SSM uses both adaptive admission control and early rejection as forms of backpressure. AIMD is used to regulate the maximum number of requests a stub can send to a particular brick; each brick can reject or ignore timed-out requests; the application of TCP, a well-studied and stable networking protocol, allows SSM components to reach capacity without collapse [39]. The goal of these mechanisms is to avoid having SSM attempt to give a functionality guarantee (``I will do it'') at the expense of predictable performance; the result is that each request requires a predictable amount of work.