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next generation unicorn sparkling distributed ledger Long-term interoperability of protocols is an important goal of the network standards process. Deployment success can depend on supporting change, which can include modifying how the protocol is used, extending the protocol, or replacing the protocol. This document presents concepts, considerations, and techniques related to protocol maintenance, such as greasing or variability. The intended audience is protocol designers and implementers. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction Long-term interoperability of protocols is an important goal of the network standards process . Protocol deployment success can depend on supporting change, which can include modifying how the protocol is used, extending the protocol, or replacing the protocol. Greasing, a technique initially designed for TLS and later adopted by other protocols such as QUIC , can help support the long-term viability of protocol extension points. In these protocols, extension codepoints are reserved only for greasing and when received must be ignored. Greasing is suitable for many protocols but not all; discusses the applicability and limitations of greasing. provides additional protocol maintenance considerations. Applications are built with the intent of serving user needs , which might only require support for a subset of protocol features. Adapting to changing user needs is a maintenance activity. For example, an HTTP deployment focused on downloads might want to add support for uploads. Changing use of the application and transport protocol features can affect the deployment's network traffic profile. If expectations have been formed around historical patterns of use i.e., ossification, introducing change might lead to deployment problems. presents considerations about how intentionally increasing the variability of protocols can mitigate some of these concerns. Protocol extensions can provide longevity in the face of changing needs or environment. However, a replacment protocol might be preferred when extensions are not adequate or feasible. A protocol replacement could aggregate common extensions and possibly make them mandatory, effectively defining a new baseline that can simplify deployment and interoperability. A replacement protocol version may or may not be compatible with other versions. A protocol may or may not have a mechanism for version selection or agility. presents considerations about designing for and/or implementing version negotiation and migration.

Conventions and Definitions 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.

Considerations for Applying Greasing Greasing can take many forms, depending on the protocol and the nature of its extension points. Many protocols register values, codepoints, or numbers in a limited space. A common approach that has developed in more recent protocols is to reserve a subset of the space for greasing (see , , or ). Values reserved for the purpose of greasing are herein referred to as grease values. Implementations that receive grease values are required to ignore them. More background to this approach is given in . This section provides concrete suggestions for its usage.

Don't Handle Grease Values as a Special Case It is assumed that endpoints should implement robust and broad extension handling. A receiver or a parser implementation should not treat grease values as individually special. Instead of identifying each grease value explicitly, it is better to have a "catch all" mechanism that can handle receipt of unknown extensions, whether grease values or not, gracefully or without error.

Use Unpredictable Grease Values It is recommended that senders pick an unpredictable grease value to include in relevant protocol elements. This ensures that receiver greasing requirements are exercised. Using predictable grease values risks ossification. To increase the variety of grease values, it is advised to reserve a large range. However, the specific size and distribution of the grease range needs to accommodate the protocol constraints. For instance, protocols that use 8-bit fields may find it too costly to dedicate many grease values, while 32-bit or 64-bit fields are likely to have no limitations.

Use Grease Values at Unpredictable Times It is recommended that senders use grease values at unpredictable times or sequence points during protocol interactions. This avoids receivers unintentionally ossifying on the occurrence of greasing in the temporal or spatial domain.

Define and Register Grease Value Ranges It is recommended that large grease value sets are allocated in protocol documents by defining a unique algorithm, to increase the chance that receiver greasing requirements are exercised. However, the choice of algorithm needs to consider the spread of values and the size of contiguous blocks between grease values. It is common for protocol extension designers to want to reserve a contiguous block of code points in order to aid iteration and experimentation. Small contiguous blocks increase the chance that such reservations might unintentionally use grease values, which could lead to interoperability failures.

Effectively Instructing IANA about grease Protocol designers might ask IANA to create new registries for their extension points. When greasing, it is recommended that only a single entry for the entire grease value set is registered. When an algorithm has been used, it should be included in the entry; see for example https://www.iana.org/assignments/http3-parameters/http3-parameters.xhtml#http3-parameters-frame-types. Grease values must not be used or registered for any other purpose. Registries should include a label to identify the protected grease value range; a label of "reserved" may be confused with other ranges that are reserved for private or experimental extensions. An implementer that conflates these two meanings may cause a new class of interoperability failure. Therefore a label such as "reserved for greasing" may help to avoid the confusion.

