PCE Working Group

Internet Engineering Task Force (IETF)                          F. Zhang
Internet-Draft
Request for Comments: 8685                                       Q. Zhao
Intended status:
Category: Standards Track                                         Huawei
Expires: December 2, 2019
ISSN: 2070-1721                                      O. Gonzalez de Dios
                                                          Telefonica I+D
                                                             R. Casellas
                                                                    CTTC
                                                                 D. King
                                                      Old Dog Consulting
                                                            June 1,
                                                           December 2019

Extensions to

   Path Computation Element Communication Protocol (PCEP) Extensions
   for the Hierarchical Path Computation Elements (PCE)
                 draft-ietf-pce-hierarchy-extensions-11 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 Hierarchical PCE H-PCE procedures.

Status of This Memo

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

   Internet-Drafts are working documents an Internet Standards Track document.

   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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum 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 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 December 2, 2019.
   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  . . . . . . . . . . . . . . . . . . . . . . . .3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .5
     1.3.  Requirements Language . . . . . . . . . . . . . . . . . .5
   2.  Requirements for the H-PCE  . . . . . . . . . . . . . . . . . . .5 Architecture
     2.1.  Path Computation Request  . . . . . . . . . . . . . . . .6 Requests
       2.1.1.  Qualification of PCEP Requests  . . . . . . . . . . .6
       2.1.2.  Multi-domain Objective Functions  . . . . . . . . . .6
       2.1.3.  Multi-domain Metrics  . . . . . . . . . . . . . . . .7
     2.2.  Parent PCE Capability Advertisement . . . . . . . . . . .7
     2.3.  PCE Domain Identification . . . . . . . . . . . . . . . .7
     2.4.  Domain Diversity  . . . . . . . . . . . . . . . . . . . .7
   3.  PCEP Extensions . . . . . . . . . . . . . . . . . . . . . . .8
     3.1
     3.1.  Applicability to PCC-PCE Communications . . . . . . . . .8
     3.2.  OPEN Object . . . . . . . . . . . . . . . . . . . . . . .8
       3.2.1.  H-PCE Capability  H-PCE-CAPABILITY TLV  . . . . . . . . . . . . . . . .8
       3.2.1.1
         3.2.1.1.  Backwards Compatibility . . . . . . . . . . . . . . .9
       3.2.2.  Domain-ID TLV . . . . . . . . . . . . . . . . . . . .10
     3.3.  RP Object . . . . . . . . . . . . . . . . . . . . . . . .11
       3.3.1.  H-PCE-FLAG TLV  . . . . . . . . . . . . . . . . . . .11
       3.3.2.  Domain-ID TLV . . . . . . . . . . . . . . . . . . . .12
     3.4.  Objective Functions . . . . . . . . . . . . . . . . . . .12
       3.4.1.  OF Codes  . . . . . . . . . . . . . . . . . . . . . .12
       3.4.2.  OF Object . . . . . . . . . . . . . . . . . . . . . .13
     3.5.  Metric  METRIC Object . . . . . . . . . . . . . . . . . . . . . .14
     3.6.  SVEC Object . . . . . . . . . . . . . . . . . . . . . . .15
     3.7.  PCEP-ERROR Object . . . . . . . . . . . . . . . . . . . .15
       3.7.1.  Hierarchy  Hierarchical PCE Error-Type  . . . . . . . . . . . . . .15
     3.8.  NO-PATH Object  . . . . . . . . . . . . . . . . . . . . .16
   4.  H-PCE Procedures  . . . . . . . . . . . . . . . . . . . . . .16
     4.1.  OPEN Procedure between Child PCE and Parent PCE . . . . .16
     4.2.  Procedure to Obtain for Obtaining the Domain Sequence . . . . . . . . . . .17
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .17
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .18
     6.1.  Control of Function and Policy  . . . . . . . . . . . . .18
       6.1.1.  Child PCE . . . . . . . . . . . . . . . . . . . . . .18
       6.1.2.  Parent PCE  . . . . . . . . . . . . . . . . . . . . .
       6.1.3.  Policy Control  . . . . . . . . . . . . . . . . . . .19
     6.2.  Information and Data Models . . . . . . . . . . . . . . .19
     6.3.  Liveness Detection and Monitoring . . . . . . . . . . . .20
     6.4.  Verify  Verifying Correct Operations . . . . . . . . . . . . . . . .20
     6.5.  Requirements On on Other Protocols . . . . . . . . . . . . .20
     6.6.  Impact On on Network Operations  . . . . . . . . . . . . . .20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .20
     7.1.  PCEP TLV Type Indicators  . . . . . . . . . . . . . . . .20
     7.2.  H-PCE-CAPABILITY TLV Flags  . . . . . . . . . . . . . . .21
     7.3.  Domain-ID TLV Domain type . . . . . . . . . . . . . . . .21 Type
     7.4.  H-PCE-FLAG TLV Flags  . . . . . . . . . . . . . . . . . .22
     7.5.  OF Codes  . . . . . . . . . . . . . . . . . . . . . . . .22
     7.6.  METRIC Object Types  . . . . . . . . . . . . . . . . . . . . . .22
     7.7.  New PCEP Error-Types and Values . . . . . . . . . . . . .23
     7.8.  New NO-PATH-VECTOR TLV Bit Flag . . . . . . . . . . . . .23
     7.9.  SVEC Flag . . . . . . . . . . . . . . . . . . . . . . . .24
     7.10.  NO-PATH VECTOR TLV Bit Flag. . . . . . . . . . . . . . .24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .24
   9. Contributing Authors . . . . . . . . . . . . . . . . . . . . .24
   10.Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .25
   11.  References  . . . . . . . . . . . . . . . . . . . . . . . . .25
     11.1.
     9.1.  Normative References . . . . . . . . . . . . . . . . . .25
     11.2.
     9.2.  Informative References . . . . . . . . . . . . . . . . .25
   Acknowledgements
   Contributors
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .28
   Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
    A1.  Implementation Status . . . . . . . . . . . . . . . . . . .28
     A1.1.  Inter-layer traffic engineering with H-PCE . . . . . . .29
     A1.2.  Telefonica Netphony (Open Source PCE)  . . . . . . . . .30
     A1.3.  H-PCE Proof of Concept developed by Huawei . . . . . . .31

