Subject: [Sipping] Query regarding m-lines in SDP

From: Amarendra Kumar <[email-masked]>
Date: Thu, 8 Sep 2005 00:35:17 +0530

Hi,

According to the rfc2327 m-lines in the SDP is mandatory. But according to
rfc3264 SDP can be sent out with out m lines as per the following statement
in rfc

In section 5.

"The offer will contain zero or more media streams (each media stream
   is described by an "m=" line and its associated attributes).  Zero
   media streams implies that the offerer wishes to communicate, but

   that the streams for the session will be added at a later time
   through a modified offer."

In section 6.

   "For each "m=" line in the offer, there MUST be a corresponding "m="
   line in the answer.  The answer MUST contain exactly the same number
   of "m=" lines as the offer.  This allows for streams to be matched up

   based on their order.  This implies that if the offer contained zero"
   "m=" lines, the answer MUST contain zero "m=" lines.

Thus if an end point receives an INVITE with zero m-lines, it will reject 
the request with 
406 Not Acceptable. 

But the same same senario with the RFC 3264 and 3725 will work
with-out any problem. There is a significant gap between 3264 and 2327. 
I didn't find any documentation for it. rfc3264 and rfc2327 contradict with 
each
other wrt m-lines. Even in the latest draft of SDP(
draft-ietf-mmusic-sdp-new-25.txt)
doesn't speak about this also. I think this a problem with rfc2327.
As part of draft draft-ietf-mmusic-sdp-new-25.txt m-line can be made 
optional, 
which will resolve the gap between rfc227 and 3264.

-- 
Rgds,
Amar
Mobile: +919886395894
The greatest enemy of best is "good." If you're willing to accept "good" 
you'll never be the "Best."
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---

From: Colin Perkins <[email-masked]>
Date: Thu, 8 Sep 2005 00:58:38 +0100

On 7 Sep 2005, at 20:05, Amarendra Kumar wrote:
> According to the rfc2327 m-lines in the SDP is mandatory.

No, it is not. The media description, including the m= line, is  
optional in both RFC 2327 and draft-ietf-mmusic-sdp-new-25.txt.  Near  
the top of page 9 of draft-ietf-mmusic-sdp-new-25.txt it is stated  
that a session description contains "Zero or more media descriptions"  
where the media descriptions, if present, contain an m= line. The  
same text is present in RFC 2327.

Colin