PCEP Extension for Stateful Inter-Domain Tunnels
draft-ietf-pce-stateful-interdomain-01

Document Type Active Internet-Draft (pce WG)
Authors Olivier Dugeon  , Julien Meuric  , Young Lee  , Daniele Ceccarelli 
Last updated 2021-02-22
Replaces draft-dugeon-pce-stateful-interdomain
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Path Computation Element Working Group                         O. Dugeon
Internet-Draft                                                 J. Meuric
Intended status: Standards Track                             Orange Labs
Expires: August 26, 2021                                          Y. Lee
                                                     Samsung Electronics
                                                           D. Ceccarelli
                                                                Ericsson
                                                       February 22, 2021

            PCEP Extension for Stateful Inter-Domain Tunnels
                 draft-ietf-pce-stateful-interdomain-01

Abstract

   This document specifies how to use a Backward Recursive or
   Hierarchical method to derive inter-domain paths in the context of
   stateful Path Computation Element (PCE).  The mechanism relies on the
   PCInitiate message to set up independent paths per domain.  Combining
   these different paths together enables them to be operated as end-to-
   end inter-domain paths, without the need for a signaling session
   between inter-domain border routers.  For this purpose, this document
   defines a new Stitching Label, new Path Setup Types, new Association
   Type, and a new PCEP communication Protocol (PCEP) Capability.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 26, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  General Assumptions . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Stitching Label . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Inter-domain LSP-TYPE . . . . . . . . . . . . . . . . . .   8
   3.  Backward Recursive PCInitiate Procedure . . . . . . . . . . .   9
     3.1.  Mode of Operation . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.3.  Completion Failure of Inter-domain Path Setup Procedure .  13
   4.  Hierarchical PCInitiate Procedure . . . . . . . . . . . . . .  14
     4.1.  Mode of Operation . . . . . . . . . . . . . . . . . . . .  14
     4.2.  Completion Failure of Inter-domain Path Setup Procedure .  16
     4.3.  Example for Stateful H-PCE Stiching Procedure . . . . . .  17
   5.  Inter-domain Path Management  . . . . . . . . . . . . . . . .  21
     5.1.  Stitching Label PCE Capabilities  . . . . . . . . . . . .  21
     5.2.  Identification of Inter-domain Paths  . . . . . . . . . .  22
     5.3.  Inter-domain Association Group  . . . . . . . . . . . . .  23
     5.4.  Modification of Inter-domain Paths  . . . . . . . . . . .  24
     5.5.  Modification of Inter-domain Paths  . . . . . . . . . . .  25
     5.6.  Tear-Down of Inter-domain Paths . . . . . . . . . . . . .  25
   6.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.1.  RSVP-TE . . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.2.  Segment Routing . . . . . . . . . . . . . . . . . . . . .  26
     6.3.  Mixing Technologies . . . . . . . . . . . . . . . . . . .  27
     6.4.  Inter-Area  . . . . . . . . . . . . . . . . . . . . . . .  27
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     7.1.  Path Setup Type Values  . . . . . . . . . . . . . . . . .  28
     7.2.  Association Type Value  . . . . . . . . . . . . . . . . .  28
     7.3.  PCEP Error Values . . . . . . . . . . . . . . . . . . . .  29
     7.4.  PCEP TLV Type Indicators  . . . . . . . . . . . . . . . .  29
     7.5.  Stitching Label PCE Capability  . . . . . . . . . . . . .  29
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   10. Disclaimer  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30

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     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   The PCE working group has produced a set of RFCs to standardize the
   behavior of the Path Computation Element ([RFC4655] and [RFC5440]) as
   a tool to help MultiProtocol Label Switching - Traffic Engineering
   (MPLS-TE)/Generalized MPLS (GMPLS) Label Switched Paths (LSPs) and
   Segment Routing paths placement.  This also includes the ability to
   compute inter-domain LSPs or Segment Routing paths following a
   distributed BRPC [RFC5441] or hierarchical H-PCE [RFC6805] approach.
   Such inter-domain paths could then serve as an Explicit Route Object
   (ERO) input for the RSVP-TE signaling to set up the tunnels within
   the underlying network.  Three kinds of inter-domain paths could be
   established:

   o  Contiguous tunnel ([RFC3209] and [RFC3473]): The RSVP-TE signaling
      crosses the boundary between two domains.  This kind of tunnel is
      not recommended mostly for security and scalability purpose.  In
      addition, the initiating domain imposes huge constraints on
      subsequent domains, because they undergo the tunnel request
      without being able to control it.

   o  Stitching tunnel ([RFC5150]): Each domain establishes in its own
      network the corresponding part of the inter-domain path
      independently.  Then, a second end-to-end RSVP-TE Path message is
      sent by the initiating domain to stitch the different tunnel parts
      to form the inter-domain path.

   o  Nesting tunnel ([RFC4206]): This is similar to the stitching mode
      but, this time, with the possibility to set up tunnel hierarchy.

   However, these inter-domain paths depend on signaling using RSVP-TE
   to be set up, but it is not common to allow signaling across
   administrative domain borders, especially in operational networks.

   For Segment Routing, issues are different as there is no signaling
   between routers.  First, a segment path depends on a stack of segment
   identifiers but, in an inter-domain path, this stack may become too
   large with respect to hardware constraint.  If Extensions for Segment
   Routing [RFC8664] takes into account the Maximum Stack Depth (MSD), a
   PCE may be unable to find a solution when it computes an end-to-end
   inter-domain path.  The second issue is related to the path
   confidentiality because all Node-SID must be stacked by the head end
   router while some of the Node-SIDs are associated to routers of the
   next domains.  It is clear that operators would not disclose details

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   of their network, which includes Node-SIDs.  Thus, it is not possible
   to stack remote labels for an end-to-end inter-domain path even if
   MSD constraint is respected.

   The purpose of this document is to take the benefit of Active
   Stateful PCE [RFC8231] and PCE-Initiated [RFC8281] modes to stitch or
   nest inter-domain paths directly using PCEP between domains' PCEs
   while avoiding the use of another signaling between inter-domain
   border nodes.  The mechanism keeps each operator free to
   independently set up their respective part of the inter-domain paths,
   i.e. the signaling (for MPLS-TE and GMPLS) is scoped on a per domain
   basis, individually.

   The PCInitiate message is used from destination domain to source
   domain, to recursively set up the end-to-end tunnel.  PCRep message
   is used to convey the specific labels or SIDs to automatically stitch
   or nest the different local LSPs.  And PCRep in conjunction with
   PCUpd messages are used to report, maintain, modify and tear down
   inter-domain paths.  This method is also applicable to Segment
   Routing to build inter-domain segment paths.  To enable this
   mechanism, this document defines a new Stitching Label, new Path
   Setup Types, new Association Type, and a new PCEP communication
   Protocol (PCEP) Capability.

   <Editor's note: the replacing encoding to use instead of PST is to be
   discussed with the WG.>

1.1.  General Assumptions

   In the remainder of this document, the same references as per BRPC
   [RFC5441] are used and the following set of assumptions are made (see
   figure below):

   o  Domain refers to administrative partitions, i.e. an IGP area or an
      Autonomous System (AS).

   o  Inter-domain path is used to refer to a path that crosses two or
      more different domains as defined previously,

   o  At least one PCE is deployed in each domain.  These PCEs are all
      active stateful-capable and can request to enforce LSPs in their
      respective domain by means of PCInitiate messages.

   o  LSRs, including border nodes, are PCC-enabled and support active
      stateful mode.  PCEP sessions are established between these
      routers and their domains' PCE.

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   o  Each PCE establishes a PCEP session with its respective neighbor
      domains' PCEs.  The way a PCE discovers its neighboring PCEs is
      out of the scope of this document.

   o  PCEs are able to compute an end-to-end path as per BRPC procedure
      [RFC5441] or as per H-PCE procedure (stateless [RFC6805] or
      stateful [RFC8751]).

   o  "Path" is a generic term to refer to both LSP setup by mean of
      RSVP-TE or Segment Path in a Segment Routing network.

