On the robustness of soft state protocols

Soft state has been a mantra of Internet protocol design for the past decade. System designers build protocols that implement soft state mechanisms based on intuition or on qualitative arguments that the design is "better", yet there has never been a formal performance evaluation study that draws the same conclusion. In fact, previous attempts [P. Ji et al., 2003 and S. Raman et al., 1999] to build such a quantitative argument have found that pure soft state protocols significantly under-perform their hard state counterparts, and that only soft-hard hybrids can match hard state protocol performance. In this paper, we argue otherwise. We develop models that provide a performance-oriented explanation and justification of the Internet designer's intuition. The novel observation is that, if network conditions are known, a hard state protocol can always be configured to outperform its soft state counterpart. However, in reality, network conditions are unpredictable, and that soft state protocols are much more resilient to unanticipated fluctuations in these conditions.