switches an MS-PW from one segment to another based
   on the PW demultiplexer, i.e., a PW label that may take one of the
   forms defined in Section 5.4.1 of RFC 3985 [1].

   Note that although Figure 4 only shows a single S-PE, a PW may
   transit more than 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 on a reciprocal path.
   Note that although the S-PE path is therefore reciprocal, the path
   taken by the PSN tunnels between the T-PEs and S-PEs might not be
   reciprocal due to choices made by the PSN routing protocol.

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4.1.  Intra-Provider Connectivity Architecture

   There is a requirement to deploy PWs edge-to-edge in large service
   provider networks (RFC 5254 [5]).  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 PW domains, where the PEs are interconnected by a
   sparse mesh of tunnels.

   Whether or not the network is partitioned into separate PW 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
   regions, core regions, or metro regions within the same provider's
   network.  PSN1 and PSN2 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 an MPLS core network.

   Alternatively, T-PE1, S-PE1, and T-PE2 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 domain
   boundary 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, PSN1 and PSN2
   would be portions of the same provider's network.

4.2.  Inter-Provider Connectivity Architecture

   Inter-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 PW-based services to CEs attached to each provider's
   network.  In addition, the following may need to be implemented on a
   per-PW basis at the provider boundary:

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RFC 5659            Multi-Segment PWE3 Architecture         October 2009

      o Operations, Administration, and Maintenance (OAM).  Note that
        this is synonymous with 'Operations and Maintenance' referred to
        in RFC 5254 [5].

      o Authentication, Authorization, and Accounting (AAA)

      o Security mechanisms

   Further security-related architectural considerations are described
   in Section 12.

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.  This is similar to the case in
   Section 4.1.1, except that additional security and policy enforcement
   measures will be required.  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

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RFC 5659            Multi-Segment PWE3 Architecture         October 2009

                |<------Multi-Segment Pseudowire------>|
                |       Provider         Provider      |
           AC   |    |<----1---->|     |<----2--->|    |  AC
            |   V    V           V     V          V    V  |
            |   +----+     +-----+     +----+     +----+  |
   +----+   |   |    |=====|     |=====|    |=====|    |  |    +----+
   |    |-------|......PW.....X....PW.....X...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

5.  PE Reference Model

5.1.  Pseudowire Pre-Processing

   Pseudowire pre-processing is applied in the T-PEs as specified in RFC
   3985.  Processing at the S-PEs is specified in the following

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.

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               |                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.

   +-----------+                                  +-----------+
   |  Emulated |                                  |  Emulated |
   |  Service  |                                  |  Service  |
   |(e.g., ATM)|<======= Emulated Service =======>|(e.g., ATM)|
   +-----------+                                  +-----------+
   | Payload   |                                  | Payload   |
   |  Encap.   |<=== Multi-segment Pseudowire ===>|  Encap.   |
   +-----------+            +--------+            +-----------+
   | PW Demux  |<PW Segment>|PW Demux|<PW Segment>| PW Demux  |
   +-----------+            +--------+            +-----------+
   |PSN Tunnel,|<PSN Tunnel>|  PSN   |<PSN Tunnel>|PSN Tunnel,|
   | PSN & PHY |            |Physical|            | PSN & PHY |
   | 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

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   is added at the sending T-PE.  The PDU is sent over PSN domain via
   the PSN transport tunnel.  The receiving S-PE swaps the existing PW
   demultiplexer for the demultiplexer of the next segment and then
   sends the PDU over transport tunnel in PSN2.  Where the ingress and
   egress PSN domains of the S-PE are of the same type, e.g., they are
   both MPLS PSNs, a simple label swap operation is performed, as
   described in Section 3.13 of RFC 3031 [3].  However, where the
   ingress and egress PSNs are of different types, e.g., MPLS and
   L2TPv3, the ingress PW demultiplexer is removed (or popped), and a
   mapping to the egress PW demultiplexer is performed and then inserted
   (or pushed).

