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. 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. 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 an idiosyncratic behavior that differs from the RFC specifications, such as the case when miscalculations affect a field value.