s) that anchors the PDP address. A record of such an association is known as the PDP context. The PDP context transfer is initiated by the MH by sending a PDP context activation message (9) to the SGSN. The SGSN, after receiving this activation message, discovers the appropriate GGSN (messages 10 and 11). It selects a GGSN capable of performing functions required for the SIP-related activities. The SGSN and the GGSN create special paths for the transfer of SIP messages to the P-CSCF, which is identified by the GGSN. The corresponding IP address of the P-CSCF is sent along with the activation accept message (messages 12-16). The SIP message exchange for re-establishing the connection is shown in Figure 17.7. The MH re-invites the CH to its new temporary address by sending a SIP INVITE message (1) through the P-CSCF, S-CSCF and the I-CSCF servers. The INVITE message uses the same call identifier as in the original call setup and contains the new IP address at the new location. Once the CH gets the updated information about the MH, it sends an acknowledge message (2) while starting to send data. 3) Mobile host moving to a wireless LAN: When an MH moves to a WLAN from another wireless LAN or a UMTS network, it goes through the following major steps to update its location with the CH: The MH goes through a DHCP registration procedure to secure a new IP address for its new location. The message exchanged in the registration procedure is shown in Figure 17.8. When the MH identifies the presence of a WLAN after receiving the characteristics beacons, it broadcasts a DHCP DISCOVER message (1) to discover the DHCP server willing to lend it with registration service. The appropriate DHCP server sends out a DHCP OFFER message (2) to offer service to the requesting MH. Correspondent Servers SIP INVITE (1) SIP INVITE (1) SIP OK (2) SIP OK (2) FIGURE 17.7 Messages involved in SIP-based mid-call terminal mobility management. © 2009 by Taylor & Francis Group, LLC SIP-Based Mobility Management and Its Performance Evaluation Corresp