The attacked computer gradually times-out uncompleted requests and reopens its slots, but your computer just fills up those cleared slots with more SYNs. Your computer never follows up on any of these requests, it just keeps sending more SYNs. (A SYN that has not yet been followed up on is a half-open connection.) This barrage of SYNs fills up all of that computer's “slots” for incoming connections, so it cannot respond to anyone else until those slots are cleared. The limit was added to prevent computers from unknowingly being used in a type of Denial-of-Service attack known as a “Syn Flood”, in which your computer sends an endless stream of connection requests (SYN's) to another computer. Windows versions prior to Windows XP SP2, did not have this limit.

Additional connections are placed in a queue, and will be opened no more than 10 at a time, until the target number of successful connections is reached. In Windows versions beginning with XP SP2, there are 10 concurrent TCP connection attempts allowed simultaneously. The detailed description can be found in Microsoft documentation:Įvent 4226, EVENT_TCPIP_TCP_CONNECT_LIMIT_REACHED You can go to XP's Admin tool, Event Viewer, look in the System tab and notice tcpip entry (appears beside a yellow warning sign). If there are more concurrent TCP connection attempts, Windows generates a warning: “EventID 4226: TCP/IP has reached the security limit imposed on the number of concurrent TCP connect attempts”. Windows XP SP2 limits the number of simultaneous TCP connection attempts to 10, at any given moment.

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