I have yet to read the JXTA docs so I may be
speaking out of school here but if JXTA is in
fact trying to define the "common core of P2P
protocols" then I whole heartedly agree with
Mr. Langley's admonition to stay well away.

Personally I think a much better approach would
be to leverage the hell out of BEEP. While not a
perfect solution for all P2P issues, e.g., it
doesn't support multicasting, it does provide a
solid, standard foundation for experimenting
with various protocol design options.