MPLS Working Group                                         M. Bocci, Ed.
Internet-Draft                                            Alcatel-Lucent
Intended status: Informational                            S. Bryant, Ed.
Expires: May 31, 2009                                      Cisco Systems
                                                          L. Levrau, Ed.
                                                       November 27, 2008

               A Framework for MPLS in Transport Networks

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   This document specifies an archiectectural framework for the
   application of MPLS in transport networks.  It describes a profile of
   MPLS that enables operational models typical in transport networks
   networks, while providing additional OAM, survivability and other
   maintenance functions not currently supported by MPLS.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC2119 [1].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Motivation and Background  . . . . . . . . . . . . . . . .  3
     1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Summary of Requirements  . . . . . . . . . . . . . . . . . . .  5
   3.  Transport Profile Overview . . . . . . . . . . . . . . . . . .  5
     3.1.  Architecture . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Addressing . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Forwarding . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Operations, Administration and Maintenance (OAM) . . . . .  9
       3.4.1.  Generic Associated Channel (G-ACH) . . . . . . . . . . 13
       3.4.2.  Generic Alert Label (GAL)  . . . . . . . . . . . . . . 15
     3.5.  Control Plane  . . . . . . . . . . . . . . . . . . . . . . 16
       3.5.1.  PW Control Plane . . . . . . . . . . . . . . . . . . . 18
       3.5.2.  LSP Control Plane  . . . . . . . . . . . . . . . . . . 18
     3.6.  Static Operation of LSPs and PWs . . . . . . . . . . . . . 19
     3.7.  Survivability  . . . . . . . . . . . . . . . . . . . . . . 19
     3.8.  Network Management . . . . . . . . . . . . . . . . . . . . 21
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 23
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 24

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1.  Introduction

1.1.  Motivation and Background

   Existing transport technologies (e.g.  SDH, ATM, OTN) have been
   designed with specific characteristics:

   o  Strictly connection oriented

      *  Long-lived connections

      *  Manually provisioned connections

   o  High level of protection and availability

   o  Quality of service

   o  Extended OAM capabilities

   The development of MPLS-TP has been driven by the carriers needing to
   evolve SONET/SDH networks to support packet based services and
   networks, and the desire to take advantage of the flexibility and
   cost benefits of packet switching technology.

   There are three objectives:

   1.  To enable MPLS to be deployed in a transport network and operated
       in a similar manner to existing transport technologies.

   2.  To enable MPLS to support packet transport services with a
       similar degree of predictability to that found in existing
       transport networks.

   3.  To create a common set of new functions that are applicable to
       both MPLS networks in general, and those blonging to the MPLS-TP

   MPLS-TP defines a profile of MPLS targeted at transport applications.
   This profile specifies the specific MPLS characteristics and
   extensions required to meet transport requirements.  An equipment
   conforming to MPLS-TP must support this profile.  An MPLS-TP
   conformant equipment MAY support additional MPLS features.  A carrier
   may deploy some of those additional features in the transport layer
   of their network if they find them to be beneficial.

   Figure 1 illustrates the range of services that MPLS-TP is intended
   to address.  Networks supporting MPLS-TP are intended to support a
   range of layer 1,layer 2 and layer 3 services, and are not limited to

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   layer 3 services only.
                                          MPLS-TP Solution exists
                                           over this spectrum

cl-ps                      Multi-Service                     co-cs & co-ps
                          (cl-ps & co-ps)                      (Label is
  |                               |                        service context)
  |                               |                                  |
  |                               |                                  |
L3 Only                 L1, L2, L3 Services                 L1, L2 Services
                        Pt-Pt, Pt-MP, MP-MP                Pt-Pt and Pt-MP

                    Figure 1: MPLS-TP Service Spectrum

1.2.  Scope

   This document specifies the high-level functionality of MPLS-TP
   required for adding transport-oriented capabilities to MPLS

1.3.  Terminology

   Term    Definition
   ------- -----------------------------------------
   LSP     Label Switched Path
   MPLS-TP MPLS Transport profile
   SDH     Synchronous Digital Hierarchy
   ATM     Asynchronous Transfer Mode
   OTN     Optical Transport Network
   cl-ps   Connectionless - Packet Switched
   co-cs   Connection Oriented - Circuit Switched
   co-ps   Connection Oriented - Packet Switched
   OAM     Operations, Adminitration and Maintenance
   G-ACH   Generic Associated Channel Header
   GAL     Generic Alert Label
   MEP     Maintenance End Point
   MIP     Maintenance Intermediate Point
   APS     Automatic Protection Switching
   SCC     Signaling Communication Channel
   MCC     Management Communication Channel
   EMF     Equipment Management Function
   FM      Fault Management
   CM      Configuration Management
   PM      Performance Management

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2.  Summary of Requirements

   This section summarizes the requirements for the MPLS transport
   profile.  Such requirements are specified in more detail in [20],
   [21], and [22].

   Solutions MUST NOT modify the MPLS forwarding architecture.

   Solutions MUST be based on existing pseudowire and LSP constructs.

   New mechanisms and capabilities added to support transport networks
   must be able to interoperate or interwork with existing MPLS and
   pseudowire control and forwarding planes.

   Point to point LSPs MAY be unidirectional or bi-directional.  It MUST
   be possible to construct congruent Bi-directional LSPs.  Point to
   multipoint LSPs are unidirectional.

