Internet-Draft PCEP extensions for CS Policies June 2024
Sidor, et al. Expires 12 December 2024 [Page]
PCE Working Group
Intended Status:
Standards Track
S. Sidor
Cisco Systems, Inc.
P. Maheshwari
Airtel India
A. Stone
L. Jalil
S. Peng
Huawei Technologies

PCEP extensions for Circuit Style Policies


This document proposes a set of extensions for Path Computation Element Communication Protocol (PCEP) for Circuit Style Policies - Segment-Routing Policy designed to satisfy requirements for connection-oriented transport services. New TLV is introduced to control path recomputation and new flag to add ability to request path with strict hops only.

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.

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

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 12 December 2024.

Table of Contents

1. Introduction

Usage of Segment-routing and PCEP in connection-oriented transport services require path persistency and hop-by-hop behavior for PCE computed paths.

Circuit-Style Policy introduced in [I-D.ietf-spring-cs-sr-policy] requires PCEP extensions, which are covered in this document.

This document:

PCEP extensions described in this document can be used with any Path Setup Type.

2. Terminology

The following terminologies are used in this document:

Explicit Route Object
Interior Gateway Protocol
Label Switched Path.
Label Switched Path Attributes.
Optical Transport Network.
Path Computation Client
Path Computation Element
Path Computation Element Protocol.
Synchronous Digital Hierarchy
Segment Identifier
Synchronous Optical Network
Segment Routing.
Segment Routing Traffic Engineering.

3. Overview of Extensions to PCEP


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 Strict-Path flag in LSP-EXTENDED-FLAG TLV. See Section 4.1 for details.
  • PATH-RECOMPUTATION-CAPABILITY - 1 bit (Bit Position 19) - If set to 1, it indicates support for PATH-RECOMPUTATION TLV. See Section 4.2 for details.

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

O-flag is proposed in the LSP-EXTENDED-FLAG TLV, which was introduced in 3.1 of [RFC9357].

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. Strict hop definition is described in Section 4.1


This document defines new TLV for the LSPA Object for encoding information whether path recomputation is allowed for delegated LSP. The TLV is optional. If the TLV is included in LSPA object, the PCE MUST NOT recompute path in cases specified by flags in the TLV. Only the first instance of this TLV SHOULD be processed, subsequent instances SHOULD 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|

Type (16 bits): the value is 72

Length (16 bits): 4 octets

MUST be set to zero by the sender and MUST be ignored by the receiver.

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 recompute path even if current path is not satisfying path computation constraints. If this flag is cleared, then the PCE SHOULD recompute path if original path is invalidated.
  • F (Force): If set to 1, the PCE MUST NOT update path. If flag is cleared, the PCE MAY update path based on explicit request from operator.

4. Operation

4.1. Strict path enforcement

PCC MAY set the O flag in LSP-EXTENDED-FLAG TLV in PCRpt message to the PCE to indicate that a path exclusively made of strict hops is required. It MUST NOT be set to 1 if one or both PCEP speakers have not set STRICT-PATH-CAPABILITY to 1 in STATEFUL-PCE-CAPABILITY TLV. If PCEP peer received LSP-EXTENDED-FLAG TLV with O flag set, but it does not support that flag, it MUST send PCErr with Error-Type = 2 (Capability not supported).

O flag cleared or LSP-EXTENDED-FLAG TLV not included indicates that a loose path is acceptable.

In PCUpd or PCInitiate messages, PCE MAY set O bit if strict path is provided.

The flag is applicable only for stateful messages. Existing O flag in RP object MAY be used to indicate similar behavior in PCReq and PCRep messages as described in as described in Section 7.4.1 of [RFC5440].

If O flag is set to 1 for both stateful and stateless messages for SR paths introduced in [RFC8664], PCE MUST use only SIDs, which will use explicitly specified adjacencies for packet forwarding. For example Adjacency SIDs MAY be used, but Prefix SIDs MUST NOT be used (even if there is only one adjacency).

4.2. Path recomputation

PCC MAY set flags in PATH-RECOMPUTATION TLV to control path computation behavior on PCE side. If 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-RECOMPUTATION 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 presence of the TLV is blocking path recomputation based on various triggers like topology update, any periodic update or changed state of other LSPs in the network. LSP path MAY be modified if forwarded packets will still use same path - for example if same path can be encoded using Adjacency and Prefix SIDs, then PCE MAY switch between various representations of same path.

