Define "server".
Define "client".
If you are looking at on the basis of multiple connections then the server is the one whose port number is stable from connection to connection (ignoring situations where both the client and server have stable ports as these are not even 0.5% of any one trace (based on the analysis of around 10,000 traces collected)).
For all practical purposes, the server is the process that is listening for incoming connections, while the client is the process that is issuing active opens to a server. There's no requirement for stable port numbers, and the roles are often flip-flopped around from their traditional interpretations (FTP-DATA, H.323, games, etc). -- Eric A. Hall http://www.ehsco.com/ Internet Core Protocols http://www.oreilly.com/catalog/coreprot/