   MPLS-TP LSPs do not merge with other LSPs at an MPLS-TP LSR.  It is
   possible to detect that a merged LSP has been created.

   It MUST be possible to forward packets solely based on switching the
   MPLS or PW label.  It MUST also be possible to establish and maintain
   LSPs and/or pseudowires both in the absence or presence of a dynamic
   control plane.  When static provisioning is used, there MUST be no
   dependency on dynamic routing or signaling.

   OAM, protection and forwarding of data packets MUST be able to
   operate without IP forwarding support.

   It MUST be possible to monitor LSPs and pseudowires through the use
   of OAM in the absence of control plane or routing functions.  In this
   case information gained from the OAM functions is used to initiate
   path recovery actions at either the PW or LSP layers.

3.  Transport Profile Overview

3.1.  Architecture

   The architecture for a transport profile of MPLS (MPLS-TP) is based
   on the MPLS-TE [2], pseudowire [3], and multi-segment pseudowire [4]
   architectures, as illustrated in Figure 2.  The primary constructs of
   the transport profie for MPLS are LSPs, while PWs are the primary
   client layer.

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          Native  |<------------Pseudowire-------------->|  Native
          Service |         PSN              PSN         |  Service
           (AC)   |     |<--cloud->|     |<-cloud-->|    |   (AC)
             |    V     V          V     V          V    V     |
             |    +----+           +-----+          +----+     |
      +----+ |    |TPE1|===========|SPE1 |==========|TPE2|     | +----+
      |    |------|..... PW.Seg't1.........PW.Seg't3.....|-------|    |
      | CE1| |    |    |           |     |          |    |     | |CE2 |
      |    |------|..... PW.Seg't2.........PW.Seg't4.....|-------|    |
      +----+ |    |    |===========|     |==========|    |     | +----+
           ^      +----+     ^     +-----+     ^    +----+       ^
           |                 |                 |                 |
           |              TE LSP            TE LSP               |
           |                                                     |
           |                                                     |
           |<---------------- Emulated Service ----------------->|

                      Figure 2: MPLS-TP Architecture

   The MPLS-TP forwarding plane is a profile of the MPLS LSP PW, and
   MS-PW forwarding architecture as detailed in section Section 3.3.

   MPLS-TP supports a comprehensive set of OAM and protection-switching
   capabilities for packet transport applications, with equivalent
   capabilities to existing SONET/SDH OAM and protection, as described
   in sections Section 3.4 and Section 3.7.  MPLS-TP may be operated
   with centralized Network Management Systems with or without the
   support of a distributed control plane as described in sections
   Section 3.5 and Section 3.8.

   MPLS-TP defines mechanisms to differentiate specific packets (e.g.
   OAM, APS, MCC or SCC) from those carrying user data packets on the
   same LSP.  These mechanisms are described in sections
   Section 3.4.2and Section 3.4.1.

3.2.  Addressing

   MPLS-TP distinguishes between adressing used to identify nodes in the
   network, and identifiers used for demultiplexing and forwarding.
   This distinction is illustrated in Figure 3.

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                             NMS                   Control/Signalling
                                 .....         .....
                            [Address]|         |   [Address]
                                     |         |
       Address = Node          |     |         |      |
       ID in forwarding plane  |     V         V      |
                               |                      |
                               |     MEP or MIP       |
                               | dmux                 |
                               | svcid                |
                               | src                  |
      OAM:                OAM     |
        dmux= [GAL/GACH]...........
                 or       ________________________________________
                IP       (________________________________________)
        svc context=ID/FEC             PWE=ID1
        SRC=IP                           .

                      Figure 3: Addressing in MPLS-TP

   Ediror's note: The figure above arose from discussions in the MPLS-TP
   design team.  It will be clarified in a future verson of this draft.

   IPv4 or IPv6 addresses are used to identify MPLS-TP nodes by default
   for network management and signaling purposes.

   In the forwarding plane, identfiers are required for the service
   context (provided by the FEC), and for OAM.  OAM requires both a
   demultiplexer and an address for the source of the OAM packet.

   For MPLS in general where IP addressing is used, IPv4 or IPv6 is used
   by default.  However, MPLS-TP must be able to operate in environments
   where IP is not used in the forwarding plane.  Therefore, the default
   mechanism for OAM demultiplexing in MPLS-TP LSPs and PWs is the
   generic associated channel.  Forwarding based on IP addresses for
   user or OAM packets is NOT REQUIRED for MPLS-TP.

   RFC 4379 [23]and BFD for MPLS LSPs [24] have defined alert mechanisms
   that enable a MPLS LSR to identify and process MPLS OAM packets when
   the OAM packets are encapsulated in an IP header.  These alert
   mechanisms are based on TTL expiration and/or use an IP destination
   address in the range 127/8.  These mechanisms are the default
   mechanisms for MPLS networks in general for identifying MPLS OAM

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   packets when the OAM packets are encapsulated in an IP header.
   MPLS-TP must not rely on these mechanisms, and thus relies on the
   GACH/GAL to demultiplex OAM packets.

3.3.  Forwarding

   MPLS-TP LSPs use the MPLS label switching operations defined in [2].
   These operations are highly optimized for performance and are not
   modified by the MPLS-TP profile.

