Internet-Draft PCEP extensions for CS Policies July 2023
Sidor, et al. Expires 7 January 2024 [Page]
PCE Working Group
Intended Status:
Standards Track
S. Sidor
Cisco Systems, Inc.
Z. Ali
Cisco Systems, Inc.
P. Maheshwari
Airtel India
R. Rokui
A. Stone
L. Jalil
S. Peng
Huawei Technologies
T. Saad
Juniper Networks
D. Voyer
Bell Canada

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 7 January 2024.

Table of Contents

1. Introduction

Usage of Segment-routing and PCEP in connection-oriented transport services require path persistancy 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 are applicable to RSVP-TE and SR-TE.

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


O-flag is proposed in the LSP-EXTENDED-FLAG TLV, which was introduced in 3.1 of [RFC9357] and extended with E-flag in [I-D.peng-pce-entropy-label-position]. TLV format will be added after assigning O flag bit position by IANA.

Type (16 bits): the value is TBD1 by IANA.

Length (16 bits): multiple of 4 octets.

O (Strict-Path): 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.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|           Type = TBD2        |             Length = 4         |
|             Reserved         |      Flags                 |P|F|

Type (16 bits): the value is TBD2 by IANA.

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. Otherwise, if this flag is cleared, then the PCE MAY 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.

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

In PCUpdate 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). the PCE MUST use Adjacency SIDs only.

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.

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 PCUpdate message with empty ERO.

TLV MAY be included in PCInitiate and PCUpdate 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. Security Considerations

No additional security measure is required.

6. IANA Considerations


[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 1
Bit Description Reference
TBD1 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 2
TLV Type TLV Name Reference

7. References

7.1. Normative References

Xiong, Q., Peng, S., and F. Qin, "PCEP Extension for SR-MPLS Entropy Label Position", Work in Progress, Internet-Draft, draft-peng-pce-entropy-label-position-09, , <>.
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, , <>.
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, , <>.
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, , <>.

7.2. Informative References

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-00, , <>.
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, .

Authors' Addresses

Samuel Sidor
Cisco Systems, Inc.
Eurovea Central 3.
Pribinova 10
811 09 Bratislava
Zafar Ali
Cisco Systems, Inc.
Praveen Maheshwari
Airtel India
Reza Rokui
Andrew Stone
Luay Jalil
Shuping Peng
Huawei Technologies
Tarek Saad
Juniper Networks
Daniel Voyer
Bell Canada