HTTP 各版本特点与区别

HTTP 各版本一览表

版本	功能	备注
0.9	仅支持 Get 仅能访问 HTML 格式资源	简单单一
1.0	新增POST，DELETE，PUT，HEADER等方式增加请求头和响应头概念，指定协议版本号，携带其他元信息（状态码、权限、缓存、内容编码）扩展传输内容格式（图片、音视频、二进制等都可以传输）	存活时间短
1.1	长连接：新增 Connection 字段,可以通过`keep-alive`保持长连接管道化：一次连接就形成一次管道，管道内进行多次有序响应。允许服务端发生多次请求，但是响应按序返回缓存处理：新增 cache-control 断点续传状态码增加	当前主流版本号存在Header 重复问题
2.0	二进制分帧：数据体和头信息可以都是二进制，统称帧多路复用与数据流：能同时发送和响应多个请求，通过数据流来传输头部压缩：对 Header 进行压缩,避免重复浪费服务器推送：服务器可以向客户端主动发送资源	2005发布

HTTP 1.1

长连接

HTTP/1.1 最大的变化就是引入了持久连接（persistent connection），在HTTP/1.1中默认开启 Connection: keep-alive，即TCP连接默认不关闭，可以被多个请求复用。
客户端和服务器发现对方一段时间没有活动，就可以主动关闭连接。不过，规范的做法是，客户端在最后一个请求时，发送Connection: close，明确要求服务器关闭TCP连接。客户端和服务器发现对方一段时间没有活动，就可以主动关闭连接。不过，规范的做法是，客户端在最后一个请求时，发送Connection: close，明确要求服务器关闭TCP连接。

断点续传

Header 字段

服务端

Accept-Ranges:表示服务器支持断点续传，并且数据传输已字节为单位

Etag:资源的唯一 tag 后端自定义，验证文件是否修改过。修改过就重新重头传输

Last-Modified:文件上次修改时间

Content-Range:返回数据范围

客户端

If-Range:服务器给的 Etag 值

Range:请求的数据范围

If-Modified-Since: 将服务器响应的 Last-Modified 保存，下次发送可以携带，后台接受判断文件是否修改，没有可以返回 304状态码，叫客户端使用缓存数据，避免重复发出资源。

流程

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.edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-u0F3a1VyhDXGrDTc .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-u0F3a1VyhDXGrDTc .marker{fill:#333}#mermaid-svg-u0F3a1VyhDXGrDTc .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-u0F3a1VyhDXGrDTc {color: rgba(0, 0, 0, 0.75);font: normal normal normal normal 16px/26px -apple-system, "SF UI Text", Arial, "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", "WenQuanYi Micro Hei", sans-serif;}ClientServer客户端向移动端请求资源开始下载 Accept-Ranges:bytes, Etag:"xxxxxx", Last-Modified: xxxxloop[中断]携带 If-Range: xxx, Range: xxx- 请求后面部分数据检查资源是否修改过，如果没有，续传 Range 部分。loop[Etag 检查]完成断点续传ClientServer

注意：断点续传后台返回状态码为 206。

Tip: 断点续传与分块编码传输(主要解决长度没有告知)不一致

HTTP2

二进制分帧

HTTP 1.1及之前版本，头信息基于文本，数据体可以是文本或者二进制，HTTP2.0 数据体和头信息可以都是二进制，统称为’帧’，即头信息帧和数据帧，且HTTP2.0把通信基本单位缩小为帧。

多路复用

通过单一的 HTTP2.0连接连续发起多重请求-响应消息，即客户端和服务器可以同时发送多个请求和响应，而不用顺序一一对应。

HTTP消息被分为独立的帧，乱序发送，接收端根据标识符号和首部将信息重新组装起来。

数据流

因为 HTTP/2 的数据包是不按顺序发送的，同一个连接里面连续的数据包，可能属于不同的回应。因此，必须要对数据包做标记，指出它属于哪个回应。HTTP/2 将每个请求或回应的所有数据包，称为一个数据流（stream）。每个数据流都有一个独一无二的编号。数据包发送的时候，都必须标记数据流ID，用来区分它属于哪个数据流。

另外还规定，客户端发出的数据流，ID一律为奇数，服务器发出的，ID为偶数。数据流发送到一半的时候，客户端和服务器都可以发送信号（RST_STREAM帧），取消这个数据流。HTTP/1.1取消数据流的唯一方法，就是关闭TCP连接。这就是说，HTTP/2 可以取消某一次请求，同时保证TCP连接还打开着，可以被其他请求使用。客户端还可以指定数据流的优先级。优先级越高，服务器就会越早回应。

头部压缩

为什么要压缩？

由于头信息使用文本，没有压缩，请求时候会来回重复传递，造成流量浪费。

头部压缩技术原理

参考HTTP2头部压缩技术介绍

头部压缩需要支持 HTTP2的浏览器和服务器之间：

维护一份相同的静态字典（包含常见的头部名称，以及常见的头部名称与值的组合）
维护一份相同的动态字典，动态添加内容（即实际的 Header 值）
支持基于静态哈夫曼码表的哈夫曼编码（Huffman Coding）

原理图：

总结：通过映射表，传递对应编码和值来达到压缩。

默认什么情况下使用同一个连接

同一个域名下的资源
不同域名但是满足两个条件：1）解析到同一个 IP；2）使用同一个证书

参考

HTTP 各版本区别，HTTP2的新特性 – 语雀 - lx70770：简述各版本特点区别

HTTP 各版本特性与区别：

HTTP文件传输-断点续传原理 – 跳墙网 - 神秘网友：头信息字段和流程列举。