   During forwarding a label is pushed to describe the processing
   operaton to be performed at the next hop at that level of
   encapsulation.  A swap of this label is an atomic operation in which
   the contents of the packet after the swapped label are opaque to the
   forwarder.  The only circumstance that disrupts a swap operation is
   TTL expiry, in which case the packet may be discarded or subjected to
   further scrutinity within the LSR.  Operations on a packet with an
   expired TTL are asynchronous to the other packets in the LSP.  Thus
   the only way to cause a P (intermediate) LSR to inspect a packet (for
   example for OAM purposes) is to set the TTL to expiry at that LSR.

   MPLS-TP PWs support the PW and MS-PW forwarding operations defined
   in[3] and [4].

   The Traffic Class field (former MPLS EXP field) follows the
   definition and processing rules of [5] and [6].  Only the pipe and
   short-pipe models are supported in MPLS-TP.

   The MPLS encapsulation format is as defined in RFC 3032[7].  Per-
   platform or the per-interface label space can be selected.  Standard
   PW encapsulation mechanisms are applicable to the different client
   layers as defined by the IETF PWE3 WG.

   MPLS-TP LSPs can be unidirectional or bidirectional point-to-point.
   As for MPLS, point-to-multipoint MPLS-TP LSPs are unidirectional.

   Point-to-multipont PWs are currently in definition in the IETF and
   may be incorporated in MPLS-TP if required.

   It MUST be possible to configure an MPLS-TP LSP such that the forward
   and backward directions of bidirectional MPLS-TP LSPs congruent: i.e.
   they follow the same path.  The pairing relationship between the
   forward and the backward directions must be known at each MEP, MIP or
   segment protection endpoint on a bidirectional LSP.

   Per-packet equal cost multi-path (ECMP) load balancing is not
   applicable to MPLS-TP LSPs, however PWs or LSPs that emulate link
   bundles may be employed, for example [25]

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   Penultimate hop popping (PHP) is disabled on MPLS-TP LSPs by default.
   The applicability of PHP to both MPLS-TP LSPs and MPLS networks in
   general providing paket transport services will be clarified in a
   future version of this draft.

   Both E-LSP and L-LSP are supported in MPLS-TP, as defined in RFC 3270

3.4.  Operations, Administration and Maintenance (OAM)

   MPLS-TP requires [21] that a set of OAM capabilities is available to
   perform fault management (e.g. fault detection and localization) and
   performance monitoring (e.g. signal quality measurement) of the
   MPLS-TP network and the services.  These capabilities are applicable
   at the section, LSP and PW layer.  The framework for OAM in MPLS-TP
   is specified in [26].

   OAM and monitoring in MPLS-TP is based on the concept of maintenance
   entities, as described in [26].  A Maintenance Entity can be viewed
   as the association of two (or more) Maintenance End Points (MEPs)
   (see example in Figure 4 ).  The MEPS that form an ME should be
   configured and managed to limit the OAM responsibilities of an OAM
   flow within a network or sub-network in the specific layer network
   that is being monitored and managed.  Each OAM flow is associated to
   a unique ME.  Each MEP within an ME resides at the boundaries of that
   ME.  An ME may also include a set of zero or more Maintenance
   Intermediate Points (MIPs), which reside within the Maintenance
   Entity.  Maintenance end points (MEPs) are capable of sourcing and
   sinking OAM flows, while maintenance intermediate points (MIPs) can
   only sink or respond to OAM flows.

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========================== End to End LSP OAM ============================
     .....                     .....         .....            .....
     '''''                     '''''         '''''            '''''

     |<-------- Carrier 1 --------->|        |<----- Carrier 2 ----->|
      ----     ---     ---      ----          ----     ---     ----
 NNI |    |   |   |   |   |    |    |  NNI   |    |   |   |   |    | NNI
-----| PE |---| P |---| P |----| PE |--------| PE |---| P |---| PE |-----
     |    |   |   |   |   |    |    |        |    |   |   |   |    |
      ----     ---     ---      ----          ----     ---     ----

      ==== Segment LSP OAM ======  == Seg't ==  === Seg't LSP OAM ===
            (Carrier 1)             LSP OAM         (Carrier 2)
      .....   .....   .....  ..........   ..........  .....    .....
      '''''   '''''   '''''  ''''''''''   ''''''''''  '''''    '''''

Note: MEPs for End-to-end LSP OAM exist outside of the scope of this figure.

                     Figure 4: Example of MPLS-TP OAM

   Editor's note: The above diagram will be clarified in the next
   version of this draft.

   The OAM architecture for MPLS-TP is illustrated in Figure 5.

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     Native  |<-------------------- PW15 --------------------->| Native
      Layer  |                                                 |  Layer
    Service  |    |<-PSN13->|    |<-PSN3X->|    |<-PSNXZ->|    | Service
       (AC1) V    V   LSP   V    V   LSP   V    V   LSP   V    V  (AC2)
             +----+   +-+   +----+         +----+   +-+   +----+
+---+        |TPE1|   | |   |SPE3|         |SPEX|   | |   |TPEZ|     +---+
|   |        |    |=========|    |=========|    |=========|    |     |   |
|   |        |    |=========|    |=========|    |=========|    |     |   |
+---+        | 1  |   |2|   | 3  |         | X  |   |Y|   | Z  |     +---+
             +----+   +-+   +----+         +----+   +-+   +----+

             |<- Subnetwork 123->|         |<- Subnetwork XYZ->|

             .------------------- PW15  PME -------------------.
             .----- PW1 TPME ----.         .---- PW5 TPME -----.
                  .---------.                   .---------.
                   PSN13 LME                     PSNXZ LME