   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 translated at the S-PE.

7.  Maintenance Reference Model

   Figure 8 shows the maintenance reference model for multi-segment

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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....X....PW Seg't3......|------|    |
   | CE1|       |    |           |     |           |    |      |CE2 |
   |    |-------|......PW.Seg't2....X....PW Seg't4......|------|    |
   +----+       |    |===========|     |===========|    |      +----+
     ^          +----+           +-----+           +----+         ^
     |        Terminating           ^            Terminating      |
     |      Provider Edge 1         |          Provider Edge 2    |
     |                              |                             |
     |                      PW switching point                    |
     |                                                            |
     |<--------------------- Emulated Service ------------------->|

               Figure 8: MS-PW Maintenance Reference Model

   RFC 3985 specifies the use of CE (end-to-end) and PSN tunnel
   signaling as well as PW/PE maintenance.  CE and PSN tunnel signaling
   is as specified in RFC 3985.  However, in the case of MS-PWs,
   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 includes the coordination of
   parameters related to each switching point as well as to the MS-PW

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 virtual circuits (VCs), the
   S-PEs do not have visibility of, and hence do not change, this level
   of multiplexing because they contain no Native Service Processor

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8.2.  Fragmentation

   If fragmentation is to be used in an MS-PW, T-PEs and S-PEs must
   satisfy themselves that fragmented PW payloads can be correctly
   reassembled for delivery to the destination attachment circuit.

   An S-PE is not required to make any attempt to reassemble a
   fragmented PW payload.  However, it may choose to do so if, for
   example, it knows that a downstream PW segment does not support

   An S-PE may fragment a PW payload using [4].

9.  Control Plane

9.1.  Setup and Placement of MS-PWs

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

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

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

   o By signaling across each segment using a dynamic maintenance

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

   For the case of dynamic choice of the PW switching points, there are
   two options for selecting the path of the MS-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 T-PE and S-PE makes a local decision as to which next-hop S-PE
     to choose to reach the target T-PE.  This choice is made either
     using locally configured information or by using a dynamic PW
     path-selection mechanism.

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

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

9.3.  Misconnection and Payload Type Mismatch

   Misconnection and payload type mismatch can occur with PWs.
   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 or the PW control
   and OAM protocols 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 an MS-PW is not misconnected into a
   local AC.  In addition, it is possible to perform an end-to-end
   connection verification to check the integrity of the PW, to verify
   the identity of S-PEs and check the correct connectivity at S-PEs,
   and to verify the identity of the T-PE.

10.  Management and Monitoring

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

   The MS-PW architecture introduces additional considerations related
   to management and monitoring, which need to be reflected in the
   design of maintenance tools and additional management objects for

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   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
   and differentiate the connectivity between T-PEs and intermediate
   S-PEs, as well as the connectivity between T-PE and T-PE.

   The second is that the set of S-PEs and P-routers along the MS-PW
   path may be less optimal than a path between the T-PEs chosen solely
   by the underlying PSN routing protocols.  This is because the S-PEs
   are chosen by the MS-PW path selection mechanism and not by the PSN
   routing protocols.  Troubleshooting mechanisms should therefore be
   provided to verify the set of S-PEs that are traversed by an MS-PW to
   reach a T-PE.

   Some of the S-PEs and the T-PEs for an MS-PW may reside in a
   different service provider's PSN domain from that of the operator who
   initiated the establishment of the MS-PW.  These situations may
   necessitate the use of remote management of the MS-PW, which is able
   to securely operate across provider boundaries.

11.  Congestion Considerations

   The following congestion considerations apply to MS-PWs.  These are
   in addition to the considerations for PWs described in RFC 3985 [1],
   [7], and 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.

