Ethereal-dev: Re: [Ethereal-dev] Re: [Ethereal-cvs] rev 14456:/trunk/epan/dissectors/: packet-

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From: "Martin Mathieson" <martin.mathieson@xxxxxxxxxxxx>
Date: Fri, 27 May 2005 20:16:44 +0100
> > - register a configurable UDP port (off by default, but needed where
setup traffic isn't captured)
> >
> >
> Just wondering: I really don't know RDT, but wouldn't this be the
> classical case for "Decode As"?
>
> Or are there other reasons as well to have a pref here?
>
> Regards, ULFL
>

There is no fixed port for RDT (and none registered with IANA), but I
understand that the helix client always begins at 6970 for both RDT and RTP
(by default RDT is used).  In my tests, the RealPlayer client has always
chosen this port.  Other clients may choose a different (fixed or not) port.
Therefore I made it optionally register a port (off by default) on a given
port number (6970 by default).

If the RTSP traffic that sets up the stream is captured, then the
conversation/session will be decoded as RDT.  Sessions can last for a long
time, so the probability of capturing RDT packets but not the RTSP setup
frame is quite high.  I looked at making it a heuristic dissector (as RTP &
RTCP are), but the protocol doesn't really lend itself to this (no
consistent signature or distinctive packet size).

"Decode As..." is still available if the setup traffic is missed and the
preference isn't switched onto the correct port...

Regards,
Martin