Existing congestion control mechanisms cannot feedback network capacity accurately to traffic sources. In this paper, we propose Adjusting Advertised Window (AAW), an accurate congestion control mechanism, based on programmable networks. After theoretical analysis of the performance of AAW, we explain the design principles. We implement AAW in P4 at intermediate nodes by modifying the advertisedWindow field at TCP headers to feedback available network capacity to traffic sources. Experiments show that AAW gets 5% and 10% increase in throughput compared to HSTCP and Reno respectively. AAW can also solve ECN-non-ECN unfairness and compared with vCC, it gains 16% increase in throughput.