9. Extensions to HTTP/3

HTTP/3 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/3 connection.

This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or fields.

Extensions are permitted to use new frame types (Section 7.2), new settings (Section 7.2.4.1), new error codes (Section 8), or new unidirectional stream types (Section 6.2). Registries are established for managing these extension points: frame types (Section 11.2.1), settings (Section 11.2.2), error codes (Section 11.2.3), and stream types (Section 11.2.4).

Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard data or abort reading on unidirectional streams that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation. However, where a known frame type is required to be in a specific location, such as the SETTINGS frame as the first frame of the control stream (see Section 6.2.1), an unknown frame type does not satisfy that requirement and SHOULD be treated as an error.

Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. Coordinating when such a revised layout comes into effect could prove complex. As such, allocating new identifiers for new definitions of existing protocol elements is likely to be more effective.

This document does not mandate a specific method for negotiating the use of an extension, but it notes that a setting (Section 7.2.4.1) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the default value MUST be defined in such a fashion that the extension is disabled if the setting is omitted.