FlightGear does have built in TCP/IP support using various options - these are just currently not used by/exposed to the MP subsystem - equally, the TCP/IP functionality (search terms: FGInterface and SGProtocol) are not exposed to scripting space - otherwise, the MP/fgms stack would have almost certainly been deprecated years ago, because all that is needed to re-implement MP/fgms in a much more generic, and efficient, form is exposing the corresponding SG/FG APIs to scripting space. In fact, most of the existing protocol could be easily made much more efficient by using a PUB/SUB approach.
However, most MP related ideas have been put on hold (read: blocked) by ongoing discussions involving HLA.
But HLA really isn't needed to make significant improvements to the MP stack, e.g. see:
http://wiki.flightgear.org/Distributed_ ... SimulationThat being said, another -orthogonal- development may be Richard's Emesary framework:
http://wiki.flightgear.org/EmesaryIn conjunction with exposing the XDR handling wrappers, this could also be used to come up with a better MP system, without people necessarily having to be familiar with C++, or intimately familiar with fgfs internals.