path: root/doc/rfc/rfc9050.txt

Internet Engineering Task Force (IETF)                             Z. Li
Request for Comments: 9050                                       S. Peng
Category: Standards Track                            Huawei Technologies
ISSN: 2070-1721                                                  M. Negi
                                                             RtBrick Inc
                                                                 Q. Zhao
                                                        Etheric Networks
                                                                 C. Zhou
                                                                     HPE
                                                               July 2021


 Path Computation Element Communication Protocol (PCEP) Procedures and
  Extensions for Using the PCE as a Central Controller (PCECC) of LSPs

Abstract

   The Path Computation Element (PCE) is a core component of Software-
   Defined Networking (SDN) systems.

   A PCE as a Central Controller (PCECC) can simplify the processing of
   a distributed control plane by blending it with elements of SDN and
   without necessarily completely replacing it.  Thus, the Label
   Switched Path (LSP) can be calculated/set up/initiated and the label-
   forwarding entries can also be downloaded through a centralized PCE
   server to each network device along the path while leveraging the
   existing PCE technologies as much as possible.

   This document specifies the procedures and Path Computation Element
   Communication Protocol (PCEP) extensions for using the PCE as the
   central controller for provisioning labels along the path of the
   static LSP.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9050.

Copyright Notice

   Copyright (c) 2021 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
   2.  Terminology
     2.1.  Requirements Language
   3.  Basic PCECC Mode
   4.  PCEP Requirements
   5.  Procedures for Using the PCE as a Central Controller (PCECC)
     5.1.  Stateful PCE Model
     5.2.  New LSP Functions
     5.3.  New PCEP Object
     5.4.  PCECC Capability Advertisement
     5.5.  LSP Operations
       5.5.1.  PCE-Initiated PCECC LSP
       5.5.2.  PCC-Initiated PCECC LSP
       5.5.3.  Central Controller Instructions
         5.5.3.1.  Label Download CCI
         5.5.3.2.  Label Cleanup CCI
       5.5.4.  PCECC LSP Update
       5.5.5.  Re-delegation and Cleanup
       5.5.6.  Synchronization of Central Controller Instructions
       5.5.7.  PCECC LSP State Report
       5.5.8.  PCC-Based Allocations
   6.  PCEP Messages
     6.1.  The PCInitiate Message
     6.2.  The PCRpt Message
   7.  PCEP Objects
     7.1.  OPEN Object
       7.1.1.  PCECC Capability Sub-TLV
     7.2.  PATH-SETUP-TYPE TLV
     7.3.  CCI Object
       7.3.1.  Address TLVs
   8.  Security Considerations
     8.1.  Malicious PCE
     8.2.  Malicious PCC
   9.  Manageability Considerations
     9.1.  Control of Function and Policy
     9.2.  Information and Data Models
     9.3.  Liveness Detection and Monitoring
     9.4.  Verify Correct Operations
     9.5.  Requirements on Other Protocols
     9.6.  Impact on Network Operations
   10. IANA Considerations
     10.1.  PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators
     10.2.  PCECC-CAPABILITY Sub-TLV's Flag Field
     10.3.  PCEP Path Setup Type Registry
     10.4.  PCEP Object
     10.5.  CCI Object Flag Field
     10.6.  PCEP-Error Object
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   The Path Computation Element (PCE) [RFC4655] was developed to offload
   the path computation function from routers in an MPLS traffic-
   engineered (TE) network.  It can compute optimal paths for traffic
   across a network and can also update the paths to reflect changes in
   the network or traffic demands.  Since then, the role and function of
   the PCE have grown to cover a number of other uses (such as GMPLS
   [RFC7025]) and to allow delegated control [RFC8231] and PCE-initiated
   use of network resources [RFC8281].

   According to [RFC7399], Software-Defined Networking (SDN) refers to a
   separation between the control elements and the forwarding components
   so that software running in a centralized system, called a
   controller, can act to program the devices in the network to behave
   in specific ways.  A required element in an SDN architecture is a
   component that plans how the network resources will be used and how
   the devices will be programmed.  It is possible to view this
   component as performing specific computations to place traffic flows
   within the network given knowledge of the availability of network
   resources, how other forwarding devices are programmed, and the way
   that other flows are routed.  This is the function and purpose of a
   PCE, and the way that a PCE integrates into a wider network control
   system (including an SDN system) is presented in [RFC7491].

   In early PCE implementations, where the PCE was used to derive paths
   for MPLS Label Switched Paths (LSPs), paths were requested by network
   elements (known as Path Computation Clients (PCCs)), and the results
   of the path computations were supplied to network elements using the
   Path Computation Element Communication Protocol (PCEP) [RFC5440].
   This protocol was later extended to allow a PCE to send unsolicited
   requests to the network for LSP establishment [RFC8281].

   The PCE was developed to derive paths for MPLS LSPs, which are
   supplied to the head end of the LSP using the PCEP.  But SDN has a
   broader applicability than signaled MPLS and GMPLS TE networks, and
   the PCE may be used to determine paths in a range of use cases.  PCEP
   has been proposed as a control protocol for use in these environments
   to allow the PCE to be fully enabled as a central controller.

   [RFC8283] introduces the architecture for the PCE as a central
   controller as an extension to the architecture described in [RFC4655]
   and assumes the continued use of PCEP as the protocol used between
   the PCE and PCC.  [RFC8283] further examines the motivations and
   applicability for PCEP as a Southbound Interface (SBI) and introduces
   the implications for the protocol.  [PCECC] describes the use cases
   for the PCECC architecture.

   A PCECC can simplify the processing of a distributed control plane by
   blending it with elements of SDN and without necessarily completely
   replacing it.  Thus, the LSP can be calculated/set up/initiated and
   the label-forwarding entries can also be downloaded through a
   centralized PCE server to each network device along the path while
   leveraging the existing PCE technologies as much as possible.

   This document specifies the procedures and PCEP extensions for using
   the PCE as the central controller for static LSPs, where LSPs can be
   provisioned as explicit label instructions at each hop on the end-to-
   end path.  Each router along the path must be told what label-
   forwarding instructions to program and what resources to reserve.
   The PCE-based controller keeps a view of the network and determines
   the paths of the end-to-end LSPs, and the controller uses PCEP to
   communicate with each router along the path of the end-to-end LSP.

   While this document is focused on the procedures for the static LSPs
   (referred to as the basic PCECC mode in Section 3), the mechanisms
   and protocol encodings are specified in such a way that extensions
   for other use cases are easy to achieve.  For example, the extensions
   for the PCECC for Segment Routing (SR) are specified in [PCECC-SR]
   and [PCECC-SRv6].

2.  Terminology

   The terminology used in this document is the same as that described
   in the [RFC8283].

2.1.  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.

3.  Basic PCECC Mode

   In this mode, LSPs are provisioned as explicit label instructions at
   each hop on the end-to-end path.  Each router along the path must be
   told what label-forwarding instructions to program and what resources
   to reserve.  The controller uses PCEP to communicate with each router
   along the path of the end-to-end LSP.

   [RFC8283] examines the motivations and applicability for the PCECC
   and use of PCEP as an SBI.  Section 3.1.2 of [RFC8283] highlights the
   use of the PCECC for label allocation along the static LSPs, and it
   simplifies the processing of a distributed control plane by blending
   it with elements of SDN and without necessarily completely replacing
   it.  This allows the operator to introduce the advantages of SDN
   (such as programmability) into the network.  Further, Section 3.3 of
   [PCECC] describes some of the scenarios where the PCECC technique
   could be useful.  Section 4 of [RFC8283] also describes the
   implications on the protocol when used as an SDN SBI.  The operator
   needs to evaluate the advantages offered by the PCECC against the
   operational and scalability needs of the PCECC.

