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rfc:rfc5659

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

Abstract

 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
 (http://trustee.ietf.org/license-info) 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.

Bocci & Bryant Informational [Page 1] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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

Bocci & Bryant Informational [Page 2] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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
 PEs.

Bocci & Bryant Informational [Page 3] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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

Bocci & Bryant Informational [Page 4] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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
 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 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

Bocci & Bryant Informational [Page 5] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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
   3985.
 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.

Bocci & Bryant Informational [Page 6] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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.

Bocci & Bryant Informational [Page 7] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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:
  +----+                                                  +----+
  |TPE1+--------------------------------------------------+TPE2|
  +----+                                                  +----+
  |<---------------------------PW----------------------------->|
  +----+              +---+           +---+               +----+
  |TPE1+--------------+SPE+-----------+SPE+---------------+TPE2|
  +----+              +---+           +---+               +----+
                     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

Bocci & Bryant Informational [Page 8] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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.,
 reachability.

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
 solutions.

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.

Bocci & Bryant Informational [Page 9] RFC 5659 Multi-Segment PWE3 Architecture October 2009

     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.

Bocci & Bryant Informational [Page 10] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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:

Bocci & Bryant Informational [Page 11] 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
 PSN2.

Bocci & Bryant Informational [Page 12] 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
 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.

Bocci & Bryant Informational [Page 13] RFC 5659 Multi-Segment PWE3 Architecture October 2009

             +------------------------------------------+
             |                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

Bocci & Bryant Informational [Page 14] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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
 pseudowires.

Bocci & Bryant Informational [Page 15] RFC 5659 Multi-Segment PWE3 Architecture October 2009

      |<------------- 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
 endpoints.

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
 (NSP).

Bocci & Bryant Informational [Page 16] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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
 reassembly.
 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
 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 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.

Bocci & Bryant Informational [Page 17] RFC 5659 Multi-Segment PWE3 Architecture October 2009

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
 MS-PWs.

Bocci & Bryant Informational [Page 18] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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

Bocci & Bryant Informational [Page 19] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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
 requirements.
 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.

Bocci & Bryant Informational [Page 20] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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
 policy.
 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
 mechanisms.
 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.

Bocci & Bryant Informational [Page 21] RFC 5659 Multi-Segment PWE3 Architecture October 2009

                |<------------- 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

Bocci & Bryant Informational [Page 22] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 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.

Bocci & Bryant Informational [Page 23] RFC 5659 Multi-Segment PWE3 Architecture October 2009

 [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
 Alcatel-Lucent
 Voyager Place, Shoppenhangers Road,
 Maidenhead, Berks, UK
 Phone: +44 1633 413600
 EMail: matthew.bocci@alcatel-lucent.com
 Stewart Bryant
 Cisco Systems
 250, Longwater,
 Green Park,
 Reading, RG2 6GB,
 United Kingdom
 EMail: stbryant@cisco.com

Bocci & Bryant Informational [Page 24]

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