 the same services when mobile. Terminal mobility can be addressed by Mobile IP [3], which has been standardized in the IETF. It allows a terminal to keep the same IP address when roaming as it does in its home network. While roaming, the terminal is reach- able by a "care of" address, which is registered in the home network. Packets des- tined for the roaming terminal are received in the home network, then tunneled to the terminal at the "care of" address, as shown in Figure 9.1. Mobile IP has the advantage of hiding the mobile nature of the terminal from layer 3 protocols and above. These protocols can then be used without any modification. SIP: Understanding the Session Initiation Protocol Mobile IP router Packets "tunneled" Indirect routed packets To "care-of" address Direct routed packets Roaming terminal Home terminal Figure 9.1 Triangular routing of IP packets in Mobile IP. For example, a TCP connection can be maintained since the terminal appears to have a constant IP address. However, Mobile IP has the disadvantage that incoming packets are not routed directly, and as a result, most efficiently. This results in increased packet latency. While this is not a problem for nonreal-time services such as Web browsing or e-mail, real-time media transport has critical requirements on packet latency. A solution that has been proposed [2] uses the fact that a protocol such as SIP already has mobility support built in. In addition, SIP is capable of handling some of the terminal mobility aspects at the application layer. This can result in more efficient RTP packet routing and better efficiency (as the additional overhead of IP packet encapsulation required by Mobile IP are avoided). The result is that mobile SIP devices are utilizing two different architec- tures. One is based on the use of Mobile IP and the other utilizes the built-in mobility support in SIP. The next sections will discuss these two approaches. As discussed in the next section, SIP is ideally suited to provide both personal and service mobility. 9