[zeromq-dev] ZMQ_MCAST_LOOP: Control multicast loopback

Steven McCoy steven.mccoy at miru.hk
Thu Jun 24 12:52:39 CEST 2010


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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