Subject: Re: SDP media alignment in SIP

From: "Stephen Sprunk" <[email-masked]>
Date: Mon, 19 Apr 1999 23:27:20 -0500

Not to mention there are some applications which might wish to change codecs
(possibly dozens of times) during a media stream.  Putting different codecs
on different ports would require unnecessary transcoding in this case.

Stephen

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-----Original Message-----
From: Jonathan Rosenberg <[email-masked]>
To: Anders Kristensen <[email-masked]>
Cc: confctrl <[email-masked]>
Date: Monday, April 19, 1999 22:56
Subject: Re: SDP media alignment in SIP

>Anders Kristensen wrote:
>>
>> On the subject of aligning SDP media streams rfc2543 says:
>>
>>    The caller and callee align their media descriptions so that the nth
>>    media stream ("m=" line) in the caller's session description
>>    corresponds to the nth media stream in the callee's description.
>>
>> As far as I can tell this is not possible in the case where the caller
>> has a single m= line citing multiple codecs (which is very common) and
>> the callee supports at least two of them but on different port numbers.
>> In this case the callee would appear to have to use multiple m= lines in
>> the response.
>
>Why would the callee do this? The port numbers are meant to demultiplex
>multiple streams, and within each stream, the payload type in the RTP
>demultiplexes the codec possibilities. Thus, the callee would respond
>with a single m line, listing the one port number it expects to receive
>the stream, along with the two codecs it supports, and their payload
>type numbers.
>
>-Jonathan R.
>--
>Jonathan D. Rosenberg                       Lucent Technologies
>Member of Technical Staff                   101 Crawfords Corner Rd.
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