An increasing number of datacenter network applications, including automated trading and high-performance computing, have stringent end-to-end latency requirements where even microsecond variations may be intolerable. The resulting fine-grained measurement demands cannot be met effectively by existing technologies, such as SNMP, NetFlow, or active probing. We propose instrumenting routers with a hash-based primitive that we call a Lossy Difference Aggregator (LDA) to measure latencies down to tens of microseconds even in the presence of packet loss. Because LDA does not modify or encapsulate the packet, it can be deployed incrementally without changes along the forwarding path. When compared to Poisson-spaced active probing with similar overheads, our LDA mechanism delivers orders of magnitude smaller relative error; active probing requires 50-60 times as much bandwidth to deliver similar levels of accuracy. Although ubiquitous deployment is ultimately desired, it may be hard to achieve in the shorter term; we discuss a partial deployment architecture called mPlane using LDAs for intrarouter measurements and localized segment measurements for interrouter measurements.