The server side Transfer Control Protocol is modified so that the server
returns a SYNACK message with the window size equal to zero if the server
is busy. When a client sends a TCP connection request and receives a
synchronization acknowledgement message with the window size equal to
zero, the client knows that the server received the connection request
and that the server is busy. The client may then send an acknowledgement
message to complete the three-way synchronization handshake, thus
successfully completing the connection. Thereafter, the client side TCP
may probe the server side TCP until a window update message is received
from the server. When the server sends a window update message to set the
window size to a non-zero size, the client knows that the server is no
longer busy and the client application may then use the TCP connection.