6.1 Differences from the classical TCP implementations

The significant changes in the negotiation phase (three-way handshake phase) from the classical TCP are;

• Any SYN segments cannot include real data because the connection is not securely established. T/TCP (Transaction TCP) in RFC1644[9] promotes real data transmissions in the SYN segment for performance reasons, however, this idea is conflicted with the Secure TCP.

• The cross-active open cannot be allowed. In the TCP protocol specification, both peer entities can send the SYN segment simultaneously. In this case, the peer entities merely return ACK segments, and then the TCP connection is established. This connection establishment is called ``cross-active open.'' However, this cross-active open does not exchange encryption keys in secure manner in the Secure TCP. Therefore, if the entity receives the SYN segment in the state ``SYN SENT,'' the state is immediately moved to the state ``CLOSED''. This state transmission avoids insecure cross-active open in the Secure TCP.