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Versions: 00 01 02 draft-ietf-pce-pcep-domain-sequence

PCE Working Group                                               D. Dhody
Internet-Draft                                                  U. Palle
Intended status: Standards Track           Huawei Technologies India Pvt
Expires: August 12, 2012                                             Ltd
                                                             R. Casellas
                                             CTTC - Centre Tecnologic de
                                          Telecomunicacions de Catalunya
                                                        February 9, 2012


               Standard Representation Of Domain Sequence
                draft-dhody-pce-pcep-domain-sequence-02

Abstract

   The ability to compute shortest constrained Traffic Engineering Label
   Switched Paths (TE LSPs) in Multiprotocol Label Switching (MPLS) and
   Generalized MPLS (GMPLS) networks across multiple domains has been
   identified as a key requirement for P2P and P2MP scenarios.  In this
   context, a domain is a collection of network elements within a common
   sphere of address management or path computational responsibility
   such as an IGP area or an Autonomous Systems.  This document
   specifies a standard representation and encoding of a domain
   sequence, which is defined as an ordered sequence of domains
   traversed to reach the destination domain.  This document also
   defines new sub-objects to be used to encode domain identifiers.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 12, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Detail Description . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Domains  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Domain-Sequence  . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Standard Representation  . . . . . . . . . . . . . . . . .  6
     3.4.  Mode of Operation  . . . . . . . . . . . . . . . . . . . .  8
     3.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.5.1.  Inter-Area Path Computation  . . . . . . . . . . . . .  9
       3.5.2.  Inter-AS Path Computation  . . . . . . . . . . . . . . 11
         3.5.2.1.  Example 1  . . . . . . . . . . . . . . . . . . . . 11
         3.5.2.2.  Example 2  . . . . . . . . . . . . . . . . . . . . 13
       3.5.3.  Boundary Node and Inter-AS-Link  . . . . . . . . . . . 15
       3.5.4.  PCE serving multiple domains . . . . . . . . . . . . . 16
       3.5.5.  P2MP . . . . . . . . . . . . . . . . . . . . . . . . . 16
       3.5.6.  HPCE . . . . . . . . . . . . . . . . . . . . . . . . . 16
       3.5.7.  Relationship to PCE Sequence . . . . . . . . . . . . . 18
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
     4.1.  New IRO Object Type  . . . . . . . . . . . . . . . . . . . 19
     4.2.  Sub-Objects  . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   6.  Manageability Considerations . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 20












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1.  Introduction

   A PCE may be used to compute end-to-end paths across multi-domain
   environments using a per-domain path computation technique [RFC5152].
   The so called backward recursive path computation (BRPC) mechanism
   [RFC5441] defines a PCE-based path computation procedure to compute
   inter-domain constrained (G)MPLS TE LSPs.  However, both per-domain
   and BRPC techniques assume that the sequence of domains to be crossed
   from source to destination is known, either fixed by the network
   operator or obtained by other means.  For inter-domain point-to-
   multi-point (P2MP) tree, [PCE-P2MP-PROCEDURES] assumes the domain-
   tree is known.

   The list of domains in a point-to-point (P2P) path or a point-to-
   multi-point (P2MP) tree is usually a constraint in the path
   computation request.  The PCE decouples the domain to determine the
   next PCE to forward the request.

   According to BRPC mechanism the PCC MAY indicate the sequence of
   domains to be traversed using the Include Route Object (IRO) defined
   in [RFC5440].

   This document proposes a standard way to represent and encode a
   domain sequence using IRO in various deployment scenarios including
   P2P, P2MP and Hierarchical PCE (HPCE) [PCE-HIERARCHY-FWK].

   The domain sequence (the set of domains traversed to reach the
   destination domain) is either administratively predetermined or
   discovered by some means (H-PCE) that is outside of the scope of this
   document.  Here the focus is only on a standard representation of the
   domain sequence in all possible scenarios.

1.1.  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 [RFC2119].

2.  Terminology

   The following terminology is used in this document.

