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For this RFC, original HTML is available from the RFC-Editor: RFC8685

PROPOSED STANDARD

Internet Engineering Task Force (IETF)                          F. Zhang
Request for Comments: 8685                                       Q. Zhao
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                      O. Gonzalez de Dios
                                                          Telefonica I+D
                                                             R. Casellas
                                                                    CTTC
                                                                 D. King
                                                      Old Dog Consulting
                                                           December 2019


   Path Computation Element Communication Protocol (PCEP) Extensions
   for the Hierarchical Path Computation Element (H-PCE) Architecture

Abstract

   The Hierarchical Path Computation Element (H-PCE) architecture is
   defined in RFC 6805.  It provides a mechanism to derive an optimum
   end-to-end path in a multi-domain environment by using a hierarchical
   relationship between domains to select the optimum sequence of
   domains and optimum paths across those domains.

   This document defines extensions to the Path Computation Element
   Communication Protocol (PCEP) to support H-PCE procedures.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8685.

Copyright Notice

   Copyright (c) 2019 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
   (https://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
     1.1.  Scope
     1.2.  Terminology
     1.3.  Requirements Language
   2.  Requirements for the H-PCE Architecture
     2.1.  Path Computation Requests
       2.1.1.  Qualification of PCEP Requests
       2.1.2.  Multi-domain Objective Functions
       2.1.3.  Multi-domain Metrics
     2.2.  Parent PCE Capability Advertisement
     2.3.  PCE Domain Identification
     2.4.  Domain Diversity
   3.  PCEP Extensions
     3.1.  Applicability to PCC-PCE Communications
     3.2.  OPEN Object
       3.2.1.  H-PCE-CAPABILITY TLV
         3.2.1.1.  Backwards Compatibility
       3.2.2.  Domain-ID TLV
     3.3.  RP Object
       3.3.1.  H-PCE-FLAG TLV
       3.3.2.  Domain-ID TLV
     3.4.  Objective Functions
       3.4.1.  OF Codes
       3.4.2.  OF Object
     3.5.  METRIC Object
     3.6.  SVEC Object
     3.7.  PCEP-ERROR Object
       3.7.1.  Hierarchical PCE Error-Type
     3.8.  NO-PATH Object
   4.  H-PCE Procedures
     4.1.  OPEN Procedure between Child PCE and Parent PCE
     4.2.  Procedure for Obtaining the Domain Sequence
   5.  Error Handling
   6.  Manageability Considerations
     6.1.  Control of Function and Policy
       6.1.1.  Child PCE
       6.1.2.  Parent PCE
       6.1.3.  Policy Control
     6.2.  Information and Data Models
     6.3.  Liveness Detection and Monitoring
     6.4.  Verifying Correct Operations
     6.5.  Requirements on Other Protocols
     6.6.  Impact on Network Operations
   7.  IANA Considerations
     7.1.  PCEP TLV Type Indicators
     7.2.  H-PCE-CAPABILITY TLV Flags
     7.3.  Domain-ID TLV Domain Type
     7.4.  H-PCE-FLAG TLV Flags
     7.5.  OF Codes
     7.6.  METRIC Object Types
     7.7.  New PCEP Error-Types and Values
     7.8.  New NO-PATH-VECTOR TLV Bit Flag
     7.9.  SVEC Flag
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   The Path Computation Element Communication Protocol (PCEP) provides a
   mechanism for Path Computation Elements (PCEs) and Path Computation
   Clients (PCCs) to exchange requests for path computation and
   responses that provide computed paths.

   The capability to compute the routes of end-to-end inter-domain MPLS
   Traffic Engineering (MPLS-TE) and GMPLS Label Switched Paths (LSPs)
   is expressed as requirements in [RFC4105] and [RFC4216].  This
   capability may be realized by a PCE [RFC4655].  The methods for
   establishing and controlling inter-domain MPLS-TE and GMPLS LSPs are
   documented in [RFC4726].

   [RFC6805] describes a Hierarchical Path Computation Element (H-PCE)
   architecture that can be used for computing end-to-end paths for
   inter-domain MPLS-TE and GMPLS LSPs.

   In the H-PCE architecture, the parent PCE is used to compute a multi-
   domain path based on the domain connectivity information.  A child
   PCE may be responsible for single or multiple domains and is used to
   compute the intra-domain path based on its own domain topology
   information.

   The H-PCE end-to-end domain path computation procedure is described
   below:

   *  A PCC sends the inter-domain Path Computation Request (PCReq)
      messages [RFC5440] to the child PCE responsible for its domain.

   *  The child PCE forwards the request to the parent PCE.

   *  The parent PCE computes the likely domain paths from the ingress
      domain to the egress domain.

   *  The parent PCE sends the intra-domain PCReq messages (between the
      domain border nodes) to the child PCEs that are responsible for
      the domains along the domain path.

   *  The child PCEs return the intra-domain paths to the parent PCE.

   *  The parent PCE constructs the end-to-end inter-domain path based
      on the intra-domain paths.

   *  The parent PCE returns the inter-domain path to the child PCE.

   *  The child PCE forwards the inter-domain path to the PCC.

   The parent PCE may be requested to provide only the sequence of
   domains to a child PCE so that alternative inter-domain path
   computation procedures, including per-domain (PD) path computation
   [RFC5152] and Backward-Recursive PCE-Based Computation (BRPC)
   [RFC5441], may be used.

   This document defines the PCEP extensions for the purpose of
   implementing H-PCE procedures, which are described in [RFC6805].

