Family of network protocols used on the internet, based on the corresponding protocols of the physical level (e.g. Ethernet or PPP). Above the physical level comes the network level with IP (Internet Protocol). Above this in turn is the transport layer with protocols like TCP (for connection-oriented communication) and UDP (for connectionless communication), and then the application layer with protocols like HTTP or FTP. For each of these levels, there are different attacks. Depending on the protective mechanisms used, attacks on the higher levels cannot be detected. A packet filter firewall operates on the network level, and is therefore unable to detect attacks on the higher levels, while an application gateway can detect attacks up to the application level.