1.  Introduction

   The Path Computation Element communication 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 PCE Path Computation Element (H-PCE)
   architecture which that can be used for computing end-to-end paths for
   inter-domain MPLS Traffic
   Engineering (TE) MPLS-TE and GMPLS Label Switched Paths (LSPs).

   Within LSPs.

   In the hierarchical PCE H-PCE architecture, the parent PCE is used to compute a multi-domain 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:

   o

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

   o domain.

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

   o PCE.

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

   o domain.

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

   o path.

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

   o PCE.

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

   o paths.

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

   o 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 per-domain (PD) path computation
   [RFC5152] and
   Backwards Recursive Path Backward-Recursive PCE-Based Computation (BRPC)
   [RFC5441], may be used.

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

1.1.  Scope

   The following functions are out of scope of for this document:

   o

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

      *

      -  via a collection of reachability information from child domain;

      *
         domains,

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

      * node, or

      -  via any other methods.

   o

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

      *

      -  YANG-based management interfaces;

      *  BGP-LS [RFC7752];

      * interfaces.

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

      -  Future extension extensions to PCEP (such as [I-D.dhodylee-pce-pcep-ls]).

   o (for example, see [PCEP-LS]).

   *  Learning of Domain domain connectivity and boundary nodes (BN) addresses,
      methods border node addresses.
      Methods to achieve this function include:

      *

      -  YANG-based management interfaces;

      * interfaces.

      -  BGP-LS [RFC7752];

      * [RFC7752].

      -  Future extension extensions to PCEP (such as [I-D.dhodylee-pce-pcep-ls]).

   o (for example, see [PCEP-LS]).

   *  Stateful PCE Operations operations.  (Refer [I-D.ietf-pce-stateful-hpce])

   o to [STATEFUL-HPCE].)

   *  Applicability of hierarchical PCE 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 LSRs Label Switching Routers
         (LSRs) is limited.  As highlighted in [RFC7399] [RFC7399], applying the hierarchical PCE
         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], [RFC5440] [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 to for the PCEP extensions
   to support the H-PCE architecture and procedures.  [RFC6805]
   identifies high-level requirements of for PCEP extensions that are
   required to support for supporting the hierarchical PCE H-PCE model.

2.1.  Path Computation Request Requests

   The Path Computation Request (PCReq) [RFC5440] 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:

   o

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

   o [RFC6805]).

   *  Multi-domain Objective Functions (OF);

   o (OFs).

   *  Multi-domain Metrics. 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:

   o are as follows:

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

   o

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

   o

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

2.1.2.  Multi-domain Objective Functions

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

   o

   *  Minimize the number of Transit Domains (MTD)
   o

   *  Minimize the number of border nodes Border Nodes (MBN)
   o

   *  Minimize the number of Common Transit Domains (MCTD)

   The PCC may specify the multi-domain Objective Function OF code to use when requesting
   inter-domain path computation, it computation.  It may also include intra-domain OFs,
   such as Minimum Cost Path (MCP) [RFC5441], [RFC5541], which must be considered
   by participating child PCEs.

2.1.3.  Multi-domain Metrics

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

   o

   *  Domain count Count (number of domains crossed);

   o crossed).

   *  Border Node count. Count.

