



PCE Working Group                                               S. Sidor
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                           P. Maheshwari
Expires: 10 August 2026                                     Airtel India
                                                                A. Stone
                                                                   Nokia
                                                                L. Jalil
                                                                 Verizon
                                                                 S. Peng
                                                     Huawei Technologies
                                                         6 February 2026


 Path Computation Element Communication Protocol (PCEP) extensions for
                         Circuit Style Policies
            draft-ietf-pce-circuit-style-pcep-extensions-13

Abstract

   Segment Routing (SR) enables a node to steer packet flows along a
   specified path without the need for intermediate per-path states, due
   to the utilization of source routing.  An SR Policy can consist of
   one or a set of candidate paths, where each candidate path is
   represented by a segment list or a set of segment lists, which are
   essentially instructions that define a source-routed path.

   This document specifies a set of extensions to the Path Computation
   Element Communication Protocol (PCEP) for Segment Routing Policies
   that are designed to satisfy requirements for connection-oriented
   transport services (Circuit-Style SR policies).  They include the
   ability to control path modification and the option to request a
   strict hop-by-hop path, being also applicable for generic SR policy
   use cases where controlling path modification or deterministic and
   persistent path requirements are applicable.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.







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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 10 August 2026.

Copyright Notice

   Copyright (c) 2026 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
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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  PCEP Extensions . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  New Flags in STATEFUL-PCE-CAPABILITY TLV  . . . . . . . .   5
     3.2.  New Flag in the LSP-EXTENDED-FLAG TLV . . . . . . . . . .   6
     3.3.  PATH-MODIFICATION TLV . . . . . . . . . . . . . . . . . .   6
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Strict Path Enforcement . . . . . . . . . . . . . . . . .   7
     4.2.  Path Modification Control . . . . . . . . . . . . . . . .   8
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .   9
     5.1.  Control of Function and Policy  . . . . . . . . . . . . .   9
     5.2.  Information and Data Models . . . . . . . . . . . . . . .   9
     5.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  10
     5.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  10
     5.5.  Requirements On Other Protocols . . . . . . . . . . . . .  10
     5.6.  Impact On Network Operations  . . . . . . . . . . . . . .  10
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Cisco . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  STATEFUL-PCE-CAPABILITY . . . . . . . . . . . . . . . . .  12
     8.2.  LSP-EXTENDED-FLAG TLV . . . . . . . . . . . . . . . . . .  12
     8.3.  PATH-MODIFICATION TLV . . . . . . . . . . . . . . . . . .  12
     8.4.  PATH-MODIFICATION TLV Flag Field  . . . . . . . . . . . .  13



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     8.5.  PCEP-Error Object . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Segment Routing (SR) [RFC8402] leverages the source routing paradigm,
   where the sender of a packet defines the path that the packet takes
   through the network.  This is achieved by encoding the path
   information as a sequence of segments within the packet header.  SR
   can be applied to both MPLS and IPv6 data planes, providing a
   flexible and scalable method for traffic engineering.

   The Path Computation Element (PCE) is a network component,
   application, or node that is capable of computing a network path or
   route based on a network graph and applying computational
   constraints.  The PCE Communication Protocol (PCEP) enables
   communication between a PCE and Path Computation Clients (PCCs),
   facilitating the computation of optimal paths for traffic flows.

   [RFC9256] introduces the concept of Segment Routing Policy (SR
   Policy), which is one or a set of candidate paths that can be used to
   steer traffic through a network.  Each candidate path is represented
   by a segment list or a set of segment lists, and the path can be
   dynamically adjusted based on network conditions and requirements.

   In connection-oriented transport services, such as those described in
   [I-D.ietf-spring-cs-sr-policy], there is a need for path persistency
   and per-hop behavior for PCE-computed paths.  This ensures that the
   paths remain stable and predictable, which is crucial for services
   that require high reliability and performance guarantees.

   To support the requirements of connection-oriented transport
   services, this document specifies extensions to PCEP to enable the
   use of Circuit Style Policies [I-D.ietf-spring-cs-sr-policy].  These
   extensions allow for the request of strict hop-by-hop paths from the
   PCE, the encoding of information to disable path modification for
   specific paths, and the clarification of the usage of existing flags
   within PCEP messages.

