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[MMUSIC] SDP for alternative encodings

Aaron Colwell <acolwell@real.com> Tue, 01 March 2005 18:07 UTC

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Date: Tue, 01 Mar 2005 10:04:21 -0800
From: Aaron Colwell <acolwell@real.com>
To: mmusic@ietf.org
Message-ID: <20050301180421.GA21638@real.com>
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Subject: [MMUSIC] SDP for alternative encodings
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I have a few questions about how to represent alternative encodings with SDP.

How do the following 2 SDPs differ in meaning?

m=audio 49230 RTP/AVP 96 97 98
a=rtpmap:96 L8/8000
a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2

m=audio 49230 RTP/AVP 96
a=rtpmap:96 L8/8000
m=audio 49230 RTP/AVP 97
a=rtpmap:97 L16/8000
m=audio 49230 RTP/AVP 98
a=rtpmap:98 L16/11025/2

If I understand the SDP spec it seems to me that this indicates that payload
types 96, 97, and 98 will all be transmitted to port 49230 and represent the
same stream. What am I missing?

The SDP Internet Draft seems to indicate that alternate streams are signalled 
by m= lines using the same port. The RTSP spec says that if the server doesn't
care about what ports are used then it should just specify 0. The SDP draft 
also mandates that m= lines with the same port have "compatible media (e.g.  
both audio or both video)". This seems to imply that the server has to specify
ports if it wants to serve an A/V stream. Is that the intention or is 0 a 
special case? If 0 is a special case then how are you supposed to specify
alternate streams when the server doesn't care about the ports used?

Aaron

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