Hi Atul, Please see inline.
Kind Regards, Noman Siddiqui ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of atul garg Sent: Wednesday, July 13, 2011 11:30 AM To: [email protected] Subject: [Sip] Codec Negotiation and renegotiataion Hello All, I have very basic question regarding the codec negotiation in SIP, I will try to summarize my queries below - 1) If A is initiating the call and sending the supported codec list say (1,2,3) A --> INVITE (SDP-> audio 1 2 3) --> B ( B supports 1 , 3, 5) A <-- 200 OK ( SDP -> what will be here 1, 2 ,3 OR 1, 3, 5) <-- B [Mohammed Noman Siddiqui] B would send 1, 3 in 200 OK if it is 1, 3, 5 then what will be the response to B ?? [Mohammed Noman Siddiqui] What do you mean by response to B. B is sending the response to A as above 2) If A is initiating the call and sending the supported codec list say (1,2,3) A --> INVITE (SDP-> audio 1 2 3) --> B ( B supports 2 , 3, 5) A <-- 200 OK ( SDP -> what will be here 2 , 3 OR 2 , 3, 5) <-- B [Mohammed Noman Siddiqui] 2,3 if it is 2 , 3 then what will be the response to B ?? if it is 2 , 3, 5 then what will be the response to B ?? 3) If A is initiating the call and sending the supported codec list say (1,2,3) A --> INVITE (SDP-> audio 1 2 3) --> B ( B supports 4, 5,6) A <-- 200 OK ( SDP -> what will be here ??) <-- B and what will be the d behaviour of A .... [Mohammed Noman Siddiqui] B would send 488 Not Acceptable here 4) Re-Negotiation - If A is initiating the call and sending the supported codec list say (1,2,3) A --> INVITE (SDP-> audio 1 2 3) --> B ( B supports 1 , 2, 3) A <-- 200 OK ( SDP audio1, 2 ,3 <-- B A --> ACK --> B ( I guess the rtp will use codec 1) [Mohammed Noman Siddiqui] Yes, RTP will use codec 1, however both sides should be able to dynamically switch the payload to either 2 and 3 Now during the call say A wants to change the codec to 2, will it send the re-invite or SDP session has some provision for it( I just want to confirm, is it possible to change the codec without sending any SIP message) [Mohammed Noman Siddiqui] Yes, dynamic payload switching based on the negotiated codecs list is permitted. Unfortunately, many devices do not support this and hence its better to do the re-negotiation. PS: I know i have made a lengthy mail, but it is very much required for me to understand some network behaviour and implementation. Regards Atul
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