





Internet Engineering Task Force (IETF)                     M. Bocci, Ed.
Request for Comments: 5921                                Alcatel-Lucent
Category: Informational                                   S. Bryant, Ed.
ISSN: 2070-1721                                            D. Frost, Ed.
                                                           Cisco Systems
                                                               L. Levrau
                                                          Alcatel-Lucent
                                                               L. Berger
                                                                    LabN
                                                               July 2010


               A Framework for MPLS in Transport Networks

Abstract

   This document specifies an architectural framework for the
   application of Multiprotocol Label Switching (MPLS) to the
   construction of packet-switched transport networks.  It describes a
   common set of protocol functions -- the MPLS Transport Profile (MPLS-
   TP) -- that supports the operational models and capabilities typical
   of such networks, including signaled or explicitly provisioned
   bidirectional connection-oriented paths, protection and restoration
   mechanisms, comprehensive Operations, Administration, and Maintenance
   (OAM) functions, and network operation in the absence of a dynamic
   control plane or IP forwarding support.  Some of these functions are
   defined in existing MPLS specifications, while others require
   extensions to existing specifications to meet the requirements of the
   MPLS-TP.

   This document defines the subset of the MPLS-TP applicable in general
   and to point-to-point transport paths.  The remaining subset,
   applicable specifically to point-to-multipoint transport paths, is
   outside the scope of this document.

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunication Union Telecommunication
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and Pseudowire Emulation Edge-to-Edge
   (PWE3) architectures to support the capabilities and functionalities
   of a packet transport network as defined by the ITU-T.










Bocci, et al.                 Informational                     [Page 1]

RFC 5921            MPLS Transport Profile Framework           July 2010


Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5921.

Copyright Notice

   Copyright (c) 2010 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 Simplified BSD License.





















Bocci, et al.                 Informational                     [Page 2]

RFC 5921            MPLS Transport Profile Framework           July 2010


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Motivation and Background  . . . . . . . . . . . . . . . .  4
     1.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
       1.3.1.  Transport Network  . . . . . . . . . . . . . . . . . .  7
       1.3.2.  MPLS Transport Profile . . . . . . . . . . . . . . . .  7
       1.3.3.  MPLS-TP Section  . . . . . . . . . . . . . . . . . . .  7
       1.3.4.  MPLS-TP Label Switched Path  . . . . . . . . . . . . .  7
       1.3.5.  MPLS-TP Label Switching Router . . . . . . . . . . . .  8
       1.3.6.  Customer Edge (CE) . . . . . . . . . . . . . . . . . . 10
       1.3.7.  Transport LSP  . . . . . . . . . . . . . . . . . . . . 10
       1.3.8.  Service LSP  . . . . . . . . . . . . . . . . . . . . . 10
       1.3.9.  Layer Network  . . . . . . . . . . . . . . . . . . . . 10
       1.3.10. Network Layer  . . . . . . . . . . . . . . . . . . . . 10
       1.3.11. Service Interface  . . . . . . . . . . . . . . . . . . 10
       1.3.12. Native Service . . . . . . . . . . . . . . . . . . . . 11
       1.3.13. Additional Definitions and Terminology . . . . . . . . 11
   2.  MPLS Transport Profile Requirements  . . . . . . . . . . . . . 11
   3.  MPLS Transport Profile Overview  . . . . . . . . . . . . . . . 12
     3.1.  Packet Transport Services  . . . . . . . . . . . . . . . . 12
     3.2.  Scope of the MPLS Transport Profile  . . . . . . . . . . . 13
     3.3.  Architecture . . . . . . . . . . . . . . . . . . . . . . . 14
       3.3.1.  MPLS-TP Native Service Adaptation Functions  . . . . . 14
       3.3.2.  MPLS-TP Forwarding Functions . . . . . . . . . . . . . 15
     3.4.  MPLS-TP Native Service Adaptation  . . . . . . . . . . . . 16
       3.4.1.  MPLS-TP Client/Server Layer Relationship . . . . . . . 16
       3.4.2.  MPLS-TP Transport Layers . . . . . . . . . . . . . . . 17
       3.4.3.  MPLS-TP Transport Service Interfaces . . . . . . . . . 18
       3.4.4.  Pseudowire Adaptation  . . . . . . . . . . . . . . . . 25
       3.4.5.  Network Layer Adaptation . . . . . . . . . . . . . . . 28
     3.5.  Identifiers  . . . . . . . . . . . . . . . . . . . . . . . 33
     3.6.  Generic Associated Channel (G-ACh) . . . . . . . . . . . . 33
     3.7.  Operations, Administration, and Maintenance (OAM)  . . . . 36
     3.8.  Return Path  . . . . . . . . . . . . . . . . . . . . . . . 38
       3.8.1.  Return Path Types  . . . . . . . . . . . . . . . . . . 39
       3.8.2.  Point-to-Point Unidirectional LSPs . . . . . . . . . . 39
       3.8.3.  Point-to-Point Associated Bidirectional LSPs . . . . . 40
       3.8.4.  Point-to-Point Co-Routed Bidirectional LSPs  . . . . . 40
     3.9.  Control Plane  . . . . . . . . . . . . . . . . . . . . . . 40
     3.10. Inter-Domain Connectivity  . . . . . . . . . . . . . . . . 43
     3.11. Static Operation of LSPs and PWs . . . . . . . . . . . . . 43
     3.12. Survivability  . . . . . . . . . . . . . . . . . . . . . . 44
     3.13. Sub-Path Maintenance . . . . . . . . . . . . . . . . . . . 45
     3.14. Network Management . . . . . . . . . . . . . . . . . . . . 47
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 48
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 49



Bocci, et al.                 Informational                     [Page 3]

RFC 5921            MPLS Transport Profile Framework           July 2010


   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 50
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 50
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 51

1.  Introduction

1.1.  Motivation and Background

   This document describes an architectural framework for the
   application of MPLS to the construction of packet-switched transport
   networks.  It specifies the common set of protocol functions that
   meet the requirements in [RFC5654], and that together constitute the
   MPLS Transport Profile (MPLS-TP) for point-to-point transport paths.
   The remaining MPLS-TP functions, applicable specifically to point-to-
   multipoint transport paths, are outside the scope of this document.

   Historically, the optical transport infrastructure -- Synchronous
   Optical Network/Synchronous Digital Hierarchy (SONET/SDH) and Optical
   Transport Network (OTN) -- has provided carriers with a high
   benchmark for reliability and operational simplicity.  To achieve
   this, transport technologies have been designed with specific
   characteristics:

   o  Strictly connection-oriented connectivity, which may be long-lived
      and may be provisioned manually, for example, by network
      management systems or direct node configuration using a command
      line interface.

   o  A high level of availability.

   o  Quality of service.

   o  Extensive Operations, Administration, and Maintenance (OAM)
      capabilities.

   Carriers wish to evolve such transport networks to take advantage of
   the flexibility and cost benefits of packet switching technology and
   to support packet-based services more efficiently.  While MPLS is a
   maturing packet technology that already plays an important role in
   transport networks and services, not all MPLS capabilities and
   mechanisms are needed in, or consistent with, the transport network
   operational model.  There are also transport technology
   characteristics that are not currently reflected in MPLS.







Bocci, et al.                 Informational                     [Page 4]

RFC 5921            MPLS Transport Profile Framework           July 2010


   There are thus two objectives for MPLS-TP:

   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.

   In order to achieve these objectives, there is a need to define a
   common set of MPLS protocol functions -- an MPLS Transport Profile --
   for the use of MPLS in transport networks and applications.  Some of
   the necessary functions are provided by existing MPLS specifications,
   while others require additions to the MPLS tool-set.  Such additions
   should, wherever possible, be applicable to MPLS networks in general
   as well as those that conform strictly to the transport network
   model.

   This document is a product of a joint Internet Engineering Task Force
   (IETF) / International Telecommunication Union Telecommunication
   Standardization Sector (ITU-T) effort to include an MPLS Transport
   Profile within the IETF MPLS and PWE3 architectures to support the
   capabilities and functionalities of a packet transport network as
   defined by the ITU-T.

1.2.  Scope

   This document describes an architectural framework for the
   application of MPLS to the construction of packet-switched transport
   networks.  It specifies the common set of protocol functions that
   meet the requirements in [RFC5654], and that together constitute the
   MPLS Transport Profile (MPLS-TP) for point-to-point MPLS-TP transport
   paths.  The remaining MPLS-TP functions, applicable specifically to
   point-to-multipoint transport paths, are outside the scope of this
   document.

1.3.  Terminology

   Term       Definition
   ---------- ----------------------------------------------------------
   AC         Attachment Circuit
   ACH        Associated Channel Header
   Adaptation The mapping of client information into a format suitable
              for transport by the server layer
   APS        Automatic Protection Switching
   ATM        Asynchronous Transfer Mode
   BFD        Bidirectional Forwarding Detection
   CE         Customer Edge



Bocci, et al.                 Informational                     [Page 5]

RFC 5921            MPLS Transport Profile Framework           July 2010


   CL-PS      Connectionless - Packet Switched
   CM         Configuration Management
   CO-CS      Connection Oriented - Circuit Switched
   CO-PS      Connection Oriented - Packet Switched
   DCN        Data Communication Network
   EMF        Equipment Management Function
   FCAPS      Fault, Configuration, Accounting, Performance, and
              Security
   FM         Fault Management
   G-ACh      Generic Associated Channel
   GAL        G-ACh Label
   LER        Label Edge Router
   LSP        Label Switched Path
   LSR        Label Switching Router
   MAC        Media Access Control
   MCC        Management Communication Channel
   ME         Maintenance Entity
   MEG        Maintenance Entity Group
   MEP        Maintenance Entity Group End Point
   MIP        Maintenance Entity Group Intermediate Point
   MPLS       Multiprotocol Label Switching
   MPLS-TP    MPLS Transport Profile
   MPLS-TP P  MPLS-TP Provider LSR
   MPLS-TP PE MPLS-TP Provider Edge LSR
   MS-PW      Multi-Segment Pseudowire
   Native     The traffic belonging to the client of the MPLS-TP network
   Service
   OAM        Operations, Administration, and Maintenance (see
              [OAM-DEF])
   OSI        Open Systems Interconnection
   OTN        Optical Transport Network
   PDU        Protocol Data Unit
   PM         Performance Monitoring
   PSN        Packet Switching Network
   PW         Pseudowire
   SCC        Signaling Communication Channel
   SDH        Synchronous Digital Hierarchy
   S-PE       PW Switching Provider Edge
   SPME       Sub-Path Maintenance Element
   SS-PW      Single-Segment Pseudowire
   T-PE       PW Terminating Provider Edge
   TE LSP     Traffic Engineered Label Switched Path
   VCCV       Virtual Circuit Connectivity Verification








Bocci, et al.                 Informational                     [Page 6]

RFC 5921            MPLS Transport Profile Framework           July 2010


1.3.1.  Transport Network

   A Transport Network provides transparent transmission of user traffic
   between attached client devices by establishing and maintaining
   point-to-point or point-to-multipoint connections between such
   devices.  The architecture of networks supporting point-to-multipoint
   connections is outside the scope of this document.  A Transport
   Network is independent of any higher-layer network that may exist
   between clients, except to the extent required to supply this
   transmission service.  In addition to client traffic, a Transport
   Network may carry traffic to facilitate its own operation, such as
   that required to support connection control, network management, and
   Operations, Administration, and Maintenance (OAM) functions.

   See also the definition of packet transport service in Section 3.1.

1.3.2.  MPLS Transport Profile

   The MPLS Transport Profile (MPLS-TP) is the subset of MPLS functions
   that meet the requirements in [RFC5654].  Note that MPLS is defined
   to include any present and future MPLS capability specified by the
   IETF, including those capabilities specifically added to support
   transport network requirements [RFC5654].

1.3.3.  MPLS-TP Section

   MPLS-TP sections are defined in [DATA-PLANE].  See also the
   definition of "section layer network" in Section 1.2.2 of [RFC5654].

1.3.4.  MPLS-TP Label Switched Path

   An MPLS-TP Label Switched Path (MPLS-TP LSP) is an LSP that uses a
   subset of the capabilities of an MPLS LSP in order to meet the
   requirements of an MPLS transport network as set out in [RFC5654].
   The characteristics of an MPLS-TP LSP are primarily that it:

   1.  Uses a subset of the MPLS OAM tools defined in [OAM-FRAMEWORK].

   2.  Supports 1+1, 1:1, and 1:N protection functions.

   3.  Is traffic engineered.

   4.  May be established and maintained via the management plane, or
       using GMPLS protocols when a control plane is used.

   5.  Is either point-to-point or point-to-multipoint.  Multipoint-to-
       point and multipoint-to-multipoint LSPs are not supported.