   ABR:  OSPF Area Border Router.  Routers used to connect two IGP
      areas.







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   AS:  Autonomous System.

   ASBR:  Autonomous System Boundary Router.

   BN:  Boundary Node, Can be an ABR or ASBR.

   BRPC:  Backward Recursive Path Computation

   Domain:  Any collection of network elements within a common sphere of
      address management or path computational responsibility.  Examples
      of domains include Interior Gateway Protocol (IGP) areas and
      Autonomous Systems (ASs).

   Domain-Seq:  An ordered sequence of domains traversed to reach the
      destination domain.

   ERO:  Explicit Route Object

   H-PCE:  Hierarchical PCE

   IGP:  Interior Gateway Protocol.  Either of the two routing
      protocols, Open Shortest Path First (OSPF) or Intermediate System
      to Intermediate System (IS-IS).

   IRO:  Include Route Object

   IS-IS:  Intermediate System to Intermediate System.

   OSPF:  Open Shortest Path First.

   PCC:  Path Computation Client: any client application requesting a
      path computation to be performed by a Path Computation Element.

   PCE:  Path Computation Element.  An entity (component, application,
      or network node) that is capable of computing a network path or
      route based on a network graph and applying computational
      constraints.

   P2MP:  Point-to-Multipoint

   P2P:  Point-to-Point

   TE LSP:  Traffic Engineering Label Switched Path.








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3.  Detail Description

3.1.  Domains

   A domain can be defined as a separate administrative or geographic
   environment within the network.  A domain may be further defined as a
   zone of routing or computational ability.  Under these definitions a
   domain might be categorized as an Antonymous System (AS) or an
   Interior Gateway Protocol (IGP) area ( as per [RFC4726] and
   [RFC4655]).  To uniquely identify a domain in the domain sequence
   both AS and Area-id is important.

3.2.  Domain-Sequence

   A domain-sequence is an ordered sequence of domains traversed to
   reach the destination domain.  In this context a Domain could be an
   Autonomous System (AS) or an IGP Area.  Note that an AS can be
   further made of multiple Area.

   Domain Sequence can be applied as a constraint and carried in path
   computation request to PCE(s).  In case of HPCE [PCE-HIERARCHY-FWK]
   Parent PCE MAY send the domain sequence as a result in path
   computation reply.

   In this context, ordered sequence is important, in a P2P path, the
   domains listed appear in the order that they are crossed.  In a P2MP
   path, the domain tree is represented as list of domain sequences.

   One main goal of the Domain-Sequence is to enable a PCE to select the
   next PCE to forward the path computation request based on the domain
   information.

   A PCC or PCE MAY add an additional constraints covering which
   Boundary Nodes (ABR or ASBR) or Border links (Inter-AS-link) MUST be
   traversed while defining a domain sequence.

   Thus a Domain-Sequence MAY be made up of one or more of -

   o  AS Number

   o  Area ID

   o  Boundary Node ID

   o  Inter-AS-Link Address






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3.3.  Standard Representation

   The IRO (Include Route Object) [RFC5440] is an optional object used
   to specify a set of specified network elements that the computed path
   MUST traverse.  [RFC5440] in its description of IRO does not
   constrain the sub-objects to be in a given particular order.  When
   considering a domain sequence, the domain relative ordering is a
   basic criterion and, as such, this document specifies a new IRO
   object type.

   We define a new type of IRO Object to define Domain Sequence.

      IRO Object-Class is 10.
      IRO Object-Type is TBD. (2 suggested value to IANA)


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                        (Subobjects)                          //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sub-objects: The IRO is made of sub-objects identical to the ones
   defined in [RFC3209], [RFC3473], and [RFC3477], where the IRO sub-
   object type is identical to the sub-object type defined in the
   related documents.  Some new sub-objects related to Domain-Sequence
   are also added in this document.


