) message to Anike to confirm a session establishment. Since Johnson knows the contact address of Anike, anike@host_b.company.com, via 200 OK and Johnson can send an ACK to Anike directly without going through the proxy server. When Johnson wants to terminate the call, Johnson sends a BYE request to Anike (8) which is confirmed by another 200 OK (9) issued by Anike. Apart from establishing sessions which causes delays, two types of terminal mobility can be identified. They are the pre-call terminal mobility and mid-call C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Proxy server johnson@host_a.uts.edu anike@host_b.telkom.com 1. INVITE sip:anike@telkom.com 6. 6 200 OK 2. INVITE sip:anike@hos_b.telkom.com 7. 7 ACK sip:anike@host_b.telkom.com 3. 180 Ringing 8. BYE sip:anike@host_b.telkom.com 4. 180 Ringing 9. 200 OK 5. 200 OK FIGURE 22.3 Basic SIP procedure. (From Nakajima, N., Dutta, A., Das, S., and Schulzinne, H. Handoff delay analysis and measurement for SIP based mobility in IPv6. International Conference on Communications (ICC'03), 26, 1085-1089, May 2003. With permission.) terminal mobility. The pre-call terminal mobility ensures that a correspondent node (CN) can reach a mobile node (MN), as in Figure 22.4 [8]. The pre- call terminal mobility scenario is akin to mobile IP. When an SIP-enabled MN moves to a foreign network (Visited Network) it registers and has a new contact address with the SIP proxy server (1). Therefore an INVITE request from a CN through the SIP server (2) is used by the server to notify the new contact address of the MN to the CN. This provides the means for the CN to send an INVITE request to the MN directly (4) with an OK message sent back by the MN (5). © 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance Correspondent Node (CN) (2) INVITE mobile Home network (3) 302 Node moved (4) INVITE (5) 200 OK SIP server (1) Register new contact address with Registrar Visited network Mobile Node (MN) FIGURE 22.4 Pre-call 