ystem. 1.6 Standardization Currently, there is a lot of work on standardizing P2P SIP. The P2P SIP work- ing group within the IETF developed a first Internet draft on basic concepts and terminology regarding P2P SIP [19]. The document includes high-level descriptions of relationships between network entities, a reference model and a discussion of open problems that should be addressed within the work- ing group. The document is expected to provide the general framework for P2P SIP. Besides, there are many efforts leading to standardization of concepts regard- ing P2P SIP. The Web site p2psip.org provides a current overview of these efforts. 1.7 Conclusion To sum up, there are two general approaches to integrate P2P mechanisms into standard SIP. Either the SIP protocol itself or the location service behav- ior is extended. The SIP extension leads to protocol changes, which prohibits its use in environments with standard SIP entities. Adapter peers, which act as some kind of gateway between P2P SIP and standard SIP environments, C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security are a solution to this. The location service extension to a P2P-LOC leads to P2P-PROXYs, which are compatible to standard SIP and thus can be seam- lessly integrated into a standard SIP environment. The direct integration into a P2P-UA leads to changes to the user agent's code. Currently, there are many efforts made to standardize P2P SIP. P2P SIP will have its field of application as Skype has already shown that VoIP using P2P mechanisms works, even within the Internet. The great strength of P2P SIP is the reduction of administrative efforts by eliminating central servers. References [1] Rosenberg, J., H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, M. Handley, and E. Schooler. 2002. SIP: Session Initiation Protocol. IETF RFC 3261, June. [2] 3GPP. 2006. IP multimedia call control protocol based on Session Initia- tion Protocol (SIP) and Session Description Protocol (SDP). TS 24.229 V