[vlc-devel] [PATCH] rtsp: clean up multicast handling

[Rémi Denis-Courmont]

On Tue, 14 Dec 2010 20:58:25 +0100, David Glaude <david.glaude at gmail.com>
wrote:
> 2010/12/14 Pierre Ynard <linkfanel at yahoo.fr>:
>>> In fact, may I suggest we remove support for custom IPv6 hop limit,
>>> and simplify IPv4 Time-To-Live to a single boolean option (1 or 255)
>>> across VLC?
>>
>> I'm afraid that users who purposefully set the TTL to a given value to
>> control the scope of the stream will miss the option, what would you
>> tell them?
> 
> Yes, it is very important to control the TTL value of multicast
> traffic you generate.

No. It is brain-dead idea. TTL is meant to avoid (infinite) loops, not to
scope traffic.

> You use that to either limit the scope of the traffic and to survive
> loops in your multicast routing table.

The correct and reliable way to scope multicast traffic is to use multicast
destination addresses of the right scope, or to filter out multicast on the
edge. You sign up for major challenges if you rely on the TTL for that
purpose.

> 255 is too big for must purpose anyway.

TTL can be too small, but it can hardly be too big.

The default multicast IPv4 TTL is 1 because many routers just cannot cope
with it at all. It's no accident that the default multicast IPv6 hop limit
is 255. I do not see any justification for any value other than 1 and 255
(and 0 if you really want loopback).

In any case, VLC does not (and should probably not) support multicast
session setup via RTSP, so there is not much point in dealing with the TTL
there.

-- 
Rémi Denis-Courmont
http://www.remlab.net
http://fi.linkedin.com/in/remidenis