【HTTP 2】启用 HTTP 2（Starting HTTP/2）

╰+哭是因爲堅強的太久メ 2022-07-26 00:04 178阅读 0赞

# 【HTTP 2】启用 HTTP 2（Starting HTTP/2） #

[四月 1, 2016][1_ 2016]  ~ [LITECODES][]

## ![Starting HTTP/2][Starting HTTP_2] ##

## 前情提要 ##

在上一篇文章《[【HTTP 2】HTTP/2 协议概述（HTTP/2 Protocol Overview）][HTTP 2_HTTP_2 _HTTP_2 Protocol Overview]》中，简单介绍了 HTTP 2 规范的文档结构以及约定和术语。

本文变对 HTTP 2 规范的第一部分进行介绍，来说明如何发起一个 HTTP 2 连接。

## 启用 HTTP/2（Starting HTTP/2） ##

HTTP 2 连接是一个运行在 [TCP][] 连接之上的应用层协议。客户端是 TCP 连接的发起者。

HTTP 2 保持了 HTTP 1.1 的风格，同样使用“http”和“https”作为 URI 的开始。并且共享相同的默认端口号：“http”使用 80 端口，“https”使用 443 端口。这意味着，相关实现要处理针对目标资源的请求，比如 http://example.org/foo 或者 https://example.com/bar，需要先知道上游服务器（该客户端当前希望建立的连接）是否支持 HTTP 2。

“http”和“https”支持 HTTP 2 的手段有所不同。“http”支持 HTTP 2 的方法在 [3.2 节][3.2]详述，而“https”支持 HTTP 2 的方法在 [3.3 节][3.3]详述。

### HTTP/2 版本标识（HTTP/2 Version Identification） ###

本文档中所定义的协议有两个标识符。

字符串“h2”标识了使用传输安全层（[TLS][]）的协议。该标识符用于 TLS 应用层协议协商（ALPN）扩展（TLS-ALPN）字段，在任何 TLS 之上的 HTTP 2 都会被该标识符标记。

“h2”字符串在 ALPN 协议标识中被序列化成两个八进制序列：0x68, 0x32。

字符串“h2c”标识了 HTTP 2 运行在明文 TCP 中。该标识符用于 HTTP 1.1 Upgrade 报文头中，在任何 TCP 之上的 HTTP 2 都会被该标识符标记。

选择“h2”或“h2c”意味着将会导致不同的传输、安全性、帧构成，以及在本文档中所描述的消息语法。

### “http”中启用 HTTP/2（Starting HTTP/2 for “http” URIs） ###

一个客户端使用“http”发起请求，并不知道下一站是否支持 HTTP 升级机制。客户端通过发送 HTTP 1.1 请求，并包括 Upgrade 报文头，以“h2c”标识。这样的 HTTP 1.1 请求必须（MUST）有且仅有一个 HTTP2-Settings 报文头字段。

例如：

<table style="box-sizing: border-box; font-size: 12px; font-weight: inherit; font-style: inherit; margin-left: 0px; vertical-align: baseline; outline: 0px; margin-top: 0px !important; margin-right: 0px !important; margin-bottom: 0px !important; padding: 0px !important; border: none !important; border-spacing: 0px !important; border-collapse: collapse !important; width: auto !important; table-layout: auto !important; background: none !important;"> 
 <tbody style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px;"> 
  <tr style="box-sizing: border-box; font-weight: inherit; font-style: inherit; outline: 0px; margin: 0px !important; padding: 0px !important; vertical-align: top !important; border: none !important; background: 0px 50%;"> 
   <td style="box-sizing: border-box; font-style: inherit; border: 0px; outline: 0px; border-collapse: collapse; font-family: Monaco, MonacoRegular, 'Courier New', monospace !important; margin: 0px !important; padding: 0px !important; vertical-align: top !important; background-image: initial; background-attachment: initial; background-color: rgb(238, 238, 238) !important; background-size: initial; background-origin: initial; background-clip: initial; background-position: 0px 50%; background-repeat: initial;"> 
    <div style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding-right: 0px; padding-left: 0px; vertical-align: baseline; border: 0px; outline: 0px; white-space: nowrap; padding-top: 5px !important; padding-bottom: 3px !important; line-height: 15px !important; background: 0px 50%;"> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       1 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       2 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       3 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       4 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       5 
     </div> 
    </div></td> 
   <td style="box-sizing: border-box; font-style: inherit; border: 0px; outline: 0px; border-collapse: collapse; width: 634px; font-family: Monaco, MonacoRegular, 'Courier New', monospace !important; margin: 0px !important; padding: 0px !important; vertical-align: top !important; background: 0px 50%;"> 
    <div style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding-right: 0px; padding-left: 0px; vertical-align: baseline; outline: 0px; white-space: pre; overflow: visible; tab-size: 4; padding-top: 5px !important; padding-bottom: 3px !important; border: none !important; line-height: 15px !important; background: none !important;"> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">GET</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">/</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">/</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 153, 153) !important;">1.1</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Host</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">server</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">.</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">example</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">.</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">com</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Connection</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Upgrade</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">,</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP2</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">-</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">Settings</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Upgrade</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">h2c</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP2</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">-</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Settings</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">&lt;</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">base64url </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">encoding </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">of </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">/</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 153, 153) !important;">2</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">SETTINGS </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">payload</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">&gt;</span> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

