For this purpose "Port Unreachable" error messages are often used, as generating them requires the adversary to send a UDP datagram to a closed port on the target. The goal of this analysis to make inferences about the type of operating system or firmware that sent the error message in reply.

This is useful for identifying unique characteristics of operating systems because the RFC-1122 expected behavior reads: "Every ICMP error message includes the Internet header and at least the first 8 data octets of the datagram that triggered the error; more than 8 octets MAY be sent [...]." This contrasts with RFC-792 expected behavior, which limited the quoted text to 64 bits (8 octets). Given the latitude in the specification the resulting RFC-1122 stack implementations often respond with a high degree of variability in the amount of data quoted in the error message because "older" or "legacy" stacks may comply with the RFC-792 specification, while other stacks may choose a longer format in accordance with RFC-1122. As a general rule most operating systems or firmware will quote the first 8 bytes of the datagram triggering the error, but some IP stacks will quote more than the first 8 bytes of data.