                    ...(H-PCE)...........................
                   .            .                        .
                  .              .                        .
       --------------           --------------           --------------
      |Domain-A .    |         |   .  Domain-B|         |   .  Domain-C|
      |        .     |         |    .         |         |    .         |
      |     PCE------+--PCEP---+---PCE--------+--PCEP---+---PCE        |
      |    /         |         |  /           |         |  /           |
      |   /          |         | /            |         | /            |
      | Src=========BNA-------BNB1===========BNB2------BNC=========Dst |
      |              |  Inter- |              |  Inter- |              |
       --------------   Domain  --------------   Domain  --------------
                        Link                     Link

           Example of the representation of 3 domains with 3 PCEs

   Operations, according to the figure above, are as follow:

   1.  The PCEs in Domain-A, Domain-B, and Domain-C communicate using
       PCEP either directly, as shown, using BRPC or with a parent PCE
       if using H-PCE.

   2.  The PCE in Domain-A selects an end-to-end domain path.  It tells
       the PCE in Domain-B that the path will be used, and that PCE
       passes the information on to the PCE in Domain-C.

   3.  Each of the PCEs use PCEP to instruct the segment head ends
       backward from destination to source:

       A.  In Domain-C, the PCE instructs the ingress Border Node, BNC,
           with the path to reach the Destination.  The instructions
           also ask BNC to provide the incoming label or SID that will
           be stitched to the intra-domain path.  Once done, PCE reports
           this label or SID to PCE of Domain-B.

       B.  In Domain-B, the PCE instructs the ingress Border Node, BNB1,
           with the path to reach the egress Border Node, BNB2.  The

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           instructions also tell BNB1 the label or SID to use on the
           inter-domain link to BNC and ask to provide the incoming
           label or SID that will be stitched to the intra-domain path.
           Once done, PCE reports this label or SID to PCE of Domain-A.

       C.  In Domain-A, the PCE instructs the Source node with the path
           to use to reach Border Node, BNA.  The instructions also
           include the label or SID to use on the inter-domain link to
           BNB1.

1.2.  Terminology

   ABR: Area Border Routers.  Routers used to connect two IGP areas
   (areas in OSPF or levels in IS-IS).

   AS: Autonomous System

   ASBR: Autonomous System Border Router.  Router used to connect
   together ASes (of the same or different service providers) via one or
   more inter-AS links.

   Border Node (BN): a boundary node is either an ABR in the context of
   inter-area TE or an ASBR in the context of inter-AS TE.

   BN-en(i): Entry BN of domain(i) connecting domain(i-1) to domain(i)
   along a determined sequence of domains.  Multiple entry BN-en(i)
   could be used to connect domain(i-1) to domain(i).

   BN-ex(i): Exit BN of domain(i) connecting domain(i) to domain(i+1)
   along a determined sequence of domains.  Multiple exit BN-ex(i) could
   be used to connect domain(i) to domain(i+1).

   Domains: Autonomous System (AS) or IGP Area.  An Autonomous System is
   composed by one or more IGP area.

   ERO(i): The Explicit Route Object scoped to domain(i)

   IGP-TE: Interior Gateway Protocol with Traffic Engineering support.
   Both OSPF-TE and IS-IS-TE are identified in this category.

   Inter-domain path: A path that crosses two or more domains through a
   pair of Border Node (BN-ex, BN-en).

   LK(i): A Link that connect BN-ex(i-1) to BN-en(i).  Note that BN-
   ex(i-1) could be connected to BN-en(i) by more than one link.  LK(i)
   identifies which of the multiple links will be used for the inter-
   domain path setup.  For inter-AS scenario, LK(i) represents the link
   between ASBR of domain i to the ASBR of domain i-1.  For inter-area

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   scenario, LK(i) is present only in IS-IS networks and represents the
   link between ABR of region L1, reciprocally L2, to the ABR of region
   L2, reciprocally L1.

   Local path: A path that does not cross a domain border.  It is set up
   either from entry BN-en, to output BN-ex or between both.  This path
   could be enforce by means of RSVP-TE signaling or Segment Routing
   labels stack.

   Local path(i): A Local path of domain(i)

   PLSP-ID(i): A PLSP-ID that identifies, in the domain(i), the local
   part of an inter-domain path.

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

   PCE(i) is a PCE within the scope of domain(i).

   PST: Path Setup Type

   R(i,j): The router j of domain i

   Stitching Label (SL): A dedicated label that is used to stitch two
   RSVP-TE LSPs or two Segment Routing paths.

   SL(i): A Stitching Label that links domain(i-1) to domain(i).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Stitching Label

   This section introduces the concept of Stitching Label that allows
   stitching and nesting of local paths in order to form an inter-domain
   path that cross several different domains.

2.1.  Definition

   The operation of stitch or nest a local path(i) to a local path(i+1)
   in order to form and inter-domain path mainly consists in defining
   the label that the output BN-ex(i) will use to send its traffic to
   the entry BN-en(i+1).  Indeed, the entry BN-en(i+1) needs to identify
   the incoming traffic (e.g.  IP packets), in order to know if this
   traffic must follow the local path(i+1) or not.  Forwarding
   Equivalent Class (FEC) could be used for that purpose.  But, when

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   stitching or nesting tunnels, the FEC is reduced to the incoming
   label that the entry BN-en(i+1) has chosen for the local path(i+1).

   In this document, we introduce the term of "Stitching Label (SL)" to
   refer to this label.  Such label is usually exchanged between output
   BN-ex(i) and entry BN-en(i+1) with the RSVP-TE signaling.  But, as we
   want to avoid to use RSVP-TE signaling due to operational
   constraints, and allow compatibility support for Segment Routing,
   this Stitching Label is here conveyed by PCEP.  In fact, the Explicit
   Route Object (ERO) and the Record Route Object (RRO) are already
   defined in order to transport (G)MPLS labels (for RSVP-TE or Segment
   Routing) in the PCEP signaling.  Thus, the Stitching Label could be
   conveyed in the ERO and RRO without any modification of PCEP nor PCEP
   Objects.

   As per RFC4003 [RFC4003], the Stitching Label will be conveyed as a
   companion of a link identifier (e.g. an IP address for numbered
   links).  In our case, this is one of the endpoint IDs of the link
   LK(i) which connects BN-ex(i) to BN-en(i+1) and carries the traffic
   from the domain(i) to domain(i+1).  It is left to implementation to
   select which of the two endpoint IDs of the link LK(i) is used.

2.2.  Inter-domain LSP-TYPE

   Even if PCEP could convey the Stitching Label, a PCC is not aware
   that a PCE requests or provides such a label.  For that purpose, this
   specification relies on the use of the PST as defined in [RFC8408]
   with new values (See IANA section of this document) defined as
   follow:

   o  TBD1: Inter-Domain TE end-to-end path is set up using Backward
      Recursive or Hierarchical method.  This new PST value MUST be set
      in a PCInitiate messages sends by a PCE(i-1) to its neighbor
      PCE(i) in the Backward Recursive method or by the Parent PCE to
      the Child PCE(i) to initiate a new inter-domain path.  In its
      response, the neighbor PCE(1) or Child PCE(i) MUST return a
      Stitching Label SL with an identifier of the associated link in
      the RRO of the PCRpt message to PCE(i-1) or Parent PCE.

   o  TBD2: Inter-Domain TE local path is set up using RSVP-TE.  This
      new PST value MUST be set in the PCInitiate message sends by a
      PCE(i) requesting to a PCC of domain(i) to initiate a new local
      path(i) which is part of an inter-domain path.  This PST value
      MUST be used by the PCE(i) only after receiving a PCInitiate
      message with an PST equal to TBD1 from a neighbor PCE(i-1) in the
      Backward Recursive method or Parent PCE in the Hierarchical
      method.  In its response, the PCC of domain(i) MUST return a

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      Stitching Label SL with the an identifier of associated link in
      the RRO of the PCRpt message.

   o  TBD3: Inter-Domain TE local path is set up using Segment Routing.
      This new PST value MUST be set in the PCInitiate message sends by
      a PCE(i) requesting to a PCC of domain(i) to initiate a new
      Segment Routing path which is part of and inter-domain Segment
      Routing path.  This PST value MUST be used by the PCE(i) only
      after receiving a PCInitiate message with an PST equal to TBD1
      from a neighbor PCE(i-1).  In its response, the PCC MUST return a
      Stitching Label SL with an identifier of the associated link in
      the RRO of the PCRpt message.

