[AG-TECH] VPCscreen, Multicast and TTL

Christoph Willing 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:

> Bonjour,
> Just to summarize, I have ran more tests on another site where  
> Multicast
> is enabled:
> * With VPCscreen only, all packets corresponding to the video stream  
> are
> 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

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