An attacker sends a UDP packet to a closed port on the target machine to
						solicit an IP Header's total length field value within the echoed 'Port
						Unreachable" error message. RFC1122 specifies that the Header of the request
						must be echoed back when an error is sent in response, but some operating
						systems and firmware alter the integrity of the original header.
						Non-standard ICMP/IP implementations result in response that are useful for
						individuating remote operating system or router firmware versions. There are
						four general response types that can be used to distinguish operating
						systems apart.1. The IP total length field may be calculated correctly.2. An operating system may add 20 or more additional bytes to the
							length calculation.3. The operating system may subtract 20 or more bytes from the correct
							length of the field4. The IP total length field is calculated with any other incorrect
							value.