off procedure to provide seamless handoff man- agement. It does not require any modifications to or addition of network entities, SO that it easy to implement. Moreover, we also propose a suitable handoff triggering method that takes advantage of current location tracking technologies. Simulations results demonstrate that the proposed architecture performs efficiently. 11.3.1 S-SIP Handoff Scheme The mechanism of S-SIP utilizes the existing multi-party conference capability of SIP to set up a new handoff connection before breaking the old connection. Here we take the same network environment discussed above as an example. As shown in Figure 11.3, the MN and CN first set up a session according to the normal SIP handshake procedure and they know the SIP address and contact address of each other. During the communication session, the MN detects that it is going to move out of the current network area or it finds a better wireless network, SO it decides to handoff to another network. It first authenticates with the new network and acquires IP/SIP addresses through the required protocols. Afterwards, the MN will send an "INVITE" message with a special "JOIN" header to the CN via the new interface. Here the "JOIN" header contains all the information relevant to the ongoing call, such as the call ID and tags of the previous connection [28]. Such information enables the CN to determine that this new connection wants to join the related ongoing connec- tion (note that the CN may have multiple ongoing connections, especially if it is a server). After some negotiation procedures, another connection is estab- lished between the MN and the CN via the new contact address. Note that these two connections follow different paths. The above procedure is normally employed to set up a conference session such that after receiving the "JOIN" message, the CN will send out packets or also forward received packets to both addresses. Since the MN's original SIP URI (note that it is different from the contact address) is also involved in a