1.1.  Scope

   The following functions are out of scope for this document:

   *  Determination of the destination domain (Section 4.5 of
      [RFC6805]):

      -  via a collection of reachability information from child
         domains,

      -  via requests to the child PCEs to discover if they contain the
         destination node, or

      -  via any other methods.

   *  Parent Traffic Engineering Database (TED) methods (Section 4.4 of
      [RFC6805]), although suitable mechanisms include:

      -  YANG-based management interfaces.

      -  BGP - Link State (BGP-LS) [RFC7752].

      -  Future extensions to PCEP (for example, see [PCEP-LS]).

   *  Learning of domain connectivity and border node addresses.
      Methods to achieve this function include:

      -  YANG-based management interfaces.

      -  BGP-LS [RFC7752].

      -  Future extensions to PCEP (for example, see [PCEP-LS]).

   *  Stateful PCE operations.  (Refer to [STATEFUL-HPCE].)

   *  Applicability of the H-PCE model to large multi-domain
      environments.

      -  The hierarchical relationship model is described in [RFC6805].
         It is applicable to environments with small groups of domains
         where visibility from the ingress Label Switching Routers
         (LSRs) is limited.  As highlighted in [RFC7399], applying the
         H-PCE model to very large groups of domains, such as the
         Internet, is not considered feasible or desirable.

1.2.  Terminology

   This document uses the terminology defined in [RFC4655] and
   [RFC5440], and the additional terms defined in Section 1.4 of
   [RFC6805].

1.3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Requirements for the H-PCE Architecture

   This section compiles the set of requirements for the PCEP extensions
   to support the H-PCE architecture and procedures.  [RFC6805]
   identifies high-level requirements for PCEP extensions that are
   required for supporting the H-PCE model.

2.1.  Path Computation Requests

   The PCReq messages [RFC5440] are used by a PCC or a PCE to make a
   path computation request to a PCE.  In order to achieve the full
   functionality of the H-PCE procedures, the PCReq message needs to
   include:

   *  Qualification of PCE requests (Section 4.8.1 of [RFC6805]).

   *  Multi-domain Objective Functions (OFs).

   *  Multi-domain metrics.

2.1.1.  Qualification of PCEP Requests

   As described in Section 4.8.1 of [RFC6805], the H-PCE architecture
   introduces new request qualifications, which are as follows:

   *  The ability for a child PCE to indicate that a PCReq message sent
      to a parent PCE should be satisfied by a domain sequence only --
      that is, not by a full end-to-end path.  This allows the child PCE
      to initiate a PD path computation per [RFC5152] or a BRPC
      procedure [RFC5441].

   *  As stated in [RFC6805], Section 4.5, if a PCC knows the egress
      domain, it can supply this information as part of the PCReq
      message.  The PCC may also want to specify the destination domain
      information in a PCEP request, if it is known.

   *  An inter-domain path computed by a parent PCE should be capable of
      disallowing re-entry into a specified domain.

2.1.2.  Multi-domain Objective Functions

   For H-PCE inter-domain path computation, there are three new OFs
   defined in this document:

   *  Minimize the number of Transit Domains (MTD)

   *  Minimize the number of Border Nodes (MBN)

   *  Minimize the number of Common Transit Domains (MCTD)

   The PCC may specify the multi-domain OF code to use when requesting
   inter-domain path computation.  It may also include intra-domain OFs,
   such as Minimum Cost Path (MCP) [RFC5541], which must be considered
   by participating child PCEs.

2.1.3.  Multi-domain Metrics

   For inter-domain path computation, there are two path metrics of
   interest.

   *  Domain Count (number of domains crossed).

   *  Border Node Count.

   A PCC may be able to limit the number of domains crossed by applying
   a limit on these metrics.  See Section 3.4 for details.

2.2.  Parent PCE Capability Advertisement

   A PCEP speaker (parent PCE or child PCE) that supports and wishes to
   use the procedures described in this document must advertise this
   fact and negotiate its role with its PCEP peers.  It does this using
   the "H-PCE Capability" TLV, as described in Section 3.2.1, in the
   OPEN object [RFC5440] to advertise its support for PCEP extensions
   for the H-PCE capability.

   During the PCEP session establishment procedure, the child PCE needs
   to be capable of indicating to the parent PCE whether it requests the
   parent PCE capability or not.

2.3.  PCE Domain Identification

   A PCE domain is a single domain with an associated PCE, although it
   is possible for a PCE to manage multiple domains simultaneously.  The
   PCE domain could be an IGP area or Autonomous System (AS).

   The PCE domain identifiers MAY be provided during the PCEP session
   establishment procedure.

2.4.  Domain Diversity

   "Domain diversity" in the context of a multi-domain environment is
   defined in [RFC6805] and described as follows:

   |  A pair of paths are domain-diverse if they do not transit any of
   |  the same domains.  A pair of paths that share a common ingress and
   |  egress are domain-diverse if they only share the same domains at
   |  the ingress and egress (the ingress and egress domains).  Domain
   |  diversity may be maximized for a pair of paths by selecting paths
   |  that have the smallest number of shared domains.

   The main motivation behind domain diversity is to avoid fate-sharing.
   However, domain diversity may also be requested to avoid specific
   transit domains due to security, geopolitical, and commercial
   reasons.  For example, a pair of paths should choose different
   transit ASes because of certain policy considerations.

   In the case when full domain diversity could not be achieved, it is
   helpful to minimize the commonly shared domains.  Also, it is
   interesting to note that other domain-diversity techniques (node,
   link, Shared Risk Link Group (SRLG), etc.) can still be applied
   inside the commonly shared domains.

3.  PCEP Extensions

   This section defines extensions to PCEP [RFC5440] to support the
   H-PCE procedures.

