The primary reason is that the party sending 200 OK get confirmation that the party sending INVITE received the 200 OK.
If A send an INVITE to B then A knows it sent an invite. A does not know that B received the INVITE. When B receives the INVITE and send 200 OK B knows it received the INVITE and B knows it sent the 200 OK. B does not know that A received the 200 OK. When A then receives the 200 OK it knows that B must have received its INVITE since B would not send 200 OK otherwise. So it knows that B received the INVITE and responded positively to it. Yet, B still does not know that A received the 200 OK. When A then send the ACK to B, A knows that the dialog is active but B does not know that yet. When B receives the ACK, B knows that A has received the 200 OK since it would not send ACK otherwise. Thus, both A and B knows that the dialog is now active. In addition; having A send ACK allows the SDP to be sent either with offer in INVITE and answer in 200 OK and empty ACK or by empty INVITE and with offer in 200 OK and then an answer in ACK. SIP supports both models. In the first model A offer a couple of possible methods of communicating with B and B picks which one to use, the other model B is the one that offer A a selection of ways to communicate with A and A picks the one to use. Alf On Mon, 2006-09-18 at 14:37 +0000, [EMAIL PROTECTED] wrote: > Hi, > > What was the intention behind proposing the need for sending ACK after > 200OK of INVITE under SIP. > > > _______________________________________________ > Sip-implementors mailing list > [email protected] > https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
