in the MN's authentication information in advance. When the MN roams into one of the neighboring networks, the authentication can be processed locally SO that the handoff delay can be reduced. A fast handoff method is proposed in [26]. In this scheme, the MN reports the new contact address to both the CN and the proxy in the old domain when it registers in the new domain. The proxy in the old domain then sets up a temporary session between the old proxy and the MN to forward the received packets; this session is terminated when the MN can reliably receive packets directly from the CN. 11.3 Seamless Vertical Handoff Support Using SIP According to the above discussions, SIP-based application layer solutions for handoff support have some overwhelming advantages over other protocols. However, the potentially long handoff delays also prevent them from becom- ing well-rounded mobility management solutions. Although several SIP-based protocols have been proposed to reduce handoff delays in different situations, C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks all of them have certain limitations. HMSIP [24] is designed to reduce intra- domain handoff delays only. Although the methods in [25] and [26] are able to shorten the handoff delay in inter-domain roaming, they both require modifi- cations to SIP operations in the terminals and networks. The method in [25] requires modifications to the existing authentication systems of all domains and may also incur heavy signalling costs to distribute the SA messages. The method in [26] requires all base stations (BSs) in the networks to be equipped with the Back-to-Back User Agents (B2BUA), which may not be preferred by some operators. Furthermore, none of these protocols can reduce handoff delays to the very small values required for seamless handoffs. In this section, we introduce a SIP-based seamless-handoff scheme (S-SIP) [27] to support seamless inter-domain roaming. Basically, this scheme employs a "make-before-break" hand