Starting HTTP/2 for "http" URIs / 为"http" URIs启用HTTP/2协议
A client that makes a request for an "http" URI without prior knowledge about support for HTTP/2 on the next hop uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client does so by making an HTTP/1.1 request that includes an Upgrade header field with the "h2c" token. Such an HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.
For example:
GET / HTTP/1.1 Host: server.example.com Connection: Upgrade, HTTP2-Settings Upgrade: h2c HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
事先不知道下一跳是否支持HTTP/2的客户端，使用HTTP的Upgrade机制( [RFC7230] 的 6.7节 )发起"http"URI请求。其做法是：客户端先发起HTTP/1.1请求，该请求包含值为"h2c"的Upgrade首部字段，还必须包含一个HTTP2-Settings( 3.2.1节 )首部字段。
例如：
GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
Requests that contain a payload body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request can block the use of the connection until it is completely sent.
在客户端能发送HTTP/2帧之前，包含负载的请求必须全部被发送。这意味着一个大的请求能阻塞连接的使用，直到其全部被发送完毕。
If concurrency of an initial request with subsequent requests is important, an OPTIONS request can be used to perform the upgrade to HTTP/2, at the cost of an additional round trip.
如果一个初始请求的后续并发请求很重要，那么可以使用OPTIONS请求来执行升级到HTTP/2的操作，代价是一个额外的往返。
A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:
HTTP/1.1 200 OK Content-Length: 243 Content-Type: text/html ...
不支持HTTP/2的服务端对请求进行响应时可以忽略Upgrade首部字段：
HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
...
A server MUST ignore an "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.
服务端必须忽略值为"h2"的Upgrade首部字段。"h2"字段表示HTTP/2使用了TLS，其协商方法在 3.3节 中描述。
A server that supports HTTP/2 accepts the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the upgrade.
For example:
HTTP/1.1 101 Switching Protocols Connection: Upgrade Upgrade: h2c [ HTTP/2 connection ...
支持HTTP/2的服务端返回101(Switching Protocols)响应，表示接受升级协议的请求。在结束101响应的空行之后，服务端可以开始发送HTTP/2帧。这些帧必须包含一个对升级请求的响应。
例如：
HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c
[ HTTP/2 connection ...
The first HTTP/2 frame sent by the server MUST be a server connection preface (Section 3.5) consisting of a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client MUST send a connection preface (Section 3.5), which includes a SETTINGS frame.
服务端发送的第一个HTTP/2帧必须是由一个 SETTINGS 帧( 6.5节 )组成的服务端连接前奏( 3.5节 )。客户端一收到101响应，也必须发送一个包含 SETTINGS 帧的连接前奏( 3.5节 )。
The HTTP/1.1 request that is sent prior to upgrade is assigned a stream identifier of 1 (see Section 5.1.1) with default priority values (Section 5.3.5). Stream 1 is implicitly "half-closed" from the client toward the server (see Section 5.1), since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.
升级之前发送的HTTP/1.1请求被分配一个流标识符1 (参见 5.1.1节 )，并被赋予默认优先级值( 5.3.5节 )。流1暗示从客户端到服务端(参见 5.1节 )是半关闭的，因为作为HTTP/1.1请求它已经完成了。HTTP/2连接开始后，流1用于响应。