or other SIP terminals. First, Source Target Source Target (a) Iterative (b) Recursive FIGURE 1.3 Routing mechanisms for dSIP. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security peers send a REGISTER message with their own calculated node ID (accord- ing to the DHT algorithm) to a so-called bootstrap peer (see Section 1.4). Therefore, dSIP specifies a peer URI that identifies the node's actual address, such as <sip:peer@10.0.0.34;peer-ID=efdab45629>. The REGISTER message is forwarded to the peer closest to the new peer's GUID according to the given metric (either iterative or recursive forwarding is used). Then, the new peer exchanges its DHT state information with the closest peer in order to learn about neighbors. In the next step, the new peer has to register its SIP URI in order to be discoverable as a SIP end-terminal. Equal to the initial peer's registration, the peer calculates the resource's (SIP URI) GUID and sends a REGISTER message to the closest peer. The registration is successful if the peer receives a 200 OK message. REGISTER messages are also used to maintain the overlay. Such mes- sages are periodically exchanged between peers for managing DHT neighbor bindings (see Section 1.2.1). Therefore, a peer calculates the GUID of a peer it wants to connect to and sends the message to the peer with the closest known GUID. This message is forwarded until it arrives at the addressed peer; then, both peers are able to exchange maintenance information. When- ever a node joins or leaves the network, messages are exchanged between peers in order to keep the network up-to-date. Session management messages are also used for maintaining the overlay. Therefore, these messages contain infor- mation about a peer's neighbor nodes as well. Two new SIP headers are intro- duced in order to maintain the overlay. A DHT-PeerID identifies the sending peer's GUID and DHT-Link describes other peers that are located in the neighborhood. The calculation of GUIDs is very expensive. Thus, calc