SYN flooding
A denial-of-service attack method in which a hostile client repeatedly sends synchronization (SYN) packets from bogus IP addresses (or legitimate IP addresses that don't respond with an acknowledgement) to every port on a server. The server interprets these as attempts to establish communication, but since the attacker doesn't return the required acknowledgement (ACK), communication is never established and the server remains busy with the hostile client. Rapidly repeated synchronization packets prevent the server from freeing itself by timing out the requests.