Downright impressive. I'm implementing the TCP/IP stack for my OS at the moment, and it's sitting at about 400 lines before TCP (ethernet, IP, DHCP, and ARP). Definitely going to see how I can improve it after this.
Let me add that most of his work was for TCP/IP over serial ..
That's impressive. Are you writing your OS for study purposes or you have something special in mind? I am interested in knowing as I am planning to do the same and might want to consult you.
If you want to talk more about it or OS dev in general, feel free to ping me on IRC (Daeken on freenode) or on AIM (bloomfilter).