   The PCEP extensions described in this document are designed to be
   compatible with any Path Setup Type and are not limited to Circuit
   Style SR policies, ensuring broad applicability across different
   network environments and use cases.




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1.1.  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.

2.  Terminology

   This document uses the following term defined in [RFC3031]:

   *  Label Switched Path (LSP)

   Note: The base PCEP specification [RFC4655] originally defined the
   use of the PCE architecture for MPLS and GMPLS networks with LSPs
   instantiated using the RSVP-TE signaling protocol.  Over time,
   support for additional path setup types such as SRv6 has been
   introduced [RFC9603].  The term "LSP" is used extensively in PCEP
   specifications, and in the context of this document, refers to a
   Candidate Path within an SR Policy, which may be an SRv6 path (still
   represented using the LSP object as specified in [RFC8231]).

   This document uses the following terms defined in [RFC5440]:

   *  Explicit Route Object (ERO)

   *  LSP Attributes (LSPA)

   *  Path Computation Client (PCC)

   *  Path Computation Element (PCE)

   *  Path Computation Element Protocol (PCEP)

   *  PCEP Peer

   *  PCEP speaker

   This document uses the following terms defined in [RFC8402]:

   *  Segment Routing (SR)

   *  Segment Identifier (SID)

   This document uses the following terms defined in [RFC9256]:

   *  SR Policy



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   This document uses the following term described in
   [I-D.ietf-spring-cs-sr-policy]:

   *  Circuit Style (CS) SR Policy

   This document defines the following term:

   *  Path Modification: Refers to the PCE instructing the PCC, via a
      PCUpd message, to use a path whose set of traversed network hops
      differs from the current path.  A PCUpd message that changes only
      the attributes or re-encodes the same hop sequence (e.g.,
      alternative SID representation) is not considered a path
      modification.

3.  PCEP Extensions

   This section specifies the PCEP extensions that enable a PCC and PCE
   to support CS SR policies.  These extensions build on the base PCEP
   [RFC5440] and the Stateful PCE extensions [RFC8231].  The mechanisms
   defined here allow a PCC or PCE to:

   *  Indicate the requirement for strict hop-by-hop paths,

   *  Signal path persistency by disabling path modification for
      specific paths,

   *  Identify and control behavior specific to CS SR policies.

   Unless explicitly stated, the procedures of existing PCEP messages
   and objects remain unchanged.  The following subsections describe the
   specific object formats, TLVs, and flag definitions introduced to
   realize this functionality.

3.1.  New Flags in STATEFUL-PCE-CAPABILITY TLV

   The STATEFUL-PCE-CAPABILITY TLV is an optional TLV introduced in
   [RFC8231] in the OPEN object for stateful PCEP peer capability
   advertisement.  This document defines the following new flags in that
   TLV:

   *  STRICT-PATH-CAPABILITY - 1 bit (Bit Position 18) - If set to 1, it
      indicates support for the Strict-Path flag in LSP-EXTENDED-FLAG
      TLV.  See Section 4.1 for details.

   *  PATH-MODIFICATION-CAPABILITY - 1 bit (Bit Position 19) - If set to
      1, it indicates support for PATH-MODIFICATION TLV.  See
      Section 4.2 for details.




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3.2.  New Flag in the LSP-EXTENDED-FLAG TLV

   The LSP-EXTENDED-FLAG TLV was introduced in Section 3.1 of [RFC9357].
   This document specifies the new Strict-Path flag in the LSP-EXTENDED-
   FLAG TLV.

   O (Strict-Path) - 1 bit (Bit Position 4): If set to 1, this indicates
   to the PCE that a path exclusively made of strict hops is required.
   The strict hop definition is described in Section 4.1

3.3.  PATH-MODIFICATION TLV

   This document defines a new TLV for the LSPA Object for encoding
   information whether path modification is allowed for a delegated LSP.
   The PATH-MODIFICATION TLV is optional.  If the TLV is included in
   LSPA object, the PCE MUST NOT modify the path in the cases specified
   by flags in the TLV.  Only the first instance of this TLV MUST be
   processed, subsequent instances MUST be ignored.