If P flag is cleared, the PCE MAY recompute if current path is not considered valid, for example after topology update resulting in path not satisfying LSP's path constraints, but it MUST NOT recompute path if current path is not optimal.

If P flag is set, the PCE MUST NOT recompute path during LSP lifetime even if path is invalidated. Only exception is explicit request from operator to recompute path

If F flag is cleared, path update triggered manually by operator or any northbound interface of PCE MAY be done. If flag is set the PCE CAN update path only to tear down LSP by sending PCUpd message with empty ERO.

TLV MAY be included in PCInitiate and PCUpd messages to indicate, which triggers will be disabled on the PCE. PCC should reflect flag values in PCRpt messages to forward requirement to other PCEs in the network.

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, requirements and considerations listed in this section apply.

5.1. Control of Function and Policy

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

5.2. Information and Data Models

An implementation SHOULD allow the operator to view the PCEP peer capability defined in this document. Section 4.1 and 4.1.1 of [I-D.ietf-pce-pcep-yang] should be extended to include that capability for PCEP peer.

Section 4.2 of [I-D.ietf-pce-pcep-yang] module should be extended to add notification for blocked recomputation satisfying specified constraints if recomputation is blocked using PATH-RECOMPUTATION TLV.

5.3. Liveness Detection and Monitoring

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

5.4. Verify Correct Operations

A PCE implementation SHOULD notify operator in case blocked recomputation for LSP, which is no longer satisfying specified constraints and it SHOULD allow the operator to view LSPs on PCE, which are not satisfying specified constraints.

5.5. Requirements On Other Protocols

The PCEP extensions defined in this document do not imply any new requirements on other protocols. Overall concept of Circuit Style policies requires interaction with other protocols, but those requirements are already 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.

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:

7. Security Considerations

The security considerations described in [RFC5440], [RFC8231], [RFC8253],[RFC8281] and [RFC8664] in itself.

Note that this specification introduces possibility to block path recomputation 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 traffic drop.

8. IANA Considerations


[RFC8231] defines the STATEFUL-PCE-CAPABILITY. IANA is requested to make the following assignment from the "STATEFUL-PCE-CAPABILITY TLV Flag Field" registry:

Table 1
Bit Description Reference


[RFC9357] defines the LSP-EXTENDED-FLAG TLV. IANA is requested to make the following assignment from the "LSP-EXTENDED-FLAG TLV Flag Field" registry:

Table 2
Bit Description Reference
4 Strict-Path Flag (O) This document


IANA is requested to make the assignment of a new value for the existing "PCEP TLV Type Indicators" registry as follows:

Table 3
TLV Type TLV Name Reference


IANA has created a new subregistry named "PATH-RECOMPUTATION TLV Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by "Standards Action" [RFC8126]. The registry contains the following codepoints, with initial values, to be assigned by IANA with the reference set to this document:

Table 4
Bit Description Reference
1 Force This document
2 Permanent This document

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
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, , <>.
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, , <>.
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, , <>.
Xiong, Q., "Label Switched Path (LSP) Object Flag Extension for Stateful PCE", RFC 9357, DOI 10.17487/RFC9357, , <>.

9.2. Informative References

Dhody, D., Beeram, V. P., Hardwick, J., and J. Tantsura, "A YANG Data Model for Path Computation Element Communications Protocol (PCEP)", Work in Progress, Internet-Draft, draft-ietf-pce-pcep-yang-25, , <>.
Schmutzer, C., Ali, Z., Maheshwari, P., Rokui, R., and A. Stone, "Circuit Style Segment Routing Policies", Work in Progress, Internet-Draft, draft-ietf-spring-cs-sr-policy-02, , <>.
Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, , <>.
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, , <>.
International Telecommunication Union, "Network node interface for the synchronous digital hierarchy (SDH)", ITU-T Recommendation G.707, .
American National Standards Institute, "Synchronous Optical Network (SONET) Basic Description including Multiplex Structure, Rates, and Formats", ANSI T1.105, .


Daniel Voyer
Bell Canada
Reza Rokui
Tarek Saad
Cisco Systems, Inc.
Zafar Ali
Cisco Systems, Inc.
Ran Chen
ZTE Corporation
Quan Xiong
ZTE Corporation
Dhruv Dhody

Authors' Addresses

Samuel Sidor
Cisco Systems, Inc.
Eurovea Central 3.
811 09 Bratislava
Praveen Maheshwari
Airtel India
Andrew Stone
Luay Jalil
Shuping Peng
Huawei Technologies