Skip to main content

Updated Rules for PCE Communication Protocol (PCEP) Object Ordering
draft-dhody-pce-pcep-object-order-06

Document Type Active Internet-Draft (individual)
Author Dhruv Dhody
Last updated 2024-07-04
RFC stream (None)
Intended RFC status (None)
Formats
Stream Stream state (No stream defined)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-dhody-pce-pcep-object-order-06
PCE Working Group                                               D. Dhody
Internet-Draft                                                    Huawei
Updates: 5440 (if approved)                                  4 July 2024
Intended status: Standards Track                                        
Expires: 5 January 2025

  Updated Rules for PCE Communication Protocol (PCEP) Object Ordering
                  draft-dhody-pce-pcep-object-order-06

Abstract

   The PCE Communication Protocol (PCEP) defines the mechanisms for the
   communication between a Path Computation Client (PCC) and a PCE, or
   among PCEs.  Such interactions include path computation requests and
   path computation replies defined in RFC 5440.  As per RFC 5440, these
   message are required to follow strict object ordering.

   This document updates RFC 5440 by relaxing the strict object ordering
   requirement in the PCEP messages.

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 5 January 2025.

Copyright Notice

   Copyright (c) 2024 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
   and restrictions with respect to this document.  Code Components

Dhody                    Expires 5 January 2025                 [Page 1]
Internet-Draft                object-order                     July 2024

   extracted from this document must include Revised BSD License text as
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Update to RFC 5440  . . . . . . . . . . . . . . . . . . . . .   3
   5.  Compatibility Considerations  . . . . . . . . . . . . . . . .   3
   6.  Open Questions  . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  Management Considerations . . . . . . . . . . . . . . . . . .   4
   8.  Other Efforts . . . . . . . . . . . . . . . . . . . . . . . .   4
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   4
     11.2.  Informative References . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Examples . . . . . . . . . . . . . . . . . . . . . .   6
   Appendix C.  When Order Matters . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   [RFC5440] describes the PCE Communication Protocol (PCEP).  PCEP
   defines the communication between a Path Computation Client (PCC) and
   a PCE, or between PCEs, enabling computation of Multiprotocol Label
   Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP)
   characteristics.

   [RFC5440] defines several PCEP messages.  For each PCEP message type,
   rules are defined that specify the set of objects that the message
   can carry using Routing Backus-Naur Form (RBNF) [RFC5511].  Further,
   [RFC5440] states that the object ordering is mandatory.  This causes
   confusion when multiple extensions add new objects in the PCEP
   messages and the respective order of these new objects is not
   specified (see [EID6627]).

   This document updates [RFC5440] to relax the strict object ordering
   requirement.

Dhody                    Expires 5 January 2025                 [Page 2]
Internet-Draft                object-order                     July 2024

2.  Conventions

   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.

3.  Motivation

   The mandatory object ordering requirement in [RFC5440] is shown to
   result in exponential complexity in terms of what each new PCEP
   extension needs to cope with in terms of reconciling all of the
   previously published RFCs, and all concurrently work in progress in
   the form of the internet-drafts.  This requirement does not lend
   itself to the extensibility of PCEP.

4.  Update to RFC 5440

   Section 6 of [RFC5440] states:

      An implementation MUST form the PCEP
      messages using the object ordering specified in this document.

   This text is updated to read as follows:

      An implementation SHOULD form the PCEP
      messages using the object ordering specified in this and
      subsequent documents when an ordering can be unambiguously
      determined; an implementation MUST be prepared to receive
      a PCEP message with objects in any order when possible.

   This update does not aim to take away the object ordering completely.
   The PCEP speaker is expected to follow the object order as specified
   unless there are valid reasons to ignore it.  It is also likely that
   the receiver can understand the object's meaning irrespective of the
   order unambiguously.

5.  Compatibility Considerations

   While one of the main objectives of the changes made by this document
   is to enable backward compatibility between PCEP extensions, there
   remains an issue of compatibility between existing implementations of
   [RFC5440] and implementations that are consistent with this document.

   It should be noted that common behaviour for checking object ordering
   in received PCEP messages is as described by the updated text
   presented in Section 4.  Thus, many implementations will still have

Dhody                    Expires 5 January 2025                 [Page 3]
Internet-Draft                object-order                     July 2024

   implemented a consistent and future-proof approach.  However, for
   completeness, it is worth noting how behaviours might interact
   between implementations.