      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 = 72           |             Length = 4        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Reserved          |      Flags                |P|F|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: PATH-MODIFICATION TLV Format

   Type (16 bits): 72.

   Length (16 bits): 4.

   Reserved (16 bits):  This field MUST be set to zero on transmission
      and MUST be ignored on receipt.

   Flags (16 bits):  This document defines the following flag bits.  The
      other bits MUST be set to zero by the sender and MUST be ignored
      by the receiver.

      *  P (Permanent): If set to 1, the PCE MUST NOT modify the path
         even if the current path does not satisfy path computation
         constraints.  If this flag is cleared, then the PCE MAY modify
         the path according to local policy if the original path is
         invalidated.  When F is set to 1, the P flag value is ignored.






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      *  F (Force): If set to 1, the PCE MUST NOT modify the path
         (exceptions description in Section 4.2).  If the flag is
         cleared, the PCE MAY update the path based on an explicit
         request from the operator.

4.  Operation

4.1.  Strict Path Enforcement

   The STRICT-PATH-CAPABILITY flag in the STATEFUL-PCE-CAPABILITY TLV
   MUST be set to 1 by both PCEP speakers during the PCEP session
   establishment to support strict hop-by-hop path enforcement.  The O
   flag MUST NOT be set to 1 if the STRICT-PATH-CAPABILITY flag was not
   set to 1 by both PCEP speakers.  If the PCEP peer received LSP-
   EXTENDED-FLAG TLV with the O flag set to 1, but it does not support
   that capability, it MUST send PCErr with Error-Type = 2 (Capability
   not supported).  To indicate that a path exclusively made of strict
   hops is required, the PCC sets the O flag to 1 in the LSP-EXTENDED-
   FLAG TLV in a PCRpt message sent to the PCE.

   The O flag set to 0 or LSP-EXTENDED-FLAG TLV not included indicates
   that a non-strictly hop-by-hop path is acceptable.

   For PCE-initiated LSPs, the PCE MAY set the O flag to 1 in PCInitiate
   or PCUpd messages.  If the PCE sets the O flag to 1, the PCC MUST
   also set the O flag to 1 in the LSP-EXTENDED-FLAG TLV in the
   corresponding PCRpt messages.  For PCC-initiated LSPs, if the PCC
   requested a strict path (by setting the O flag to 1 in the PCRpt
   message), the PCE MUST set the O flag to 1 in the corresponding PCUpd
   message.  Even if the PCC did not request a strict path, the PCE MAY
   set the O flag to 1 in the PCUpd message if the computed path is a
   strict hop-by-hop path.

   The flag is applicable only for stateful messages.  Existing O flag
   in Request Parameters (RP) object can be used to indicate similar
   behavior in PCReq and PCRep messages as described in Section 7.4.1 of
   [RFC5440].  For RSVP-TE, [RFC5440] already defines the strict/loose
   indication for stateless PCEP; this document extends a corresponding
   indication to stateful messages via the LSP-EXTENDED-FLAG TLV.

   If the O flag is set to 1 (either in the LSP-EXTENDED-FLAG TLV for
   stateful messages or in the RP object for stateless messages) for SR
   paths introduced in [RFC8664], the PCE MUST use only Segment
   Identifiers (SIDs) that explicitly specify adjacencies for packet
   forwarding.  Adjacency SIDs should be used, but Prefix SIDs must not
   be used (even if there is only one adjacency).





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4.2.  Path Modification Control

   A PCC MAY set flags in PATH-MODIFICATION TLV to control path
   modification behavior on the PCE side.  If the PATH-MODIFICATION TLV
   is not included, then the PCE MAY use local policy to trigger path
   computation or LSP path update.