   <Editor's note: the replacing encoding to use instead of PST is to be
   discussed with the WG.>

3.  Backward Recursive PCInitiate Procedure

   This section describes how to set up inter-domain paths that cross
   different domains by using a Backward Recursive method.  It is
   compatible with the inter-domain path computation by means of the
   BRPC procedure as describe in RFC5441 [RFC5441].

3.1.  Mode of Operation

   This section describes how PCInitiate and PCRpt messages are combined
   between PCE in order to set up inter-domain paths between a source
   domain(1) to a destination domain(n).  S and D are respectively the
   source and destination of the inter-domain path.  Domain(1) and
   domain(n) are different and connected through 0 (i.e. direct
   connection when n = 2) or more intermediate domains denoted domain(i)
   with i = [2, n-1].

   First, the PCE(1) runs standard BRPC algorithm as per RFC5441
   [RFC5441] with its neighbor PCEs in order to compute the inter-domain
   path from S to D, where S and D are respectively a node in the
   domain(1) and domain(n).  Path Key confidentiality as per RFC5520
   [RFC5520] SHOULD be used to obfuscate the detailed ERO(i) of the
   different domains(i).  The resulting ERO is in the form {S, PKS(1),
   BN-ex(1), ..., BN-en(i), PKS(i), BN-ex(i), ..., BN-en(n), PKS(n), D}
   when Path Key is used and of the form {S, R(1,1), ..., R(1,k), BN-
   ex(1), ..., BN-en(i), R(i,1), ..., R(i,l), BN-ex(i), ..., BN-en(n),
   R(n,1), ..., R(n,m), D} otherwise . As subsequent domains are not
   aware about the computed end-to-end ERO in case of Virtual Source
   Path trees (VSPTs), the final ERO selected by the PCE(1) MUST be sent
   in the PCInitiate message to indicate to the subsequent PCEs which
   path has been finally chosen.  PCE(1) MUST ensure that this ERO is
   self comprehensive by subsequent PCEs.  Indeed, when a PCE(i)

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   receives the ERO, it MUST be able to verify that this ERO matches its
   own scope and to determine the PCE(i+1).  When Path Key is used, PCEs
   MUST encode the Path Key with a reachable IP address so that previous
   PCEs in the AS chain are able to join them.  When Path Key is not
   used, the PCEs MUST be able to retrieve an IP address of the next PCE
   corresponding to the ERO (e.g., relying on a per prefix table).

   The complete procedure with Path Key follows the different steps
   described below:

   Steps 1: Initialization

   Once ERO(S, D) is computed, PCE(1) sends a PCInitiate message to
   PCE(2) containing an ERO equal to {S, PKS(2), ..., PKS(i), ...,
   PKS(n), D}, PST = TBD1 and End-Points Object = (S, D).  The ERO
   corresponds to the one PCE(1) has received from PCE(2) during the
   BRPC process in which only Path Key are kept.  In case of multiple
   EROs, i.e. VSPT, PCE(1) has chosen one of them and used the selected
   one for the PCInitiate message.  PKS(i) could be replaced by the full
   ERO description if Path Key is not used by PCE(i).

   When PCE(i) receives a PCInitiate message from domain(i-1) with PST =
   TBD1 and ERO = {PKS(i), PKS(i+1), ..., PKS(n), D)}, it sends a
   PCInitiate message to PCE(i+1) with a popped ERO and records its
   received PKS(i) part.  All PCE(i)s generate the appropriate
   PCInitiate message to PCE(i+1) up to PCE(n), i.e. to the destination
   domain(n).

   Steps 2: Actions taken at the destination domain(n) by PCE(n)

   1.  When a PCInitiate message reaches the destination domain(n),
       PCE(n) retrieves the ERO from the PKS(n) if necessary and sends
       to BN-en(n) a PCInitiate message with the ERO(n) = {BN-en(n),
       ..., D}, PST = TBD2 and End-Points Object = {BN(n), D} in order
       to inform the PCC BN-en(n) that this local path(n) is part of an
       inter-domain path.

   2.  When the PCC BN-en(n) receives the PCInitiate message from its
       PCE(n), it sets up the local path from entry BN-en(n) to D by
       means of RSVP-TE signaling with the given ERO(n).

   3.  Once the tunnel is set up, BN-en(n) chooses a free label for the
       Stitching Label SL(n) and adds a new entry in its MPLS L(F)IB
       with this SL(n) label.  Then, it sends a PCRpt message to its
       PCE(n) with an RRO equal to {[LK(n), SL(n)], RRO(n)} and PLSP-
       ID(n).

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   4.  Once PCE(n) receives the PCRpt from the PCC BN-en(n) with the
       RRO, PLSP-ID and PST = TBD2, it sends to the PCE(n-1) a PCRpt
       containing the RRO equal to {[LK(n), SL(n)]} and PLSP-ID(n).
       PCE(n) MAY add {PKS(n), D} in the RRO.

   Steps i: Actions performed by all intermediate domains(i), for i = 2
   to n-1

   1.  When the PCE(i) receives a PCRpt message from domain(i+1) with
       PST = TBD1, RRO = {[LK(i+1), SL(i+1)]} and PLSP-ID(i+1), it
       retrieves the ERO(i) from the PKS(i), recorded in step 1, and
       sends to the PCC BN-en(i) a PCInitiate message with ERO =
       {ERO(i), [LK(i+1), SL(i+1)]}, PST = TBD2 and End-Points Object =
       {BN-en(i), BN-ex(i)} in order to inform the PCC BN-en(i) that
       this local path(i) is part of an inter-domain path.

   2.  When the PCC BN-en(i) receives the PCInitiate message from its
       PCE(i), it sets up the local path from BN-en(i) to BN-ex(i) by
       means of RSVP-TE signaling with the given ERO(i).

   3.  Egress Control mechanism, as per RFC4003 section 2.1 [RFC4003],
       is used to instruct the egress node of domain(i), i.e. BN-ex(i),
       to forward packets belonging to this tunnel with the Stitching
       Label.  Both the Stitching Label and the identifier of the
       interface are carried in the ERO = {..., [LK(i+1), SL(i+1)]} as
       the last SubObject in conformance to [RFC4003].  As a result, BN-
       ex(i) installs in its MPLS L(F)IB the SWAP instruction to label
       SL(i+1) with forward to LK(i+1).

   4.  Once the tunnel is set up, PCC BN-en(i) chooses a free label for
       the Stitching Label SL(i) and adds a new entry in its MPLS L(F)IB
       with this SL(i) label.  Then, it sends a PCRpt message to its
       PCE(i) with an RRO equal to {[LK(i), SL(i)], RRO(i)} and PLSP-
       ID(i).

   5.  Once PCE(i) receives the PCRpt from the PCC BN-en(i) with the RRO
       and PST = TBD2, it sends to PCE(i-1) a PCRpt message containing
       the RRO equal to {[LK(i), SL(i)]} and the PLSP-ID(i).  PCE(i) MAY
       add {PKS(i), ..., PKS(n)} in the RRO.

   Steps n: Actions performed at the source domain(1) by PCE(1)

   Once PCE(1) receives the PCRpt message from PCE(2) with the RRO
   containing the label SL(2), it sends a PCInitiate message to PCC node
   S with ERO equal to {ERO(1), [LK(2), SL(2)]}, PST = 0 and End-Points
   Object = {S, BN-ex(1)}. This time, the PST is equal to 0 as the PCC S
   does not need to return a Stitching Label SL, because it is the head-

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   end of the inter-domain path.  A usual PCRpt message is sent back to
   PCE(1) by the PCC node S.

