User Datagram Protocol UDP

Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Some applications such as Trivial File Transfer Protocol (TFTP) may add rudimentary reliability mechanisms into the application layer as needed. Most often, UDP applications do not require reliability mechanisms and may even be hindered by them. Streaming media, real-time multiplayer games and voice over IP (VoIP) are examples of applications that often use UDP. Another example is the Simple Network Management Protocol (SNMP) or Routing Information Protocol (RIP). These two use UDP instead of TCP because of the low bandwidth cost. It wouldn't be very effective to overload a network with our management software packets or with our routing information update packets.

Because it's the fastest way to transfer data over internet, UDP is the main method used by hackers to do a Denial Of Service (DOS) attack. If a computer receives many UDP packets, it will check for the availability of that port and notify the sender with a Destination Unreachable message. The host floods itself, sending reply ICMP packets to the originating UDP sender. Usually this kind of attack comes with IP spoofing (hiding the origin IP of the sender) too, so the attacker remains in anonymity and doesn't get flooded by the replies. The best measure to prevent this is to get a firewall that blocks any echo replies (like Destination Unreachable) and prevents you from self-flooding your network connection.

Exactly like TCP, UDP uses port numbers to handle multiple connections too. (See TCP for more details).

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