http protocols defining what gets sent between web server and browser

hi... this might not be a php question but might still provide interest... I'm working on a project, and need to know if there's anyone who's a guru with Web Server/Client interactions. Basically, I'm trying to get a much better/deeper understanding of the HTTP protocols defining the information that is sent/transfered between the web server/client browser apps. I'm interested in understanding what the various information is that gets transfered between the apps, as well as understanding what information can be spoofed/altered on the client side, as it goes back to the server. I know you can get the querystring information from the get/put/request/etc... I'm more interested in any other information that gets transferred as potentially part of the header structure, like machine id, mac address, date/time, etc.... I'm interested in whether this can be spoofed/altered, and potentially rendered invalid by a 'man in the middle' type of attack. Searching google isn't getting me what i really want!! So, if you have the skills/expertise in this area, and you're willing to talk to me for a few minutes, I'd appreciate it. As stated, the underlying reason for the questions is to get a better understanding of 'man in the middle attacks' as this applies to web server apps. Thanks bruce bedouglas@earthlink.net