[vlc-devel] [PATCH] live555: fix the subtitles issue

Gilles Chanteperdrix gilles.chanteperdrix at xenomai.org
Mon Dec 30 14:57:40 CET 2013

On 12/30/2013 02:52 PM, Rémi Denis-Courmont wrote:
> Le lundi 30 décembre 2013, 14:40:30 Gilles Chanteperdrix a écrit :
>>> You _cannot_ use RTSP with unicast UDP behind a NAT (unless the NAT has an
>>> RTSP ALG). Period.
>> No, it can be made to work. I know how, I already explained it on this list.
> To traverse a NAT or a stateful firewall, you need cooperation from either the 
> server (hole punching) or the NAT/firewall (ALG or port forwarding). The RTSP 
> specification has no provisions for the  server sid.
> So no, you cannot reliably traverse a NAT with the standardized RTSP protocol.

I am going to try and explain what I am doing very simply.

Do you know how NAT traversal works for any UDP connection, like e.g.
DNS requests?

I am doing the same thing. It requires a little modification of the
client, which is already implemented in live555 and ffmpeg, because it
seems to be required even if the firewall has ALG. And a modification of
the server to use that supplementary information sent by the client. And
the server can still obey the standard if it does not receive the
information. No harm done.


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