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Definition: UDP (User Datagram Protocol) is a simple OSI transport layer protocol for client/server network applications based on Internet Protocol (IP). UDP is the main alternative to TCP and one of the oldest network protocols in existence, introduced in 1980.

UDP is often used in videoconferencing applications or computer games specially tuned for real-time performance. To achieve higher performance, the protocol allows individual packets to be dropped (with no retries) and UDP packets to be received in a different order than they were sent as dictated by the application.

UDP Datagrams

UDP network traffic is organized in the form of datagrams. A datagram comprises one message unit. The first eight (8) bytes of a datagram contain header information and the remaining bytes contain message data.

A UDP datagram header consists of four (4) fields of two bytes each:

  • source port number
  • destination port number
  • datagram size
  • checksum
UDP port numbers allow different applications to maintain their own channels for data similar to TCP. UDP port headers are two bytes long; therefore, valid UDP port numbers range from 0 to 65535.

The UDP datagram size is a count of the total number of bytes contained in header and data sections. As the header length is a fixed size, this field effectively tracks the length of the variable-sized data portion (sometimes called payload). The size of datagrams varies depending on the operating environment but has a maximum of 65535 bytes.

UDP checksums protect message data from tampering. The checksum value represents an encoding of the datagram data calculated first by the sender and later by the receiver. Should an individual datagram be tampered with or get corrupted during transmission, the UDP protocol detects a checksum calculation mismatch. In UDP, checksumming is optional as opposed to TCP where checksums are mandatory.

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