[vlc-devel] Streaming wizard issues

Marian Ďurkovič md at bts.sk
Mon Nov 30 11:14:38 CET 2009

On Mon, Nov 30, 2009 at 10:50:12AM +0100, jpd at videolan.org wrote:
> On Mon, Nov 30, 2009 at 10:15:06AM +0100, Marian ??urkovi?? wrote:
> > 1) multicast doesn't get beyond a router which is not *explicitly*
> > configured to route it. Any local network is effectively isolated by
> > this even without TTL=1 hack
> That's still no reason to set up this and the next network for instant
> flooding the moment it does get enabled.

Multicast router does not flood anything behind it even if it's enabled
to route multicast. It's not a switch. It only starts forwarding when 
asked to do so, via IGMP or PIM join. 

> > 2) problem with TTL=0 happens even on largest internet-grade routers like
> > Cisco 7600, which are certainly well capable of routing multicast
> Apparently not, but then it also was Cisco software that would duly
> decrement-by-one-and-propagate packets incoming with TTY=0. A big name
> is no guarantee of well-behavedness.

It's nothing regarding a name. High throughput is only achievable by HW
forwarding, and ASICs have limited capabilities. Of course it works,
but it's a big difference, wheter your box does 400 Mpps or only 100 kpps
just because you enabled a feature which requires SW forwarding. 

> > 3) usage of TTL scoping is discouraged by latest RFCs, I don't have time
> > to look it up ATM but courmisch can certainly provide a reference to you,
> > since he already changed SAP announcer to *not* use TTL scoping quite some
> > time ago. TTL scoping sucks big time, it shouldn't be used anymore.
> Then at most default to the system-wide default. Do provide references
> when you do have the time.

OK, for example RFC4566 (July 2006):

"Although the TTL MUST be specified, its use to scope multicast traffic
is deprecated; applications SHOULD use an administratively scoped address

> But I think you are mixing up ``providing
> a safe default'' and ``depening on TTY=N for your production use'' here.
> The default in VLC isn't ment for production, but to provide a sane
> default, and in the case of multicast with its flooding potential,

It's not a safe default, it's a broken default. There's no flooding
potential in a properly configured L3 multicast network. If noone
asks the stream, it shouldn't get beyond the first router. 

TTL=1 might work in a home LAN for playing, but in any network consisting
of more than one L3 segment, users can't get RTP working without tweaking
the TTL field.

   With kind regards,


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