PCE Working Group
Internet Engineering Task Force (IETF)                          D. Dhody
Internet-Draft
Request for Comments: 7897                                      U. Palle
Intended status:
Category: Experimental                               Huawei Technologies
Expires: June 9, 2016
ISSN: 2070-1721                                              R. Casellas
                                                                    CTTC
                                                        December 7, 2015
                                                               June 2016

                           Domain Subobjects
     for the Path Computation Element (PCE) Communication Protocol (PCEP).
                 draft-ietf-pce-pcep-domain-sequence-12 (PCEP)

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.  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 Interior
   Gateway Protocol (IGP) area or an Autonomous System (AS).  This
   document specifies a representation and encoding of a Domain-
   Sequence, domain
   sequence, which is defined as an ordered sequence of domains
   traversed to reach the destination domain to be used by Path
   Computation Elements (PCEs) to compute inter-domain constrained
   shortest paths across a predetermined sequence of domains . domains.  This
   document also defines new subobjects to be used to encode domain
   identifiers.

Status of This Memo

   This Internet-Draft document is submitted in full conformance with the
   provisions of BCP 78 not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and BCP 79.

   Internet-Drafts are working documents
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list  It represents the consensus of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid 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 maximum candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of six months this document, any errata,
   and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
   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 June 9, 2016.
   http://www.rfc-editor.org/info/rfc7897.

Copyright Notice

   Copyright (c) 2015 2016 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Detail Description  . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Domains . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Domain-Sequence  Domain Sequence . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Domain-Sequence  Domain Sequence Representation  . . . . . . . . . . . . .   7
     3.4.  Include Route Object (IRO)  . . . . . . . . . . . . . . .   7   8
       3.4.1.  Subobjects  . . . . . . . . . . . . . . . . . . . . .   8
         3.4.1.1.  Autonomous system System . . . . . . . . . . . . . . . .   8
         3.4.1.2.  IGP Area  . . . . . . . . . . . . . . . . . . . .   9
       3.4.2.  Update in IRO specification Specification . . . . . . . . . . . . .  10
       3.4.3.  IRO for Domain-Sequence Domain Sequence . . . . . . . . . . . . . . .  10
         3.4.3.1.  PCC Procedures  . . . . . . . . . . . . . . . . .  11
         3.4.3.2.  PCE Procedures  . . . . . . . . . . . . . . . . .  11
     3.5.  Exclude Route Object (XRO)  . . . . . . . . . . . . . . .  12  13
       3.5.1.  Subobjects  . . . . . . . . . . . . . . . . . . . . .  13
         3.5.1.1.  Autonomous system System . . . . . . . . . . . . . . . .  13
         3.5.1.2.  IGP Area  . . . . . . . . . . . . . . . . . . . .  14
     3.6.  Explicit Exclusion Route Subobject (EXRS) . . . . . . . .  15
     3.7.  Explicit Route Object (ERO) . . . . . . . . . . . . . . .  16
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     4.1.  Inter-Area Path Computation . . . . . . . . . . . . . . .  16
     4.2.  Inter-AS Path Computation . . . . . . . . . . . . . . . .  18
       4.2.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . .  19
       4.2.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . .  21
     4.3.  Boundary Node and Inter-AS-Link Inter-AS Link . . . . . . . . . . . . .  23
     4.4.  PCE Serving multiple Multiple Domains  . . . . . . . . . . . . . .  24
     4.5.  P2MP  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     4.6.  Hierarchical PCE  . . . . . . . . . . . . . . . . . . . .  26
   5.  Other Considerations  . . . . . . . . . . . . . . . . . . . .  26
     5.1.  Relationship to PCE Sequence  . . . . . . . . . . . . . .  26
     5.2.  Relationship to RSVP-TE . . . . . . . . . . . . . . . . .  26
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  New Subobjects  . . . . . . . . . . . . . . . . . . . . .  27
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  28
     8.1.  Control of Function and Policy  . . . . . . . . . . . . .  28
     8.2.  Information and Data Models . . . . . . . . . . . . . . .  28
     8.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  29  28
     8.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  29
     8.5.  Requirements On on Other Protocols . . . . . . . . . . . . .  29
     8.6.  Impact On on Network Operations  . . . . . . . . . . . . . .  29
   9.  Acknowledgments  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
   10.
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     9.2.  Informative References  . . . . . . .  30
     10.1.  Normative References . . . . . . . . . .  31
   Acknowledgments . . . . . . . .  30
     10.2.  Informative References . . . . . . . . . . . . . . . . .  31  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   A Path Computation Element (PCE) may be used to compute end-to-end
   paths across multi-domain environments using a per-domain path
   computation technique [RFC5152].  The backward recursive path
   computation Backward-Recursive PCE-Based
   Computation (BRPC) mechanism [RFC5441] also defines a PCE-based path
   computation procedure to compute an inter-domain constrained path for
   (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, known and is either fixed by the network operator or obtained by
   other means.  Also  Also, for inter-domain point-to-multi-point point-to-multipoint (P2MP) tree
   computation, it is assumed per [RFC7334] assumes that the domain-tree domain tree is
   known in a priori.

   The list of domains (Domain-Sequence) (domain sequence) in point-to-point (P2P) or a
   domain tree in point-to-multipoint (P2MP) P2MP is usually a constraint in inter-domain path
   computation procedure.

   The Domain-Sequence domain sequence (the set of domains traversed to reach the
   destination domain) is either administratively predetermined or
   discovered by some means like H-PCE. Hierarchical PCE (H-PCE).

   [RFC5440] defines the Include Route Object (IRO) and the Explicit
   Route Object (ERO).  [RFC5521] defines the Exclude Route Object (XRO)
   and the Explicit Exclusion Route Subobject subobject (EXRS).  The use of an
   Autonomous System (AS) (albeit with a 2-Byte 2-byte AS number) as an abstract
   node representing a domain is defined in [RFC3209].  In the current
   document, we specify new subobjects to include or exclude domains
   including an IGP area or an Autonomous Systems (4-Byte AS (4 bytes as per [RFC6793]).

   Further, the domain identifier may simply act as a delimiter to
   specify where the domain boundary starts and ends in some cases.

   This is a companion document to Resource ReserVation Reservation Protocol -
   Traffic Engineering (RSVP-TE) extensions for the domain identifiers
   [DOMAIN-SUBOBJ].
   [RFC7898].

1.1.  Scope

   The procedures described in this document are experimental.  The
   experiment is intended to enable research for the usage of Domain-
   Sequence the domain
   sequence at the PCEs for inter-domain paths.  For this purpose purpose, this
   document specifies new domain subobjects as well as how they
   incorporate with existing subobjects to represent a Domain-Sequence. domain sequence.

