对于客户端而言：
初始的状态是处于 CLOSED 状态。CLOSED 并不是一个真实的状态，而是一个假想的起点和终点。
客户端调用 connect 以后会发送 SYN 同步报文给服务端，然后进入 SYN-SENT 阶段，客户端将保持这个阶段直到它收到了服务端的确认包。
如果在 SYN-SENT 状态收到了服务端的确认包，它将发送确认服务端 SYN 报文的 ACK 包，同时进入 ESTABLISHED 状态，表明自己已经准备好发送数据。
node2:/root/test#time python t18.py
Traceback (most recent call last):
File "t18.py", line 6, in <module>
s.connect(("1.1.1.1",8089))
File "/usr/local/python27/lib/python2.7/socket.py", line 224, in meth
return getattr(self._sock,name)(*args)
socket.error: [Errno 110] Connection timed out
real 1m3.031s
user 0m0.024s
sys 0m0.006s
Last login: Sun Mar 8 11:05:42 2020 from 192.168.137.1
node2:/root#netstat -na | grep 1.1.1.1
tcp 0 1 192.168.137.3:36674 1.1.1.1:8089 SYN_SENT
node2:/root#
node2:/root#tcpdump -S -i eth1 '((tcp) and (port 8089) and (host 1.1.1.1))'
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
12:09:35.772552 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199217467 ecr 0,nop,wscale 7], length 0
12:09:36.772292 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199218467 ecr 0,nop,wscale 7], length 0
12:09:38.772543 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199220467 ecr 0,nop,wscale 7], length 0
12:09:42.772616 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199224467 ecr 0,nop,wscale 7], length 0
12:09:50.772741 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199232467 ecr 0,nop,wscale 7], length 0
12:10:06.772496 IP node2.36705 > one.one.one.one.8089: Flags [S], seq 4166753538, win 14600, options [mss 1460,sackOK,TS val 199248467 ecr 0,nop,wscale 7], length 0
SYN 包重传了 6 次，这个值是由/proc/sys/net/ipv4/tcp_syn_retries决定的， 在我的 Centos 机器上，这个值等于 6
cat /proc/sys/net/ipv4/tcp_syn_retries
6