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

2.2.  Parent PCE Capability Advertisement

   A PCEP Speaker (Parent speaker (parent PCE or Child child PCE) that supports and wishes to
   use the procedures described in this document must advertise
   the 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 object [RFC5440] to advertise its support for PCEP extensions
   for the H-PCE
   Capability. 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 PCE, although it
   is possible for a PCE to manage multiple domains simultaneously.  The
   PCE domain could be an IGP area or AS. Autonomous System (AS).

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

2.4.  Domain Diversity

   In

   "Domain diversity" in the context of a multi-domain environment, Domain Diversity environment is
   defined in [RFC6805] and described as "A 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." domains.

   The main motivation behind domain diversity is to avoid fate sharing,
   but it can fate-sharing.
   However, domain diversity may also be because of some geo-political reasons requested to avoid specific
   transit domains due to security, geopolitical, and commercial relationships that would require domain diversity.
   reasons.  For example, a pair of paths should choose different
   transit Autonomous
   System (AS) ASes because of some 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 scope of diversity domain-diversity techniques (node,
   link, SRLG 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

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 RP Request Parameters
   (RP) object
   (Section 3.3) ([RFC5440] and Section 3.3), the Objective Function (OF) OF codes
   (Section 3.4.1), and objects the OF object (Section 3.4). 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

   Two

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

3.2.1.  H-PCE Capability  H-PCE-CAPABILITY TLV

   The H-PCE-CAPABILITY TLV is an optional TLV associated with the OPEN
   Object
   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= TBD1               Type=13         |            Length=4           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Flags                               |P|
   +---------------------------------------------------------------+

                   Figure 1: H-PCE-CAPABILITY TLV format Format

   The type of the TLV is TBD1 (to be assigned by IANA), 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
         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. P-flag.  The parent PCE MUST include
   this TLV and unset the P flag. P-flag.

   The setting of the P flag (parent P-flag (Parent PCE request 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 PCC-to-child-PCE relationship, neither entity would set the P
   flag. P-flag.

   If both peers attempt to set the P flag 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" 1 (PCEP session establishment failure)
   as per [RFC5440].

   If the PCE understands the H-PCE path computation request PCReq message but did not advertise
   its H-PCE capability, it MUST send a PCErr message with
   Error-Type=TBD8 ("H-PCE error") Error-Type=28
   (H-PCE Error) and Error-Value=1 ("H-PCE (H-PCE Capability not advertised").

3.2.1.1 advertised).

3.2.1.1.  Backwards Compatibility

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

   That means that a 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 that
   does not support this document but that receives an Open Message message
   containing an Open Object OPEN object that includes an H-PCE-CAPABILITIES TLV H-PCE-CAPABILITY TLV, it
   will ignore that TLV and will continue to attempt to establish a PCEP
   session. It will, however,  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 a an H-PCE relationship), the
   receiving PCE will ignore the H-PCE related parameters because they
   are all encoded in TLVs within 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
   inform
   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= TBD2               Type=14         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Domain Type   |                  Reserved                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                          Domain ID                          //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Domain-ID TLV format Format

   The type of the TLV is TBD2 (to be assigned by IANA), 14, and it has a variable Length of the value
   portion.  The value part comprises: comprises the following:

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

      *

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

      *

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

      *

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

      *

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

      Reserved:  Zero at transmission; ignored at the on receipt.

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

   In the case where a PCE serves more than one domain, multiple Domain-ID 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 object
   [RFC5440] to indicate the H-PCE path computation request 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= TBD3               Type=15         |             Length=4          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Flags                             |D|S|
   +---------------------------------------------------------------+

                      Figure 3: H-PCE-FLAG TLV format Format

   The type of the TLV is TBD3 (to be assigned by IANA), 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
         If set, will signal that the child PCE wishes to get only the
         domain sequence in the path computation
      reply. Path Computation Reply (PCRep) message
         [RFC5440].  Refer to Section 3.7 of [RFC7897] for details.

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

   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 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
   (list of)
   managed domains domain (or a list of managed domains) and is described in
   Section 3.3.1. 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.  The  Section 3.2.2 also
   defines the format and procedure of for this TLV are
   defined in Section 3.2.2. and the procedure for using it.

   If a Domain-id Domain-ID TLV is used in the RP object, object and the destination is
   not actually in the indicated domain, then the parent PCE should
   respond with a NO-PATH object and NO-PATH VECTOR 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 3.7). 3.8).

3.4.  Objective Functions

3.4.1.  OF Codes

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

   o  MTD

   *  MTD

      Name:  Minimize the number of Transit Domains (MTD)

      *  Objective Function Code - TBD4 (to be assigned by IANA)

      *

      OF code:  12

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

      *  Objective functions

      -  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)}.

         + {Dpi, (i=1...K)}.

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

   o  MBN

   *  MBN

      Name:  Minimize the number of border nodes.

