Our implementation is interoperable with the classical TCP.
In a case a sender uses the Secure TCP and its receiver uses the classical TCP, the response for the SYN segment sent by the sender is a normal SYN+ACK segment that does not include any negotiation data. Using this characteristic, the sender can determine the receiver uses the classical TCP. Hence, the sender can easily decide whether it goes ahead to establish a connection in the classical TCP or shuts down the negotiation.
In the current implementation, if the receiver uses the classical TCP, the sender will try to establish a connection using the classical TCP. This may be a drawback in some cases. We discuss this problem in the later section.