Bocci, et al.                 Informational                     [Page 7]

RFC 5921            MPLS Transport Profile Framework           July 2010


   6.  It is either unidirectional, associated bidirectional, or co-
       routed bidirectional (i.e., the forward and reverse components of
       a bidirectional LSP follow the same path, and the intermediate
       nodes are aware of their association).  These are further defined
       in [DATA-PLANE].

   Note that an MPLS LSP is defined to include any present and future
   MPLS capability, including those specifically added to support the
   transport network requirements.

   See [DATA-PLANE] for further details on the types and data-plane
   properties of MPLS-TP LSPs.

   The lowest server layer provided by MPLS-TP is an MPLS-TP LSP.  The
   client layers of an MPLS-TP LSP may be network-layer protocols, MPLS
   LSPs, or PWs.  The relationship of an MPLS-TP LSP to its client
   layers is described in detail in Section 3.4.

1.3.5.  MPLS-TP Label Switching Router

   An MPLS-TP Label Switching Router (LSR) is either an MPLS-TP Provider
   Edge (PE) router or an MPLS-TP Provider (P) router for a given LSP,
   as defined below.  The terms MPLS-TP PE router and MPLS-TP P router
   describe logical functions; a specific node may undertake only one of
   these roles on a given LSP.

   Note that the use of the term "router" in this context is historic
   and neither requires nor precludes the ability to perform IP
   forwarding.

1.3.5.1.  Label Edge Router

   An MPLS-TP Label Edge Router (LER) is an LSR that exists at the
   endpoints of an LSP and therefore pushes or pops the LSP label, i.e.,
   does not perform a label swap on the particular LSP under
   consideration.

1.3.5.2.  MPLS-TP Provider Edge Router

   An MPLS-TP Provider Edge (PE) router is an MPLS-TP LSR that adapts
   client traffic and encapsulates it to be transported over an MPLS-TP
   LSP.  Encapsulation may be as simple as pushing a label, or it may
   require the use of a pseudowire.  An MPLS-TP PE exists at the
   interface between a pair of layer networks.  For an MS-PW, an MPLS-TP
   PE may be either an S-PE or a T-PE, as defined in [RFC5659] (see
   below).  A PE that pushes or pops an LSP label is an LER for that
   LSP.




Bocci, et al.                 Informational                     [Page 8]

RFC 5921            MPLS Transport Profile Framework           July 2010


   The term Provider Edge refers to the node's role within a provider's
   network.  A provider edge router resides at the edge of a given
   MPLS-TP network domain, in which case it has links to another MPLS-TP
   network domain or to a CE, except for the case of a pseudowire
   switching provider edge (S-PE) router, which is not restricted to the
   edge of an MPLS-TP network domain.

1.3.5.3.  MPLS-TP Provider Router

   An MPLS-TP Provider router is an MPLS-TP LSR that does not provide
   MPLS-TP PE functionality for a given LSP.  An MPLS-TP P router
   switches LSPs that carry client traffic, but does not adapt client
   traffic and encapsulate it to be carried over an MPLS-TP LSP.  The
   term Provider Router refers to the node's role within a provider's
   network.  A provider router does not have links to other MPLS-TP
   network domains.

1.3.5.4.  Pseudowire Switching Provider Edge Router (S-PE)

   RFC 5659 [RFC5659] defines an S-PE as:

      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.

1.3.5.5.  Pseudowire Terminating Provider Edge (T-PE) Router

   RFC 5659 [RFC5659] defines a T-PE as:

      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.



Bocci, et al.                 Informational                     [Page 9]

RFC 5921            MPLS Transport Profile Framework           July 2010


1.3.6.  Customer Edge (CE)

   A Customer Edge (CE) is the client function that sources or sinks
   native service traffic to or from the MPLS-TP network.  CEs on either
   side of the MPLS-TP network are peers and view the MPLS-TP network as
   a single link.

1.3.7.  Transport LSP

   A Transport LSP is an LSP between a pair of PEs that may transit zero
   or more MPLS-TP provider routers.  When carrying PWs, the Transport
   LSP is equivalent to the PSN tunnel LSP in [RFC3985] terminology.

1.3.8.  Service LSP

   A service LSP is an LSP that carries a single client service.

1.3.9.  Layer Network

   A layer network is defined in [G.805] and described in [RFC5654].  A
   layer network provides for the transfer of client information and
   independent operation of the client OAM.  A layer network may be
   described in a service context as follows: one layer network may
   provide a (transport) service to a higher client layer network and
   may, in turn, be a client to a lower-layer network.  A layer network
   is a logical construction somewhat independent of arrangement or
   composition of physical network elements.  A particular physical
   network element may topologically belong to more than one layer
   network, depending on the actions it takes on the encapsulation
   associated with the logical layers (e.g., the label stack), and thus
   could be modeled as multiple logical elements.  A layer network may
   consist of one or more sublayers.

1.3.10.  Network Layer

   This document uses the term Network Layer in the same sense as it is
   used in [RFC3031] and [RFC3032].  Network-layer protocols are
   synonymous with those belonging to Layer 3 of the Open System
   Interconnect (OSI) network model [X.200].

1.3.11.  Service Interface

   The packet transport service provided by MPLS-TP is provided at a
   service interface.  Two types of service interfaces are defined:

   o  User-Network Interface (UNI) (see Section 3.4.3.1).

   o  Network-Network Interface (NNI) (see Section 3.4.3.2).



Bocci, et al.                 Informational                    [Page 10]

RFC 5921            MPLS Transport Profile Framework           July 2010


   A UNI service interface may be a Layer 2 interface that carries only
   network layer clients.  MPLS-TP LSPs are both necessary and
   sufficient to support this service interface as described in
   Section 3.4.3.  Alternatively, it may be a Layer 2 interface that
   carries both network-layer and non-network-layer clients.  To support
   this service interface, a PW is required to adapt the client traffic
   received over the service interface.  This PW in turn is a client of
   the MPLS-TP server layer.  This is described in Section 3.4.2.

   An NNI service interface may be to an MPLS LSP or a PW.  To support
   this case, an MPLS-TP PE participates in the service interface
   signaling.

1.3.12.  Native Service

   The native service is the client layer network service that is
   transported by the MPLS-TP network, whether a pseudowire or an LSP is
   used for the adaptation (see Section 3.4).

1.3.13.  Additional Definitions and Terminology

   Detailed definitions and additional terminology may be found in
   [RFC5654] and [ROSETTA-STONE].

2.  MPLS Transport Profile Requirements

   The requirements for MPLS-TP are specified in [RFC5654], [RFC5860],
   and [NM-REQ].  This section provides a brief reminder to guide the
   reader.  It is not normative or intended as a substitute for these
   documents.

   MPLS-TP must not modify the MPLS forwarding architecture and must be
   based on existing pseudowire and LSP constructs.

   Point-to-point LSPs may be unidirectional or bidirectional, and it
   must be possible to construct congruent bidirectional LSPs.

   MPLS-TP LSPs do not merge with other LSPs at an MPLS-TP LSR and it
   must be possible to detect if 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 and protection mechanisms, and forwarding of data packets, must
   be able to operate without IP forwarding support.



Bocci, et al.                 Informational                    [Page 11]

RFC 5921            MPLS Transport Profile Framework           July 2010


   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.  MPLS Transport Profile Overview

3.1.  Packet Transport Services

   One objective of MPLS-TP is to enable MPLS networks to provide packet
   transport services with a similar degree of predictability to that
   found in existing transport networks.  Such packet transport services
   exhibit a number of characteristics, defined in [RFC5654]:

   o  In an environment where an MPLS-TP layer network is supporting a
      client layer network, and the MPLS-TP layer network is supported
      by a server layer network then operation of the MPLS-TP layer
      network must be possible without any dependencies on either the
      server or client layer network.

   o  The service provided by the MPLS-TP network to a given client will
      not fall below the agreed level as a result of the traffic loading
      of other clients.

   o  The control and management planes of any client network layer that
      uses the service is isolated from the control and management
      planes of the MPLS-TP layer network, where the client network
      layer is considered to be the native service of the MPLS-TP
      network.

   o  Where a client network makes use of an MPLS-TP server that
      provides a packet transport service, the level of coordination
      required between the client and server layer networks is minimal
      (preferably no coordination will be required).

   o  The complete set of packets generated by a client MPLS(-TP) layer
      network using the packet transport service, which may contain
      packets that are not MPLS packets (e.g., IP or CLNS
      (Connectionless Network Service) packets used by the control/
      management plane of the client MPLS(-TP) layer network), are
      transported by the MPLS-TP server layer network.

   o  The packet transport service enables the MPLS-TP layer network
      addressing and other information (e.g., topology) to be hidden
      from any client layer networks using that service, and vice-versa.






Bocci, et al.                 Informational                    [Page 12]

RFC 5921            MPLS Transport Profile Framework           July 2010


   These characteristics imply that a packet transport service does not
   support a connectionless packet-switched forwarding mode.  However,
   this does not preclude it carrying client traffic associated with a
   connectionless service.

3.2.  Scope of the MPLS Transport Profile

   Figure 1 illustrates the scope of MPLS-TP.  MPLS-TP solutions are
   primarily intended for packet transport applications.  MPLS-TP is a
   strict subset of MPLS, and comprises only those functions that are
   necessary to meet the requirements of [RFC5654].  This includes MPLS
   functions that were defined prior to [RFC5654] but that meet the
   requirements of [RFC5654], together with additional functions defined
   to meet those requirements.  Some MPLS functions defined before
   [RFC5654] such as Equal Cost Multi-Path (ECMP), LDP signaling when
   used in such a way that it creates multipoint-to-point LSPs, and IP
   forwarding in the data plane are explicitly excluded from MPLS-TP by
   that requirements specification.

   Note that MPLS as a whole will continue to evolve to include
   additional functions that do not conform to the MPLS Transport
   Profile or its requirements, and thus fall outside the scope of
   MPLS-TP.

  |<============================== MPLS ==============================>|
                                                     { Post-RFC5654    }
                                                     { non-Transport   }
                                                     {   Functions     }
  |<========== Pre-RFC5654 MPLS ===========>|
  {      ECMP       }
  { LDP/non-TE LSPs }
  {  IP forwarding  }

                    |<======== MPLS-TP ============>|
                                       { Additional }
                                       {  Transport }
                                       {  Functions }

                        Figure 1: Scope of MPLS-TP

   MPLS-TP can be used to construct packet networks and is therefore
   applicable in any packet network context.  A subset of MPLS-TP is
   also applicable to ITU-T-defined packet transport networks, where the
   transport network operational model is deemed attractive.







Bocci, et al.                 Informational                    [Page 13]

RFC 5921            MPLS Transport Profile Framework           July 2010


3.3.  Architecture

   MPLS-TP comprises the following architectural elements:

   o  A standard MPLS data plane [RFC3031] as profiled in [DATA-PLANE].

   o  Sections, LSPs, and PWs that provide a packet transport service
      for a client network.

   o  Proactive and on-demand Operations, Administration, and
      Maintenance (OAM) functions to monitor and diagnose the MPLS-TP
      network, as outlined in [OAM-FRAMEWORK].

   o  Control planes for LSPs and PWs, as well as support for static
      provisioning and configuration, as outlined in [CP-FRAMEWORK].

   o  Path protection mechanisms to ensure that the packet transport
      service survives anticipated failures and degradations of the
      MPLS-TP network, as outlined in [SURVIVE-FWK].

   o  Control-plane-based restoration mechanisms, as outlined in
      [SURVIVE-FWK].

   o  Network management functions, as outlined in [NM-FRAMEWORK].

   The MPLS-TP architecture for LSPs and PWs includes the following two
   sets of functions:

   o  MPLS-TP native service adaptation

   o  MPLS-TP forwarding

   The adaptation functions interface the native service (i.e., the
   client layer network service) to MPLS-TP.  This includes the case
   where the native service is an MPLS-TP LSP.

   The forwarding functions comprise the mechanisms required for
   forwarding the encapsulated native service traffic over an MPLS-TP
   server layer network, for example, PW and LSP labels.

3.3.1.  MPLS-TP Native Service Adaptation Functions

   The MPLS-TP native service adaptation functions interface the client
   layer network service to MPLS-TP.  For pseudowires, these adaptation
   functions are the payload encapsulation described in Section 4.4 of
   [RFC3985] and Section 6 of [RFC5659].  For network layer client
   services, the adaptation function uses the MPLS encapsulation format
   as defined in [RFC3032].



Bocci, et al.                 Informational                    [Page 14]

RFC 5921            MPLS Transport Profile Framework           July 2010


   The purpose of this encapsulation is to abstract the data plane of
   the client layer network from the MPLS-TP data plane, thus
   contributing to the independent operation of the MPLS-TP network.