   As per Section 3.1.2 of [RFC8283], the PCE-based controller will take
   responsibility for managing some part of the MPLS label space for
   each of the routers that it controls and may take wider
   responsibility for partitioning the label space for each router and
   allocating different parts for different uses.  The PCC MUST NOT make
   allocations from the label space set aside for the PCE to avoid
   overlap and collisions of label allocations.  It is RECOMMENDED that
   the PCE makes allocations (from the label space set aside for the
   PCE) for all nodes along the path.  For the purpose of this document,
   it is assumed that the exclusive label range to be used by a PCE is
   known and set on both PCEP peers.  A future extension could add the
   capability to advertise this range via a possible PCEP extension as
   well (see [PCE-ID]).  The rest of the processing is similar to the
   existing stateful PCE mechanism.

   This document also allows a case where the label space is maintained
   by the PCC and the labels are allocated by it.  In this case, the PCE
   should request the allocation from the PCC, as described in
   Section 5.5.8.

4.  PCEP Requirements

   The following key requirements should be considered when designing
   the PCECC-based solution:

   1.  A PCEP speaker supporting this document needs to have the
       capability to advertise its PCECC capability to its peers.

   2.  A PCEP speaker needs means to identify PCECC-based LSPs in the
       PCEP messages.

   3.  PCEP procedures need to allow for PCC-based label allocations.

   4.  PCEP procedures need to provide a means to update (or clean up)
       label entries downloaded to the PCC.

   5.  PCEP procedures need to provide a means to synchronize the labels
       between the PCE and the PCC via PCEP messages.

5.  Procedures for Using the PCE as a Central Controller (PCECC)

5.1.  Stateful PCE Model

   Active stateful PCE is described in [RFC8231].  A PCE as a Central
   Controller (PCECC) reuses the existing active stateful PCE mechanism
   as much as possible to control LSPs.

5.2.  New LSP Functions

   Several new functions are required in PCEP to support the PCECC.
   This document extends the existing messages to support the new
   functions required by the PCECC:

   PCInitiate:  A PCEP message described in [RFC8281].  A PCInitiate
      message is used to set up a PCE-initiated LSP based on a PCECC
      mechanism.  It is also extended for Central Controller
      Instructions (CCI) (download or clean up the label-forwarding
      instructions in the context of this document) on all nodes along
      the path, as described in Section 6.1.

   PCRpt:  A PCEP message described in [RFC8231].  A PCRpt message is
      used to send the PCECC LSP Reports.  It is also extended to report
      the set of CCI (label-forwarding instructions in the context of
      this document) received from the PCE, as described in Section 6.2.
      Section 5.5.6 describes the use of a PCRpt message during
      synchronization.

   PCUpd:  A PCEP message described in [RFC8231].  A PCUpd message is
      used to send the PCECC LSP Updates.

   The new functions defined in this document are mapped onto the PCEP
   messages, as shown in Table 1.

              +================================+============+
              | Function                       | Message    |
              +================================+============+
              | PCECC Capability advertisement | Open       |
              +--------------------------------+------------+
              | Label entry Add                | PCInitiate |
              +--------------------------------+------------+
              | Label entry Clean up           | PCInitiate |
              +--------------------------------+------------+
              | PCECC-Initiated LSP            | PCInitiate |
              +--------------------------------+------------+
              | PCECC LSP Update               | PCUpd      |
              +--------------------------------+------------+
              | PCECC LSP State Report         | PCRpt      |
              +--------------------------------+------------+
              | PCECC LSP Delegation           | PCRpt      |
              +--------------------------------+------------+
              | PCECC Label Report             | PCRpt      |
              +--------------------------------+------------+

               Table 1: Functions Mapped to the PCEP Messages

5.3.  New PCEP Object

   This document defines a new PCEP object called CCI (Section 7.3) to
   specify the Central Controller Instructions.  In the scope of this
   document, this is limited to label-forwarding instructions.  Future
   documents can create new CCI object-types for other types of Central
   Controller Instructions.  The CC-ID is the unique identifier for the
   CCI in PCEP.  The PCEP messages are extended in this document to
   handle the PCECC operations.

5.4.  PCECC Capability Advertisement

   During the PCEP initialization phase, PCEP speakers (PCE or PCC)
   advertise their support of and willingness to use PCEP extensions for
   the PCECC using these elements in the OPEN message:

   *  a new Path Setup Type (PST) (Section 7.2) in the PATH-SETUP-TYPE-
      CAPABILITY TLV to indicate support for PCEP extensions for the
      PCECC - 2 (Traffic engineering path is set up using PCECC mode)

   *  a new PCECC-CAPABILITY sub-TLV (Section 7.1.1) with the L bit set
      to '1' inside the PATH-SETUP-TYPE-CAPABILITY TLV to indicate a
      willingness to use PCEP extensions for the PCECC-based Central
      Controller Instructions for label download

   *  the STATEFUL-PCE-CAPABILITY TLV [RFC8231] (with the I flag set
      [RFC8281])

   The new PST is to be listed in the PATH-SETUP-TYPE-CAPABILITY TLV by
   all PCEP speakers that support the PCEP extensions for the PCECC in
   this document.

   The new PCECC-CAPABILITY sub-TLV is included in the PATH-SETUP-TYPE-
   CAPABILITY TLV in the OPEN object to indicate a willingness to use
   the PCEP extensions for the PCECC during the established PCEP
   session.  Using the L bit in this TLV, the PCE shows the intention to
   function as a PCECC server, and the PCC shows a willingness to act as
   a PCECC client for label download instructions (see Section 7.1.1).

   If the PCECC-CAPABILITY sub-TLV is advertised and the STATEFUL-PCE-
   CAPABILITY TLV is not advertised, or is advertised without the I flag
   set, in the OPEN object, the receiver MUST:

   *  send a PCErr message with Error-Type=19 (Invalid Operation) and
      Error-value=17 (Stateful PCE capability was not advertised) and

   *  terminate the session.

   If a PCEP speaker receives the PATH-SETUP-TYPE-CAPABILITY TLV with
   the PCECC PST but without the PCECC-CAPABILITY sub-TLV, it MUST:

   *  send a PCErr message with Error-Type=10 (Reception of an invalid
      object) and Error-value=33 (Missing PCECC Capability sub-TLV) and

   *  terminate the PCEP session.

   The PCECC-CAPABILITY sub-TLV MUST NOT be used without the
   corresponding PST being listed in the PATH-SETUP-TYPE-CAPABILITY TLV.
   If it is present without the corresponding PST listed in the PATH-
   SETUP-TYPE-CAPABILITY TLV, it MUST be ignored.

   If one or both speakers (PCE and PCC) have not indicated support and
   willingness to use the PCEP extensions for the PCECC, the PCEP
   extensions for the PCECC MUST NOT be used.  If a PCECC operation is
   attempted when both speakers have not agreed in the OPEN messages,
   the receiver of the message MUST:

   *  send a PCErr message with Error-Type=19 (Invalid Operation) and
      Error-value=16 (Attempted PCECC operations when PCECC capability
      was not advertised) and

   *  terminate the PCEP session.

   A legacy PCEP speaker (that does not recognize the PCECC Capability
   sub-TLV) will ignore the sub-TLV in accordance with [RFC8408] and
   [RFC5440].  As per [RFC8408], the legacy PCEP speaker, on receipt of
   an unsupported PST in a Request Parameter (RP) / Stateful PCE Request
   Parameter (SRP) object, will:

   *  send a PCErr message with Error-Type=21 (Invalid traffic
      engineering path setup type) and Error-value=1 (Unsupported path
      setup type) and

   *  terminate the PCEP session.

5.5.  LSP Operations

   The PCEP messages pertaining to a PCECC MUST include the PATH-SETUP-
   TYPE TLV [RFC8408] in the SRP object [RFC8231] with the PST set to
   '2' to clearly identify that the PCECC LSP is intended.

5.5.1.  PCE-Initiated PCECC LSP

   The LSP instantiation operation is defined in [RFC8281].  In order to
   set up a PCE-initiated LSP based on the PCECC mechanism, a PCE sends
   a PCInitiate message with the PST set to '2' for the PCECC (see
   Section 7.2) to the ingress PCC.

