MPLS-TP IDs: MPLS Transport Profile (MPLS-TP) Identifiers

Internet Engineering Task Force (IETF)                          M. Bocci
Request for Comments: 6370                                Alcatel-Lucent
Category: Standards Track                                     G. Swallow
ISSN: 2070-1721                                                    Cisco
                                                                 E. Gray
                                                                Ericsson
                                                          September 2011


              MPLS Transport Profile (MPLS-TP) Identifiers

Abstract

   This document specifies an initial set of identifiers to be used in
   the Transport Profile of Multiprotocol Label Switching (MPLS-TP).
   The MPLS-TP requirements (RFC 5654) require that the elements and
   objects in an MPLS-TP environment are able to be configured and
   managed without a control plane.  In such an environment, many
   conventions for defining identifiers are possible.  This document
   defines identifiers for MPLS-TP management and Operations,
   Administration, and Maintenance (OAM) functions compatible with IP/
   MPLS conventions.

   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.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6370.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
      1.2. Requirements Language ......................................4
      1.3. Notational Conventions .....................................4
   2. Named Entities ..................................................5
   3. Uniquely Identifying an Operator - the Global_ID ................5
   4. Node and Interface Identifiers ..................................6
   5. MPLS-TP Tunnel and LSP Identifiers ..............................7
      5.1. MPLS-TP Point-to-Point Tunnel Identifiers ..................8
      5.2. MPLS-TP LSP Identifiers ....................................9
           5.2.1. MPLS-TP Co-Routed Bidirectional LSP Identifiers .....9
           5.2.2. MPLS-TP Associated Bidirectional LSP Identifiers ....9
      5.3. Mapping to RSVP Signaling .................................10
   6. Pseudowire Path Identifiers ....................................11
   7. Maintenance Identifiers ........................................13
      7.1. Maintenance Entity Group Identifiers ......................13
           7.1.1. MPLS-TP Section MEG_IDs ............................13
           7.1.2. MPLS-TP LSP MEG_IDs ................................13
           7.1.3. Pseudowire MEG_IDs .................................14
      7.2. Maintenance Entity Group End Point Identifiers ............14
           7.2.1. MPLS-TP Section MEP_IDs ............................14
           7.2.2. MPLS-TP LSP_MEP_ID .................................15
           7.2.3. MEP_IDs for Pseudowires ............................15
      7.3. Maintenance Entity Group Intermediate Point Identifiers ...15
   8. Security Considerations ........................................15
   9. References .....................................................16
      9.1. Normative References ......................................16
      9.2. Informative References ....................................17

1.  Introduction

   This document specifies an initial set of identifiers to be used in
   the Transport Profile of Multiprotocol Label Switching (MPLS-TP).
   The MPLS-TP requirements (RFC 5654 [7]) require that the elements and
   objects in an MPLS-TP environment are able to be configured and
   managed without a control plane.  In such an environment, many
   conventions for defining identifiers are possible.  This document
   defines identifiers for MPLS-TP management and OAM functions
   compatible with IP/MPLS conventions.  That is, the identifiers have
   been chosen to be compatible with existing IP, MPLS, GMPLS, and
   Pseudowire definitions.

   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.

1.1.  Terminology

   AGI: Attachment Group Identifier

   AII: Attachment Interface Identifier

   AS: Autonomous System

   ASN: Autonomous System Number

   EGP: Exterior Gateway Protocol

   FEC: Forwarding Equivalence Class

   GMPLS: Generalized Multiprotocol Label Switching

   IGP: Interior Gateway Protocol

   LSP: Label Switched Path

   LSR: Label Switching Router

   MEG: Maintenance Entity Group

   MEP: Maintenance Entity Group End Point

   MIP: Maintenance Entity Group Intermediate Point
   MPLS: Multiprotocol Label Switching

   NNI: Network-to-Network Interface

   OAM: Operations, Administration, and Maintenance

   PW: Pseudowire

   RSVP: Resource Reservation Protocol

   RSVP-TE: RSVP Traffic Engineering

   SAII: Source AII

   SPME: Sub-Path Maintenance Entity

   T-PE: Terminating Provider Edge

   TAII: Target AII

1.2.  Requirements Language

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

1.3.  Notational Conventions

   All multiple-word atomic identifiers use underscores (_) between the
   words to join the words.  Many of the identifiers are composed of a
   set of other identifiers.  These are expressed by listing the latter
   identifiers joined with double-colon "::" notation.

