I have a problem,

INVITE: M=audio 3456 RTP/AVP 2 9 15 
200: m=audio 5004 RTP/AVP 2 9

Every side give the coders that it can receive (reverseChannels) 
But how would the UAC will know what was picked by the UAS for transmission
(forwardChannel). 

In other words: lets say I have a machine that can do payload types 2 9 15 
But it must know what algorithm to load before its start receiving. 

In one hand I want to advertise I can do all of the above, to increase
compatibility to other vendors. 

On the other hand I would like to know what the remote is going to send me
without looking at the payload type field in the RTP packet header. 

How can I signal that in SIP/SDP? 

In H.323 you have TCS and than OLC & OLCA 
Or in fastStart: 
OLC & OLCA for forwardChannel 
OLCA and then OLC for reverseChannel 

But in SIP the remote side does not pick from your list and tells you what
will be transmitted, it just start transmitting. 

So if you compare it to H.323 its: 
TCS from one side 
TCS from the other side 

And start transition without saying what will I transmit. 
This can cause many interop problems. 
Do you know a solution for that problem? 
Do I interpret the standard correctly?

Thanks,

Eran.

Eran Shen

Intel Corporation
Telecommunications and Embedded Group 
P.O Box 58, Migdal Tefen, Israel 
Tel: +972-4-9105020 
<mailto:[EMAIL PROTECTED]> 



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