   MPLS-TP is itself a client of an underlying server layer.  MPLS-TP is
   thus also bounded by a set of adaptation functions to this server
   layer network, which may itself be MPLS-TP.  These adaptation
   functions provide encapsulation of the MPLS-TP frames and for the
   transparent transport of those frames over the server layer network.
   The MPLS-TP client inherits its Quality of Service (QoS) from the
   MPLS-TP network, which in turn inherits its QoS from the server
   layer.  The server layer therefore needs to provide the necessary QoS
   to ensure that the MPLS-TP client QoS commitments can be satisfied.

3.3.2.  MPLS-TP Forwarding Functions

   The forwarding functions comprise the mechanisms required for
   forwarding the encapsulated native service traffic over an MPLS-TP
   server layer network, for example, PW and LSP labels.

   MPLS-TP LSPs use the MPLS label switching operations and Time-to-Live
   (TTL) processing procedures defined in [RFC3031], [RFC3032], and
   [RFC3443], as profiled in [DATA-PLANE].  These operations are highly
   optimized for performance and are not modified by the MPLS-TP
   profile.

   In addition, MPLS-TP PWs use the SS-PW and optionally the MS-PW
   forwarding operations defined in [RFC3985] and [RFC5659].

   Per-platform label space is used for PWs.  Either per-platform, per-
   interface, or other context-specific label space [RFC5331] may be
   used for LSPs.

   MPLS-TP forwarding is based on the label that identifies the
   transport path (LSP or PW).  The label value specifies the processing
   operation to be performed by 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 event that interrupts a swap operation is TTL
   expiry.  This is a fundamental architectural construct of MPLS to be
   taken into account when designing protocol extensions (such as those
   for OAM) that require packets to be sent to an intermediate LSR.

   Further processing to determine the context of a packet occurs when a
   swap operation is interrupted in this manner, or a pop operation
   exposes a specific reserved label at the top of the stack, or the





Bocci, et al.                 Informational                    [Page 15]

RFC 5921            MPLS Transport Profile Framework           July 2010


   packet is received with the GAL (Section 3.6) at the top of stack.
   Otherwise, the packet is forwarded according to the procedures in
   [RFC3032].

   MPLS-TP supports Quality of Service capabilities via the MPLS
   Differentiated Services (Diffserv) architecture [RFC3270].  Both
   E-LSP and L-LSP MPLS Diffserv modes are supported.

   Further details of MPLS-TP forwarding can be found in [DATA-PLANE].

3.4.  MPLS-TP Native Service Adaptation

   This document describes the architecture for two native service
   adaptation mechanisms, which provide encapsulation and demultiplexing
   for native service traffic traversing an MPLS-TP network:

   o  A PW

   o  An MPLS LSP

   MPLS-TP uses IETF-defined pseudowires to emulate certain services,
   for example, Ethernet, Frame Relay, or PPP / High-Level Data Link
   Control (HDLC).  A list of PW types is maintained by IANA in the
   "MPLS Pseudowire Type" registry.  When the native service adaptation
   is via a PW, the mechanisms described in Section 3.4.4 are used.

   An MPLS LSP can also provide the adaptation, in which case any native
   service traffic type supported by [RFC3031] and [RFC3032] is allowed.
   Examples of such traffic types include IP packets and MPLS-labeled
   packets.  Note that the latter case includes TE-LSPs [RFC3209] and
   LSP-based applications such as PWs, Layer 2 VPNs [RFC4664], and Layer
   3 VPNs [RFC4364].  When the native service adaptation is via an MPLS
   label, the mechanisms described in Section 3.4.5 are used.

3.4.1.  MPLS-TP Client/Server Layer Relationship

   The relationship between the client layer network and the MPLS-TP
   server layer network is defined by the MPLS-TP network boundary and
   the label context.  It is not explicitly indicated in the packet.  In
   terms of the MPLS label stack, when the native service traffic type
   is itself MPLS-labeled, then the S bits of all the labels in the
   MPLS-TP label stack carrying that client traffic are zero; otherwise,
   the bottom label of the MPLS-TP label stack has the S bit set to 1.
   In other words, there can be only one S bit set in a label stack.

   The data-plane behavior of MPLS-TP is the same as the best current
   practice for MPLS.  This includes the setting of the S bit.  In each
   case, the S bit is set to indicate the bottom (i.e., innermost) label



Bocci, et al.                 Informational                    [Page 16]

RFC 5921            MPLS Transport Profile Framework           July 2010


   in the label stack that is contiguous between the MPLS-TP LSP and its
   payload, and only one label stack entry (LSE) contains the S bit
   (Bottom of Stack bit) set to 1.  Note that this best current practice
   differs slightly from [RFC3032], which uses the S bit to identify
   when MPLS label processing stops and network layer processing starts.

   The relationship of MPLS-TP to its clients is illustrated in
   Figure 2.  Note that the label stacks shown in the figure are divided
   between those inside the MPLS-TP network and those within the client
   network when the client network is MPLS(-TP).  They illustrate the
   smallest number of labels possible.  These label stacks could also
   include more labels.

   PW-Based               MPLS Labeled                 IP
   Services                  Services                Transport
 |------------|  |-----------------------------|  |------------|

   Emulated        PW over LSP      IP over LSP         IP
   Service
                  +------------+
                  | PW Payload |
                  +------------+  +------------+               (CLIENTS)
                  |PW Lbl(S=1) |  |     IP     |
 +------------+   +------------+  +------------+  +------------+
 | PW Payload |   |LSP Lbl(S=0)|  |LSP Lbl(S=1)|  |     IP     |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 |PW Lbl (S=1)|   |LSP Lbl(S=0)|  |LSP Lbl(S=0)|  |LSP Lbl(S=1)|
 +------------+   +------------+  +------------+  +------------+
 |LSP Lbl(S=0)|         .               .               .
 +------------+         .               .               .      (MPLS-TP)
        .               .               .               .
        .
        .

~~~~~~~~~~~ denotes Client <-> MPLS-TP layer boundary

                  Figure 2: MPLS-TP - Client Relationship

3.4.2.  MPLS-TP Transport Layers

   An MPLS-TP network consists logically of two layers: the Transport
   Service layer and the Transport Path layer.

   The Transport Service layer provides the interface between Customer
   Edge (CE) nodes and the MPLS-TP network.  Each packet transmitted by
   a CE node for transport over the MPLS-TP network is associated at the
   receiving MPLS-TP Provider Edge (PE) node with a single logical
   point-to-point connection at the Transport Service layer between this



Bocci, et al.                 Informational                    [Page 17]

RFC 5921            MPLS Transport Profile Framework           July 2010


   (ingress) PE and the corresponding (egress) PE to which the peer CE
   is attached.  Such a connection is called an MPLS-TP Transport
   Service Instance, and the set of client packets belonging to the
   native service associated with such an instance on a particular CE-PE
   link is called a client flow.

   The Transport Path layer provides aggregation of Transport Service
   Instances over MPLS-TP transport paths (LSPs), as well as aggregation
   of transport paths (via LSP hierarchy).

   Awareness of the Transport Service layer need exist only at PE nodes.
   MPLS-TP Provider (P) nodes need have no awareness of this layer.
   Both PE and P nodes participate in the Transport Path layer.  A PE
   terminates (i.e., is an LER with respect to) the transport paths it
   supports, and is responsible for multiplexing and demultiplexing of
   Transport Service Instance traffic over such transport paths.

3.4.3.  MPLS-TP Transport Service Interfaces

   An MPLS-TP PE node can provide two types of interface to the
   Transport Service layer.  The MPLS-TP User-Network Interface (UNI)
   provides the interface between a CE and the MPLS-TP network.  The
   MPLS-TP Network-Network Interface (NNI) provides the interface
   between two MPLS-TP PEs in different administrative domains.

   When MPLS-TP is used to provide a transport service for, e.g., IP
   services that are a part of a Layer 3 VPN, then packets are
   transported in the same manner as specified in [RFC4364].

3.4.3.1.  User-Network Interface

   The MPLS-TP User-Network interface (UNI) is illustrated in Figure 3.
   The UNI for a particular client flow may or may not involve signaling
   between the CE and PE, and if signaling is used, it may or may not
   traverse the same attachment circuit that supports the client flow.
















Bocci, et al.                 Informational                    [Page 18]

RFC 5921            MPLS Transport Profile Framework           July 2010


    :          User-Network Interface        :           MPLS-TP
    :<-------------------------------------->:           Network <----->
    :                                        :
   -:-------------             --------------:------------------
    :             |           |              : Transport        |
    :             |           |  Transport   :   Path           |
    :             |           |   Service    : Mux/Demux        |
    :             |           |   Control    :    --            |
    :             |           |    Plane     :   |  |  Transport|
    : ----------  | Signaling |  ----------  :   |  |    Path   |
    :|Signaling |_|___________|_|Signaling | :   |  |    --------->
    :|Controller| |           | |Controller| :   |  |   |
    : ----------  |           |  ----------  :   |  |    --------->
    :      :......|...........|......:       :   |  |           |
    :             |  Control  |              :   |  |  Transport|
    :             |  Channel  |              :   |  |    Path   |
    :             |           |              :   |  |    --------->
    :             |           |              :   |  |  -+----------->TSI
    :             |           |  Transport   :   |  | |  --------->
    :             |  Client   |   Service    :   |  | |         |
    :             |  Traffic  |  Data Plane  :   |  | |         |
    : ----------  |  Flows    |  --------------  |  | |Transport|
    :|Signaling |-|-----------|-|Client/Service|-|  |-   Path   |
    :|Controller|=|===========|=|    Traffic   | |  |    --------->
    : ----------  |           | |  Processing  |=|  |===+===========>TSI
    :      |      |           |  --------------  |  |    --------->
    :      |______|___________|______|       :   |  |           |
    :             | Data Link |              :   |  |           |
    :             |           |              :    --            |
    :             |           |              :        Transport |
    :             |           |              :         Service  |
    :             |           |              :        Data Plane|
   ---------------             ---------------------------------
   Customer Edge Node              MPLS-TP Provider Edge Node


    TSI = Transport Service Instance

                   Figure 3: MPLS-TP PE Containing a UNI












Bocci, et al.                 Informational                    [Page 19]

RFC 5921            MPLS Transport Profile Framework           July 2010


        --------------From UNI------->            :
       -------------------------------------------:------------------
      |                     | Client Traffic Unit :                  |
      | Link-Layer-Specific | Link Decapsulation  : Service Instance |
      |    Processing       |         &           :    Transport     |
      |                     |  Service Instance   :  Encapsulation   |
      |                     |   Identification    :                  |
       -------------------------------------------:------------------
                                                  :
                                                  :
       -------------------------------------------:------------------
      |                     |                     : Service Instance |
      |                     |                     :    Transport     |
      | Link-Layer-Specific | Client Traffic Unit :  Decapsulation   |
      |    Processing       | Link Encapsulation  :        &         |
      |                     |                     : Service Instance |
      |                     |                     :  Identification  |
       -------------------------------------------:------------------
        <-------------To UNI ---------            :

       Figure 4: MPLS-TP UNI Client-Server Traffic Processing Stages

   Figure 4 shows the logical processing steps involved in a PE both for
   traffic flowing from the CE to the MPLS-TP network (left to right),
   and from the network to the CE (right to left).

   In the first case, when a packet from a client flow is received by
   the PE from the CE over the data-link, the following steps occur:

   1.  Link-layer-specific pre-processing, if any, is performed.  An
       example of such pre-processing is the PREP function illustrated
       in Figure 3 of [RFC3985].  Such pre-processing is outside the
       scope of MPLS-TP.

   2.  The packet is extracted from the data-link frame, if necessary,
       and associated with a Transport Service Instance.  At this point,
       UNI processing has completed.

   3.  A transport service encapsulation is associated with the packet,
       if necessary, for transport over the MPLS-TP network.

   4.  The packet is mapped to a transport path based on its associated
       Transport Service Instance, the transport path encapsulation is
       added, if necessary, and the packet is transmitted over the
       transport path.






Bocci, et al.                 Informational                    [Page 20]

RFC 5921            MPLS Transport Profile Framework           July 2010


   In the second case, when a packet associated with a Transport Service
   Instance arrives over a transport path, the following steps occur:

   1.  The transport path encapsulation is disposed of.

   2.  The transport service encapsulation is disposed of and the
       Transport Service Instance and client flow identified.

   3.  At this point, UNI processing begins.  A data-link encapsulation
       is associated with the packet for delivery to the CE based on the
       client flow.

   4.  Link-layer-specific postprocessing, if any, is performed.  Such
       postprocessing is outside the scope of MPLS-TP.

3.4.3.2.  Network-Network Interface

   The MPLS-TP NNI is illustrated in Figure 5.  The NNI for a particular
   Transport Service Instance may or may not involve signaling between
   the two PEs; and if signaling is used, it may or may not traverse the
   same data-link that supports the service instance.






























Bocci, et al.                 Informational                    [Page 21]

RFC 5921            MPLS Transport Profile Framework           July 2010


                   :      Network-Network Interface    :
                   :<--------------------------------->:
                   :                                   :
       ------------:-------------         -------------:------------
      |  Transport :             |       |             : Transport  |
      |    Path    : Transport   |       |  Transport  :   Path     |
      |  Mux/Demux :  Service    |       |   Service   : Mux/Demux  |
      |      --    :  Control    |       |   Control   :    --      |
      |     |  |   :   Plane     |Sig-   |    Plane    :   |  |     |
      |TP   |  |   : ----------  | naling|  ---------- :   |  |   TP|
    <---    |  |   :|Signaling |_|_______|_|Signaling |:   |  |    --->
   TSI<-+-  |  |   :|Controller| |       | |Controller|:   |  |   |
    <---  | |  |   : ----------  |       |  ---------- :   |  |    --->
      |   | |  |   :      :......|.......|......:      :   |  |     |
      |   | |  |   :             |Control|             :   |  |     |
      |TP | |  |   :             |Channel|             :   |  |   TP|
    <---  | |  |   :             |       |             :   |  |    --->
        | | |  |   :             |       |             :   |  |  -+->TSI
    <---  | |  |   : Transport   |       |  Transport  :   |  | |  --->
      |   | |  |   :  Service    |Service|   Service   :   |  | |   |
      |   | |  |   : Data Plane  |Traffic|  Data Plane :   |  | |   |
      |   | |  |  -------------  | Flows |  -------------  |  | |   |
      |TP  -|  |-|   Service   |-|-------|-|   Service   |-|  |-  TP|
    <---    |  | |   Traffic   | |       | |   Traffic   | |  |    --->
   TSI<=+===|  |=|  Processing |=|=======|=|  Processing |=|  |===+=>TSI
    <---    |  |  -------------  |       |  -------------  |  |    --->
      |     |  |   :      |______|_______|______|      :   |  |     |
      |     |  |   :             | Data  |             :   |  |     |
      |      --    :             | Link  |             :    --      |
      |            :             |       |             :            |
       --------------------------         --------------------------
       MPLS-TP Provider Edge Node         MPLS-TP Provider Edge Node


    TP  = Transport Path
    TSI = Transport Service Instance

                  Figure 5: MPLS-TP PE Containing an NNI













Bocci, et al.                 Informational                    [Page 22]

RFC 5921            MPLS Transport Profile Framework           July 2010


                                                   :
        --------------From NNI------->             :
       --------------------------------------------:------------------
      |                     | Service Traffic Unit :                  |
      | Link-Layer-Specific |  Link Decapsulation  : Service Instance |
      |    Processing       |          &           :  Encapsulation   |
      |                     |   Service Instance   :  Normalization   |
      |                     |    Identification    :                  |
       --------------------------------------------:------------------
                                                   :
                                                   :
       --------------------------------------------:------------------
      |                     |                      : Service Instance |
      |                     |                      :  Identification  |
      | Link-Layer-Specific | Service Traffic Unit :        &         |
      |    Processing       |  Link Encapsulation  : Service Instance |
      |                     |                      :  Encapsulation   |
      |                     |                      :  Normalization   |
       --------------------------------------------:------------------
        <-------------To NNI ---------             :

          Figure 6: MPLS-TP NNI Service Traffic Processing Stages

   Figure 6 shows the logical processing steps involved in a PE for
   traffic flowing both from the peer PE (left to right) and to the peer
   PE (right to left).

   In the first case, when a packet from a Transport Service Instance is
   received by the PE from the peer PE over the data-link, the following
   steps occur:

   1.  Link-layer specific pre-processing, if any, is performed.  Such
       pre-processing is outside the scope of MPLS-TP.

   2.  The packet is extracted from the data-link frame if necessary,
       and associated with a Transport Service Instance.  At this point,
       NNI processing has completed.

   3.  The transport service encapsulation of the packet is normalized
       for transport over the MPLS-TP network.  This step allows a
       different transport service encapsulation to be used over the NNI
       than that used in the internal MPLS-TP network.  An example of
       such normalization is a swap of a label identifying the Transport
       Service Instance.







Bocci, et al.                 Informational                    [Page 23]

RFC 5921            MPLS Transport Profile Framework           July 2010


   4.  The packet is mapped to a transport path based on its associated
       Transport Service Instance, the transport path encapsulation is
       added, if necessary, and the packet is transmitted over the
       transport path.

   In the second case, when a packet associated with a Transport Service
   Instance arrives over a transport path, the following steps occur:

   1.  The transport path encapsulation is disposed of.

   2.  The Transport Service Instance is identified from the transport
       service encapsulation, and this encapsulation is normalized for
       delivery over the NNI (see Step 3 above).

   3.  At this point, NNI processing begins.  A data-link encapsulation
       is associated with the packet for delivery to the peer PE based
       on the normalized Transport Service Instance.

   4.  Link-layer-specific postprocessing, if any, is performed.  Such
       postprocessing is outside the scope of MPLS-TP.

3.4.3.3.  Example Interfaces

   This section considers some special cases of UNI processing for
   particular transport service types.  These are illustrative, and do
   not preclude other transport service types.

3.4.3.3.1.  Layer 2 Transport Service

   In this example the MPLS-TP network is providing a point-to-point
   Layer 2 transport service between attached CE nodes.  This service is
   provided by a Transport Service Instance consisting of a PW
   established between the associated PE nodes.  The client flows
   associated with this Transport Service Instance are the sets of all
   Layer 2 frames transmitted and received over the attachment circuits.

   The processing steps in this case for a frame received from the CE
   are:

   1.  Link-layer specific pre-processing, if any, is performed,
       corresponding to the PREP function illustrated in Figure 3 of
       [RFC3985].

   2.  The frame is associated with a Transport Service Instance based
       on the attachment circuit over which it was received.

   3.  A transport service encapsulation, consisting of the PW control
       word and PW label, is associated with the frame.



Bocci, et al.                 Informational                    [Page 24]

RFC 5921            MPLS Transport Profile Framework           July 2010


   4.  The resulting packet is mapped to an LSP, the LSP label is
