UMTS. This approach is subject to the same limitations as the tight coupling architecture. In NG heterogeneous wireless networks, the mobile node (MN) initiates handoffs when it roams between different RANs. To differentiate the hor- izontal handoff that happens in homogeneous networks, a handoff taking place between heterogeneous access networks is referred as a vertical hand- off. Vertical handoffs require the heterogeneous wireless networks to redirect in a timely manner ongoing connections to RANs employing different wire- less technologies. Horizontal handoff management is normally implemented at the link layer because it involves RANs employing homogeneous wireless technologies, which belong to the same administrative domain. However, the same function cannot be applied to vertical handoff between heterogeneous RANs that are usually administrative-separated. Therefore, vertical handoff management should operate at a higher layer in order to provide a uniform management capability. Several procedures should also be executed during a vertical handoff, such as re-authentication in the new RAN, updating the addresses in the home network and transferring the context information to C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security the new domain. The MN should also inform the corresponding node (CN) that communicates with the MN during the handoff in order to keep the connection going with minimal disruption. All these actions will incur a heav- ier signalling cost and more QoS degradation than horizontal handoffs. For example, some inflight packets may be lost during the handoff or the connec- tion may even be terminated if the handoff takes too long. Moreover, vertical handoffs in NG heterogeneous wireless networks are not necessarily compul- sory. For example, vertical handoffs from UMTS to WLAN (downward hand- off) are normally executed to achieve better QoS or lower access costs rather than for the purpose of maintaining the ongoing connections, while vertical handoffs 