   The label-forwarding instructions (see Section 5.5.3) from the PCECC
   are sent after the initial PCInitiate and PCRpt message exchange with
   the ingress PCC, as per [RFC8281] (see Figure 1).  This is done so
   that the PCEP-specific identifier for the LSP (PLSP-ID) and other LSP
   identifiers can be obtained from the ingress and can be included in
   the label-forwarding instruction in the next set of PCInitiate
   messages along the path, as described below.

   An LSP-IDENTIFIERS TLV [RFC8231] MUST be included for the PCECC LSPs;
   it uniquely identifies the LSP in the network.  Note that the fields
   in the LSP-IDENTIFIERS TLV are described for the RSVP-signaled LSPs
   but are applicable to the PCECC LSP as well.  The LSP object is
   included in the CCI (label download Section 7.3) to identify the
   PCECC LSP for this instruction.  The PLSP-ID is the original
   identifier used by the ingress PCC, so a transit/egress Label
   Switching Router (LSR) could have multiple Central Controller
   Instructions that have the same PLSP-ID.  The PLSP-ID in combination
   with the source (in the LSP-IDENTIFIERS TLV) MUST be unique.  The
   PLSP-ID is included for maintainability reasons to ease debugging.
   As per [RFC8281], the LSP object could also include the SPEAKER-
   ENTITY-ID TLV to identify the PCE that initiated these instructions.
   Also, the CC-ID is unique in each PCEP session, as described in
   Section 7.3.

   On receipt of a PCInitiate message for the PCECC LSP, the PCC
   responds with a PCRpt message with the status set to 'Going-up' and
   carrying the assigned PLSP-ID (see Figure 1).  The ingress PCC also
   sets the D (Delegate) flag (see [RFC8231]) and C (Create) flag (see
   [RFC8281]) in the LSP object.  When the PCE receives this PCRpt
   message with the PLSP-ID, it assigns labels along the path and sets
   up the path by sending a PCInitiate message to each node along the
   path of the LSP, as per the PCECC technique.  The CC-ID uniquely
   identifies the Central Controller Instructions within a PCEP session.
   Each node along the path (PCC) responds with a PCRpt message to
   acknowledge the CCI with the PCRpt messages including the CCI and LSP
   objects.

   The ingress node would receive one CCI object with the O bit (out-
   label) set.  The transit node(s) would receive two CCI objects with
   the in-label CCI without the O bit set and the out-label CCI with the
   O bit set.  The egress node would receive one CCI object without the
   O bit set (see Figure 1).  A node can determine its role based on the
   setting of the O bit in the CCI object(s) and the LSP-IDENTIFIERS TLV
   in the LSP object.

   The LSP deletion operation for the PCE-initiated PCECC LSP is the
   same as defined in [RFC8281].  The PCE should further perform the
   label entry cleanup operation, as described in Section 5.5.3.2, for
   the corresponding LSP.

                 +-------+                              +-------+
                 |PCC    |                              |  PCE  |
                 |ingress|                              +-------+
          +------|       |                                  |
          | PCC  +-------+                                  |
          | transit| |                                      |
   +------|        | |<--PCInitiate,PLSP-ID=0,PST=2---------| PCECC LSP
   |PCC   +--------+ |                                      | Initiate
   |egress  |  |     |----PCRpt,PLSP-ID=2,D=1,C=1---------->| PCECC LSP
   +--------+  |     |       (GOING-UP)                     |
       |       |     |                                      |
       |<-------PCInitiate,CC-ID=X,PLSP-ID=2----------------| Label
       |       |     |                                      | download
       |--------PCRpt,CC-ID=X,PLSP-ID=2-------------------->| CCI
       |       |     |                                      |
       |       |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=2-----| Label
       |       |     |                                      | download
       |       |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=2--------->| CCI
       |       |     |                                      |
       |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label
       |       |     |                                      | download
       |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI
       |       |     |                                      |
       |       |     |<---PCUpd,PLSP-ID=2,PST=2,D=1---------| PCECC LSP
       |       |     |      (UP)                            | Update
       |       |     |----PCRpt,PLSP-ID=2,D=1,C=1---------->|
       |       |     |      (UP)                            |

                     Figure 1: PCE-Initiated PCECC LSP

   Once the label operations are completed, the PCE MUST send a PCUpd
   message to the ingress PCC.  As per [RFC8231], the PCUpd message is
   with the D flag set.

   The PCECC LSPs are considered to be 'up' by default (on receipt of a
   PCUpd message from the PCE).  The ingress could further choose to
   deploy a data-plane check mechanism and report the status back to the
   PCE via a PCRpt message to make sure that the correct label
   instructions are made along the path of the PCECC LSP (and it is
   ready to carry traffic).  The exact mechanism is out of scope of this
   document.

   In the case where the label allocations are made by the PCC itself
   (see Section 5.5.8), the PCE could request an allocation to be made
   by the PCC; then, the PCC would send a PCRpt message with the
   allocated label encoded in the CC-ID object (as shown in Figure 2) in
   the configuration sequence from the egress towards the ingress along
   the path.

                 +-------+                              +-------+
                 |PCC    |                              |  PCE  |
                 |ingress|                              +-------+
          +------|       |                                  |
          | PCC  +-------+                                  |
          | transit| |                                      |
   +------|        | |<--PCInitiate,PLSP-ID=0,PST=2,--------| PCECC LSP
   |PCC   +--------+ |                                      | Initiate
   |egress  |  |     |----PCRpt,PLSP-ID=2,D=1,C=1---------->| PCECC LSP
   +--------+  |     |       (GOING-UP)                     |
       |       |     |                                      |
       |<-------PCInitiate,CC-ID=X,PLSP-ID=2----------------| Label
       |       |     |     C=1,O=0                          | download
       |--------PCRpt,CC-ID=X,PLSP-ID=2-------------------->| CCI
       |       |     |     Label=L1                         |
       |       |<------PCInitiate,PLSP-ID=2,----------------| Labels
       |       |     |            CC-ID=Y1,C=1,O=0          | download
       |       |     |            CC-ID=Y2,C=0,O=1,L1       | CCI
       |       |-------PCRpt,PLSP-ID=2--------------------->|
       |       |     |       CC-ID=Y1,O=0,Label=L2          |
       |       |     |       CC-ID=Y2,O=1                   |
       |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label
       |       |     |                C=0,O=1,L2            | download
       |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI
       |       |     |                                      |
       |       |     |<---PCUpd,PLSP-ID=2,PST=2,D=1---------| PCECC LSP
       |       |     |      (UP)                            | Update

             Figure 2: PCE-Initiated PCECC LSP (PCC Allocation)

   In this example, it should be noted that the request is made to the
   egress node with the C bit set in the CCI object to indicate that the
   label allocation needs to be done by the egress, and the egress
   responds with the allocated label to the PCE.  The PCE further
   informs the transit PCC without setting the C bit to '1' in the CCI
   object for the out-label, but the C bit is set to '1' for the in-
   label, so the transit node makes the label allocation (for the in-
   label) and reports to the PCE.  Similarly, the C bit is unset towards
   the ingress to complete all the label allocations for the PCECC LSP.

5.5.2.  PCC-Initiated PCECC LSP

   In order to set up an LSP based on the PCECC mechanism where the LSP
   is configured at the PCC, a PCC MUST delegate the LSP by sending a
   PCRpt message with the PST set for the PCECC (see Section 7.2) and D
   (Delegate) flag (see [RFC8231]) set in the LSP object (see Figure 3).

   When a PCE receives the initial PCRpt message with the D flag and PST
   set to '2', it SHOULD calculate the path and assign labels along the
   path in addition to setting up the path by sending a PCInitiate
   message to each node along the path of the LSP, as per the PCECC
   technique (see Figure 3).  The CC-ID uniquely identifies the CCI
   within a PCEP session.  Each PCC further responds with the PCRpt
   messages, including the CCI and LSP objects.

