What if an HTTP/1.1 client talk to an HTTP/2 only server and what if an HTTP/2 client talk to an HTTP/1.1 only server?

Question

HTTP/2 is definitely the future trend because it is now the standard of HTTP protocol. As we can see in Can I use, 70.15 percent of browsers support the HTTP/2. But HTTP/2 i

Answer 1

It's important to take in consideration that the most implementations of HTTP/2 uses it over TLS 1.2 with ALPN protocol (Application Layer Protocol Negotiation). Thus the client just start the standard TLS connection. As the part of such communication the client sends "Client Hello" to the server with some information:

It's like: "Hi, Tom! It's Bob. I speak German, Russian and English. Let's talk a little". And the server send "Server Hello":

"Hi, Bob! I suggest to speak German or English". Then the client send one more short message "OK, then let's speak German" and he start to speak German without waiting of any response from the server:

The whole communication looks like on the picture below

Because both the client and the server start the communication just using TLS 1.2, which the both know. They start the main communication after the protocol negotiation. Thus the problem which you describe could not exist in the practice.