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How does a browser know if a site supports HTTP/2?
Asked Answered
http2
R

3

39

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

Reyreyes answered 12/5, 2016 at 8:40 Comment(1)
in the RFC there's quite a good explanation how this works: tools.ietf.org/html/rfc7540#section-3.2 – Josiejosler
P
46

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

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.

Parasitize answered 2/8, 2018 at 2:50 Comment(0)
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6

The chrome browser will only send a HTTP/1.1 Request to the website. As the website is HTTP/2 Enabled, it will send a message to the browser that it supports HTTP/2. The server upgrades the communication protocol between it and the server to HTTP/2 if it finds the browser capable of recognizing HTTP/2. So, it is generally the server which converts a request to the HTTP/2 Connection. The browser just complies with the upgrade policy of the server. The chrome browser displays that you have a HTTP/2 connection with the server or website, only after the server upgrades the communication protocol.

Spinose answered 13/5, 2016 at 6:55 Comment(0)
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2

The string "h2" identifies the protocol where HTTP/2 uses Transport Layer >Security (TLS) [TLS12]. This identifier is used in the TLS application-layer protocol negotiation (ALPN) >extension [TLS-ALPN] field and in any place where HTTP/2 over TLS is identified.

If server support http2.0 browser will find that server is support http2.0 in TLS application-layer protocol negotiation. refer link!

Bobbette answered 19/7, 2017 at 3:43 Comment(0)

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