3.2.  Example

   In the figure below, two different domains S and D are interconnected
   through BN respectively BN-S and BN-D.  PE-S and PE-D are edge
   routers.  All routers in the figure are connected to their respective
   PCE through PCEP.  In this example, we consider that PCE(S) needs to
   set up an inter-domain path between PE-S and PE-D acting as source
   and destination of the path.  To simplify the figure, neither
   intermediate routers between (PE-S, BN-S), (BN-D and PE-D), nor RSVP-
   TE messages are represented, but they are all presents.  The
   following notation is used (in this example, we use the PKS for the
   sake of simplicity):

   o  PKS(D) = Path Key corresponding to the path from BN(D) to PE-D

   o  ERO(D) = Explicit Route Object corresponding to the path from
      BN(D) to PE-D, retrieved from PKS(D)

   o  RRO(D) = Record Route Object of the local path(D) from BN(D) to
      PE-D

   o  SL(D) = Stitching Label for the local path from BN(D) to PE-D

   o  ERO(S) = Explicit Route Object corresponding to the path from PE-S
      to BN(S)

   o  RRO(S) = Record Route Object of local path(S) from PE-S to BN(S)

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     PE-S      PCE-S                           BN-D      PCE-D
      |          |                              |          |
      |        [ -------- Standard BRPC exchange ------------]
      |          |                              |          |
      |          | PCInitiate(ERO={PKS(D)}, PST = TBD1)
      |          | --------------------------------------> |
      |          |                              |          |
      |          |             PCInitiate(ERO = ERO(D), PST = TBD2)
      |          |                              | <------- |
      |          |                              |          |
      |          |         PCRpt(RRO = {SL(D), RRO(D)}, PST = TBD2)
      |          |                              |  ------> |
      |          |                              |          |
      |        PCRpt(RRO = {SL(D), PKS(D)}, PST = TBD1, PLSP-ID(D))
      |          | <-------------------------------------- |
      |          |                              |          |
      |  PCInitiate(ERO={ERO(S), SL(D), BN(D)}, PST = 0)
      | <------- |                              |          |
      |          |                              |          |
      |  PCRpt(RRO={RRO(S)}, PST = 0)      |          |
      | -------> |                              |          |
      |          |                              |          |

     +----------------------+                  +----------------------+
     |                      |                  |                      |
     |       +------+       |     PCEP         |       +------+       |
     | +---->|PCE(S)|<-------------------------------->|PCE(D)|       |
     | |     +------+       |                  |       +------+       |
     | |         ^          |                  |        ^  ^          |
     | |         |          |                  |        |  |          |
     | |PCEP     |          |                  |        |  |          |
     | |         |PCEP      |                  |   PCEP |  | PCEP     |
     | v         |          |                  |        |  |          |
   (PE-S)        +------> (BN-S) <---------> (BN-D)<----+  +----> (PE-D)
     |                      |  Inter-Domain    |                      |
     |     Domain (S)       |   Link           |   Domain (D)         |
     +----------------------+                  +----------------------+

    [--- LSP Tunnel (S) ---][---- SL label ----][--- LSP Tunnel (D) ---]

           Example of inter-domain path setup between two domains

3.3.  Completion Failure of Inter-domain Path Setup Procedure

   In case of error during path setup, PCRpt and or PCErr messages MUST
   be used to signal the problem to the neighbor PCE domain backward.
   In particular, if the new PST values defined in this document are not

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   supported by the neighbor PCE or the PCC, the PCE, respectively the
   PCC, MUST return a PCErr message with Error-Type = 21 (TE path setup
   error) and Error-Value = 1 (Unsupported path setup type) to its
   neighbor PCE.  If a PCE(i) receives a PCInitiate message from its
   peer PCE(i-1) without PST set to TBD1 or PST set to a value different
   from TBD1, it MUST return a PCErr message with Error-Type = 21 (TE
   path setup error) and Error-Value = 1 (Unsupported path setup type)
   to its peer PCE(i-1).

   Following a PCInitiate message with PST set to TBD1, if a PCC or a
   PCE returns no RRO, or an RRO without the Stitching Label SL and an
   identifier of the associated link, the PCE MUST return a PCErr
   message with Error-Type = 21 (TE path setup error) and Error-Value =
   TBD5 (Mandatory Stitching Label missing in the RRO).

   In case of completion failure, the PCE(i) MUST propagate the PCErr
   message up to the PCE(1).  In turn, PCE(1) MUST send a PCInitate
   message (R flag set in the SRP Object as per [RFC8281]) to tear down
   this inter-domain path from its neighbor PCEs.  PCE(i) MUST propagate
   the PCInitiate message and remove its local path by means of
   PCInitiate message to its PCC BN-en(i) and send back PCRpt message to
   PCE(i-1).

   In case of error in domain(i+1), PCE(i) MAY add the AS number of
   domain(i+1) in the RRO to identify the faulty domain.

4.  Hierarchical PCInitiate Procedure

   This section describes how to set up inter-domain paths that cross
   different domains by using a hierarchical method.  It is compatible
   with inter-domain path computation as described in [RFC6805].

4.1.  Mode of Operation

   This section describes how PCInitiate and PCRpt messages are combined
   between PCEs in order to set up inter-domain paths between a source
   domain(1) to a destination domain(n).  S and D are respectively the
   source and destination of the inter-domain path.  Domain(1) and
   domain(n) are different and connected through 0 or more intermediate
   domains denoted domain(i) with i = (2, n-1).  Domains are directly
   connected when n = 2.

   First, the Parent PCE contacts its Child PCE as per [RFC6805] in
   order to compute the inter-domain path from S to D, where S and D are
   respectively a node in the domain(1) and domain(n).  Path Key
   confidentiality as per RFC5520 [RFC5520] SHOULD be used to obfuscate
   the detailed ERO(i) of the different domains(i).  The resulting ERO
   is of the form (S, PKS(1), BN-ex(1), ..., BN-en(i), PKS(i), BN-ex(i),

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   ..., BN-en(n), PKS(n), D) when Path Key is used and of the form {S,
   R(1,1), ..., R(1,k), BN-ex(1), ..., BN-en(i), R(i,1), ..., R(i,l),
   BN-ex(i), ..., BN-en(n), R(n,1), ..., R(n,m), D} otherwise.

   The complete procedure with Path Key follow the different steps
   described below:

   Step 1: Initialization

   1.  The Parent PCE sends a PCInitiate message to Child PCE(n) with an
       ERO = {PKS(n)} and End-Points = {BN-en(n), D}. Then, PCE(n)
       retrieves the ERO from the PKS(n) (if necessary) and sends to BN-
       en(n) a PCInitiate message with the ERO(n) = {BN-en(n), ..., D},
       PST = TBD2 and End-Points Object = {BN-en(n), D} in order to
       inform the PCC BN-en(n) that this local path(n) is part of an
       inter-domain path.

   2.  When the PCC BN-en(n) receives the PCInitiate message from its
       PCE(n), it sets up the local path from the entry BN-en(n) to D by
       means of RSVP-TE signaling with the given ERO(n).

   3.  Once the path is set up, it chooses a free label for the
       Stitching Label SL(n) and adds a new entry in its MPLS L(F)IB
       with this SL(n) label.  Then, it sends a PCRpt message to its
       PCE(n) with an RRO equal to {[LK(n), SL(n)], RRO(n)} and PLSP-
       ID(n).

   4.  Once PCE(n) receives the PCRpt from the PCC BN-en(n) with the
       RRO, PLSP-ID and PST = TBD2, it sends to its Parent PCE a PCRpt
       containing the RRO equal to {[LK(n), SL(n)]} and PLSP-ID(n).
       PCE(n) MAY add PKS(n) in the RRO.

   Steps i: Actions performed for all intermediate domains(i), for i =
   n-1 to 2

   1.  The Parent PCE sends a PCInitiate message to Child PCE(i) with
       PST = TBD1, ERO = {PKS(i), [LK(i+1), SL(i+1)]} and End-Points =
       {BN-en(i), BN-ex(i)}

   2.  Then, PCE(i) retrieves the ERO from the PKS(i) if necessary and
       sends to the PCC BN-en(i) a PCInitiate message with ERO =
       {ERO(i), [LK(i+1), SL(i+1)]}, PST = TBD2 and End-Points Object =
       {BN-en(i), BN-ex(i)} in order to inform the PCC BN-en(i) that
       this local path(i) is part of an inter-domain path.

   3.  When the PCC BN-en(i) receives the PCInitiate message from its
       PCE(i), it sets up the local path from BN-en(i) to BN-ex(i) by
       means of RSVP-TE signaling with the given ERO(i).

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   4.  Egress Control mechanism, as per RFC4003 section 2.1 [RFC4003],
       is used to instruct the egress node of domain(i), i.e. BN-ex(i)
       to forward packets belonging to this tunnel with the Stitching
       Label.  Both the Label Stitching and an identifier of the
       outgoing interface are carried in the ERO = {..., [LK(i+1),
       SL(i+1)]} as the last SubObject in conformance to [RFC4003].  So
       that, BN-ex(i) installs in its MPLS L(F)IB the SWAP instruction
       to label SL(i+1) with forward to LK(i+1) instead of the usual POP
       instruction.

