This is one of the reasons why the pace at which services are cre- ated for SIP is much faster than for networks other than the Internet. Mid-session Transactions That Do Not Change the State of the Session We have already seen several examples of how SIP is used to establish ses- sions. Once a session is established, the core SIP specification provides a means to change the parameters of the sessions through re-INVITEs and a means to terminate the session through a BYE. Certainly, in some situa- tions, the parties involved in a session may need to exchange information that does not influence the state of the session in any way. However, core SIP does not provide a means of sending information to the remote party without changing session state. In this case, the means proves to be an extension for mid-session information. Exchange of this kind of information can be typically found when SIP interworks with other signalling protocols. One such case is billing information, where the session provider regularly needs to transfer information related to the session that does not modify its parameters. SIP can carry it only by means of extension. How It's Done A new SIP method called INFO was defined [RFC 2976] to provide this functionality. INFO transports mid-session information that does not affect state. INFOs typically carry information in the message body that is transferred from end to end between UAs. Figure 6-6 illus- trates Bob and Laura exchanging INFOs during an ongoing session. Chapter 6 Laura Figure 6-6 INFOs do not change (1) INVITE the state of the (2) 200 OK session. (3) ACK (4) INFO (5) 200 OK (6) INFO (7) 200 OK (8) BYE (9) 200 OK Multiple Message Bodies You already know that SIP can locate a user and deliver a session descrip- tion to the user at his or her current location. However, sometimes it's important to deliver more than a session description. For instance, Bob has a SIP UA in his laptop. When Bob receives a call, apart from receiving the session description that will enable him to establish t