       pushed, and the packet is transmitted over the outbound interface
       associated with the LSP.

   For PW packets received over the LSP, the steps are performed in the
   reverse order.

3.4.3.3.2.  IP Transport Service

   In this example, the MPLS-TP network is providing a point-to-point IP
   transport service between CE1, CE2, and CE3, as follows.  One point-
   to-point Transport Service Instance delivers IPv4 packets between CE1
   and CE2, and another instance delivers IPv6 packets between CE1 and
   CE3.

   The processing steps in this case for an IP packet received from CE1
   are:

   1.  No link-layer-specific processing is performed.

   2.  The IP packet is extracted from the link-layer frame and
       associated with a Service LSP based on the source MAC address
       (CE1) and the IP protocol version.

   3.  A transport service encapsulation, consisting of the Service LSP
       label, is associated with the packet.

   4.  The resulting packet is mapped to a tunnel LSP, the tunnel LSP
       label is pushed, and the packet is transmitted over the outbound
       interface associated with the LSP.

   For packets received over a tunnel LSP carrying the Service LSP
   label, the steps are performed in the reverse order.

3.4.4.  Pseudowire Adaptation

   MPLS-TP uses pseudowires to provide a Virtual Private Wire Service
   (VPWS), a Virtual Private Local Area Network Service (VPLS), a
   Virtual Private Multicast Service (VPMS), and an Internet Protocol
   Local Area Network Service (IPLS).  VPWS, VLPS, and IPLS are
   described in [RFC4664].  VPMS is described in [VPMS-REQS].

   If the MPLS-TP network provides a layer 2 interface (that can carry
   both network-layer and non-network-layer traffic) as a service
   interface, then a PW is required to support the service interface.
   The PW is a client of the MPLS-TP LSP server layer.  The architecture
   for an MPLS-TP network that provides such services is based on the
   MPLS [RFC3031] and pseudowire [RFC3985] architectures.  Multi-segment



Bocci, et al.                 Informational                    [Page 25]

RFC 5921            MPLS Transport Profile Framework           July 2010


   pseudowires may optionally be used to provide a packet transport
   service, and their use is consistent with the MPLS-TP architecture.
   The use of MS-PWs may be motivated by, for example, the requirements
   specified in [RFC5254].  If MS-PWs are used, then the MS-PW
   architecture [RFC5659] also applies.

   Figure 7 shows the architecture for an MPLS-TP network using single-
   segment PWs.  Note that, in this document, the client layer is
   equivalent to the emulated service described in [RFC3985], while the
   Transport LSP is equivalent to the Packet Switched Network (PSN)
   tunnel of [RFC3985].

            |<----------------- Client Layer ------------------->|
            |                                                    |
            |          |<-------- Pseudowire -------->|          |
            |          |      encapsulated, packet    |          |
            |          |      transport service       |          |
            |          |                              |          |
            |          |          Transport           |          |
            |          |    |<------ LSP ------->|    |          |
            |          V    V                    V    V          |
            V    AC    +----+      +-----+       +----+     AC   V
      +-----+    |     | PE1|=======\   /========| PE2|     |    +-----+
      |     |----------|.......PW1.| \ / |............|----------|     |
      | CE1 |    |     |    |      |  X  |       |    |     |    | CE2 |
      |     |----------|.......PW2.| / \ |............|----------|     |
      +-----+  ^ |     |    |=======/   \========|    |     | ^  +-----+
            ^  |       +----+   ^  +-----+       +----+       |  ^
            |  |      Provider  |     ^         Provider      |  |
            |  |       Edge 1   |     |           Edge 2      |  |
     Customer  |                |  P Router                   | Customer
      Edge 1   |             TE LSP                           |  Edge 2
               |                                              |
               |                                              |
         Native service                                 Native service

            Figure 7: MPLS-TP Architecture (Single Segment PW)

   Figure 8 shows the architecture for an MPLS-TP network when multi-
   segment pseudowires are used.  Note that as in the SS-PW case,
   P-routers may also exist.










Bocci, et al.                 Informational                    [Page 26]

RFC 5921            MPLS Transport Profile Framework           July 2010


     |<--------------------- Client Layer ------------------------>|
     |                                                             |
     |                  Pseudowire encapsulated,                   |
     |    |<---------- Packet Transport Service ------------->|    |
     |    |                                                   |    |
     |    |              Transport               Transport    |    |
     | AC |     |<-------- LSP1 --------->|    |<--LSP2-->|   | AC |
     | |  V     V                         V    V          V   V |  |
     V |  +----+              +-----+    +----+          +----+ |  V
 +---+ |  |TPE1|===============\   /=====|SPE1|==========|TPE2| |  +---+
 |   |----|......PW1-Seg1.... | \ / | ......X...PW1-Seg2......|----|   |
 |CE1| |  |    |              |  X  |    |    |          |    | |  |CE2|
 |   |----|......PW2-Seg1.... | / \ | ......X...PW2-Seg2......|----|   |
 +---+  ^ |    |===============/   \=====|    |==========|    | | ^+---+
        | +----+     ^        +-----+    +----+     ^    +----+   |
        |            |           ^                  |             |
        |          TE LSP        |                TE LSP          |
        |                      P-router                           |
 Native Service                                          Native Service


 PW1-segment1 and PW1-segment2 are segments of the same MS-PW,
 while PW2-segment1 and PW2-segment2 are segments of another MS-PW.

             Figure 8: MPLS-TP Architecture (Multi-Segment PW)

   The corresponding MPLS-TP protocol stacks including PWs are shown in
   Figure 9.  In this figure, the Transport Service layer [RFC5654] is
   identified by the PW demultiplexer (Demux) label, and the Transport
   Path layer [RFC5654] is identified by the LSP Demux Label.





















Bocci, et al.                 Informational                    [Page 27]

RFC 5921            MPLS Transport Profile Framework           July 2010


  +-------------------+    /===================\   /===================\
  |  Client Layer     |    H     OAM PDU       H   H     OAM PDU       H
  /===================\    H-------------------H   H-------------------H
  H     PW Encap      H    H      GACh         H   H      GACh         H
  H-------------------H    H-------------------H   H-------------------H
  H   PW Demux (S=1)  H    H PW Demux (S=1)    H   H    GAL (S=1)      H
  H-------------------H    H-------------------H   H-------------------H
  H Trans LSP Demux(s)H    H Trans LSP Demux(s)H   H Trans LSP Demux(s)H
  \===================/    \===================/   \===================/
  |    Server Layer   |    |   Server Layer    |   |   Server Layer    |
  +-------------------+    +-------------------+   +-------------------+

      User Traffic                PW OAM                  LSP OAM

 Note: H(ighlighted) indicates the part of the protocol stack considered
 in this document.

              Figure 9: MPLS-TP Label Stack Using Pseudowires

   PWs and their associated labels may be configured or signaled.  See
   Section 3.11 for additional details related to configured service
   types.  See Section 3.9 for additional details related to signaled
   service types.

3.4.5.  Network Layer Adaptation

   MPLS-TP LSPs can be used to transport network-layer clients.  This
   document uses the term Network Layer in the same sense as it is used
   in [RFC3031] and [RFC3032].  The network-layer protocols supported by
   [RFC3031] and [RFC3032] can be transported between service
   interfaces.  Support for network-layer clients follows the MPLS
   architecture for support of network-layer protocols as specified in
   [RFC3031] and [RFC3032].

   With network-layer adaptation, the MPLS-TP domain provides either a
   unidirectional or bidirectional point-to-point connection between two
   PEs in order to deliver a packet transport service to attached
   customer edge (CE) nodes.  For example, a CE may be an IP, MPLS, or
   MPLS-TP node.  As shown in Figure 10, there is an attachment circuit
   between the CE node on the left and its corresponding provider edge
   (PE) node (which provides the service interface), a bidirectional LSP
   across the MPLS-TP network to the corresponding PE node on the right,
   and an attachment circuit between that PE node and the corresponding
   CE node for this service.

   The attachment circuits may be heterogeneous (e.g., any combination
   of SDH, PPP, Frame Relay, etc.) and network-layer protocol payloads
   arrive at the service interface encapsulated in the Layer 1 / Layer 2



Bocci, et al.                 Informational                    [Page 28]

RFC 5921            MPLS Transport Profile Framework           July 2010


   encoding defined for that access link type.  It should be noted that
   the set of network-layer protocols includes MPLS, and hence MPLS-
   encoded packets with an MPLS label stack (the client MPLS stack) may
   appear at the service interface.

   The following figures illustrate the reference models for network-
   layer adaptation.  The details of these figures are described further
   in the following paragraphs.

            |<------------- Client Network Layer --------------->|
            |                                                    |
            |          |<----------- Packet --------->|          |
            |          |         Transport Service    |          |
            |          |                              |          |
            |          |                              |          |
            |          |          Transport           |          |
            |          |    |<------ LSP ------->|    |          |
            |          V    V                    V    V          |
            V    AC    +----+      +-----+       +----+     AC   V
      +-----+    |     | PE1|=======\   /========| PE2|     |    +-----+
      |     |----------|..Svc LSP1.| \ / |............|----------|     |
      | CE1 |    |     |    |      |  X  |       |    |     |    | CE2 |
      |     |----------|..Svc LSP2.| / \ |............|----------|     |
      +-----+  ^ |     |    |=======/   \========|    |     | ^  +-----+
            ^  |       +----+  ^   +-----+       +----+     | |  ^
            |  |      Provider |       ^         Provider     |  |
            |  |       Edge 1  |       |          Edge 2      |  |
      Customer |               |    P Router                  | Customer
       Edge 1  |             TE LSP                           |  Edge 2
               |                                              |
               |                                              |
         Native service                                 Native service

         Figure 10: MPLS-TP Architecture for Network-Layer Clients

















Bocci, et al.                 Informational                    [Page 29]

RFC 5921            MPLS Transport Profile Framework           July 2010


    |<--------------------- Client Layer ------------------------>|
    |                                                             |
    |                                                             |
    |    |<---------- Packet Transport Service ------------->|    |
    |    |                                                   |    |
    |    |              Transport               Transport    |    |
    | AC |     |<-------- LSP1 --------->|    |<--LSP2-->|   | AC |
    | |  V     V                         V    V          V   V |  |
    V |  +----+              +-----+    +----+          +----+ |  V
+---+ |  | PE1|===============\   /=====| PE2|==========| PE3| |  +---+
|   |----|......svc-lsp1.... | \ / | .....X....svc-lsp1......|----|   |
|CE1| |  |    |              |  X  |    |    |          |    | |  |CE2|
|   |----|......svc-lsp2.... | / \ | .....X....svc-lsp2......|----|   |
+---+  ^ |    |===============/   \=====|    |==========|    | | ^+---+
       | +----+     ^        +-----+    +----+     ^    +----+   |
       |            |           ^         ^        |             |
       |          TE LSP        |         |      TE LSP          |
       |                      P-router    |                      |
Native Service               (LSR for     |               Native Service
                             T'port LSP1) |
                                          |
                                  LSR for Service LSPs
                                  LER for Transport LSPs

   Figure 11: MPLS-TP Architecture for Network Layer Adaptation, Showing
                           Service LSP Switching

   Client packets are received at the ingress service interface.  The PE
   pushes one or more labels onto the client packets that are then label
   switched over the transport network.  Correspondingly, the egress PE
   pops any labels added by the MPLS-TP networks and transmits the
   packet for delivery to the attached CE via the egress service
   interface.


















Bocci, et al.                 Informational                    [Page 30]

RFC 5921            MPLS Transport Profile Framework           July 2010


                           /===================\
                           H     OAM PDU       H
  +-------------------+    H-------------------H   /===================\
  |  Client Layer     |    H      GACh         H   H     OAM PDU       H
  /===================\    H-------------------H   H-------------------H
  H    Encap Label    H    H      GAL (S=1)    H   H      GACh         H
  H-------------------H    H-------------------H   H-------------------H
  H   SvcLSP Demux    H    H SvcLSP Demux (S=0)H   H    GAL (S=1)      H
  H-------------------H    H-------------------H   H-------------------H
  H Trans LSP Demux(s)H    H Trans LSP Demux(s)H   H Trans LSP Demux(s)H
  \===================/    \===================/   \===================/
  |   Server Layer    |    |   Server Layer    |   |   Server Layer    |
  +-------------------+    +-------------------+   +-------------------+

      User Traffic           Service LSP OAM             LSP OAM


 Note: H(ighlighted) indicates the part of the protocol stack considered
 in this document.

           Figure 12: MPLS-TP Label Stack for IP and LSP Clients

   In the figures above, the Transport Service layer [RFC5654] is
   identified by the Service LSP (SvcLSP) demultiplexer (Demux) label,
   and the Transport Path layer [RFC5654] is identified by the Transport
   (Trans) LSP Demux Label.  Note that the functions of the
   Encapsulation Label (Encap Label) and the Service Label (SvcLSP
   Demux) shown above may alternatively be represented by a single label
   stack entry.  Note that the S bit is always zero when the client
   layer is MPLS-labeled.  It may be necessary to swap a service LSP
   label at an intermediate node.  This is shown in Figure 11.

   Within the MPLS-TP transport network, the network-layer protocols are
   carried over the MPLS-TP network using a logically separate MPLS
   label stack (the server stack).  The server stack is entirely under
   the control of the nodes within the MPLS-TP transport network and it
   is not visible outside that network.  Figure 12 shows how a client
   network protocol stack (which may be an MPLS label stack and payload)
   is carried over a network layer client service over an MPLS-TP
   transport network.

   A label may be used to identify the network-layer protocol payload
   type.  Therefore, when multiple protocol payload types are to be
   carried over a single service LSP, a unique label stack entry needs
   to be present for each payload type.  Such labels are referred to as
   "Encapsulation Labels", one of which is shown in Figure 12.  An
   Encapsulation Label may be either configured or signaled.




Bocci, et al.                 Informational                    [Page 31]

RFC 5921            MPLS Transport Profile Framework           July 2010


   Both an Encapsulation Label and a Service Label should be present in
   the label stack when a particular packet transport service is
   supporting more than one network-layer protocol payload type.  For
   example, if both IP and MPLS are to be carried, then two
   Encapsulation Labels are mapped on to a common Service Label.

