UDP (User Datagram Protocol) - an explanation

Part of the TCP/IP protocol suite that provides means for applications to access connectionless features of IP. Operates at layer 4 of OSI reference model and provides for exchange of datagrams without acknowledgements or guaranteed delivery.User Datagram Protocol (UDP) supports the network at the transport layer. User Datagram Protocol (UDP) is an unreliable connection-less protocol and is defined by RFC 768 and 1122. It is a datagram service. There is no guarantee that the data will reach its destination. UDP is meant to provide serivce with very little transmission overhead. It adds very little to IP datapackets except for some error checking and port direction (Remember, UDP encapsulates IP packets). The following protocols or services use UDP:

DNS
SNMP
BOOTP
TFTP
NFS
RPC
RIP

The UDP header includes:

Source port number (16 bits) - An optional field
Destination port number (16 bits)
UDP length (16 bits)
UDP checksum (16 bits)

This is followed by data. The UDP checksum includes UDP data, not just the header as with IP message formats. For UDP and TCP checksum calculation a 12 byte pseudo header is included which contains some fields form the IP message header. This header is not transmitted as part of UDP or TCP, but is only used to help compute the checksum as a means of being sure that the data has arrived at the correct IP address. This is the TCP/UDP pseudo header:

Source IP address (32 bits)
Destination IP address (32 bits)
blank filler(0) (8 bits)
Protocol (8 bits)
UDP length (16 bits)

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