   The following sub-object types are used.
                Type   Sub-object
                 1     IPv4 prefix
                 2     IPv6 prefix
                 4     Unnumbered Interface ID
                 32    Autonomous system number (2 Byte)
                 TBD   Autonomous system number (4 Byte)
                 TBD   OSPF Area id
                 TBD   ISIS Area id

   [RFC3209] defines sub-objects for IPv4, IPv6 and unnumbered Interface
   ID, which in the context of domain-sequence is used to specify
   Boundary Node (ABR/ASBR) and Inter-AS-Links.

   [RFC3209] also defines 2 octet AS number.

   To support 4 octet AS number [RFC4893] following subobject is



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   defined:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |L|    Type     |     Length    |         Reserved              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          AS Id (4 bytes)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Since the length of Area-id is different for OSPF and ISIS, we
   propose different sub-objects.

  For OSPF, the area-id is a 32 bit number. The Subobject looks
          0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |L|    Type     |     Length    |         Reserved              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Area Id (4 bytes)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  The length if fixed.

   For ISIS, the area-id is of variable length and thus the length of
   the Subobject is variable.  The Area-id is as described in ISIS by
   ISO standard [ISO 10589].  The Length MUST be at least 4, and MUST be
   a multiple of 4.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |L|    Type     |     Length    |         Reserved              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       //                        ISIS Area ID                         //
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The above sub-objects in various combinations can be used to encode
   the domain-sequence.  When the domain-sequence is used as a
   constraint in path computation request it is carried in IRO Domain
   Sequence Object Type.  The same sub-objects and their encoding can be
   used in ERO and path reply message when the domain sequence is
   computed from Parent PCE.

   All other rules of PCEP objects and message processing is as per



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   [RFC5440].

3.4.  Mode of Operation

   A domain sequence IRO object constraints or defines the domains
   involved in a muti-domain path computation, typically involving two
   or more collaborative PCEs.

   Consequently, a Domain-Sequence can be used:

   1.  by a PCE in order to discover or select the next PCE in a
       collaborative path computation, such as in BRPC [RFC5441];

   2.  by the Parent PCE to return the domain sequence when unknown,
       this can further be an input to BRPC procedure;

   3.  By a PCC (or PCE) to constraint the domains used in a H-PCE path
       computation, explicitly specifying which domains to be expanded;

   A domain sequence can have varying degrees on granularity; it is
   possible to have a domain sequence composed of, uniquely, AS
   identifiers.  It is also possible to list the involved areas for a
   given AS.

   In any case, the mapping between domains and responsible PCEs is not
   defined in this document.  It is assumed that a PCE that needs to
   obtain a "next PCE" from a domain sequence is able to do so (e.g. via
   administrative configuration, or discovery).

   The following algorithm can be applied to select the next domain and,
   if need be, the PCE responsible for that domain.  Note the PCC select
   the PCE(1) based on its own domain information.



















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       START
       Get the first Sub-Object S1 from the Domain-Sequence
       IF S1's Type is Area (OSPF or ISIS)
          IF S1's Domain is same as current PCE's Area
             Remove S1 from Domain-Sequence and Goto START
          ELSE
             Find the next PCE based on S1's Area within the AS
          ENDIF
       ELSEIF S1's Type is AS (2 or 4 Byte)
          IF S1's Domain is same as current PCE's AS
             Remove S1 from Domain-Sequence and Goto START
          ELSE
            Get the next Sub-Object S2 from the Domain-Sequence
            IF the S2 is NULL or S2's type is AS
               Find the next PCE based on S1's Domain (AS) only
            ELSEIF S1's Type is Area
               Find the next PCE based on S1's Domain (AS)
               and S2's Domain (Area)
            ELSE
            ENDIF
          ENDIF
       ENDIF
       IF Domain-Sequence is empty or next PCE is not found
          Send PCRep with NO-Path
       ENDIF


   If the Sub-Object is of other type representing Boundary Node or
   Inter-As-Link, it is not used to select the next PCE, but used only
   while applying BRPC or any other inter-domain procedure.

