I don&#x27;t know if it was a kernel issue, but I was able to reproduce the problem in another DC by setting somaxconn artificially low (to 2 IIRC) in order to artificially force Linux into SYN cookie mode, and that reproduced this issue as well.<p>Given my knowledge of TCP and SYN cookies, I was not able to come up with another theory for why individual TCP connections would be stuck with a tiny window, because it seems that the entire idea behind congestion control is to increase the window once the congestion is gone ... That&#x27;s why I think it <i>might</i> be a kernel issue. Given the complexity of TCP, I&#x27;d say there is an at least equally high chance I just don&#x27;t know enough ;)