While workload characterization and performance of web systems have been studied extensively, reliability has received much less attention. In this paper, we propose a framework for session reliability modeling which integrates the user view represented by the session layer and the system view represented by the service layer. A unique characteristic of the session layer is that, in addition to the user navigation patterns, it incorporates the session length in number of requests and allows us to account for heavy-tailed workloads shown to exist in real web systems. The service layer is focused on the request reliability as it is observed at the service provider side. It considers the multifier web server architecture and the way components interact in serving each request. Within this framework, we develop a session reliability model and solve it using simulation. Instead of the traditional one-factor-at-a-time sensitivity analysis, we use statistical design and analysis of experiments, which allow us to identify the factors and interactions that have statistically significant effect on session reliability. Our findings indicate that session reliability, which accounts for the distribution of failed requests within sessions, provides better representation of the user perceived quality than the request-based reliability.