SYN
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SYN (synchronize) is a type of packet used by the Transmission Control Protocol (TCP) when initiating a new connection to sychronize the sequence numbers on two connecting computers. The SYN is acknowledged by a SYN/ACK by the responding computer.
A type of denial of service attack known as a SYN flood involves sending large numbers of SYN packets and ignoring the return, thereby forcing the server to keep track of a large number of half-open connections.