RFC8786



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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  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: [email protected]