   Although congestion within a single provider's network can be
   mitigated by suitable engineering of the network so that the traffic
   imposed by PWs can never cause congestion in the underlying PSN, a
   significant number of MS-PWs are expected to be deployed for inter-
   provider services.  In this case, there may be no way of a provider
   who initiates the establishment of an MS-PW at a T-PE guaranteeing
   that it will not cause congestion in a downstream PSN.  A specific
   PSN may be able to protect itself from excess PW traffic by policing
   all PWs at the S-PE at the provider border.  However, this may not be

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   effective when the PSN tunnel across a provider utilizes the transit
   services of another provider that cannot distinguish PW traffic from
   ordinary, TCP-controlled IP traffic.

   Each segment of an MS-PW therefore needs to implement congestion
   detection and congestion control mechanisms where it is not possible
   to explicitly provision sufficient capacity to avoid congestion.

   In many cases, only the T-PEs may have sufficient information about
   each PW to fairly apply congestion control.  Therefore, T-PEs need to
   be aware of which of their PWs are causing congestion in a downstream
   PSN and of their native service characteristics, and to apply
   congestion control accordingly.  S-PEs therefore need to propagate
   PSN congestion state information between their downstream and
   upstream directions.  If the MS-PW transits many S-PEs, it may take
   some time for congestion state information to propagate from the
   congested PSN segment to the source T-PE, thus delaying the
   application of congestion control.  Congestion control in the S-PE at
   the border of the congested PSN can enable a more rapid response and
   thus potentially reduce the duration of congestion.

   In addition to protecting the operation of the underlying PSN,
   consistent QoS and traffic engineering mechanisms should be used on
   each segment of an 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., TC bits in either the label switched path (LSP) or PW label for
   an MPLS PSN or the DS field of the outer IP header for L2TPv3.

12.  Security Considerations

   The security considerations described in RFC 3985 [1] apply here.
   Detailed security requirements for MS-PWs are specified in RFC 5254
   [5].  This section describes the architectural implications of those

   The security implications for T-PEs are similar to those for PEs in
   single-segment pseudowires.  However, S-PEs represent a point in the
   network where the PW label is exposed to additional processing.  An
   S-PE or T-PE must trust that the context of the MS-PW is maintained
   by a downstream S-PE.  OAM tools must be able to verify the identity
   of the far end T-PE to the satisfaction of the network operator.
   Additional consideration needs to be given to the security of the
   S-PEs, both at the data plane and the control plane, particularly
   when these are dynamically selected and/or when the MS-PW transits
   the networks of multiple operators.

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   An implicit trust relationship exists between the initiator of an
   MS-PW, the T-PEs, and the S-PEs along the MS-PW's path.  That is, the
   T-PE trusts the S-PEs to process and switch PWs without compromising
   the security or privacy of the PW service.  An S-PE should not select
   a next-hop S-PE or T-PE unless it knows it would be considered
   eligible, as defined in Section 1.3, by the originator of the MS-PW.
   For dynamically placed MS-PWs, this can be achieved by allowing the
   T-PE to explicitly specify the path of the MS-PW.  When the MS-PW is
   dynamically created by the use of a signaling protocol, an S-PE or
   T-PE should determine the authenticity of the peer entity from which
   it receives the request and the compliance of that request with

   Where an MS-PW crosses a border between one provider and another
   provider, the MS-PW segment endpoints (S-PEs or T-PEs) or, for the
   PSN tunnel, P-routers typically reside on the same nodes as the
   Autonomous System Border Router (ASBRs) interconnecting the two
   providers.  In either case, an S-PE in one provider is connected to a
   limited number of trusted T-PEs or S-PEs in the other provider.  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

   Directly interconnecting the S-PEs/T-PEs using a physically secure
   link and enabling signaling and routing authentication between the
   S-PEs/T-PEs eliminates the possibility of receiving an MS-PW
   signaling message or packet from an untrusted peer.  The S-PEs/T-PEs
   represent security policy enforcement points for the MS-PW, while the
   ASBRs represent security policy enforcement points for the provider's
   PSNs.  This architecture is illustrated in Figure 9.

