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Routing PCE PCEP Path Computation Element Stateful PCE Flags 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 .

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Table of Contents

• .  Introduction
• .  Requirements Language
• .  Updated Procedures
  • .  Advice for Specification of New Flags
  • .  Flags Field of the SRP Object
• .  Compatibility Considerations
• .  Management Considerations
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction 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. specifies a set of extensions to PCEP to enable stateful control of LSPs within and across PCEP sessions in compliance with . 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 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, 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.

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 when, and only when, they appear in all capitals, as shown here.

Updated Procedures

Advice for Specification of New Flags 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 is updated to read as follows:

• The PCEP objects defined in this document are compliant with the PCEP object format defined in . 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.

Flags Field of the SRP Object 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.

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 . 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 or ) 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.

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.

Security Considerations 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.

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 .

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. Although PCEP explicitly makes no assumptions regarding the information available to the PCE, it also makes no provisions for PCE control of timing and sequence of path computations within and across PCEP sessions. This document describes a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. The extensions for stateful PCE provide active control of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs) via PCEP, for a model where the PCC delegates control over one or more locally configured LSPs to the PCE. This document describes the creation and deletion of PCE-initiated LSPs under the stateful PCE model. Informative References The PCE model is described in the "PCE Architecture" document and facilitates path computation requests from Path Computation Clients (PCCs) to Path Computation Elements (PCEs). This document specifies generic requirements for a communication protocol between PCCs and PCEs, and also between PCEs where cooperation between PCEs is desirable. Subsequent documents will specify application-specific requirements for the PCE communication protocol. This memo provides information for the Internet community. This document specifies the Path Computation Element (PCE) Communication Protocol (PCEP) for communications between a Path Computation Client (PCC) and a PCE, or between two PCEs. Such interactions include path computation requests and path computation replies as well as notifications of specific states related to the use of a PCE in the context of Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering. PCEP is designed to be flexible and extensible so as to easily allow for the addition of further messages and objects, should further requirements be expressed in the future. [STANDARDS-TRACK] A stateful Path Computation Element (PCE) retains information about the placement of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs). When a PCE has stateful control over LSPs, it may send indications to LSP head-ends to modify the attributes (especially the paths) of the LSPs. A Path Computation Client (PCC) that has set up LSPs under local configuration may delegate control of those LSPs to a stateful PCE. There are use cases in which a stateful PCE may wish to obtain control of locally configured LSPs that it is aware of but have not been delegated to the PCE. This document describes an extension to the Path Computation Element Communication Protocol (PCEP) to enable a PCE to make requests for such control.

Acknowledgements Thanks to the authors of for exposing the need for this work. Thanks to and for discussing the solution. Additional thanks to for his Shepherd's review. Thanks to and for helpful comments during IESG review.

Author's Address Old Dog Consulting

adrian@olddog.co.uk