Internet-Draft STATE-REPORTING-EXTENSIONS October 2024
Sidor, et al. Expires 19 April 2025 [Page]
Workgroup:
PCE Working Group
Internet-Draft:
draft-sidor-pce-lsp-state-reporting-extensions-00
Published:
Intended Status:
Standards Track
Expires:
Authors:
S. Sidor
Cisco Systems, Inc.
Z. Ali
Cisco Systems, Inc.
C. Li
Huawei Technologies

LSP State Reporting Extensions in Path Computation Element Communication Protocol (PCEP)

Abstract

Path Computation Element Communication Protocol (PCEP) is a protocol defined in multiple RFCs for enabling communication between a Path Computation Elements (PCEs) and Path Computation Clients (PCCs).

Although PCEP defines various LSP identifiers, attributes, and constraints, there are operational attributes available on the PCC that can enhance path computation and improve the debugging experience, which are not currently supported in PCEP.

This document proposes extensions to PCEP to include:

These extensions aim to address the existing gaps and enhance the overall functionality and operational efficiency of PCEP

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 https://datatracker.ietf.org/drafts/current/.

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 19 April 2025.

Table of Contents

1. Introduction

A Stateful Path Computation Element (PCE) maintains comprehensive information on the current network state, including computed Label Switched Path (LSPs), reserved network resources, and the pending path computation requests. This information is critical for computing path for traffic-engineering LSPs and any associated or dependent LSPs.

[RFC9604] outlines the usage of Binding labels/ Segment Identifiers (SID) usage for an RSVP-TE-signaled Label Switched Paths (LSPs) and Segment-routing Traffic Engineering paths. It specifies the possibility of a PCE explicitly requesting the allocation of a specific binding value by a PCC. However, [RFC9604] only considers the option of rejecting entire request if the specified binding value is unavailable, but section 6.2 of [RFC9256] allows also fallback to a dynamically allocated binding value. This document introduces the possibility for a PCC to accept such request and include originally specified binding value for which allocation failed, as well as a binding value binding value allocated from the dynamic range as a fallback.

This document specifies a set of extensions to Path Computation Element Communication Protocol(PCEP) to enhance the accuracy of path computations by considering whether the Binding label/SID of an LSP can be utilized in the path computation for another LSP, based on LSP transit eligibility, for example as described in case of LSP stitching in [I-D.ietf-pce-stateful-interdomain].

Additionally, this document introduces the ability to encode information regarding whether a path included in an Explicit Route Object (ERO) was specified explicitly or it is the result of dynamic path computation executed by a PCE or PCC. Such information can help in debuggability and can be used by other PCEs in the network to avoid triggering unnecessary path computations for LSPs where it is not intended (e.g., PCE-initiated LSPs with explicit path).

The mechanisms described in this document are applicable to all path setup types.

2. Terminology

The following terminologies are used in this document:

BSID:
Binding Segment Identifier.
ERO:
Explicit Route Object.
IGP:
Interior Gateway Protocol.
NAI:
Node or Adjacency Identifier.
P2P:
Point-to-Point.
PCE:
Path Computation Element.
PCEP:
Path Computation Element Protocol.
SID:
Segment Identifier.
SR:
Segment Routing.
SR-TE:
Segment Routing Traffic Engineering.
LSP:
Label Switched Path.
LSPA:
Label Switched Path Attributes.

3. Overview of Extensions to PCEP

3.1. STATEFUL-PCE-CAPABILITY TLV Flag

A new flag is proposed in the STATEFUL-PCE-CAPABILITY TLV, originally defined in Section 5.4 of [RFC8231].

  • F (Fallback): If set, indicates that the PCEP peer supports dynamic fallback to a binding value if the specific binding value requested by the PCE is unavailable, as detailed in Section 4.1 of this document.

3.2. LSP-EXTENDED-FLAG TLV Flags

New flags are introduced in the LSP-EXTENDED-FLAG TLV, which was initially defined in Section 3.1 of [RFC9357].

  • E (Explicit): If set, indicates that the path encoded in the Explicit Route Object (ERO) is explicitly specified and not dynamically computed by the PCEP peer.

  • T (Transit Eligible): If set, indicates that the binding value of the LSP can be used in paths computed for other LSPs.

3.3. TE-PATH-BINDING TLV Flag

A new flag proposed in the TE-PATH-BINDING TLV, which was originally defined in Section 4 of [RFC9604].

  • A (Allocated): If set, indicates that the binding value encoded in the TLV represents an allocated binding value, as described in Section 4.1 of this document.

4. Operation

The PCEP protocol extensions defined in Section 3.3 of this draft MUST NOT be used if one or both PCEP speakers have not indicated support for the extensions using the F flag in the STATEFUL-PCE-CAPABILITY TLV in their respective OPEN messages.