3.1.  Applicability to PCC-PCE Communications

   Although the extensions defined in this document are intended
   primarily for use between a child PCE and a parent PCE, they are also
   applicable for communications between a PCC and its PCE.

   Thus, the information that may be encoded in a PCReq can be sent from
   a PCC towards the child PCE.  This includes the Request Parameters
   (RP) object ([RFC5440] and Section 3.3), the OF codes
   (Section 3.4.1), and the OF object (Section 3.4.2).  A PCC and a
   child PCE could also exchange the H-PCE capability (Section 3.2.1)
   during its session.

   This allows a PCC to request paths that transit multiple domains
   utilizing the capabilities defined in this document.

3.2.  OPEN Object

   This document defines two new TLVs to be carried in an OPEN object.
   This way, during the PCEP session establishment, the H-PCE capability
   and domain information can be advertised.

3.2.1.  H-PCE-CAPABILITY TLV

   The H-PCE-CAPABILITY TLV is an optional TLV associated with the OPEN
   object [RFC5440] to exchange the H-PCE capability of PCEP speakers.

   Its format is shown in the following figure:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Type=13         |            Length=4           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Flags                               |P|
   +---------------------------------------------------------------+

                   Figure 1: H-PCE-CAPABILITY TLV Format

   The type of the TLV is 13, and it has a fixed length of 4 octets.

   The value comprises a single field -- Flags (32 bits):

      P (Parent PCE Request bit):
         If set, will signal that the child PCE wishes to use the peer
         PCE as a parent PCE.

   Unassigned bits MUST be set to 0 on transmission and MUST be ignored
   on receipt.

   The inclusion of this TLV in an OPEN object indicates that the H-PCE
   extensions are supported by the PCEP speaker.  The child PCE MUST
   include this TLV and set the P-flag.  The parent PCE MUST include
   this TLV and unset the P-flag.

   The setting of the P-flag (Parent PCE Request bit) would mean that
   the PCEP speaker wants the peer to be a parent PCE, so in the case of
   a PCC-to-child-PCE relationship, neither entity would set the P-flag.

   If both peers attempt to set the P-flag, then the session
   establishment MUST fail, and the PCEP speaker MUST respond with a
   PCErr message using Error-Type 1 (PCEP session establishment failure)
   as per [RFC5440].

   If the PCE understands the H-PCE PCReq message but did not advertise
   its H-PCE capability, it MUST send a PCErr message with Error-Type=28
   (H-PCE Error) and Error-Value=1 (H-PCE Capability not advertised).

3.2.1.1.  Backwards Compatibility

   Section 7.1 of [RFC5440] specifies the following requirement:
   "Unrecognized TLVs MUST be ignored."

   The OPEN object [RFC5440] contains the necessary PCEP information
   between the PCE entities, including session information and PCE
   capabilities via TLVs (including if H-PCE is supported).  If the PCE
   does not support this document but receives an Open message
   containing an OPEN object that includes an H-PCE-CAPABILITY TLV, it
   will ignore that TLV and continue to attempt to establish a PCEP
   session.  However, it will not include the TLV in the Open message
   that it sends, so the H-PCE relationship will not be created.

   If a PCE does not support the extensions defined in this document but
   receives them in a PCEP message (notwithstanding the fact that the
   session was not established as supporting an H-PCE relationship), the
   receiving PCE will ignore the H-PCE related parameters because they
   are all encoded in TLVs in standard PCEP objects.

3.2.2.  Domain-ID TLV

   The Domain-ID TLV, when used in the OPEN object, identifies the
   domains served by the PCE.  The child PCE uses this mechanism to
   provide the domain information to the parent PCE.

   The Domain-ID TLV is defined below:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Type=14         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Domain Type   |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                          Domain ID                          //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Domain-ID TLV Format

   The type of the TLV is 14, and it has a variable Length of the value
   portion.  The value part comprises the following:

      Domain Type (8 bits):  Indicates the domain type.  Four types of
         domains are currently defined:

         Type=1:   The Domain ID field carries a 2-byte AS number.
                   Padded with trailing zeros to a 4-byte boundary.

         Type=2:   The Domain ID field carries a 4-byte AS number.

         Type=3:   The Domain ID field carries a 4-byte OSPF area ID.

         Type=4:   The Domain ID field carries a 2-byte Area-Len and a
                   variable-length IS-IS area ID.  Padded with trailing
                   zeros to a 4-byte boundary.

      Reserved:  Zero at transmission; ignored on receipt.

      Domain ID (variable):  Indicates an IGP area ID or AS number as
         per the Domain Type field.  It can be 2 bytes, 4 bytes, or
         variable length, depending on the domain identifier used.  It
         is padded with trailing zeros to a 4-byte boundary.  In the
         case of IS-IS, it includes the Area-Len as well.

   In the case where a PCE serves more than one domain, multiple Domain-
   ID TLVs are included for each domain it serves.

3.3.  RP Object

3.3.1.  H-PCE-FLAG TLV

   The H-PCE-FLAG TLV is an optional TLV associated with the RP object
   [RFC5440] to indicate the H-PCE PCReq message and options.

   Its format is shown in the following figure:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Type=15         |             Length=4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Flags                             |D|S|
   +---------------------------------------------------------------+

                      Figure 3: H-PCE-FLAG TLV Format

   The type of the TLV is 15, and it has a fixed length of 4 octets.

   The value comprises a single field -- Flags (32 bits):

      D (Disallow Domain Re-entry bit):
         If set, will signal that the computed path does not enter a
         domain more than once.

      S (Domain Sequence bit):
         If set, will signal that the child PCE wishes to get only the
         domain sequence in the Path Computation Reply (PCRep) message
         [RFC5440].  Refer to Section 3.7 of [RFC7897] for details.