   The experiment will end two years after the RFC is published.  At
   that point, the RFC authors will attempt to determine how widely this
   has been implemented and deployed.

   This document does not change the procedures for handling existing
   subobjects in PCEP. the PCE Communication Protocol (PCEP).

   The new subobjects introduced by this document will not be understood
   by legacy implementations.  If a legacy implementation receives one
   of the subobjects that it does not understand in a PCEP object, the
   legacy implementation will behave according to the rules for a
   malformed object as described in Section 3.4.3. per [RFC5440].  Therefore, it is assumed that
   this experiment will be conducted only when both the PCE and the PCC Path
   Computation Client (PCC) form part of the experiment.  It is possible
   that a PCC or PCE can operate with peers peers, some of which form part of
   the experiment and some that do not.  In this case, since no
   capabilities exchange is used to identify which nodes can use these
   extensions, manual configuration should be used to determine which
   peerings form part of the experiment.

   When the result results of implementation and deployment are available, this
   document will be updated and refined, and then it could be moved from
   Experimental to Standard Standards Track.

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 [RFC2119].

2.  Terminology

   The following terminology is used in this document.

   ABR:  OSPF  Area Border Router.  Routers used to connect two IGP
      areas. areas
      (Open Shortest Path First (OSPF) or Intermediate System to
      Intermediate System (IS-IS).

   AS:  Autonomous System. System

   ASBR:  Autonomous System Border Router

   BN:  Boundary Router.

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

   BRPC:  Backward Recursive Path  Backward-Recursive PCE-Based Computation

   Domain:  As per [RFC4655], any collection of network elements within
      a common sphere of address management or path computational
      responsibility.  Examples of domains include Interior Gateway
      Protocol (IGP) IGP area and Autonomous System (AS).

   Domain-Sequence: AS.

   Domain Sequence:  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)
      protocols: OSPF or Intermediate System
      to Intermediate System (IS-IS). IS-IS.

   IRO:  Include Route Object

   IS-IS:  Intermediate System to Intermediate System. System

   OSPF:  Open Shortest Path First. First

   PCC:  Path Computation Client: any 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
   RSVP:  Resource Reservation Protocol

   TE LSP:  Traffic Engineering Label Switched Path. Path

   XRO:  Exclude Route Object

3.  Detail Description

3.1.  Domains

   [RFC4726] and [RFC4655] define a domain as a separate administrative
   or geographic environment within the network.  A domain could be
   further defined as a zone of routing or computational ability.  Under
   these
   definitions definitions, a domain might be categorized as an AS or an IGP
   area.  Each AS can be made of several IGP areas.  In order to encode
   a
   Domain-Sequence, domain sequence, it is required to uniquely identify a domain in
   the
   Domain-Sequence. domain sequence.  A domain can be uniquely identified by area-id or an
   area-id, AS number number, or both.

3.2.  Domain-Sequence  Domain Sequence

   A Domain-Sequence domain sequence is an ordered sequence of domains traversed to
   reach the destination domain.

   A Domain-Sequence domain sequence can be applied as a constraint and carried in a
   path computation request to a PCE(s).  A Domain-Sequence domain sequence can also be
   the result of a path computation.  For example, in the case of
   Hierarchical PCE (H-PCE) H-PCE
   [RFC6805], Parent a parent PCE could send the Domain-
   Sequence domain sequence as a result in
   a path computation reply.

   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 a list of
   Domain-Sequences.
   domain sequences.

   A Domain-Sequence domain sequence enables a PCE to select the next domain and the PCE
   serving that domain to forward the path computation request based on
   the domain information.

   Domain-Sequence

   A domain sequence can include Boundary Nodes boundary nodes (ABR or ASBR) or Border border
   links (Inter-AS-links) (inter-AS links) to be traversed as an additional constraint.

   Thus

   Thus, a Domain-Sequence domain sequence can be made up of one or more of - the
   following:

   o  AS Number

   o  Area ID
   o  Boundary Node ID

   o  Inter-AS-Link  Inter-AS Link Address

   These are encoded in the new subobjects defined in this document as
   well as in the existing subobjects to that represent a Domain-Sequence. domain sequence.

   Consequently, a Domain-Sequence domain sequence can be used: used by:

   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 parent PCE to return the Domain-Sequence domain sequence when unknown; this
       can then be an input to the BRPC procedure [RFC6805];

   3.  by  a Path Computation Client (PCC) PCC or a PCE, PCE to constrain the domains used in inter-domain path
       computation, explicitly specifying which domains to be expanded
       or excluded; and

   4.  by  a PCE in the per-domain path computation model [RFC5152] to
       identify the next domain.

3.3.  Domain-Sequence  Domain Sequence Representation

   Domain-Sequence

   A domain sequence appears in PCEP messages, notably in - in:

   o  Include Route Object (IRO): As per [RFC5440], IRO can be used to
      specify a set of network elements to be traversed to reach the
      destination, which includes subobjects used to specify the Domain-
      Sequence. domain
      sequence.

   o  Exclude Route Object (XRO): As per [RFC5521], XRO can be used to
      specify certain abstract nodes, to be excluded from the whole
      path, which includes include subobjects used to specify the Domain-Sequence. domain
      sequence.

   o  Explicit Exclusion Route Subobject (EXRS): As per [RFC5521], EXRS
      can be used to specify exclusion of certain abstract nodes
      (including domains) between a specific pair of nodes.  EXRS are is a
      subobject inside the IRO.

   o  Explicit Route Object (ERO): As per [RFC5440], ERO can be used to
      specify a computed path in the network.  For example, in the case
      of H-PCE [RFC6805], a Parent parent PCE can send the Domain-Sequence domain sequence as a
      result,
      result in a path computation reply using ERO.

3.4.  Include Route Object (IRO)

   As per [RFC5440], IRO (Include Route Object) can be used to specify that the computed path
   needs to traverse a set of specified network elements or abstract
   nodes.

3.4.1.  Subobjects

   Some subobjects are defined in [RFC3209], [RFC3473], [RFC3477] [RFC3477], and
   [RFC4874], but new subobjects related to Domain-Sequence domain sequence are needed.

   This document extends the support for 4-Byte 4-byte AS numbers and IGP
   Areas.

                Type   Subobject
                 TBD1  Autonomous system
   areas.

                 Value  Description
                 -----  ----------------
                 5      4-byte AS number (4 Byte)
                 TBD2
                 6      OSPF Area id
                 TBD3  ISIS ID
                 7      IS-IS Area id ID

   Note: The twins of these Identical subobjects are carried in RSVP-TE messages as defined
   in [DOMAIN-SUBOBJ]. [RFC7898].

3.4.1.1.  Autonomous system System

   [RFC3209] already defines 2 byte 2-byte AS number. numbers.