   Note: The Encapsulation Label may be omitted when the service LSP is
   supporting only one network-layer protocol payload type.  For
   example, if only MPLS labeled packets are carried over a service,
   then the Service Label (stack entry) provides both the payload type
   indication and service identification.  The Encapsulation Label
   cannot have any of the reserved label values [RFC3032].

   Service labels are typically carried over an MPLS-TP Transport LSP
   edge-to-edge (or transport path layer).  An MPLS-TP Transport LSP is
   represented as an LSP Transport Demux label, as shown in Figure 12.
   Transport LSP is commonly used when more than one service exists
   between two PEs.

   Note that, if only one service exists between two PEs, the functions
   of the Transport LSP label and the Service LSP Label may be combined
   into a single label stack entry.  For example, if only one service is
   carried between two PEs, then a single label could be used to provide
   both the service indication and the MPLS-TP Transport LSP.
   Alternatively, if multiple services exist between a pair of PEs, then
   a per-client Service Label would be mapped on to a common MPLS-TP
   Transport LSP.

   As noted above, the Layer 2 and Layer 1 protocols used to carry the
   network-layer protocol over the attachment circuits are not
   transported across the MPLS-TP network.  This enables the use of
   different Layer 2 and Layer 1 protocols on the two attachment
   circuits.

   At each service interface, Layer 2 addressing needs to be used to
   ensure the proper delivery of a network-layer packet to the adjacent
   node.  This is typically only an issue for LAN media technologies
   (e.g., Ethernet) that have Media Access Control (MAC) addresses.  In
   cases where a MAC address is needed, the sending node sets the
   destination MAC address to an address that ensures delivery to the
   adjacent node.  That is, the CE sets the destination MAC address to
   an address that ensures delivery to the PE, and the PE sets the
   destination MAC address to an address that ensures delivery to the
   CE.  The specific address used is technology type specific and is not
   specified in this document.  In some technologies, the MAC address
   will need to be configured.





Bocci, et al.                 Informational                    [Page 32]

RFC 5921            MPLS Transport Profile Framework           July 2010


   Note that when two CEs, which peer with each other, operate over a
   network layer transport service and run a routing protocol such as
   IS-IS or OSPF, some care should be taken to configure the routing
   protocols to use point-to-point adjacencies.  The specifics of such
   configuration is outside the scope of this document.  See [RFC5309]
   for additional details.

   The CE-to-CE service types and corresponding labels may be configured
   or signaled.

3.5.  Identifiers

   Identifiers are used to uniquely distinguish entities in an MPLS-TP
   network.  These include operators, nodes, LSPs, pseudowires, and
   their associated maintenance entities.  MPLS-TP defined two types of
   sets of identifiers: those that are compatible with IP, and those
   that are compatible with ITU-T transport-based operations.  The
   definition of these sets of identifiers is outside the scope of this
   document and is provided by [IDENTIFIERS].

3.6.  Generic Associated Channel (G-ACh)

   For correct operation of OAM mechanisms, it is important that OAM
   packets fate-share with the data packets.  In addition, in MPLS-TP it
   is necessary to discriminate between user data payloads and other
   types of payload.  For example, a packet may be associated with a
   Signaling Communication Channel (SCC) or a channel used for a
   protocol to coordinate path protection state.  This is achieved by
   carrying such packets in either:

   o  A generic control channel associated to the LSP, PW, or section,
      with no IP encapsulation, e.g., in a similar manner to
      Bidirectional Forwarding Detection for Virtual Circuit
      Connectivity Verification (VCCV-BFD) with PW ACH encapsulation
      [RFC5885]).

   o  An IP encapsulation where IP capabilities are present, e.g., PW
      ACH encapsulation with IP headers for VCCV-BFD [RFC5885] or IP
      encapsulation for MPLS BFD [RFC5884].

   MPLS-TP makes use of such a generic associated channel (G-ACh) to
   support Fault, Configuration, Accounting, Performance, and Security
   (FCAPS) functions by carrying packets related to OAM, a protocol used
   to coordinate path protection state, SCC, MCC or other packet types
   in-band over LSPs, PWs, or sections.  The G-ACh is defined in
   [RFC5586] and is similar to the Pseudowire Associated Channel
   [RFC4385], which is used to carry OAM packets over pseudowires.  The
   G-ACh is indicated by an Associated Channel Header (ACH), similar to



Bocci, et al.                 Informational                    [Page 33]

RFC 5921            MPLS Transport Profile Framework           July 2010


   the Pseudowire VCCV control word; this header is present for all
   sections, LSPs, and PWs that make use of FCAPS functions supported by
   the G-ACh.

   As specified in [RFC5586], the G-ACh must only be used for channels
   that are an adjunct to the data service.  Examples of these are OAM,
   a protocol used to coordinate path protection state, MCC, and SCC,
   but the use is not restricted to these 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.

   At the server layer, bandwidth and QoS commitments apply to the gross
   traffic on the LSP, PW, or section.  Since the G-ACh traffic is
   indistinguishable from the user data traffic, protocols using the
   G-ACh need to take into consideration the impact they have on the
   user data with which they are sharing resources.  Conversely,
   capacity needs to be made available for important G-ACh uses such as
   protection and OAM.  In addition, the security and congestion
   considerations described in [RFC5586] apply to protocols using the
   G-ACh.

   Figure 13 shows the reference model depicting how the control channel
   is associated with the pseudowire protocol stack.  This is based on
   the reference model for VCCV shown in Figure 2 of [RFC5085].


























Bocci, et al.                 Informational                    [Page 34]

RFC 5921            MPLS Transport Profile Framework           July 2010


          +-------------+                                +-------------+
          |  Payload    |           < FCAPS >            |  Payload    |
          +-------------+                                +-------------+
          |   Demux /   |         < ACH for PW >         |   Demux /   |
          |Discriminator|                                |Discriminator|
          +-------------+                                +-------------+
          |     PW      |             < PW >             |     PW      |
          +-------------+                                +-------------+
          |    PSN      |             < LSP >            |    PSN      |
          +-------------+                                +-------------+
          |  Physical   |                                |  Physical   |
          +-----+-------+                                +-----+-------+
                |                                              |
                |             ____     ___       ____          |
                |           _/    \___/   \    _/    \__       |
                |          /               \__/         \_     |
                |         /                               \    |
                +--------|        MPLS-TP Network          |---+
                          \                               /
                           \   ___      ___     __      _/
                            \_/   \____/   \___/  \____/

     Figure 13: PWE3 Protocol Stack Reference Model Showing the G-ACh

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




















Bocci, et al.                 Informational                    [Page 35]

RFC 5921            MPLS Transport Profile Framework           July 2010


          +-------------+                                +-------------+
          |  Payload    |           < FCAPS >            |   Payload   |
          +-------------+                                +-------------+
          |Discriminator|         < ACH on LSP >         |Discriminator|
          +-------------+                                +-------------+
          |Demultiplexer|         < GAL on LSP >         |Demultiplexer|
          +-------------+                                +-------------+
          |    PSN      |            < LSP >             |    PSN      |
          +-------------+                                +-------------+
          |  Physical   |                                |  Physical   |
          +-----+-------+                                +-----+-------+
                |                                              |
                |             ____     ___       ____          |
                |           _/    \___/   \    _/    \__       |
                |          /               \__/         \_     |
                |         /                               \    |
                +--------|        MPLS-TP Network          |---+
                          \                               /
                           \   ___      ___     __      _/
                            \_/   \____/   \___/  \____/

      Figure 14: MPLS Protocol Stack Reference Model Showing the LSP
                        Associated Control Channel

3.7.  Operations, Administration, and Maintenance (OAM)

   The MPLS-TP OAM architecture supports a wide range of OAM functions
   to check continuity, to verify connectivity, to monitor path
   performance, and to generate, filter, and manage local and remote
   defect alarms.  These functions are applicable to any layer defined
   within MPLS-TP, i.e., to MPLS-TP sections, LSPs, and PWs.

   The MPLS-TP OAM tool-set is able to operate without relying on a
   dynamic control plane or IP functionality in the data path.  In the
   case of an MPLS-TP deployment in a network in which IP functionality
   is available, all existing IP/MPLS OAM functions (e.g., LSP Ping,
   BFD, and VCCV) may be used.  Since MPLS-TP can operate in
   environments where IP is not used in the forwarding plane, the
   default mechanism for OAM demultiplexing in MPLS-TP LSPs and PWs is
   the Generic Associated Channel (Section 3.6).  Forwarding based on IP
   addresses for OAM or user data packets is not required for MPLS-TP.

   [RFC4379] and BFD for MPLS LSPs [RFC5884] have defined alert
   mechanisms that enable an 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 for IPv4 and that same range
   embedded as IPv4-mapped IPv6 addresses for IPv6 [RFC4379].  When the



Bocci, et al.                 Informational                    [Page 36]

RFC 5921            MPLS Transport Profile Framework           July 2010


   OAM packets are encapsulated in an IP header, these mechanisms are
   the default mechanisms for MPLS networks (in general) for identifying
   MPLS OAM packets, although the mechanisms defined in [RFC5586] can
   also be used.  MPLS-TP is able to operate in environments where IP
   forwarding is not supported, and thus the G-ACh/GAL is the default
   mechanism to demultiplex OAM packets in MPLS-TP in these
   environments.

   MPLS-TP supports a comprehensive set of OAM capabilities for packet
   transport applications, with equivalent capabilities to those
   provided in SONET/SDH.

   MPLS-TP requires [RFC5860] that a set of OAM capabilities is
   available to perform fault management (e.g., fault detection and
   localization) and performance monitoring (e.g., packet delay and loss
   measurement) of the LSP, PW, or section.  The framework for OAM in
   MPLS-TP is specified in [OAM-FRAMEWORK].

   MPLS-TP OAM packets share the same fate as their corresponding data
   packets, and are identified through the Generic Associated Channel
   mechanism [RFC5586].  This uses a combination of an Associated
   Channel Header (ACH) and a G-ACh Label (GAL) to create a control
   channel associated to an LSP, section, or PW.

   OAM and monitoring in MPLS-TP is based on the concept of maintenance
   entities, as described in [OAM-FRAMEWORK].  A Maintenance Entity (ME)
   can be viewed as the association of two Maintenance Entity Group End
   Points (MEPs).  A Maintenance Entity Group (MEG) is a collection of
   one or more MEs that belongs to the same transport path and that are
   maintained and monitored as a group.  The MEPs that form an ME limit
   the OAM responsibilities of an OAM flow to within the domain of a
   transport path or segment, in the specific layer network that is
   being monitored and managed.

   A MEG may also include a set of Maintenance Entity Group Intermediate
   Points (MIPs).

   A G-ACh packet may be directed to an individual MIP along the path of
   an LSP or MS-PW by setting the appropriate TTL in the label stack
   entry for the G-ACh packet, as per the traceroute mode of LSP Ping
   [RFC4379] and the vccv-trace mode of [SEGMENTED-PW].  Note that this
   works when the location of MIPs along the LSP or PW path is known by
   the MEP.  There may be circumstances where this is not the case,
   e.g., following restoration using a facility bypass LSP.  In these
   cases, tools to trace the path of the LSP may be used to determine
   the appropriate setting for the TTL to reach a specific MIP.





Bocci, et al.                 Informational                    [Page 37]

RFC 5921            MPLS Transport Profile Framework           July 2010


   Within an LSR or PE, MEPs and MIPs can only be placed where MPLS
   layer processing is performed on a packet.  The MPLS architecture
   mandates that MPLS layer processing occurs at least once on an LSR.

   Any node on an LSP can send an OAM packet on that LSP.  Likewise, any
   node on a PW can send OAM packets on a PW, including S-PEs.

   An OAM packet can only be received to be processed at an LSP
   endpoint, a PW endpoint (T-PE), or on the expiry of the TTL in the
   LSP or PW label stack entry.

3.8.  Return Path

   Management, control, and OAM protocol functions may require response
   packets to be delivered from the receiver back to the originator of a
   message exchange.  This section provides a summary of the return path
   options in MPLS-TP networks.  Although this section describes the
   case of an MPLS-TP LSP, it is also applicable to a PW.

   In this description, U and D are LSRs that terminate MPLS-TP LSPs
   (i.e., LERs), and Y is an intermediate LSR along the LSP.  Note that
   U is the upstream LER, and D is the downstream LER with respect to a
   particular direction of an LSP.  This reference model is shown in
   Figure 15.

                 LSP         LSP

           U ========= Y ========= D

          LER         LSR         LER

           ---------> Direction of user traffic flow

                  Figure 15: Return Path Reference Model

   The following cases are described for the various types of LSPs:

   Case 1  Return path packet transmission from D to U

   Case 2  Return path packet transmission from Y to U

   Case 3  Return path packet transmission from D to Y

   Note that a return path may not always exist (or may exist but be
   disabled), and that packet transmission in one or more of the above
   cases may not be possible.  In general, the existence and nature of
   return paths for MPLS-TP LSPs is determined by operational
   provisioning.



Bocci, et al.                 Informational                    [Page 38]

RFC 5921            MPLS Transport Profile Framework           July 2010


3.8.1.  Return Path Types

   There are two types of return path that may be used for the delivery
   of traffic from a downstream node D to an upstream node U.  Either:

   a.  The LSP between U and D is bidirectional, and therefore D has a
       path via the MPLS-TP LSP to return traffic back to U, or

   b.  D has some other unspecified means of directing traffic back to
       U.

   The first option is referred to as an "in-band" return path, the
   second as an "out-of-band" return path.

   There are various possibilities for "out-of-band" return paths.  Such
   a path may, for example, be based on ordinary IP routing.  In this
   case, packets would be forwarded as usual to a destination IP address
   associated with U.  In an MPLS-TP network that is also an IP/MPLS
   network, such a forwarding path may traverse the same physical links
   or logical transport paths used by MPLS-TP.  An out-of-band return
   path may also be indirect, via a distinct Data Communication Network
   (DCN) (provided, for example, by the method specified in [RFC5718]);
   or it may be via one or more other MPLS-TP LSPs.

3.8.2.  Point-to-Point Unidirectional LSPs

   Case 1  If an in-band return path is required to deliver traffic from