   5.  Once the tunnel is set up, PCC BN-en(i) chooses a free label for
       the Stitching Label SL(i) and adds a new entry in its MPLS L(F)IB
       with this SL(i) label.  Then, it sends a PCRpt message to its
       PCE(i) with an RRO equal to {[LK(i), SL(i)], RRO(i)} and PLSP-
       ID(i).

   6.  Once PCE(i) receives the PCRpt from the PCC BN-en(i) with the RRO
       and PST = TBD2, it sends to its Parent PCE a PCRpt message
       containing the RRO equal to {[LK(i), SL(i)]} and the PLSP-ID(i).
       PCE(i) MAY add PKS(i) in the RRO.

   7.  Once the Parent PCE receives the PCRpt from the Child PCE(i), it
       stores the corresponding PLSP-ID for this inter-domain path part.

   Steps n: Actions performed to the source domain(1)

   Finally, the Parent PCE sends a last PCInitiate message to its Child
   PCE(1) with PST = TBD1, ERO = {PKS(1), [LK(2), SL(2)]} and End-Points
   = {S, BN-ex(1)}. In turn, Child PCE(1) sends a PCInitiate message to
   PCC node S with ERO equal to {ERO(1), [LK(2), SL(2)]}, PST = 0 and
   End-Points Object = {S, BN-ex(1)}. This time, the PST is equal to 0
   as the PCC S does not need to return a Stitching Label SL, because it
   is the head-end of the inter-domain path.  A usual PCRpt message is
   sent back to PCE(1) by the PCC node S.  In turn, Child PCE(1) sends a
   final PCRpt message to the Parent PCE with the PSLP-ID(1).  PCE(1)
   MAY add {S, BN-ex(1)} in the RRO as a loose path.

4.2.  Completion Failure of Inter-domain Path Setup Procedure

   In case of error during path set up, PCRpt and or PCError messages
   MUST be used to signal the problem to the Parent PCE.  In particular,
   if the new PST values defined in this document are not supported by
   the Child PCE or the PCC, the Child PCE, respectively the PCC, MUST
   return a PCErr message with Error-Type = 21 (TE path setup error) and
   Error-Value = 1 (Unsupported path setup type) to its Parent PCE.  If
   Child PCE(i) receives a PCInitiate message from its Parent PCE
   without PST set to TBD1 or PST set to a value different from TBD1, it
   MUST return a PCErr message with Error-Type = 21 (TE path setup

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   error) and Error-Value = 1 (Unsupported path setup type) to its
   Parent PCE.

   Following a PCInitiate message with PST set to TBD1, if a Child PCE
   or a PCC returns no RRO, or an RRO without the Stitching Label SL and
   an identifier of the associated link, the Parent PCE, respectively
   the Child PCE, MUST return a PCErr message with Error-Type = 21 (TE
   path setup error) and Error-Value = TBD5 (Mandatory Stitching Label
   missing in the RRO).

   In case of completion failure, the Parent PCE MUST send a PCInitate
   message (R flag set in the SRP Object as per [RFC8281]) to tear down
   this inter-domain path from the Child PCEs that already set up their
   respective part of the inter-domain path.  Child PCE(i) MUST remove
   its local path by means of PCInitiate message with R flag set to 1 to
   its PCC BN-en(i) and send back a PCRpt message to the Parent PCE.

4.3.  Example for Stateful H-PCE Stiching Procedure

   Taking the sample hierarchical domain topology example from [RFC6805]
   as the reference topology for the entirety of this section.

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   -----------------------------------------------------------------
   |   Domain 5                                                      |
   |                              -------                            |
   |                             |P-PCE 5|                           |
   |                              -------                            |
   |                                                                 |
   |    ----------------     ----------------     ----------------   |
   |   | Domain 1       |   | Domain 2       |   | Domain 3       |  |
   |   |                |   |                |   |                |  |
   |   |      -------   |   |      -------   |   |      -------   |  |
   |   |     |C-PCE 1|  |   |     |C-PCE 2|  |   |     |C-PCE 3|  |  |
   |   |      -------   |   |      -------   |   |      -------   |  |
   |   |                |   |                |   |                |  |
   |   |            ----|   |----        ----|   |----            |  |
   |   |           |BN11+---+BN21|      |BN23+---+BN31|           |  |
   |   |   -        ----|   |----        ----|   |----        -   |  |
   |   |  |S|           |   |                |   |           |D|  |  |
   |   |   -        ----|   |----        ----|   |----        -   |  |
   |   |           |BN12+---+BN22|      |BN24+---+BN32|           |  |
   |   |            ----|   |----        ----|   |----            |  |
   |   |                |   |                |   |                |  |
   |   |         ----   |   |                |   |   ----         |  |
   |   |        |BN13|  |   |                |   |  |BN33|        |  |
   |    -----------+----     ----------------     ----+-----------   |
   |                \                                /               |
   |                 \       ----------------       /                |
   |                  \     |                |     /                 |
   |                   \    |----        ----|    /                  |
   |                    ----+BN41|      |BN42+----                   |
   |                        |----        ----|                       |
   |                        |                |                       |
   |                        |      -------   |                       |
   |                        |     |C-PCE 4|  |                       |
   |                        |      -------   |                       |
   |                        |                |                       |
   |                        | Domain 4       |                       |
   |                         ----------------                        |
   |                                                                 |
    -----------------------------------------------------------------

           Hierarchical domain topology from RFC6805

   Section 3.3.1 of [RFC8751] describes the per-domain stitched LSP mode
   and list all the steps needed.  To support SL-based stitching, using
   the reference architecture described in the figure above, the steps
   are modified as follows (note that we do not use PKS in this example
   for simplicity):

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   Step 1: initialization

   The P-PCE (PCE5) is requested to initiate a path.  Steps 4 to 10 of
   section 4.6.2 of [RFC6805] are executed to determine the end-to-end
   path, which are split into per-domain paths, e.g.  {S-BN41,
   BN41-BN33, BN33-D}.

   Step 2: Path (BN33-D) at C-PCE3:

   1.  The P-PCE (P-PCE5) sends the initiate request to the C-PCE
       (C-PCE3) via PCInitiate message for path (BN33-D) with
       ERO={BN33..D} and PST = TBD1.

   2.  C-PCE3 further propagates the initiate message to BN33 with the
       ERO and PST = TBD2/TBD3 based on the setup type.

   3.  BN33 initiates the setup of the path and reports to the status
       ("GOING-UP") to C-PCE3.

   4.  C-PCE3 further reports the status of the path to the P-PCE
       (P-PCE5)

   5.  The node BN33 notifies the path state to C-PCE3 when the state is
       "UP"; it also sends the Stitching Label (SL33) in the RRO as
       {SL33,BN33..D}.

   6.  C-PCE3 further reports the status of the path to the P-PCE
       (P-PCE5) as well as sends the Stitching Label (SL33) in the RRO
       as {LK33,SL33,BN33..D}.

   Step 3: Path (BN41-BN33) at C-PCE4

   1.  The P-PCE (P-PCE5) sends the initiate request to the C-PCE
       (C-PCE4) via PCInitiate message for path (BN41-BN33) with
       ERO={BN41..BN42,LK33,SL33,BN33} and PST = TBD1.

   2.  C-PCE4 further propagates the initiate message to BN41 with the
       ERO and PST = TBD2/TBD3 based on the setup type.  In case of
       RSVP_TE, the node BN41 encode the Stitching Label SL33 as part of
       the ERO to make sure the node BN42 uses the label SL33 towards
       node BN33.  In case of SR, the label SL33 is part of the label
       stack pushed at node BN41.

   3.  BN41 initiates the setup of the path and reports the path status
       ("GOING-UP") to C-PCE4.

   4.  C-PCE4 further reports the status of the path to the P-PCE
       (P-PCE5).

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   5.  The node BN41 notifies the path state to C-PCE4 when the state is
       "UP"; it also sends the Stitching Label (SL41) in RRO as
       {LK41,SL41,BN41..BN33}.

   6.  C-PCE4 further reports the status of the to the P-PCE (P-PCE5) as
       well as sends the Stitching Label (SL41) in the RRO as
       {LK41,SL41,BN41..BN33}.

   Step 3: Path (S-BN41) at C-PCE1

   1.  The P-PCE (P-PCE5) sends the initiate request to the C-PCE
       (C-PCE1) via PCInitiate message for path (S-BN41) with
       ERO={S..BN13,LK41,SL41,BN41}.

