On 7/13/11 11:29 AM, atul garg wrote:
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
if it is 1, 3, 5 then what will be the response to B ??
The answer could include 1; 1,3; 1,3,5; or I suppose 1,5; 3,5.
A doesn't "respond" to B.
If the answer contains both 1,3 then A could use either 1, 3, or both
when sending to B, and B can use either 1, 3, or both when sending to A.
So if B only wants to use one codec it would be well advised to only
mention one in the answer.
If B mentioned codec 5, it isn't really useful immediately. Its just an
FYI for A. It could be treated as a hint, in case A does support 5 and
just didn't mention it.
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
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 ??
This isn't much different from (1).
B could answer with any of the things you say.
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
Typically B would respond with 488.
If it wants, it could respond 200 and refuse the media stream (port 0).
That is probably not a good idea unless it has further plans - e.g. to
make another offer of some sort.
and what will be the d behaviour of A ....
If the response is 488, A gives up.
If response is 200 with refused media, it might want to wait and see
what happens next. Or else it might as well send a BYE.
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)
rtp could be 1, 2, 3, or a combination.
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)
PS: I know i have made a lengthy mail, but it is very much required for
me to understand some network behaviour and implementation.
If 1,2,3 negotiated, it can just start using 2.
But I think this will break a number of implementations that make
unjustified assumptions. I know there are big vendors with products that
can only support one codec and must renegotiate to switch, and assume
the first one in the answer is the one to be used.
So you would be well advised to send a new offer with just 2 if that is
the one you want to use.
Thanks,
Paul
Regards
Atul
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_______________________________________________
Sip mailing list https://www.ietf.org/mailman/listinfo/sip
This list is essentially closed and only used for finishing old business.
Use [email protected] for questions on how to develop a SIP
implementation.
Use [email protected] for new developments on the application of sip.
Use [email protected] for issues related to maintenance of the core SIP
specifications.