在客户端发送 HTTP 2 帧之前，包含 Payload 的请求必须（MUST）被完整的发送。这意味着一个较大的请求将会阻塞连接，直到它被完全发送完毕。

如果初始请求与后续请求的并发非常重要，OPTIONS 请求可以用于升级至 HTTP 2 ，不过需要额外增加一次通信的成本。

不支持 HTTP 2 的服务器可以视 Upgrade 报文头不存在，进行如下响应：

<table style="font-size:12px; font-weight:inherit; font-style:inherit; margin-left:0px; vertical-align:baseline; outline:0px; margin-top:0px!important; margin-right:0px!important; margin-bottom:0px!important; padding:0px!important; border:none!important; border-spacing:0px!important; border-collapse:collapse!important; width:auto!important; table-layout:auto!important; background:none!important"> 
 <tbody style="font-weight:inherit; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px"> 
  <tr style="font-weight:inherit; font-style:inherit; outline:0px; margin:0px!important; padding:0px!important; vertical-align:top!important; border:none!important; background:0px 50%"> 
   <td style="font-style:inherit; border:0px; outline:0px; border-collapse:collapse; font-family:Monaco,MonacoRegular,'Courier New',monospace!important; margin:0px!important; padding:0px!important; vertical-align:top!important; background-color:rgb(238,238,238)!important"> 
    <div style="font-weight:inherit; font-style:inherit; margin:0px; padding-right:0px; padding-left:0px; vertical-align:baseline; border:0px; outline:0px; white-space:nowrap; padding-top:5px!important; padding-bottom:3px!important; line-height:15px!important; background:0px 50%"> 
     <div style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border-top-width:0px; border-right-width:1px!important; border-bottom-width:0px; border-left-width:0px; border-right-style:solid!important; border-right-color:rgb(222,222,222)!important; outline:0px; text-align:right!important; height:inherit; line-height:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important; background:0px 50%">
       1 
     </div> 
     <div style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border-top-width:0px; border-right-width:1px!important; border-bottom-width:0px; border-left-width:0px; border-right-style:solid!important; border-right-color:rgb(222,222,222)!important; outline:0px; text-align:right!important; height:inherit; line-height:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important; background:0px 50%">
       2 
     </div> 
     <div style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border-top-width:0px; border-right-width:1px!important; border-bottom-width:0px; border-left-width:0px; border-right-style:solid!important; border-right-color:rgb(222,222,222)!important; outline:0px; text-align:right!important; height:inherit; line-height:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important; background:0px 50%">
       3 
     </div> 
     <div style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border-top-width:0px; border-right-width:1px!important; border-bottom-width:0px; border-left-width:0px; border-right-style:solid!important; border-right-color:rgb(222,222,222)!important; outline:0px; text-align:right!important; height:inherit; line-height:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important; background:0px 50%">
       4 
     </div> 
     <div style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border-top-width:0px; border-right-width:1px!important; border-bottom-width:0px; border-left-width:0px; border-right-style:solid!important; border-right-color:rgb(222,222,222)!important; outline:0px; text-align:right!important; height:inherit; line-height:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important; background:0px 50%">
       5 
     </div> 
    </div> </td> 
   <td style="font-style:inherit; border:0px; outline:0px; border-collapse:collapse; width:634px; font-family:Monaco,MonacoRegular,'Courier New',monospace!important; margin:0px!important; padding:0px!important; vertical-align:top!important; background:0px 50%"> 
    <div style="font-weight:inherit; font-style:inherit; margin:0px; padding-right:0px; padding-left:0px; vertical-align:baseline; outline:0px; white-space:pre; overflow:visible; padding-top:5px!important; padding-bottom:3px!important; border:none!important; line-height:15px!important; background:none!important"> 
     <div style="font-family:inherit; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important; background:0px 50%"> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">HTTP</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">/</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,153,153)!important">1.1</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important"> </span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,153,153)!important">200</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important"> </span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:teal!important">OK</span> 
     </div> 
     <div style="font-family:inherit; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important; background:0px 50%"> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">Content</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">-</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">Length</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">:</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important"> </span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,153,153)!important">243</span> 
     </div> 
     <div style="font-family:inherit; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important; background:0px 50%"> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">Content</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">-</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">Type</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">:</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important"> </span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,45,122)!important">text</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(0,111,224)!important">/</span> 
      <span style="font-family:inherit; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important">html</span> 
     </div> 
     <div style="font-family:inherit; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important; background:0px 50%">
       &nbsp; 
     </div> 
     <div style="font-family:inherit; font-style:inherit; margin:0px; padding:0px 5px; vertical-align:baseline; border:0px; outline:0px; height:inherit; font-size:inherit!important; font-weight:inherit!important; line-height:inherit!important; background:0px 50%"> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(51,51,51)!important">.</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(51,51,51)!important">.</span> 
      <span style="font-family:inherit; font-size:inherit!important; font-weight:inherit!important; font-style:inherit; margin:0px; padding:0px; vertical-align:baseline; border:0px; outline:0px; height:inherit; line-height:inherit!important; color:rgb(51,51,51)!important">.</span> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