   2.  C-PCE1 further propagates the initiate message to node S with the
       ERO.  In case of RSVP-TE, node S encodes the Stitching Label SL41
       as part of the ERO to make sure the node BN13 uses the label SL41
       towards node BN41.  In case of SR, the label SL41 is part of the
       label stack pushed at node S.

   3.  S initiates the setup of the path and reports the path status
       ("GOING-UP") to C-PCE1.

   4.  C-PCE1 further reports the status of the path to the P-PCE
       (P-PCE5)

   5.  The node S notifies the path state to C-PCE1 when the state is
       "UP".

   6.  C-PCE1 further reports the status of the path to the P-PCE
       (P-PCE5).

   In this way, per-domain paths are stitched together using the
   Stitching Label (SL).  The per-domain paths MUST be set up from the
   destination domain towards the source domain one after the other.

   Once the per-domain path is set up, the entry BN chooses a free label
   for the Stitching Label SL and adds a new entry in its MPLS L(F)IB
   with this SL label.  The SL from the destination domain is propagated
   to adjacent transit domain, towards the source domain at each step.
   This happens from the entry BN to C-PCE then to the P-PCE, and vice-
   versa.  In case of RSVP-TE, the entry BN further propagates the SL
   label to the exit BN via RSVP-TE.  In case of SR, the SL label is
   pushed as part of the SR label stack.

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5.  Inter-domain Path Management

   This section describes how inter-domain paths could be managed.

5.1.  Stitching Label PCE Capabilities

   A PCE needs to know if its neighbor PCEs as well as PCCs are able to
   configure and provide a Stitching Label.  The STITCHING-LABEL-PCE-
   CAPABILITY TLV is an optional TLV for use in the OPEN object for
   Stitching Label PCE capability advertisement.  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=TBD7       |            Length=4           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Flags                       |I|R|S|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           STITCHING-LABEL-PCE-CAPABILITY TLV Format

   The Type (16 bits) of the TLV is TBD7.  The Length field is 16 bits
   long and has a fixed value of 4.

   The value comprises a single 32 bits "Flags" field:

   R (RSVP-TE-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1 by a PCC,
   the R flag indicates that the PCC is able to provide Stitching
   Labels, for RSVP-TE inter-domain paths, when requested by a PCE.  If
   set to 1 by a PCE, the R flag indicates that the domain controlled by
   this PCE is able to set up inter-domain paths by means of RSVP-TE
   signaling.

   S (SEGMENT-ROUTING-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1
   by a PCC, the S flag indicates that the PCC is able to provide
   Stitching Labels, for Segment-Routing inter-domain paths, when
   requested by a PCE.  If set to 1 by a PCE, the R flag indicates that
   the domain controlled by this PCE is able to set up inter-domain
   paths by means of Segment Routing.

   I (INTER-DOMAIN-STITCHING-LABEL-CAPABILITY - 1 bit): if set to 1 by a
   PCE, the I flag indicates that the domain is supporting Stitching
   Label to set up inter-domain paths.  This flag is reserved for PCEP
   session established between PCEs and MUST be kept unset by a PCC.

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   Unassigned bits are considered reserved.  They MUST be set to 0 on
   transmission and MUST be ignored on receipt.

   PCCs MUST set the R and/or S flags and MUST NOT set the I flag when
   adding the Stitching Label Capability to the PCEP Open Message.  The
   RSVP-TE-STITCHING-LABEL-CAPABILITY, respectively SEGMENT-ROUTING-
   STITCHING-LABEL-CAPABILITY, flag must be set by both the PCC and PCE
   in order to enable the configuration of Stitching Labels with RSVP-
   TE, respectively with Segment-Routing.

   A PCE MUST set the I flag when establishing a PCEP session with a
   neighbor PCE when adding Stitching Label Capability to the PCEP Open
   Message.  It MAY set R and/or S flags depending if the operator would
   like to keep confidential the technology used to set up inter-domain
   paths or not.  The INTER-DOMAIN-STITCHING-LABEL-CAPABILITY flag must
   be set by both PCEs in order to enable inter-domain paths
   instantiation by means of Stitching Label.

5.2.  Identification of Inter-domain Paths

   First, in order to manage inter-domain paths composed by the
   stitching or nesting of local paths, it is important to identify
   them.  For this purpose, the PLSP-ID managed by the PCEs are combined
   to one provided by PCCs to form a global identifier as follow:

   o  PCE(i) in the Backward Recursive method or the Child PCE in
      Hierarchical method MUST create a new unique PLSP-ID for this
      inter-domain path part and MUST send it in the PCRpt message, to
      the PCE(i-1), respectively the Parent PCE.  In addition this new
      PLSP-ID MUST be associated to the one received from the PCC that
      instantiates the local path part for further reference.

   o  In the Hierarchical mode, the Parent PCE MUST store and associate
      the different PLSP-ID(i)s received from the different Child
      PCE(i)s in order to identify the different part of the inter-
      domain paths.

   o  In the Backward Recursive method, PCE(i) MUST store and associate
      its PLSP-ID(i) and the PLSP-ID(i+1) it received from the PCE(i+1).
      PCE(n), i.e. the last one in the chain, does not need to perform
      such association.

   Further reference to the inter-domain path will use this PLSP-ID(i).
   In the Backward Recursive method, PCE(i) MUST replace the PLSP-ID(i)
   by PLSP-ID(i+1) in the PCUpd, PCRpt or PCinitiate message before
   propagating it to PCE(i+1); and PCE(i) MUST replace the PLSP-ID(i+1)
   by PLSP-ID(i) in the PCRpt message before propagating it to the

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   PCE(i-1).  In the Hierarchical method, the Parent PCE MUST use the
   corresponding PLSP-ID(i) of the Child PCE(i).

5.3.  Inter-domain Association Group

   In case of failure, a PCE(i) will received PCRpt messages from its
   PCCs and neighbors PCE(i+1) to synchronize the Inter-domain paths.
   In addition, it may received PCInitiate messages from its previous
   neighbors PCE(i-1) to re-initiate its inter-domain path part.  As the
   PCE(i) may loose the PLSP-ID association, a new association group
   (within Association Object) is used to ease the association of the
   different parts of the inter-domain path: the local part and the PCE-
   to-PCE part.  The use of the Association Object is MANDATORY in the
   Backward Recursive method and OPTIONAL in the Hierarchical method.

   For that purpose, a new Inter-Domain Association Type with value TBD4
   is defined.  The first PCE in the Backward Recursive chain (the one
   which received the initial request) MUST send the PCInitiate message
   with an Association Object as follows:

   o  Association Type field MUST be set to new value TBD4

   o  Association ID MUST be set to a unique value.  In case the
      Association ID field is too short or wraps, the first PCE MAY use
      the Extended Association ID to increase the number of association
      groups.  The Association ID is managed locally by the PCE and does
      not need to be coordinated with neighbor or remote PCEs.

   o  IPV4 or IPv6 association source MUST be set to the IP address
      which identifies PCE(1) in domain(1).

   o  The Global Association Source TLV MUST be present and set with the
      ASN number of domain(1).  It allows to create a globally unique
      association scope without putting constraint on operator's IP
      association source.  Thus the IP Association Source is associated
      with the Global Association source to form a unique identifier.

   o  Extended Association ID MAY be present and MANDATORY if
      association ID is too short or wraps.

   Subsequent PCE(i), for i = 2 to n, MUST send this Association Object
   as is to the local PCC and the neighbor PCE(i+1).

   In case of error with the association group, a PCErr message MUST be
   raised with Error = 26 (Association Error) and Error value set
   accordingly.  A new Error value TBD6 is defined to identify
   association of inter-domain paths.

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   In the Hierarchical method, the Parent PCE MAY act as the initiator
   of the Association and send to the Child PCEs an Association Object
   that follows the same rules as for the Backward Recursive method.  In
   turn, Child PCEs MUST propagate the Association Object to the local
   PCCs as is.

