What will happen if server implement spdy/3 while browser only support spdy/2?

Question

What will happen if I add support for spdy/3 on my server, while my users only have spdy/2 in their browsers? Will the protocol degrade gracefully to spdy/2 or it will only use plain https?

Answer 1

Servers that supports SPDY such as Jetty advertise the protocols that they are able to support via NPN, and usually there always is "http/1.1" among them.

If your server advertises "spdy/3","http/1.1", and the browser only supports "spdy/2", then the browser (being a browser means it supports HTTP by default) will fallback to "http/1.1" over TLS.

There is no degradation to spdy/2 if the server does not support it (spdy/2 and spdy/3 are binary incompatible). If a protocol negotiation is not possible, the connection is closed.