nvestigated in [34], where ker- nel modifications based on the design practices of MIPL IPv6 [35], such as Aggresive Router Selection, have been done to reduce some key delay com- ponents involved in the handoff. The modifications have been shown to pay off for mid-call mobility support for real-time communication. However, it is still not as fast as the network-layer mobility implemented by MIPL IPv6. The extra delay in the case of SIP mobility has been suspected due to the application-layer processing of SIP based handoff messages in contrast to the operating system level message processing in case of the network layer mobility implemented with MIPL IPv6. Several other attempts in the higher layers have been made to minimize packet loss during the handoff period. RTP translators have been used in conjunction with SIP servers to forward packets during the transient handoff period [36]. The SIP servers however, need to take up additional responsibility to configure the RTP tranlators, in this case. Alternatively, a B2BUA has been used to establish a transient supplementary connection during the handoff © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security period to minimize packet loss. Similarly, a transparent B2BUA functionality compounded with the packet filters and replicators has been proposed [40] in the base stations of cellular networks, again to minimize the packet loss during SIP-based handoff in cellular networks. As mentioned before, SIP was originally designed for UDP-based multimedia-based sessions. Hence, SIP-based mobility provides mobility sup- port to UDP-based applications only. There are, however, numerous already existing applications that are based on TCP, and SIP alone could not provide mobility support to them. There have been a number of proposals to solve this problem. For example, SIP has been complemented with Mobile IP [37] to handle mobility for both UDP- and TCP-based applications. The combination of SIP and Mobile IP has been further leveraged to 