ongs to a home network with a SIP server providing a registrar service. Each time the MH changes location, it registers with the home network's registrar service. This is in principle similar to Mobile IP home registration. For ongoing sessions, the MH sends a re-INVITE message to the corresponding CH using the same call identifier as in the original setup. The former procedure takes care of pre- call mobility, while the latter enables mid-call mobility. High-level messaging of SIP-based mid-call mobility management is depicted in Figure 17.3. The new contact information (e.g., URI for future contact) is put in the Contact field of the SIP message to redirect the subsequent SIP messages to the cur- rent location. The data traffic flow is redirected by updating the transport address field in the Session Description Protocol (SDP) [26] part of the re- INVITE message. For mid-call mobility, the CH starts sending data to the new location as soon as it gets the re-INVITE message. Hence, the handoff SIP Home Correspondent registrar Insession mobility network 1 - SIP INVITE 2 SIP OK 3 Data Location update Internet a) SIP REGISTER b) SIP OK Wireless access network Mobile host moves during Mobile and ongoing session FIGURE 17.3 SIP-based mid-call terminal mobility management. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security BSS-Base station subsystem (BTS+BSC) SIP Server BSC-Base station controller BTS-Base tranceiver station SGSN-Serving GPRS support node GGSN-Gateway GPRS support node HLR-Home location register Correspondent host VLR-Visitor location register Internet MH-Mobile host P-CSCF-Proxy call session call control function S-CSCF-Serving call session call control function I-CSCF-Interrogating call session call control function IMS-IP multimedia subsystem I-CSCF DHCP Server P-CSCF S-CSCF Gateway to WLAN ----Ethernet Access Point Access Access Point Point Map-D Wireless LAN (WLAN) FIGURE 17.4 4G Architecture considered for case study. delay is essentially the one-way delay fo