   If a PCEP speaker does not recognize the PATH-MODIFICATION TLV, it
   MUST ignore the TLV based on Section 7.1 of [RFC5440].  If a PCEP
   speaker recognizes the TLV but does not support the TLV, it MUST send
   PCErr with Error-Type = 2 (Capability not supported).  The LSP path
   MAY be modified, if the change results in a semantically equivalent
   path representation (e.g., a different SID list) that preserves the
   exact sequence of traversed network links.  If the same path can be
   encoded using Adjacency, Binding, Prefix, or other SIDs, then PCE MAY
   switch between various representations of the same path.

   The PATH-MODIFICATION TLV defines the path modification behavior for
   an LSP.  It is important to note that regardless of the flag settings
   described below, a PCE can always initiate an update to tear down the
   LSP (e.g., by sending a PCUpd message with an empty ERO) or to bring
   it up again with the same path it had before being torn down.  The P
   and F flags specifically restrict the PCE's ability to initiate a
   path modification:

   TLV present, P=0, F=0:
      The PCE MUST NOT modify the path in response to various triggers
      (E.g. topology updates, periodic reoptimization timers, or changes
      in the state of other LSPs) if the current path remains valid and
      meets all constraints (e.g. it is not the most optimal path, but
      it is still valid and satisfies all constraints including bounds).
      However, the PCE MAY modify the path if:

      *  The current path is invalidated (e.g., due to a topology change
         that makes it non-compliant with LSP constraints).

      *  An operator explicitly triggers a path modification via an
         implementation-specific mechanism (e.g., a Command Line
         Interface (CLI) or northbound Application Programming Interface
         (API) on the PCE).

   P flag set (P=1):
      The PCE MUST NOT modify the path due to network or optimization
      triggers, even if the path becomes invalidated or no longer
      satisfies its constraints.  A path modification MAY be initiated
      if explicitly triggered by an operator.  This behavior applies
      only when F=0; when F=1, the value of P is ignored.




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   F flag set (F=1):
      The PCE MUST NOT modify the path for any reason, including in
      response to an explicit operator trigger.

   A PCE includes the PATH-MODIFICATION TLV in PCInitiate and PCUpd
   messages to define which triggers will be disabled for an LSP.  When
   a PCC receives and applies behavior specified by flags in the TLV, it
   MUST reflect the active flag values in the PATH-MODIFICATION TLV of
   its PCRpt messages for that LSP.  By including this TLV, the PCC
   ensures that the LSP's path modification policy is consistently
   communicated to all connected PCEs.

   When a PCC receives a PCUpd message with a path modification for an
   LSP, where such a modification is blocked by flags in the PATH-
   MODIFICATION TLV (e.g., the F flag is set to 1), it MUST reject the
   update and maintain the existing path for the LSP.The PCC MUST also
   send a PCErr message to the PCE with Error-Type=19 ("Invalid
   Operation") and Error-Value=TBD1 ("Path modification is blocked by
   constraint").

5.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC8231] and [RFC8281] apply to PCEP protocol extensions
   defined in this document.  In addition, the requirements and
   considerations listed in this section apply.

5.1.  Control of Function and Policy

   A PCE or PCC implementation SHOULD allow the capability of supporting
   PCEP extensions introduced in this document to be enabled/disabled as
   part of the global configuration.

   When path modification is restricted (e.g., when the P flag is set to
   1 and F flag is set to 0), the PCE relies on an explicit operator
   trigger to modify the path if it becomes invalid.  Therefore, a PCE
   implementation SHOULD provide a mechanism to allow an operator to
   explicitly trigger path modification for a specific LSP.

5.2.  Information and Data Models

   An implementation SHOULD allow an operator to view the PCEP peer
   capability defined in this document.  Section 4.1 and 4.1.1 of
   [RFC9826] should be extended to include that capability for PCEP
   peer.






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   Section 4.2 of [RFC9826] module SHOULD be extended to add
   notification for blocked path modification that satisfies specified
   constraints if path modification is blocked using the PATH-
   MODIFICATION TLV.

5.3.  Liveness Detection and Monitoring

   Circuit-Style Policy draft [I-D.ietf-spring-cs-sr-policy] describes
   connectivity verification and path validity considerations for
   Circuit Style Policies.