服务器必须（MUST）忽略 Upgrade 报文头中的“h2”标识。“h2”标识意味着基于 TLS 的 HTTP/2，应该参考 [3.3 节][3.3]进行协商。

支持 HTTP 2 的服务器接受了升级请求，并以状态码 101（转换协议）进行响应。在 101 响应的空行后，服务器可以开始发送 HTTP 2 帧。这些帧必须（MUST）包括升级请求对应的响应。

例如：

<table style="box-sizing: border-box; font-size: 12px; font-weight: inherit; font-style: inherit; margin-left: 0px; vertical-align: baseline; outline: 0px; margin-top: 0px !important; margin-right: 0px !important; margin-bottom: 0px !important; padding: 0px !important; border: none !important; border-spacing: 0px !important; border-collapse: collapse !important; width: auto !important; table-layout: auto !important; background: none !important;"> 
 <tbody style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px;"> 
  <tr style="box-sizing: border-box; font-weight: inherit; font-style: inherit; outline: 0px; margin: 0px !important; padding: 0px !important; vertical-align: top !important; border: none !important; background: 0px 50%;"> 
   <td style="box-sizing: border-box; font-style: inherit; border: 0px; outline: 0px; border-collapse: collapse; font-family: Monaco, MonacoRegular, 'Courier New', monospace !important; margin: 0px !important; padding: 0px !important; vertical-align: top !important; background-image: initial; background-attachment: initial; background-color: rgb(238, 238, 238) !important; background-size: initial; background-origin: initial; background-clip: initial; background-position: 0px 50%; background-repeat: initial;"> 
    <div style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding-right: 0px; padding-left: 0px; vertical-align: baseline; border: 0px; outline: 0px; white-space: nowrap; padding-top: 5px !important; padding-bottom: 3px !important; line-height: 15px !important; background: 0px 50%;"> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       1 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       2 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       3 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       4 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border-top-width: 0px; border-right-width: 1px !important; border-bottom-width: 0px; border-left-width: 0px; border-right-style: solid !important; border-right-color: rgb(222, 222, 222) !important; outline: 0px; text-align: right !important; height: inherit; line-height: inherit !important; min-width: 1.2em !important; color: rgb(170, 170, 170) !important; background: 0px 50%;">
       5 
     </div> 
    </div></td> 
   <td style="box-sizing: border-box; font-style: inherit; border: 0px; outline: 0px; border-collapse: collapse; width: 634px; font-family: Monaco, MonacoRegular, 'Courier New', monospace !important; margin: 0px !important; padding: 0px !important; vertical-align: top !important; background: 0px 50%;"> 
    <div style="box-sizing: border-box; font-weight: inherit; font-style: inherit; margin: 0px; padding-right: 0px; padding-left: 0px; vertical-align: baseline; outline: 0px; white-space: pre; overflow: visible; tab-size: 4; padding-top: 5px !important; padding-bottom: 3px !important; border: none !important; line-height: 15px !important; background: none !important;"> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">/</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 153, 153) !important;">1.1</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 153, 153) !important;">101</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">Switching </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">Protocols</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Connection</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: teal !important;">Upgrade</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">Upgrade</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">:</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important;">h2c</span> 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;">
       &nbsp; 
     </div> 
     <div style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px 5px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important; background: 0px 50%;"> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">[</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 45, 122) !important;">HTTP</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;">/</span> 
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      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; font-size: inherit !important; font-weight: inherit !important; line-height: inherit !important;">connection</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(0, 111, 224) !important;"> </span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">.</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">.</span> 
      <span style="box-sizing: border-box; font-family: inherit; font-size: inherit !important; font-weight: inherit !important; font-style: inherit; margin: 0px; padding: 0px; vertical-align: baseline; border: 0px; outline: 0px; height: inherit; line-height: inherit !important; color: rgb(51, 51, 51) !important;">.</span> 
     </div> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

