As I mentioned above, I agree that that needing to pass maximum message size out of band makes some things more difficult. Whether that was the right tradeoff in terms of convenience vs protocol complexity I think has yet to be seen. At any rate, an extension to perform this in band should be trivial. Perhaps I will try writing one to test out my extension handling code.

I do see your point on the "endless streaming" section of the RFC. Stating that "(section 5.4) The primary purpose of fragmentation is to allow sending a message that is of unknown size when the message is started without having to buffer that message." implies that a web socket implementation should support this sort of operation. Indeed, if you want to support sending messages of unknown size you must expose an interface more complicated than the default message based one.

That said, a message only implementation that does not allow sending unknown sized messages is 100% compliant with both the spec and receiving such messages. The RFC probably could have made this fact more clear. I believe that endless streaming mode will not be a common use case and have not implemented it in my generic WebSocket library. I do believe, however, that fragmentation of messages provides important benefits even without unknown size sends. Once you have message fragmentation there is no additional protocol cost to allow unknown size sends.