Blind attacks on a protocol connection, such as a TCP connection, are prevented by inserting checksums computed during protocol connection establishment handshake into data sent through the connection and invalidating data sent through the connection that lacks the protocol setup information checksums. Reset attacks are prevented by invalidating reset requests unless a master checksum computed from the protocol setup information checksums is included with the reset request. Checksums computed from protocol setup information have improved robustness by including a random number with the protocol setup information.