An attacker uses a technique to generate an ICMP Error message (Port
						Unreachable, Destination Unreachable, Redirect, Source Quench, Time
						Exceeded, Parameter Problem) from a target and then analyze the integrity of
						data returned or "Quoted" from the originating request that generated the
						error message. For this purpose "Port Unreachable" error messages are often
						used, as generating them requires the attacker to send a UDP datagram to a
						closed port on the target. When replying with an ICMP error message some
						IP/ICMP stack implementations change aspects of the IP header, change or
						reverse certain byte orders, reset certain field values to default values
						which differ between operating system and firmware implementations, and make
						other changes. Some IP/ICMP stacks are decidedly broken, indicating a
						idiosyncratic behavior that differs from the RFC specifications, such as the
						case when miscalculations affect a field value. A tremendous amount of
						information about the host operating system can be deduced from its
						'echoing' characteristics. Notably, inspection of key protocol header
						fields, including the echoed header fields of the encapsulating protocol can
						yield a wealth of data about the host operating system or firmware version.