 form of secu- rity and QoS. No similar solutions exist for providing the security mechanisms that subscribers are familiar with over PSTN and Internet. There are however several proposals that have been published that are aimed at addressing the identified security concerns in SIP. This chapter provides a summary of the QoS performance of SIP, SIP security issues and the proposed solutions. 22.2 QoS Performance of SIP SIP was not developed originally with QoS considerations in mind. SIP is never used in isolation. It has to work in agreement with other QoS determining protocols such as DiffServ and real-time protocols such as RTP, RTCP, RTSP and RSVP. The IETF has provided several QoS solutions in IntServ, RSVP and DiffServ. The RSVP is an end-to-end QoS solution but it is relatively hard to design due to its high cost. RSVP cannot provide guaranteed QoS in a wireless environment because of the dynamic links and significant bit error rates (BER) that result. Therefore, the QoS performance of SIP is intricately woven to the QoS performances of these protocols. The QoS performance of SIP alone can be measured by how fast it establishes and maintains sessions through its signaling mechanisms under TCP (UDP) and either IPv4 or IPv6. This section summarizes these two approaches to the QoS performance of SIP. To handle QoS, extension to SIP proxy server is normally undertaken [1]. 22.2.1 Measuring Session Based QoS Performance The key QoS performance measures for SIP sessions include communication time, handoff delay when mobile nodes are involved in and between neigh- boring subnets, data volume exchanged during the SIP session time, jitter and BER. For real-time traffic, these parameters refer to end-to-end perfor- mance. In non-real-time traffic, guaranteed bit rates are used. An SIP session normally consists of several ON and OFF periods. Most data types manifest these times. Three data types are considered, audio, data and video. For real-time traffic such as VoIP, the ON time refers to talk periods and the OFF t