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UDP is used directly for protocols such as the Domain Name System (DNS) or the Network Time Protocol (NTP), which consists on isolated request-response transactions between hosts, where the negotiation and maintenance of TCP connections would be prohibitive.
There are other protocols layered on top of UDP, for example the Real-time Transport Protocol (RTP) used in real-time media applications. UDT,VFER, RBUDP, Tsunami, and Hurricane are examples of UDP-based bulk transport protocols.
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