   To support 4 byte 4-byte AS number numbers as per [RFC6793] [RFC6793], the following
   subobject is 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                      AS Number (4 bytes)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L: The L bit is an attribute of the subobject as defined in [RFC3209]
      [RFC3209], and its usage in the IRO subobject updated is defined in [IRO-UPDATE].
      [RFC7896].

   Type:  (TBD1 by IANA) indicating  5 (indicating a 4-Byte 4-byte AS Number. number).

   Length:  8 (Total (total length of the subobject in bytes).

   Reserved:  Zero at transmission, transmission; ignored at receipt.

   AS-ID:

   AS Number:  The 4-Byte 4-byte AS Number. number.  Note that if 2-Byte 2-byte AS numbers are
      in use, the low order low-order bits (16 through 31) MUST be used used, and the high
      order
      high-order bits (0 through 15) MUST be set to zero.

3.4.1.2.  IGP Area

   Since the length and format of Area-id Area ID is different for OSPF and
   ISIS,
   IS-IS, the following two subobjects are defined: defined below:

   For OSPF, the area-id Area ID is a 32 bit 32-bit number.  The subobject is encoded
   as follows:

    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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    OSPF Area Id ID (4 bytes)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L: The L bit is an attribute of the subobject as defined in [RFC3209]
      [RFC3209], and its usage in the IRO subobject updated is defined in [IRO-UPDATE].
      [RFC7896].

   Type:  (TBD2 by IANA) indicating  6 (indicating a 4-Byte 4-byte OSPF Area ID. ID).

   Length:  8 (Total (total length of the subobject in bytes).

   Reserved:  Zero at transmission, transmission; ignored at receipt.

   OSPF Area Id: ID:  The 4-Byte 4-byte OSPF Area ID.

   For IS-IS, the area-id Area ID is of variable length and thus length; thus, the length of the Subobject
   subobject is variable.  The Area-id Area ID is as described in IS-IS by the
   ISO standard [ISO10589].  The subobject is encoded as follows:

    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    |  Area-Len     |  Reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        IS-IS Area ID                        //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   L: The L bit is an attribute of the subobject as defined in [RFC3209]
      [RFC3209], and its usage in the IRO subobject updated is defined in [IRO-UPDATE].
      [RFC7896].

   Type:  (TBD3 by IANA) indicating  7 (indicating the IS-IS Area ID. ID).

   Length:  Variable.  The Length length MUST be at least 8, 8 and MUST be a
      multiple of 4.

   Area-Len:  Variable (Length (length of the actual (non-padded) IS-IS Area
      Identifier area
      identifier in octets; Valid valid values are from 1 to 13 13, inclusive).

   Reserved:  Zero at transmission, transmission; ignored at receipt.

   IS-IS Area Id: ID:  The variable-length IS-IS area identifier.  Padded
      with trailing zeroes to a four-byte 4-byte boundary.

3.4.2.  Update in IRO specification Specification

   [RFC5440] describes IRO as an optional object used to specify network
   elements to be traversed by the computed path.  It further state states
   that the L bit of such subobject has no meaning within an IRO.  It
   also
   did does not mention if IRO is an ordered or un-ordered unordered list of
   subobjects.

   An update to the IRO specification [IRO-UPDATE] [RFC7896] makes IRO as an ordered
   list, as well as
   list and includes support for loose bit (L-bit) is added. the L bit.

   The use of IRO for Domain-Sequence, the domain sequence assumes the updated
   specification is being used for IRO, as per [IRO-UPDATE]. [RFC7896].

3.4.3.  IRO for Domain-Sequence Domain Sequence

   The subobject type for IPv4, IPv6, and unnumbered Interface ID IDs can
   be used to specify Boundary Nodes boundary nodes (ABR/ASBR) and Inter-AS-Links. inter-AS links.  The
   subobject type for the AS Number (2 or 4 Byte) bytes) and the IGP Area area are
   used to specify the domain identifiers in the Domain-Sequence. domain sequence.

   The IRO can incorporate the new domain subobjects with the existing
   subobjects in a sequence of traversal.

   Thus

   Thus, an IRO, comprising subobjects, that represents a Domain-
   Sequence, define domain
   sequence defines the domains involved in an inter-domain path
   computation, typically involving two or more collaborative PCEs.

   A Domain-Sequence domain sequence can have varying degrees of granularity.  It is
   possible to have a Domain-Sequence domain sequence composed of, uniquely, AS
   identifiers.  It is also possible to list the involved IGP 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 domain sequence is able to do so (e.g. (e.g.,
   via administrative configuration, configuration or discovery).

3.4.3.1.  PCC Procedures

   A PCC builds an IRO to encode the Domain-Sequence, domain sequence, so that the
   cooperating PCEs could compute an inter-domain shortest constrained
   path across the specified sequence of domains.

   A PCC may intersperse Area area and AS subobjects with other subobjects
   without change to the previously specified processing of those
   subobjects in the IRO.

3.4.3.2.  PCE Procedures

   If a PCE receives an IRO in a Path Computation request Request (PCReq)
   message that contains the subobjects defined in this document, document that it
   does not recognize, it will respond according to the rules for a
   malformed object as per [RFC5440].  The PCE MAY also include the IRO
   in the PCErr PCEP Error (PCErr) message as per [RFC5440].

   The interpretation of Loose the L bit (L bit) is as per section Section 4.3.3.1 of
   [RFC3209] (as per [IRO-UPDATE]). [RFC7896]).

   In a Path Computation reply Reply (PCRep), PCE MAY also supply IRO (with
   Domain-Sequence
   domain sequence information) with the NO-PATH object indicating that
   the set of elements (domains) of the request's IRO prevented the PCEs
   from finding a path.

   The following processing rules apply for Domain-Sequence a domain sequence in IRO - IRO:

   o  When a PCE parses an IRO, it interprets each subobject according
      to the AS number associated with the preceding subobject.  We call
      this the "current AS".  Certain subobjects modify the current AS,
      as follows.

      *  The current AS is initialized to the AS number of the PCC.

      *  If the PCE encounters an AS subobject, then it updates the
         current AS to this new AS number.

      *  If the PCE encounters an Area area subobject, then it assumes that
         the area belongs to the current AS.

      *  If the PCE encounters an IP address that is globally routable,
         then it updates the current AS to the AS that owns this IP
         address.  This document does not define how the PCE learns
         which AS owns the IP address.

      *  If the PCE encounters an IP address that is not globally
         routable, then it assumes that it belongs to the current AS.

      *  If the PCE encounters an unnumbered link, then it assumes that
         it belongs to the current AS.

   o  When a PCE parses an IRO, it interprets each subobject according
      to the Area ID associated with the preceding subobject.  We call
      this the "current Area". area".  Certain subobjects modify the current
      Area,
      area, as follows.

