If you've ever been responsible for a network, you know that sinkingfeeling: your pager has gone off at 2 a.m., the network is broken, and you can't figure out why by using a dial-in connection from home. You drive into the office, dig out your protocol analyzer, and spend the next fourhours trying to put things back together before the staff shows up for work. When this happens, you often find yourself looking at the low-level guts of the Internet protocols: you're deciphering individual packets, trying to figure out what is (or isn't) happening. Until now, the only real guide to the protocols.