   Where the same identifier type is used multiple times in a
   concatenation, they are qualified by a prefix joined to the
   identifier by a dash (-).  For example, A1-Node_ID is the Node_ID of
   a node referred to as A1.

   The notation defines a preferred ordering of the fields.
   Specifically, the designation A1 is used to indicate the lower sort
   order of a field or set of fields and Z9 is used to indicate the
   higher sort order of the same.  The sort is either alphanumeric or
   numeric depending on the field's definition.  Where the sort applies
   to a group of fields, those fields are grouped with {...}.

   Note, however, that the uniqueness of an identifier does not depend
   on the ordering, but rather, upon the uniqueness and scoping of the
   fields that compose the identifier.  Further, the preferred ordering
   is not intended to constrain protocol designs by dictating a
   particular field sequence (for example, see Section 5.2.1) or even
   what fields appear in which objects (for example, see Section 5.3).

2.  Named Entities

   In order to configure, operate, and manage a transport network based
   on the MPLS Transport Profile, a number of entities require
   identification.  Identifiers for the following entities are defined
   in this document:

      *  Global_ID

      *  Node

      *  Interface

      *  Tunnel

      *  LSP

      *  PW

      *  MEG

      *  MEP

      *  MIP

   Note that we have borrowed the term "tunnel" from RSVP-TE (RFC 3209
   [2]) where it is used to describe an entity that provides a logical
   association between a source and destination LSR.  The tunnel, in
   turn, is instantiated by one or more LSPs, where the additional LSPs
   are used for protection or re-grooming of the tunnel.

3.  Uniquely Identifying an Operator - the Global_ID

   The Global_ID is defined to uniquely identify an operator.  RFC 5003
   [3] defines a globally unique Attachment Interface Identifier (AII).
   That AII is composed of three parts: a Global_ID that uniquely
   identifies an operator, a prefix, and, finally, an attachment circuit
   identifier.  We have chosen to use that Global ID for MPLS-TP.
   Quoting from RFC 5003, Section 3.2:

      The global ID can contain the 2-octet or 4-octet value of the
      provider's Autonomous System Number (ASN).  It is expected that
      the global ID will be derived from the globally unique ASN of the
      autonomous system hosting the PEs containing the actual AIIs.  The
      presence of a global ID based on the operator's ASN ensures that
      the AII will be globally unique.

   A Global_ID is an unsigned 32-bit value and MUST be derived from a
   4-octet AS number assigned to the operator.  Note that 2-octet AS
   numbers have been incorporated in the 4-octet by placing the 2-octet
   AS number in the low-order octets and setting the two high-order
   octets to zero.

   ASN 0 is reserved and cannot be assigned to an operator.  An
   identifier containing a Global_ID of zero means that no Global_ID is
   specified.  Note that a Global_ID of zero is limited to entities
   contained within a single operator and MUST NOT be used across an
   NNI.

   The Global_ID is used solely to provide a globally unique context for
   other MPLS-TP identifiers.  While the AS number used in the Global_ID
   MUST be one that the operator is entitled to use, the use of the
   Global_ID is not related to the use of the ASN in protocols such as
   BGP.

4.  Node and Interface Identifiers

   An LSR requires identification of the node itself and of its
   interfaces.  An interface is the attachment point to a server
   (sub-)layer, e.g., MPLS-TP section or MPLS-TP tunnel.

   We call the identifier associated with a node a "Node Identifier"
   (Node_ID).  The Node_ID is a unique 32-bit value assigned by the
   operator within the scope of a Global_ID.  The structure of the
   Node_ID is operator-specific and is outside the scope of this
   document.  However, the value zero is reserved and MUST NOT be used.
   Where IPv4 addresses are used, it may be convenient to use the Node's
   IPv4 loopback address as the Node_ID; however, the Node_ID does not
   need to have any association with the IPv4 address space used in the
   operator's IGP or EGP.  Where IPv6 addresses are used exclusively, a
   32-bit value unique within the scope of a Global_ID is assigned.

