> At the very minimum you would need a centralized discovery server. That's not necessarily true. You could use a DHT. Or have decentralized discovery servers,

> At the very minimum you would need a centralized discovery server. That's not necessarily true. You could use a DHT. Or have decentralized discovery servers, i.e. your username is user@example.com and then example.com is contacted to coordinate direct communication with user. Or for local networks, use multicast service discovery to find local users.