communicating with the MH, by sending an INVITE message about the terminal's new IP address and updated session description. In principle, this is similar to the Mobile IP route optimization [7] strategy. The CH starts sending data to the new location as soon as it gets the re-INVITE message. Hence, the handoff delay is essentially the one-way delay for sending an INVITE message from the MH to the CH. Here the "home" refers to the redirect or SIP server in the home network of the MH. The MH needs to register with the redirect server in the © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security home network for future calls. However, in mid-call mobility management, before sending the SIP re-INVITE message, there are some procedures that need to be completed to get the MH attached to the wireless access network infrastructure. For example, an MH attaches to the GPRS radio access of a UMTS network using the GPRS attach and Packet Data Protocol (PDP) context activation procedure, while for the wireless LAN it uses DHCP to attach to the WLAN. Now, mobility in such a heterogeneous networking environment can give rise to the following four cases: (1) MH moves from a UMTS network to another UMTS network, (2) MH moves from a UMTS network to a WLAN network, (3) MH moves from a WLAN network to a UMTS network, and (4) MH moves from a WLAN network to another WLAN network. Since we are concerned with the analysis of the delay incurred in the vertical handoff procedure, the above four cases can be mapped to only two cases of interest: MH moving to a UMTS network MH moving to a WLAN This is because the handoff delay is caused mainly by the message exchange that occurs while an MH attaches to a new access network (either UMTS or WLAN in our case) followed by the location update. These two cases are discussed in more details as follows. 2) Mobile host moving to a UMTS network: When an MH moves to a UMTS network from another UMTS network or a wireless LAN, it performs two key functions to initiate