Wireshark-users: [Wireshark-users] newbie question about tcp three-way handshaking

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From: 温金超 <wenjinchao0418@xxxxxxxxx>
Date: Tue, 22 Jan 2013 10:41:49 +0800
Hi guys,

Is should be three-way handshaking or not when trying to establishe a new connection?
but I tcpdump following:
 
19:23:12.688758 IP 10.1.164.64.51350 > 10.13.220.4.80: S 3779651860:3779651860(0) win 8192 <mss 1260,nop,wscale 8,nop,nop,sackOK>
19:23:12.688776 IP 10.13.220.4.80 > 10.1.164.64.51350: S4133937230:4133937230(0) ack 3779651861 win 5840 <mss 1460,nop,nop,sackOK,nop,wscale 9>
19:23:12.688779 IP 10.13.220.4.80 > 10.1.164.64.51350: S4133937230:4133937230(0) ack 3779651861 win 5840 <mss 1460,nop,nop,sackOK,nop,wscale 9>
19:23:12.689716 IP 10.1.164.64.51350 > 10.13.220.4.80: . ack 1 win 260

10.13.220.4 ack twice for syn. any idea ?

Thanks for any suggesstion.

jinchao