There's more to TCP, of course. You won't learn all about TCP in this short article (try Doug Comer's book Internetworking with TCP/IP, Volume 1, Principles, Protocols, a nd Architecture. Second Edition(Prentice Hall, 1991 ISBN 0-13-468505-9) or W. Richard Steven'sTCP/IP Illustrated, Volume 1 (Addison-Wesley, 1993, ISBN 0-201-63346-9). For now, it's important to understand that TCP packets include flag bits that get set to indicate conditions. When you readShimomura's account of the attack , he makes reference to several flags.