Clarify HTTP/2 and HTTP/3 pseudo-headers interaction with ASGI

In addition to “normal” HTTP headers, HTTP/2 and HTTP/3 use pseudo-headers starting with “:”, some of which have natural mappings to ASGI scope variables :

• :method: => this maps to scope["method"]
• :path: => this maps to scope["raw_path"]
• :scheme: => this maps to scope["scheme"]

A more problematic pseudo-header is :authority which is the equivalent of HTTP/1.1’s Host header.

It would be good to clarify in the ASGI specs:

• how should the :authority pseudo-header be conveyed by the server to the ASGI app: should servers append a synthetic Host header?

• my feeling is that pseudo-headers should be purge from what is passed into scope[“headers”], do we want to specify this in the specs?

Another interesting pseudo-header is the :protocol pseudo-header which is used to handshake WebSockets over HTTP/2, and very likely also over HTTP/3.

[andrewgodwin]

I’ve long held the position that ASGI’s position is not to be a low-level HTTP library but a useful abstraction that works for 95% of apps, and so given that I agree we should ban pseudo-headers from going through and we should synthesise a Host header.

[pgjones]

See #123.