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RFC 7897

Experimental
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Domain Subobjects for the Path Computation Element Communication Protocol (PCEP)

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Internet Engineering Task Force (IETF)                          D. Dhody
Request for Comments: 7897                                      U. Palle
Category: Experimental                               Huawei Technologies
ISSN: 2070-1721                                              R. Casellas
                                                                    CTTC
                                                               June 2016


                           Domain Subobjects
     for the Path Computation Element Communication Protocol (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, 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.  This
   document also defines new subobjects to be used to encode domain
   identifiers.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7897.

Page 2 
Copyright Notice

   Copyright (c) 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
   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.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Detail Description  . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Domains . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Domain Sequence . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Domain Sequence Representation  . . . . . . . . . . . . .   7
     3.4.  Include Route Object (IRO)  . . . . . . . . . . . . . . .   8
       3.4.1.  Subobjects  . . . . . . . . . . . . . . . . . . . . .   8
         3.4.1.1.  Autonomous System . . . . . . . . . . . . . . . .   8
         3.4.1.2.  IGP Area  . . . . . . . . . . . . . . . . . . . .   9
       3.4.2.  Update in IRO Specification . . . . . . . . . . . . .  10
       3.4.3.  IRO for Domain Sequence . . . . . . . . . . . . . . .  11
         3.4.3.1.  PCC Procedures  . . . . . . . . . . . . . . . . .  11
         3.4.3.2.  PCE Procedures  . . . . . . . . . . . . . . . . .  11
     3.5.  Exclude Route Object (XRO)  . . . . . . . . . . . . . . .  13
       3.5.1.  Subobjects  . . . . . . . . . . . . . . . . . . . . .  13
         3.5.1.1.  Autonomous System . . . . . . . . . . . . . . . .  14
         3.5.1.2.  IGP Area  . . . . . . . . . . . . . . . . . . . .  14
     3.6.  Explicit Exclusion Route Subobject (EXRS) . . . . . . . .  16
     3.7.  Explicit Route Object (ERO) . . . . . . . . . . . . . . .  16
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  Inter-Area Path Computation . . . . . . . . . . . . . . .  17
     4.2.  Inter-AS Path Computation . . . . . . . . . . . . . . . .  19
       4.2.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . .  20
       4.2.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . .  22
     4.3.  Boundary Node and Inter-AS Link . . . . . . . . . . . . .  25
     4.4.  PCE Serving Multiple Domains  . . . . . . . . . . . . . .  25
     4.5.  P2MP  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     4.6.  Hierarchical PCE  . . . . . . . . . . . . . . . . . . . .  27

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   5.  Other Considerations  . . . . . . . . . . . . . . . . . . . .  27
     5.1.  Relationship to PCE Sequence  . . . . . . . . . . . . . .  27
     5.2.  Relationship to RSVP-TE . . . . . . . . . . . . . . . . .  27
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     6.1.  New Subobjects  . . . . . . . . . . . . . . . . . . . . .  28
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  29
     8.1.  Control of Function and Policy  . . . . . . . . . . . . .  29
     8.2.  Information and Data Models . . . . . . . . . . . . . . .  29
     8.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  30
     8.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  30
     8.5.  Requirements on Other Protocols . . . . . . . . . . . . .  30
     8.6.  Impact on Network Operations  . . . . . . . . . . . . . .  30
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

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 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 and is either fixed by the network operator or obtained by
   other means.  Also, for inter-domain point-to-multipoint (P2MP) tree
   computation, it is assumed per [RFC7334] that the domain tree is
   known a priori.

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

   The domain sequence (the set of domains traversed to reach the
   destination domain) is either administratively predetermined or
   discovered by some means like 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 (EXRS).  The use of an
   Autonomous System (albeit with a 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 AS (4 bytes as per [RFC6793]).

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   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 Protocol -
   Traffic Engineering (RSVP-TE) extensions for the domain identifiers
   [RFC7898].

1.1.  Scope

   The procedures described in this document are experimental.  The
   experiment is intended to enable research for the usage of the domain
   sequence at the PCEs for inter-domain paths.  For this purpose, this
   document specifies new domain subobjects as well as how they
   incorporate with existing subobjects to represent a 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 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 per [RFC5440].  Therefore, it is assumed that
   this experiment will be conducted only when both the PCE and the Path
   Computation Client (PCC) form part of the experiment.  It is possible
   that a PCC or PCE can operate with 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 results of implementation and deployment are available, this
   document will be updated and refined, and then it could be moved from
   Experimental to 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].

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2.  Terminology

   The following terminology is used in this document.

