ulated resource and peer GUIDs are attached to messages as part of the headers for performance reasons. However, whenever messages result in changing the overlay structure, network entities have to check if the GUIDs are hashed correctly. For connecting standard SIP user agents to the overlay, dSIP suggests SO- called adapter peers, which act like gateways between standard SIP and the overlay. 1.3.2 Extension of the SIP Location Service Beside changing the SIP protocol itself as described before, another approach is an extension of the SIP location service. In contrast to a standard SIP location service, a P2P SIP location service (P2P-LOC) stores contact infor- mation in a P2P network. Therefore, the location service interface provides means for registration as well as for discovery of SIP-URI's contact infor- mation. The P2P-LOC can either be integrated into the SIP proxy server (P2P-PROXY) or directly into the user agent (P2P-UA). A hybrid approach with P2P-PROXYs and P2P-UAs is possible as well. As shown in Figure 1.4, an integration into the SIP user agent leads to reduced communication in comparison to standard SIP (see Figure 1.2); messages are saved due to user- agent-internal location of the target's contact information. C 2009 by Taylor & Francis Group, LLC SIP in Peer-to-Peer Networks With SIPPEER, Singh and Schulzrinne developed a P2P SIP adaptor that allows for participation into a P2P network without changes in the user agent by implementing a P2P-PROXY [9, 10]. SIPPEER should run on the same host as the user agent and has to be configured as an outbound proxy. Then, all messages of the user agent are automatically sent through SIPPEER, which provides an extended location service on basis of the OpenDHT [11] infras- tructure for registration and discovery (OpenDHT uses the Bamboo [12] DHT implementation as underlying infrastructure). Schmidt et al. developed another P2P-LOC on the basis of JXTA, an open and generic P2P infrastructure [6, 13]. Their P2P-LOC can either be inte- grated into a P2P-PROXY