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==== Applications ==== NIC hardware timestamping is essential to IEEE 1588 clock synchronization. Applications at hyperscale also include congestion control and distributed applications. Delay based TCP congestion control takes network RTT as input signal. Measurement must be more precise than network delay, which in data centers can be tens of microseconds. Hyperscale deployment of advanced congestion control requires a significantly higher measurement rate than for PTP clock synchronization, since RTT estimates are per-connection and measurements taken on every packet. NIC hardware timestamps also enable latency measurement of the NIC datapath itself. Incast is a significant concern in hyperscale environments. Concurrent connection establishment can cause queue build up in a NIC if the host CPU, memory or peripheral bus are out of resources. Latency instrumentation can give an earlier and more informative signal than drops alone. Finally, distributed systems increasingly rely on high precision clock synchronization to offer strongly consistent scalable storage_id[sundial]. Microsoft FaRMv2_id[farm] and CockroachDB are two examples. Serializability in such databases depends on strict event ordering based on timestamps. Transactions can be committed only after a time uncertainty bound has elapsed. Key to scaling transaction rate is bounding this uncertainty. <span id="traffic-shaping"></span>
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