Permissible states for extension frames #591

[Martin Thomson <[e-mail]>]

We don't really say this, but the implication is that an extension
frame can appear anywhere, for any stream. Worst case, you can have
frames appearing with any stream number at any time.

Are we OK with this?

Or... do we want to limit the sending of extension frames on streams
somehow. Note that the most permissibly constrained frame type is
PRIORITY, which can appear in any state other than "idle". A similar
constraint would be relatively easy to enact.

A tighter scope, like just "open" and the sending-permitted
"half-closed" variant, would be even easier to enact, but might reduce
the utility of extension frames.

-- https://github.com/http2/http2-spec/issues/591