What's the standard format for representing true and false values in an HTTP header?

Question

I need to add a custom header to an HTTP request, and its value's type will be boolean. However, HTTP header values are all strings. So what is the common practice for representing a boolean in this case? 0/1, false/true, False/True, off/on, etc?

E.g.

My-Header: 1

Or what's generally recommended, i.e. the most commonly used convention?

Answer 1

UPDATE: it might make the most sense to use a header exists vs doesn't exist convention as implied here

HTTP Header Operation http header .EXISTS

Returns a Boolean TRUE if an instance of the specified header type exists. Following is an example:

http.req.header("CacheControl").exists

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i think the answer is ?0 vs ?1

Structured Field Values for HTTP > 3.3.6. Booleans

Boolean values can be conveyed in Structured Fields.

The ABNF for a Boolean is:

sf-boolean = "?" boolean

boolean = "0" / "1"

A Boolean is indicated with a leading "?" character followed by a "1" for a true value or "0" for false. For example:

Example-Boolean: ?1

check out Sec-Fetch-User:

Syntax

Sec-Fetch-User: ?0

Sec-Fetch-User: ?1

Values

The value is a Boolean Structured Header.

?0 The navigation request was triggered by a user activation.

?1 The navigation request was triggered by something other than a user activation.

but fwiw, Save-Data also seems to be a standard/official "boolean" header that accepts values "on" vs "off"

Save-Data

A boolean that indicates the user agent's preference for reduced data usage.

(https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers)

A numerical value indicating whether the client wants to opt in to reduced data usage mode. on indicates yes, while off (the default) indicates no.

also confusingly, the ABNF page actually says:

boolean = ( "true" / "1" ) /

( "false" / "0" )