3. The SETTINGS_ENABLE_CONNECT_PROTOCOL SETTINGS Parameter

This document adds a new SETTINGS parameter to those defined by [RFC7540], Section 6.5.2.

The new parameter name is SETTINGS_ENABLE_CONNECT_PROTOCOL. The value of the parameter MUST be 0 or 1.

Upon receipt of SETTINGS_ENABLE_CONNECT_PROTOCOL with a value of 1, a client MAY use the Extended CONNECT as defined in this document when creating new streams. Receipt of this parameter by a server does not have any impact.

A sender MUST NOT send a SETTINGS_ENABLE_CONNECT_PROTOCOL parameter with the value of 0 after previously sending a value of 1.

Using a SETTINGS parameter to opt into an otherwise incompatible protocol change is a use of "Extending HTTP/2" defined by Section 5.5 of [RFC7540]. Specifically, the addition a new pseudo-header field, ":protocol", and the change in meaning of the :authority pseudo-header field in Section 4 require opt-in negotiation. If a client were to use the provisions of the extended CONNECT method defined in this document without first receiving a SETTINGS_ENABLE_CONNECT_PROTOCOL parameter, a non-supporting peer would detect a malformed request and generate a stream error (Section 8.1.2.6 of [RFC7540]).