Re: [hybi] WebSocket subprotocol parameters

[Tobias Oberstein <[e-mail]>]

> > My impression was that subprotocol primarily changed the interpretation
> of messages payloads by the end applications themselves. WebSocket
> messages allow essentially arbitrary data so you can define whatever
> parameter setting mechanism you like with whatever grammar makes sense
> for your subprotocol and send the parameters in the opening “hello”
> message. Defining a grammar in the socket spec to allow application
> protocols to configure themselves seems limiting and out of scope.
> 
> Yes indeed. It has always been my impression that even subprotocols
> themselves are rather unnecessary. On the server side, just use a different
> URI/IRI as endpoint. On the client side, it's all JS anyway.

No quite, since using a different URL does not allow the server to communicate back
it's subprotocol selection, e.g.

Sec-WebSocket-Protocol: foo.msgpack, foo.json

allows the client to announce the "foo" protocol in 2 variants (MsgPack and JSON serialization)
and the server to _select_ one of those, and communicate back it's selection to the client.

An URL

ws://server.com?serializers=msgpack;json

does not allow the server to communicate back it's selection.

Further, as an example, imagine a subprotocol that allows message batching with length prefixes, that wants to negotiate the number of bytes used for the length prefix.

Sec-WebSocket-Protocol: foo; max_len_prefix=4

A server that wants to use "len_prefix=3" cannot communicate that - neither with URL (since there is no "URL response"), and not with Sec-WebSocket-Protocol,
since the server _must_ choose one subprotocol announced by the client _exactly_.

It cannot answer

Sec-WebSocket-Protocol: foo; len_prefix=3

Since the number of bytes per length prefix in batched messages is needed to _parse_ already the first WebSocket message at application level,
negotiating at the WS application message level does not work either (or gets complicated).

So I still think having Sec-WebSocket-Protocol work like Sec-WebSocket-Extension would have been useful.

/Tobias