Half-open limit fix is a program designed to change the maximum number of concurrent half-open outbound TCP connections (connection attempts) in the Windows system file tcpip.sys.
Microsoft first introduced this limit in Windows XP SP2 (Service Pack 2) and is present in all later versions of Windows. This was done to try to slow the spreading of viruses and malware from system to system and also to reduce the impact of infected systems participating in DoS (Denial of Service) attacks.
This limit makes it impossible for Windows systems to have more than 10 concurrent half-open outbound connections. After 10, new connection attempts are put in a queue and forced to wait.