           D back to U, it is recommended for reasons of operational
           simplicity that point-to-point unidirectional LSPs be
           provisioned as associated bidirectional LSPs (which may also
           be co-routed) whenever return traffic from D to U is
           required.  Note that the two directions of such an LSP may
           have differing bandwidth allocations and QoS characteristics.
           The discussion below for such LSPs applies.

   As an alternative, an out-of-band return path may be used.

   Case 2  In this case, only the out-of-band return path option is
           available.  However, an additional out-of-band possibility is
           worthy of note here: if D is known to have a return path to
           U, then Y can arrange to deliver return traffic to U by first
           sending it to D along the original LSP.  The mechanism by
           which D recognizes the need for and performs this forwarding
           operation is protocol specific.

   Case 3  In this case, only the out-of-band return path option is
           available.  However, if D has a return path to U, then (in a
           manner analogous to the previous case) D can arrange to



Bocci, et al.                 Informational                    [Page 39]

RFC 5921            MPLS Transport Profile Framework           July 2010


           deliver return traffic to Y by first sending it to U along
           that return path.  The mechanism by which U recognizes the
           need for and performs this forwarding operation is protocol
           specific.

3.8.3.  Point-to-Point Associated Bidirectional LSPs

   For Case 1, D has a natural in-band return path to U, the use of
   which is typically preferred for return traffic, although out-of-band
   return paths are also applicable.

   For Cases 2 and 3, the considerations are the same as those for
   point-to-point unidirectional LSPs.

3.8.4.  Point-to-Point Co-Routed Bidirectional LSPs

   For all of Cases 1, 2, and 3, a natural in-band return path exists in
   the form of the LSP itself, and its use is preferred for return
   traffic.  Out-of-band return paths, however, are also applicable,
   primarily as an alternative means of delivery in case the in-band
   return path has failed.

3.9.  Control Plane

   A distributed dynamic control plane may be used to enable dynamic
   service provisioning in an MPLS-TP network.  Where the requirements
   specified in [RFC5654] can be met, the MPLS Transport Profile uses
   existing standard control-plane protocols for LSPs and PWs.

   Note that a dynamic control plane is not required in an MPLS-TP
   network.  See Section 3.11 for further details on statically
   configured and provisioned MPLS-TP services.

   Figure 16 illustrates the relationship between the MPLS-TP control
   plane, the forwarding plane, the management plane, and OAM for point-
   to-point MPLS-TP LSPs or PWs.















Bocci, et al.                 Informational                    [Page 40]

RFC 5921            MPLS Transport Profile Framework           July 2010


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

   Note:
      1) NMS may be centralized or distributed.  Control plane is
         distributed.
      2) 'Edge' functions refers to those functions present at
         the edge of a PSN domain, e.g., native service processing or
         classification.
      3) The control plane may be transported over the server
         layer, an LSP, or a G-ACh.

           Figure 16: MPLS-TP Control Plane Architecture Context

   The MPLS-TP control plane is based on existing MPLS and PW control
   plane protocols, and is consistent with the Automatically Switched
   Optical Network (ASON) architecture [G.8080].  MPLS-TP uses:

   o  Generalized MPLS (GMPLS) signaling ([RFC3945], [RFC3471],
      [RFC3473]) for LSPs, and

   o  Targeted LDP (T-LDP) signaling ([RFC4447], [SEGMENTED-PW],
      [DYN-MS-PW]) for pseudowires.

   MPLS-TP requires that any control-plane traffic be capable of being
   carried over an out-of-band signaling network or a signaling control
   channel such as the one described in [RFC5718].  Note that while
   T-LDP signaling is traditionally carried in-band in IP/MPLS networks,
   this does not preclude its operation over out-of-band channels.
   References to T-LDP in this document do not preclude the definition
   of alternative PW control protocols for use in MPLS-TP.




Bocci, et al.                 Informational                    [Page 41]

RFC 5921            MPLS Transport Profile Framework           July 2010


   PW control (and maintenance) takes place separately from LSP tunnel
   signaling.  The main coordination between LSP and PW control will
   occur within the nodes that terminate PWs.  The control planes for
   PWs and LSPs may be used independently, and one may be employed
   without the other.  This translates into the four possible scenarios:
   (1) no control plane is employed; (2) a control plane is used for
   both LSPs and PWs; (3) a control plane is used for LSPs, but not PWs;
   (4) a control plane is used for PWs, but not LSPs.  The PW and LSP
   control planes, collectively, need to satisfy the MPLS-TP control
   plane requirements reviewed in the MPLS-TP Control Plane Framework
   [CP-FRAMEWORK].  When client services are provided directly via LSPs,
   all requirements must be satisfied by the LSP control plane.  When
   client services are provided via PWs, the PW and LSP control planes
   operate in combination, and some functions may be satisfied via the
   PW control plane, while others are provided to PWs by the LSP control
   plane.

   Note that if MPLS-TP is being used in a multi-layer network, a number
   of control protocol types and instances may be used.  This is
   consistent with the MPLS architecture, which permits each label in
   the label stack to be allocated and signaled by its own control
   protocol.

   The distributed MPLS-TP control plane may provide the following
   functions:

   o  Signaling

   o  Routing

   o  Traffic engineering and constraint-based path computation

   In a multi-domain environment, the MPLS-TP control plane supports
   different types of interfaces at domain boundaries or within the
   domains.  These include the User-Network Interface (UNI), Internal
   Network-Network Interface (I-NNI), and External Network-Network
   Interface (E-NNI).  Note that different policies may be defined that
   control the information exchanged across these interface types.

   The MPLS-TP control plane is capable of activating MPLS-TP OAM
   functions as described in the OAM section of this document
   Section 3.7, 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 Section 3.12.




Bocci, et al.                 Informational                    [Page 42]

RFC 5921            MPLS Transport Profile Framework           July 2010


   Examples of the MPLS-TP data-plane connectivity patterns are LSPs
   utilizing the fast reroute backup methods as defined in [RFC4090] and
   ingress-to-egress 1+1 or 1:1 protected LSPs.

   The MPLS-TP control plane provides functions to ensure its own
   survivability and to enable it to recover gracefully from failures
   and degradations.  These include graceful restart and hot redundant
   configurations.  Depending on how the control plane is transported,
   varying degrees of decoupling between the control plane and data
   plane may be achieved.  In all cases, however, the control plane is
   logically decoupled from the data plane such that a control-plane
   failure does not imply a failure of the existing transport paths.

3.10.  Inter-Domain Connectivity

   A number of methods exist to support inter-domain operation of
   MPLS-TP, including the data-plane, OAM, and configuration aspects,
   for example:

   o  Inter-domain TE LSPs [RFC4726]

   o  Multi-segment Pseudowires [RFC5659]

   o  LSP stitching [RFC5150]

   o  Back-to-back attachment circuits [RFC5659]

   An important consideration in selecting an inter-domain connectivity
   mechanism is the degree of layer network isolation and types of OAM
   required by the operator.  The selection of which technique to use in
   a particular deployment scenario is outside the scope of this
   document.

3.11.  Static Operation of LSPs and PWs

   A PW or LSP may be statically configured without the support of a
   dynamic control plane.  This may be either by direct configuration of
   the PEs/LSRs or via a network management system.  Static operation is
   independent for a specific PW or LSP instance.  Thus, it should be
   possible for a PW to be statically configured, while the LSP
   supporting it is set up by a dynamic control plane.  When static
   configuration mechanisms are used, care must be taken to ensure that
   loops are not created.  Note that the path of an LSP or PW may be
   dynamically computed, while the LSP or PW itself is established
   through static configuration.






Bocci, et al.                 Informational                    [Page 43]

RFC 5921            MPLS Transport Profile Framework           July 2010


3.12.  Survivability

   The survivability architecture for MPLS-TP is specified in
   [SURVIVE-FWK].

   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 ([RFC4090]), while pseudowire
   redundancy [PW-REDUNDANCY] supports scenarios where the protection
   for the PW cannot be fully provided by the underlying LSP (i.e.,
   where the backup PW terminates on a different target PE node than the
   working PW in dual-homing scenarios, or where protection of the S-PE
   is required).  Additionally, GMPLS provides a well-known set of
   control-plane-driven protection and restoration mechanisms [RFC4872].
   MPLS-TP provides additional protection mechanisms that are optimized
   for both linear topologies and ring topologies and that operate in
   the absence of a dynamic control plane.  These are specified in
   [SURVIVE-FWK].

   Different protection schemes apply to different deployment topologies
   and operational considerations.  Such protection schemes may provide
   different levels of resiliency, for example:

   o  two concurrent traffic paths (1+1).

   o  one active and one standby path with guaranteed bandwidth on both
      paths (1:1).

   o  one active path and a standby path the resources of which are
      shared by one or more other active paths (shared protection).

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

   The characteristics of MPLS-TP resiliency mechanisms are as follows:

   o  Optimized for linear, ring, or meshed topologies.

   o  Use OAM mechanisms to detect and localize network faults or
      service degenerations.

   o  Include protection mechanisms to coordinate and trigger protection
      switching actions in the absence of a dynamic control plane.

   o  MPLS-TP recovery schemes are applicable to all levels in the
      MPLS-TP domain (i.e., section, LSP, and PW) providing segment and
      end-to-end recovery.



Bocci, et al.                 Informational                    [Page 44]

RFC 5921            MPLS Transport Profile Framework           July 2010


   o  MPLS-TP recovery mechanisms support the coordination of protection
      switching at multiple levels to prevent race conditions occurring
      between a client and its server layer.

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

   o  MPLS-TP supports revertive and non-revertive behavior.

3.13.  Sub-Path Maintenance

   In order to monitor, protect, and manage a portion (i.e., segment or
   concatenated segment) of an LSP, a hierarchical LSP [RFC3031] can be
   instantiated.  A hierarchical LSP instantiated for this purpose is
   called a Sub-Path Maintenance Element (SPME).  Note that by
   definition an SPME does not carry user traffic as a direct client.

   An SPME is defined between the edges of the portion of the LSP that
   needs to be monitored, protected or managed.  The SPME forms an
   MPLS-TP Section [DATA-PLANE] that carries the original LSP over this
   portion of the network as a client.  OAM messages can be initiated at
   the edge of the SPME and sent to the peer edge of the SPME or to a
   MIP along the SPME by setting the TTL value of the LSE at the
   corresponding hierarchical LSP level.  A P router only pushes or pops
   a label if it is at the end of a SPME.  In this mode, it is an LER
   for the SPME.

   For example, in Figure 17, two SPMEs are configured to allow
   monitoring, protection, and management of the LSP concatenated
   segments.  One SPME is defined between LER2 and LER3, and a second
   SPME is set up between LER4 and LER5.  Each of these SPMEs may be
   monitored, protected, or managed independently.

   |<============================= LSP =============================>|

          |<---- Carrier 1 ---->|       |<---- Carrier 2 ---->|

 |LER1|---|LER2|---|LSR|---|LER3|-------|LER4|---|LSR|---|LER5|---|LER6|

          |====== SPME =========|       |====== SPME =========|
                 (Carrier 1)                 (Carrier 2)

 Note 1: LER2, LER3, LER4, and LER5 are with respect to the SPME,
         but LSRs with respect to the LSP.
 Note 2: The LSP terminates in LERs outside of Carrier 1 and
         Carrier 2, for example, LER1 and LER6.

                 Figure 17: SPMEs in Inter-Carrier Network



Bocci, et al.                 Informational                    [Page 45]

RFC 5921            MPLS Transport Profile Framework           July 2010


   The end-to-end traffic of the LSP, including data traffic and control
   traffic (OAM, Protection Switching Control, management and signaling
   messages) is tunneled within the hierarchical LSP by means of label
   stacking as defined in [RFC3031].

   The mapping between an LSP and a SPME can be 1:1, in which case it is
   similar to the ITU-T Tandem Connection Element [G.805].  The mapping
   can also be 1:N to allow aggregated monitoring, protection, and
   management of a set of LSP segments or concatenated LSP segments.
   Figure 18 shows a SPME that is used to aggregate a set of
   concatenated LSP segments for the LSP from LERx to LERt and the LSP
   from LERa to LERd.  Note that such a construct is useful, for
   example, when the LSPs traverse a common portion of the network and
   they have the same Traffic Class.

   The QoS aspects of a SPME are network specific.  [OAM-FRAMEWORK]
   provides further considerations on the QoS aspects of OAM.

  |LERx|--|LSRy|-+                                      +-|LSRz|--|LERt|
                 |                                      |
                 |  |<---------- Carrier 1 --------->|  |
                 |  +-----+   +---+   +---+    +-----+  |
                 +--|     |---|   |---|   |----|     |--+
                    |LER1 |   |LSR|   |LSR|    |LER2 |
                 +--|     |---|   |---|   |----|     |--+
                 |  +-----+   +---+   + P +    +-----+  |
                 |  |============ SPME ==============|  |
  |LERa|--|LSRb|-+            (Carrier 1)               +-|LSRc|--|LERd|

          Figure 18: SPME for a Set of Concatenated LSP Segments

   SPMEs can be provisioned either statically or using control-plane
   signaling procedures.  The make-before-break procedures which are
   supported by MPLS allow the creation of a SPME on existing LSPs in-
   service without traffic disruption, as described in [SURVIVE-FWK].  A
   SPME can be defined corresponding to one or more end-to-end LSPs.
   New end-to-end LSPs that are tunneled within the SPME can be set up,
   which may require coordination across administrative boundaries.
   Traffic of the existing LSPs is switched over to the new end-to-end
   tunneled LSPs.  The old end-to-end LSPs can then be torn down.

   Hierarchical label stacking, in a similar manner to that described
   above, can be used to implement Sub-Path Maintenance Elements on
   pseudowires, as described in [OAM-FRAMEWORK].







Bocci, et al.                 Informational                    [Page 46]

RFC 5921            MPLS Transport Profile Framework           July 2010


3.14.  Network Management

   The network management architecture and requirements for MPLS-TP are
   specified in [NM-FRAMEWORK] and [NM-REQ].  These derive from the
   generic specifications described in ITU-T G.7710/Y.1701 [G.7710] for
   transport technologies.  They also incorporate the OAM requirements
   for MPLS Networks [RFC4377] and MPLS-TP Networks [RFC5860] and expand
   on those requirements to cover the modifications necessary for fault,
   configuration, performance, and security in a transport network.

   The Equipment Management Function (EMF) of an MPLS-TP Network Element
   (NE) (i.e., LSR, LER, PE, S-PE, or T-PE) 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.  The Network Management System (NMS) can be used to
   provision and manage an end-to-end connection across a network.
   Maintenance operations are run on a connection (LSP or PW) in a
   manner that is independent of the provisioning mechanism.  Segments
