http://dast.nlanr.net/Training/DCWJuly99/kai_tcpip/sld008.htm I looked at this page of one of the links you sent me. Notice at the Internet and Transport levels it simply says, Application data (datagram ?), TCP header, could this be a datagram, or maybe a packet. Then at the Internet level, application data, TCP header and IP header. Now according to rfc 1122, we know that - o application data, tcp header o application data, tcp header, and ip header. Are both datagrams. Wow IMHO what a mix up. Unless you know the name of the protocol and where you know (or think) it is in TCP/IP, you're lost.