HTTP 500之后，会出现一段时间的TCP "TIME_WAIT"状态。因为500之后，服务器主动关闭了连接，导致TCP进入了"TIME_WAIT"状态。这段时间持续的长度为2个MSL时间长度。
『MSL在RFC 1122上建议是2分钟，而源自berkeley的TCP实现传统上使用30秒。』
```
caikiki@ubuntu:~$ netstat -pant
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN -
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN -
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN -
tcp 0 0 192.168.170.190:22 192.168.170.1:50463 ESTABLISHED -
tcp 0 0 192.168.170.190:22 192.168.170.1:50656 ESTABLISHED -
tcp6 0 0 :::80 :::* LISTEN -
tcp6 0 0 :::22 :::* LISTEN -
tcp6 0 0 192.168.170.190:80 192.168.170.1:63422 TIME_WAIT -
caikiki@ubuntu:~$ date
Sun Jan 21 08:55:16 PST 2018
# 一分钟之后
caikiki@ubuntu:~$ netstat -pant
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 127.0.0.1:3306 0.0.0.0:* LISTEN -
tcp 0 0 127.0.0.1:6379 0.0.0.0:* LISTEN -
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN -
tcp 0 0 192.168.170.190:22 192.168.170.1:50463 ESTABLISHED -
tcp 0 0 192.168.170.190:22 192.168.170.1:50656 ESTABLISHED -
tcp6 0 0 :::80 :::* LISTEN -
tcp6 0 0 :::22 :::* LISTEN -
caikiki@ubuntu:~$ date
Sun Jan 21 08:56:16 PST 2018
```
参考：
http://blog.oldboyedu.com/tcp-wait/