      *  Objective Function Code - TBD5 (to be assigned by IANA)
      * Border Nodes (MBN)

      OF code:  13

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

      *  Objective functions

      -  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)}.

         + {Lpi, (i=1...K)}.

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

         +

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

         + sum{D(Lpi), (i=1...K-1)}.

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

   There is one objective function OF that applies to a set of synchronized path computation requests PCReq messages
   to increase the domain diversity:

   o  MCTD

   *  MCTD

      Name:  Minimize the number of Common Transit Domains

      *  Objective Function Code - TBD13 (to be assigned by IANA)

      * (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)}.

         + {Dpi, (i=1...K)}.

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

         + {Dpi, (i=1...L)}.

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

3.4.2.  OF Object

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

   The new OF codes described in Section 3.4.1 are applicable at to the
   inter-domain path computation performed by the parent PCE, it 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. 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 OF code in the
   OF-LIST
   OF-List TLV is considered.  The parent PCE MUST use this OF code in
   the OF object when sending the intra domain path computation request intra-domain PCReq message to the
   child PCE.  If the OF list OF-List TLV is included in the OF Object, object, the OF Code
   code inside the OF Object object MUST include one of the H-PCE
   Objective Functions OFs defined
   in this document, the document.  The OF Code code inside the
   OF List OF-List TLV MUST NOT
   include an H-PCE Objective Function. 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=TBD15 Error-Value=23 (Incompatible OF codes in H-PCE).

   If the Objective Functions OFs defined in this document are unknown or unsupported by a
   PCE, then the procedure as defined in [RFC5541] [RFC5440] is followed.

3.5.  Metric  METRIC Object

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

   o  T=TBD6: the H-PCE model:

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

   o  T=TBD7: crossed).

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

   The domain count Domain Count metric type of the METRIC object encodes the number
   of domains crossed in the path.  The border node count 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 domain domains or border nodes crossing requirement. being crossed.  As per [RFC5440], in this
   case, the B bit 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 as
   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 B-flag is cleared, and the C flag C-flag is set.

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

3.6.  SVEC Object

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

   The following new bit is added to the Flags field:

   o

      Domain Diverse O-bit - TBD14 : when 18:
         When set, this indicates that the computed paths corresponding
         to the requests specified by the
      following any RP objects that might be
         provided MUST NOT have any transit domains in common.

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

   When domain diverse O bit the Domain Diverse O-bit is set, it is applied to the transit
   domains.  The other bit in SVEC object (N, L, L (Link diverse), N (Node
   diverse), S etc.) (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.  Hierarchy  Hierarchical PCE Error-Type

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

    +------------+-----------------------------------------+

        +------------+--------------------------------------------+
        | Error-Type | Meaning                                    |
    +------------+-----------------------------------------+
        +============+============================================+
        | TBD8 28         |       H-PCE error Error                          |
        |            | Error-value=1:       Error-Value=1: H-PCE capability Capability      |
        |            | was       not advertised                       |
        |            | Error-value=2: parent       Error-Value=2: Parent PCE capability Capability |
        |            |       cannot be provided                   |
    +------------+-----------------------------------------+

                           Figure 4:
        +------------+--------------------------------------------+

                            Table 1: H-PCE error 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.

   Three

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

   o object.

      Bit number TBD9: 22:  When set, the parent PCE indicates that the
                     destination domain unknown;

   o is unknown.

      Bit number TBD10: 21:  When set, the parent PCE indicates unresponsive that one or
                     more child PCE(s);

   o PCEs are unresponsive.

      Bit number TBD11: 20:  When set, the parent PCE indicates that no available
      resource
                     resources are available in one or more domains.

   o

      Bit number TBD12: 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 (parent
   P-flag (Parent PCE request 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, IDs to the parent
   PCE,
   PCE by specifying them in the Domain-ID TLVs in the OPEN object.
   This object is carried in the OPEN Open message during the PCEP session
   initialization procedure procedure.

   The OF codes defined in this document can be carried in the OF-list OF-List
   TLV of the OPEN object.  If the OF-list OF-List TLV carries the OF codes, it
   means that the PCE is capable of implementing the corresponding
   objective functions. 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 Objective Function. OF.

   When a child PCE sends a PCReq to a peer PCE, which PCE that requires parental
   activity and H-PCE capability flags TLV the H-PCE-CAPABILITY TLV but which these items were not included 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 the
   error-type=TBD8 Error-Type=28 (H-PCE error) Error) and error-value=1 Error-Value=1 (H-PCE capability was
   Capability not advertised) in the PCEP-ERROR object.

   When a specific child PCE sends a PCReq to a peer PCE, 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 the error-type=TBD8 Error-Type=28 (H-PCE error) Error) and error-value=2 Error-Value=2 (Parent
   PCE capability Capability cannot be provided) in the PCEP-ERROR object.

