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Routing PCE Working Group The PCE Communication Protocol (PCEP) defines the mechanisms for the communication between a Path Computation Client (PCC) and a PCE, or among PCEs. Such interactions include path computation requests and path computation replies defined in RFC 5440. As per RFC 5440, these messages are required to follow strict object ordering. This document updates RFC 5440 by relaxing the strict object ordering requirement in the PCEP messages.

Introduction describes the PCE Communication Protocol (PCEP). PCEP defines the communication between a Path Computation Client (PCC) and a PCE, or between PCEs, enabling computation of Multiprotocol Label Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP) characteristics. defines several PCEP messages. For each PCEP message type, rules are defined that specify the set of objects that the message can carry using Routing Backus-Naur Form (RBNF) . Further, states that the object ordering is mandatory. This occurs when multiple extensions add new objects in the PCEP messages and the respective order of these new objects is not specified (see ). This document updates to relax the strict object ordering requirement.

Conventions 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.

Motivation The mandatory object ordering requirement in is shown to result in exponential complexity in terms of what each new PCEP extension needs to cope with in terms of reconciling all of the previously published RFCs, and all concurrently work in progress in the form of the internet-drafts. This requirement does not lend itself to the extensibility of PCEP.

Update to RFC 5440 states: This text is updated to read as follows: This update does not aim to take away the object ordering completely. The PCEP speaker is expected to follow the object order as specified unless there are valid reasons to ignore it. It is also likely that the receiver can understand the object's meaning irrespective of the order unambiguously.

Compatibility Considerations While one of the main objectives of the changes made by this document is to enable backward compatibility between PCEP extensions, there remains an issue of compatibility between existing implementations of and implementations that are consistent with this document. It should be noted that common behaviour for checking object ordering in received PCEP messages is as described by the updated text presented in . Thus, many implementations will still have implemented a consistent and future-proof approach. However, for completeness, it is worth noting how behaviours might interact between implementations. The messages generated by an implementation of this document, when received by a legacy implementation with a strict interpretation of object ordering, MAY lead to error handling. It is interesting to note that the does not define an Error-Type and Error-value corresponding to this error condition.

Open Questions

• Should a new flag or a TLV in Open Message be added to exchange this capability? Not sure if this is strictly needed, if we can live with .

Management Considerations Implementations receiving set objects that they consider out of order MAY log this. That could be helpful for diagnosing backward compatibility issues.

Other Efforts In the past, there have been efforts to consolidate and update the RBNF, such as in . This document relaxes the object ordering only; it does not take on the various other issues or the need to consolidate the RBNF for all PCEP extensions. There have been proposals to consolidate the RBNF for the PCEP message in a single place in GitHub and use modern data modelling tools to represent PCEP extensions. They might be taken up in parallel.

Security Considerations This document does not raise any security issues.

IANA Considerations This document does not require any IANA actions.

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. 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] Several protocols have been specified in the Routing Area of the IETF using a common variant of the Backus-Naur Form (BNF) of representing message syntax. However, there is no formal definition of this version of BNF. There is value in using the same variant of BNF for the set of protocols that are commonly used together. This reduces confusion and simplifies implementation. Updating existing documents to use some other variant of BNF that is already formally documented would be a substantial piece of work. This document provides a formal definition of the variant of BNF that has been used (that we call Routing BNF) and makes it available for use by new protocols. [STANDARDS-TRACK] 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. Informative References This document specifies a CLASSTYPE object to support Diffserv-Aware Traffic Engineering (DS-TE) where path computation is performed with the aid of a Path Computation Element (PCE). [STANDARDS-TRACK] 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. CTTC Coriant Old Dog Consulting Huawei Technologies Huawei Technologies The PCEP protocol has been defined in [RFC5440] and later extended in several RFCs. This document aims at documenting inconsistencies when implementing a set of extensions and at providing a reference, complete and formal RBNF grammar for PCEP messages, including object ordering and precedence rules. n.d.

Acknowledgments Thanks to John Scudder for the motivation behind this document. Thanks to Oscar Gonzalez de Dios and Cyril Margaria for raising errata on this topic. Thanks to the author of for highlighting the issue.

Examples As described in , for the PCReq message, the CLASSTYPE object is encoded after the END-POINTS object in . Whereas in , the LSP object is encoded just after the END-POINTS object. So it is not known which of the following orders is expected. [][]... or ...[][]... ]]> This update requires the receiver to be able to accept both of these.

When Order Matters There are cases where the ordering between objects is important. For instance, PCRpt message includes <path> with some attributes that say BANDWIDTH can be part of both <actual-attribute-list> and <intended-attribute-list>. ::= [] ]]> An important factor to distinguish between the actual and intended attribute list is the presence of RRO (i.e. <actual-path>) and the order of objects in the PCRpt message. If the RRO is present, any attributes encoded before it are to be considered as part of <actual-attribute-list> and those after it, as part of <intended-attribute-list>. If the RRO is absent, all attributes are part of <intended-attribute-list>. Thus, the approach taken by this document is to say that ordering is relaxed in cases where there is no ambiguity.