HTTP协议探究（序章） - HelloWorld开发者社区

1 HTTP协议基于TCP协议

（1）TCP三次握手连接

HTTP客户端（Chrome浏览器）：
- IP：192.168.1.47
- 端口：59875
- MSS：1460
HTTP服务器（Nginx服务器）：
- IP：45.76.37.162
- 端口：80
- MSS：1452
1 8.893857 192.168.1.47 45.76.37.162 TCP 66 59875 → 80 [SYN] Seq=0 Win=64240 Len=0 MSS=1460 WS=256 SACK_PERM=1 2 9.151584 45.76.37.162 192.168.1.47 TCP 66 80 → 59875 [SYN, ACK] Seq=0 Ack=1 Win=29200 Len=0 MSS=1452 SACK_PERM=1 WS=128 3 9.151807 192.168.1.47 45.76.37.162 TCP 54 59875 → 80 [ACK] Seq=1 Ack=1 Win=66560 Len=0

（2）第一个HTTP请求

4     9.152317    192.168.1.47    45.76.37.162    HTTP 461    GET / HTTP/1.1 

# 具体报文
Hypertext Transfer Protocol
    GET / HTTP/1.1\r\n
    Host: 45.76.37.162\r\n
    Connection: keep-alive\r\n
    Cache-Control: max-age=0\r\n
    Upgrade-Insecure-Requests: 1\r\n
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36\r\n
    Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8\r\n
    Accept-Encoding: gzip, deflate\r\n
    Accept-Language: zh-CN,zh;q=0.9\r\n
    \r\n

（3）第一个HTTP响应（HTML）

# ACK报文告知客户端我已收到请求，你可以准备接收我的数据了。
5    9.409851    45.76.37.162    192.168.1.47    TCP    60    80 → 59875 [ACK] Seq=1 Ack=408 Win=30336 Len=0
6    9.410305    45.76.37.162    192.168.1.47    TCP    1506    80 → 59875 [ACK] Seq=1 Ack=408 Win=30336 Len=1452 [TCP segment of a reassembled PDU]
7    9.410551    45.76.37.162    192.168.1.47    TCP    1506    80 → 59875 [ACK] Seq=1453 Ack=408 Win=30336 Len=1452 [TCP segment of a reassembled PDU]
8    9.410552    45.76.37.162    192.168.1.47    HTTP    1089    HTTP/1.1 200 OK  (text/html)
9    9.410580    192.168.1.47    45.76.37.162    TCP    54    59875 → 80 [ACK] Seq=408 Ack=3940 Win=66560 Len=0 # 客户端返回收到确认

# 服务器返回的最后一个报文的部分HTTP报文(省略实体内容)
Hypertext Transfer Protocol
    HTTP/1.1 200 OK\r\n
    Server: nginx/1.12.2\r\n
    Date: Wed, 21 Nov 2018 06:57:21 GMT\r\n
    Content-Type: text/html\r\n
    Content-Length: 3700\r\n # HTTP报文长为3700
    Last-Modified: Tue, 06 Mar 2018 09:26:21 GMT\r\n
    Connection: keep-alive\r\n
    ETag: "5a9e5ebd-e74"\r\n
    Accept-Ranges: bytes\r\n
    \r\n
    [HTTP response 1/2]
    [Time since request: 0.258235000 seconds]
    [Request in frame: 103]
    [Next request in frame: 114] # 这里证明了一次TCP请求可以发送多次HTTP报文
    [Next response in frame: 120]
    File Data: 3700 bytes

HTTP报文太长了，一个TCP段无法发送完，于是分成了n份，n为多少呢？
- HTTP报文长为3700，TCP MSS为1452，3700为HTTP实体大小 + HTTP首部（239）才是HTTP包的总大小 = 3939，3939 / 1452 = 3次。
TCP客户端怎么知道TCP服务端，分段发送已经完成？
- 客户端HTTP请求时，发送PSH,ACK报文
- 服务器先返回ACK报文，告知收到
- 接着正式发送分段的HTTP报文（每次都是ACK）
- 最后一段HTTP报文为PSH,ACK报文
为什么HTTP请求时，发送的是PSH,ACK报文？
- 因为只放ACK报文，服务器无法确认客户端是否发送完毕了。
- 简而言之，PSH报文能够告知对方，我已发送完毕，请处理。

（4）第二个请求

10    9.419095    192.168.1.47    45.76.37.162    HTTP    404    GET /nginx-logo.png HTTP/1.1 

# 具体报文
Hypertext Transfer Protocol
    GET /nginx-logo.png HTTP/1.1\r\n
    Host: 45.76.37.162\r\n
    Connection: keep-alive\r\n
    User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36\r\n
    Accept: image/webp,image/apng,image/*,*/*;q=0.8\r\n
    Referer: http://45.76.37.162/\r\n
    Accept-Encoding: gzip, deflate\r\n
    Accept-Language: zh-CN,zh;q=0.9\r\n
    \r\n

（5）第二个响应（图片）

11    9.679782    45.76.37.162    192.168.1.47    HTTP    660    HTTP/1.1 200 OK  (PNG)

# 具体报文（省略实体报文）
Hypertext Transfer Protocol
    HTTP/1.1 200 OK\r\n
    Server: nginx/1.12.2\r\n
    Date: Wed, 21 Nov 2018 06:57:21 GMT\r\n
    Content-Type: image/png\r\n
    Content-Length: 368\r\n
    Last-Modified: Tue, 06 Mar 2018 09:26:21 GMT\r\n
    Connection: keep-alive\r\n
    ETag: "5a9e5ebd-170"\r\n
    Accept-Ranges: bytes\r\n
    \r\n
    [HTTP response 2/2]
    [Time since request: 0.260687000 seconds]
    File Data: 368 bytes