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                  |<------------- MS-PW ---------------->|
                  |       Provider         Provider      |
             AC   |    |<----1---->|     |<----2--->|    |  AC
              |   V    V           V     V          V    V  |
              |   +----+     +-----+     +----+     +----+  |
      +---+   |   |    |=====|     |=====|    |=====|    |  |    +---+
      |   |-------|......PW.....X....PW.....X...PW.......|-------|   |
      |CE1|   |   |    |Seg 1|     |Seg 2|    |Seg 3|    |  |    |CE2|
      +---+   |   |    |=====|     |=====|    |=====|    |  |    +---+
          ^       +----+     +-----+  ^  +----+     +----+       ^
          |       T-PE1       S-PE1   |   S-PE2     T-PE2        |
          |                    ASBR   |    ASBR                  |
          |                           |                          |
          |                  Physically secure link              |
          |                                                      |
          |                                                      |
          |<------------------- Emulated Service --------------->|

       Figure 9: Directly Connected Inter-Provider Reference Model

   Alternatively, the P-routers for the PSN tunnel may reside on the
   ASBRs, while the S-PEs or T-PEs reside behind the ASBRs within each
   provider's network.  A limited number of trusted inter-provider PSN
   tunnels interconnect the provider networks.  This is illustrated in
   Figure 10.

                |<-------------- MS-PW -------------------->|
                |          Provider          Provider       |
            AC  |    |<------1----->|   |<-----2------->|   |  AC
             |  V    V              V   V               V   V  |
             |  +---+     +---+  +--+   +--+  +---+     +---+  |
      +---+  |  |   |=====|   |===============|   |=====|   |  |   +---+
      |   |-----|.....PW....X.......PW..............PW....X.|------|   |
      |CE1|  |  |   |Seg 1|   |    Seg 2      |   |Seg 3|   |  |   |CE2|
      +---+  |  |   |=====|   |===============|   |=====|   |  |   +---+
          ^     +---+     +---+  +--+ ^ +--+  +---+     +---+      ^
          |      T-PE1    S-PE1  ASBR | ASBR  S-PE2     T-PE2      |
          |                           |                            |
          |                           |                            |
          |                Trusted Inter-AS PSN Tunnel             |
          |                                                        |
          |                                                        |
          |<------------------- Emulated Service ----------------->|

      Figure 10: Indirectly Connected Inter-Provider Reference Model

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

   Where an S-PE provides interconnection between different providers,
   security considerations that are similar to the security
   considerations for 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 are switched correctly to the
   appropriate outgoing PW segment, rather than to a local AC.  Other
   mechanisms for PW endpoint verification may also be used to confirm
   the correct PW connection prior to enabling the attachment circuits.

13.  Acknowledgments

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

14.  References

14.1.  Normative References

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

   [2] Andersson, L. and T. Madsen, "Provider Provisioned Virtual
       Private Network (VPN) Terminology", RFC 4026, March 2005.

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

   [4] Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-Edge
       (PWE3) Fragmentation and Reassembly", RFC 4623, August 2006.

14.2.  Informative References

   [5] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
       "Requirements for Multi-Segment Pseudowire Emulation Edge-to-Edge
       (PWE3)", RFC 5254, October 2008.

   [6] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
       Sprecher, N., and S. Ueno, "Requirements of an MPLS Transport
       Profile", RFC 5654, September 2009.

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   [7] Bryant, S., Davie, B., Martini, L., and E. Rosen, "Pseudowire
       Congestion Control Framework", Work in Progress, June 2009.

   [8] Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
       Transport Networks", Work in Progress, August 2009.

Authors' Addresses

   Matthew Bocci
   Voyager Place, Shoppenhangers Road,
   Maidenhead, Berks, UK
   Phone: +44 1633 413600

   Stewart Bryant
   Cisco Systems
   250, Longwater,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom

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