Subject: [SIP] comments on SDP section/bis-03

From: Shan Lu <[email-masked]>
Date: Thu, 31 May 2001 13:45:05 -0400

Hi,

1. From Section B.3.1 (3rd paragraph):

"If the stream in the answer is marked as sendrecv, it MUST contain at 
least one codec the answerer is willing to both send and receive with, 
from amongst those listed in the offer. The stream MAY indicate 
additional codes, not listed in the corresponding stream in the offer, 
that the answerer is willing to receive with."

My comment is on the last sentence. It seems rather odd that somebody 
can list a codec as recv only in a media stream that is send and recv.  
Is this a typo? The last sentence should read as: "The stream ..., that 
the answerer is willing to send and receive with."

2. Immediately after the quoted sentences, I propose that we add the 
following text:

"If more than one codec are listed in the answer, the answerer and 
offerer may choose different codecs to send media, leading to asymmetric 
codecs being used for send and receive. Furthermore,  even for the same 
codec, the answerer and offerer may choose different packetization sizes 
because the indication of packetization time is not mandatory in SDP."

I'd rather see that the SIP spec requires only one codec listed in the 
answer and mandates usage of ptime, for reasons that I elaborated in my 
earlier postings. If people out there think otherwise, at least we need 
to point out the consequences. 

3. From Section B.4 (4th paragraph):

"..., the new SDP MUST have a matching media section for each media 
section in the previous SDP. ... This matching is necessary in order for 
the answerer to determine which stream in the new SDP corresponds to a 
stream in the previous SDP."

I think the idea of "matching media" is completely bogus as described in 
bis-03. Compare the 2nd and 3rd examples in Section B.5. Are the first m 
lines in these two SDPs supposed to match with each other? What if the 
media has changed? Consider the following two SDPs:

SDP1:
m=audio 2400 RTP/AVP 0
m=video 3600 RTP/AVP 31

SDP2:
m=video 3600 RTP/AVP 31
m=audio 2402 RTP/AVP 0

The only thing that has changed is the audio port from 2400 to 2402. But 
since I am allowed to change media type, I can change the order of the m 
lines. We can always refuse/delete a media stream by setting the port to 
0. I don't understand why one needs to change the media type.

Regards,

Shan Lu

NexTone Communications

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