3.5.  Examples

3.5.1.  Inter-Area Path Computation

   In an inter-area path computation where ingress and egress belong to
   different IGP area, the domain sequence MAYBE represented using a
   ordered list of AREA sub-objects.  AS number MAYBE skipped, as area
   information is enough to select the next PCE.












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 +-------------------+                            +-------------------+
 |                   |                            |                   |
 |           +--+    |                            |     +--+          |
 |  +--+     |  |    |                            |     |  |          |
 |  |  |     +--+    |                            |     +--+   +--+   |
 |  +--*             +                            +            |  |   |
 |                   |                            |            +--+   |
 |         *--+      +                            +                   |
 |         |  |      |                            |     +--+          |
 |         +--+      |                            |     |  |          |
 |                   |+--------------------------+|     +--+          |
 |                  ++++                       +-++                   |
 |                  ||||         +--+          | ||                   |
 | Area 2           ++++         |  |          +-++  Area 4           |
 +-------------------+|          +--+            |+-------------------+
                      |                          |
                      |                +--+      |
                      |    +--+        |  |      |
                      |    |  |        +--+      |
                      |    +--+                  |
                      |                          |
                      |                          |
                      |                          |
                      |                          |
                      |           +--+           |
                      |           |  |           |
                      |           +--+           |
  +------------------+|                          |+--------------------+
  |                  ++-+                      +-++                    |
  |                  || |                      | ||                    |
  |                  ++-+    Area 0            +-++                    |
  |                  |+--------------------------+|     +--+           |
  |          +--+    |                            |     |  |           |
  |          |  |    |                            |     +--+           |
  | +--+     +--+    |                            |                    |
  | |  |             +                            +            +--+    |
  | +--+             |                            |            |  |    |
  |                  +                            +            +--+    |
  |       +--+       |                            |                    |
  |       |  |       |                            |     +--+           |
  |       +--+       |                            |     |  |           |
  |                  |                            |     +--+           |
  |                  |                            |                    |
  | Area 1           |                            |  Area 5            |
  +------------------+                            +--------------------+

AS Number is 100.




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                   Figure 1: Inter-Area Path Computation

   This could be represented as <IRO> as:

     +---------+ +---------+ +---------+ +---------+
     |IRO      | |Sub      | |Sub      | |Sub      |
     |Object   | |Object   | |Object   | |Object   |
     |Header   | |Area 2   | |Area 0   | |Area 4   |
     |         | |         | |         | |         |
     |         | |         | |         | |         |
     +---------+ +---------+ +---------+ +---------+


     +---------+ +---------+ +---------+ +---------+ +---------+
     |IRO      | |Sub      | |Sub      | |Sub      | |Sub      |
     |Object   | |Object As| |Object   | |Object   | |Object   |
     |Header   | |100      | |Area 2   | |Area 0   | |Area 4   |
     |         | |         | |         | |         | |         |
     |         | |         | |         | |         | |         |
     +---------+ +---------+ +---------+ +---------+ +---------+

   AS is optional and it MAY be skipped.  PCE should be able to
   understand both notations.

3.5.2.  Inter-AS Path Computation

   In inter-AS path computation, where ingress and egress belong to
   different AS, the domain sequence is represented using an ordered
   list of AS sub-objects.  The domain sequence MAY further include
   decomposed area information in AREA sub-objects.

3.5.2.1.  Example 1

   As shown in Figure 2, where AS to be made of a single area, the area
   subobject MAY be skipped in the domain sequence as AS is enough to
   uniquely identify the next domain and PCE.