4.2.  Procedure to Obtain 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 which that receives the PCReq message tries to
   compute a domain sequence for it (instead of the E2E end-to-end path).
   If the domain path computation succeeds succeeds, the parent PCE sends a PCRep
   message
   which that carries the domain sequence in the Explicit Route Object
   (ERO) to the child PCE.  Refer to [RFC7897] for more details about
   domain
   sub-objects subobjects in the ERO.  Otherwise, it sends a PCReq message which
   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.
   This fact could be determined when
   When the child PCE sends a PCReq that requires parental activity, and could result in a
   negative response in the form of a PCEP Error (PCErr) message and indicate the hierarchy PCE error-
   type=TBD8 that
   includes H-PCE Error-Type=28 (H-PCE error) Error) and suitable error-value. an applicable Error-
   Value (Section 3.7) might result.

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

   o

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

   o egress.

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

   o
      time.

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

   In this case, the parent PCE MAY need to send a negative path
   computation reply PCRep
   message specifying the reason. reason for the failure.  This can be achieved
   by including the NO-PATH object in the PCRep message.  Extension  An extension
   to the NO-PATH object is needed in order to include the aforementioned reasons described
   defined in Section 3.7. 3.8.

6.  Manageability Considerations

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

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

   o  multi-domains

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

   o  multiple provider.

   *  Multiple ASes within in different service provider networks, it 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 and of H-PCE function will need to be carefully managed in an H-PCE
   network. 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
   do
   does not form part of the hierarchical PCE 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 behaviour behavior of the child and
   parent PCE are PCEs is described in the following sub-sections. 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 path computation requests 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 for the (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
   [RFC5394]. 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

   A

   [RFC7420] provides a MIB module for PCEP was published as RFC 7420 [RFC7420] that and describes managed
   objects for modelling the modeling of PCEP communication.  A YANG module for
   PCEP has also been proposed [I-D.ietf-pce-pcep-yang].

   Additionally, [PCEP-YANG].

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

   o

   *  parent PCE configuration and status,

   o

   *  child PCE configuration and information,

   o

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

   o

   *  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, operational and an alternate
   transit domain is not available, then the failure must be reported.

6.4.  Verify  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:

   o

   *  number of child PCE requests,

   o

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

   o

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

   o

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

6.5.  Requirements On on Other Protocols

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

6.6.  Impact On on Network Operations

   The hierarchical PCE 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.  This document requests  IANA actions to allocate has allocated code points for the protocol elements
   defined in this document.

7.1.  PCEP TLV Type Indicators

   IANA Manages the PCEP TLV code point registry (see [RFC5440]).  This
   is maintained as maintains the "PCEP TLV Type Indicators" sub-registry of subregistry (see
   [RFC5440]) within the "Path Computation Element Protocol (PCEP)
   Numbers" registry.

   This document defines three new PCEP TLVs.

   IANA is requested to make has allocated the following allocation: three new PCEP TLVs:

                  +------+------------------+-----------+
                  | Type | TLV name                  References
         -----------------------------------------------
         TBD1 Name         | Reference |
                  +======+==================+===========+
                  | 13   | H-PCE-CAPABILITY TLV      This I-D
         TBD2 | RFC 8685  |
                  +------+------------------+-----------+
                  | 14   | Domain-ID TLV             This I-D
         TBD3        | RFC 8685  |
                  +------+------------------+-----------+
                  | 15   | H-PCE-FLAG TLV            This I-D       | RFC 8685  |
                  +------+------------------+-----------+

                           Table 2: New PCEP TLVs

7.2.  H-PCE-CAPABILITY TLV Flags

   This document requests that a new sub-registry, named "H-PCE-
   CAPABILITY

   IANA has created the "H-PCE-CAPABILITY TLV Flag Field", is created 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 to be assigned by Standards Action [RFC8126].  Each
   registered bit should be tracked with include the following qualities:

   o information:

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

   o

   *  Capability description

   o

   *  Defining RFC

   The following values are value is defined in this document:

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

                     Table 3: Parent PCE Request Bit

7.3.  Domain-ID TLV Domain type

   This document requests that a new sub-registry, named Type

   IANA has created the "Domain-ID TLV Domain type", is created Type" subregistry within
   the "Path Computation Element Protocol (PCEP) Numbers" registry to
   manage the Domain-Type Domain Type field of the Domain-ID TLV.  The allocation
   policy for this new sub-registry 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     | 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

   This document requests that a new sub-registry, named

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

   o information:

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

   o

   *  Capability description

   o

   *  Defining RFC

   The following values are defined in this document:

          +-----+----------------------------------+-----------+
          | Bit | Description                      | Reference
         -----------------------------------------------
         31      S (Domain             This I.D.
                    Sequence bit) |
          +=====+==================================+===========+
          | 30  | D (Disallow Domain    This I.D. 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 registry list of Objective Function OFs (described in [RFC5541]) at in the sub-registry
   "Objective Function".  Three new
   Objective Functions have been defined in this document. Function" subregistry within the "Path Computation Element
   Protocol (PCEP) Numbers" registry.

