What is the underlying transport for D-Bus?
This is remarkably similar to the question DBus query. And the answer from Googling was sockets - either for TCP/IP or Unix Domain.
Apparently, IPC or TCP/IP:
http://www.freedesktop.org/wiki/Software/dbus
Update:
I mean, multiple IPC methods on different OS's, plus TCP/IP.
http://dbus.freedesktop.org/doc/dbus-daemon.1.html shows that the unix reference edition uses both unix domain sockets and tcp/ip.