[vlc-devel] Issue 66 -- NAT traversal of RTSP

[Rémi Denis-Courmont]

Le mercredi 17 novembre 2010 11:50:54 John Wimer, vous avez écrit :
> Back in the real world, NATs usually don't inspect the RTSP traffic. One
> technique used to convince a NAT to open a port is to send so-called
> punch packets. These truncated packets come from the listening rtp and
> rtcp ports and are directed to the server's sending rtp and rtcp ports.

That will create a mapping on the NAT between the internal client ports and 
some unknown ports on the Internet side of the NAT.

> All RTSP servers that I know of simply discard the packets as corrupt.

Sure.

> The simple act of sending a UDP packet through the NAT usually creates a
> port forward to the client. It's a simple system, frequently works, and
> has no major drawbacks.

It would work if you could convince the RTSP server to send RTP/RTCP packets 
to where the packets came from, instead of to the client_port pair in the 
Transport field. But there are no standard transports with such semantics. And 
my suggestion to this end were more or less ignored on the IETF MMUSIC working 
group a few years back.

In other words, this technique works if you have a stateful firewall but no 
NAT. It also work for a few totally idiotic NATs that preserve port numbers.

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