[zeromq-dev] ZMQ, multicast and MTU

Jim Hague jim.hague at acm.org
Fri Nov 20 22:27:56 CET 2015

Hi all,

I am wondering how people usually handle MTU issues with multicast using EPGM.

I am working on a deployment with several different multicast destinations. 
Several are on a local LAN segment, and the interface MTU is fine and dandy.

Some, though, go over an external WAN with a significantly smaller MTU.

I don't want to have to configure the interfaces to the smaller MTU, but I 
don't see a way to limit datagram size for destinations over the external WAN 
without altering the interface MTU. Ideally, then, I'd like to be able to 
configure a per-destination MTU. This would also mean that if in future 
another destination is added with an even more restrictive MTU I can add that 
without needing to change interface configuration on sending machines.

I guess I'd need to set the PGM socket option PGM_MTU to achieve this, but 
that's not exposed in ZMQ. Indeed, it seems to be set in ZMQ, but it's nailed 
to 1500 (pgm_max_tpdu).

I'm thinking of creating a patch to add a ZMQ socket option to set PGM TPDU, 
but before I do, am I missing something obvious? How do people normally handle 
this scenario?
Jim Hague - jim.hague at acm.org          Never trust a computer you can't lift.

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