Abstract: Bursty traffic and thousands of concurrent flows incur inevitable network congestion in datacenter networks (DCNs) and then affect the overall performance. Various transport protocols are developed to mitigate the network congestion, including reactive and proactive protocols. Reactive schemes use different congestion signals, such as explicit congestion notification (ECN) and round trip time (RTT), to handle the network congestion after congestion arises. However, with the growth of scale and link speed in datacenters, reactive schemes encounter a significant problem of slow responding to congestion. On the contrary, proactive protocols (e.g., credit-reservation protocols) are designed to avoid congestion before it occurs, and they have the advantages of zero data loss, fast convergence and low buffer occupancy. But credit-reservation protocols have not been widely deployed in current DCNs (e.g., Microsoft, Amazon), which mainly deploy ECN-based protocols, such as data center transport control protocol (DCTCP) and data center quantized congestion notification (DCQCN). And in an actual deployment scenario, it is hard to guarantee one protocol to be deployed in every server at one time. When credit-reservation protocol is deployed to DCNs step by step, the network will be converted to multi-protocol state and will face the following fundamental challenges:1) unfairness, 2) high buffer occupancy, and 3) heavy tail latency. Therefore, we propose Harmonia, aiming for converging ECN-based and credit-reservation protocols to fairness with minimal modification. To the best of our knowledge, Harmonia is the first to address the trouble of harmonizing proactive and reactive congestion control. Targeting the common ECN-based protocols-DCTCP and DCQCN, Harmonia leverages forward ECN and RTT to deliver real-time congestion information and redefines feedback control. After the evaluation, the results show that Harmonia effectively solves the unfair link allocation, eliminating the timeouts and addressing the buffer overflow.