Here's a really interesting document I stumbled across yesterday, especially having written part of a TCP/IP implementation in years gone by. It was originally published a couple of years ago, but an updated version was released earlier this month which now includes details of changes in Windows Server 2003 SP1. The whitepaper goes into details of the implementation of the TCP/IP stack for Windows Server 2003, which is, all but a few differences, the same as in Windows XP - where there are differences, they are highlighted in the document.

It goes into way more detail than most IT Pro's are ever likely to need, but I'm sure you'll find some value in it. Here's the download link.