Hi Hans, > Are there any papers or documents describing the binary protocol used by ZMQ? > Or is the source code the only documentation? See zmq_tcp(7) for TCP framing, zmq_pgm(7) for PGM/EPGM framing etc. As for aggregation of backpaths in REQ/REP see here: http://www.zeromq.org/tutorials:xreq-and-xrep Martin