[zeromq-dev] ZMQ_MCAST_LOOP: Control multicast loopback

Martin Sustrik sustrik at 250bpm.com
Thu Jun 24 14:34:31 CEST 2010

Hi Steven,

> 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.

This would only confuse the readers. Let's assume they have no idea how 
reliable multicast works (or even what it is supposed to be), never 
heard of NACKs etc.

If you want your message to be heard and understood you should express 
it in very simple terms.

> A side note, the more advanced functionality being developed in OpenPGM 
> 5 such as congestion control completely fails on multicast loop.

AFAICS the message is: "Never use multicast loopback."

If so, wouldn't it make sense to disable multicast loopback entirely?


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