   IANA is requested to make has allocated the following allocations: OFs:

        +------------+-------------------------------+-----------+
        | Code Point | Name                          | Reference
       ------------------------------------------------------
       TBD4     Minimum |
        +============+===============================+===========+
        | 12         | Minimize the number of        | RFC 8685  |
        |            | Transit      This I.D. Domains (MTD)
       TBD5         |           |
        +------------+-------------------------------+-----------+
        | 13         | Minimize the number of Border      This I.D. | RFC 8685  |
        |            | Nodes (MBN)
       TBD13                   |           |
        +------------+-------------------------------+-----------+
        | 14         | Minimize the number of         This I.D. Common | RFC 8685  |
        |            | Transit Domains (MCTD)        |           |
        +------------+-------------------------------+-----------+

                          Table 6: New OF Codes

7.6.  METRIC Object Types

   IANA maintains one sub-registry for the "METRIC object Object T field".  Two new
   metric types are defined in this document Field" subregistry [RFC5440]
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry.

   The following two new metric types for the METRIC object
   (specified are defined
   in [RFC5440]).

   IANA is requested to make the following allocations: this document:

             +-------+--------------------------+-----------+
             | Value | Description              | Reference
       ----------------------------------------------------------
       TBD6 |
             +=======+==========================+===========+
             | 20    | Domain Count metric            This I.D.
       TBD7      | RFC 8685  |
             +-------+--------------------------+-----------+
             | 21    | Border Node Count metric       This I.D. | RFC 8685  |
             +-------+--------------------------+-----------+

                     Table 7: New METRIC Object Types

7.7.  New PCEP Error-Types and Values

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

   IANA is requested to make has allocated the following allocations:

       Error-Type following:

   +-----------+---------------------------------------------+---------+
   |Error-Type | Meaning and error values  Reference
       ------------------------------------------------------
       TBD8 Error Values                    |Reference|
   +===========+=============================================+=========+
   | 28        |       H-PCE Error                    This I.D.

                Error-value=1                           |RFC 8685 |
   |           |       Error-Value=1: H-PCE Capability       |         |
   |           |       not advertised

                Error-value=2                        |         |
   |           |       Error-Value=2: Parent PCE Capability  |         |
   |           |       cannot be provided                    |         |
   +-----------+---------------------------------------------+---------+
   | 10        |       Reception of an invalid object [RFC5440]

                Error-value=TBD15:        |RFC 5440 |
   |           |       Error-Value=23: Incompatible  This I.D. OF codes |         |
   |           |       in H-PCE                              |RFC 8685 |
   +-----------+---------------------------------------------+---------+

                  Table 8: New PCEP Error-Types and Values

7.8.  New NO-PATH-VECTOR TLV Bit Flag

   IANA maintains a sub-registry 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 is requested to assign three has allocated the following four new bit flag as follows: flags:

          +------------+----------------------------+-----------+
          | Bit Number      Name Flag | Description                | Reference
      ------------------------------------------------------
      TBD9 |
          +============+============================+===========+
          | 22         | Destination Domain domain unknown  This I.D.
      TBD10 | RFC 8685  |
          +------------+----------------------------+-----------+
          | 21         | Unresponsive child PCE(s)   This I.D.
      TBD11  | RFC 8685  |
          +------------+----------------------------+-----------+
          | 20         | No available resource in    This I.D.   | RFC 8685  |
          |            | one or more domain
      TBD12 domains        |           |
          +------------+----------------------------+-----------+
          | 19         | Destination is not found    This I.D.   | RFC 8685  |
          |            | in the indicated domain. domain    |           |
          +------------+----------------------------+-----------+

                     Table 9: PCEP NO-PATH Object Flags

7.9.  SVEC Flag

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

   IANA is
   requested to assign one has allocated the following new bit flag as follows: flag:

             +------------+----------------------+-----------+
             | Bit Number      Name Flag | Description          | Reference
      ------------------------------------------------------
      TBD14 |
             +============+======================+===========+
             | 18         | Domain Diverse O-bit        This I.D. | RFC 8685  |
             +------------+----------------------+-----------+

                       Table 10: Domain Diverse O-Bit

8.  Security Considerations

   The hierarchical PCE 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]. [RFC8253] [RFC8446].

   Any multi-domain operation necessarily involves the exchange of
   information across domain boundaries.  This may represent a
   significant security and confidentiality risk 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 information as is possible.

