g., JXTA allows this by introducing specific router peers [13]). Performance is another issue that has to be addressed by the infrastruc- ture. However, as only session establishment is based on a P2P discovery and multimedia communication traffic is transferred directly, the requirements to latency of the P2P infrastructure are quite low. 1.5 Field of Application of P2P SIP P2P SIP can be used for a broad range of applications. Bryan et al. wrote an IETF Internet draft summarizing possible use cases [18]. Amongst others, P2P SIP can be used for providing a P2P VoIP ser- vice comparable to Skype. However, P2P SIP builds on a standard proto- col, whereas Skype uses proprietary protocols. As Skype, a P2P SIP VoIP provider may rely on more or less central entities, for instance for accounting C 2009 by Taylor & Francis Group, LLC SIP in Peer-to-Peer Networks and identity management. Nevertheless, in the future there might also be an open global P2P VoIP network in which anybody connected to the Internet is able to join with a compatible device (this requires standard protocols as proposed by P2P SIP). P2P SIP can also be used for providing anonymous communication. As there are no central servers within a P2P SIP infrastructure users cannot be easily monitored. Another application are ad-hoc networks, connecting a set of devices with each other spontaneously. Examples are spontaneous meeting networks, spontaneous emergency and crisis networks, or even networks inside the developing world. P2P SIP can provide a VoIP service to such environ- ments. With regard to fault tolerance, P2P SIP can be used for the implementation of redundant standard SIP proxy servers. Therefore, SIP providers have to provide server farms, which are able to use P2P SIP for data synchronization. P2P SIP may also be used as failover in case of server crashes. Last but not least, P2P SIP can be an alternative for internal VoIP networks in small companies. Often, these companies do not have the administrative staff to manage a central SIP-based s