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

by

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/..
  • …

Client Hello

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

enter image description here

Conclusion

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

Leave a Comment

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)