[Sip] Codec Negotiation and renegotiataion

[atul garg <[e-mail]>]

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 ??
 
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 ??
 
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 ....
   
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)
 
  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.
 
Regards
Atul