   An LSR can support multiple layers (e.g., hierarchical LSPs) and the
   Node_ID belongs to the multiple-layer context, i.e., it is applicable
   to all LSPs or PWs that originate on, have an intermediate point on,
   or terminate on the node.

   In situations where a Node_ID needs to be globally unique, this is
   accomplished by prefixing the identifier with the operator's
   Global_ID.

   The term "interface" is used for the attachment point to an MPLS-TP
   section.  Within the context of a particular node, we call the
   identifier associated with an interface an "Interface Number"
   (IF_Num).  The IF_Num is a 32-bit unsigned integer assigned by the
   operator and MUST be unique within the scope of a Node_ID.  The
   IF_Num value 0 has special meaning (see Section 7.3, MIP Identifiers)
   and MUST NOT be used to identify an MPLS-TP interface.

   Note that IF_Num has no relation with the ifNum object defined in RFC
   2863 [8].  Further, no mapping is mandated between IF_Num and ifIndex
   in RFC 2863.

   An "Interface Identifier" (IF_ID) identifies an interface uniquely
   within the context of a Global_ID.  It is formed by concatenating the
   Node_ID with the IF_Num.  That is, an IF_ID is a 64-bit identifier
   formed as Node_ID::IF_Num.

   This convention was chosen to allow compatibility with GMPLS.  The
   GMPLS signaling functional description [4] requires interface
   identification.  GMPLS allows three formats for the Interface_ID.
   The third format consists of an IPv4 address plus a 32-bit unsigned
   integer for the specific interface.  The format defined for MPLS-TP
   is consistent with this format, but uses the Node_ID instead of an
   IPv4 address.

   If an IF_ID needs to be globally unique, this is accomplished by
   prefixing the identifier with the operator's Global_ID.

   Note that MPLS-TP supports hierarchical sections.  The attachment
   point to an MPLS-TP section at any (sub-)layer requires a node-unique
   IF_Num.

5.  MPLS-TP Tunnel and LSP Identifiers

   In MPLS, the actual transport of packets is provided by Label
   Switched Paths (LSPs).  A transport service may be composed of
   multiple LSPs.  Further, the LSPs providing a service may change over
   time due to protection and restoration events.  In order to clearly
   identify the service, we use the term "MPLS-TP Tunnel" or simply
   "tunnel" for a service provided by (for example) a working LSP and
   protected by a protection LSP.  The "Tunnel Identifier" (Tunnel_ID)
   identifies the transport service and provides a stable binding to the
   client in the face of changes in the data-plane LSPs used to provide
   the service due to protection or restoration events.  This section
   defines an MPLS-TP Tunnel_ID to uniquely identify a tunnel, and an
   MPLS-TP LSP Identifier (LSP_ID) to uniquely identify an LSP
   associated with a tunnel.

   For the case where multiple LSPs (for example) are used to support a
   single service with a common set of end points, using the Tunnel_ID
   allows for a trivial mapping between the server and client layers,
   providing a common service identifier that may be either defined by
   or used by the client.

   Note that this usage is not intended to constrain protection schemes,
   and may be used to identify any service (protected or unprotected)
   that may appear to the client as a single service attachment point.
   Keeping the Tunnel_ID consistent across working and protection LSPs
   is a useful construct currently employed within GMPLS.  However, the
   Tunnel_ID for a protection LSP MAY differ from that used by its
   corresponding working LSP.

5.1.  MPLS-TP Point-to-Point Tunnel Identifiers

   At each end point, a tunnel is uniquely identified by the end point's
   Node_ID and a locally assigned tunnel number.  Specifically, a
   "Tunnel Number" (Tunnel_Num) is a 16-bit unsigned integer unique
   within the context of the Node_ID.  The motivation for each end point
   having its own tunnel number is to allow a compact form for the
   MEP_ID.  See Section 7.2.2.

   Having two tunnel numbers also serves to simplify other signaling
   (e.g., setup of associated bidirectional tunnels as described in
   Section 5.3).

