| Internet-Draft | Protocol Greasing | June 2025 |
| Pardue | Expires 8 December 2025 | [Page] |
Active use and maintenance of network protocols is an important way to ensure that protocols remain interoperable and extensible over time. Techniques such as intentionally exercising extension points with non-meaningful values (referred to as "grease") or adding variability to how protocol elements are used help generate this active use.¶
Grease and variability are used across various protocols developed by the IETF. This document discusses considerations when designing and deploying grease and variability mechanisms, and provides advice for making them as effective as possible.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://intarchboard.github.io/draft-protocol-greasing/draft-edm-protocol-greasing.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-edm-protocol-greasing/.¶
Source for this draft and an issue tracker can be found at https://github.com/intarchboard/draft-protocol-greasing.¶
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Section 3 of [VIABILITY] discusses "active use" as a category of techniques that protocol designers and implementers employ to ensure that protocol extension mechanisms are exercised and can be used in the future. This ability to change (to handle protocol updates and extensions) is an important factor in the success of protocol deployment, as discussed in [SUCCESS].¶
Active use of protocol features and extensions often requires intentional efforts beyond what would organically occur in deployments. Some extension points do not frequently see new values being used, but are still important to be usable in the future. Some patterns of protocol usage might be relatively static without specific efforts to ensure that they can change in the future.¶
One key techique for intentional use is "grease", or "greasing". Greasing was initially designed for TLS [GREASE] and was later adopted by other protocols such as QUIC [QUIC]. In these protocols, extension codepoints are reserved only for greasing and must be ignored upon receipt. Greasing is suitable for many protocols but not all; Section 3.3 of [VIABILITY] discusses the applicability and limitations of greasing.¶
While it is becoming more common, designing and applying grease is not necessarily trivial. There are best practices, and some common pitfalls to avoid, that have been developed by the protocols using grease thus far. Section 3 takes these learnings and provides considerations for new protocol design and deployment.¶
Separate from greasing using reserved values, protocol deployments can intentionally add variability to ensure that network endpoints and middleboxes do not end up ossifying certain patterns. 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. Section 4 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. Section 5 presents considerations about designing for and/or implementing version negotiation and migration.¶
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.¶
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 [GREASE], Section 18.1 of [QUIC], or Section 7.2.8 of [HTTP/3]). 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 Section 3.3 of [VIABILITY]. This section provides concrete suggestions for its usage.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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.¶
There are intrinsic and well-documented issues related to testing version negotiation of protocols; see [EXTENSIBILITY] and Sections 2.1 and 3.2 of [VIABILITY]. This section will be expanded with advice for protocol designers and implementers about how to approach these problems.¶
The considerations in [MAINTENANCE], [GREASE], [END-USERS], and [VIABILITY] all apply to the topics discussed in this document.¶
The use of protocol features, extensions, and versions can already allow fingerprinting [PRIVCON]. 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.¶
This document has no IANA actions itself. Guidance on how other documents can effectively instruct IANA about protocol greasing is provided in Section 3.4.1¶
This work is a summary of the topics discussed during EDM meetings. The contributors at those meetings are thanked.¶