The primary idea behind a TCP reset attack is to falsely terminate an established TCP connection. Lets imagine an established TCP connection from host A to host B. Now, a third host, C, spoofs a packet that matches the source port and IP address of host A, the destination port and IP address of host B, and the current sequence number of the active TCP connection between host A and host B. Host C sets the RST bit on the spoofed packet, so when received by host B, host B immediately terminates the connection. This results in a denial of service, until the connection can be reestablished. However, the severity of such an attack is different from application to application.