[AG-TECH] VPCscreen, Multicast and TTL

Philippe d'Anfray Philippe.d-Anfray at cea.fr
Thu Apr 1 08:40:49 CDT 2010


I tried using just vic with a multicast address between "our" two sites.
It appears that the threshold for the TTL here is... 35 (!)
(below the video gets lost somewhere)

If the routers use the policy I read here
....."The IP multicast routing protocol uses the *Time To Live (TTL)*
field of IP datagrams to decide how "far" from a sending host a given
multicast packet should be forwarded.".....

Then the TTL is used as an "a priori scope"  (and not only as a hop count)
it is normal that we receive nothing even if the number of hops between
the two sites here is only 8.



Christoph Willing a écrit :
>> * 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.
> 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?

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