   Once the CCI (label operations) are completed, the PCE MUST send the
   PCUpd message to the ingress PCC.  As per [RFC8231], this PCUpd
   message should include the path information calculated by the PCE.

   Note that the PCECC LSPs MUST be delegated to a PCE at all times.

   The LSP deletion operation for the PCECC LSPs is the same as defined
   in [RFC8231].  If the PCE receives a PCRpt message for LSP deletion,
   then it does label the cleanup operation, as described in
   Section 5.5.3.2, for the corresponding LSP.

   The basic PCECC LSP setup sequence is as shown in Figure 3.

                  +-------+                             +-------+
                  |PCC    |                             |  PCE  |
                  |ingress|                             +-------+
           +------|       |                                 |
           | PCC  +-------+                                 |
           | transit| |                                     |
    +------|        | |---PCRpt,PLSP-ID=1,PST=2,D=1-------->| PCECC LSP
    |PCC   +--------+ |                                     |
    |egress  |  |     |                                     |
    +--------+  |     |                                     |
        |       |     |                                     |
        |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label
        |       |     |     L1,O=0                          | download
        |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI
        |       |     |                                     |
        |       |<------PCInitiate,PLSP-ID=1,---------------| Labels
        |       |     |            CC-ID=Y1,O=0,L2          | download
        |       |     |            CC-ID=Y2,O=1,L1          | CCI
        |       |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=1-------->|
        |       |     |                                     |
        |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label
        |       |     |                L2,O=1               | download
        |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI
        |       |     |                                     |
        |       |     |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC LSP
        |       |     |                                     | Update
        |       |     |                                     |

                     Figure 3: PCC-Initiated PCECC LSP

   In the case where the label allocations are made by the PCC itself
   (see Section 5.5.8), the PCE could request an allocation to be made
   by the PCC; then, the PCC would send a PCRpt message with the
   allocated label encoded in the CC-ID object, as shown in Figure 4.

                  +-------+                             +-------+
                  |PCC    |                             |  PCE  |
                  |ingress|                             +-------+
           +------|       |                                 |
           | PCC  +-------+                                 |
           | transit| |                                     |
    +------|        | |---PCRpt,PLSP-ID=1,PST=2,D=1-------->| PCECC LSP
    |PCC   +--------+ |                                     |
    |egress  |  |     |                                     |
    +--------+  |     |                                     |
        |       |     |                                     |
        |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label
        |       |     |     C=1                             | download
        |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI
        |       |     |     Label=L1                        |
        |       |<------PCInitiate,PLSP-ID=1,---------------| Labels
        |       |     |            CC-ID=Y1,C=1             | download
        |       |     |            CC-ID=Y2,C=0,L1          | CCI
        |       |-------PCRpt,PLSP-ID=1-------------------->|
        |       |     |       CC-ID=Y1,Label=L2             |
        |       |     |       CC-ID=Y2                      |
        |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label
        |       |     |                C=0,L2               | download
        |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI
        |       |     |                                     |
        |       |     |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC LSP
        |       |     |                                     | Update
        |       |     |                                     |

             Figure 4: PCC-Initiated PCECC LSP (PCC Allocation)

      |  Note:
      |  
      |  The O bit is set as before (and thus not included).

   In the case where the label allocations are made by the PCC itself
   (see Section 5.5.8), the procedure remains the same, with just an
   additional constraint on the configuration sequence.

   The rest of the PCC-initiated PCECC LSP setup operations are the same
   as those described in Section 5.5.1.

5.5.3.  Central Controller Instructions

   The new CCI for the label operations in PCEP are done via the
   PCInitiate message (Section 6.1) by defining a new PCEP object for
   CCI operations.  The local label range of each PCC is assumed to be
   known by both the PCC and the PCE.

5.5.3.1.  Label Download CCI

   In order to set up an LSP based on the PCECC, the PCE sends a
   PCInitiate message to each node along the path to download the label
   instructions, as described in Sections 5.5.1 and 5.5.2.

   The CCI object MUST be included, along with the LSP object in the
   PCInitiate message.  The LSP-IDENTIFIERS TLV MUST be included in the
   LSP object.  The SPEAKER-ENTITY-ID TLV SHOULD be included in the LSP
   object.

   If a node (PCC) receives a PCInitiate message that includes a label
   to download (as part of CCI) that is out of the range set aside for
   the PCE, it MUST send a PCErr message with Error-Type=31 (PCECC
   failure) and Error-value=1 (Label out of range) and MUST include the
   SRP object to specify the error is for the corresponding label update
   via a PCInitiate message.  If a PCC receives a PCInitiate message but
   fails to download the label entry, it MUST send a PCErr message with
   Error-Type=31 (PCECC failure) and Error-value=2 (Instruction failed)
   and MUST include the SRP object to specify the error is for the
   corresponding label update via a PCInitiate message.

   A new PCEP object for CCI is defined in Section 7.3.

5.5.3.2.  Label Cleanup CCI

   In order to delete an LSP based on the PCECC, the PCE sends Central
   Controller Instructions via a PCInitiate message to each node along
   the path of the LSP to clean up the label-forwarding instruction.

   If the PCC receives a PCInitiate message but does not recognize the
   label in the CCI, the PCC MUST generate a PCErr message with Error-
   Type=19 (Invalid operation) and Error-value=18 (Unknown Label) and
   MUST include the SRP object to specify the error is for the
   corresponding label cleanup (via a PCInitiate message).

   The R flag in the SRP object defined in [RFC8281] specifies the
   deletion of the label entry in the PCInitiate message.

                  +-------+                              +-------+
                  |PCC    |                              |  PCE  |
                  |ingress|                              +-------+
           +------|       |                                  |
           | PCC  +-------+                                  |
           | transit| |                                      |
    +------|        | |                                      |
    |PCC   +--------+ |                                      |
    |egress  |  |     |                                      |
    +--------+  |     |                                      |
        |       |     |                                      |
        |<-------PCInitiate,CC-ID=X,PLSP-ID=2----------------| Label
        |       |     |                   R=1                | cleanup
        |--------PCRpt,CC-ID=X,PLSP-ID=2-------------------->| CCI
        |       |     |              R=1                     |
        |       |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=2-----| Label
        |       |     |                          R=1         | cleanup
        |       |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=2--------->| CCI
        |       |     |                         R=1          |
        |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=2-----| Label
        |       |     |                              R=1     | cleanup
        |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=2--------->| CCI
        |       |     |                         R=1          |
        |       |     |<--PCInitiate,PLSP-ID=2,PST=2,R=1-----| PCECC LSP
        |       |     |                                      | remove

                          Figure 5: Label Cleanup

   As per [RFC8281], following the removal of the label-forwarding
   instruction, the PCC MUST send a PCRpt message.  The SRP object in
   the PCRpt message MUST include the SRP-ID-number from the PCInitiate
   message that triggered the removal.  The R flag in the SRP object
   MUST be set.

   In the case where the label allocation is made by the PCC itself (see
   Section 5.5.8), the removal procedure remains the same, adding the
   sequence constraint.

5.5.4.  PCECC LSP Update

   The update is done as per the make-before-break procedures, i.e., the
   PCECC first updates new label instructions based on the updated path
   and then informs the ingress to switch traffic before cleaning up the
   former instructions.  New CC-IDs are used to identify the updated
   instructions; the identifiers in the LSP object uniquely identify the
   existing LSP.  Once new instructions are downloaded, the PCE further
   updates the new path at the ingress, which triggers the traffic
   switch on the updated path.  The ingress PCC acknowledges with a
   PCRpt message, on receipt of the PCRpt message, the PCE does the
   cleanup operation for the former LSP, as described in
   Section 5.5.3.2.

