Subject: Re: Question regarding SDP and MIME codec registration...

From: Henning Schulzrinne <[email-masked]>
Date: Fri, 21 Apr 2000 11:48:27 -0400

I should have read Mike's message more carefully - what I said is wrong,
as are the other examples given for SDP selection of codecs provided
earlier by others. Note that the SDP parameter is the *clock* rate, not
the bit rate. This parameter is included to allow receivers that do not
understand the details of the encoding to deal with RTP/RTCP information
measured in clock samples. (For example, an RTP recorder doesn't need to
understand the media encoding, but needs to know what the clock rate is
so that it can play back the recorded data without the network jitter.)

For all G.723 and G.729 varieties, the clock (sampling) rate is 8000 Hz.
Thus, different versions of the codec need to be labeled and cannot be
distinguished by the clock rate parameter. Thus, the rtpmap format would
be something like

a=rtpmap:100 vnd.nuera.ecelp7470/8000

Using the third parameter (as in vnd.nuera.ecelp/8000/7470) for the bit
rate is probably not a good idea since this parameter is reserved for
the number of audio channels. RFC 2327 also says explicitly
"Codec-specific parameters should be added in other attributes."

I would also argue that using the third parameter for the bit rate
complicates implementations, as they now have to look in three places to
create a table entry. Encoding it as part of the name seems easier,
particularly given that the number of rates seems to be small.

Hope this clarifies things.
-- 
Henning Schulzrinne   http://www.cs.columbia.edu/~hgs

---

From: Mark Handley <[email-masked]>
Date: Fri, 21 Apr 2000 09:12:36 -0700

>"Codec-specific parameters should be added in other attributes."

Note that there is also a b= field in SDP specifically for specifying
bandwidths.  

Cheers,
	Mark