5.4.  Modification of Inter-domain Paths

   For the Backward Recursive method, each domain manages their
   respective local path part of an inter-domain path independently of
   each other.  In particular, Stitching Label(i) is managed by
   domain(i) and is of interest of domain(i-1) only.  Thus, Stitching
   Label SL(i) is not supposed to be propagated to other domains.  The
   same behavior apply to PLSP-ID(i).  In the Hierarchical method, the
   Parent PCE MUST ensure the correct distribution of Stitching Label
   SL(i) to Child PCE(i-1).  The PLSP-ID(i) is kept for the usage of the
   Parent PCE and thus is not propagated.  Only the Association Object
   defined in section 5.2 is propagated if it is present.

   If PCE(i) needs to modify its local path(i) with a PCUpd message to
   the PCC BN-en(i), once the PCRpt message received from the PCC BN-
   en(i), it MUST sends a new PCRpt message to advertise the
   modification.  This message is targeted to its neighbor PCE(i-1) in
   the Backward Recursive method, respectively to the Parent PCE in the
   Hierarchical method.  In this case PLSP-ID(i) is used to identify the
   inter-domain path.  PCE(i-1), respectively the Parent PCE, MUST
   propagate the PCRpt message if the modification implies the upstream
   domain, e.g. if the PCRpt indicates that the Stitching Label SL(i)
   has changed.

   PCE(1), respectively the Parent PCE, could modify the inter-domain
   path.  For that purpose, it MUST send a PCUpd message to its neighbor
   PCEs, respectively Child PCE, using the PLSP-ID it received.  Each
   PCE(i) MUST process the PCUpd message the same way they process the
   PCInitiate message as define in section 3.1 for the Backward
   Recursive method and in section 4.1 for the Hierarchical method.

   In case a failure appear in domain(i), e.g. path becoming down,
   PCE(i) MUST sends a PCRpt message to its neighbor PCE(i-1),
   respectively its Parent PCE to advertise the problem in its local
   part of the inter-domain path.  Once PCE(1), respectively the Parent
   PCE, receives this PCRpt message indicating that the path is down, it
   is up to the PCE(1), respectively the Parent PCE to take appropriate
   correction e.g. start a new path computation to update the ERO.

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5.5.  Modification of Inter-domain Paths

   Modification of local path, BN-en(i) and BN-ex(i) is left for further
   study.

5.6.  Tear-Down of Inter-domain Paths

   The tear-down of an inter-domain path is only possible by the inter-
   domain path initiator i.e. PCE(1).  For the Backward Recursive
   method, a PCInitiate message with R flag set to 1, PLSP-ID set
   accordingly to section 5.1 and the Association Object with R flag set
   to 1, is sent by PCE(1) to PCE(n) through PCE(i), and processed the
   same way as described in section 3.1.  For the Hierarchical method, a
   PCInitiate message with R flag set to 1 is sent by the Parent PCE to
   each Child PCE(i) with corresponding PLSP-ID(i), and processed
   according to section 4.1.  Each domain PCE(i) is responsible to tear
   down its part of the path and the PCC MUST release both the Stitching
   label SL in its L(F)IB and the path when it receives the PCInitiate
   message with the R flag set to 1 and the corresponding PLSP-ID.  The
   Association Group MUST also be removed by the PCC and PCE(i).

6.  Applicability

   The newly introduce Stitching Label SL serves to stitch or nest part
   of local paths to form an inter-domain path.  Each domain is free to
   decide if the incoming path is stitched or nested and how the path is
   enforced, e.g. through RSVP-TE or Segment Routing.  At the peering
   point, the Border Node BN-ex(i) MUST encapsulate the packet with the
   Stitching Label, i.e. the MPLS label prior to send them to the next
   Border Node BN-en(i+1).  Thus, only RSVP-TE and Segment Routing over
   MPLS technology are detailed in the following sections.

6.1.  RSVP-TE

   In case of RSVP-TE, the Border Node BN-ex(i) needs to received the
   Stitching Label from BN-en(i) through the RSVP-TE message and install
   in its L(F)IB a SWAP instruction to the Stitching Label and forward
   it to the next Border Node BN-en(i+1).  For that purpose, the Egress
   Control mechanism, as per RFC4003 section 2.1 [RFC4003], is
   RECOMMENDED to instruct the Border Node BN-ex(i) of this action.
   Other mechanisms to program the L(F)IB could be used, e.g.  NETCONF.

   As the Stitching Label could serves to stitch or nest tunnels, a
   domain(i) may decide to nest the incoming LSPs into a higher
   hierarchy of LSPs for a Traffic Engineering purpose.  A PCE(i) may
   also decide to group local LSPs part of inter-domain paths into a
   higher hierarchical LSP to carry all these local paths from a BN-
   en(i) to a BN-ex(i).

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6.2.  Segment Routing

   To use Segment Routing instead of RSVP-TE to set up the local LSP
   tunnels as defined in [RFC8664], PCE(i) MUST send a PCInitiate
   message with PST = TBD3 instead of TBD2 to advertise its respective
   PCC that the local path is enforce by means of Segment Routing.

   The Stitching Label SL(i+1) will be inserted into the label stack in
   order to become the top label in the stack when the packet reaches
   BN-en(i+1).  Thus, the Stitching Label SL(i+1) serves as a FEC entry
   for BN-en(i+1) to identify the packets that follow the next Segment
   Path.  For that purpose, BN-en(i+1) MUST install in its MPLS L(F)IB
   an instruction to replace the incoming Stitching Label SL(i+1) by the
   label stack given by the ERO(i+1) plus the Stitching Label SL(i+2),
   if any.

   When a packet reaches BN-ex(i), the last label in the stack before
   the label SL(i+1) corresponds to a SID that allows to reach BN-
   en(i+1).  When there are multiple interfaces between Border Nodes,
   BN-ex(i) needs to know how to send the packets to BN-en(i+1).
   Similarly to the Egress Control mechanism used with RSVP-TE, it is
   RECOMMENDED to use the inter-domain SID defined as per draft Egress
   Peer Engineering [I-D.ietf-idr-bgpls-segment-routing-epe] for that
   purpose.  The inter-domain SID is announced by BN-ex(i) to PCE(i)
   through BGP-LS for each interface that connect BN-ex(i) to neighbors
   BN-en(i+1).  Thus, the label stack will end with {BN-ex(i) SID,
   Inter-Domain SID, SL(i+1)} and should be processed as follows:

   o  The penultimate router of domain(i) pops its node SID, and sends
      the packet to the next node designated by the top label in the
      label stack, i.e. the node SID of BN-ex(i) or the adjacency SID of
      the link between the router and BN-ex(i).

   o  BN-ex(i) pops its node SID or its adjacency SID and looks up the
      next label in the stack, i.e. the inter-domain SID which
      corresponds to the interface to BN-en(i+1).  BN-ex(i) pops this
      inter-domain SID as well and sends the packet to BN-ex(i) through
      the corresponding interface.

   o  BN-en(i+1) looks up the top label which is the Stitching Label
      SL(i+1), pops it and replaces it by the sub-sequent label stack.

   Other mechanisms, e.g.  NETCONF, could be used to configure the
   inter-domain SID on exit Border Nodes.

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6.3.  Mixing Technologies

   During the instantiation procedure, if PCE(i) decides to reuse a
   local tunnel which is not yet part of an inter-domain tunnel, it
   SHOULD send a PCUpd message with PST = TBD2 to the PCC BN-en(i), in
   order to request a Stitching Label SL(i), and new ERO(i) to add the
   Stitching Label SL(i+1) and the associated link to the previous ERO.

   [RFC8453] describes framework for Abstraction and Control of TE
   Networks (ACTN), where each Physical Network Controller (PNC) is
   equivalent to C-PCE and the Multi-Domain Service Coordinator (MDSC)
   to the P-PCE.  The per-domain stitched LSP as per the Hierarchical
   PCE architecture described in Section 3.3.1 and Section 4.1 of
   [RFC8751] is well suited for ACTN.  The Stitching Label mechanism as
   described in this document is well suited for ACTN when per-domain
   LSPs need to be stitched to form an E2E tunnel or a VN Member.  It is
   to be noted that certain VNs require isolation from other clients.
   The SL mechanism described in this document can be applicable to the
   VN isolation use-case by uniquely identifying the concatenated
   stitching labels across multi-domain only to a certain VN member or
   an E2E tunnel.

