An oft-repeated adage among telecommunication providers goes, "There are ve things that matter: reliability, reliability, reliability, time to market, and cost. If you can't do all ve, at least do the rst three. " Yet, designing and operating reliable networks and services is a Herculean task. Building truly reliable components is unacceptably expensive, forcing us to c- struct reliable systems out of unreliable components. The resulting systems are inherently complex, consisting of many different kinds of components running a variety of different protocols that interact in subtle ways....