5.4.  Verify Correct Operations

   A PCE implementation SHOULD notify the operator in case of blocked
   path modification for an LSP that no longer satisfies specified
   constraints.  It SHOULD also allow the operator to view LSPs on the
   PCE that does not satisfy specified constraints.

5.5.  Requirements On Other Protocols

   The PCEP extensions defined in this document do not imply any new
   requirements on other protocols.  The overall concept of Circuit
   Style policies requires interaction with other protocols, but those
   requirements are described in [I-D.ietf-spring-cs-sr-policy].

5.6.  Impact On Network Operations

   The mechanisms defined in [RFC5440], [RFC8231], and [RFC8281] also
   apply to the PCEP extensions defined in this document.

6.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to RFC 7942.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.




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   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

6.1.  Cisco

   *  Organization: Cisco Systems

   *  Implementation: IOS-XR PCC and PCE.

   *  Description: PCEP extensions supported using VENDOR-INFORMATION
      Object.

   *  Maturity Level: Production.

   *  Coverage: Partial.

   *  Contact: ssidor@cisco.com

7.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8253],[RFC8281] and [RFC8664] are applicable to this document.

   Note that this specification introduces the possibility to block path
   modification after various topology events.  This creates an
   additional vulnerability if the security mechanisms of [RFC5440],
   [RFC8231], and [RFC8281] are not used.  If there is no integrity
   protection on the session, then an attacker could block path updates
   from PCE potentially resulting in a traffic drop.

   As per [RFC8231], it is RECOMMENDED that these PCEP extensions can
   only be activated on authenticated and encrypted sessions across PCEs
   and PCCs belonging to the same administrative authority, using
   Transport Layer Security (TLS) [RFC8253][I-D.ietf-pce-pceps-tls13] as
   per the recommendations and best current practices in [RFC9325].

8.  IANA Considerations

   IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
   registry at <https://www.iana.org/assignments/pcep>.







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8.1.  STATEFUL-PCE-CAPABILITY

   [RFC8231] defines the STATEFUL-PCE-CAPABILITY.  IANA is requested to
   confirm the following allocations within the "STATEFUL-PCE-CAPABILITY
   TLV Flag Field" registry of the "Path Computation Element Protocol
   (PCEP) Numbers" registry group:

          +=====+==============================+===============+
          | Bit | Description                  | Reference     |
          +=====+==============================+===============+
          |  18 | STRICT-PATH-CAPABILITY       | This document |
          +-----+------------------------------+---------------+
          |  19 | PATH-MODIFICATION-CAPABILITY | This document |
          +-----+------------------------------+---------------+

                                 Table 1

8.2.  LSP-EXTENDED-FLAG TLV

   [RFC9357] defines the LSP-EXTENDED-FLAG TLV.  IANA is requested to
   confirm the following allocation within the "LSP-EXTENDED-FLAG TLV
   Flag Field" registry of the "Path Computation Element Protocol (PCEP)
   Numbers" registry group:

              +=====+======================+===============+
              | Bit | Description          | Reference     |
              +=====+======================+===============+
              |  4  | Strict-Path Flag (O) | This document |
              +-----+----------------------+---------------+

                                 Table 2

8.3.  PATH-MODIFICATION TLV

   IANA is requested to confirm the following allocation within the
   "PCEP TLV Type Indicators" registry of the "Path Computation Element
   Protocol (PCEP) Numbers" registry group:

           +==========+=======================+===============+
           | TLV Type | TLV Name              | Reference     |
           +==========+=======================+===============+
           |    72    | PATH-MODIFICATION TLV | This document |
           +----------+-----------------------+---------------+

                                 Table 3






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8.4.  PATH-MODIFICATION TLV Flag Field

   IANA is requested to create a new registry named "PATH-MODIFICATION
   TLV Flag Field" within the "Path Computation Element Protocol (PCEP)
   Numbers" registry group.  Values are to be assigned by "IETF Review"
   [RFC8126].  Each bit should be tracked with the following qualities:

