One of the unique features of WinRoute's packet filter is the ability to check for SYN/ACK TCP flags. Before all TCP connections there is an initial connection, the 'hand-shake'. This process begins when the connecting client sends a packet to the server with a synchronization (SYN) flag. The server then replies with a synchronize and acknowledgment (SYN/ACK). The client and server both proceed with acknowledgments and the connection is established. WinRoute denotes the SYN and ACK flags with the terms 'establishing' and 'established'. Note that If both flags are checked WinRoute will apply the rule only for the server response packet, where both flags are set in the TCP header.