问题:

The SYN packet has the same source dest IP address & port with the established connection, so what will happen in this case?

回答1:

The server will silently drop the packet since it already has a connection in the ESTABLISHED state, one of the four values from (client-ip, src-port, server-ip, dest-port) must be different for the new SYN to be accepted.

回答2:

The server will attempt a new connection. in tech terms it will send a syn,ack packet and wait for the client to finish the tcp handshake and open the connection. http://en.wikipedia.org/wiki/Transmission_Control_Protocol will explain the process alot better than me.

the server will send some information to identify the connection in its syn,ack packet. and that information is used to keep that connection seperate from others.

Most the time, the ports will not be the same but when it is, it can cause problems with low grade nat routers, They try to rewrite that ports that are used, and can get the connections confused.