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

9.  Contributing Authors

      Xian Zhang
      Huawei
      EMail: zhang.xian@huawei.com

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

      EMail: dhruv.ietf@gmail.com

10.Acknowledgements

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

11.  References

11.1.

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>.

11.2.

9.2.  Informative References

   [RFC4105]  Le Roux, J., J.-L., Ed., Vasseur, J., 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. J.-P. Vasseur, Ed., "MPLS Inter-Autonomous
              System 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., 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., 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,
              <http://www.rfc-editor.org/info/rfc7399>.
              <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>.

   [I-D.ietf-pce-pcep-yang]

   [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)", draft-ietf-pce-pcep-
              yang-11 (work Work in progress), March 2019.

   [I-D.ietf-pce-stateful-hpce] 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., King, D., and O. Dios, D. King,
              "Hierarchical Stateful Path Computation Element (PCE).", draft-ietf-pce-stateful-hpce-07 (work (PCE)",
              Work in
              progress), April 2019.

   [I-D.dhodylee-pce-pcep-ls] 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.", draft-
              dhodylee-pce-pcep-ls-13 (work Work in progress), February 2019.
              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, Karnataka  560066
   India
   Email: dhruv.ietf@gmail.com

Authors' Addresses

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

   EMail:
   Huawei

   Email: zhangfatai@huawei.com

   Quintin Zhao
   Huawei
   125 Nagog Technology Park
   Acton, MA 01719
   USA

   EMail: quintin.zhao@huawei.com
   United States of America

   Email: quintinzhao@gmail.com

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

   EMail:

   Email: oscar.gonzalezdedios@telefonica.com

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

   EMail:

   Email: ramon.casellas@cttc.es

   Daniel King
   Old Dog Consulting
   UK

   EMail:
   United Kingdom

   Email: daniel@olddog.co.uk

Appendix

A1. Implementation Status
   The H-PCE architecture and protocol procedures describe in this I-D
   were implemented and tested for a variety of optical research
   applications.

   The Appendix should be removed before publication.

A1.1.  Inter-layer traffic engineering with H-PCE

   This work was led by:

   o  Ramon Casellas [ramon.casellas@cttc.es]

   o  Centre Tecnologic de Telecomunicacions de Catalunya (CTTC)

   The H-PCE instances (parent and child) were multi-threaded
   asynchronous processes.  Implemented in C++11, using C++ Boost
   Libraries.  The targeted system used to deploy and run H-PCE
   applications was a POSIX system (Debian GNU/Linux operating system).

   Some parts of the software may require a Linux Kernel, the
   availability of a Routing Controller running collocated in the same
   host and the usage of libnetfilter / libipq and GNU/Linux firewalling
   capabilities.  Most of the functionality, including algorithms is
   done by means of plugins (e.g., as shared libraries or .so files in
   Unix systems).

   The CTTC PCE supports the H-PCE architecture, but also supports
   stateful PCE with active capabilities, as an OpenFlow controller, and
   has dedicated plugins to support monitoring, BRPC, P2MP, path keys,
   back end PCEs.  Management of the H-PCE entities was supported via
   HTTP and CLI via Telnet.

   Further details of the H-PCE prototyping and experimentation can be
   found in the following scientific papers:

      R.  Casellas, R.  Martinez, R.  Munoz, L.  Liu, T.  Tsuritani, I.
      Morita, "Inter-layer traffic engineering with hierarchical-PCE in
      MPLS-TP over wavelength switched optical networks" , Optics
      Express, Vol. 20, No. 28, December 2012.

      R.  Casellas, R.  Martinez, R.  Munoz, L.  Liu, T.  Tsuritani, I.
      Morita, M.  Msurusawa, "Dynamic virtual link mesh topology
      aggregation in multi-domain translucent WSON with hierarchical-
      PCE", Optics Express Journal, Vol. 19, No. 26, December 2011.

      R.  Casellas, R.  Munoz, R.  Martinez, R.  Vilalta, L.  Liu, T.
      Tsuritani, I.  Morita, V.  Lopez, O.  Gonzalez de Dios, J.  P.
      Fernandez-Palacios, "SDN based Provisioning Orchestration of
      OpenFlow/GMPLS Flexi-grid Networks with a Stateful Hierarchical
      PCE", in Proceedings of Optical Fiber Communication Conference and
      Exposition (OFC), 9-13 March, 2014, San Francisco (EEUU).
      Extended Version to appear in Journal Of Optical Communications
      and Networking January 2015

      F.  Paolucci, O.  Gonzalez de Dios, R.  Casellas, S.  Duhovnikov,
      P.  Castoldi, R.  Munoz, R.  Martinez, "Experimenting Hierarchical
      PCE Architecture in a Distributed Multi-Platform Control Plane
      Testbed" , in Proceedings of Optical Fiber Communication
      Conference and Exposition (OFC) and The National Fiber Optic
      Engineers Conference (NFOEC), 4-8 March, 2012, Los Angeles,
      California (USA).

