Simplest C++ library that supports distributed messaging

I need to do something relatively simple, and I don't really want to install a MOM.

1. 
   

   There are several programs that "register" with a central "service" server through TCP. The only function of the server is to call back all the registered clients when they all in turn say "DONE". So it is a kind of "join" for distributed processes.

   
   


2. 
   

   When all clients say "DONE" (they can be done at totally different times), the central server messages them all saying "ALL-COMPLETE"

   
   

So this is a kind of distributed asynchronous Observer Pattern. The server has to keep track of where the clients are somehow. It is ok to pass the IP address to the server etc. It is constructable with things like Boost::Signal etc, but I don't want to reivent the wheel.

Is there a very simple system that does this?