      *  The current Area area is initialized to the Area ID of the PCC.

      *  If the current AS is changed, the current Area area is reset and
         need
         needs to be determined again by a current or subsequent
         subobject.

      *  If the PCE encounters an Area area subobject, then it updates the
         current Area area to this new Area ID.

      *  If the PCE encounters an IP address that belongs to a different
         area, then it updates the current Area area to the Area area that has
         this IP address.  This document does not define how the PCE
         learns which Area area has the IP address.

      *  If the PCE encounters an unnumbered link that belongs to a
         different area, then it updates the current Area to the Area area
         that has this link.

      *  Otherwise, it assumes that the subobject belongs to the current
         Area.
         area.

   o  In case the current PCE is not responsible for the path
      computation in the current AS or Area, area, then the PCE selects the
      "next PCE" in the domain-sequence domain sequence based on the current AS and
      Area.
      area.

   Note that it is advised that, that PCC should use AS and Area subobject area subobjects
   while building the domain-sequence domain sequence in IRO and avoid using other
   mechanism
   mechanisms to change the "current AS" and "current Area" area" as described
   above.

3.5.  Exclude Route Object (XRO)

   The Exclude Route Object (XRO)

   XRO [RFC5521] is an optional object used to specify exclusion of
   certain abstract nodes or resources from the whole path.

3.5.1.  Subobjects

   Some subobjects are to be used in XRO as defined in [RFC3209],
   [RFC3477], [RFC4874], and [RFC5520], but new subobjects related to Domain-
   Sequence
   domain sequence are needed.

   This document extends the support for 4-Byte 4-byte AS numbers and IGP
   Areas.

                Type   Subobject
                 TBD1  Autonomous system
   areas.

                 Value  Description
                 -----  ----------------
                 5      4-byte AS number (4 Byte)
                 TBD2
                 6      OSPF Area id
                 TBD3  ISIS ID
                 7      IS-IS Area id ID

   Note: The twins of these Identical subobjects are carried in RSVP-TE messages as defined
   in [DOMAIN-SUBOBJ]. [RFC7898].

3.5.1.1.  Autonomous system System

   The new subobjects to support 4 byte 4-byte AS numbers and the IGP (OSPF / ISIS) Area (OSPF/IS-
   IS) area MAY also be used in the XRO to specify exclusion of certain
   domains in the path computation procedure.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |X|    Type     |     Length    |         Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          AS-ID                      AS Number (4 bytes)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The X-bit indicates whether the exclusion is mandatory or desired.

   0: indicates that the AS specified MUST be excluded from the path
      computed by the PCE(s).

   1: indicates that the AS specified SHOULD be avoided from the inter-
      domain path computed by the PCE(s), but it MAY be included subject
      to PCE policy and the absence of a viable path that meets the
      other constraints.

   All other fields are consistent with the definition in Section 3.4.

3.5.1.2.  IGP Area

   Since the length and format of Area-id the Area ID is different for OSPF and
   ISIS,
   IS-IS, the following two subobjects are defined:

   For OSPF, the area-id Area ID is a 32 bit 32-bit number.  The subobject is encoded
   as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |X|    Type     |     Length    |         Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    OSPF Area Id ID (4 bytes)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The X-bit indicates whether the exclusion is mandatory or desired.

   0: indicates that the OSFF Area OSPF area specified MUST be excluded from the
      path computed by the PCE(s).

   1: indicates that the OSFF Area OSPF area specified SHOULD be avoided from the
      inter-domain path computed by the PCE(s), but it MAY be included
      subject to PCE policy and the absence of a viable path that meets
      the other constraints.

   All other fields are consistent with the definition in Section 3.4.

   For IS-IS, the area-id Area ID is of variable length and thus length; thus, the length of the
   subobject is variable.  The Area-id Area ID is as described in IS-IS by the
   ISO standard [ISO10589].  The subobject is encoded as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |X|    Type     |     Length    |  Area-Len     |  Reserved     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        IS-IS Area ID                        //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The X-bit indicates whether the exclusion is mandatory or desired.

   0: indicates that the ISIS Area IS-IS area specified MUST be excluded from the
      path computed by the PCE(s).

   1: indicates that the ISIS Area IS-IS area specified SHOULD be avoided from the
      inter-domain path computed by the PCE(s), but it MAY be included
      subject to PCE policy and the absence of a viable path that meets
      the other constraints.

   All other fields are consistent with the definition in Section 3.4.

   All the processing rules are as per [RFC5521].

   Note that, that if a PCE receives an XRO in a PCReq message that contains
   subobjects defined in this document, document that it does not recognize, it
   will respond according to the rules for a malformed object as per
   [RFC5440].

   IGP Area area subobjects in the XRO are local to the current AS.  In case
   of
   multi-AS path computation to exclude excludes an IGP area in a different AS, the
   IGP Area area subobject should be part of Explicit Exclusion Route
   Subobject (EXRS) EXRS in the IRO to specify the
   AS in which the IGP area is to be excluded.  Further  Further, policy may be
   applied to prune/ignore
   Area area subobjects in XRO after a "current AS"
   change during path computation.

3.6.  Explicit Exclusion Route Subobject (EXRS)

   The EXRS [RFC5521] is used to specify exclusion of certain abstract
   nodes between a specific pair of nodes.

   The EXRS subobject can carry any of the subobjects defined for inclusion in the XRO, thus
   XRO; thus, the new subobjects to support 4 byte 4-byte AS numbers and the
   IGP (OSPF / ISIS) Area IS-IS) area can also be used in the EXRS.  The meanings
   of the fields of the new XRO subobjects are unchanged when the
   subobjects are included in an EXRS, except that the scope of the
   exclusion is limited to the single hop between the previous and
   subsequent elements in the IRO.

   The EXRS subobject should be interpreted in the context of the current AS and
   current Area area of the preceding subobject in the IRO.  The EXRS subobject does
   not change the current AS or current Area. area.  All other processing
   rules are as per [RFC5521].

   Note that, that if a PCE that supports the EXRS in an IRO, IRO parses an IRO,
   and encounters an EXRS that contains subobjects defined in this
   document,
   document that it does not recognize, it will act according to the
   setting of the X-bit in the subobject as per [RFC5521].

3.7.  Explicit Route Object (ERO)

   The Explicit Route Object (ERO)

   ERO [RFC5440] is used to specify a computed path in the network.
   PCEP ERO subobject types correspond to RSVP-TE ERO subobject types as
   defined in [RFC3209], [RFC3473], [RFC3477], [RFC4873], [RFC4874], and
   [RFC5520].  The subobjects related to Domain-Sequence the domain sequence are further
   defined in [DOMAIN-SUBOBJ]. [RFC7898].