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                                     +---------------------------------+
                                     |AS 200                           |
                                     |            +------+             |
                                     |            |      |             |
      +------------------------+     |            |      |   +------+  |
      | AS 100                 |     |            +------+   |      |  |
      |               +------+ |     | +------+              |      |  |
      |               |      +-+-----+-+      |              +------+  |
      |               |      | |     | |      |                        |
      |               +------+ |     | +------+                        |
      | +------+               |     |              +------+           |
      | |      |               |     |              |      |           |
      | |      |               |     |              |      |           |
      | +------+               |     |              +------+           |
      |                        |     |                                 |
      |               +------+ |     | +------+                        |
      |               |      +-+-----+-+      |               +------+ |
      |               |      | |     | |      |               |      | |
      |               +------+ |     | +------+               |      | |
      |                        |     |                        +------+ |
      |                        |     |                                 |
      |                        |     |                                 |
      |       +------+         |     |              +------+           |
      |       |      |         |     |              |      |           |
      |       |PCE   |         |     |              |PCE   |           |
      |       +------+         |     |              +------+           |
      |                        |     |                                 |
      +------------------------+     |                                 |
                                     +---------------------------------+

  Both AS are made of Area 0.

                    Figure 2: Inter-AS Path Computation


















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   This could be represented as <IRO> as:

       +---------+  +---------+  +---------+
       |IRO      |  |Sub      |  |Sub      |
       |Object   |  |Object As|  |Object As|
       |Header   |  |100      |  |200      |
       |         |  |         |  |         |
       |         |  |         |  |         |
       +---------+  +---------+  +---------+



       +---------+  +---------+  +---------+  +---------+  +---------+
       |IRO      |  |Sub      |  |Sub      |  |Sub      |  |Sub      |
       |Object   |  |Object As|  |Object   |  |Object As|  |Object   |
       |Header   |  |100      |  |Area 0   |  |200      |  |Area 0   |
       |         |  |         |  |         |  |         |  |         |
       |         |  |         |  |         |  |         |  |         |
       +---------+  +---------+  +---------+  +---------+  +---------+
   Area is optional and it MAY be skipped.  PCE should be able to
   understand both notations.

3.5.2.2.  Example 2

   As shown in Figure 3, where AS 200 is made up of multiple areas and
   multiple domain-sequence exist, PCE MAY include both AS and AREA
   subobject to uniquely identify the next domain and PCE.
























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                  |
                  |  +-------------+                +----------------+
                  |  |Area 2       |                |Area 4          |
                  |  |         +--+|                |          +--+  |
                  |  |         |  ||                |          |  |  |
                  |  |  +--+   +--+|                |   +--+   +--+  |
                  |  |  |  |       |                |   |  |         |
                  |  |  *--+       |                |   +--+         |
                  |  | /      +--+ |                |          +--+  |
                  |  |/       |  | |                |          |  |  |
                  |  /        +--+ |                |   +--+   +--+  |
                  | /|  +--+       |+--------------+|   |  |         |
                  |/ |  |  |       ++-+          +-++   +--+         |
   +-------------+/  |  +--+       || |          | ||                |
   |             /|  |             ++-+          +-++                |
   |         +--*||  +-------------+|              |+----------------+
   |         |  |||                 |     +--+     |
   |         +--+||                 |     |  |     |
   |    +--+     ||                 |     +--+     |
   |    |  |     ||                 |              |
   |    +--+     ||                 |              |
   |             ||                 |     +--+     |
   |+--+         ||                 |     |  |     |
   ||  |         ||                 |     +--+     |
   |+--+         ||                 |              |
   |             ||                 |     +--+     |
   |    +--+     ||  +------------+ |     |  |     |+----------------+
   |    |  |     ||  |Area 3      +-++    +--+   +-++ Area 5         |
   |    +--+     ||  |            | ||           | ||                |
   |             ||  |            +-++           +-++                |
   |         +--+||  |       +--+ | |  Area 0      ||   +--+         |
   |         |  |||  |       |  | | +--------------+|   |  |         |
   |         +--*||  |       +--+ |                 |   +--+         |
   |             \|  |            |                 |          +--+  |
   |Area 1       |\  |   +--+     |                 |   +--+   |  |  |
   +-------------+|\ |   |  |     |                 |   |  |   +--+  |
                  | \|   +--+  +--+                 |   +--+         |
                  |  \         |  |                 |                |
                  |  |\        +--+                 |          +--+  |
                  |  | \ +--+     |                 |          |  |  |
                  |  |  \|  |     |                 |          +--+  |
                  |  |   *--+     |                 |                |
                  |  |            |                 |                |
                  |  +------------+                 +----------------+
                  |
                  |
       As 100     |  AS 200
                  |