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

   AS:  Autonomous System

   ASBR:  Autonomous System Border Router

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

   BRPC:  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 IGP area and 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: OSPF or 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

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   RSVP:  Resource Reservation Protocol

   TE LSP:  Traffic Engineering Label Switched 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, 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, it is required to uniquely identify a domain in
   the domain sequence.  A domain can be uniquely identified by an
   area-id, AS number, or both.

3.2.  Domain Sequence

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

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

   A 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.

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

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   Thus, a 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 Address

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

   Consequently, a domain sequence can be used by:

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

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

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

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

3.3.  Domain Sequence Representation

   A domain sequence appears in PCEP messages, notably 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.

   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 include subobjects used to specify the 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 is a
      subobject inside the IRO.

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   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 PCE can send the domain sequence as a
      result in a path computation reply using ERO.

3.4.  Include Route Object (IRO)

   As per [RFC5440], IRO 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], and
   [RFC4874], but new subobjects related to domain sequence are needed.

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

                 Value  Description
                 -----  ----------------
                 5      4-byte AS number
                 6      OSPF Area ID
                 7      IS-IS Area ID

   Note: Identical subobjects are carried in RSVP-TE messages as defined
   in [RFC7898].

3.4.1.1.  Autonomous System

   [RFC3209] already defines 2-byte AS numbers.

   To support 4-byte AS numbers as per [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 Number (4 bytes)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Type:  5 (indicating a 4-byte AS number).

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

   Reserved:  Zero at transmission; ignored at receipt.

   AS Number:  The 4-byte AS number.  Note that if 2-byte AS numbers are
      in use, the low-order bits (16 through 31) MUST be used, and the
      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 is different for OSPF and
   IS-IS, the following two subobjects are defined below:

   For OSPF, the Area ID is a 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 (4 bytes)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Type:  6 (indicating a 4-byte OSPF Area ID).

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

   Reserved:  Zero at transmission; ignored at receipt.

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

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   For IS-IS, the Area ID is of variable length; thus, the length of the
   subobject is variable.  The 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], and its usage in the IRO subobject is defined in
      [RFC7896].

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

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

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

   Reserved:  Zero at transmission; ignored at receipt.

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

3.4.2.  Update in IRO Specification

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

   An update to the IRO specification [RFC7896] makes IRO as an ordered
   list and includes support for the L bit.

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

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3.4.3.  IRO for Domain Sequence

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

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

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

   A domain sequence can have varying degrees of granularity.  It is
   possible to have a 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 is able to do so (e.g.,
   via administrative configuration or discovery).

3.4.3.1.  PCC Procedures

   A PCC builds an IRO to encode the 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 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 (PCReq)
   message that contains the subobjects defined in this 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 PCEP Error (PCErr) message as per [RFC5440].

   The interpretation of the L bit is as per Section 4.3.3.1 of
   [RFC3209] (as per [RFC7896]).

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   In a Path Computation Reply (PCRep), PCE MAY also supply IRO (with
   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 a domain sequence in 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 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".  Certain subobjects modify the current
      area, as follows.

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

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

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

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

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

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

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

3.5.  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 are needed.

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

                 Value  Description
                 -----  ----------------
                 5      4-byte AS number
                 6      OSPF Area ID
                 7      IS-IS Area ID

   Note: Identical subobjects are carried in RSVP-TE messages as defined
   in [RFC7898].

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3.5.1.1.  Autonomous System

   The new subobjects to support 4-byte AS numbers and the IGP
   (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 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 the Area ID is different for OSPF and
   IS-IS, the following two subobjects are defined:

   For OSPF, the Area ID is a 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 (4 bytes)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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   1: indicates that the 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 is of variable length; thus, the length of the
   subobject is variable.  The 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 IS-IS area specified MUST be excluded from the
      path computed by the PCE(s).

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

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

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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 can carry any of the subobjects defined for inclusion in the
   XRO; thus, the new subobjects to support 4-byte AS numbers and the
   IGP (OSPF / 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 should be interpreted in the context of the current AS and
   current area of the preceding subobject in the IRO.  The EXRS does
   not change the current AS or current area.  All other processing
   rules are as per [RFC5521].

   Note that if a PCE that supports the EXRS in an IRO parses an IRO,
   and encounters an EXRS that contains subobjects defined in this
   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)

   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 the domain sequence are further
   defined in [RFC7898].

   The new subobjects to support 4-byte AS numbers and the IGP
   (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 H-PCE [RFC6805], a parent PCE can be requested to find the
   domain sequence.  Refer to the example in Section 4.6 of this
   document.  The ERO in reply from the parent PCE can then be used in
   per-domain path computation or BRPC.

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


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