                 +-------+                             +-------+
                 |PCC    |                             |  PCE  |
                 |ingress|                             +-------+
          +------|       |                                 |
          | PCC  +-------+                                 |
          | transit| |                                     |
   +------|        | |                                     |
   |PCC   +--------+ |                                     |
   |egress  |  |     |                                     |
   +--------+  |     |                                     |
       |       |     |                                     | New Path
       |<------ PCInitiate,CC-ID=XX,PLSP-ID=1 -------------| for LSP
       |       |     |                                     | trigger
       |--------PCRpt,CC-ID=XX,PLSP-ID=1------------------>| new CCI
       |       |     |                                     |
       |       |<------PCInitiate,CC-ID=YY1,YY2,PLSP-ID=1--| Label
       |       |     |                                     | download
       |       |-------PCRpt,CC-ID=YY1,YY2,PLSP-ID=1------>| CCI
       |       |     |                                     |
       |       |     |<----PCInitiate,CC-ID=ZZ,PLSP-ID=1---| Label
       |       |     |                                     | download
       |       |     |-----PCRpt,CC-ID=ZZ,PLSP-ID=1------->| CCI
       |       |     |                                     |
       |       |     |<---PCUpd,PLSP-ID=1,PST=2,D=1--------| PCECC
       |       |     |    SRP=S                            | LSP Update
       |       |     |                                     |
       |       |     |---PCRpt,PLSP-ID=1,PST=2,D=1-------->| Trigger
       |       |     |       (SRP=S)                       | Delete
       |       |     |                                     | former CCI
       |       |     |                                     |
       |<-------PCInitiate,CC-ID=X,PLSP-ID=1---------------| Label
       |       |     |                   R=1               | cleanup
       |--------PCRpt,CC-ID=X,PLSP-ID=1------------------->| CCI
       |       |     |              R=1                    |
       |       |<------PCInitiate,CC-ID=Y1,Y2,PLSP-ID=1----| Label
       |       |     |                              R=1    | cleanup
       |       |-------PCRpt,CC-ID=Y1,Y2,PLSP-ID=1-------->| CCI
       |       |     |                         R=1         |
       |       |     |<----PCInitiate,CC-ID=Z,PLSP-ID=1----| Label
       |       |     |                              R=1    | cleanup
       |       |     |-----PCRpt,CC-ID=Z,PLSP-ID=1-------->| CCI
       |       |     |                         R=1         |

                         Figure 6: PCECC LSP Update

   The modified PCECC LSPs are considered to be 'up' by default.  The
   ingress could further choose to deploy a data-plane check mechanism
   and report the status back to the PCE via a PCRpt message.  The exact
   mechanism is out of scope of this document.

   In the case where the label allocations are made by the PCC itself
   (see Section 5.5.8), the procedure remains the same.

5.5.5.  Re-delegation and Cleanup

   As described in [RFC8281], a new PCE can gain control over an
   orphaned LSP.  In the case of a PCECC LSP, the new PCE MUST also gain
   control over the CCI in the same way by sending a PCInitiate message
   that includes the SRP, LSP, and CCI objects and carries the CC-ID and
   PLSP-ID identifying the instructions that it wants to take control
   of.

   Further, as described in [RFC8281], the State Timeout Interval timer
   ensures that a PCE crash does not result in automatic and immediate
   disruption for the services using PCE-initiated LSPs.  Similarly the
   Central Controller Instructions are not removed immediately upon PCE
   failure.  Instead, they are cleaned up on the expiration of this
   timer.  This allows for network cleanup without manual intervention.
   The PCC MUST support the removal of CCI as one of the behaviors
   applied on expiration of the State Timeout Interval timer.

   In the case of the PCC-initiated PCECC LSP, the control over the
   orphaned LSP at the ingress PCC is taken over by the mechanism
   specified in [RFC8741] to request delegation.  The control over the
   CCI is described above using [RFC8281].

5.5.6.  Synchronization of Central Controller Instructions

   The purpose of CCI synchronization (labels in the context of this
   document) is to make sure that the PCE's view of CCI (labels) matches
   with the PCC's label allocation.  This synchronization is performed
   as part of the LSP State Synchronization, as described in [RFC8231]
   and [RFC8232].

   As per LSP State Synchronization [RFC8231], a PCC reports the state
   of its LSPs to the PCE using PCRpt messages and, as per [RFC8281],
   the PCE would initiate any missing LSPs and/or remove any LSPs that
   are not wanted.  The same PCEP messages and procedures are also used
   for the CCI synchronization.  The PCRpt message includes the CCI and
   the LSP object to report the label-forwarding instructions.  The PCE
   would further remove any unwanted instructions or initiate any
   missing instructions.

5.5.7.  PCECC LSP State Report

   As mentioned before, an ingress PCC MAY choose to apply any
   Operations, Administration, and Maintenance (OAM) mechanism to check
   the status of the LSP in the data plane and MAY further send its
   status in a PCRpt message to the PCE.

5.5.8.  PCC-Based Allocations

   The PCE can request the PCC to allocate the label using the
   PCInitiate message.  The C flag in the CCI object is set to '1' to
   indicate that the allocation needs to be made by the PCC.  The PCC
   MUST try to allocate the label and MUST report to the PCE via a PCRpt
   or PCErr message.

   If the value of the label is 0 and the C flag is set to '1', it
   indicates that the PCE is requesting the allocation to be made by the
   PCC.  If the label is 'n' and the C flag is set to '1' in the CCI
   object, it indicates that the PCE requests a specific value 'n' for
   the label.  If the allocation is successful, the PCC MUST report via
   the PCRpt message with the CCI object.  If the value of the label in
   the CCI object is invalid, it MUST send a PCErr message with Error-
   Type=31 (PCECC failure) and Error-value=3 (Invalid CCI).  If it is
   valid but the PCC is unable to allocate it, it MUST send a PCErr
   message with Error-Type=31 (PCECC failure) and Error-value=4 (Unable
   to allocate the specified CCI).

   If the PCC wishes to withdraw or modify the previously assigned
   label, it MUST send a PCRpt message without any label or with the
   label containing the new value, respectively, in the CCI object.  The
   PCE would further trigger the label cleanup of the older label, as
   per Section 5.5.3.2.

6.  PCEP Messages

   As defined in [RFC5440], a PCEP message consists of a common header
   followed by a variable-length body made of a set of objects that can
   be either mandatory or optional.  An object is said to be mandatory
   in a PCEP message when the object must be included for the message to
   be considered valid.  For each PCEP message type, a set of rules is
   defined, which specifies the set of objects that the message can
   carry.  An implementation MUST form the PCEP messages using the
   object ordering specified in this document.

   The LSP-IDENTIFIERS TLV MUST be included in the LSP object for the
   PCECC LSP.

   The message formats in this document are specified using Routing
   Backus-Naur Form (RBNF) encoding, as specified in [RFC5511].

6.1.  The PCInitiate Message

   The PCInitiate message [RFC8281] can be used to download or remove
   the labels; this document extends the message, as shown below.

        <PCInitiate Message> ::= <Common Header>
                                 <PCE-initiated-lsp-list>

   Where:

   *  <Common Header> is defined in [RFC5440].

        <PCE-initiated-lsp-list> ::= <PCE-initiated-lsp-request>
                                     [<PCE-initiated-lsp-list>]

        <PCE-initiated-lsp-request> ::=
                               (<PCE-initiated-lsp-instantiation>|
                                <PCE-initiated-lsp-deletion>|
                                <PCE-initiated-lsp-central-control>)

        <PCE-initiated-lsp-central-control> ::= <SRP>
                                                <LSP>
                                                <cci-list>

        <cci-list> ::=  <CCI>
                        [<cci-list>]

   Where:

   *  <PCE-initiated-lsp-instantiation> and <PCE-initiated-lsp-deletion>
      are as per [RFC8281].

   *  The LSP and SRP object is defined in [RFC8231].

