[vlc-devel] Streaming wizard issues

jpd at videolan.org jpd at videolan.org
Mon Nov 30 10:02:26 CET 2009

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.

So, a default TTL of 1 is fine for multicast as that limits the damage
to the gateway(s) on the local network, also easing finding the source
of the offending multicast packets. If multicast is too hard for the
user, then I don't want that user to touch multicast.

For broadcast a TTL other than 1 has not made sense since people
discovered that directed broadcast wasn't such a great idea. For other
packets there is a system default, which is not supposed to be 255 but
more likely 32 or 64. I see no reason why we should have our own default
for unicast.

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