Wireshark-bugs: [Wireshark-bugs] [Bug 5208] RTP dynamic payload type can not be dissected correc
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From: bugzilla-daemon@xxxxxxxxxxxxx
Date: Fri, 02 Dec 2016 23:52:31 +0000

changed bug 5208


What Removed Added
CC   peter@lekensteyn.nl

Comment # 18 on bug 5208 from 
I'm currently having a look at the SDP dissector which seems to assume that
port zero means "no media". But that is only true for responses in the
Offer/Answer model. For RTSP the value is merely an optional recommendation
which can be set to zero (see comment 4).

And from the SDP Offer/Answer model RFC we find where the above "ignore media
with port zero" comes from (https://tools.ietf.org/html/rfc3264#section-5.1):

   [..] sendrecv is the default. [..]
   For recvonly and sendrecv streams, the port number and address in the
   offer indicate where the offerer would like to receive the media
   stream. [..]
   A port number of zero in the offer indicates that the
   stream is offered but MUST NOT be used.  This has no useful semantics
   in an initial offer, but is allowed for reasons of completeness,
   since the answer can contain a zero port indicating a rejected stream
   (Section 6).  Furthermore, existing streams can be terminated by
   setting the port to zero (Section 8).  In general, a port number of
   zero indicates that the media stream is not wanted.

I'll first try to refactor the SDP dissector to reduce duplication in
setup_sdp_transport and dissect_sdp. Maybe the SDP dissector should provide a
means to export the parsed SDP to the parent dissector (SDP, RTSP, etc.). RTSP
would then mainly be interested in the media and should probably not register
any recommended ports since the client may ignore them (instead, rely on the
DESCRIBE response).

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