ocol for NG wireless networks. In SIP, the MN is identified with a unique logical SIP address in the format of an e-mail address, which is also called a "SIP URI"; e.g., node.name@ip. domain. When the MN roams into a foreign domain, it obtains a new contact address with the domain name of the foreign domain. The MN also registers the new contact address with the registration server in the home domain through the SIP proxy. Figure 11.2a shows the procedure of setting up a new session from a CN to MN B. In this case, the CN sends an "INVITE" message via its SIP proxy to the home network of the MN. The address of the home network can be figured out from the public SIP address of MN B. This request is captured by the SIP proxy in the MN's home network. After checking the registration server, the proxy in the MN's home network determines that MN B is currently visiting a foreign network. Then, the SIP proxy returns a "moved temporarily" message to the CN with the current contact address of MN B attached. The CN's SIP proxy re-sends the "INVITE" message MN B directly using the new contact address. The MN returns an "OK" message and negotiates session parameters with the CN. The CN completes the negotiation by sending an "ACK" message to confirm that it accepts the modification. When the negotiation is done, the session has been directly set up between the CN and MN B. As shown in Figure 11.2b, if MN B roams to a new domain in the middle of the session, it will notify the CN of its new contact address and the updated session description via another "INVITE" message. The CN then re-establishes the session with MN B at the new location. In contrast with the mobility solutions at other layers, SIP can be easily deployed in the network without requiring the modification of network entities or end-user protocol stacks. Moreover, SIP can be easily extended or enhanced due to its operation at the application layer and the use of text-based signalling messages. Having been accepted as a standard protocol in both the Internet © 