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User Datagram Protocol (UDP) is a connectionless and unreliable transport layer protocol. UDP adds only application multiplexing and optional data checksumming on top of an IP datagram.

The UDP header consists of only 4 header fields of which two are optional. The source and destination port fields are 16-bit fields that identify the sending and receiving process. The port fields are followed by length field specified as numer of bytes of the UDP datagram including the data. The remaining header field is a optional 16-bit checksum field covering the header and data.

Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Since no sequence numer is included in the UDP header packet loss, duplicates and the right packet ordering can not be recognized in UDP.