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
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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