PCEP SRP flags rules: Updated Rules for Processing Stateful PCE Request Parameters Flags
Unassigned flags in the PCEP SRP object must be sent as zero and ignored when received.
Farrel, A., "Updated Rules for Processing Stateful PCE Request Parameters Flags", RFC 8786, DOI 10.17487/RFC8786, May 2020, <https://www.rfc-editor.org/info/rfc8786>.
@misc{rfc8786,
series = {Request for Comments},
number = 8786,
howpublished = {RFC 8786},
publisher = {RFC Editor},
doi = {10.17487/RFC8786},
url = {https://www.rfc-editor.org/info/rfc8786},
author = {A. Farrel},
title = {{Updated Rules for Processing Stateful PCE Request Parameters Flags}},
pagetotal = 6,
year = 2020,
month = may,
} RFC text
Contents
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 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 extracted from this document must 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
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§1The Gap in RFC 8231
The Path Computation Element Communication Protocol (PCEP, RFC 5440) carries messages between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between PCEs. RFC 8231 adds extensions for stateful PCEs, including the Stateful PCE Request Parameters (SRP) object.
The SRP object has a Flags field of 32 bits, and RFC 8281 defines an IANA registry for tracking assigned flags. RFC 8231 did not say how to set unassigned flags in transmitted messages, nor how to process unassigned or unknown flags in received messages. It also gave no guidance to authors of future specifications on how new flags should interact with existing ones.
The first rulebook gave the form a row of checkboxes but never said what to do with the ones nobody uses yet. This new rule fills that gap.
§3The Updated Rules
Two pieces of RFC 8231 are replaced. First, in Section 7 the P and I flags of the PCEP objects MUST be set to 0 on transmission and SHOULD be ignored on receipt, since they relate only to path computation requests. Future specifications that define additional flags are expected to describe how the new flags interact with existing ones, especially when both are set.
Second, the SRP Flags text, which used to say "None defined yet", now reads: this document defines no flags; unassigned flags MUST be set to zero on transmission and MUST be ignored on receipt; and implementations that do not understand a particular flag MUST ignore it.
Rule one: if a box has no meaning yet, leave it empty. Rule two: if you see a ticked box you do not know, pretend you did not see it.
§4Compatibility
An implementation that follows RFC 8786 sets all unknown flag bits to zero, so it causes no problem for an older implementation even if that one inspects the bits. It also does not inspect unknown bits, so it is unaffected by how older implementations set them.
One risk remains. An RFC 8231 implementation might set an unassigned flag, while a current or future specification such as RFC 8281 or RFC 8741 gives that flag a meaning. The RFC says this cannot be fixed in old implementations. Since most implementations set unused flags to zero, the risk is small enough that there is no need to obsolete the Flags field.
Most people already left unused boxes empty, so changing the rule rarely causes trouble. A few older programs might tick boxes by accident, but that is rare.
§5Logging Unknown Flags
An implementation that receives set SRP flags it does not recognize MAY log this. The RFC notes the log could help diagnose backward compatibility problems with future features that use those flags.
If you meet a ticked box you do not know, you may write a note about it. The note can help someone work out later what went wrong.
§6Security and IANA
This document does not change the security considerations of RFC 8231. It says that defining expected behavior may improve network stability and so reduce the attack surface: ignoring unset bits makes it harder to attack implementations by randomly tweaking bits, and leaving undefined bits unset protects the network against new definitions of those bits.
For IANA, the reference for the "SRP Object Flag Field" subregistry of the PCEP Numbers registry was updated to list this document in addition to RFC 8281.
When everyone follows the same rule, there are fewer surprises, so it is harder for a troublemaker to cause chaos by flipping random switches.