Internet-Draft PCEP extensions for Circuit Style Polici March 2022
Sidor, et al. Expires 5 September 2022 [Page]
PCE Working Group
Intended Status:
Standards Track
S. Sidor
Cisco Systems, Inc.
Z. Ali
Cisco Systems, Inc.
P. Maheshwari
Airtel India

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 triggers 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 5 September 2022.

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.schmutzer-pce-cs-sr-policy] requires PCEP extensions, which are covered in this document.

This document:

  • Introduces possibility to request strict path from the PCE by extending LSP-EXTENDED-FLAG TLV
  • Adding new TLV for encoding blocked path recomputation triggers to the PCE is introduced, to be carried inside the LSP object, which is defined in [RFC8231].
  • Clarifies usage of existing O-flag from RP object in Segment-routing

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 5.1.2 of [I-D.ietf-pce-lsp-extended-flags] and extended with E-flag in [I-D.peng-pce-entropy-label-position]

The format of the LSP-EXTENDED-FLAG is as follows:

       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=TBD1           |          Length               |
       |                                                           |O|E|
       //                 LSP Extended Flags                          //
       |                                                               |

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 can be found in Section 4.1


This document defines new TLV for the LSP Object for encoding information about blocked path recomputation triggers.

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                 |T|P|

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.

  • T (Topology-change): If set to 1, the PCE MUST NOT trigger recomputation as a result of received updated topology information.
  • P (Periodic-timer): If set to 1, the PCE MUST NOT trigger recomputation based on any periodic timer.

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, when the O bit is set, this indicates that 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], the PCE MUST use Adjacency SIDs only.

4.2. Path computation triggers

PCC MAY set flags in RECOMPUTATION-TRIGGERS-TLV to block specific triggers. If TLV is not included or all flags are set to 0, then the PCE MAY use any event to start path computation.

Disabled recomputation triggered by topology event is not blocking path computation started based PCRpt or based on updated state of associated LSP.

If trigger blocked by specific flag is not supported or allowed on the PCE, then PCE MAY ignore received flag value. The PCE SHOULD reflect blocked triggers in PCUpdate message.

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


[I-D.ietf-pce-lsp-extended-flags] 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., "LSP Object Flag Extension of Stateful PCE", Work in Progress, Internet-Draft, draft-ietf-pce-lsp-extended-flags-01, , <>.
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-07, , <>.
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, , <>.

7.2. Informative References

Schmutzer, C., Filsfils, C., Ali, Z., and F. Clad, "Circuit Style Segment Routing Policies", Work in Progress, Internet-Draft, draft-schmutzer-pce-cs-sr-policy-00, , <>.

Authors' Addresses

Samuel Sidor
Cisco Systems, Inc.
Eurovea Central 3.
Pribinova 10
811 09 Bratislava
Zafar Ali
Cisco Systems, Inc.
Praveen Maheshwari
Airtel India