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                    Figure 3: Inter-AS Path Computation

The domain sequence can be carried in IRO as shown below:

 +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
 |IRO    | |Sub    | |Sub    | |Sub    | |Sub    | |Sub    | |Sub    |
 |Object | |Object | |Object | |Object | |Object | |Object | |Object |
 |Header | |As 100 | |Area 1 | |AS 200 | |Area 3 | |Area 0 | |Area 4 |
 |       | |       | |       | |       | |       | |       | |       |
 +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
Combination of both AS and Area uniquely identify a domain in the domain
sequence.

   Note that an Area domain identifier always belongs to the previous AS
   that appear before it or, if no AS sub-objects are present, it is
   assumed to be the current AS.

   If the area information cannot be provided, PCE MAY forward the path
   computation request to the next PCE based on AS only.  If multiple
   PCEs of different area domain exist, PCE MAY apply local policy to
   select the next PCE.  Furthermore the domain sequence (list of areas
   within AS) in the next PCE MAYBE pre-administered or MAYBE discovered
   via some mechanism (ex.  HPCE).

3.5.3.  Boundary Node and Inter-AS-Link

   A PCC or PCE MAY add additional constraints covering which Boundary
   Nodes (ABR or ASBR) or Border links (Inter-AS-link) MUST be traversed
   while defining a domain sequence.  In which case the Boundary Node or
   Link MAY be encoded as a part of the domain-sequence using the
   existing sub-objects.

   Boundary Node (ABR / ASBR) can be encoded using the IPv4 or IPv6
   prefix sub-objects.  The Inter-AS link can be encoded using the IPv4
   or IPv6 prefix or unnumbered interface sub-objects.

   For Figure 1, an ABR to be traversed can be specified as:



        +---------+ +---------+ +---------++---------+ +---------+
        |IRO      | |Sub      | |Sub      ||Sub      | |Sub      |
        |Object   | |Object   | |Object   ||Object   | |Object   |
        |Header   | |Area 2   | |IPv4     ||Area 0   | |Area 4   |
        |         | |         | |x.x.x.x  ||         | |         |
        |         | |         | |         ||         | |         |
        +---------+ +---------+ +---------++---------+ +---------+




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   For Figure 2, an inter-AS-link to be traversed can be specified as:


          +---------+  +---------+ +---------+ +---------+ +---------+
          |IRO      |  |Sub      | |Sub      | |Sub      | |Sub      |
          |Object   |  |Object As| |Object   | |Object   | |Object As|
          |Header   |  |100      | |IPv4     | |IPv4     | |200      |
          |         |  |         | |x.x.x.x  | |x.x.x.x  | |         |
          |         |  |         | |         | |         | |         |
          +---------+  +---------+ +---------+ +---------+ +---------+

3.5.4.  PCE serving multiple domains

   A single PCE MAYBE responsible for multiple domains; for example PCE
   function deployed on an ABR.  Domain sequence should have no impact
   on this.  PCE which can support 2 adjacent domains can internally
   handle this situation without any impact on the neighboring domains.

3.5.5.  P2MP

   In case of P2MP the path domain tree is nothing but a series of
   Domain Sequences, as shown in the below figure:

      D1-D3-D6, D1-D3-D5 and D1-D2-D4.
                  D1
                 /  \
                D2  D3
               /   /  \
              D4  D5  D6

3.5.6.  HPCE

   As per [PCE-HIERARCHY-FWK], consider a case as shown in Figure 4
   consisting of multiple child PCEs and a parent PCE.

