Considerations for Increasing Protocol Variability Greasing can maintain protocol extensibility by falsifying active use of its extension points. However, greasing alone does not ensure positive use of extension mechanisms. A protocol may define a wide-ranging extension capability that remains unused in the absence of real use cases. This can lead to ossification that does not expect extensions, leading to interoperability problems later on. Long-term maintenance and interoperability can be ensured by exercising extension points positively. To some extent this can be thought of as protocol fuzzing. This might be difficult to exercise because varying the protocol elements might change the outcome of interactions, leading to real errors. However, some protocols allow elements to be be safely changed, as shown in the following examples.

Example: QUIC frames QUIC packets contain frames. Receivers might build expectations on the longitudinal aspects of packets or frames - size, ordering, frequency, etc. A sender can quite often manipulate these parameters and stay compliant to the requirements of the QUIC protocol. A QUIC stream is an ordered reliable byte stream that is serialized as a sequence of STREAM frames with a length and offset. Receivers are expected to reassemble frames, which could arrive in any order, into an ordered reliable byte stream that is readable by applications. A form of positive testing is for a sender to unpredictably order the STREAM frames that it transmits. For example, varying the sequence order of offset values. This allows to exercise the QUIC reassembly features of the receiver with the expectation that no failure would occur. However, doing this may introduce delay or stream head-of-line blocking that affects the performance aspects of a transmission, which may not be acceptable for a given use case. As such, positive testing might be most appropriate to use in a subset of connections, or phases within a connection.

Considerations for Protocol Versions There are intrinsic and well-documented issues related to testing version negotiation of protocols; see and Sections and of . This section will be expanded with advice for protocol designers and implementers about how to approach these problems.

Security Considerations The considerations in , , , and all apply to the topics discussed in this document. The use of protocol features, extensions, and versions can already allow fingerprinting . Any techniques that change parameters in any way, including but not limited to those discussed in this document, can affect fingerprinting. A deeper analysis of this topic has been deemed out of scope.

IANA Considerations This document has no IANA actions itself. Guidance on how other documents can effectively instruct IANA about protocol greasing is provided in

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. Informative References The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC and describes how HTTP/2 extensions can be ported to HTTP/3. The main goal of the networking standards process is to enable the long-term interoperability of protocols. This document describes active protocol maintenance, a means to accomplish that goal. By evolving specifications and implementations, it is possible to reduce ambiguity over time and create a healthy ecosystem. The robustness principle, often phrased as "be conservative in what you send, and liberal in what you accept", has long guided the design and implementation of Internet protocols. However, it has been interpreted in a variety of ways. While some interpretations help ensure the health of the Internet, others can negatively affect interoperability over time. When a protocol is actively maintained, protocol designers and implementers can avoid these pitfalls. The Internet community has specified a large number of protocols to date, and these protocols have achieved varying degrees of success. Based on case studies, this document attempts to ascertain factors that contribute to or hinder a protocol's success. It is hoped that these observations can serve as guidance for future protocol work. This memo provides information for the Internet community. This document describes GREASE (Generate Random Extensions And Sustain Extensibility), a mechanism to prevent extensibility failures in the TLS ecosystem. It reserves a set of TLS protocol values that may be advertised to ensure peers correctly handle unknown values. This document defines the core of the QUIC transport protocol. QUIC provides applications with flow-controlled streams for structured communication, low-latency connection establishment, and network path migration. QUIC includes security measures that ensure confidentiality, integrity, and availability in a range of deployment circumstances. Accompanying documents describe the integration of TLS for key negotiation, loss detection, and an exemplary congestion control algorithm. The ability to change protocols depends on exercising the extension and version-negotiation mechanisms that support change. This document explores how regular use of new protocol features can ensure that it remains possible to deploy changes to a protocol. Examples are given where lack of use caused changes to be more difficult or costly. This document explains why the IAB believes that, when there is a conflict between the interests of end users of the Internet and other parties, IETF decisions should favor end users. It also explores how the IETF can more effectively achieve this. This document discusses architectural issues related to the extensibility of Internet protocols, with a focus on design considerations. It is intended to assist designers of both base protocols and extensions. Case studies are included. A companion document, RFC 4775 (BCP 125), discusses procedures relating to the extensibility of IETF protocols. This document is not an Internet Standards Track specification; it is published for informational purposes. This document offers guidance for developing privacy considerations for inclusion in protocol specifications. It aims to make designers, implementers, and users of Internet protocols aware of privacy-related design choices. It suggests that whether any individual RFC warrants a specific privacy considerations section will depend on the document's content.

Acknowledgments This work is a summary of the topics discussed during EDM meetings. The contributors at those meetings are thanked.