[AG-TECH] Unicast-Multicast bridges?

Chris Greenhalgh cmg at Cs.Nott.AC.UK
Tue Feb 12 06:16:21 CST 2002

>>> Omer Rana (Cardiff) and I have been trying it out with some success,
>>> although unfortunately we've hit a problem. The unicast end is able to
>>> receive audio just fine, but not send it. The unicast end can see all 
>>> the
>>> rat members (including themselves). The multicast end can't see the 
>>> unicast
>>> end in rat. (We haven't tried out video.)

> I recall seeing this happen once or twice in the past, but never really 
> looked into it.  I suspect some OS-related problem wrt. multicast 
> packets not getting looped back as they should be.

I agree that lack of loopback is the most likely thing. This tends to be 
OS-specific (what the default is, how to control it). A possible 
workaround is to try dedicated machine(s) for each multicasting process. 
(i.e. without running any of the mbone tools on reflector/bridge 
machine(s)). There may also be options for the apps and/or patches to 
make sure loopback is enabled.

A related problem that I've seen is some apps (reflectors and/or 
multicast tools) don't set REUSE_ADDR and/or REUSE_PORT socket options, 
preventing subsequent applications from also getting stuff on the same 
address/port. Usually you'd get a diagnostic message from the subsequent 
app when this happens, e.g. "bind failure".


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