                   .--.  .--.     .--------.     .--.  .--.
               Sec12 SME Sec23 SME Sec3X SME SecXY SME SecYZ SME

TPE1: Terminating Provider Edge 1     SPE2: Switching Provider Edge 3
TPEX: Terminating Provider Edge X     SPEZ: Switching Provider Edge Z

   .---. ME     .     MEP    ====   LSP      .... PW

SME: Section Maintenance Entity
LME: LSP Maintenance Entity
PME: PW Maintenance Entity

                     Figure 5: MPLS-TP OAM archtecture

   The following MPLS-TP MEs are specified in [26]:

   o  A Section Maintenance Entity (SME), allowing monitoring and
      management of MPLS-TP Sections (between MPLS LSRs).

   o  A LSP Maintenance Entity (LME), allowing monitoring and management
      of an end-to-end LSP (between LERs).

   o  A PW Maintenance Entity (PME), allowing monitoring and management
      of an end-to-end SS/MS-PWs (between T-PEs).

   o  An LSP Tandem Connection Maintenance Entity (TLME), allowing
      monitoring and management of an LSP Tandem Connection (or LSP

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      Segment) between any LER/LSR along the LSP. o A MS-PW Tandem
      Connection Maintenance Entity (TPME), allows monitoring and
      management of a SS/MS-PW Tandem Connection (or PW Segment) between
      any T-PE/S-PE along the (MS-)PW.

   Individual MIPs along the path of an LSP or PW are addressed by
   setting the appropriate TTL in the label for the OAM packet, as per
   [27].  Note that this works when the location of MIPs along the LSP
   or PW path is known by the MEP.  There may be cases where this is not
   the case in general MPLS networks e.g. following restoration using a
   facility bypass LSP.

   The following is a high level summary of the classes of OAM functions
   that MPLS-TP supports.  These are intended to be applicable to any
   layer defined within MPLS- TP, i.e.  MPLS Section, LSP and PW:

   o  Continuity Check

   o  Connectivity verification

   o  Performance monitoring

   o  Alarm suppression

   o  Remote Integrity

   For all of the above listed functions except alarm suppression, both
   "continuous" and "on-demand" operation SHOULD be supported.

   Performance monitoring includes means for both "packet loss
   measurement" and "delay measurement".

   It is REQUIRED that MPLS-TP OAM packets share the same fate as their
   corresponding data packets and that a means exists to identify OAM
   packets.  The document[8] proposes specific mechanisms relying on the
   combination of the 'Generic Alert Label (GAL)' and Generic Associated
   Channel Header for MPLS Sections and LSPs and using the Generic
   Associated Channel Header only for MPLS PWs.  This is described in
   more detail elsewhere in this document Section 3.4.1 and
   Section 3.4.2.

   The MPLS-TP OAM toolset needs to be able to operate without relying
   on a dynamic control plane or IP functionality in the datapath.  In
   the case of MPLS-TP deployment with IP functionality, all existing
   IP-MPLS OAM functions, e.g.  LSP-Ping, BFD and VCCV, may be used.
   This does not preculde the use of other OAM tools in an IP
   addressable network.

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   One use of OAM mechanisms is to detect link failures, node failures
   and performance outside the required specification which then may be
   used to trigger recovery actions, according to the requirements of
   the service.

3.4.1.  Generic Associated Channel (G-ACH)

   MPLS-TP makes use of a generic associated channel (G-ACH) to support
   Fault, Configuration, Accounting, Performance and Security (FCAPS)
   functions by carrying packets related to OAM, APS, SCC, MCC or other
   packet types in band over LSPs or PWs.  The G-ACH is defined in
   [8]and it is similar to the PWE3 Associated Channel, which is used to
   carry OAM packets across pseudowires.  The G-ACH is indicated by a
   generic associated channel header, similar to the PWE3 VCCV control
   word, and this is present for all LSPs and PWs making use of FCAPS
   functions supported by the G-ACH.

   The G-ACH MUST only be used for channels that are an adjunct to the
   data service.  Examples of these are OAM, APS, MCC and SCC, but the
   use is not resticted to those names services.  The G-ACH MUST NOT be
   used to carry additional data for use in the forwarding path, i.e. it
   MUST NOT be used as an alternative to a PW control word, or to define
   a PW type.

   The messages transfered over the G-ACH MUST conform to the security
   and congestion considerations described in [8].  They must also take
   into consideration the throughput, latency and congestion
   requirements of the main data channel.

   Figure 1 shows the reference model depicting how the control channel
   is associated with the pseudowire protocol stack, as per [9].