   Unassigned bits MUST be set to 0 on transmission and MUST be ignored
   on receipt.

   The presence of the TLV indicates that the H-PCE-based path
   computation is requested as per this document.

3.3.2.  Domain-ID TLV

   The Domain-ID TLV, carried in an OPEN object, is used to indicate a
   managed domain (or a list of managed domains) and is described in
   Section 3.2.2.  This TLV, when carried in an RP object, indicates the
   destination domain ID.  If a PCC knows the egress domain, it can
   supply this information in the PCReq message.  Section 3.2.2 also
   defines the format for this TLV and the procedure for using it.

   If a Domain-ID TLV is used in the RP object and the destination is
   not actually in the indicated domain, then the parent PCE should
   respond with a NO-PATH object and the NO-PATH-VECTOR TLV should be
   used.  A new bit number is assigned to indicate "Destination is not
   found in the indicated domain" (see Section 3.8).

3.4.  Objective Functions

3.4.1.  OF Codes

   [RFC5541] defines a mechanism to specify an OF that is used by a PCE
   when it computes a path.  Three new OFs are defined for the H-PCE
   model; these are:

   *  MTD

      Name:  Minimize the number of Transit Domains (MTD)

      OF code:  12

      Description:  Find a path P such that it passes through the least
         number of transit domains.

      -  OFs are formulated using the following terminology:

         o  A network comprises a set of N domains {Di, (i=1...N)}.

         o  A path P passes through K unique domains {Dpi, (i=1...K)}.

         o  Find a path P such that the value of K is minimized.

   *  MBN

      Name:  Minimize the number of Border Nodes (MBN)

      OF code:  13

      Description:  Find a path P such that it passes through the least
         number of border nodes.

      -  OFs are formulated using the following terminology:

         o  A network comprises a set of N links {Li, (i=1...N)}.

         o  A path P is a list of K links {Lpi, (i=1...K)}.

         o  D(Lpi) is a function that determines if the links Lpi and
            Lpi+1 belong to different domains.  D(Li) = 1 if link Li and
            Li+1 belong to different domains; D(Lk) = 0 if link Lk and
            Lk+1 belong to the same domain.

         o  The number of border nodes in a path P is denoted by B(P),
            where B(P) = sum{D(Lpi), (i=1...K-1)}.

         o  Find a path P such that B(P) is minimized.

   There is one OF that applies to a set of synchronized PCReq messages
   to increase the domain diversity:

   *  MCTD

      Name:  Minimize the number of Common Transit Domains (MCTD)

      OF code:  14

      Description:  Find a set of paths such that it passes through the
         least number of common transit domains.

      -  A network comprises a set of N domains {Di, (i=1...N)}.

      -  A path P passes through K unique domains {Dpi, (i=1...K)}.

      -  A set of paths {P1...Pm} has L transit domains that are common
         to more than one path {Dpi, (i=1...L)}.

      -  Find a set of paths such that the value of L is minimized.

3.4.2.  OF Object

   The OF object [RFC5541] is carried in a PCReq message so as to
   indicate the desired/required OF to be applied by the PCE during path
   computation.  As per Section 3.2 of [RFC5541], a single OF object may
   be included in a PCReq message.

   The new OF codes described in Section 3.4.1 are applicable to the
   inter-domain path computation performed by the parent PCE.  It is
   also necessary to specify the OF code that may be applied for the
   intra-domain path computation performed by the child PCE.  To
   accommodate this, the OF-List TLV (described in Section 2.1 of
   [RFC5541]) is included in the OF object as an optional TLV.

   The OF-List TLV allows the encoding of multiple OF codes.  When this
   TLV is included inside the OF object, only the first OF code in the
   OF-List TLV is considered.  The parent PCE MUST use this OF code in
   the OF object when sending the intra-domain PCReq message to the
   child PCE.  If the OF-List TLV is included in the OF object, the OF
   code inside the OF object MUST include one of the H-PCE OFs defined
   in this document.  The OF code inside the OF-List TLV MUST NOT
   include an H-PCE OF.  If this condition is not met, the PCEP speaker
   MUST respond with a PCErr message with Error-Type=10 (Reception of an
   invalid object) and Error-Value=23 (Incompatible OF codes in H-PCE).

   If the OFs defined in this document are unknown or unsupported by a
   PCE, then the procedure as defined in [RFC5440] is followed.

3.5.  METRIC Object

   The METRIC object is defined in Section 7.8 of [RFC5440] and is
   comprised of the metric-value field, the metric type (the T field),
   and flags (the Flags field).  This document defines the following
   types for the METRIC object for the H-PCE model:

      T=20:  Domain Count metric (number of domains crossed).

      T=21:  Border Node Count metric (number of border nodes crossed).

   The Domain Count metric type of the METRIC object encodes the number
   of domains crossed in the path.  The Border Node Count metric type of
   the METRIC object encodes the number of border nodes in the path.  If
   a domain is re-entered, then the domain should be double counted.

   A PCC or child PCE MAY use the metric in a PCReq message for an
   inter-domain path computation, meeting the requirement for the number
   of domains or border nodes being crossed.  As per [RFC5440], in this
   case, the B-bit is set to suggest a bound (a maximum) for the metric
   that must not be exceeded for the PCC to consider the computed path
   acceptable.

   A PCC or child PCE MAY also use this metric to ask the PCE to
   optimize the metric during inter-domain path computation.  In this
   case, the B-flag is cleared, and the C-flag is set.

