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If I have a client trying to connect to a TCP server, and the client sends a couple of retransmits trying to establish a connection, is there ANY difference in the SYNs it will send? If the server SYN-ACKs on the 3rd packet, does the client have any indication as to which copy of the SYN packet the reply is for?

I've been advised by a vendor that an appliance we have will abort a connection attempt if it recieves a SYN-ACK for the original packet after it has retransmitted it, and this sounds impossible to me. This same guy also told be a retransmitted packet would have a different source port to the original, so I have pretty low confidence in anything he tells me!

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He's almost certainly confusing rejection of stale packets from old connections with handling duplicate SYNs. – David Schwartz Feb 1 '12 at 9:16

The retransmitted syn from the client will be having the same source port as that of the previous syn packets .

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