      R.  Casellas, R.  Martinez, R.  Munoz, L.  Liu, T.  Tsuritani, I.
      Morita, M.  Tsurusawa, "Dynamic Virtual Link Mesh Topology
      Aggregation in Multi-Domain Translucent WSON with Hierarchical-
      PCE", in Proceedings of 37th European Conference and Exhibition on
      Optical Communication (ECOC 2011), 18-22 September 2011, Geneve (
      Switzerland).

      R.  Casellas, R.  Munoz, R.  Martinez, "Lab Trial of Multi-Domain
      Path Computation in GMPLS Controlled WSON Using a Hierarchical
      PCE", in Proceedings of OFC/NFOEC Conference (OFC2011), 10 March
      2011, Los Angeles (USA).

A1.2.  Telefonica Netphony (Open Source PCE)

   The Telefonica Netphony PCE is an open source Java-based
   implementation of a Path Computation Element, with several flavours,
   and a Path Computation Client.  The PCE follows a modular
   architecture and allows to add customized algorithms.  The PCE has
   also stateful and remote initiation capabilities.  In current
   version, three components can be built, a domain PCE (aka child PCE),
   a parent PCE (ready for the H-PCE architecture) and a PCC (path
   computation client).

   This work was led by:

   o  Oscar Gonzalez de Dios [oscar.gonzalezdedios@telefonica.com]

   o  Victor Lopez Alvarez [victor.lopezalvarez@telefonica.com]

   o  Telefonica I+D, Madrid, Spain

   The PCE code is publicly available in a GitHub repository:

   o  https://github.com/telefonicaid/netphony-pce
   The PCEP protocol encodings are located in the following repository:

   o  https://github.com/telefonicaid/netphony-network protocols

   The traffic engineering database and a BGP-LS speaker to fill the
   database is located in:

   o  https://github.com/telefonicaid/netphony-topology

   The parent and child PCE are multi-threaded java applications.  The
   path computation uses the jgrapht free Java class library (0.9.1)
   that provides mathematical graph-theory objects and algorithms.
   Current version of netphony PCE runs on java 1.7 and 1.8, and has
   been tested in GNU/Linux, Mac OS-X and Windows environments.  The
   management of the parent and domain PCEs is supported though CLI via
   Telnet, and configured via XML files.

   Further details of the netphony H-PCE prototyping and experimentation
   can be found in the following research papers:

   o  O.  Gonzalez de Dios, R.  Casellas, F.  Paolucci, A.  Napoli, L.
      Gifre, A.  Dupas, E, Hugues-Salas, R.  Morro, S.  Belotti, G.
      Meloni, T.  Rahman, V.P Lopez, R.  Martinez, F.  Fresi, M.  Bohn,
      S.  Yan, L.  Velasco, . Layec and J.  P.  Fernandez-Palacios:
      Experimental Demonstration of Multivendor and Multidomain EON With
      Data and Control Interoperability Over a Pan-European Test Bed, in
      Journal of Lightwave Technology, Dec. 2016, Vol. 34, Issue 7, pp.
      1610-1617.

   o  O.  Gonzalez de Dios, R.  Casellas, R.  Morro, F.  Paolucci, V.
      Lopez, R.  Martinez, R.  Munoz, R.  Villalta, P.  Castoldi:
      "Multi-partner Demonstration of BGP-LS enabled multi-domain EON,
      in Journal of Optical Communications and Networking, Dec. 2015,
      Vol. 7, Issue 12, pp.  B153-B162.

   o  F.  Paolucci, O.  Gonzalez de Dios, R.  Casellas, S.  Duhovnikov,
      P.  Castoldi, R.  Munoz, R.  Martinez, "Experimenting Hierarchical
      PCE Architecture in a Distributed Multi-Platform Control Plane
      Testbed" , in Proceedings of Optical Fiber Communication
      Conference and Exposition (OFC) and The National Fiber Optic
      Engineers Conference (NFOEC), 4-8 March, 2012, Los Angeles,
      California (USA).

A1.3.  H-PCE Proof of Concept developed by Huawei

   Huawei developed this H-PCE on the Huawei Versatile Routing Platform
   (VRP) to experiment with the hierarchy of PCE.  Both end to end path
   computation as well as computation for domain-sequence are supported.

   This work was led by:

   o  Udayasree Pallee [udayasreereddy@gmail.com]

   o  Dhruv Dhody [dhruv.ietf@gmail.com]

   o  Huawei Technologies, Bangalore, India

   Further work on stateful H-PCE [I-D.ietf-pce-stateful-hpce] is being
   carried out on ONOS.