HTTP/2

来自WHY42

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

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