4.1. Binding Label/SID Dynamic Fallback

[RFC9604] specifies the possibility for a PCE to explicitly request the allocation of a specific binding value by a PCC. If the specified binding value is unavailable, the entire request MUST be rejected. However, if both PCEP peers advertised support for the F flag in STATEFUL-PCE-CAPABILITY TLV and the specified binding value in the PCInitiate or PCUpdate message is unavailable, the PCC MUST fallback to binding value allocation from the dynamic range, as described in Section 6.2 of [RFC9256]. Since the originally requested binding value and the allocated binding value are different, two instances of the TE-PATH-BINDING TLV MUST be included in the PCRpt message:

  • A TLV instance with requested binding value with the A flag cleared

  • A TLV instance with allocated binding value with the A flag set

4.2. Explicit or Dynamic Path

[RFC9256] describes various types of Segment Routing (SR) Policy Candidate Paths and methods to identify them. Specifically, Sections 5.1 and 5.2 are describing explicit and dynamic candidate paths, but there is currently no way to encode this information in PCEP. A similar limitation applies to LSPs of other path setup types.

For instance, if an operator requests the creation of a PCE-Initiated Candidate Path with an Explicit Path, then such path will be encoded in the ERO object of the PCInitiate message sent to the PCC. If the delegation of such LSP is transferred to another PCE, the new PCE will not know whether the path of the LSP was computed dynamically or explicitly specified by the operator.

Even if similar problem does not exist for LSPs originated on the PCC, information about the type of path may be valuable for other purposes, such as debuggability.

For LSPs initiated by PCC, the E flag value is initially set by the PCC in the PCRpt message and the PCE MUST set the flag value in PCUpdate messages for such LSP based on the last reported state.

For PCE-initiated LSPs, the E flag value is initially set by the PCE in PCInitiate message but MAY be modified in the PCUpdate messages. The PCC MUST set the flag value in PCRpt messages for such LSP based on the value received from the last PCInitiate or PCUpdate message.

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 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 introduced in Section 3.1 for PCEP peer.

5.3. Verify Correct Operations

Operation verification requirements already listed in [RFC5440], [RFC8231], [RFC8281] and [RFC8664] are applicable to mechanisms defined in this document.

5.4. 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".

7. Security Considerations

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

8. IANA Considerations

8.1. STATEFUL-PCE-CAPABILITY TLV Flag

IANA maintains a registry, named "STATEFUL-PCE-CAPABILITY TLV Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the STATEFUL-PCE-CAPABILITY TLV. IANA is requested to make the following assignment:

Table 1
Bit Description Reference
 
TBA1 F (Fallback) This document

8.2. TE-PATH-BINDING TLV Flag

IANA maintains a registry, named "TE-PATH-BINDING TLV Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the TE-PATH-BINDING TLV. IANA is requested to make the following assignment:

Table 2
Bit Description Reference
 
TBA2 A (Allocated) This document

8.3. LSP-EXTENDED-FLAG TLV Flags

IANA maintains a registry, named "LSP-EXTENDED-FLAG TLV Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry group to manage the Flags field of the LSP-EXTENDED-FLAG TLV. IANA is requested to make the following assignment:

Table 3
Bit Description Reference
 
TBA3 E (Explicit) This document
TBA4 T (Transit Eligible) This document

9. References

9.1. Normative References

[I-D.ietf-pce-pcep-yang]
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, , <https://datatracker.ietf.org/doc/html/draft-ietf-pce-pcep-yang-25>.
[I-D.ietf-pce-stateful-interdomain]
Dugeon, O., Meuric, J., Lee, Y., and D. Ceccarelli, "PCEP Extension for Stateful Inter-Domain Tunnels", Work in Progress, Internet-Draft, draft-ietf-pce-stateful-interdomain-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-pce-stateful-interdomain-05>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <https://www.rfc-editor.org/info/rfc5440>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc8281>.
[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, , <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, , <https://www.rfc-editor.org/info/rfc9256>.
[RFC9357]
Xiong, Q., "Label Switched Path (LSP) Object Flag Extension for Stateful PCE", RFC 9357, DOI 10.17487/RFC9357, , <https://www.rfc-editor.org/info/rfc9357>.
[RFC9604]
Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S., and C. Li, Ed., "Carrying Binding Label/SID in PCE-Based Networks", RFC 9604, DOI 10.17487/RFC9604, , <https://www.rfc-editor.org/info/rfc9604>.

9.2. Informative References

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

Appendix A. Contributors

Rajesh Melarcode Venkateswaran
Cisco Systems, Inc.
Email: rmelarco@cisco.com

Authors' Addresses

Samuel Sidor
Cisco Systems, Inc.
Eurovea Central 3.
Pribinova 10
811 09 Bratislava
Slovakia
Zafar Ali
Cisco Systems, Inc.
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China