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                                +--------+
                                | Parent |
                                | PCE    |
                                +--------+

 +-------------------+                            +-------------------+
 |           +--+    |                            |     +--+          |
 |  +--+     |  |    |                            |     |  |          |
 |  |  |     +--+    |                            |     +--+   +--+   |
 |  +--*             +                            +            |  |   |
 |                   |                            |            +--+   |
 |         *--+      +                            +                   |
 |         |  |      |                            |     +--+          |
 |         +--+      |                            |     |  |          |
 |                   |+--------------------------+|     +--+          |
 |                  ++++                       +-++                   |
 |                  ||||         +--+          | ||                   |
 | Area 2           ++++         |  |          +-++  Area 4           |
 +-------------------+|          +--+            |+-------------------+
                      |                +--+      |
                      |    +--+        |  |      |
                      |    |  |        +--+      |
                      |    +--+                  |
                      |                          |
                      |           +--+           |
                      |           |  |           |
                      |           +--+           |
  +------------------+|                          |+--------------------+
  |                  ++-+                      +-++                    |
  |                  || |                      | ||                    |
  |                  ++-+    Area 0            +-++                    |
  |                  |+--------------------------+|     +--+           |
  |          +--+    |                            |     |  |           |
  |          |  |    |                            |     +--+           |
  | +--+     +--+    |                            |                    |
  | |  |             +                            +            +--+    |
  | +--+             |                            |            |  |    |
  |                  +                            +            +--+    |
  |       +--+       |                            |                    |
  |       |  |       |                            |     +--+           |
  |       +--+       |                            |     |  |           |
  |                  |                            |     +--+           |
  | Area 1           |                            |  Area 5            |
  +------------------+                            +--------------------+

                        Figure 4: Hierarchical PCE

   In HPCE implementation the initiator PCE - PCE(1) can request the



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   parent PCE to determine the domain sequence and return in the path
   computation reply message (PCRep), using the ERO Object.  The ERO can
   contain an ordered sequence of sub-object such as AS and Area (OSPF/
   ISIS).  In this case, the PCRep would carry the domain sequence
   result as:



     +---------+ +---------+ +---------+ +---------+
     |ERO      | |Sub      | |Sub      | |Sub      |
     |Object   | |Object   | |Object   | |Object   |
     |Header   | |Area 2   | |Area 0   | |Area 4   |
     |         | |         | |         | |         |
     |         | |         | |         | |         |
     +---------+ +---------+ +---------+ +---------+


     +---------+ +---------+ +---------+ +---------+ +---------+
     |ERO      | |Sub      | |Sub      | |Sub      | |Sub      |
     |Object   | |Object As| |Object   | |Object   | |Object   |
     |Header   | |100      | |Area 2   | |Area 0   | |Area 4   |
     |         | |         | |         | |         | |         |
     |         | |         | |         | |         | |         |
     +---------+ +---------+ +---------+ +---------+ +---------+

   Note that, in the case of ERO objects, no new PCEP object type is
   required since the ordering constraint is assumed.

3.5.7.  Relationship to PCE Sequence

   [RFC5886] and [PCE-P2MP-PROCEDURES] along with Domain Sequence
   introduces the concept of PCE-Sequence, where a sequence of PCEs,
   based on the domain sequence, should be decided and attached in the
   PCReq at the very beginning of path computation.  An alternative
   would be to use domain sequences, which simplifies as explained
   below:

   Advantages

   o  All PCE must be aware of all other PCEs in all domain for PCE-
      Sequence.  There is no clear method for this.  In domain-sequence
      PCE should be aware of the domains and not all the PCEs serving
      the domain.  PCE needs to be aware of the neighboring PCEs as done
      by discovery protocols.

   o  There maybe multiple PCE in a domain, the selection of PCE should
      not be made at the PCC/PCE(1).  This decision is made only at the
      neighboring PCE which is aware of state of PCEs via notification



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      messages.

   o  Domain sequence would be compatible to P2P inter-domain BRPC
      method as described in [RFC5441].

4.  IANA Considerations

4.1.  New IRO Object Type

   IANA has defined a registry for Domain-Sequence.

   IRO Object-Class 10

   IRO Object-Type 2

4.2.  Sub-Objects

   IANA has defined a registry for following sub-objects.

