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My pc is running Windows server 2003 sp2. Whenever I establish an internet connection (e.g., with internet explorer to google.com), my host sends the syn packet twice. It is (probably not) a retransmit as the two syn packet are sent within a 0.01 ms interval (according to wireshark timestamping).

I wrote a short java program that simply do a socket.connect and I have the same thing, but if I run the program on another machine (with Windows XP), it is fine (no duplicate syn).

Anyone has a clue what this is about ?

A bit more data: When I initiate a tcp connection with google.com, my pc sends 2 syn, then I get only 1 syn/ack (which I think is normal), but then, my pc will send the ack twice as well.

Any help is welcome !

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Are you running Wireshark on the same machine? Try running it on another machine on the network. –  Michael Hampton Oct 17 '12 at 1:14
    
Can you perform this test with Nagling disabled? There is a tcpip registry setting, TcpAckFrequency, set it to 1. –  Greg Askew Oct 17 '12 at 1:22

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