由服务器发送的第一个 HTTP 2 帧必须（MUST）是一个服务器连接序言（Connection Preface）（参考 3.5 章），并且包含一个 SETTINGS 帧（参考章节 6.5）。根据接收的 101 响应，客户端必须（MUST）发送一个同样包含 SETTINGS 帧的连接序言（Connection Preface）（参考章节 3.5）。

在升级之前所发送的 HTTP 1.1 请求，将被分配一个流标识符 “1”（参考章节 5.1.1），其优先级为默认级别（参考章节 5.3.5）。流 “1” 从 HTTP 1.1 请求完成后，是一个从客户端至服务器的隐性“半关闭”的流（参考章节 5.1）。在开始 HTTP 2 连接后，流 “1” 被用于响应。

#### HTTP2-Settings 报文头（HTTP2-Settings Header Field） ####

一个 HTTP 1.1 请求升级至 HTTP 2，必须（MUST）包含有且仅有一个 HTTP2-Settings 报文头。HTTP2-Settings 报文头是一个特定于连接的报头字段，其中的参数用于管理 HTTP 2 连接，在服务器接收升级请求时预先提供的。

如果没有提供 HTTP2-Settings 报文头，或者提供了超过一个该报文头，服务器不可以（MUST NOT）升级该连接至 HTTP 2。服务器也不可以（MUST NOT）发送这个报文头。

HTTP2-Settings 报文头中的内容，是一个以 Base64 编码的 SETTINGS 帧的 Payload（即：URL 和安全文件名的 Base64 描述（[RFC4648][]）的[第 5 章][5]，任何结尾的‘=’都将被忽略）。token68 的 ABNF（RFC5234）产物在 [RFC7235][] 有所定义。

由于升级（Upgrade）只应用于直接连接，发送 HTTP2-Settings 的客户端必须（MUST）将 HTTP2-Settings 作为连接选项，包含在 Connection 报文头中，以防止它被转发（参考 [RFC7230章节 6.1][RFC7230_ 6.1]）。

服务器解码并解释这些值，并将其看做其他 SETTINGS 帧。由于 101 响应作为了隐式确认，所以显式确认这些设置（参考章节 6.5.3）是没必要的。在升级请求中提供这些值，使得一个客户端有机会在接收服务器发 送的任何帧之前提供参数信息。

### “https”中启用 HTTP/2（Starting HTTP/2 for “https” URIs） ###

一个发起“https”请求的客户端，需要用到 TLS（[TLS1.2][TLS]） 和应用层协议协商（ALPN） 扩展（TLS-ALPN）。

TLS 之上的 HTTP 2 使用“h2”作为协议标识符。客户端一定不能（MUST NOT）发送“h2c”协议标识符，服务器也一定不能（MUST NOT）选择“h2c”标识符使用。“h2c”协议标识符描述了不适用 TLS 的协议。

一旦 TLS 协商完成，客户端和服务器都必须（MUST）发送一个连接序言（Connection preface）。

### 先验下启用 HTTP/2（Starting HTTP/2 with Prior Knowledge） ###

客户端可以通过其他方式判断服务器是否支持 HTTP 2。例如 ALT-SVC，它描述了一种机制使得 HTTP 具有广播能力。

客户端必须（MUST）发送连接序言（Connection preface，参考章节 3.5），然后可以（MAY）立即发送 HTTP 2 帧给服务器。服务端可以通过已存在的连接序言识别出这个连接。这只对基于明文 TCP 的 HTTP 2 连接建立有影响，TLS 之上的 HTTP 2 必须（MUST）使用 TLS 协议协商（TLS-ALPN）。

