### HTTP http是由客户端和服务端请求和答应的标准。通常有客户端发起请求，服务器指定端口建立TCP连接，服务端返回状态行，以及响应消息。 比如请求头 ``` Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9 Accept-Encoding: gzip, deflate Accept-Language: zh-CN,zh;q=0.9 Cache-Control: max-age=0 Connection: keep-alive Cookie: mysid=24189d47508c8c36504c9675196e9d35; PHPSESSID=68v7ff9oem0e9nnok62lf123kh; sec_defend=9f5ac718f59bda04c4b5c8c559dad343a970620d12abe86195c090b18b406632; sec_defend_time=1 Host: xxxx.com Referer: http://xxxx.com/ Upgrade-Insecure-Requests: 1 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.198 Safari/537.36 ``` 比如响应头 ``` Cache-Control: no-store, no-cache, must-revalidate Connection: keep-alive Content-Encoding: gzip Content-Type: text/html; charset=UTF-8 Date: Mon, 17 Jan 2022 08:09:21 GMT Expires: Thu, 19 Nov 1981 08:52:00 GMT Pragma: no-cache Server: nginx Transfer-Encoding: chunked Vary: Accept-Encoding ```