arding mechanism or a direct integration of P2P techniques into the user agent (e.g., Schmidt et al. [6]). C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security The protocol extension leads to changes in the SIP headers to support P2P overlays, which restricts backward compatibility to standard SIP. An inte- gration of P2P mechanisms into the SIP message forwarding mechanism or directly into the SIP user agent entails changes to SIP entities but not to the protocol itself. This maintains compatibility to standard SIP. In the next section, we provide background on the basic technologies needed for P2P SIP. On this basis, we present possible integration approaches of P2P mechanisms into SIP: either the protocol or standard SIP network entities are extended. Furthermore, we will describe issues that have to be considered in P2P SIP environments. At the end of the chapter, we describe possible fields of application as well as current standardization efforts regarding P2P SIP. 1.2 Background This section describes fundamental technologies, which are the basis for P2P SIP. First of all, we introduce P2P networking topologies: unstructured and structured topologies. Then, we provide background on standard SIP. 1.2.1 Peer-to-Peer Networks P2P networks evolved from the client-server paradigm, in which clients con- sume server resources. However, the client-server paradigm has some draw- backs such as bad scalability, single-point-of-failure on the server side, and unused resources within the network. In P2P networks, peers are equal, i.e., they have client and server roles at the same time. Thus, a crash of part of the P2P system has no severe consequences for the whole system; other parts of the system are able to overtake the crashed part's tasks. P2P systems are separated into two groups: hybrid and pure P2P systems. While hybrid P2P systems have client-server components, pure P2P systems do not have any single-point-of-failure due to client-server components. There are two types of p