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          +-------------+                                +-------------+
          |  Layer2     |                                |  Layer2     |
          |  Emulated   |       < Emulated Service >     |  Emulated   |
          |  Services   |                                |  Services   |
          +-------------+                                +-------------+
          |             |            VCCV/PW             |             |
          |Demultiplexer|    < Associated Channel >      |Demultiplexer|
          +-------------+                                +-------------+
          |    PSN      |          < PSN Tunnel >        |    PSN      |
          +-------------+                                +-------------+
          |  Physical   |                                |  Physical   |
          +-----+-------+                                +-----+-------+
                |                                              |
                |             ____     ___       ____          |
                |           _/    \___/   \    _/    \__       |
                |          /               \__/         \_     |
                |         /                               \    |
                +--------|      MPLS/MPLS-TP Network       |---+
                          \                               /
                           \   ___      ___     __      _/
                            \_/   \____/   \___/  \____/

      Figure 6: PWE3 Protocol Stack Reference Model including the PW
                        Associated Control Channel

   PW associated channel messages are encapsulated using the PWE3
   encapsulation, so that they are handled and processed in the same
   manner (or in some cases, an analogous manner) as the PW PDUs for
   which they provide a control channel.

   Figure 2 shows the reference model depicting how the control channel
   is associated with the LSP protocol stack.

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          +-------------+                                +-------------+
          |             |                                |             |
          |  Payload    |          < Service >           |   Payload   |
          |  Services   |                                |             |
          +-------------+                                +-------------+
          |             |            LSP                 |             |
          |Demultiplexer|     < Associated Channel >     |Demultiplexer|
          +-------------+                                +-------------+
          |    GAL      |                                |    GAL      |
          +-------------+                                +-------------+
          |    PSN      |            < LSP >             |    PSN      |
          +-------------+                                +-------------+
          |  Physical   |                                |  Physical   |
          +-----+-------+                                +-----+-------+
                |                                              |
                |             ____     ___       ____          |
                |           _/    \___/   \    _/    \__       |
                |          /               \__/         \_     |
                |         /                               \    |
                +--------|      MPLS/MPLS-TP Network       |---+
                          \                               /
                           \   ___      ___     __      _/
                            \_/   \____/   \___/  \____/

      Figure 7: MPLS Protocol Stack Reference Model including the LSP
                        Associated Control Channel

   LSP associated channel messages are encapsulated using a generic
   associated control channel header (G-ACH).  The presence of the GE-
   ACH is indicated by the inclusion of an additional 'Generic Alert
   Label (GAL)'.  This arrangement means that both normal data packets
   and packets carrying an ACH are carried over LSPs in a similar

   Note that where a traffic engineered LSP is used the paths will be
   identical.  If for any reason a non-traffic engineered path (for
   example an LDP path) were to be used the ECMP behaviour may be
   modified by the presence of the GAL.

3.4.2.  Generic Alert Label (GAL)

   For correct operation of the OAM it is important that the OAM packets
   fate share with the data packets.  In addition in MPSL-TP it is
   necessary to indicate that the payload carried over an LSP is not
   user data.  For example the packet may contain Signaling
   Communication Channel (SCC), or Automatic Protecton Switching (APS)
   data.  The presence of the ACH indicates that the packet is not user
   data and identifies its type.

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   PWE3 uses the first nibble of the control word to provide the initial
   discrimination between data packets and "other" packets [10].  When
   the first nibble of a pseudwire packet has a value of one, then the
   first 32 bits that follow the bottom of stack have a defined format
   called an ACH, and which further defines the content of the
   pseudowire packet.  For MPLS-TP this mechanism is further generalized
   to apply to also apply to LSPs and MPLS sections [8].

   When the OAM, or a similar message is carried over an LSP, rather
   than over a pseudowire, it is necessary to provide an indication in
   the packet that the payload is something other than a regular data
   packet.  This is acheived by including ia new reserved label in the
   label stack.  This reserved label is referred to as the 'Generic
   Alert Label (GAL)', and is defined in [8].  When a GAL is found
   anywhere within the label stack it indicates that the payload begins
   with an ACH.  Note however that MPLS-TP forwarding follows the normal
   MPLS model, and that a GAL is invisible to an LSR unless it is the
   label being popped.  The only circumstance under which the label
   stack may be inspected for a GAL is when the TTL has expired.  Any
   MPLS-TP component which intentionally triggers this inspection must
   assume that the inspection to be asynchronous with respect to the
   forwarding of other packets.

   In MPLS-TP, the 'Generic Alert Label (GAL)' always appears at the
   bottom of the label stack (i.e.  S bit set to 1), however this does
   not preclude its use elsewhere in the label stack in other

3.5.  Control Plane

   The MPLS-TP may utilize a distributed control plane to enable fast,
   dynamic and reliable service provisioning in multi-vendor and multi-
   domain environments using standardized protocols that guarantee

   Figure 8 illustrates the relationshop between the MPLS-TP control
   plane, the forwarding plane, the management plane, and OAM.

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 |                                                                        |
 |                   Network Management System and/or                     |
 |                                                                        |
 |           Control Plane for Point to Point Connections                 |
 |                                                                        |
              |      |           |       |          |    |
  ............|......|.....  ....|.......|....  ....|....|...............
            +---+    |    :  : +---+     |   :  : +---+  |              :
  :         |OAM|    |    :  : |OAM|     |   :  : |OAM|  |              :
  :         +---+    |    :  : +---+     |   :  : +---+  |              :
  :           |      |    :  :   |       |   :  :   |    |              :
 \: +----+   +----------+ :  : +----------+  :  : +----------+   +----+ :/
 /: +----+   |          | :  : |          |  :  : |          |   +----+ :\
  :          +----------+ :  : +----------+  :  : +----------+          :
  '''''''''''''''''''''''''  '''''''''''''''''   ''''''''''''''''''''''''

   1) NMS may be centralised or distributed. Control plane is distributed
   2) 'Edge' functions refers to those functions present at the edge of
      a PSN domain, e.g. NSP or classification.
   3) OAM functions are described in more detail below.