   The concatenation of the two end point identifiers serves as the full
   identifier.  Using the A1/Z9 convention, the format of a Tunnel_ID
   is:

      A1-{Node_ID::Tunnel_Num}::Z9-{Node_ID::Tunnel_Num}

   Where the Tunnel_ID needs to be globally unique, this is accomplished
   by using globally unique Node_IDs as defined above.  Thus, a globally
   unique Tunnel_ID becomes:

      A1-{Global_ID::Node_ID::Tunnel_Num}::Z9-{Global_ID::Node_ID::
      Tunnel_Num}

   When an MPLS-TP Tunnel is configured, it MUST be assigned a unique
   IF_ID at each end point.  As usual, the IF_ID is composed of the
   local Node_ID concatenated with a 32-bit IF_Num.

5.2.  MPLS-TP LSP Identifiers

   This section defines identifiers for MPLS-TP co-routed bidirectional
   and associated bidirectional LSPs.  Note that MPLS-TP Sub-Path
   Maintenance Entities (SPMEs), as defined in RFC 5921 [9], are also
   LSPs and use these same forms of identifiers.

5.2.1.  MPLS-TP Co-Routed Bidirectional LSP Identifiers

   A co-routed bidirectional LSP can be uniquely identified by a single
   LSP number within the scope of an MPLS-TP Tunnel_ID.  Specifically,
   an LSP Number (LSP_Num) is a 16-bit unsigned integer unique within
   the Tunnel_ID.  Thus, the format of an MPLS-TP co-routed
   bidirectional LSP_ID is:

      A1-{Node_ID::Tunnel_Num}::Z9-{Node_ID::Tunnel_Num}::LSP_Num

   Note that the uniqueness of identifiers does not depend on the A1/Z9
   sort ordering.  Thus, the identifier:

      Z9-{Node_ID::Tunnel_Num}::A1-{Node_ID::Tunnel_Num}::LSP_Num

   is synonymous with the one above.

   At the data-plane level, a co-routed bidirectional LSP is composed of
   two unidirectional LSPs traversing the same links in opposite
   directions.  Since a co-routed bidirectional LSP is provisioned or
   signaled as a single entity, a single LSP_Num is used for both
   unidirectional LSPs.  The unidirectional LSPs can be referenced by
   the identifiers:

      A1-Node_ID::A1-Tunnel_Num::LSP_Num::Z9-Node_ID and

      Z9-Node_ID::Z9-Tunnel_Num::LSP_Num::A1-Node_ID, respectively.

   Where the LSP_ID needs to be globally unique, this is accomplished by
   using globally unique Node_IDs as defined above.  Thus, a globally
   unique LSP_ID becomes:

      A1-{Global_ID::Node_ID::Tunnel_Num}::Z9-{Global_ID::
      Node_ID::Tunnel_Num}::LSP_Num

5.2.2.  MPLS-TP Associated Bidirectional LSP Identifiers

   For an associated bidirectional LSP, each of the unidirectional LSPs
   from A1 to Z9 and Z9 to A1 require LSP_Nums.  Each unidirectional LSP
   is uniquely identified by a single LSP number within the scope of the
   ingress's Tunnel_Num.  Specifically, an "LSP Number" (LSP_Num) is a
   16-bit unsigned integer unique within the scope of the ingress's
   Tunnel_Num.  Thus, the format of an MPLS-TP associated bidirectional
   LSP_ID is:

      A1-{Node_ID::Tunnel_Num::LSP_Num}::
      Z9-{Node_ID::Tunnel_Num::LSP_Num}

   At the data-plane level, an associated bidirectional LSP is composed
   of two unidirectional LSPs between two nodes in opposite directions.
   The unidirectional LSPs may be referenced by the identifiers:

      A1-Node_ID::A1-Tunnel_Num::A1-LSP_Num::Z9-Node_ID and

      Z9-Node_ID::Z9-Tunnel_Num::Z9-LSP_Num::A1-Node_ID, respectively.

   Where the LSP_ID needs to be globally unique, this is accomplished by
   using globally unique Node_IDs as defined above.  Thus, a globally
   unique LSP_ID becomes:

      A1-{Global_ID::Node_ID::Tunnel_Num::LSP_Num}::
      Z9-{Global_ID::Node_ID::Tunnel_Num::LSP_Num}

5.3.  Mapping to RSVP Signaling

   This section is informative and exists to help understand the
   structure of the LSP IDs.

   GMPLS [5] is based on RSVP-TE [2].  This section defines the mapping
   from an MPLS-TP LSP_ID to RSVP-TE.  At this time, RSVP-TE has yet to
   be extended to accommodate Global_IDs.  Thus, a mapping is only made
   for the network unique form of the LSP_ID and assumes that the
   operator has chosen to derive its Node_IDs from valid IPv4 addresses.

