A Transmission Control Protocol/Internet Protocol (TCP/IP) technology that enables an application to send a message to one of several applications running in a destination machine. UDP is stateless — it differentiates sources and destinations within hosts and provides no other services. Often services do not use predefined port numbers, so filtering on the basis of "well-known ports" will not work. See TCP/IP.
_U_ser _D_atagram _P_rotocol. An IP protocol characterized by a packet-oriented approach, without correction, retries, throttling, or any effort made to guarantee that the other application even get the data. Also known as Unreliable Datagram Protocol because no guarantee is made about packets being received. In theory, UDP packets are the first ones dropped by an overloaded router; in all cases, this should be the working assumption. This protocol is a very thin layer on top of IP, and is therefore useful for implementing protocol in an application layer. Networked games (such as [xpilot]?) typically rely on this protocol for performance (and a late packet is worse than an out-of-order or missing packet).