   The new subobjects to support 4 byte 4-byte AS numbers and the IGP (OSPF / ISIS) Area (OSPF/IS-
   IS) area can also be used in the ERO to specify an abstract node (a
   group of nodes whose internal topology is opaque to the ingress node
   of the LSP).  Using this concept of abstraction, an explicitly routed
   LSP can be specified as a sequence of domains.

   In case of Hierarchical PCE H-PCE [RFC6805], a Parent parent PCE can be requested to find the Domain-Sequence.
   domain sequence.  Refer to the example in Section 4.6. 4.6 of this
   document.  The ERO in reply from the parent PCE can then be used in Per-Domain
   per-domain path computation or BRPC.

   If a PCC receives an ERO in a PCRep message that contains a subobject
   defined in this document, document that it does not recognize, it will respond
   according to the rules for a malformed object as per [RFC5440].

4.  Examples

   The examples in this section are for illustration purposes only; only to
   highlight how the new subobjects could be encoded.  They are not
   meant to be an exhaustive list of all possible usecases use cases and
   combinations.

4.1.  Inter-Area Path Computation

   In an inter-area path computation where the ingress and the egress
   nodes belong to different IGP areas within the same AS, the Domain-
   Sequence domain
   sequence could be represented using a an ordered list of Area area
   subobjects.

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

                   Figure 1: Inter-Area Path Computation
   The AS Number is 100.

   If the ingress is in Area area 2, the egress is in Area 4 area 4, and transit is
   through
   Area 0.  Some area 0, here are some possible way ways a PCC can encode the IRO:

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

     or

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

     or

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

   The Domain-Sequence domain sequence can further include encompassing AS information
   in the AS subobject.

4.2.  Inter-AS Path Computation

   In inter-AS path computation, where the ingress and egress belong to
   different AS, ASes, the Domain-Sequence domain sequence could be represented using an
   ordered list of AS subobjects.  The Domain-Sequence domain sequence can further
   include decomposed area information in the Area area subobject.

4.2.1.  Example 1

   As shown in Figure 2, where AS has a single area, the AS subobject in
   the
   domain-sequence domain sequence can uniquely identify the next domain and PCE.

              AS A                AS E                AS C
         <------------->      <---------->      <------------->

                  A4----------E1---E2---E3---------C4
                 /           /                       \
               /            /                          \
             /            /       AS B                   \
           /            /      <---------->                \
     Ingress------A1---A2------B1---B2---B3------C1---C2------Egress
           \                                    /          /
             \                                /          /
               \                            /          /
                 \                        /          /
                  A3----------D1---D2---D3---------C3

                              <---------->
                                  AS D

     * All AS ASes have one area (area 0)

                    Figure 2: Inter-AS Path Computation

   If the ingress is in AS A, the egress is in AS C C, and transit is
   through AS B.
   Some B, here are some possible way ways a PCC can encode the IRO:

   +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object |
   |Header | |AS B   | |AS C   |
   |       | |       | |       |
   +-------+ +-------+ +-------+

   or

   +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object |
   |Header | |AS A   | |AS B   | |AS C   |
   |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+

   or

   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object | |Object |object | |Object |object | |Object |object |
   |Header | |AS A   | |Area 0 | |AS B   | |Area 0 | |AS C   | |Area 0 |
   |       | |       | |       | |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+ +-------+

   Note that to get a domain disjoint path, the ingress could also
   request the backup path with - with:

   +-------+ +-------+
   |XRO    | |Sub    |
   |Object | |Object |
   |Header | |AS B   |
   |       | |       |
   +-------+ +-------+

   As described in Section 3.4.3, a domain subobject in IRO changes the
   domain information associated with the next set of subobjects; subobjects till
   you encounter a subobject that changes the domain too.  Consider the
   following IRO:

   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object | |Object |object | |Object |object |
   |Header | |AS B   | |IP     | |IP     | |AS C   | |IP     |
   |       | |       | |B1     | |B3     | |       | |C1     |
   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   On processing subobject "AS B", it changes the AS of the subsequent
   subobjects till we encounter another subobject "AS C" which that changes
   the AS for its subsequent subobjects.

   Consider another IRO:

   +-------+ +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object | |Object |object |
   |Header | |AS D   | |IP     | |IP     | |IP     |
   |       | |       | |D1     | |D3     | |C3     |
   +-------+ +-------+ +-------+ +-------+ +-------+

   Here as well, on processing "AS D", it changes the AS of the
   subsequent subobjects till you encounter another subobject "C3" which
   belong that
   belongs in another AS and changes the AS for its subsequent
   subobjects.

   Further description for the Boundary Node boundary node and Inter-AS-Link inter-AS link can be
   found in Section 4.3.

4.2.2.  Example 2

   In Figure 3, AS 200 is made up of multiple areas.

                  |
                  |  +-------------+                +----------------+
                  |  |Area 2       |                |Area 4          |
                  |  |         +--+|                |          +--+  |
                  |  |         |  ||                |          | B|  |
                  |  |  +--+   +--+|                |   +--+   +--+  |
                  |  |  |  |       |                |   |  |         |
                  |  |  +--+       |                |   +--+         |
                  |  |        +--+ |                |          +--+  |
                  |  |        |  | |                |          |  |  |
                  |  |        +--+ |                |   +--+   +--+  |
                  |  |  +--+       |+--------------+|   |  |         |
                  |  |  |  |       +--+          +--+   +--+         |
   +-------------+|  |  +--+       |  |          |  |                |
   |             ||  |             +--+          +--+                |
   |         +--+||  +-------------+|              |+----------------+
   |         |  |||                 |     +--+     |
   |         +--+||                 |     |  |     |
   |    +--+     ||                 |     +--+     |
   |    |  |  +---+                +--+            |
   |    +--+  |   |----------------|  |            |
   |          +---+   Inter-AS     +--+   +--+     |
   |+--+         ||    Links        |     |  |     |
   ||A |      +---+                +--+   +--+     |
   |+--+      |   |----------------|  |            |
   |          +---+                +--+   +--+     |
   |    +--+     ||  +------------+ |     |  |     |+----------------+
   |    |  |     ||  |Area 3      +--+    +--+   +--+ Area 5         |
   |    +--+     ||  |            |  |           |  |                |
   |             ||  |            +--+           +--+                |
   |         +--+||  |       +--+ | |  Area 0      ||   +--+         |
   |         |  |||  |       |  | | +--------------+|   |  |         |
   |         +--+||  |       +--+ |                 |   +--+         |
   |             ||  |            |                 |          +--+  |
   |Area 0       ||  |   +--+     |                 |   +--+   |  |  |
   +-------------+|  |   |  |     |                 |   |  |   +--+  |
                  |  |   +--+  +--+                 |   +--+         |
                  |  |         |  |                 |                |
                  |  |         +--+                 |          +--+  |
                  |  |   +--+     |                 |          | C|  |
                  |  |   |  |     |                 |          +--+  |
                  |  |   +--+     |                 |                |
                  |  |            |                 |                |
                  |  +------------+                 +----------------+
                  |
                  |
       AS 100     |  AS 200
                  |

                    Figure 3: Inter-AS Path Computation

   For LSP (A-B), where ingress A is in (AS 100, Area area 0), egress B is in
   (AS 200, Area 4) area 4), and transit is through (AS 200, Area 0).  Some area 0), here are
   some possible
   way ways a PCC can encode the IRO:

   +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object |
   |Header | |AS 200 | |Area 0 | |Area 4 |
   |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+

   or

   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object | |Object |object | |Object |object |
   |Header | |AS 100 | |Area 0 | |AS 200 | |Area 0 | |Area 4 |
   |       | |       | |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   For LSP (A-C), where ingress A is in (AS 100, Area area 0), egress C is in
   (AS 200, Area 5) area 5), and transit is through (AS 200, Area 0).  Some area 0), here are
   some possible
   way ways a PCC can encode the IRO:

   +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object |
   |Header | |AS 200 | |Area 0 | |Area 5 |
   |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+

   or

   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+
   |IRO    | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   | |Sub |Sub-   |
   |Object | |Object |object | |Object |object | |Object |object | |Object |object | |Object |object |
   |Header | |AS 100 | |Area 0 | |AS 200 | |Area 0 | |Area 5 |
   |       | |       | |       | |       | |       | |       |
   +-------+ +-------+ +-------+ +-------+ +-------+ +-------+

4.3.  Boundary Node and Inter-AS-Link Inter-AS Link

   A PCC or PCE can include additional constraints covering which
   Boundary Nodes
   boundary nodes (ABR or ASBR) or Border border links (Inter-AS-link) (inter-AS link) to be
   traversed while defining a Domain-Sequence. domain sequence.  In which case case, the
   Boundary Node
   boundary node or Link link can be encoded as a part of the Domain-
   Sequence. domain
   sequence.

   Boundary Nodes (ABR / ASBR) nodes (ABR/ASBR) can be encoded using the IPv4 or IPv6
   prefix subobjects subobjects, usually the with a loopback address of 32 and 128 a prefix
   length of 128, respectively.  An Inter-AS inter-AS link can be encoded using
   the IPv4 or IPv6 prefix subobjects or unnumbered interface
   subobjects.

   For Figure 1, an ABR (say (say, 203.0.113.1) to be traversed can be
   specified in IRO as:

        +---------+ +---------+ +---------++---------+ +---------+
        |IRO      | |Sub |Sub-     | |Sub      ||Sub |Sub-     ||Sub-     | |Sub |Sub-     |
        |Object   | |Object |object   | |Object   ||Object |object   ||object   | |Object |object   |
        |Header   | |Area 2   | |IPv4     ||Area 0   | |Area 4   |
        |         | |         | |203.0.   ||         | |         |
        |         | |         | |112.1    ||         | |         |
        +---------+ +---------+ +---------++---------+ +---------+
   For Figure 3, an inter-AS-link (say inter-AS link (say, 198.51.100.1 - 198.51.100.2) to
   be traversed can be specified as:

          +---------+  +---------+ +---------+ +---------+
          |IRO      |  |Sub  |Sub-     | |Sub |Sub-     | |Sub |Sub-     |
          |Object   |  |Object  |object AS| |Object |object   | |Object |object AS|
          |Header   |  |100      | |IPv4     | |200      |
          |         |  |         | |198.51.  | |         |
          |         |  |         | |100.2    | |         |
          +---------+  +---------+ +---------+ +---------+

4.4.  PCE Serving multiple Multiple Domains

   A single PCE can be responsible for multiple domains; for example example,
   PCE function deployed on an ABR could be responsible for multiple
   areas.  A PCE which that can support adjacent domains can internally handle
   those domains in the Domain-Sequence domain sequence without any impact on the other
   domains in the Domain-Sequence. domain sequence.

4.5.  P2MP

   [RFC7334] describes an experimental inter-domain P2MP path
   computation mechanism where the path domain tree is described as a
   series of Domain-Sequences, domain sequences; an example is shown in the below figure: figure below:

                           +----------------+
                           |                |Domain D1
                           |        R       |
                           |                |
                           |        A       |
                           |                |
                           +-B------------C-+
                            /              \
                           /                \
                          /                  \
          Domain D2      /                    \ Domain D3
          +-------------D--+             +-----E----------+
          |                |             |                |
          |  F             |             |                |
          |          G     |             |       H        |
          |                |             |                |
          |                |             |                |
          +-I--------------+             +-J------------K-+
           /\                             /              \
          /  \                           /                \
         /    \                         /                  \
        /      \                       /                    \
       /        \                     /                      \
      /          \                   /                        \
     / Domain D4  \      Domain D5  /              Domain D6   \
   +-L-------------W+       +------P---------+      +-----------T----+
   |                |       |                |      |                |
   |                |       |  Q             |      |   U            |
   |  M        O    |       |         S      |      |                |
   |                |       |                |      |          V     |
   |          N     |       |   R            |      |                |
   +----------------+       +----------------+      +----------------+

                       Figure 4: Domain Tree Example

   The domain tree can be represented as a series of domain-sequence - domain sequences:

   o  Domain D1, Domain D3, Domain D6

   o  Domain D1, Domain D3, Domain D5

   o  Domain D1, Domain D2, Domain D4

   The domain sequence handling described in this document could be
   applied to the P2MP path domain tree.

4.6.  Hierarchical PCE

   In case of H-PCE [RFC6805], the parent PCE can be requested to
   determine the Domain-Sequence domain sequence and return it in the path computation
   reply, using the ERO. .  For the example in section Section 4.6 of [RFC6805],
   the Domain-Sequence domain sequence can possibly appear as:

   +---------+ +---------+ +---------+ +---------+
   |ERO      | |Sub |Sub-     | |Sub |Sub-     | |Sub |Sub-     |
   |Object   | |Object |object   | |Object |object   | |Object |object   |
   |Header   | |Domain 1 | |Domain 2 | |Domain 3 |
   |         | |         | |         | |         |
   |         | |         | |         | |         |
   +---------+ +---------+ +---------+ +---------+

   or

   +---------+ +---------+ +---------+
   |ERO      | |Sub |Sub-     | |Sub |Sub-     |
   |Object   | |Object |object   | |Object |object   |
   |Header   | |BN 21    | |Domain 3 |
   |         | |         | |         |
   |         | |         | |         |
   +---------+ +---------+ +---------+

5.  Other Considerations

5.1.  Relationship to PCE Sequence

   Instead of a Domain-Sequence, domain sequence, a sequence of PCEs MAY be enforced by
   policy on the PCC, and this constraint can be carried in the PCReq
   message (as defined in [RFC5886]).

   Note that PCE-Sequence PCE Sequence can be used along with Domain-Sequence domain sequence, in
   which case PCE-Sequence PCE Sequence MUST have higher precedence in selecting the
   next PCE in the inter-domain path computation procedures.

5.2.  Relationship to RSVP-TE

   [RFC3209] already describes the notion of abstract nodes, where an
   abstract node is a group of nodes whose internal topology is opaque
   to the ingress node of the LSP.  It further defines a subobject for
   AS but with a 2-Byte 2-byte AS Number.

   [DOMAIN-SUBOBJ] number.

   [RFC7898] extends the notion of abstract nodes by adding new
   subobjects for IGP Areas areas and 4-byte AS numbers.  These subobjects can
   be included in Explicit Route Object (ERO), Exclude Route object
   (XRO) ERO, XRO, or Explicit Exclusion Route Subobject (EXRS) EXRS in RSVP-TE.

   In any case case, subobject type types defined in RSVP-TE are identical to the
   subobject type types defined in the related documents in PCEP.

6.  IANA Considerations

6.1.  New Subobjects

   IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
   registry at <http://www.iana.org/assignments/pcep>.  Within this registry
   registry, IANA maintains two sub-registries:

   o  IRO Subobjects (see IRO Subobjects at
      http://www.iana.org/assignments/pcep)

   o  XRO Subobjects (see XRO Subobjects at
      http://www.iana.org/assignments/pcep)

   Upon approval of this document,

   IANA is requested to make has made identical additions to these those registries as follows:

   Subobject Type

   Value   Description        Reference
   TBD1      4 byte
   -----   ----------------   -------------------
   5       4-byte AS number        [This I.D.][DOMAIN-SUBOBJ]
   TBD2   RFC 7897, [RFC7898]
   6       OSPF Area ID            [This I.D.][DOMAIN-SUBOBJ]
   TBD3       RFC 7897, [RFC7898]
   7       IS-IS Area ID           [This I.D.][DOMAIN-SUBOBJ]

   Further upon approval of this document,      RFC 7897, [RFC7898]

   Further, IANA is requested to add has added a reference to this document to the new RSVP
   numbers that are registered by [DOMAIN-SUBOBJ]. [RFC7898], as shown on
   <http://www.iana.org/assignments/rsvp-parameters>.

7.  Security Considerations

   The protocol extensions defined in this document do not substantially
   change the nature of PCEP.  Therefore, the security considerations
   set out in [RFC5440] apply unchanged.  Note that further security
   considerations for the use of PCEP over TCP are presented in
   [RFC6952].

   This document specifies a representation of Domain-Sequence the domain sequence and
   new subobjects, which could be used in inter-domain PCE scenarios as
   explained in [RFC5152], [RFC5441], [RFC6805], [RFC7334] [RFC7334], etc.  The
   security considerations set out in each of these mechanisms remain
   unchanged by the new subobjects and Domain-Sequence domain sequence representation in
   this document.

   But the new subobjects do allow finer and more specific control of
   the path computed by a cooperating PCE(s).  Such control increases
   the risk if a PCEP message is intercepted, modified, or spoofed
   because it allows the attacker to exert control over the path that
   the PCE will compute or to make the path computation impossible.
   Consequently, it is important that implementations conform to the
   relevant security requirements of [RFC5440].  These mechanisms
   include:

   o  Securing the PCEP session messages using TCP security techniques
      (Section 10.2 of [RFC5440]).  PCEP implementations SHOULD also
      consider the additional security provided by the TCP
      Authentication Option (TCP-AO) [RFC5925] or Transport Layer
      Security (TLS) [PCEPS].

   o  Authenticating the PCEP messages to ensure the message is messages are intact
      and sent from an authorized node (Section 10.3 of [RFC5440]).

   o  PCEP operates over TCP, so it is also important to secure the PCE
      and PCC against TCP denial-of-service attacks.  Section 10.7.1 of
      [RFC5440] outlines a number of mechanisms for minimizing the risk
      of TCP-based denial-of-service attacks against PCEs and PCCs.

   o  In inter-AS scenarios, attacks may be particularly significant
      with commercial commercial- as well as service-level implications.

   Note, however, that the Domain-Sequence domain sequence mechanisms also provide the
   operator with the ability to route around vulnerable parts of the
   network and may be used to increase overall network security.

8.  Manageability Considerations

8.1.  Control of Function and Policy

   The exact behaviour behavior with regards to desired inclusion and exclusion of
   domains MUST be available for examination by an operator and MAY be
   configurable.  Manual configurations is are needed to identify which
   PCEP peers understand the new domain subobjects defined in this
   document.

8.2.  Information and Data Models

   A MIB module for management of the PCEP is being specified in a
   separate document [RFC7420].  This document does not imply any new
   extension to the current MIB module.

8.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to aside from those already listed
   in [RFC5440].

8.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to aside from those already listed in
   [RFC5440].

8.5.  Requirements On on Other Protocols

   In case of per-domain path computation [RFC5152], where the full path
   of an inter-domain TE LSP cannot be, or be determined (or is not determined determined)
   at the ingress node, a signaling message can use the domain
   identifiers.  The Subobjects subobjects defined in this document SHOULD be
   supported by RSVP-
   TE.  [DOMAIN-SUBOBJ] RSVP-TE.  [RFC7898] extends the notion of abstract nodes
   by adding new subobjects for IGP Areas areas and 4-byte AS numbers.

   Apart from this, mechanisms defined in this document do not imply any
   requirements on other protocols in addition to aside from those already listed in
   [RFC5440].

8.6.  Impact On on Network Operations

   The mechanisms described in this document can provide the operator
   with the ability to exert finer and more specific control of the path
   computation by inclusion or exclusion of domain subobjects.  There
   may be some scaling benefit when a single domain subobject may
   substitute for many subobjects and can reduce the overall message
   size and processing.

   Backward compatibility issues associated with the new subobjects
   arise when a PCE does not recognize them, in which case PCE responds
   according to the rules for a malformed object as per [RFC5440].  For
   successful operations operations, the PCEs in the network would need to be
   upgraded.

9.  Acknowledgments

   Authors would like to especially thank Adrian Farrel for his detailed
   reviews as well as providing text to be included in the document.

   Further, we would like to thank Pradeep Shastry, Suresh Babu, Quintin
   Zhao, Fatai Zhang, Daniel King, Oscar Gonzalez, Chen Huaimo,
   Venugopal Reddy, Reeja Paul, Sandeep Boina, Avantika Sergio Belotti
   and Jonathan Hardwick  References

9.1.  Normative References

   [ISO10589]
              International Organization for their useful comments Standardization,
              "Information technology -- Telecommunications and suggestions.

