multiple source message generators; in a way similar to attack architectures employed against web servers (Figure 18.5 and Figure 18.6). Note that in SIP architectures both servers and end-user terminals are susceptible to any kind of flooding attack. End user devices are more susceptible to single source flooding attacks than SIP servers. This is because their resources and processing capabilities are limited; for instance, SIP phones are only able to process a small number of sessions simultaneously. The adversary may generate hundreds of SIP invita- tion requests, over a specific time interval, and send them to the end-terminal (either directly to the terminal's IP address or indirectly via the corresponding proxy). This will not only cause the terminal to ring continuously but it will also disrupt its normal operation by eventually crashing it. A single source Request 1 Request 2 Request N Malicious user Target FIGURE 18.5 Single source attack architecture. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Attacker Command Command Attack Innocent Innocent server server Slave 1 Slave n Attack target FIGURE 18.6 Multiple source attack architecture. attack can be also launched against a SIP server even though it is far more difficult to consume its resources by utilizing only a single source message generator. Alternatively, the aggressor may generate spoofed SIP invitation messages, containing a new URI (which corresponds to a new session), in an attempt to gradually consume the recourses of a stateful SIP proxy. This kind of attack exploits the stateful operational mode of the proxy, or in other words its need to store each transaction's context consuming three Kbytes per request [6], i.e., for every new session, and keep it alive until the transaction ends. Thus, depending on the implementation and configuration, a stateful proxy should be able to maintain a transaction active for a duration of a few seconds up to some minutes. Consequently, the concurrent existence of a