



Internet Engineering Task Force (IETF)                         A. Farrel
Request for Comments: 8786                            Old Dog Consulting
Updates: 8231                                                   May 2020
Category: Standards Track                                               
ISSN: 2070-1721


   Updated Rules for Processing Stateful PCE Request Parameters Flags

Abstract

   Extensions to the Path Computation Element Communication Protocol
   (PCEP) to support stateful Path Computation Elements (PCEs) are
   defined in RFC 8231.  One of the extensions is the Stateful PCE
   Request Parameters (SRP) object.  That object includes a Flags field
   that is a set of 32 bit flags, and RFC 8281 defines an IANA registry
   for tracking assigned flags.  However, RFC 8231 does not explain how
   an implementation should set unassigned flags in transmitted
   messages, nor how an implementation should process unassigned,
   unknown, or unsupported flags in received messages.

   This document updates RFC 8231 by defining the correct behaviors.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8786.

Copyright Notice

   Copyright (c) 2020 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Updated Procedures
     3.1.  Advice for Specification of New Flags
     3.2.  Flags Field of the SRP Object
   4.  Compatibility Considerations
   5.  Management Considerations
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

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

   [RFC8231] specifies a set of extensions to PCEP to enable stateful
   control of LSPs within and across PCEP sessions in compliance with
   [RFC4657].  It includes mechanisms to effect Label Switched Path
   (LSP) State Synchronization between PCCs and PCEs, delegation of
   control over LSPs to PCEs, and PCE control of timing and sequence of
   path computations within and across PCEP sessions.

   One of the extensions defined in [RFC8231] is the Stateful PCE
   Request Parameters (SRP) object.  That object includes a Flags field
   that is a set of 32 bit flags, and RFC 8281 defines an IANA registry
   for tracking assigned flags.  However, RFC 8231 does not explain how
   an implementation should set unassigned flags in transmitted
   messages, nor how an implementation should process unassigned or
   unknown flags in received messages.

   Furthermore, [RFC8231] gives no guidance to the authors of future
   specifications about how to describe the interaction between flags
   that have already been defined and flags being defined in the new
   specifications.

   This document updates RFC 8231 by defining the correct behaviors.

2.  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.

3.  Updated Procedures

3.1.  Advice for Specification of New Flags

   Section 7 of [RFC8231] introduces changes to existing PCEP objects
   and defines new PCEP objects and TLVs in support of stateful PCE
   functionality.  That text does not advise future specifications on
   how to describe the interaction between flags that may be defined.

   The text in Section 7 of [RFC8231] is updated to read as follows:

      The PCEP objects defined in this document are compliant with the
      PCEP object format defined in [RFC5440].  The P and I flags of the
      PCEP objects defined in the current document MUST be set to 0 on
      transmission and SHOULD be ignored on receipt since they are
      exclusively related to path computation requests.

      The sections that follow define PCEP objects and TLVs that contain
      Flags fields, and some flag values are defined.  Future
      specifications may define further flags, and each new
      specification that defines additional flags is expected to
      describe the interaction between these new flags and any existing
      flags.  In particular, new specifications are expected to explain
      how to handle the cases when both new and pre-existing flags are
      set.

3.2.  Flags Field of the SRP Object

   Section 7.2 of [RFC8231] defines the PCEP SRP object.  It describes
   the Flags field as:

      Flags (32 bits): None defined yet.

   This document updates that text as follows:

      Flags (32 bits): This document does not define any flags.
      Unassigned flags MUST be set to zero on transmission and MUST be
      ignored on receipt.  Implementations that do not understand any
      particular flag MUST ignore the flag.

4.  Compatibility Considerations

   While one of the main objectives of the changes made by this document
   is to enable backward compatibility, there remains an issue of
   compatibility between existing implementations of RFC 8231 and
   implementations that are consistent with this document.

   It should be noted that common behavior for Flags fields is as
   described by the updated text presented in Section 3.  Thus, many
   implementations, lacking guidance from RFC 8231, will still have
   implemented a consistent and future-proof approach.  However, for
   completeness, it is worth noting how behaviors might interact between
   implementations.

   SRP objects generated by an implementation of this document will set
   all unknown flag bits to zero and will therefore cause no issues to
   an older implementation even if it inspects those bits.  Similarly,
   an implementation of this document will not inspect any unknown flag
   bits and will therefore be unaffected by older implementations no
   matter how they set the flags.

   There will remain an issue with compatibility between implementations
   and how they set the flags.  An implementation of RFC 8231 might set
   any of the unassigned flags, but an implementation of a future or
   current specification (such as [RFC8281] or [RFC8741]) assigns
   specific meanings to a flag if set.  That problem cannot be fixed in
   old implementations by any amount of documentation and can only be
   handled for future specifications by obsoleting the Flags field and
   using a new technique.  Fortunately, however, most implementations
   will have been constructed to set unused flags to zero, which is
   consistent with the behavior described in this document, and so the
   risk of bad interactions is sufficiently small that there is no need
   to obsolete the existing Flags field.

5.  Management Considerations

   Implementations receiving set SRP flags that they do not recognize
   MAY log this.  That could be helpful for diagnosing backward
   compatibility issues with future features that utilize those flags.

6.  Security Considerations

   [RFC8231] sets out security considerations for PCEP when used for
   communication with a stateful PCE.  This document does not change
   those considerations.

   However, by defining the expected behavior of implementations, this
   document may improve the stability of networks and thus reduce the
   attack surface.  That is, by reminding implementations to ignore
   unset bits, it is less possible to attack them by randomly tweaking
   bits.  Furthermore, by reminding implementations to leave undefined
   bits unset, the network is future-proofed against new definitions of
   previously undefined bits.

7.  IANA Considerations

   IANA maintains a registry called the "Path Computation Element
   Protocol (PCEP) Numbers" registry with a subregistry called "SRP
   Object Flag Field".  IANA has updated the reference for that
   subregistry to list this document in addition to [RFC8281].

8.  References

8.1.  Normative References

   [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/info/rfc2119>.

   [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/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, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [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, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

8.2.  Informative References

   [RFC4657]  Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

   [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/info/rfc5440>.

   [RFC8741]  Raghuram, A., Goddard, A., Karthik, J., Sivabalan, S., and
              M. Negi, "Ability for a Stateful Path Computation Element
              (PCE) to Request and Obtain Control of a Label Switched
              Path (LSP)", RFC 8741, DOI 10.17487/RFC8741, March 2020,
              <https://www.rfc-editor.org/info/rfc8741>.

Acknowledgements

   Thanks to the authors of [RFC8741] for exposing the need for this
   work.  Thanks to Dhruv Dhody and Julien Meuric for discussing the
   solution.  Additional thanks to Hariharan Ananthakrishnan for his
   Shepherd's review.  Thanks to Benjamin Kaduk and Alvaro Retana for
   helpful comments during IESG review.

Author's Address

   Adrian Farrel
   Old Dog Consulting

   Email: adrian@olddog.co.uk
