SYN Flood Attack - Transport Layer

A SYN flood is a form of denial-of-service attack in which an attacker sends a succession of SYN requests to the system of a target in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic.
A SYN flood attack works by not responding to the server with the expected ACK code. The malicious client can either simply not send the expected ACK, or by spoofing the source IP address in the SYN, causing the server to send the SYN-ACK to a falsified IP address, which will not send an ACK because it "knows" that it never sent a SYN.
The server will wait for the acknowledgement for some time, as simple network congestion could also be the cause of the missing ACK.
In an attack, the half-open connections created by the malicious client bind resources on the server and may eventually exceed the resources available on the server. At that point, the server cannot connect to any clients, whether legitimate or otherwise. This effectively denies service to legitimate clients. Some systems may also malfunction or crash when other operating system functions are starved of resources in this way.