                 Type   Sub-object
                 TBD    AS Number (4 Byte)
                 TBD    OSPF Area id
                 TBD    ISIS Area id

5.  Security Considerations

   This document specifies a standard representation of domain sequence,
   which is used in all inter-domain PCE scenarios as explained in other
   RFC and drafts.  It does not introduce any new security
   considerations.

6.  Manageability Considerations

   TBD

7.  Acknowledgments

   We would like to thank Pradeep Shastry, Suresh babu, Quintin Zhao,
   Fatai Zhang, Daniel King, Oscar Gonzalez and Chen Huaimo for their
   useful comments and suggestions.

8.  References

8.1.  Normative References

   [RFC2119]              Bradner, S., "Key words for use in RFCs to
                          Indicate Requirement Levels", BCP 14,
                          RFC 2119, March 1997.



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   [ISO 10589]            ISO, "Intermediate system to Intermediate
                          system routeing information exchange protocol
                          for use in conjunction with the Protocol for
                          providing the Connectionless-mode Network
                          Service (ISO 8473)", ISO/IEC 10589:2002.

8.2.  Informative References

   [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.

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

   [RFC3477]              Kompella, K. and Y. Rekhter, "Signalling
                          Unnumbered Links in Resource ReSerVation
                          Protocol - Traffic Engineering (RSVP-TE)",
                          RFC 3477, January 2003.

   [RFC4655]              Farrel, A., Vasseur, J., and J. Ash, "A Path
                          Computation Element (PCE)-Based Architecture",
                          RFC 4655, August 2006.

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

   [RFC4893]              Vohra, Q. and E. Chen, "BGP Support for Four-
                          octet AS Number Space", RFC 4893, May 2007.

   [RFC5152]              Vasseur, JP., Ayyangar, A., and R. Zhang, "A
                          Per-Domain Path Computation Method for
                          Establishing Inter-Domain Traffic Engineering
                          (TE) Label Switched Paths (LSPs)", RFC 5152,
                          February 2008.

   [RFC5440]              Vasseur, JP. and JL. Le Roux, "Path
                          Computation Element (PCE) Communication
                          Protocol (PCEP)", RFC 5440, March 2009.

   [RFC5441]              Vasseur, JP., Zhang, R., Bitar, N., and JL. Le
                          Roux, "A Backward-Recursive PCE-Based
                          Computation (BRPC) Procedure to Compute



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                          Shortest Constrained Inter-Domain Traffic
                          Engineering Label Switched Paths", RFC 5441,
                          April 2009.

   [RFC5886]              Vasseur, JP., Le Roux, JL., and Y. Ikejiri, "A
                          Set of Monitoring Tools for Path Computation
                          Element (PCE)-Based Architecture", RFC 5886,
                          June 2010.

   [PCE-P2MP-PROCEDURES]  Zhao, Q., Dhody, D., Ali, Z., Saad,, T.,
                          Sivabalan,, S., and R. Casellas, "PCE-based
                          Computation Procedure To Compute Shortest
                          Constrained P2MP Inter-domain Traffic
                          Engineering Label Switched Paths (draft-ietf-
                          pce-pcep-inter-domain-p2mp-procedures-02)",
                          February 2012.

   [PCE-HIERARCHY-FWK]    King, D. and A. Farrel, "The Application of
                          the Path Computation Element Architecture to
                          the Determination of a Sequence of Domains in
                          MPLS and GMPLS.
                          (draft-ietf-pce-hierarchy-fwk-00)",
                          October 2011.

Authors' Addresses

   Dhruv Dhody
   Huawei Technologies India Pvt Ltd
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: dhruv.dhody@huawei.com


   Udayasree Palle
   Huawei Technologies India Pvt Ltd
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: udayasree.palle@huawei.com









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   Ramon Casellas
   CTTC - Centre Tecnologic de Telecomunicacions de Catalunya
   Av. Carl Friedrich Gauss n7
   Castelldefels, Barcelona  08860
   SPAIN

   EMail: ramon.casellas@cttc.es












































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