It was thus said that the Great Roger Merchberger once stated: If you have the money, "TCP/IP Illustrated" Volumes I and II by Stevens is a good start (Volume III isn't really worth it). Volume I is the actual specification and Volume II goes over the BSD implementation. If, on the other hand, you don't have the money but you do have the time to read technical documentation, the TCP/IP spec is spelled out in RFC791, RFC950, RFC919, RFC922, RFC792, RFC1112, RFC768 and RFC793. IP and UDP aren't that complicated, but TCP can get involved. You might also want to check out the PPP spec in RFC1661 (and others, bleh). -spc (Have fun ... )