   The messages generated by an implementation of this document when
   received by a legacy implementation with a strict interpretation of
   object ordering MAY lead to error handling.  It is interesting to
   note that the [RFC5440] does not define an Error-Type and Error-value
   corresponding to this error condition.

6.  Open Questions

   *  Should a new flag or a TLV in Open Message be added to exchange
      this capability?  Not sure if this is strictly needed if we can
      live with Section 5.

7.  Management Considerations

   Implementations receiving set objects that they consider out of order
   MAY log this.  That could be helpful for diagnosing backward
   compatibility issues.

8.  Other Efforts

   In the past, there have been efforts to consolidate and update the
   RBNF such as in [I-D.cmfg-pce-pcep-grammar].  This document document
   relaxes the object ordering only, it does not take on the various
   other issues or the need to consolidate the RBNF for all PCEP
   extensions.  There have been proposals to consolidate the RBNF for
   the PCEP message in a single place in GitHub and use modern data
   modelling tools to represent PCEP extensions.  They might be taken up
   in parallel.

9.  Security Considerations

   This document does not raise any security issues.

10.  IANA Considerations

   This document does not require any IANA actions.

11.  References

11.1.  Normative References

Dhody                    Expires 5 January 2025                 [Page 4]
Internet-Draft                object-order                     July 2024

   [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/rfc/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/rfc/rfc5440>.

   [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
              Used to Form Encoding Rules in Various Routing Protocol
              Specifications", RFC 5511, DOI 10.17487/RFC5511, April
              2009, <https://www.rfc-editor.org/rfc/rfc5511>.

   [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/rfc/rfc8174>.

11.2.  Informative References

   [EID6627]  "Errata ID: 6627", n.d.,
              <https://www.rfc-editor.org/errata/eid6627>.

   [I-D.cmfg-pce-pcep-grammar]
              Casellas, R., Margaria, C., Farrel, A., de Dios, O. G.,
              Dhody, D., and X. Zhang, "Current issues with existing
              RBNF notation for PCEP messages and extensions", Work in
              Progress, Internet-Draft, draft-cmfg-pce-pcep-grammar-02,
              10 January 2014, <https://datatracker.ietf.org/doc/html/
              draft-cmfg-pce-pcep-grammar-02>.

   [RFC5455]  Sivabalan, S., Ed., Parker, J., Boutros, S., and K.
              Kumaki, "Diffserv-Aware Class-Type Object for the Path
              Computation Element Communication Protocol", RFC 5455,
              DOI 10.17487/RFC5455, March 2009,
              <https://www.rfc-editor.org/rfc/rfc5455>.

   [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/rfc/rfc8231>.

Dhody                    Expires 5 January 2025                 [Page 5]
Internet-Draft                object-order                     July 2024

Appendix A.  Acknowledgments

   Thanks to John Scudder for the motivation behind this document.
   Thanks to Oscar Gonzalez de Dios and Cyril Margaria for raising
   errata on this topic.  Thanks to the author of
   [I-D.cmfg-pce-pcep-grammar] for highlighting the issue.

Appendix B.  Examples

   As described in [EID6627], for the PCReq message, the CLASSTYPE
   object is encoded after the END-POINTS object in [RFC5455].  Whereas
   in [RFC8231], the LSP object is encoded just after the END-POINTS
   object.  So it is not known which of the below orders is expected.

   ...<END-POINTS>[<LSP>][<CLASSTYPE>]...

   or

   ...<END-POINTS>[<CLASSTYPE>][<LSP>]...

   This update requires the receiver to be able to accept both of these.

Appendix C.  When Order Matters

   There are cases where the ordering between objects is important.  For
   instance, PCRpt message [RFC8231] includes <path> with some
   attributes that say BANDWIDTH can be part of both <actual-attribute-
   list> and <intended-attribute-list>.

       Where:
         <path>::= <intended-path>
                   [<actual-attribute-list><actual-path>]
                   <intended-attribute-list>

   An important factor to distinguish between the actual and intended
   attribute list is the presence of RRO (i.e. <actual-path>) and the
   order of objects in the PCRpt message.

   If the RRO is present, any attributes encoded before it, are to be
   considered as part of <actual-attribute-list> and those after it, as
   part of <intended-attribute-list>.

   If the RRO is absent, all attributes are part of <intended-attribute-
   list>.

   Thus the approach taken by this document is to say that ordering is
   relaxed in cases where there is no ambiguity.

Dhody                    Expires 5 January 2025                 [Page 6]
Internet-Draft                object-order                     July 2024

Author's Address

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

Dhody                    Expires 5 January 2025                 [Page 7]