Like what? I'm not familiar with any daemons that expose this info on a network interface. Maybe identd? AFAIK that can expose the username, none of the other fields, but is typically set to an alternate identifier.

Or do you mean shared mainframes with many users on the same system? That'd still be local to the system, you just happened to have access to that system.

Are you sure you're not just thinking of ldap/directory lookups?