           Figure 8: MPLS-TP Control Plane Architecture Context

   The MPLS-TP control plane is based on a combination of the MPLS
   control plane for pseudowires and the GMPLS control plane for MPLS-TP
   LSPs, respectively.  More specifically, LDP is used for PW signaling
   and GMPLS based RSVP-TE for LSP signaling.  The distributed MPLS-TP
   control plane provides the following basic functions:

   o  Signaling

   o  Routing

   o  Traffic engineering and constraint-based path computation

   In a multi-domain environment, the MPLS-TP control plane may provide
   different types of interfaces at domain boundaries or within the
   domains such as UNI, I-NNI, and E-NNI where different policies are in
   place that control what kind of information is exchanged across these
   different types of interfaces.

   Editor's note: Isn't the following a managment plane operation.  I
   can't think of a routing protocol triggering an OAM message.  Or do

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   we mean that the control plane is capable of reacting to OAM events?
   Control plane and OAM are independent.

   The MPLS-TP control plane is capable of activating MPLS-TP OAM
   functions as described in the OAM section of this document
   Section 3.4 e.g. for fault detection and localization in the event of
   a failure in order to efficiently restore failed transport paths.

   The MPLS-TP control plane supports all MPLS-TP data plane
   connectivity patterns that are needed for establishing transport
   paths including protected paths as described in the survivability
   section Section 3.7 of this document.  Examples of the MPLS-TP data
   plane connectivity patterns are LSPs utilizing the fast reroute
   backup methods as defined in [11] and ingress-to-egress 1+1 or 1:1
   protected LSPs.

   Moreover, the MPLS-TP control plane needs to be capable of performing
   fast restoration in the event of network failures.

   The MPLS-TP control plane provides features to ensure its own
   survivavbility and to enable it to recover gracefully from failures
   and degredations.  These include graceful restart and hot redundant
   configurations.  The MPLS-TP control plane is largely decoupled from
   the MPLS-TP data plane such that failures in the control plane do not
   impact the data plane and vice versa.

3.5.1.  PW Control Plane

   An MPLS-TP packet transport network provides many of its transport
   services in the form of single-segment or multi-segment pseudowires
   following the PWE3 architecture as defined in [3] and [4] .  The
   setup and maintenance of single-segment or multi- segment pseudowires
   is based on the Label Distribution Protocol (LDP) as per [12] and the
   use of LDP in this manner is applicable to PWs used to provide MPLS
   transport services.

   It shall be noted that multi-segment pseudowire signaling is still
   work in progress.  The control plane supporting multi-segment
   pseudowires is based on [13].

3.5.2.   LSP Control Plane

   Editors note: The following must be reviewed by a CP specialist.  Lou
   will review and provide comments.

   MPLS-TP provider edge nodes aggregate multiple pseudowires and carry
   them across the MPLS-TP network through MPLS-TP tunnels (MPLS-TP
   LSPs).  The generalized MPLS (GMPLS) protocol suite already supports

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   packet-switched capable (PSC) technologies and is therefore used as
   control plane for MPLS-TP transport paths (LSPs).  The LSP control
   plane includes:

   o  RSVP-TE for signalling

   o  OSPF-TE for routing

   o  ISIS-TE for routing

   RSVP-TE signaling in support of GMPLS as defined in [14]is used for
   the setup, modification, and release of MPLS-TP transport paths and
   protection paths.  It supports unidirectional, bi-directional and
   multicast types of LSPs.  The route of a transport path is typically
   calculated in the ingress node of a domain and the RSVP explicit
   route object (ERO) is utilized for the setup of the transport path
   exactly following the given route.  GMPLS based MPLS-TP LSPs must be
   able to interoperate with RSVP-TE based MPLS-TE LSPs, as per [28]

   OSPF-TE routing in support of GMPLS as defined in [15] is used for
   carrying link state information in a MPLS-TP network.

   For routing scalability reasons, parallel physical links in an MPLS-
   TP network are typically bundled into TE-links as defined in [16]and
   the OSPF-TE routing protocol disseminates link state information on a
   TE-link basis.

3.6.  Static Operation of LSPs and PWs

   Where a control plane is not used to set up and manage PWs or LSPs,
   the following considerations apply.  Static configuration of the PW
   or LSP, either by direct configuration of the PEs/LSRs, or via a
   network management station must take care that loops to not form on
   an active LSP.  The OAM would normally detect a break in end to end
   connectivity as a consequence of a loop, and withdraw the LSP from
   use.  However the colateral damage that a loop can during the time
   taken to detect the failure is severe.  Therefore an LSP should not
   be brought into operation until it certain that loops do not exist.

3.7.  Survivability

   Survivability requirements for MPLS-TP are apecified in [29].

   A wide variety of resiliency schemes have been developed to meet the
   various network and service survivability objectives.  For example,
   as part of the MPLS/PW paradigms, MPLS provides methods for local
   repair using back-up LSP tunnels ([11]), while pseudowire redundancy
   [17]supports scenarios where the protection for the PW can not be

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   fully provided by the PSN layer (i.e. where the backup PW terminates
   on a different target PE node than the working PW).  Additionally,
   GMPLS provides a set of control plane driven well known protection
   and restoration mechanisms [14].  Finally, as part of the transport
   networks and applications paradigms, APS-based linear and ring
   protection mechanisms are defined in [18]and [30].

