. However, TCP-Migrate involves the following modifications to TCP: A token contained in the Migrate option is negotiated between both ends during the initial connection establishment. Thus, a TCP connection can © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security be identified by a triple: <source IP address, destination IP address, token A new Migrate TCP option included in SYN flags identifies a SYN packet as part of a previously established connection, rather than a request for a new connection. The previously broken TCP connection can be resumed after the token information exchange between the client and the server. To track an MH, the hostname to address (A-record) mapping in the DNS must be updated dynamically once the MH changes the point of attachment. This scheme relies on some secure DNS update proto- cols [18]. Also, some DHCP servers can issue a DNS update at the client boot time when assigning a new IP address to a known MH, according to the MAC-to-DNS table. It is required to modify the transport layer implementation for all the hosts, regardless of whether they are mobile or static. This is difficult considering the number of hosts already in operation today. In addi- tion to this major problem, there exist several other problems with this approach. For example, during the DNS update period, the packets des- tined to the MHs may be lost, especially for the hosts that move fre- quently. A caching time to live (TTL) is associated with each DNS resource record, forcing a choice between long TTLs and long periods of inaccessibility, or short TTLs and increased load on the DNS servers due to ineffective resolver caching. 17.4.4 Application-Layer Mobility SIP is capable of handling terminal, session, personal, and service mobil- ity [19]. Since we are discussing terminal mobility support that allows a device to move between subnets while being reachable to other hosts and maintain- ing any ongoing session, we focus on the terminal mobility feature of SIP only. Further