[vlc-devel] Streaming wizard issues

Marian Ďurkovič md at bts.sk
Mon Nov 30 10:15:06 CET 2009

On Mon, Nov 30, 2009 at 10:02:26AM +0100, jpd at videolan.org wrote:
> On Mon, Nov 30, 2009 at 09:02:41AM +0100, Marian ??urkovi?? wrote:
> > One additional item:
> > * TTL is by default set to 1, which was supposed to limit multicast
> > to the LAN scope. But it's highly confusing for users and multicast
> > scoping shouldn't be done via TTL hacks as it might put too much
> > burden on routers where TTL expires. TTL should be set to 255 by
> > default.
> I disagree. Strongly.
> The way to limit multicast is either through "administratively scoped
> addresses", which requires supporting router configuration on the local
> network, or through TTL. If the routers can't cope, it is certainly not
> appropriate to set the TTL higher, causing even more routers to have to
> do multicast routing lookups. For if they cannot deal with expiring TTL
> on multicast packets gracefully they certainly cannot be expected to
> deal with multicast routing. That might mean such routers will simply
> treat the multicast packet like broadcast, with undesirable results.

Sorry, you got it all wrong.

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

2) problem with TTL=0 happens even on largest internet-grade routers like
Cisco 7600, which are certainly well capable of routing multicast

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.

  With kind regards,


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