   These schemes have different scopes.  They are protecting against
   link and/or node failures and can be applied end-to-end or on a
   segment of the considered connection.

   These protection schemes propose different levels of resiliency (e.g.
   1+1, 1:1, shared).

   The applicability of any given scheme to meet specific requirements
   is outside the current scope of this document.

   MPLS-TP resiliency mechanisms characteristics are listed below

   o  Linear, ring and meshed protection schemes are supported.

   o  As with all network layer protection schemes, MPLS-TP recovery
      mechanisms (protection and restoration), rely on OAM mechanisms to
      detect and localize network faults or service degenerations.

   o  APS-based protection mechanisms (linear and ring) rely on MPLS-TP
      APS mechanisms to coordinate and trigger protection switching

   o  MPLS-TP recovery schemes are designed to be applicable at various
      levels (MPLS section, LSP and PW), providing segment and end-to-
      end recovery.

   o  MPLS-TP recovery mechanisms support means for avoiding race
      conditions in switching activity triggered by a fault condition
      detected both at server layer and at MPLS-TP layer.

   o  MPLS-TP recovery mechanisms can be data plane, control plane or
      management plane based.

   o  MPLS-TP allows for revertive and non-revertive behavior

   o  Multiple resiliency mechanisms can be applied to any connection

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3.8.  Network Management

   The network management architecture and requirements for MPLS-TP are
   specified in [22].  It derives from the generic specifications
   described in ITU-T G.7710/Y.1701 [19]for transport technologies.  It
   also leverages on the OAM requirements for MPLS Networks [31] and
   MPLS-TP Networks [21]and expands on the requirements to cover the
   modifications necessary for fault, configuration, performance, and

   The Equipment Management Function (EMF) of a MPLS-TP NE provides the
   means through which a management system manages the NE.  The
   Management Communication Channel (MCC), realized by the G-ACH,
   provides a logical operations channel between NEs for transferring
   Management information.  For the management interface from a
   management system to a MPLS-TP NE, there is no restriction on which
   management protocol should be used.  It is allowed to provision and
   manage an end-to-end connection across a network where some segments
   are create/managed, for examples by Netconf or SNMP and other
   segments by XML or CORBA interfaces.  It is allowed to run
   maintenance operations on a connection which is independent of the
   provisioning mechanism.  An MPLS-TP NE is not required to offer more
   than one standard management interface.  In MPLS-TP. the EMF MUST be
   capable of statically provisioning LSPs for an LSR or LER, and PWs
   for a PE, as per Section 3.6.

   The Fault Management (FM) functions within the EMF of an MPLS-TP NE
   enable the supervision, detection, validation, isolation, correction,
   and alarm handling of abnormal operation of the MPLS-TP network and
   its environment.  Supervision for transmission (such as continuity,
   connectivity, etc.), software processing, hardware, and environment
   are essential for FM.  Alarm handling includes alarm severity
   assignment, alarm suppression/aggregation/correlation, alarm
   reporting control, and alarm reporting.

   Configuration Management (CM) provides functions to exercise control
   over, identify, collect data from, and provide data to MPLS-TP NEs.
   In addition to general configuration for hardware, software
   protection switching, alarm reporting control, and date/time setting,
   the EMF of the MPLS-TP NE also supports the configuration of
   maintenance entity identifiers (such as MEP ID and MIP ID).  The EMF
   also supports configuration of the OAM parameters as part of
   connectivity management to meet specific operational requirements,
   such as whether one-time on-demand or periodically based on a
   specified frequency.

   The Performance Management (PM) functions within the EMF of an MPLS-
   TP NE supports the evaluation and reporting upon the behaviour of the

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   equipment, NE, and network with the objective of providing coherent
   and consistent interpretation of the network behaviour, in particular
   for hybrid network which consists of multiple transport technologies.
   Packet loss measurement and delay measurement are collected so that
   they can be used to detect performance degradation.  Performance
   degradation is reported via fault management for corrective actions
   (e.g. protection switch) and via performance monitoring for Service
   Level Agreement (SLA) verification and billing.  The performance data
   collection mechanisms should be flexible to be configured to operate
   on-demand or proactively.

4.  Security Considerations

   The introduction of MPLS-TP into transport networks means that the
   security considerations applicable to both MPLS and PWE3 apply to
   those transport networks.  Furthermore, when general MPLS networks
   that utilise functionality outside of the strict MPLS-TP profile are
   used to support packet transport services, the security
   considerations of that additional functionality also apply.

   Specific security considerations for MPLS-TP will be detailed in
   documents covering specific aspects on the MPLS-TP architecture.

5.  IANA Considerations

   IANA considerations resulting from specific elements of MPLS-TP
   functionality will be detailed in the documents specifying that

   This document introduces no additional IANA considerations in itself.

6.  Acknowledgements

   The editors wish to thank the following for their contribution to
   this document:

   o  Dieter Beller

   o  Italo Busi

   o  Hing-Kam Lam

   o  Marc Lasserre

   o  Vincenzo Sestito

   o  Martin Vigoureux

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7.  References

7.1.  Normative References

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

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

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

   [4]   Bocci, M. and S. Bryant, "An Architecture for Multi-Segment
         Pseudowire Emulation Edge-to-Edge",
         draft-ietf-pwe3-ms-pw-arch-05 (work in progress),
         September 2008.

