The SYN flood attack sends TCP connections requests faster than a machine can process them.
Attacker creates a random source address for each packet SYN flag set in each packet is a request to open a new connection to the server from the spoofed IP address
Victim responds to spoofed IP address, then waits for confirmation that never arrives (timeout wait is about 3 minutes)
Victim’s connection table fills up waiting for replies and ignores new connections Legitimate users are ignored and will not be able to access the server
How do you protect your network against SYN Flood attacks?

A.
Stack Tweaking. TCP stacks can be tweaked in order to reduce the effect of SYN floods.

B.
Check the incoming packet’s IP address with the SPAM database on the Internet and enable the filter using ACLs at the Firewall

C.
RST cookies – The server sends a wrong SYN/ACK back to the client. The client should then generate a RST packet telling the server that something is wrong. At this point, the server knows the client is valid and will now accept incoming connections from that client normally

D.
Micro Blocks. Instead of allocating a complete connection, simply allocate a micro-record of 16- bytes for the incoming SYN object

E.
SYN cookies. Instead of allocating a record, send a SYN-ACK with a carefully constructed sequence number generated as a hash of the clients IP address, port number, and other information. When the client responds with a normal ACK, that special sequence number will be included, which the server then verifies. Thus, the server first allocates memory on the third packet of the handshake, not the first

Explanation:
Reduce the timeout before a stack frees up the memory allocated for a connection
All above helps protecting against SYN flood attacks. Most TCP/IP stacks today are already tweaked to make it harder to perform a SYN flood DOS attack against a target.