While reviewing the running processes on my Linux desktop, I noticed a process named mDNSResponder. I have seen this process numerous times on my OS X desktops, but have never taken the time to determine what function it served. Being the curious guy I am, I read through the mDNSResponder(1) manual page, and found the following description:
“The mDNSResponder daemon publishes and browses available services on a link according to the IETF Zeronconf (aka “Rendezvous”) draft standard. Only one instance of mDNSResponder needs to run on a host. Once started, mDNSResponder runs continuously.”
This explains how iTunes is able to discover airtunes capable airport express base stations, and is most likely the foundation of peer-to-eer service discovery in Apple networks.