   The parent PCE MAY use the metric in a PCRep message along with a NO-
   PATH object in the case where the PCE cannot compute a path that
   meets this constraint.  A PCE MAY also use this metric to send the
   computed end-to-end metric value in a reply message.

3.6.  SVEC Object

   [RFC5440] defines the Synchronization Vector (SVEC) object, which
   includes flags for the potential dependency between the set of PCReq
   messages (Link, Node, and SRLG diverse).  This document defines a new
   flag (the O-bit) for domain diversity.

   The following new bit is added to the Flags field:

      Domain Diverse O-bit - 18:
         When set, this indicates that the computed paths corresponding
         to the requests specified by any RP objects that might be
         provided MUST NOT have any transit domains in common.

   The Domain Diverse O-bit can be used in H-PCE path computation to
   compute synchronized domain-diverse end-to-end paths or diverse
   domain sequences.

   When the Domain Diverse O-bit is set, it is applied to the transit
   domains.  The other bit in SVEC object L (Link diverse), N (Node
   diverse), S (SRLG diverse), etc. MAY be set and MUST still be applied
   in the ingress and egress shared domain.

3.7.  PCEP-ERROR Object

3.7.1.  Hierarchical PCE Error-Type

   A new PCEP Error-Type [RFC5440] is used for the H-PCE extension as
   defined below:

   +------------+------------------------------------------------------+
   | Error-Type | Meaning                                              |
   +============+======================================================+
   | 28         | H-PCE Error                                          |
   |            |                                                      |
   |            |       Error-Value=1: H-PCE Capability not            |
   |            |       advertised                                     |
   |            |                                                      |
   |            |       Error-Value=2: Parent PCE Capability cannot    |
   |            |       be provided                                    |
   +------------+------------------------------------------------------+

                            Table 1: H-PCE Error

3.8.  NO-PATH Object

   To communicate the reason(s) for not being able to find a multi-
   domain path or domain sequence, the NO-PATH object can be used in the
   PCRep message.  [RFC5440] defines the format of the NO-PATH object.
   The object may contain a NO-PATH-VECTOR TLV to provide additional
   information about why a path computation has failed.

   This document defines four new bit flags in the "NO-PATH-VECTOR TLV
   Flag Field" subregistry.  These flags are to be carried in the Flags
   field in the NO-PATH-VECTOR TLV carried in the NO-PATH object.

      Bit number 22:  When set, the parent PCE indicates that the
                      destination domain is unknown.

      Bit number 21:  When set, the parent PCE indicates that one or
                      more child PCEs are unresponsive.

      Bit number 20:  When set, the parent PCE indicates that no
                      resources are available in one or more domains.

      Bit number 19:  When set, the parent PCE indicates that the
                      destination is not found in the indicated domain.

4.  H-PCE Procedures

   The H-PCE path computation procedure is described in [RFC6805].

4.1.  OPEN Procedure between Child PCE and Parent PCE

   If a child PCE wants to use the peer PCE as a parent, it MUST set the
   P-flag (Parent PCE Request flag) in the H-PCE-CAPABILITY TLV inside
   the OPEN object carried in the Open message during the PCEP session
   initialization procedure.

   The child PCE MAY also report its list of domain IDs to the parent
   PCE by specifying them in the Domain-ID TLVs in the OPEN object.
   This object is carried in the Open message during the PCEP session
   initialization procedure.

   The OF codes defined in this document can be carried in the OF-List
   TLV of the OPEN object.  If the OF-List TLV carries the OF codes, it
   means that the PCE is capable of implementing the corresponding OFs.
   This information can be used for selecting a proper parent PCE when a
   child PCE wants to get a path that satisfies a certain OF.

   When a child PCE sends a PCReq to a peer PCE that requires parental
   activity and the H-PCE-CAPABILITY TLV but these items were not taken
   into account in the session establishment procedure described above,
   the peer PCE SHOULD send a PCErr message to the child PCE and MUST
   specify Error-Type=28 (H-PCE Error) and Error-Value=1 (H-PCE
   Capability not advertised) in the PCEP-ERROR object.

   When a specific child PCE sends a PCReq to a peer PCE that requires
   parental activity and the peer PCE does not want to act as the parent
   for it, the peer PCE SHOULD send a PCErr message to the child PCE and
   MUST specify Error-Type=28 (H-PCE Error) and Error-Value=2 (Parent
   PCE Capability cannot be provided) in the PCEP-ERROR object.

4.2.  Procedure for Obtaining the Domain Sequence

   If a child PCE only wants to get the domain sequence for a multi-
   domain path computation from a parent PCE, it can set the Domain Path
   Request bit in the H-PCE-FLAG TLV in the RP object carried in a PCReq
   message.  The parent PCE that receives the PCReq message tries to
   compute a domain sequence for it (instead of the end-to-end path).
   If the domain path computation succeeds, the parent PCE sends a PCRep
   message that carries the domain sequence in the Explicit Route Object
   (ERO) to the child PCE.  Refer to [RFC7897] for more details about
   domain subobjects in the ERO.  Otherwise, it sends a PCReq message
   that carries the NO-PATH object to the child PCE.

5.  Error Handling

   A PCE that is capable of acting as a parent PCE might not be
   configured or willing to act as the parent for a specific child PCE.
   When the child PCE sends a PCReq that requires parental activity, a
   negative response in the form of a PCEP Error (PCErr) message that
   includes H-PCE Error-Type=28 (H-PCE Error) and an applicable Error-
   Value (Section 3.7) might result.

   Additionally, the parent PCE may fail to find the multi-domain path
   or domain sequence for one or more of the following reasons:

   *  A child PCE cannot find a suitable path to the egress.

   *  The parent PCE does not hear from a child PCE for a specified
      time.