   GMPLS and RSVP-TE signaling use a 5-tuple to uniquely identify an LSP
   within an operator's network.  This tuple is composed of a Tunnel
   End-point Address, Tunnel_ID, Extended Tunnel ID, Tunnel Sender
   Address, and (RSVP) LSP_ID.  RFC 3209 allows some flexibility in how
   the Extended Tunnel ID is chosen, and a direct mapping is not
   mandated.  One convention that is often used, however, is to populate
   this field with the same value as the Tunnel Sender Address.  The
   examples below follow that convention.  Note that these are only
   examples.

   For a co-routed bidirectional LSP signaled from A1 to Z9, the mapping
   to the GMPLS 5-tuple is as follows:

      *  Tunnel End-point Address = Z9-Node_ID

      *  Tunnel_ID = A1-Tunnel_Num

      *  Extended Tunnel_ID = A1-Node_ID

      *  Tunnel Sender Address = A1-Node_ID

      *  (RSVP) LSP_ID = LSP_Num

   An associated bidirectional LSP between two nodes A1 and Z9 consists
   of two unidirectional LSPs, one from A1 to Z9 and one from Z9 to A1.

   In situations where a mapping to the RSVP-TE 5-tuples is required,
   the following mappings are used.  For the A1 to Z9 LSP, the mapping
   would be:

      *  Tunnel End-point Address = Z9-Node_ID

      *  Tunnel_ID = A1-Tunnel_Num

      *  Extended Tunnel_ID = A1-Node_ID

      *  Tunnel Sender Address = A1-Node_ID

      *  (RSVP) LSP_ID = A1-LSP_Num

   Likewise, the Z9 to A1 LSP, the mapping would be:

      *  Tunnel End-point Address = A1-Node_ID

      *  Tunnel_ID = Z9-Tunnel_Num

      *  Extended Tunnel_ID = Z9-Node_ID

      *  Tunnel Sender Address = Z9-Node_ID

      *  (RSVP) LSP_ID = Z9-LSP_Num

6.  Pseudowire Path Identifiers

   Pseudowire signaling (RFC 4447 [6]) defines two FECs used to signal
   pseudowires.  Of these, the Generalized PWid FEC (type 129) along
   with AII Type 2 as defined in RFC 5003 [3] fits the identification
   requirements of MPLS-TP.

   In an MPLS-TP environment, a PW is identified by a set of identifiers
   that can be mapped directly to the elements required by the
   Generalized PWid FEC (type 129) and AII Type 2.  To distinguish this
   identifier from other Pseudowire Identifiers, we call this a
   Pseudowire Path Identifier (PW_Path_ID).

   The AII Type 2 is composed of three fields.  These are the Global_ID,
   the Prefix, and the AC_ID.  The Global_ID used in this document is
   identical to the Global_ID defined in RFC 5003.  The Node_ID is used
   as the Prefix.  The AC_ID is as defined in RFC 5003.

   To complete the Generalized PWid FEC (type 129), all that is required
   is an Attachment Group Identifier (AGI).  That field is exactly as
   specified in RFC 4447.  A (bidirectional) pseudowire consists of a
   pair of unidirectional LSPs, one in each direction.  Thus, for
   signaling, the Generalized PWid FEC (type 129) has a notion of Source
   AII (SAII) and Target AII (TAII).  These terms are used relative to
   the direction of the LSP, i.e., the SAII is assigned to the end that
   allocates the PW label for a given direction, and the TAII to the
   other end.

   In a purely configured environment, when referring to the entire PW,
   this distinction is not critical.  That is, a Generalized PWid FEC
   (type 129) of AGIa::AIIb::AIIc is equivalent to AGIa::AIIc::AIIb.

   We note that in a signaled environment, the required convention in
   RFC 4447 is that at a particular end point, the AII associated with
   that end point comes first.  The complete PW_Path_ID is:

      AGI::A1-{Global_ID::Node_ID::AC_ID}::
      Z9-{Global_ID::Node_ID::AC_ID}.