   may be created or managed by, for example, Netconf [RFC4741], SNMP
   [RFC3411], or CORBA (Common Object Request Broker Architecture)
   interfaces, but not all segments need to be created or managed using
   the same type of interface.  Where an MPLS-TP NE is managed by an
   NMS, at least one of these standard management mechanisms is required
   for interoperability, but this document imposes no restriction on
   which of these standard management protocols is used.  In MPLS-TP,
   the EMF needs to support statically provisioning LSPs for an LSR or
   LER, and PWs for a PE, as well as any associated MEPs and MIPs, as
   per Section 3.11.

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

   Configuration Management (CM) provides functions to control,
   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 Maintenance Entity Group Endpoint (MEP) ID and
   MEG Intermediate Point (MIP) ID).  The EMF also supports the
   configuration of OAM parameters as a part of connectivity management
   to meet specific operational requirements.  These may specify whether



Bocci, et al.                 Informational                    [Page 47]

RFC 5921            MPLS Transport Profile Framework           July 2010


   the operational mode is one-time on-demand or is periodic at a
   specified frequency.

   The Performance Management (PM) functions within the EMF of an
   MPLS-TP NE support the evaluation and reporting of the behavior of
   the NEs and the network.  One particular requirement for PM is to
   provide coherent and consistent interpretation of the network
   behavior in a hybrid network that uses multiple transport
   technologies.  Packet loss measurement and delay measurements may be
   collected and used to detect performance degradation.  This is
   reported via fault management to enable corrective actions to be
   taken (e.g., protection switching) and via performance monitoring for
   Service Level Agreement (SLA) verification and billing.  Collection
   mechanisms for performance data should be capable of operating on-
   demand or proactively.

4.  Security Considerations

   The introduction of MPLS-TP into transport networks means that the
   security considerations applicable to both MPLS [RFC3031] and PWE3
   [RFC3985] apply to those transport networks.  When an MPLS function
   is included in the MPLS transport profile, the security
   considerations pertinent to that function apply to MPLS-TP.
   Furthermore, when general MPLS networks that utilize functionality
   outside of the strict MPLS Transport Profile are used to support
   packet transport services, the security considerations of that
   additional functionality also apply.

   For pseudowires, the security considerations of [RFC3985] and
   [RFC5659] apply.

   MPLS-TP nodes that implement the G-ACh create a Control Channel (CC)
   associated with a pseudowire, LSP, or section.  This control channel
   can be signaled or statically configured.  Over this control channel,
   control channel messages related to network maintenance functions
   such as OAM, signaling, or network management are sent.  Therefore,
   three different areas are of concern from a security standpoint.

   The first area of concern relates to control plane parameter and
   status message attacks, that is, attacks that concern the signaling
   of G-ACh capabilities.  MPLS-TP Control Plane security is discussed
   in [RFC5920].

   A second area of concern centers on data-plane attacks, that is,
   attacks on the G-ACh itself.  MPLS-TP nodes that implement the G-ACh
   mechanisms are subject to additional data-plane denial-of-service
   attacks as follows:




Bocci, et al.                 Informational                    [Page 48]

RFC 5921            MPLS Transport Profile Framework           July 2010


      An intruder could intercept or inject G-ACh packets effectively
      disrupting the protocols carried over the G-ACh.

      An intruder could deliberately flood a peer MPLS-TP node with
      G-ACh messages to deny services to others.

      A misconfigured or misbehaving device could inadvertently flood a
      peer MPLS-TP node with G-ACh messages that could result in denial
      of services.  In particular, if a node has either implicitly or
      explicitly indicated that it cannot support one or all of the
      types of G-ACh protocol, but is sent those messages in sufficient
      quantity, it could result in a denial of service.

   To protect against these potential (deliberate or unintentional)
   attacks, multiple mitigation techniques can be employed:

      G-ACh message throttling mechanisms can be used, especially in
      distributed implementations that have a centralized control-plane
      processor with various line cards attached by some control-plane
      data path.  In these architectures, G-ACh messages may be
      processed on the central processor after being forwarded there by
      the receiving line card.  In this case, the path between the line
      card and the control processor may become saturated if appropriate
      G-ACh traffic throttling is not employed, which could lead to a
      complete denial of service to users of the particular line card.
      Such filtering is also useful for preventing the processing of
      unwanted G-ACh messages, such as those which are sent on unwanted
      (and perhaps unadvertised) control channel types.

   A third and last area of concern relates to the processing of the
   actual contents of G-ACh messages.  It is necessary that the
   definition of the protocols using these messages carried over a G-ACh
   include appropriate security measures.

   Additional security considerations apply to each MPLS-TP solution.
   These are discussed further in [SEC-FRAMEWORK].

   The security considerations in [RFC5920] apply.

5.  IANA Considerations

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

   This document introduces no additional IANA considerations in itself.





Bocci, et al.                 Informational                    [Page 49]

RFC 5921            MPLS Transport Profile Framework           July 2010


6.  Acknowledgements

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

   o  Rahul Aggarwal

   o  Dieter Beller

   o  Malcolm Betts

   o  Italo Busi

   o  John E Drake

   o  Hing-Kam Lam

   o  Marc Lasserre

   o  Vincenzo Sestito

   o  Nurit Sprecher

   o  Martin Vigoureux

   o  Yaacov Weingarten

   o  The participants of ITU-T SG15

7.  References

7.1.  Normative References

   [G.7710]         ITU-T, "Common equipment management function
                    requirements", ITU-T Recommendation G.7710/Y.1701,
                    July 2007.

   [G.805]          ITU-T, "Generic Functional Architecture of Transport
                    Networks", ITU-T Recommendation G.805, November
                    1995.

   [RFC3031]        Rosen, E., Viswanathan, A., and R. Callon,
                    "Multiprotocol Label Switching Architecture", RFC
                    3031, January 2001.

   [RFC3032]        Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                    Farinacci, D., Li, T., and A. Conta, "MPLS Label
                    Stack Encoding", RFC 3032, January 2001.



Bocci, et al.                 Informational                    [Page 50]

RFC 5921            MPLS Transport Profile Framework           July 2010


   [RFC3270]        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.

   [RFC3473]        Berger, L., "Generalized Multi-Protocol Label
                    Switching (GMPLS) Signaling Resource ReserVation
                    Protocol-Traffic Engineering (RSVP-TE) Extensions",
                    RFC 3473, January 2003.

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

   [RFC4090]        Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
                    Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
                    May 2005.

   [RFC4385]        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.

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

   [RFC4872]        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.

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

   [RFC5586]        Bocci, M., Vigoureux, M., and S. Bryant, "MPLS
                    Generic Associated Channel", RFC 5586, June 2009.

7.2.  Informative References

   [CP-FRAMEWORK]   Andersson, L., Berger, L., Fang, L., Bitar, N.,
                    Takacs, A., Vigoureux, M., Bellagamba, E., and E.
                    Gray, "MPLS-TP Control Plane Framework", Work in
                    Progress, March 2010.





Bocci, et al.                 Informational                    [Page 51]

RFC 5921            MPLS Transport Profile Framework           July 2010


   [DATA-PLANE]     Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
                    Profile Data Plane Architecture", Work in Progress,
                    July 2010.

   [DYN-MS-PW]      Martini, L., Bocci, M., Balus, F., Bitar, N., Shah,
                    H., Aissaoui, M., Rusmisel, J., Serbest, Y., Malis,
                    A., Metz, C., McDysan, D., Sugimoto, J., Duckett,
                    M., Loomis, M., Doolan, P., Pan, P., Pate, P.,
                    Radoaca, V., Wada, Y., and Y. Seo, "Dynamic
                    Placement of Multi Segment Pseudo Wires", Work in
                    Progress, October 2009.

   [G.8080]         ITU-T, "Architecture for the automatically switched
                    optical network (ASON)", ITU-T Recommendation
                    G.8080/Y.1304, 2005.

   [IDENTIFIERS]    Bocci, M. and G. Swallow, "MPLS-TP Identifiers",
                    Work in Progress, March 2010.

   [NM-FRAMEWORK]   Mansfield, S., Ed., Gray, E., Ed., and H. Lam, Ed.,
                    "MPLS-TP Network Management Framework", Work in
                    Progress, February 2010.

   [NM-REQ]         Mansfield, S. and K. Lam, "MPLS TP Network
                    Management Requirements", Work in Progress, October
                    2009.

   [OAM-DEF]        Andersson, L., Helvoort, H., Bonica, R., Romascanu,
                    D., and S. Mansfield, "The OAM Acronym Soup", Work
                    in Progress, June 2010.

   [OAM-FRAMEWORK]  Busi, I., Ed., Niven-Jenkins, B., Ed., and D. Allan,
                    Ed., "MPLS-TP OAM Framework", Work in Progress,
                    April 2010.

   [PW-REDUNDANCY]  Muley, P., "Pseudowire (PW) Redundancy", Work in
                    Progress, May 2010.

   [RFC3209]        Awduche, D., Berger, L., Gan, D., Li, T.,
                    Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions
                    to RSVP for LSP Tunnels", RFC 3209, December 2001.

   [RFC3411]        Harrington, D., Presuhn, R., and B. Wijnen, "An
                    Architecture for Describing Simple Network
                    Management Protocol (SNMP) Management Frameworks",
                    STD 62, RFC 3411, December 2002.





Bocci, et al.                 Informational                    [Page 52]

RFC 5921            MPLS Transport Profile Framework           July 2010


   [RFC3443]        Agarwal, P. and B. Akyol, "Time To Live (TTL)
                    Processing in Multi-Protocol Label Switching (MPLS)
                    Networks", RFC 3443, January 2003.

   [RFC3471]        Berger, L., "Generalized Multi-Protocol Label
                    Switching (GMPLS) Signaling Functional Description",
                    RFC 3471, January 2003.

   [RFC3945]        Mannie, E., "Generalized Multi-Protocol Label
                    Switching (GMPLS) Architecture", RFC 3945, October
                    2004.

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

   [RFC4377]        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.

   [RFC4379]        Kompella, K. and G. Swallow, "Detecting Multi-
                    Protocol Label Switched (MPLS) Data Plane Failures",
                    RFC 4379, February 2006.

   [RFC4664]        Andersson, L. and E. Rosen, "Framework for Layer 2
                    Virtual Private Networks (L2VPNs)", RFC 4664,
                    September 2006.

   [RFC4726]        Farrel, A., Vasseur, J., and A. Ayyangar, "A
                    Framework for Inter-Domain Multiprotocol Label
                    Switching Traffic Engineering", RFC 4726, November
                    2006.

   [RFC4741]        Enns, R., "NETCONF Configuration Protocol", RFC
                    4741, December 2006.

   [RFC5150]        Ayyangar, A., Kompella, K., Vasseur, JP., and A.
                    Farrel, "Label Switched Path Stitching with
                    Generalized Multiprotocol Label Switching Traffic
                    Engineering (GMPLS TE)", RFC 5150, February 2008.

   [RFC5254]        Bitar, N., Bocci, M., and L. Martini, "Requirements
                    for Multi-Segment Pseudowire Emulation Edge-to-Edge
                    (PWE3)", RFC 5254, October 2008.

   [RFC5309]        Shen, N. and A. Zinin, "Point-to-Point Operation
                    over LAN in Link State Routing Protocols", RFC 5309,
                    October 2008.



Bocci, et al.                 Informational                    [Page 53]

RFC 5921            MPLS Transport Profile Framework           July 2010


   [RFC5331]        Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS
                    Upstream Label Assignment and Context-Specific Label
                    Space", RFC 5331, August 2008.

   [RFC5654]        Niven-Jenkins, B., Brungard, D., Betts, M.,
                    Sprecher, N., and S. Ueno, "Requirements of an MPLS
                    Transport Profile", RFC 5654, September 2009.

   [RFC5659]        Bocci, M. and S. Bryant, "An Architecture for Multi-
                    Segment Pseudowire Emulation Edge-to-Edge", RFC
                    5659, October 2009.

   [RFC5718]        Beller, D. and A. Farrel, "An In-Band Data
                    Communication Network For the MPLS Transport
                    Profile", RFC 5718, January 2010.

   [RFC5860]        Vigoureux, M., Ward, D., and M. Betts, "Requirements
                    for Operations, Administration, and Maintenance
                    (OAM) in MPLS Transport Networks", RFC 5860, May
                    2010.

   [RFC5884]        Aggarwal, R., Kompella, K., Nadeau, T., and G.
                    Swallow, "Bidirectional Forwarding Detection (BFD)
                    for MPLS Label Switched Paths (LSPs)", RFC 5884,
                    June 2010.

   [RFC5885]        Nadeau, T. and C. Pignataro, "Bidirectional
                    Forwarding Detection (BFD) for the Pseudowire
                    Virtual Circuit Connectivity Verification (VCCV)",
                    RFC 5885, June 2010.

   [RFC5920]        Fang, L., Ed., "Security Framework for MPLS and
                    GMPLS Networks", RFC 5920, July 2010.

   [ROSETTA-STONE]  Sprecher, N., "A Thesaurus for the Terminology used
                    in Multiprotocol Label Switching Transport Profile
                    (MPLS-TP) drafts/RFCs and ITU-T's Transport Network
                    Recommendations.", Work in Progress, May 2010.

   [SEC-FRAMEWORK]  Fang, L. and B. Niven-Jenkins, "Security Framework
                    for MPLS-TP", Work in Progress, March 2010.

   [SEGMENTED-PW]   Martini, L., Nadeau, T., Metz, C., Bocci, M., and M.
                    Aissaoui, "Segmented Pseudowire", Work in Progress,
                    June 2010.






Bocci, et al.                 Informational                    [Page 54]

RFC 5921            MPLS Transport Profile Framework           July 2010


   [SURVIVE-FWK]    Sprecher, N. and A. Farrel, "Multiprotocol Label
                    Switching Transport Profile Survivability
                    Framework", Work in Progress, June 2010.

   [VPMS-REQS]      Kamite, Y., JOUNAY, F., Niven-Jenkins, B., Brungard,
                    D., and L. Jin, "Framework and Requirements for
                    Virtual Private Multicast Service (VPMS)", Work in
                    Progress, October 2009.

   [X.200]          ITU-T, "Information Technology - Open Systems
                    Interconnection - Basic reference Model: The Basic
                    Model", ITU-T Recommendation X.200, 1994.







































Bocci, et al.                 Informational                    [Page 55]

RFC 5921            MPLS Transport Profile Framework           July 2010


Authors' Addresses

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

   EMail: matthew.bocci@alcatel-lucent.com


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

   EMail: stbryant@cisco.com


   Dan Frost (editor)
   Cisco Systems

   EMail: danfrost@cisco.com


   Lieven Levrau
   Alcatel-Lucent
   7-9, Avenue Morane Sulnier
   Velizy  78141
   France

   EMail: lieven.levrau@alcatel-lucent.com


   Lou Berger
   LabN Consulting, L.L.C.

   EMail: lberger@labn.net












Bocci, et al.                 Informational                    [Page 56]

