How does a browser know if a site supports HTTP/2?

Question

If I type out https://http2.golang.org/ the chrome browser will automatically send the HTTP/2 request. How is this done?

Answer 1

Take stackoverflow for example, when the browser sends a request to stackoverflow.com, it has to do the following steps:

1. DNS lookup. find the ip address of stackoverflow.
2. TCP/IP handshake
3. TLS handshake.
4. HTTP request/response (Application Protocol).
5. ....

TLS handshake

Regarding step3 TLS handshake, there is an nice explanation by @Oleg.

In order to inspect the detail of TCP/IP packet, You may need use some tools to capture packets. e.g. Wireshark.

Client sends ClientHello to server, which carries several things

• supported cipher suite. which cipher suites do you like?
• supported TLS version.
• a random number.
• the supported Application Protocols. e.g. HTTP/2, HTTP 1.1/ Spdy/..
• ...

Server responds SeverHello, which carries

• chosen cipher suite.
• chosen TLS version.
• a random generated number
• And, chosen application protocols in TLS application-layer protocol negotiation (ALPN), e.g. HTTP/2

Conclusion

HTTP2 request/response happens in step4. Before that, browser has already know whether sever support HTTP/2 through TLS handshake.