Re: HTTP2 server-side stream creation

[Amos Jeffries <[e-mail]>]

On 8/07/2015 7:49 p.m., Cory Benfield wrote:
> On 7 July 2015 at 23:43, Fedor Indutny wrote:
>> Obviously, the most straightforward way is to do a PUSH_PROMISE on
>> existing client-initiated stream, but it appears to me that the
>> server-initiated streams created using HEADERS frame are valid
>> too.
> 
>>From section 8.1 of RFC 7540[0]:
> 
>> A client sends an HTTP request on a new stream, using a previously
>> unused stream identifier (Section 5.1.1).  A server sends an HTTP
>> response on the same stream as the request.
> 
> My reading is that this forbids a 'server' from sending a HEADERS
> frame first, because servers send responses on already-opened streams.
> 
> You could pretty easily construct a semantic for this that essentially
> turns HTTP/2 into a peer-to-peer communication stream, with both sides
> of the connection being free to issue requests. This could plausibly
> be very valuable in systems that use HTTP/2 as an RPC transport. I
> suspect most clients will currently not allow that behaviour, however,
> so if you wanted it it might be best to propose it as a negotiated
> HTTP/2 extension, per section 5.5 of RFC 7540[1]. If you (or anyone
> else on the list) think this is interesting I'd be happy to co-author
> a draft to propose it.

Technically.

However, HTTP/1.x currently still exists in the world. A surprisingly
large number of connections one way or another pass over at least one
HTTP/1.x transit hop. So for now any implementations will have to cope
with translation to HTTP/1.x where server requests are not possible.

HTTP/2 was designed with that in mind, thus it does not define
server-initiated semantics. But also does not forbid them outright,
since extensions or HTTP/3 may one day have a need to define it.

If you have a strong use case for it I suggest writing up a SETTINGS
extension which can be negotiated between two endpoints. That way 2->1.1
gateway devices can negotiate its absence for the 1.1 hops and things
still work.

HTH
Amos