ontact address. 3. The MN must re-register its new contact address with its corresponding SIP server using the SIP REGISTER method. 4. After the re-registration, if a correspondence node (CN) wants to estab- lish a session with the MN, the CN uses the SIP INVITE method to invite the MN. 5. The INVITE request is routed to the SIP server located in the MN's home network. 6. The SIP server responses that the MN has moved temporarily to the re-registered contact address. 7. The INVITE request is finally re-routed to the MN. After the pre-call handoff operation, the MN's reachability is recovered, i.e., any CN can invite and create a session with the MN. Thus, the pre-call oper- ation guarantees that an MN can be reachable even if the MN roams the Internet. 4.1.2 Mid-Call Operation Mid-call means that an MN moves to a new visited network during an on- going session. The mid-call operation utilizes the SIP INVITE method to recover the original session between the MN and the CN. Figure 4.2 depicts Correspondence Node (CN) 4. Re-invite 1. Invite 2. 200 OK Mobile Node (MN) Mobile 3. Move Node (MN) Visited network Home network FIGURE 4.2 The mid-call operation. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security the mid-call handoff operation. An MN is supposed to be located in its home network. The detailed steps are described as follows: 1. The MN sends an INVITE request to a CN for session establishment. 2. The CN accepts the INVITE request and responds with an OK message. The session between the MN and the CN is established. 3. The MN roams to another network and acquires a new contact address. 4. The MN must re-invite the corresponding CN for session recovery using the SIP INVITE method. After the mid-call operation, the CN is able to realize the MN's movement and get the MN's new contact address. Hence, the mid-call operation guarantees that an MN can keep sessions continuous even if the MN is roaming. In spite of the pre-call operation or the mid-call operation, SIP keeps data ex