tcpdump 分析 TCP 通信过程

step1: connect(client)

14:26:17.213856 IP 172.25.40.208.49040 > 172.25.40.208.9999: S 2796874307:2796874307(0) win 32767 <mss 16396,sackOK,timestamp 166610063 166607508,nop,wscale 2>
14:26:17.213876 IP 172.25.40.208.9999 > 172.25.40.208.49040: S 2805061524:2805061524(0) ack 2796874308 win 32767 <mss 16396,sackOK,timestamp 166610063 166610063,nop,wscale 2>
14:26:17.213887 IP 172.25.40.208.49040 > 172.25.40.208.9999: . ack 1 win 8192 <nop,nop,timestamp 166610063 166610063>

step2: send(client)

14:26:59.682802 IP 172.25.40.208.49040 > 172.25.40.208.9999: P 1:10(9) ack 1 win 8192 <nop,nop,timestamp 166620679 166610063>
14:26:59.682867 IP 172.25.40.208.9999 > 172.25.40.208.49040: . ack 10 win 8192 <nop,nop,timestamp 166620679 166620679>

step3: close(server)

14:28:16.448889 IP 172.25.40.208.9999 > 172.25.40.208.49040: F 1:1(0) ack 44 win 8192 <nop,nop,timestamp 166639869 166628957>
14:28:16.449045 IP 172.25.40.208.49040 > 172.25.40.208.9999: . ack 2 win 8192 <nop,nop,timestamp 166639870 166639869>

netstat

client: CLOSE_WAIT

server: FIN_WAIT2

step4: send(client)

14:45:24.783124 IP 172.25.40.208.37241 > 172.25.40.208.9999: P 8:13(5) ack 2 win 8192 <nop,nop,timestamp 166896937 166892750>
14:45:24.783142 IP 172.25.40.208.9999 > 172.25.40.208.37241: R 3992527880:3992527880(0) win 0

此时客户端依然可以发送数据（这里发送了 5 个字节），服务端已经关闭连接，所以内核回包为 “RESET”.

正常的连接断开

显然，上面的连接断开是非正常的连接断开，下面是正常的连接断开。

client close, server read 0 and close

16:11:48.289783 IP 172.25.40.208.35642 > 172.25.40.208.9999: F 5:5(0) ack 1 win 8192 <nop,nop,timestamp 168192733 168190498>
16:11:48.289870 IP 172.25.40.208.9999 > 172.25.40.208.35642: F 1:1(0) ack 6 win 8192 <nop,nop,timestamp 168192733 168192733>
16:11:48.289879 IP 172.25.40.208.35642 > 172.25.40.208.9999: . ack 2 win 8192 <nop,nop,timestamp 168192733 168192733>

内核将连接断开的第 2 和 3 个包合并为一个包了。

client close, server read 0, sleep, then close

16:38:28.395910 IP 172.25.40.208.35219 > 172.25.40.208.9999: F 5:5(0) ack 1 win 8192 <nop,nop,timestamp 168592734 168591624>
16:38:28.433255 IP 172.25.40.208.9999 > 172.25.40.208.35219: . ack 6 win 8192 <nop,nop,timestamp 168592744 168592734>
16:38:29.397193 IP 172.25.40.208.9999 > 172.25.40.208.35219: F 1:1(0) ack 6 win 8192 <nop,nop,timestamp 168592985 168592734>
16:38:29.397215 IP 172.25.40.208.35219 > 172.25.40.208.9999: . ack 2 win 8192 <nop,nop,timestamp 168592985 168592985>

发现客户端关闭连接后，sleep 一下再 close 则第 2 和 3 个包不会合并。