I have a project idea... A program that analyzes tcp packages... But I need to learn a bit more about the TCP/IP protocol to actually get it going...

I got myself equipped with some good books, such as "TCP/IP Illustrated", but I would be interested in some good ( online? ) documentation regarding TCP package structure for various protocols...

I'm not interested in the header ( I know what that looks like already). I would like to know the body structure. (e.g. for the yahoo protocol, there are fields containing an ID and a value, separated by the bit value C0 80 if I remember correctly).

Can anybody suggest some good documentation on this subject?

Thanks for reading. Any other suggestions, ideas or tips are welcomed

Well the data contained in a TCP segment is going to be the header + data of whatever application layer protocol is being used, so you'll want to look up the details of the specific protocol you're interested in.

Yup, that's the idea...
I'm searching for documentation regarding the most important protocols out there... http, ftp, etc.
Just the basic package structure, not the whole tutorials on how to implement it in a program, or anything...

EDIT: Ok, a bit more details...
I'm thinking of writing a program that identifies the type of package it is, and that displays (when possible) it's contents. E.G. you capture a package with tcpdump, and: if it's a yahoo protocol package, creates a log file with the conversations. If it's http protocol, getting a website, it saves the website and all it's contents, making a user able to view it, etc. It shouldn't be difficult, but it's a lot of work; it's a good opportunity for me to learn about the TCP/IP protocols and networks.

don't mean to burst your bubble, but ethereal already does this.

hmm, more details please?

Indeed it does. It's called Wireshark now.