   Thanks to Jonathan Hardwick for shepherding this document.

   Thanks
              information exchange between systems -- Intermediate
              System to Deborah Brungard Intermediate System intra-domain routeing
              information exchange protocol for being use in conjunction with
              the Responsible AD.

   Thanks to Amanda Baber for IANA Review.

   Thanks to Joel Halpern for Gen-ART Review.

   Thanks to Klaas Wierenga for SecDir Review.

   Thanks to Spencer Dawkins and Barry Leiba protocol for comments during providing the
   IESG Review.

10.  References

10.1.  Normative References connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <http://www.rfc-editor.org/info/rfc3209>.

   [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation Protocol-
              Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
              DOI 10.17487/RFC3473, January 2003,
              <http://www.rfc-editor.org/info/rfc3473>.

   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003,
              <http://www.rfc-editor.org/info/rfc3477>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <http://www.rfc-editor.org/info/rfc5440>.

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,
              <http://www.rfc-editor.org/info/rfc5441>.

   [RFC5521]  Oki, E., Takeda, T., and A. Farrel, "Extensions to the
              Path Computation Element Communication Protocol (PCEP) for
              Route Exclusions", RFC 5521, DOI 10.17487/RFC5521, April
              2009, <http://www.rfc-editor.org/info/rfc5521>.

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,
              <http://www.rfc-editor.org/info/rfc6805>.

   [ISO10589]
              ISO, "Intermediate system to Intermediate system routing
              information exchange protocol for use in conjunction with
              the Protocol for providing the Connectionless-mode Network
              Service (ISO 8473)", ISO/IEC 10589:2002, 1992.

   [IRO-UPDATE]

   [RFC7896]  Dhody, D., "Update to the Include Route Object (IRO)
              specification
              Specification in the Path Computation Element communication
              Communication Protocol (PCEP. (draft-ietf-pce-iro-update-02)", May 2015.

   [DOMAIN-SUBOBJ] (PCEP)", RFC 7896,
              DOI 10.17487/RFC7896, June 2016,
              <http://www.rfc-editor.org/info/rfc7896>.

   [RFC7898]  Dhody, D., Palle, U., Kondreddy, V., and R. Casellas,
              "Domain Subobjects for Resource ReserVation Reservation Protocol -
              Traffic Engineering (RSVP-TE). (draft-ietf-teas-rsvp-te-
              domain-subobjects-05)", November 2015.

10.2. (RSVP-TE)", RFC 7898,
              DOI 10.17487/RFC7898, June 2016,
              <http://www.rfc-editor.org/info/rfc7898>.

9.2.  Informative References

   [PCEPS]    Lopez, D., Dios, O., Wu, W., and D. Dhody, "Secure
              Transport for PCEP", Work in Progress, draft-ietf-pce-
              pceps-09, November 2015.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <http://www.rfc-editor.org/info/rfc4655>.

   [RFC4726]  Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework for
              Inter-Domain Multiprotocol Label Switching Traffic
              Engineering", RFC 4726, DOI 10.17487/RFC4726, November
              2006, <http://www.rfc-editor.org/info/rfc4726>.

   [RFC4873]  Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel,
              "GMPLS Segment Recovery", RFC 4873, DOI 10.17487/RFC4873,
              May 2007, <http://www.rfc-editor.org/info/rfc4873>.

   [RFC4874]  Lee, CY., Farrel, A., and S. De Cnodder, "Exclude Routes -
              Extension to Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE)", RFC 4874, DOI 10.17487/RFC4874,
              April 2007, <http://www.rfc-editor.org/info/rfc4874>.

   [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
              Per-Domain Path Computation Method for Establishing Inter-
              Domain Traffic Engineering (TE) Label Switched Paths
              (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
              <http://www.rfc-editor.org/info/rfc5152>.

   [RFC5520]  Bradford, R., Ed., Vasseur, JP., and A. Farrel,
              "Preserving Topology Confidentiality in Inter-Domain Path
              Computation Using a Path-Key-Based Mechanism", RFC 5520,
              DOI 10.17487/RFC5520, April 2009,
              <http://www.rfc-editor.org/info/rfc5520>.

   [RFC5886]  Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set of
              Monitoring Tools for Path Computation Element (PCE)-Based
              Architecture", RFC 5886, DOI 10.17487/RFC5886, June 2010,
              <http://www.rfc-editor.org/info/rfc5886>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <http://www.rfc-editor.org/info/rfc5925>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <http://www.rfc-editor.org/info/rfc6793>.

   [RFC6952]  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
              BGP, LDP, PCEP, and MSDP Issues According to the Keying
              and Authentication for Routing Protocols (KARP) Design
              Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
              <http://www.rfc-editor.org/info/rfc6952>.

   [RFC7334]  Zhao, Q., Dhody, D., King, D., Ali, Z., and R. Casellas,
              "PCE-Based Computation Procedure to Compute Shortest
              Constrained Point-to-Multipoint (P2MP) Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 7334,
              DOI 10.17487/RFC7334, August 2014,
              <http://www.rfc-editor.org/info/rfc7334>.

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <http://www.rfc-editor.org/info/rfc7420>.

   [PCEPS]    Lopez, D., Dios, O., Wu, W., and D. Dhody, "Secure
              Transport

Acknowledgments

   The authors would like to especially thank Adrian Farrel for PCEP", draft-ietf-pce-pceps-06 (work his
   detailed reviews as well as providing text to be included in
              progress), November 2015. the
   document.

   Further, we would like to thank Pradeep Shastry, Suresh Babu, Quintin
   Zhao, Fatai Zhang, Daniel King, Oscar Gonzalez, Chen Huaimo,
   Venugopal Reddy, Reeja Paul, Sandeep Boina, Avantika Sergio Belotti,
   and Jonathan Hardwick for their useful comments and suggestions.

   Thanks to Jonathan Hardwick for shepherding this document.

   Thanks to Deborah Brungard for being the responsible AD.

   Thanks to Amanda Baber for the IANA review.

   Thanks to Joel Halpern for the Gen-ART review.

   Thanks to Klaas Wierenga for the SecDir review.

   Thanks to Spencer Dawkins and Barry Leiba for comments during the
   IESG review.

Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560037  560066
   India

   EMail:

   Email: dhruv.ietf@gmail.com

   Udayasree Palle
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560037  560066
   India

   EMail:

   Email: udayasree.palle@huawei.com

   Ramon Casellas
   CTTC
   Av. Carl Friedrich Gauss n7
   Castelldefels, Barcelona  08860
   Spain

   EMail:

   Email: ramon.casellas@cttc.es