[twic]

It is, but since you don't control the receive buffer size in the browser, or the TCP window size (or, with many web frameworks, the socket buffer size in the server!), you can't rely on socket buffers giving you timely feedback. By the time the server's send buffer fills, there is already masses of stale data buffered in between you and the client.

In my apps i do indeed detect the socket buffers filling, just as you suggest, but pretty much only as a way of detecting completely wedged clients.