Abstract: An important class of datacenter applications, called Online Data-Intensive (OLDI) applications, includes Web search, online retail, and advertisement. To achieve good user experience, OLDI applications operate under soft-real-time constraints (e.g., 300 ms latency) which imply deadlines for network communication within the applications. Further, OLDI applications typically employ tree-based algorithms which, in the common case, result in bursts of children-to-parent traffic with tight deadlines. Recent work on datacenter network protocols is either deadline-agnostic (DCTCP) or is deadline-aware (D3) but suffers under bursts due to race conditions. Further, D3 has the practical drawbacks of requiring changes to the switch hardware and not being able to coexist with legacy TCP. We propose Deadline-Aware Datacenter TCP (D2TCP), a novel transport protocol, which handles bursts, is deadline-aware, and is readily deployable. In designing D2TCP, we make two contributions: (1) D2TCP uses a distributed and reactive approach for bandwidth allocation which fundamentally enables D2TCP’s properties. (2) D2TCP employs a novel congestion avoidance algorithm, which uses ECN feedback and deadlines to modulate the congestion window via a gamma-correction function. Using a small-scale implementation and at-scale simulations, we show that D2TCP reduces the fraction of missed deadlines compared to DCTCP and D3 by 75% and 50%, respectively.