   When a PCInitiate message is used for the CCI (labels), the SRP, LSP,
   and CCI objects MUST be present.  The SRP object is defined in
   [RFC8231]; if the SRP object is missing, the receiving PCC MUST send
   a PCErr message with Error-Type=6 (Mandatory Object missing) and
   Error-value=10 (SRP object missing).  The LSP object is defined in
   [RFC8231], and if the LSP object is missing, the receiving PCC MUST
   send a PCErr message with Error-Type=6 (Mandatory Object missing) and
   Error-value=8 (LSP object missing).  The CCI object is defined in
   Section 7.3, and if the CCI object is missing, the receiving PCC MUST
   send a PCErr message with Error-Type=6 (Mandatory Object missing) and
   Error-value=17 (CCI object missing).  More than one CCI object MAY be
   included in the PCInitiate message for a transit LSR.

   To clean up entries, the R (remove) bit MUST be set in the SRP object
   to be encoded along with the LSP and CCI objects.

   The CCI object received at the ingress node MUST have the O bit (out-
   label) set.  The CCI object received at the egress MUST have the O
   bit unset.  If this is not the case, the PCC MUST send a PCErr
   message with Error-Type=31 (PCECC failure) and Error-value=3 (Invalid
   CCI).  Other instances of the CCI object, if present, MUST be
   ignored.

   For the point-to-point (P2P) LSP setup via the PCECC technique, at
   the transit LSR, two CCI objects are expected for incoming and
   outgoing labels associated with the LSP object.  If any other CCI
   object is included in the PCInitiate message, it MUST be ignored.  If
   the transit LSR did not receive two CCI objects, with one of them
   having the O bit set and another with the O bit unset, it MUST send a
   PCErr message with Error-Type=31 (PCECC failure) and Error-value=3
   (Invalid CCI).

   Note that, on receipt of the PCInitiate message with CCI object, the
   ingress, egress, or transit role of the PCC is identified via the
   ingress and egress IP address encoded in the LSP-IDENTIFIERS TLV.

6.2.  The PCRpt Message

   The PCRpt message can be used to report the labels that were
   allocated by the PCE to be used during the State Synchronization
   phase or as an acknowledgment to a PCInitiate message.

         <PCRpt Message> ::= <Common Header>
                             <state-report-list>

   Where:

         <state-report-list> ::= <state-report>[<state-report-list>]

         <state-report> ::= (<lsp-state-report>|
                             <central-control-report>)

         <lsp-state-report> ::= [<SRP>]
                                <LSP>
                                <path>

         <central-control-report> ::= [<SRP>]
                                      <LSP>
                                      <cci-list>

         <cci-list> ::=  <CCI>
                         [<cci-list>]

   Where:

   *  <path> is as per [RFC8231], and the LSP and SRP objects are also
      defined in [RFC8231].

   When a PCRpt message is used to report the CCI (labels), the LSP and
   CCI objects MUST be present.  The LSP object is defined in [RFC8231],
   and if the LSP object is missing, the receiving PCE MUST send a PCErr
   message with Error-Type=6 (Mandatory Object missing) and Error-
   value=8 (LSP object missing).  The CCI object is defined in
   Section 7.3, and if the CCI object is missing, the receiving PCE MUST
   send a PCErr message with Error-Type=6 (Mandatory Object missing) and
   Error-value=17 (CCI object missing).  Two CCI objects can be included
   in the PCRpt message for a transit LSR.

7.  PCEP Objects

   The PCEP objects defined in this document are compliant with the PCEP
   object format defined in [RFC5440].

7.1.  OPEN Object

   This document defines a new PST (2) to be included in the PATH-SETUP-
   TYPE-CAPABILITY TLV in the OPEN object.  Further, a new sub-TLV for
   the PCECC capability exchange is also defined.

7.1.1.  PCECC Capability Sub-TLV

   The PCECC-CAPABILITY sub-TLV is an optional TLV for use in the OPEN
   object in the PATH-SETUP-TYPE-CAPABILITY TLV when the Path Setup Type
   list includes the PCECC Path Setup Type 2.  A PCECC-CAPABILITY sub-
   TLV MUST be ignored if the PST list does not contain PST=2.

   Its format is shown in Figure 7.

       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=1          |          Length=4             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Flags                           |L|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 7: PCECC Capability Sub-TLV

   The type of the TLV is 1, and it has a fixed length of 4 octets.

   The value comprises a single field: Flags (32 bits).  Currently, the
   following flag bit is defined:

   L bit (Label):  If set to '1' by a PCEP speaker, the L flag indicates
      that the PCEP speaker will support and is willing to handle the
      PCEC-based Central Controller Instructions for label download.
      The bit MUST be set to '1' by both a PCC and a PCE for the PCECC
      label download/report on a PCEP session.

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

7.2.  PATH-SETUP-TYPE TLV

   The PATH-SETUP-TYPE TLV is defined in [RFC8408]; this document
   defines a new PST value:

   PST=2:  Path is set up via the PCECC mode.

   On a PCRpt/PCUpd/PCInitiate message, the PST=2 in the PATH-SETUP-TYPE
   TLV in the SRP object MUST be included for an LSP set up via the
   PCECC-based mechanism.

7.3.  CCI Object

   The CCI object is used by the PCE to specify the forwarding
   instructions (label information in the context of this document) to
   the PCC and MAY be carried within a PCInitiate or PCRpt message for
   label download/report.

   CCI Object-Class is 44.

   CCI Object-Type is 1 for the MPLS label.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            CC-ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Reserved1            |             Flags         |C|O|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Label                 |     Reserved2         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                        Optional TLV                         //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 8: CCI Object

   The fields in the CCI object are as follows:

   CC-ID:  A PCEP-specific identifier for the CCI information.  A PCE
      creates a CC-ID for each instruction; the value is unique within
      the scope of the PCE and is constant for the lifetime of a PCEP
      session.  The values 0 and 0xFFFFFFFF are reserved and MUST NOT be
      used.  Note that [SECURITY-ID] gives advice on assigning transient
      numeric identifiers, such as the CC-ID, so as to minimize security
      risks.

   Reserved1 (16 bit):  Set to 'zero' while sending; ignored on receipt.

   Flags (16 bit):  A field used to carry any additional information
      pertaining to the CCI.  Currently, the following flag bits are
      defined:

      *  O bit (out-label) : If the bit is set to '1', it specifies the
         label is the out-label, and it is mandatory to encode the next-
         hop information (via Address TLVs (Section 7.3.1) in the CCI
         object).  If the bit is not set, it specifies the label is the
         in-label, and it is optional to encode the local interface
         information (via Address TLVs in the CCI object).

      *  C Bit (PCC allocation): If the bit is set to '1', it indicates
         that the label allocation needs to be done by the PCC for the
         Central Controller Instruction.  A PCE sets this bit to request
         the PCC to make an allocation from its label space.  A PCC
         would set this bit to indicate that it has allocated the label
         and report it to the PCE.

      *  All unassigned bits MUST be set to 'zero' at transmission and
         ignored at receipt.

   Label (20-bit):  The label information.

   Reserved2 (12 bit):  Set to 'zero' while sending; ignored on receive.

7.3.1.  Address TLVs

   [RFC8779] defines the IPV4-ADDRESS, IPV6-ADDRESS, and UNNUMBERED-
   ENDPOINT TLVs for the use of Generalized Endpoint.  The same TLVs can
   also be used in the CCI object to associate the next-hop information
   in the case of an outgoing label and local interface information in
   the case of an incoming label.  The next-hop information encoded in
   these TLVs needs to be a directly connected IP address/interface
   information.  If the PCC is not able to resolve the next-hop
   information, it MUST reject the CCI and respond with a PCErr message
   with Error-Type=31 (PCECC failure) and Error-value=5 (Invalid next-
   hop information).

8.  Security Considerations

   As per [RFC8283], the security considerations for a PCE-based
   controller are a little different from those for any other PCE
   system.  That is, the operation relies heavily on the use and
   security of PCEP, so consideration should be given to the security
   features discussed in [RFC5440] and the additional mechanisms
   described in [RFC8253].  It further lists the vulnerability of a
   central controller architecture, such as a central point of failure,
   denial of service, and a focus for interception and modification of
   messages sent to individual Network Elements (NEs).