   *  The OFs specified in the path request cannot be met.

   In this case, the parent PCE MAY need to send a negative PCRep
   message specifying the reason for the failure.  This can be achieved
   by including the NO-PATH object in the PCRep message.  An extension
   to the NO-PATH object is needed in order to include the reasons
   defined in Section 3.8.

6.  Manageability Considerations

   General PCE and PCEP management/manageability considerations are
   discussed in [RFC4655] and [RFC5440].  There are additional
   management considerations for the H-PCE model; these are described in
   [RFC6805] and repeated in this section.

   The administrative entity responsible for the management of the
   parent PCEs must be determined for the following cases:

   *  Multiple domains (e.g., IGP areas or multiple ASes) in a single
      service provider network.  The management responsibility for the
      parent PCE would most likely be handled by the service provider.

   *  Multiple ASes in different service provider networks.  It may be
      necessary for a third party to manage the parent PCEs according to
      commercial and policy agreements from each of the participating
      service providers.

6.1.  Control of Function and Policy

   Control of H-PCE function will need to be carefully managed via
   configuration and interaction policies, which may be controlled via a
   policy module on the H-PCE.  A child PCE will need to be configured
   with the address of its parent PCE.  It is expected that there will
   only be one or two parents of any child.

   The parent PCE also needs to be aware of the child PCEs for all child
   domains that it can see.  This information is most likely to be
   configured (as part of the administrative definition of each domain).

   Discovery of the relationships between parent PCEs and child PCEs
   does not form part of the H-PCE architecture.  Mechanisms that rely
   on advertising or querying PCE locations across domain or provider
   boundaries are undesirable for security, scaling, commercial, and
   confidentiality reasons.  The specific behavior of the child and
   parent PCEs is described in the following subsections.

6.1.1.  Child PCE

   Support of the hierarchical procedure will be controlled by the
   management organization responsible for each child PCE.  A child PCE
   must be configured with the address of its parent PCE in order for it
   to interact with its parent PCE.  The child PCE must also be
   authorized to peer with the parent PCE.

6.1.2.  Parent PCE

   The parent PCE MUST only accept PCReq messages from authorized child
   PCEs.  If a parent PCE receives requests from an unauthorized child
   PCE, the request SHOULD be dropped.  This means that a parent PCE
   MUST be able to cryptographically authenticate requests from child
   PCEs.

   Multi-party shared key authentication schemes are not recommended for
   inter-domain relationships because of (1) the potential for
   impersonation and repudiation and (2) operational difficulties should
   revocation be required.

   The choice of authentication schemes to employ may be left to
   implementers of the H-PCE architecture and are not discussed further
   in this document.

6.1.3.  Policy Control

   It may be necessary to maintain H-PCE policy [RFC5394] via a policy
   control module on the parent PCE.  This would allow the parent PCE to
   apply commercially relevant constraints such as SLAs, security,
   peering preferences, and monetary costs.

   It may also be necessary for the parent PCE to limit the end-to-end
   path selection by including or excluding specific domains based on
   commercial relationships, security implications, and reliability.

6.2.  Information and Data Models

   [RFC7420] provides a MIB module for PCEP and describes managed
   objects for the modeling of PCEP communication.  A YANG module for
   PCEP has also been proposed [PCEP-YANG].

   An H-PCE MIB module or an additional data model will also be required
   for reporting parent PCE and child PCE information, including:

   *  parent PCE configuration and status,

   *  child PCE configuration and information,

   *  notifications to indicate session changes between parent PCEs and
      child PCEs, and

   *  notification of parent PCE TED updates and changes.

6.3.  Liveness Detection and Monitoring

   The hierarchical procedure requires interaction with multiple PCEs.
   Once a child PCE requests an end-to-end path, a sequence of events
   occurs that requires interaction between the parent PCE and each
   child PCE.  If a child PCE is not operational and an alternate
   transit domain is not available, then the failure must be reported.

6.4.  Verifying Correct Operations

   Verifying the correct operation of a parent PCE can be performed by
   monitoring a set of parameters.  The parent PCE implementation should
   provide the following parameters monitored at the parent PCE:

   *  number of child PCE requests,

   *  number of successful H-PCE procedure completions on a per-PCE-peer
      basis,

   *  number of H-PCE procedure-completion failures on a per-PCE-peer
      basis, and

   *  number of H-PCE procedure requests from unauthorized child PCEs.

6.5.  Requirements on Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.

6.6.  Impact on Network Operations

   The H-PCE procedure is a multiple-PCE path computation scheme.
   Subsequent requests to and from the child and parent PCEs do not
   differ from other path computation requests and should not have any
   significant impact on network operations.

7.  IANA Considerations

   IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
   registry.  IANA has allocated code points for the protocol elements
   defined in this document.

7.1.  PCEP TLV Type Indicators

   IANA maintains the "PCEP TLV Type Indicators" subregistry (see
   [RFC5440]) within the "Path Computation Element Protocol (PCEP)
   Numbers" registry.

   IANA has allocated the following three new PCEP TLVs:

                  +------+------------------+-----------+
                  | Type | TLV Name         | Reference |
                  +======+==================+===========+
                  | 13   | H-PCE-CAPABILITY | RFC 8685  |
                  +------+------------------+-----------+
                  | 14   | Domain-ID        | RFC 8685  |
                  +------+------------------+-----------+
                  | 15   | H-PCE-FLAG       | RFC 8685  |
                  +------+------------------+-----------+

                           Table 2: New PCEP TLVs

7.2.  H-PCE-CAPABILITY TLV Flags

   IANA has created the "H-PCE-CAPABILITY TLV Flag Field" subregistry
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry to manage the Flag field in the H-PCE-CAPABILITY TLV of the
   PCEP OPEN object.

