HTTP/2

HTTP/2 Discovery

h2

HTTP/2 over TLS

h2c

HTTP/2 is run over cleartext TCP

Starting HTTP/2 for "http" URIs

如果客户端无法事先知道服务端是否支持http2，可以通过HTTP Upgrade mechanism（定义在HTTP/1.1）中来实现，这样在之后的请求中可以采取HTTP/2。

GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

这样的话，如果请求中由payload必须一次性发完。然后才能升级到HTTP/2。另一种办法是通过OPTIONS请求 来判断，这样做会多一次请求。

如果服务端不支持HTTP/2，返回中则不包含Upgrade header:

HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html

否则，返回一个101 (Switching Protocols)响应：

HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c