   In the PCECC operations, the PCEP sessions are also required to the
   internal routers, thus increasing the resources required for the
   session management at the PCE.

   The PCECC extension builds on the existing PCEP messages; thus, the
   security considerations described in [RFC5440], [RFC8231], and
   [RFC8281] continue to apply.  [RFC8253] specifies the support of
   Transport Layer Security (TLS) in PCEP, as it provides support for
   peer authentication, message encryption, and integrity.  It further
   provides mechanisms for associating peer identities with different
   levels of access and/or authoritativeness via an attribute in X.509
   certificates or a local policy with a specific accept-list of X.509
   certificates.  This can be used to check the authority for the PCECC
   operations.  Additional considerations are discussed in following
   sections.

8.1.  Malicious PCE

   In this extension, the PCE has complete control over the PCC to
   download/remove the labels and can cause the LSPs to behave
   inappropriately and cause a major impact to the network.  As a
   general precaution, it is RECOMMENDED that this PCEP extension be
   activated on mutually authenticated and encrypted sessions across
   PCEs and PCCs belonging to the same administrative authority, using
   TLS [RFC8253], as per the recommendations and best current practices
   in BCP 195 [RFC7525].

   Further, an attacker may flood the PCC with the PCECC-related
   messages at a rate that exceeds either the PCC's ability to process
   them or the network's ability to send them, by either spoofing
   messages or compromising the PCE itself.  [RFC8281] provides a
   mechanism to protect the PCC by imposing a limit.  The same can be
   used for the PCECC operations as well.

   As specified in Section 5.5.3.1, a PCC needs to check if the label in
   the CCI object is in the range set aside for the PCE; otherwise, it
   MUST send a PCErr message with Error-Type=31 (PCECC failure) and
   Error-value=1 (Label out of range).

8.2.  Malicious PCC

   The PCECC mechanism described in this document requires the PCE to
   keep labels (CCI) that it downloads and relies on the PCC responding
   (with either an acknowledgment or an error message) to request for
   LSP instantiation.  This is an additional attack surface by placing a
   requirement for the PCE to keep a CCI/label replica for each PCC.  It
   is RECOMMENDED that PCE implementations provide a limit on resources
   (in this case the CCI) a single PCC can occupy.  [RFC8231] provides a
   notification mechanism when such threshold is reached.

9.  Manageability Considerations

9.1.  Control of Function and Policy

   A PCE or PCC implementation SHOULD allow the PCECC capability to be
   enabled/disabled as part of the global configuration.  Section 6.1 of
   [RFC8664] list various controlling factors regarding the Path Setup
   Type.  They are also applicable to the PCECC Path Setup Types.
   Further, Section 6.2 of [RFC8664] describes the migration steps when
   the Path Setup Type of an existing LSP is changed.

9.2.  Information and Data Models

   [RFC7420] describes the PCEP MIB; this MIB can be extended to get the
   PCECC capability status.

   The PCEP YANG module [PCEP-YANG] could be extended to enable/disable
   the PCECC capability.

9.3.  Liveness Detection and Monitoring

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

9.4.  Verify Correct Operations

   The operator needs the following information to verify that PCEP is
   operating correctly with respect to the PCECC Path Setup Type.

   *  An implementation SHOULD allow the operator to view whether the
      PCEP speaker sent the PCECC PST capability to its peer.

   *  An implementation SHOULD allow the operator to view whether the
      peer sent the PCECC PST capability.

   *  An implementation SHOULD allow the operator to view whether the
      PCECC PST is enabled on a PCEP session.

   *  If one PCEP speaker advertises the PCECC PST capability, but the
      other does not, then the implementation SHOULD create a log to
      inform the operator of the capability mismatch.

   *  If a PCEP speaker rejects a CCI, then it SHOULD create a log to
      inform the operator, giving the reason for the decision (local
      policy, label issues, etc.).

9.5.  Requirements on Other Protocols

   PCEP extensions defined in this document do not put new requirements
   on other protocols.

9.6.  Impact on Network Operations

   PCEP extensions defined in this document do not put new requirements
   on network operations.

10.  IANA Considerations

10.1.  PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators

   [RFC8408] detailed the creation of the "PATH-SETUP-TYPE-CAPABILITY
   Sub-TLV Type Indicators" subregistry.  Further, IANA has allocated
   the following codepoint:

                 +=======+==================+===========+
                 | Value | Meaning          | Reference |
                 +=======+==================+===========+
                 | 1     | PCECC-CAPABILITY | RFC 9050  |
                 +-------+------------------+-----------+

                   Table 2: PATH-SETUP-TYPE-CAPABILITY
                   Sub-TLV Type Indicators Subregistry
                                 Addition

10.2.  PCECC-CAPABILITY Sub-TLV's Flag Field

   This document defines the PCECC-CAPABILITY sub-TLV; IANA has created
   a new subregistry to manage the value of the PCECC-CAPABILITY sub-
   TLV's 32-bit Flag field.  New values are to be assigned by Standards
   Action [RFC8126].  Each bit should be tracked with the following
   qualities:

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

   *  capability description

   *  defining RFC

   Currently, there is one allocation in this registry.

                     +======+============+===========+
                     | Bit  | Name       | Reference |
                     +======+============+===========+
                     | 0-30 | Unassigned | RFC 9050  |
                     +------+------------+-----------+
                     | 31   | Label      | RFC 9050  |
                     +------+------------+-----------+

                        Table 3: Initial Contents of
                        the PCECC-CAPABILITY Sub-TLV
                                Subregistry

10.3.  PCEP Path Setup Type Registry

   [RFC8408] created a subregistry within the "Path Computation Element
   Protocol (PCEP) Numbers" registry called "PCEP Path Setup Types".
   IANA has allocated a new codepoint within this registry, as follows:

            +=======+============================+===========+
            | Value | Description                | Reference |
            +=======+============================+===========+
            | 2     | Traffic engineering path   | RFC 9050  |
            |       | is set up using PCECC mode |           |
            +-------+----------------------------+-----------+

               Table 4: Path Setup Type Registry Codepoint
                                 Addition

10.4.  PCEP Object

   IANA has allocated new codepoints in the "PCEP Objects" subregistry
   for the CCI object as follows:

     +==============+=============+=====================+===========+
     | Object-Class | Name        | Object-Type         | Reference |
     | Value        |             |                     |           |
     +==============+=============+=====================+===========+
     | 44           | CCI Object- | 0: Reserved         | RFC 9050  |
     |              | Type        | 1: MPLS Label       |           |
     |              |             | 2-15: Unassigned    |           |
     +--------------+-------------+---------------------+-----------+

               Table 5: PCEP Objects Subregistry Additions

10.5.  CCI Object Flag Field

   IANA has created a new subregistry to manage the Flag field of the
   CCI object called "CCI Object Flag Field for MPLS Label".  New values
   are to be assigned by Standards Action [RFC8126].  Each bit should be
   tracked with the following qualities:

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

   *  capability description

   *  defining RFC

   Two bits are defined for the CCI Object flag field in this document
   as follows:

        +======+======================================+===========+
        | Bit  | Description                          | Reference |
        +======+======================================+===========+
        | 0-13 | Unassigned                           |           |
        +------+--------------------------------------+-----------+
        | 14   | C Bit - PCC allocation               | RFC 9050  |
        +------+--------------------------------------+-----------+
        | 15   | O Bit - Specifies label is out-label | RFC 9050  |
        +------+--------------------------------------+-----------+

           Table 6: CCI Object Flag Field for MPLS Label Initial
                                  Contents

10.6.  PCEP-Error Object

   IANA has allocated new error types and error values within the "PCEP-
   ERROR Object Error Types and Values" subregistry of the "Path
   Computation Element Protocol (PCEP) Numbers" registry for the
   following errors:

      +============+===========+=======================+===========+
      | Error-Type | Meaning   | Error-value           | Reference |
      +============+===========+=======================+===========+
      | 6          | Mandatory | 17: CCI object        | RFC 9050  |
      |            | Object    | missing               |           |
      |            | missing   |                       |           |
      +------------+-----------+-----------------------+-----------+
      | 10         | Reception | 33: Missing PCECC     | RFC 9050  |
      |            | of an     | Capability sub-TLV    |           |
      |            | invalid   |                       |           |
      |            | object    |                       |           |
      +------------+-----------+-----------------------+-----------+
      | 19         | Invalid   | 16: Attempted PCECC   | RFC 9050  |
      |            | Operation | operations when PCECC |           |
      |            |           | capability was not    |           |
      |            |           | advertised            |           |
      |            |           |                       |           |
      |            |           | 17: Stateful PCE      |           |
      |            |           | capability was not    |           |
      |            |           | advertised            |           |
      |            |           |                       |           |
      |            |           | 18: Unknown Label     |           |
      +------------+-----------+-----------------------+-----------+
      | 31         | PCECC     | 1: Label out of range | RFC 9050  |
      |            | failure   |                       |           |
      |            |           | 2: Instruction failed |           |
      |            |           |                       |           |
      |            |           | 3: Invalid CCI        |           |
      |            |           |                       |           |
      |            |           | 4: Unable to allocate |           |
      |            |           | the specified CCI     |           |
      |            |           |                       |           |
      |            |           | 5: Invalid next-hop   |           |
      |            |           | information           |           |
      +------------+-----------+-----------------------+-----------+

       Table 7: PCEP-ERROR Object Error Types and Values Additions

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>.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, DOI 10.17487/RFC5511, April
              2009, <https://www.rfc-editor.org/info/rfc5511>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8779]  Margaria, C., Ed., Gonzalez de Dios, O., Ed., and F.
              Zhang, Ed., "Path Computation Element Communication
              Protocol (PCEP) Extensions for GMPLS", RFC 8779,
              DOI 10.17487/RFC8779, July 2020,
              <https://www.rfc-editor.org/info/rfc8779>.

11.2.  Informative References

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

   [RFC7025]  Otani, T., Ogaki, K., Caviglia, D., Zhang, F., and C.
              Margaria, "Requirements for GMPLS Applications of PCE",
              RFC 7025, DOI 10.17487/RFC7025, September 2013,
              <https://www.rfc-editor.org/info/rfc7025>.

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

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

   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,
              <https://www.rfc-editor.org/info/rfc7491>.

   [RFC8232]  Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
              and D. Dhody, "Optimizations of Label Switched Path State
              Synchronization Procedures for a Stateful PCE", RFC 8232,
              DOI 10.17487/RFC8232, September 2017,
              <https://www.rfc-editor.org/info/rfc8232>.

   [RFC8283]  Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An
              Architecture for Use of PCE and the PCE Communication
              Protocol (PCEP) in a Network with Central Control",
              RFC 8283, DOI 10.17487/RFC8283, December 2017,
              <https://www.rfc-editor.org/info/rfc8283>.

   [RFC8741]  Raghuram, A., Goddard, A., Karthik, J., Sivabalan, S., and
              M. Negi, "Ability for a Stateful Path Computation Element
              (PCE) to Request and Obtain Control of a Label Switched
              Path (LSP)", RFC 8741, DOI 10.17487/RFC8741, March 2020,
              <https://www.rfc-editor.org/info/rfc8741>.

   [PCECC]    Li, Z. (., Dhody, D., Zhao, Q., Ke, K., Khasanov, B.,
              Fang, L., Zhou, C., Zhang, B., Rachitskiy, A., and A.
              Gulida, "The Use Cases for Path Computation Element (PCE)
              as a Central Controller (PCECC).", Work in Progress,
              Internet-Draft, draft-ietf-teas-pcecc-use-cases-07, 8
              March 2021, <https://datatracker.ietf.org/doc/html/draft-
              ietf-teas-pcecc-use-cases-07>.

   [PCEP-YANG]
              Dhody, D., Ed., Hardwick, J., Beeram, V., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-16, 22 February
              2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pce-pcep-yang-16>.

   [PCECC-SR] Li, Z., Peng, S., Negi, M. S., Zhao, Q., and C. Zhou,
              "PCEP Procedures and Protocol Extensions for Using PCE as
              a Central Controller (PCECC) for Segment Routing (SR) MPLS
              Segment Identifier (SID) Allocation and Distribution.",
              Work in Progress, Internet-Draft, draft-ietf-pce-pcep-
              extension-pce-controller-sr-02, 25 March 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              pcep-extension-pce-controller-sr-02>.

   [PCECC-SRv6]
              Li, Z., Peng, S., Geng, X., and M. S. Negi, "PCEP
              Procedures and Protocol Extensions for Using PCE as a
              Central Controller (PCECC) for SRv6", Work in Progress,
              Internet-Draft, draft-dhody-pce-pcep-extension-pce-
              controller-srv6-06, 21 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-dhody-pce-
              pcep-extension-pce-controller-srv6-06>.

   [PCE-ID]   Li, C., Chen, M., Wang, A., Cheng, W., and C. Zhou, "PCE
              Controlled ID Space", Work in Progress, Internet-Draft,
              draft-li-pce-controlled-id-space-08, 22 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-li-pce-
              controlled-id-space-08>.

   [SECURITY-ID]
              Gont, F. and I. Arce, "Security Considerations for
              Transient Numeric Identifiers Employed in Network
              Protocols", Work in Progress, Internet-Draft, draft-gont-
              numeric-ids-sec-considerations-06, 5 December 2020,
              <https://datatracker.ietf.org/doc/html/draft-gont-numeric-
              ids-sec-considerations-06>.

Acknowledgments

   We would like to thank Robert Tao, Changjing Yan, Tieying Huang,
   Avantika, and Aijun Wang for their useful comments and suggestions.

   Thanks to Julien Meuric for shepherding this document and providing
   valuable comments.  Thanks to Deborah Brungard for being the
   responsible AD.

   Thanks to Victoria Pritchard for a very detailed RTGDIR review.
   Thanks to Yaron Sheffer for the SECDIR review.  Thanks to Gyan Mishra
   for the Gen-ART review.

   Thanks to Alvaro Retana, Murray Kucherawy, Benjamin Kaduk, Roman
   Danyliw, Robert Wilton, Éric Vyncke, and Erik Kline for the IESG
   review.

Contributors

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

   Email: dhruv.ietf@gmail.com


   Satish Karunanithi
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: satishk@huawei.com


   Adrian Farrel
   Old Dog Consulting
   United Kingdom

   Email: adrian@olddog.co.uk


   Xuesong Geng
   Huawei Technologies
   China

   Email: gengxuesong@huawei.com


   Udayasree Palle

   Email: udayasreereddy@gmail.com


   Katherine Zhao
   Futurewei Technologies

   Email: katherine.zhao@futurewei.com


   Boris Zhang
   Telus Ltd.
   Toronto
   Canada

   Email: boris.zhang@telus.com


   Alex Tokar
   Cisco Systems
   Slovakia

   Email: atokar@cisco.com


Authors' Addresses

   Zhenbin Li
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing
   100095
   China

   Email: lizhenbin@huawei.com


   Shuping Peng
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing
   100095
   China

   Email: pengshuping@huawei.com


   Mahendra Singh Negi
   RtBrick Inc
   N-17L, 18th Cross Rd, HSR Layout
   Bangalore 560102
   Karnataka
   India

   Email: mahend.ietf@gmail.com


   Quintin Zhao
   Etheric Networks
   1009 S Claremont St.
   San Mateo, CA 94402
   United States of America

   Email: qzhao@ethericnetworks.com


   Chao Zhou
   HPE

   Email: chaozhou_us@yahoo.com