[zeromq-dev] Unreliable multicast was: udp not enabled on Mac OS X 10.6

Chris Wong chris at chriswongstudio.com
Fri Mar 5 19:17:33 CET 2010

On Mar 5, 2010, at 12:04 PM, Martin Sustrik wrote:

> Martin Lucina wrote:
>> Yes, but once you introduce sequence numbers you're going to need a
>> handshake to initialise those, which means a "virtual connection" of some
>> sort, and if you want to do that using multicast I guess you'll end up with
>> something like PGM anyway :-)
> No. You need no negotiation. Receiver just has to handle first seqnum it 
> sees as beginning of the stream and wait for following packet. Any 
> larger seqnum means the stream is disrupted at that point. Any lower 
> seqnum means the sender was restarted.
> It may cause some hickups when packets are reordered, but that doesn't 
> really matter, it's unreliable anyway.

Yes.  My intent isn't trying to use Zeromq to build reliability on top of the unreliable multicast.  It's more as an abstract framework for switching the underlying transport to deliver "multicast" message.

> Of course, multiple senders would make a mess.

I think that should be pushed to the application layer to decide.  Zeromq as a framework wouldn't have a better idea on how to handle that anyway.  After all, we're talking about multicast...

>> I guess the answer depends on what applications Chris has in mind.
> Definitely.

The applications I have in mind belong to the category where message is idempotent and only the latest "version" is important. So, if message was lost, a later version will arrive in the next period anyway.  Old version that arrives out of sequence will be discarded by the app.


> Martin
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