In the four-layer model, "Application" encompasses "Layer 7", "Layer 6", and "Layer 5" from the seven-layer OSI model. I always considered TCP headers or IP headers as data formats that make up the overall protocol. They have to be processed. Therefore, they are essentially "application data". Anyway, it gets confusing when you start trying to differentiate between what is an algorithm, what is a data format, what is a file format, and what is a protocol. The terms should be a lot clearer. For instance, some people think algorithms and protocols are the same. Other people think data formats and protocols are the same. Additionally, some people have trouble differentiating between what is a data format and a file format. Does data format mean the data is generated on-the-fly whereas a file format is a static file? In the logical sense, something like XML still has to be processed so to me it's just another data format but if you think of it in its "statically and physically stored" sense it's a file format. Mind boggling... :-) Brian B. >>> "Bill Cunningham" <billcu@citynet.net> 09/30/02 12:34AM >>> 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. - This message was passed through ietf_censored@carmen.ipv6.cselt.it, which is a sublist of ietf@ietf.org. Not all messages are passed. Decisions on what to pass are made solely by Raffaele D'Albenzio.