I'd like to update this entry in the zmq_setsockopt(3) manpage, "The *ZMQ_MCAST_LOOP* option shall control whether data sent via multicast transports using the specified *socket* can also be received by the sending host via loopback. A value of zero disables the loopback functionality, while the default value of 1 enables the loopback functionality. Leaving multicast loopback enabled when it is not required can have a negative impact on performance. Where possible, disable *ZMQ_MCAST_LOOP* in production environments."
When enabled multicast loop can completely disable reliable delivery. Negative acknowledgements, NAKs, are sent from receiver to sender as unicast messages; unicast packets are not broadcast to all listening sockets hence back channel communication is only functional to the first opened transport. A side note, the more advanced functionality being developed in OpenPGM 5 such as congestion control completely fails on multicast loop. -- Steve-o
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