  Homa is a new transport protocol for datacenter networks. It provides
exceptionally low latency, especially for workloads with a high volume of very
short messages, and it also supports large messages and high network
utilization. Homa uses in-network priority queues to ensure low latency for
short messages; priority allocation is managed dynamically by each receiver and
integrated with a receiver-driven flow control mechanism. Homa also uses
controlled overcommitment of receiver downlinks to ensure efficient bandwidth
utilization at high load. Our implementation of Homa delivers 99th percentile
round-trip times less than 15{\mu}s for short messages on a 10 Gbps network
running at 80% load. These latencies are almost 100x lower than the best
published measurements of an implementation. In simulations, Homa's latency is
roughly equal to pFabric and significantly better than pHost, PIAS, and NDP for
almost all message sizes and workloads. Homa can also sustain higher network
loads than pFabric, pHost, or PIAS.