   [5]   Andersson, L. and R. Asati, ""EXP field" renamed to "Traffic
         Class field"", draft-ietf-mpls-cosfield-def-07 (work in
         progress), November 2008.

   [6]   Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen, P.,
         Krishnan, R., Cheval, P., and J. Heinanen, "Multi-Protocol
         Label Switching (MPLS) Support of Differentiated Services",
         RFC 3270, May 2002.

   [7]   Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
         D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
         RFC 3032, January 2001.

   [8]   Vigoureux, M., Bocci, M., Ward, D., Swallow, G., and R.
         Aggarwal, "MPLS Generic Associated Channel",
         draft-bocci-mpls-tp-gach-gal-00 (work in progress),
         October 2008.

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

   [10]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
         "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use
         over an MPLS PSN", RFC 4385, February 2006.

   [11]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute Extensions to
         RSVP-TE for LSP Tunnels", RFC 4090, May 2005.

   [12]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron,

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         "Pseudowire Setup and Maintenance Using the Label Distribution
         Protocol (LDP)", RFC 4447, April 2006.

   [13]  Martini, L., Bocci, M., Bitar, N., Shah, H., Aissaoui, M., and
         F. Balus, "Dynamic Placement of Multi Segment Pseudo Wires",
         draft-ietf-pwe3-dynamic-ms-pw-08 (work in progress), July 2008.

   [14]  Lang, J., Rekhter, Y., and D. Papadimitriou, "RSVP-TE
         Extensions in Support of End-to-End Generalized Multi-Protocol
         Label Switching (GMPLS) Recovery", RFC 4872, May 2007.

   [15]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support of
         Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203,
         October 2005.

   [16]  Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in
         MPLS Traffic Engineering (TE)", RFC 4201, October 2005.

   [17]  Muley, P. and M. Bocci, "Pseudowire (PW) Redundancy",
         draft-ietf-pwe3-redundancy-01 (work in progress),
         September 2008.

   [18]  "ITU-T Recommendation G.8131/Y.1382 (02/07) " Linear protection
         switching for Transport MPLS (T-MPLS) networks"", 2005.

   [19]  "ITU-T Recommendation G.7710/Y.1701 (07/07), "Common equipment
         management function requirements"", 2005.

7.2.  Informative References

   [20]  Niven-Jenkins, B., Brungard, D., Betts, M., and N. Sprecher,
         "MPLS-TP Requirements",
         draft-jenkins-mpls-mpls-tp-requirements-01 (work in progress),
         October 2008.

   [21]  Vigoureux, M., Ward, D., Betts, M., Bocci, M., and I. Busi,
         "Requirements for OAM in MPLS Transport Networks",
         draft-vigoureux-mpls-tp-oam-requirements-01 (work in progress),
         November 2008.

   [22]  Mansfield, S., Lam, K., and E. Gray, "MPLS TP Network
         Management Requirements", draft-gray-mpls-tp-nm-req-01 (work in
         progress), October 2008.

   [23]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
         Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.

   [24]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, "BFD

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         For MPLS LSPs", draft-ietf-bfd-mpls-07 (work in progress),
         June 2008.

   [25]  Bryant, S., Filsfils, C., and U. Drafz, "Load Balancing Fat
         MPLS Pseudowires", draft-bryant-filsfils-fat-pw-02 (work in
         progress), July 2008.

   [26]  Busi, I. and B. Niven-Jenkins, "MPLS-TP OAM Framework and
         Overview", draft-busi-mpls-tp-oam-framework-00 (work in
         progress), October 2008.

   [27]  Nadeau, T., Metz, C., Duckett, M., Bocci, M., Balus, F., and L.
         Martini, "Segmented Pseudo Wire",
         draft-ietf-pwe3-segmented-pw-09 (work in progress), July 2008.

   [28]  Kumaki, K., "Interworking Requirements to Support Operation of
         MPLS-TE over GMPLS Networks", RFC 5146, March 2008.

   [29]  Sprecher, N., Farrel, A., and V. Kompella, "Multiprotocol Label
         Switching Transport Profile Survivability Framework",
         draft-sprecher-mpls-tp-survive-fwk-00 (work in progress),
         July 2008.

   [30]  "Draft ITU-T Recommendation G.8132/Y.1382, "T-MPLS shared
         protection ring",", 2005.

   [31]  Nadeau, T., Morrow, M., Swallow, G., Allan, D., and S.
         Matsushima, "Operations and Management (OAM) Requirements for
         Multi-Protocol Label Switched (MPLS) Networks", RFC 4377,
         February 2006.

Authors' Addresses

   Matthew Bocci (editor)
   Voyager Place, Shoppenhangers Road
   Maidenhead, Berks  SL6 2PJ
   United Kingdom

   Phone: +44-207-254-5874

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   Stewart Bryant (editor)
   Cisco Systems
   250 Longwater Ave
   Reading  RG2 6GB
   United Kingdom

   Phone: +44-208-824-8828

   Lieven Levrau (editor)
   7-9, Avenue Morane Sulnier
   Velizy  78141

   Phone: +33-6-33-86-1916

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Full Copyright Statement

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