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(1) Short for User Datagram Protocol.

UDP, a connectionless protocol that, like TCP, runs on top of IP networks. Unlike TCP/IP, UDP/IP provides very few error recovery services, offering instead a direct way to send and receive datagrams over an IP network. It's used primarily for broadcasting messages over a network.

RFC 1122, section 4.1:

The User Datagram Protocol offers only a minimal transport service -- non-guaranteed datagram delivery -- and gives applications direct access to the datagram service of the IP layer. UDP is used by applications that do not require the level of service of TCP or that wish to use communications services (e.g., multicast or broadcast delivery) not available from TCP.

UDP is almost a null protocol; the only services it provides over IP are checksumming of data and multiplexing by port number. Therefore, an application program running over UDP must deal directly with end-to-end communication problems that a connection-oriented protocol would have handled -- e.g., retransmission for reliable delivery, packetization and reassembly, flow control, congestion avoidance, etc., when these are required. The fairly complex coupling between IP and TCP will be mirrored in the coupling between UDP and many applications using UDP.

RFC 768


UDP-Header (8 Bytes lang + Daten)
Quell-Port Ziel-Port Länge Prüfsumme Daten
16 Bits 16 Bits 16 Bits 16 Bits variabel

Related: Networking

(2) Short for Usenet Death Penalty

Abbreviated UDP, a Usenet penalty enforced by system administrators of an ISP or NSP against other service providers. Usenet is by nature cooperative, and the UDP is one way that administrators can handle uncooperative members. UDPs are typically issued to ISP/NSPs that have users that are spamming and that have not stopped the users from doing so even after repeated requests for the administrators to deal with the spam. When a UDP is issued, any messages posted to Usenet from that site will be cancelled. UDPs are also issued to individual spammers.

Related: Usenet Newsgroups

Start listening on UDP port: nc -u localhost 55555

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