   New values are assigned by Standards Action [RFC8126].  Each
   registered bit should include the following information:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following value is defined in this document:

             +-----+----------------------------+-----------+
             | Bit | Description                | Reference |
             +=====+============================+===========+
             | 31  | P (Parent PCE Request bit) | RFC 8685  |
             +-----+----------------------------+-----------+

                     Table 3: Parent PCE Request Bit

7.3.  Domain-ID TLV Domain Type

   IANA has created the "Domain-ID TLV Domain Type" subregistry within
   the "Path Computation Element Protocol (PCEP) Numbers" registry to
   manage the Domain Type field of the Domain-ID TLV.  The allocation
   policy for this new subregistry is IETF Review [RFC8126].

   The following values are defined in this document:

                 +-------+-------------------------------+
                 | Value | Meaning                       |
                 +=======+===============================+
                 | 0     | Reserved                      |
                 +-------+-------------------------------+
                 | 1     | 2-byte AS number              |
                 +-------+-------------------------------+
                 | 2     | 4-byte AS number              |
                 +-------+-------------------------------+
                 | 3     | 4-byte OSPF area ID           |
                 +-------+-------------------------------+
                 | 4     | Variable-length IS-IS area ID |
                 +-------+-------------------------------+
                 | 5-255 | Unassigned                    |
                 +-------+-------------------------------+

                   Table 4: Parameters for Domain-ID TLV
                                Domain Type

7.4.  H-PCE-FLAG TLV Flags

   IANA has created the "H-PCE-FLAG TLV Flag Field" subregistry within
   the "Path Computation Element Protocol (PCEP) Numbers" registry to
   manage the Flag field in the H-PCE-FLAG TLV of the PCEP RP object.
   New values are to be assigned by Standards Action [RFC8126].  Each
   registered bit should include the following information:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following values are defined in this document:

          +-----+----------------------------------+-----------+
          | Bit | Description                      | Reference |
          +=====+==================================+===========+
          | 30  | D (Disallow Domain Re-entry bit) | RFC 8685  |
          +-----+----------------------------------+-----------+
          | 31  | S (Domain Sequence bit)          | RFC 8685  |
          +-----+----------------------------------+-----------+

              Table 5: New H-PCE-FLAG TLV Flag Field Entries

7.5.  OF Codes

   IANA maintains a list of OFs (described in [RFC5541]) in the
   "Objective Function" subregistry within the "Path Computation Element
   Protocol (PCEP) Numbers" registry.

   IANA has allocated the following OFs:

        +------------+-------------------------------+-----------+
        | Code Point | Name                          | Reference |
        +============+===============================+===========+
        | 12         | Minimize the number of        | RFC 8685  |
        |            | Transit Domains (MTD)         |           |
        +------------+-------------------------------+-----------+
        | 13         | Minimize the number of Border | RFC 8685  |
        |            | Nodes (MBN)                   |           |
        +------------+-------------------------------+-----------+
        | 14         | Minimize the number of Common | RFC 8685  |
        |            | Transit Domains (MCTD)        |           |
        +------------+-------------------------------+-----------+

                          Table 6: New OF Codes

7.6.  METRIC Object Types

   IANA maintains the "METRIC Object T Field" subregistry [RFC5440]
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry.

   The following two new metric types for the METRIC object are defined
   in this document:

             +-------+--------------------------+-----------+
             | Value | Description              | Reference |
             +=======+==========================+===========+
             | 20    | Domain Count metric      | RFC 8685  |
             +-------+--------------------------+-----------+
             | 21    | Border Node Count metric | RFC 8685  |
             +-------+--------------------------+-----------+

                     Table 7: New METRIC Object Types

7.7.  New PCEP Error-Types and Values

   IANA maintains a list of Error-Types and Error-Values for use in PCEP
   messages.  This list is maintained in the "PCEP-ERROR Object Error
   Types and Values" subregistry within the "Path Computation Element
   Protocol (PCEP) Numbers" registry.

   IANA has allocated the following:

   +------------+------------------------------------------+-----------+
   | Error-Type | Meaning and Error Values                 | Reference |
   +============+==========================================+===========+
   | 28         | H-PCE Error                              | RFC 8685  |
   |            |                                          |           |
   |            |       Error-Value=1: H-PCE Capability    |           |
   |            |       not advertised                     |           |
   |            |                                          |           |
   |            |       Error-Value=2: Parent PCE          |           |
   |            |       Capability cannot be provided      |           |
   +------------+------------------------------------------+-----------+
   | 10         | Reception of an invalid object           | RFC 5440  |
   |            |                                          |           |
   |            |       Error-Value=23: Incompatible OF    | RFC 8685  |
   |            |       codes in H-PCE                     |           |
   +------------+------------------------------------------+-----------+

                  Table 8: New PCEP Error-Types and Values

7.8.  New NO-PATH-VECTOR TLV Bit Flag

   IANA maintains the "NO-PATH-VECTOR TLV Flag Field" subregistry, which
   contains a list of bit flags carried in the PCEP NO-PATH-VECTOR TLV
   in the PCEP NO-PATH object as defined in [RFC5440].