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

   *  Description

   *  Reference

   The registry contains the following codepoints, with initial values,
   to be assigned by IANA with the reference set to this document:

                  +======+=============+===============+
                  | Bit  | Description | Reference     |
                  +======+=============+===============+
                  | 0-13 | Unassigned  |               |
                  +------+-------------+---------------+
                  |  14  | Permanent   | This document |
                  +------+-------------+---------------+
                  |  15  | Force       | This document |
                  +------+-------------+---------------+

                                 Table 4

8.5.  PCEP-Error Object

   IANA is requested to allocate new error types and error values within
   the "PCEP-ERROR Object Error Types and Values" sub-registry of the
   PCEP Numbers registry for the following errors.

     +============+===========+==========================+===========+
     | Error-Type | Meaning   | Error-Value              | Reference |
     +============+===========+==========================+===========+
     |     19     | Invalid   | TBD1:Path modification   | This      |
     |            | Operation | is blocked by constraint | Document  |
     +------------+-----------+--------------------------+-----------+

                                  Table 5

9.  References

9.1.  Normative References





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   [I-D.ietf-pce-pceps-tls13]
              Dhody, D., Turner, S., and R. Housley, "Updates for PCEPS:
              TLS Connection Establishment Restrictions", Work in
              Progress, Internet-Draft, draft-ietf-pce-pceps-tls13-04, 9
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-pce-pceps-tls13-04>.

   [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>.

   [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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.



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   [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>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC9325]  Sheffer, Y., Saint-Andre, P., and T. Fossati,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
              2022, <https://www.rfc-editor.org/info/rfc9325>.

   [RFC9357]  Xiong, Q., "Label Switched Path (LSP) Object Flag
              Extension for Stateful PCE", RFC 9357,
              DOI 10.17487/RFC9357, February 2023,
              <https://www.rfc-editor.org/info/rfc9357>.

9.2.  Informative References

   [I-D.ietf-spring-cs-sr-policy]
              Schmutzer, C., Ali, Z., Maheshwari, P., Rokui, R., and A.
              Stone, "Circuit Style Segment Routing Policy", Work in
              Progress, Internet-Draft, draft-ietf-spring-cs-sr-policy-
              14, 28 January 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              cs-sr-policy-14>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.





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   [RFC9603]  Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
              and Y. Zhu, "Path Computation Element Communication
              Protocol (PCEP) Extensions for IPv6 Segment Routing",
              RFC 9603, DOI 10.17487/RFC9603, July 2024,
              <https://www.rfc-editor.org/info/rfc9603>.

   [RFC9826]  Dhody, D., Ed., Beeram, V., Hardwick, J., and J. Tantsura,
              "A YANG Data Model for the Path Computation Element
              Communication Protocol (PCEP)", RFC 9826,
              DOI 10.17487/RFC9826, September 2025,
              <https://www.rfc-editor.org/info/rfc9826>.

Contributors

   Daniel Voyer
   Bell Canada
   Email: daniel.voyer@bell.ca


   Reza Rokui
   Ciena
   Email: rrokui@ciena.com


   Tarek Saad
   Cisco Systems, Inc.
   Email: tsaad.net@gmail.com


   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com


   Ran Chen
   ZTE Corporation
   Email: chen.ran@zte.com.cn


   Quan Xiong
   ZTE Corporation
   Email: xiong.quan@zte.com.cn


   Dhruv Dhody
   Huawei
   Email: dhruv.ietf@gmail.com




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   Christian Schmutzer
   Cisco Systems, Inc.
   Email: cschmutz@cisco.com


Authors' Addresses

   Samuel Sidor
   Cisco Systems, Inc.
   Eurovea Central 3.
   811 09 Bratislava
   Slovakia
   Email: ssidor@cisco.com


   Praveen Maheshwari
   Airtel India
   Email: Praveen.Maheshwari@airtel.com


   Andrew Stone
   Nokia
   Email: andrew.stone@nokia.com


   Luay Jalil
   Verizon
   Email: luay.jalil@verizon.com


   Shuping Peng
   Huawei Technologies
   Email: pengshuping@huawei.com


















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