How many SETTINGS frame in ALPS and what order with respect to other frames?

[bencebeky]

If I read section 3, "Use of ALPS in HTTP", correctly, it does not specify how many SETTINGS frames can be sent in ALPS, nor does it say anything about what the order of SETTINGS frame(s) has to be with request to potential other frames explicitly allowed by other extensions. I think we should consider spelling these out explicitly. I suggest to ban more than one SETTINGS frame. I'm fine with arbitrary frame order, although requiring that if there is a SETTINGS frame then it MUST be the first frame is also a reasonable option.

[davidben]

At least for HTTP/3, it seems to me SETTINGS first and at most one would align best with assumptions other HTTP/3 frames may be making. Not sure about HTTP/2 since that protocol already allows multiple and later SETTINGS. But a similar requirement also seems plausible?

shrug I don't feel very strongly. @vasilvv?

[vasilvv]

Requiring one SETTINGS frame go first sounds reasonable.