   As each operator is free to enforce the tunnel with its technology
   choice, it is a local policy decision for PCE(i) to instantiate the
   local part of the end-to-end tunnel by either RSVP-TE or Segment
   Routing.  Thus, the PST value (i.e.  TBD2 or TBD3) used in the
   PCinitiate message sent by the PCE(i) to the local PCC is determined
   by the local policy.  How the local policy decision is set in the PCE
   is out of the scope of this document.  This flexibility is allowed
   because the SL principle allows to mix (data plane) technologies
   between domains.  For example, a domain(i) could use RSVP-TE while
   domain(i+1) uses SR.  The SL could serve to stitch indifferently
   Segment Paths and RSVP-TE tunnels.  Indeed, the SL will be part of
   the label stack in order to become the top label in the stack when
   reaching the BN-en(i+1).  This SL could be swapped as usual if the
   next domain uses RSVP-TE tunnels.  When the upstream domain uses an
   RSVP-TE tunnel, the SL will serve as a key for the BN-en(i+1) to
   determine which label stack it must use on top of the packet for a
   Segment Routing path.

6.4.  Inter-Area

   If use cases for inter-AS are easily identifiable, this is less
   evident for inter-area.  However, two scenarios have been identified:

   o  Paths between levels for IS-IS networks.

   o  Reduction of labels stack depth for Segment Routing.

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   Thus, the SL could be used to stitch or nest independent tunnels
   deployed through different IS-IS levels, even if there are controlled
   by the same PCE.  IS-IS levels are considered as domains but under
   the control of the same PCE.  In this scenario, there is no exchange
   between PCEs (it remains internal and implementation matter) and new
   TLVs are only applicable between the PCE and PCCs.  The PCE requests
   to the different PCCs it identifies (i.e.  BNs of the different IS-IS
   levels) to set up SLs and propagated them.

   In large-scale networks, MSD could constraints the path computation
   in the possibility of path selection i.e. explicit expression of a
   path could exceeded the MSD.  The SL could be used to split a too
   long explicit path regarding the MSD constraints.  In this scenario,
   there is also no communications between PCEs and new TLVs are only
   used between PCE and PCCs.

7.  IANA Considerations

7.1.  Path Setup Type Values

   [RFC8408] defines the PATH-SETUP-TYPE TLV.  IANA is requested to
   allocate new code points in the PCEP PATH-SETUP-TYPE TLV PST field
   registry, as follows:

   +-------+-----------------------------------------------+-----------+
   | Value | Description                                   | Reference |
   +-------+-----------------------------------------------+-----------+
   | TBD1  | Inter-domain TE end-to-end path is set up     | This      |
   |       | using the Backward           Recursive method | Document  |
   | TBD2  | Inter-domain TE local path is set up using    | This      |
   |       | RSVP-TE signaling                             | Document  |
   | TBD3  | Inter-domain TE local path is set up using    | This      |
   |       | Segment Routing                               | Document  |
   +-------+-----------------------------------------------+-----------+

7.2.  Association Type Value

   PCE Association Group [RFC8697] defines the ASSOCIATION Object and
   requests that IANA creates a registry to manage the value of the
   Association Type value.  IANA is requested to allocate a new code
   point in the PCEP ASSOCIATION GROUP TLV Association Type field
   registry, as follows:

   +------------------+--------------------------------+
   | Association Type | Description                    |
   +------------------+--------------------------------+
   | TBD4             | Inter-domain Association Group |
   +------------------+--------------------------------+

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7.3.  PCEP Error Values

   IANA is requested to allocate code-points in the PCEP-ERROR Object
   Error Values registry for a new error-value of Error-Type 21 Invalid
   TE path setup and new error-value of Error-Type 26 Association Error:

   +------------+-------------+----------------------------------------+
   | Error-Type | Error-Value | Description                            |
   +------------+-------------+----------------------------------------+
   | 21         | TBD5        | Mandatory Stitching Label missing in   |
   |            |             | the RRO                                |
   | 26         | TBD6        | Error in association of Inter-domain   |
   |            |             | LSPs                                   |
   +------------+-------------+----------------------------------------+

7.4.  PCEP TLV Type Indicators

   IANA is requested to allocate a new TLV Type Indicator for the
   "Stitching Label PCE Capability" within the "PCEP TLV Type
   Indicators" subregistry of the "Path Computation Element Protocol
   (PCEP) Numbers" registry:

   +-------+--------------------------------+---------------+
   | Value | Description                    | Reference     |
   +-------+--------------------------------+---------------+
   | TBD7  | STITCHING-LABEL-PCE-CAPABILITY | This Document |
   +-------+--------------------------------+---------------+

7.5.  Stitching Label PCE Capability

   IANA is requested to allocate a new subregistry, named "STITCHING-
   LABEL-PCE-CAPABILITY TLV Flag Field", within the "Path Computation
   Element Protocol (PCEP) Numbers" registry, to manage the Flag field
   in the STITCHING-LABEL-PCE-CAPABILITY TLV of the PCEP OPEN object
   (class = 1).  New values are assigned by Standards Action [RFC8126].
   Each bit should be tracked with the following qualities:

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

   o  Capability description

   o  Defining RFC

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   +-------+--------------------------------------+---------------+
   | Value | Description                          | Reference     |
   +-------+--------------------------------------+---------------+
   | 31    | RSVP-TE-STITCHING-CAPABILITY         | This Document |
   | 30    | SEGMENT-ROUTING-STITCHING-CAPABILITY | This Document |
   | 29    | INTER-DOMAIN-STITCHING-CAPABILITY    | This Document |
   +-------+--------------------------------------+---------------+

8.  Security Considerations

   No modification of PCE protocol (PCEP) has been requested by this
   draft which does not introduce any issue regarding security.
   Concerning the PCEP session between PCEs, authors recommend to use
   the secured version of PCEP as defined in PCEPS [RFC8253] or use any
   other secured tunnel mechanism, e.g.  IPsec tunnel to transport PCEP
   session between PCEs.

9.  Acknowledgements

   The authors want to thanks PCE's WG members, and in particular Dhruv
   Dhody who greatly contributed to the Hierarchical section of this
   document and Quan Xiong for his advice.

10.  Disclaimer

   This work has been performed in the framework of the H2020-ICT-2014
   project 5GEx (Grant Agreement no. 671636), which is partially funded
   by the European Commission.  This information reflects the
   consortium's view, but neither the consortium nor the European
   Commission are liable for any use that may be done of the information
   contained therein.

11.  References

11.1.  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>.

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   [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>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8408]  Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
              Hardwick, "Conveying Path Setup Type in PCE Communication
              Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
              July 2018, <https://www.rfc-editor.org/info/rfc8408>.

   [RFC8697]  Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
              Dhody, D., and Y. Tanaka, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Establishing
              Relationships between Sets of Label Switched Paths
              (LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
              <https://www.rfc-editor.org/info/rfc8697>.

11.2.  Informative References

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

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

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

   [RFC4003]  Berger, L., "GMPLS Signaling Procedure for Egress
              Control", RFC 4003, DOI 10.17487/RFC4003, February 2005,
              <https://www.rfc-editor.org/info/rfc4003>.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206,
              DOI 10.17487/RFC4206, October 2005,
              <https://www.rfc-editor.org/info/rfc4206>.

   [RFC4655]  Farrel, A., Vasseur, J., 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>.

   [RFC5150]  Ayyangar, A., Kompella, K., Vasseur, JP., and A. Farrel,
              "Label Switched Path Stitching with Generalized
              Multiprotocol Label Switching Traffic Engineering (GMPLS
              TE)", RFC 5150, DOI 10.17487/RFC5150, February 2008,
              <https://www.rfc-editor.org/info/rfc5150>.

   [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>.

   [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>.

   [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>.

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   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [RFC8751]  Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
              "Hierarchical Stateful Path Computation Element (PCE)",
              RFC 8751, DOI 10.17487/RFC8751, March 2020,
              <https://www.rfc-editor.org/info/rfc8751>.

Authors' Addresses

   Olivier Dugeon
   Orange Labs
   2, Avenue Pierre Marzin
   Lannion  22307
   France

   Email: olivier.dugeon@orange.com

   Julien Meuric
   Orange Labs
   2, Avenue Pierre Marzin
   Lannion  22307
   France

   Email: julien.meuric@orange.com

   Young Lee
   Samsung Electronics

   Email: younglee.tx@gmail.com

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   Daniele Ceccarelli
   Ericsson
   Torshamnsgatan, 48
   Stockholm
   Sweden

   Email: daniele.ceccarelli@ericsson.com

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