DoS protection against flooding of new sessions is beneficial against high-volume single-session and multiple-session attacks. In a single-session attack, an attacker uses a single session to target a device behind the firewall. If a Security rule allows the traffic, the session is established and the attacker initiates an attack by sending packets at a very high rate with the same source IP address and port number, destination IP address and port number, and protocol, trying to overwhelm the target. In a multiple-session attack, an attacker uses multiple sessions (or connections per second [cps]) from a single host to launch a DoS attack.

DoS Protection Profiles and Policy Rules work together to provide protection against flooding of many incoming SYN, UDP, ICMP, and ICMPv6 packets, and other types of IP packets. You determine what thresholds constitute flooding. In general, the DoS Protection profile sets the thresholds at which the firewall generates a DoS alarm, takes action such as Random Early Drop, and drops additional incoming connections. A DoS Protection policy rule that is set to protect (rather than to allow or deny packets) determines the criteria for packets to match (such as source address) in order to be counted toward the thresholds. This flexibility allows you to block certain traffic, or allow certain traffic and treat other traffic as DoS traffic. When the incoming rate exceeds your maximum threshold, the firewall blocks incoming traffic from the source address.