ZyXEL 4-port Prestige 792H Router 91-004-342001 Manual Do Utilizador

Prestige 792H G.SHDSL Router 

Firewalls  

8-5 

 

Under normal circumstances, the application that initiates a session sends a SYN (synchronize) packet to the 
receiving server. The receiver sends back an ACK (acknowledgment) packet and its own SYN, and then the 
initiator responds with an ACK (acknowledgment). After this handshake, a connection is established.  

2-a  SYN Attack floods a targeted system with a series of SYN packets. Each packet causes the 
targeted system to issue a SYN-ACK response. While the targeted system waits for the ACK that 
follows the SYN-ACK, it queues up all outstanding SYN-ACK responses on what is known as a 
backlog queue. SYN-ACKs are moved off the queue only when an ACK comes back or when an 
internal timer (which is set at relatively long intervals) terminates the three-way handshake. Once 
the queue is full, the system will ignore all incoming SYN requests, making the system unavailable 
for legitimate users.