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User Datagram protocol (UDP)

A UDP datagram is used in Network File System (NFS), DNS, SNMP, TFTP etc. and has no handshaking, flow control or windowing capability, it is a fire and forget type protocol. The segment looks like this:

UDP header

The checksum field includes a 12-byte 'pseudo header' that includes the source and destination IP addresses, the 8-bit reserved field containing 0, the 8-bit protocol ID and the 16-bit UDP length field. The pseudo header is useful to check that the IP datagram arrived at the correct station.

UDP is ideal for applications that want to send datagrams one at a time and for applications that perform their own error recovery. There is no sequencing with UDP so you do not want UDP datagrams to become fragmenting therefore the MTU of the LLC needs to be larger than that taken up by the UDP datagram including the UDP and IP headers and the data.

An application can use a UDP port number and another application can use the same port number for a TCP session from the same IP address. Very often the port numbers in RFC 1700 RFC1700 are the same for TCP and UDP. The advantages of UDP over TCP are that it does not have to wait for acknowledgements, keep packets in memory for re-ordering purposes or perform handshaking, it is therefore quicker and has less overhead than TCP.

RFC 768 describes UDP in detail.

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