**2.2 Related work**

With the success of SIP, there have already been a number of studies addressing aspects of performance evaluation and modeling of SIP. Chebbo et al. describe in (Chebbo et al. 2003) a modeling tool with which it is possible to estimate the number of required SIP entities for supporting certain traffic. Gurbani et al. present in (Gurbani et al. 2005) a theoretical model of a SIP server using queuing theory. This model is then used to evaluate the performance of a SIP server in terms of response time and number of served requests. Wu et al. analyze in (Wu et al.2003) the usage of SIP for carrying telephony information in terms of queuing delay and delay variations.

In general, these studies aim at investigating the performance of SIP servers in terms of the number of SIP sessions that can be supported by a SIP server or the processing delays at such servers. In contrast, in our work we do not aim at modeling the performance of a SIP server but to investigate the performance of SIP in terms of the number of messages and amount of time needed by SIP for establishing a session in lossy environments.

Fathi et al. (Fathi et al.2006) present a model of SIP in VoIP networks and investigate the effects of mobility on the performance of session establishment using SIP. The used model is however rather simplified and is only applicable to stateless SIP proxies which have no notion of transactions. Alam et al. (Alam et al.2005) discuss different performance model for SIP deployment scenarios in mobile networks. This involves providing models for evaluating the performance of push-to-talk applications or the effects of different mobility concepts. The work does not however provide for a model of how SIP itself deals with losses. Sisalem et al. (Sisalem et al. 2008) provided a theoretical model of the effects of losses and delays on the performance of SIP. While that work is providing the basis for our work here, it is rather limited to simple SIP networks as are discussed in IETF. The work in this chapter takes the multi-hop nature of IMS into account as well as the SIP specifications.
