Performance analysis of TLS Web servers

Abstract

TLS is the protocol of choice for securing today's e-commerce and online transactions but adding TLS to a Web server imposes a significant overhead relative to an insecure Web server on the same platform. We perform a comprehensive study of the performance costs of TLS. Our methodology is to profile TLS Web servers with trace-driven workloads, replace individual components inside TLS with no-ops, and measure the observed increase in server throughput. We estimate the relative costs of each TLS processing stage, identifying the areas for which future optimizations would be worthwhile. Our results show that while the RSA operations represent the largest performance cost in TLS Web servers, they do not solely account for TLS overhead. RSA accelerators are effective for e-commerce site workloads since they experience low TLS session reuse. Accelerators appear to be less effective for sites where all the requests are handled by a TLS server because they have a higher session reuse rate. In this case, investing in a faster CPU might provide a greater boost in performance. Our experiments show that having a second CPU is at least as useful as an RSA accelerator. Our results seem to suggest that, as CPUs become faster, the cryptographic costs of TLS will become dwarfed by the CPU costs of the nonsecurity aspects of a Web server. Optimizations aimed at general purpose Web servers should continue to be a focus of research and would benefit secure Web servers as well.