[zeromq-dev] Precise meaning of ZMQ_MULTICAST_HOPS?

CFK cfkaran2 at gmail.com
Fri Aug 31 22:59:31 CEST 2012

Hi all, I need to know the precise meaning of ZMQ_MULTICAST_HOPS in the
latest version of ZMQ, and I'm hoping someone can fill me in (and maybe
update the documentation so others will know it as well).

Some background: I'm trying to use PUB-SUB over multicast on a mobile
ad-hoc network (batman-adv,
http://www.open-mesh.org/projects/batman-adv/wiki/Quick-start-guide, for
anyone that's interested), to simulate reliable 1-hop broadcast for an
experiment we're working on.  To make this work, my plan is to have every
node publish to its own multicast address, and to subscribe to every other
node's address.  So far, this seems to work in my test code (only tested
under 2.2).  Unfortunately, I discovered that I can't limit how many hops a
message will be routed under 2.2, which means that under batman, every
connected node gets every message.  The 3.x series has the
ZMQ_MULTICAST_HOPS socket option, which looks like it would the right thing
to use, but its documentation at
http://api.zeromq.org/3-3:zmq-setsockopt#toc18 says two slightly different
things; in the synopsis, it says that this limits the TTL, which would
limit packets to whatever hop diameter I want, but the full documentation
talks about 'local network', which makes me think of local subnet, which is
an entirely different thing.  So, is it a true TTL, or is it for the local
subnet?  If it is for the local subnet, what is the best/easiest way to
make all multicast packets have a true TTL?

Cem Karan
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