[vlc-devel] Streaming wizard issues

Marian Ďurkovič md at bts.sk
Tue Dec 1 12:09:44 CET 2009

On Tue, Dec 01, 2009 at 11:05:11AM +0100, jpd at videolan.org wrote:
> On Tue, Dec 01, 2009 at 08:15:02AM +0100, Marian ??urkovi?? wrote:
> > I said "even" the biggest - meaning that packets with TTL=1 cause
> > *any* router to perform extra processing in SW.
> But _only_ the first router. The number affected explodes with TTL>1.

What? Always, only a *single* router which sees TTL=1 is affected, because
the ASIC copies such packets to CPU. If initial TTL is 2, then not the first,
but the second router is affected, and so on... 
> Routers are prohibited from sending ICMP in response to multicast.[1]
> If they do then they're broken.

Again, ASIC copies packets with TTL=1 to the CPU. Of course, CPU does not
send ICMP for multicast, but it gets the copy anyway and needs to process it.

> No, I'm looking for a sensible default that doesn't cause too much
> trouble in adverse conditions.

You never gave real-world example where this really helps. This is
just hypothetical help to non-existent problem, I'm afraid.

> Different issue than setting TTL=1, which IMO, and for all the reasons
> already detailed, should be not only possible, but the default. If you
> propose to reduce choice to only TTL=1 and TTL=255, then that would be
> sensible.

Might be better than what's available now, but still, default of TTL=1
is clearly *invalid* for IPv6 and unnecessary for IPv4.

> The RFC you quoted implicitly assumes a ``production environment'',
> obviously with a TTL>1, but that isn't what I'm after, as explained
> above.

Any decent application is supposed to work correctly in standards-based
environment out of the box. That's what standards are all about. If every
application introduces strange config knobs, we'll be doing nothing else
than seeking what to tweak just to get basic things working. 

Bottom line: This debate does not make sense anymore. I've expressed my
opinion in a clear way, using the state-of-the-art knowledge, but you still
seem to believe multicast is a bad monster and everyone needs to be protected
from it by ugly hacks. Please realize, that people on multicast-enabled
networks view multicast as yet another thing that is expected to "just work"
and TTL=1 is clearly preventing that.   

   With kind regards,


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