This memo describes Datacenter TCP (DCTCP), an improvement to TCP congestion control for datacenter traffic. DCTCP uses improved Explicit Congestion Notification (ECN) processing to estimate the fraction of bytes that encounter congestion, rather than simply detecting that some congestion has occurred. DCTCP then scales the TCP congestion window based on this estimate. This method achieves high burst tolerance, low latency, and high throughput with shallow- buffered switches.