URE 1.4 Message flow of P2P SIP session establishment with extended location service. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Issues in P2P SIP There are several issues in P2P SIP that have to be considered. First of all, there are general issues that are common to all P2P systems. The system has to ensure that IDs are globally unique within the whole system. Other- wise, the overlay structure and consequently the message routing does not work. Another common issue is the so-called first-contact-problem targeting the bootstrapping mechanism for the overlay. In order to join the network, a new node has to know what participating peers (i.e., bootstrap peers) to contact. Recent literature provides many different ways to solve this issue. For instance, a fixed Web address can be used to provide random bootstrap peers or broadcasting mechanisms are used. Another severe problem within P2P infrastructures is security. To solve this, either the protocol ensures security or there has to be some trust relationship between the participating entities. A severe issue in P2P SIP is identity management. The infrastructure has to ensure that identities cannot be stolen, i.e., that SIP-URIs cannot be used by unauthorized entities. Skype, for instance, solves this by introducing a central entity for identity management. However, in the case that the system should not rely on central entities, ensuring identity is very difficult. For instance, Seedorf tried to approach this issue with self-certifying SIP URIs [16]. A common problem with SIP is Network Address Translation (NAT), which is, for instance, used for locating users behind firewalls. However, there are sol- utions for NAT traversal, e.g., Simple traversal of UDP over NATs (STUN [17]). For P2P SIP solutions based on SIP messages only, these standard NAT traversal mechanism can be reused [5]. If P2P SIP approaches use particular P2P systems, these approaches have to ensure that these systems are capa- ble of NAT traversal (e.