同样的，服务器必须（MUST） 发送连接序言（参考章节 3.5）。

如果没有其他信息，对 HTTP 2 之前的支持并不代表着指定服务器一定会在之后的连接中支持 HTTP 2。比如，服务器配置有可能改变，或者服务器集群中不同的实例配置有差异，亦或者网络状况的变化。

### HTTP 2 连接序言（HTTP/2 Connection Preface） ###

在 HTTP 2 中，每个终端都需要发送一个连接序言作为协议的最终确认以及 HTTP 2 的连接初始设置。客户端和服务器均需要发送一个不同的连接序言。

客户端连接序言以 24 个字节序列开始，以十六进制表示为：

也就是说，连接序言以字符串 PRI \* HTTP/2.0\\r\\n\\r\\nSM\\r\\n\\r\\n  开头。这个序列后必须（MUST）跟着一个 SETTINGS 帧（参考章节 6.5），其可以（MAY）为空。客户端收到 101 （协议转换）响应后，立刻发送客户端连接序言。或者作为 TLS 连接的第一个应用数据字节。如果在语言验证服务器支持 HTTP 2 的情况下启用 HTTP 2 连接，客户端连接序言在连接建立后发送。

> 注意：客户端连接序言是用来让大部分 HTTP 1.1 或 HTTP 1.0 服务端及中介端不试图进一步处理帧。注意这并不能解决【TALKING】中提到的问题。

服务器连接序言包含一个可能是空的 SETTINGS 帧（参考章节 6.5），它必须在 HTTP 2 连接中首个发送。

从一个节点接收到的连接序言中所包含的 SETTINGS 帧必须在连接序言发送后被确认（参考章节 6.5.3）。

为了避免不必要的延迟，允许客户端在发送客户端连接序言之后立即发送其他额外的帧，不需要等待收到服务器端连接序言。不过，值得注意的是，服务端连接序言 SETTINGS 帧可能包含一些关于期望客户端如何与服务器端通信的必须修改的参数。在收到这些 SETTINGS 帧后，客户端应当遵守所有设置的参数。在有些配置中，服务器可能在客户端发送其他额外帧之前传输 SETTINGS，以提供一个避免这些问题的机会。

客户端和服务器必须（MUST）将一个非法的连接序言当做一个 PROTOCOL\_ERROR 类型的连接错误（Connection error，参考章节 5.4.1）。在这个案例中，当一个节点不支持 HTTP 2 而出现非法序言时，GOAWAY 帧（参考章节 6.8）可以（MAY）被省略。

--------------------

本文以 [CC BY-NC-SA 3.0 CN][] 协议共享，转载、共享及二次创作时请保留原文出处及链接，请勿用于商业用途。

本文链接：[http://litecodes.com/dev/http-2-spec-starting-http-2/][1_ 2016]

本系列文章将会在我的 [GitBook：http2-spec-zh][GitBook_http2-spec-zh] 同步更新，

下一篇文章将会翻译协议的第三部分：HTTP Frames（HTTP 帧），不要错过哟~

[1_ 2016]: http://litecodes.com/dev/http-2-spec-starting-http-2/
[LITECODES]: http://litecodes.com/author/litecodes/
[Starting HTTP_2]: http://litecodes.com/wp-content/uploads/2016/03/http2.jpg
[HTTP 2_HTTP_2 _HTTP_2 Protocol Overview]: http://litecodes.com/dev/http-2-spec-protocol-overview/
[TCP]: https://tools.ietf.org/html/rfc793
[3.2]: http://httpwg.org/specs/rfc7540.html#discover-http
[3.3]: http://httpwg.org/specs/rfc7540.html#discover-https
[TLS]: https://tools.ietf.org/html/rfc5246
[RFC4648]: https://tools.ietf.org/html/rfc4648
[5]: https://tools.ietf.org/html/rfc4648#section-5
[RFC7235]: https://tools.ietf.org/html/rfc7235#section-2.1
[RFC7230_ 6.1]: http://httpwg.org/specs/rfc7230.html#header.connection
[CC BY-NC-SA 3.0 CN]: http://creativecommons.org/licenses/by-nc-sa/3.0/cn/
[GitBook_http2-spec-zh]: https://litecodes.gitbooks.io/http2-spec-zh/content/