   IANA has allocated the following four new bit flags:

          +------------+----------------------------+-----------+
          | Bit Number | Description                | Reference |
          +============+============================+===========+
          | 22         | Destination domain unknown | RFC 8685  |
          +------------+----------------------------+-----------+
          | 21         | Unresponsive child PCE(s)  | RFC 8685  |
          +------------+----------------------------+-----------+
          | 20         | No available resource in   | RFC 8685  |
          |            | one or more domains        |           |
          +------------+----------------------------+-----------+
          | 19         | Destination is not found   | RFC 8685  |
          |            | in the indicated domain    |           |
          +------------+----------------------------+-----------+

                     Table 9: PCEP NO-PATH Object Flags

7.9.  SVEC Flag

   IANA maintains the "SVEC Object Flag Field" subregistry, which
   contains a list of bit flags carried in the PCEP SVEC object as
   defined in [RFC5440].

   IANA has allocated the following new bit flag:

             +------------+----------------------+-----------+
             | Bit Number | Description          | Reference |
             +============+======================+===========+
             | 18         | Domain Diverse O-bit | RFC 8685  |
             +------------+----------------------+-----------+

                       Table 10: Domain Diverse O-Bit

8.  Security Considerations

   The H-PCE procedure relies on PCEP and inherits the security
   considerations defined in [RFC5440].  As PCEP operates over TCP, it
   may also make use of TCP security mechanisms, such as the TCP
   Authentication Option (TCP-AO) [RFC5925] or Transport Layer Security
   (TLS) [RFC8253] [RFC8446].

   Any multi-domain operation necessarily involves the exchange of
   information across domain boundaries.  This may represent a
   significant security and confidentiality risk, especially when the
   child domains are controlled by different commercial concerns.  PCEP
   allows individual PCEs to maintain the confidentiality of their
   domain path information using path-keys [RFC5520], and the H-PCE
   architecture is specifically designed to enable as much isolation of
   information related to domain topology and capabilities as possible.

   For further considerations regarding the security issues related to
   inter-AS path computation, see [RFC5376].

9.  References

9.1.  Normative References

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

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

   [RFC5541]  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
              Objective Functions in the Path Computation Element
              Communication Protocol (PCEP)", RFC 5541,
              DOI 10.17487/RFC5541, June 2009,
              <https://www.rfc-editor.org/info/rfc5541>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [RFC4105]  Le Roux, J.-L., Ed., Vasseur, J.-P., Ed., and J. Boyle,
              Ed., "Requirements for Inter-Area MPLS Traffic
              Engineering", RFC 4105, DOI 10.17487/RFC4105, June 2005,
              <https://www.rfc-editor.org/info/rfc4105>.

   [RFC4216]  Zhang, R., Ed. and J.-P. Vasseur, Ed., "MPLS Inter-
              Autonomous System (AS) Traffic Engineering (TE)
              Requirements", RFC 4216, DOI 10.17487/RFC4216, November
              2005, <https://www.rfc-editor.org/info/rfc4216>.

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

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

   [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,
              <https://www.rfc-editor.org/info/rfc5152>.

   [RFC5376]  Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
              Requirements for the Path Computation Element
              Communication Protocol (PCECP)", RFC 5376,
              DOI 10.17487/RFC5376, November 2008,
              <https://www.rfc-editor.org/info/rfc5376>.

   [RFC5394]  Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
              "Policy-Enabled Path Computation Framework", RFC 5394,
              DOI 10.17487/RFC5394, December 2008,
              <https://www.rfc-editor.org/info/rfc5394>.

   [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,
              <https://www.rfc-editor.org/info/rfc5520>.

   [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,
              <https://www.rfc-editor.org/info/rfc5441>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc6805>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [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,
              <https://www.rfc-editor.org/info/rfc7420>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7897]  Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects
              for the Path Computation Element Communication Protocol
              (PCEP)", RFC 7897, DOI 10.17487/RFC7897, June 2016,
              <https://www.rfc-editor.org/info/rfc7897>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [PCEP-YANG]
              Dhody, D., Ed., Hardwick, J., Beeram, V., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
              2019,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.

   [STATEFUL-HPCE]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
              "Hierarchical Stateful Path Computation Element (PCE)",
              Work in Progress, Internet-Draft, draft-ietf-pce-stateful-
              hpce-15, 20 October 2019, <https://tools.ietf.org/html/
              draft-ietf-pce-stateful-hpce-15>.

   [PCEP-LS]  Dhody, D., Lee, Y., and D. Ceccarelli, "PCEP Extension for
              Distribution of Link-State and TE Information.", Work in
              Progress, Internet-Draft, draft-dhodylee-pce-pcep-ls-14,
              21 October 2019, <https://tools.ietf.org/html/draft-
              dhodylee-pce-pcep-ls-14>.

Acknowledgements

   The authors would like to thank Mike McBride, Kyle Rose, and Roni
   Even for their detailed review, comments, and suggestions, which
   helped improve this document.

Contributors

   The following people contributed substantially to the content of this
   document and should be considered coauthors:

   Xian Zhang
   Huawei

   Email: zhang.xian@huawei.com


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

   Email: dhruv.ietf@gmail.com


Authors' Addresses

   Fatai Zhang
   Huawei
   Huawei Base, Bantian, Longgang District
   Shenzhen, 518129
   China

   Email: zhangfatai@huawei.com


   Quintin Zhao
   Huawei
   125 Nagog Technology Park
   Acton, MA 01719
   United States of America

   Email: quintinzhao@gmail.com


   Oscar Gonzalez de Dios
   Telefonica I+D
   Don Ramon de la Cruz 82-84
   28045 Madrid
   Spain

   Email: oscar.gonzalezdedios@telefonica.com


   Ramon Casellas
   CTTC
   Av. Carl Friedrich Gauss n.7
   Castelldefels Barcelona
   Spain

   Email: ramon.casellas@cttc.es


   Daniel King
   Old Dog Consulting
   United Kingdom

   Email: daniel@olddog.co.uk


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