As mentioned in the previous section, the current implementation of the Secure TCP provides the interoperability with the classical TCP. In the current implementation, when a sender finds out a receiver uses the classical TCP, it will try to establish the connection in the classical TCP, not the Secure TCP. However, in some cases, this interoperability may cause some troubles, because the classical TCP transfers all the data in the plaintext. Thus, there must be a notification mechanism for application programs to indicate if the receiver uses the classical TCP.