   In a signaled environment the LSP from A1 to Z9 would be initiated
   with a label request from A1 to Z9 with the fields of the Generalized
   PWid FEC (type 129) completed as follows:

      AGI = AGI
      SAII = A1-{Global_ID::Node_ID::AC_ID}
      TAII = Z9-{Global_ID::Node_ID::AC_ID}

   The LSP from Z9 to A1 would signaled with:

      AGI = AGI
      SAII = Z9-{Global_ID::Node_ID::AC_ID}
      TAII = A1-{Global_ID::Node_ID::AC_ID}

7.  Maintenance Identifiers

   In MPLS-TP, a Maintenance Entity Group (MEG) represents an entity
   that requires management and defines a relationship between a set of
   maintenance points.  A maintenance point is either a Maintenance
   Entity Group End Point (MEP), a Maintenance Entity Group Intermediate
   Point (MIP), or a Pseudowire Segment End Point.  Within the context
   of a MEG, MEPs and MIPs must be uniquely identified.  This section
   defines a means of uniquely identifying Maintenance Entity Groups and
   Maintenance Entities.  It also uniquely defines MEPs and MIPs within
   the context of a Maintenance Entity Group.

7.1.  Maintenance Entity Group Identifiers

   Maintenance Entity Group Identifiers (MEG_IDs) are required for
   MPLS-TP sections, LSPs, and Pseudowires.  The formats were chosen to
   follow the IP-compatible identifiers defined above.

7.1.1.  MPLS-TP Section MEG_IDs

   MPLS-TP allows a hierarchy of sections.  See "MPLS-TP Data Plane
   Architecture" (RFC 5960 [10]).  Sections above layer 0 are MPLS-TP
   LSPs.  These use their MPLS-TP LSP MEG IDs defined in Section 7.1.2.

   IP-compatible MEG_IDs for MPLS-TP sections at layer 0 are formed by
   concatenating the two IF_IDs of the corresponding section using the
   A1/Z9 ordering.  For example:

      A1-IF_ID::Z9-IF_ID

   Where the Section_MEG_ID needs to be globally unique, this is
   accomplished by using globally unique Node_IDs as defined above.
   Thus, a globally unique Section_MEG_ID becomes:

      A1-{Global_ID::IF_ID}::Z9-{Global_ID::IF_ID}

7.1.2.  MPLS-TP LSP MEG_IDs

   A MEG pertains to a unique MPLS-TP LSP.  IP compatible MEG_IDs for
   MPLS-TP LSPs are simply the corresponding LSP_IDs; however, the A1/Z9
   ordering MUST be used.  For bidirectional co-routed LSPs, the format
   of the LSP_ID is found in Section 5.2.1.  For associated
   bidirectional LSPs, the format is in Section 5.2.2.

   We note that while the two identifiers are syntactically identical,
   they have different semantics.  This semantic difference needs to be
   made clear.  For instance, if both an MPLS-TP LSP_ID and MPLS-TP LSP
   MEG_IDs are to be encoded in TLVs, different types need to be
   assigned for these two identifiers.

7.1.3.  Pseudowire MEG_IDs

   For Pseudowires, a MEG pertains to a single PW.  The IP-compatible
   MEG_ID for a PW is simply the corresponding PW_Path_ID; however, the
   A1/Z9 ordering MUST be used.  The PW_Path_ID is described in
   Section 6.  We note that while the two identifiers are syntactically
   identical, they have different semantics.  This semantic difference
   needs to be made clear.  For instance, if both a PW_Path_ID and a
   PW_MEG_ID are to be encoded in TLVs, different types need to be
   assigned for these two identifiers.

7.2.  Maintenance Entity Group End Point Identifiers

7.2.1.  MPLS-TP Section MEP_IDs

   IP-compatible MEP_IDs for MPLS-TP sections above layer 0 are their
   MPLS-TP LSP_MEP_IDs.  See Section 7.2.2.

   IP-compatible MEP_IDs for MPLS-TP sections at layer 0 are simply the
   IF_IDs of each end of the section.  For example, for a section whose
   MEG_ID is:

      A1-IF_ID::Z9-IF_ID

   the Section MEP_ID at A1 would be:

      A1-IF_ID

   and the Section MEP_ID at Z9 would be:

      Z9-IF_ID.

