**2.3.2 SIP in IMS**

All IP voice and multimedia call signaling in IMS will be performed by SIP providing a basis for rapid new service introductions and integration with fixed network IP services. With regard to the SIP messages we distinguish between requests and responses. A request indicates the user's wishing to start a session (INVITE request) or terminate a session (BYE request). We further distinguish between session initiating requests and in-dialog requests. The INVITE request used to establish a session between two users is a session initiating request. The BYE sent for terminating this session would be an in-dialog request. Responses can either be final or provisional. Final responses can indicate that a request was successfully received and processed by the destination. Alternatively, a final response can indicate that the request could not be processed by the destination or by some proxy in between or that the session could not be established for some reason. Provisional responses indicate that the session establishment is in progress, e.g., the destination phone is ringing but the user did not pick up the phone yet. A SIP proxy acts in either stateful or stateless mode. In the stateful mode, the proxy forwards an incoming request to its destination and keeps state information about the forwarded request until either a response is received for this request or a timer expires. When used over an unreliable transport protocol such as UDP, if the proxy did not receive a response after some time, it will resend the request. In the stateless mode, the proxy would forward the request without maintaining any state information. In this case the user agent would be responsible for retransmitting the request if no responses were received. SIP uses an exponential retransmission behavior. So if a sender of a SIP message does not receive a response after some time, it will resend the request after some waiting time. In case no response was received for the retransmission, the sender increases the waiting time and tries again and so up to a certain number of retransmissions, (Rosenberg et al.2002) (29.328 2008).In general one can distinguish between two retransmission modes in SIP:
