The article focuses on the details of the seamless Session Initiation Protocol (SIP) designed for multimedia applications. SIP is designed to provide the required application-level protocol abstraction of the mobility support for multimedia applications in the next-generation heterogeneous wireless networks. However, the approach with SIP endures from undesirable delay and packet loss, which is damaging to applications like voice over Internet protocol (IP) or streaming video that call for quality of service requirements. Moreover, IP convergence has led to the existence of various IP-based wireless access technologies such as General Packet Radio Service, wireless local area network, as well as the advancement of next-generation technology like Universal Mobile Telecommunications System.