[AG-TECH] VPCscreen, Multicast and TTL
c.willing at uq.edu.au
Thu Apr 1 06:02:13 CDT 2010
On 01/04/2010, at 7:37 PM, Philippe d'Anfray wrote:
> Just to summarize, I have ran more tests on another site where
> is enabled:
> * With VPCscreen only, all packets corresponding to the video stream
> given a TTL of 16
> * With VPCscreen and a Video Producer I can see packets with a TTL
> of 16
> and others with a TTL of 127
> Only those with a TTL of 127 can be received outside the site.
> Has someone used VPCScreen successfully in Multicast ?
Yes, as per one of my earlier emails, our results are somewhat
different to yours - our use of vpcscreen with multicast shows no
problems so far.
Although it is clearly wrong that vpcscreen's ttl is somehow reset to
16 even when you explicitly set it to 127, I would nevertheless expect
that a ttl of 16 is more than sufficient for most multicast sessions
within a country - try an ordinary ping to one of your regular
destinations - does it take more than 16 hops? So, although there is
something wrong with vpcscreen, I think the issue is made worse by
some router misconfiguration somewhere in your area (perhaps wrongly
passing only multicast traffic with ttl of, say 32).
How to fix vpcscreen? I had a quick look through the source code for
obvious problems yesterday and found one potential issue but haven't
had a chance to investigate it yet - maybe in the next few days.
BTW you could confirm that the reduced ttl is actually the cause of
your original problem with vpcscreen by running vic (which works as
expected with multicast, I think?) with a lower ttl too - does it then
exhibit the same problem of not reaching the destination when you set
its ttl to 16?
Christoph Willing +61 7 3365 8316
QCIF Access Grid Manager
University of Queensland
More information about the ag-tech