Silly window syndrome is a problem in computer networking caused by poorly-implemented TCP flow control. If a server with this problem is unable to process all incoming data, it requests that its clients reduce the amount of data they send at a time (the "window" setting on a TCP packet). If the server continues to be unable to process all incoming data, the window becomes smaller and smaller, sometimes to the point that the data transmitted is smaller than the packet header, making data transmission extremely inefficient. The name of this problem is due to the window size shrinking to a "silly" value.

Since there is a certain amount of overheadassociated with processing each packet, the increased number of packets means increased overhead to process a decreasing amount of data. The end result is thrashing.