2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security and public telecommunication networks, SIP is considered as an attractive candidate to support mobility in NG networks. Despite the advantages discussed above, SIP-based handoff schemes can suffer from long handoff delays due to its operation at the highest layer. The resulting handoff delays have been shown to be unacceptable for support- ing real-time multimedia services [23]; consequently many schemes have been proposed to deal with this problem. In hierarchical mobile SIP (HMSIP) [24], a global network proxy is set up for each domain to manage the roaming inside the domain. When an MN roams between different sub-nets of the domain, its registration messages are sent to the global network proxy instead of its home proxy. The global network proxy will check all incoming packets and redirect them to the current address of the MN. In this way, the signalling messages for intra-domain roaming are localized in the foreign network, and the handoff delay will thereby be greatly shortened. HMSIP only reduces delays of handoffs between sub-nets of the same domain, which can cooperate easily due to homogeneous underlying technologies. How- ever, handoffs between different domains are more difficult to address for the following reasons: First, it is difficult to localize signalling messages of inter- domain handoffs since it is commercially impractical to set up intermediate nodes between heterogeneous networks. Second, the above scheme reduces the handoff delay by shortening the registration period. However, inter-domain handoffs involve some additional procedures such as "authenticating to the new domain." The handoff delays could remain unacceptable even if the reg- istration period is very short. Several schemes have also been proposed to reduce the delays of inter- domain handoffs. In [25], the MN establishes security associations (SAs) with all neighboring domains whenever a session is set up SO that neighboring networks can obta