In many ways, our current approach to Internet congestion control arose from the need to deal with a series of congestion collapses. While some of those innovations were explicitly intended to prevent congestion collapse, others helped give TCP better congestion control more generally. In this paper we try to identify the aspects of congestion control that are necessary and sufficient to prevent congestion collapse. We argue that — in the context of a basic congestion control framework that includes such features as sliding window, reasonably accurate RTTs, and backing off retransmit timers — we need two and only two relatively straightforward conditions on retransmit timers to prevent congestion collapse. Making it easy to tell if a congestion control algorithm is congestion-collapse resistant will be important as congestion control turns away from the current paradigm and adopts rather different approaches, as is starting to occur.