   Where the Section MEP_ID needs to be globally unique, this is
   accomplished by using globally unique Node_IDs as defined above.
   Thus, a globally unique Section MEP_ID becomes:

      Global_ID::IF_ID.

7.2.2.  MPLS-TP LSP_MEP_ID

   In order to automatically generate MEP_IDs for MPLS-TP LSPs, we use
   the elements of identification that are unique to an end point.  This
   ensures that MEP_IDs are unique for all LSPs within an operator.
   When Tunnels or LSPs cross operator boundaries, these are made unique
   by pre-pending them with the operator's Global_ID.

   The MPLS-TP LSP_MEP_ID is:

      Node_ID::Tunnel_Num::LSP_Num

   where the Node_ID is the node in which the MEP is located and
   Tunnel_Num is the tunnel number unique to that node.  In the case of
   co-routed bidirectional LSPs, the single LSP_Num is used at both
   ends.  In the case of associated bidirectional LSPs, the LSP_Num is
   the one unique to where the MEP resides.

   In situations where global uniqueness is required, this becomes:

      Global_ID::Node_ID::Tunnel_Num::LSP_Num

7.2.3.  MEP_IDs for Pseudowires

   Like MPLS-TP LSPs, Pseudowire end points (T-PEs) require MEP_IDs.  In
   order to automatically generate MEP_IDs for PWs, we simply use the
   AGI plus the AII associated with that end of the PW.  Thus, a MEP_ID
   for a Pseudowire T-PE takes the form:

      AGI::Global_ID::Node_ID::AC_ID

   where the Node_ID is the node in which the MEP is located and the
   AC_ID is the AC_ID of the Pseudowire at that node.

7.3.  Maintenance Entity Group Intermediate Point Identifiers

   For a MIP that is associated with a particular interface, we simply
   use the IF_ID (see Section 4) of the interfaces that are cross-
   connected.  This allows MIPs to be independently identified in one
   node where a per-interface MIP model is used.  If only a per-node MIP
   model is used, then one MIP is configured.  In this case, the MIP_ID
   is formed using the Node_ID and an IF_Num of 0.

8.  Security Considerations

   This document describes an information model and, as such, does not
   introduce security concerns.  Protocol specifications that describe
   use of this information model, however, may introduce security risks
   and concerns about authentication of participants.  For this reason,
   the writers of protocol specifications for the purpose of describing
   implementation of this information model need to describe security
   and authentication concerns that may be raised by the particular
   mechanisms defined and how those concerns may be addressed.

   Uniqueness of the identifiers from this document is guaranteed by the
   assigner (e.g., a Global_ID is unique based on the assignment of ASNs
   from IANA and both a Node_ID and an IF_Num are unique based on the
   assignment by an operator).  Failure by an assigner to use unique
   values within the specified scoping for any of the identifiers
   defined herein could result in operational problems.  For example, a
   non-unique MEP value could result in failure to detect a mis-merged
   LSP.

   Protocol specifications that utilize the identifiers defined herein
   need to consider the implications of guessable identifiers and, where
   there is a security implication, SHOULD give advice on how to make
   identifiers less guessable.

9.  References

9.1.  Normative References

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

   [2]   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.

   [3]   Metz, C., Martini, L., Balus, F., and J. Sugimoto, "Attachment
         Individual Identifier (AII) Types for Aggregation", RFC 5003,
         September 2007.

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

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

   [6]   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.

9.2.  Informative References

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

   [8]   McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
         RFC 2863, June 2000.

   [9]   Bocci, M., Bryant, S., Frost, D., Levrau, L., and L. Berger, "A
         Framework for MPLS in Transport Networks", RFC 5921, July 2010.

   [10]  Frost, D., Bryant, S., and M. Bocci, "MPLS Transport Profile
         Data Plane Architecture", RFC 5960, August 2010.

Authors' Addresses

   Matthew Bocci
   Alcatel-Lucent
   Voyager Place, Shoppenhangers Road
   Maidenhead, Berks  SL6 2PJ
   UK

   EMail: matthew.bocci@alcatel-lucent.com


   George Swallow
   Cisco

   EMail: swallow@cisco.com


   Eric Gray
   Ericsson
   900 Chelmsford Street
   Lowell, Massachussetts  01851-8100

   EMail: eric.gray@ericsson.com