User Datagram Protocol (UDP)

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UDP is defined in RFC 768. UDP is used by applications that only need a connectionless, best effort transport service. The service provided by UDP is an unreliable service that provides no guarantees for delivery and no protection from duplication of data (if this arises due to software errors). The simplicity of UDP reduces the overhead and may be adequate in many cases where good connectivity is expected.

A variant of UDP, UDP-Lite [RFC3828] was introduced into the Internet Protocol Suite in 2004. This has been designed to provide more optimized transport for unicast/multicast multimedia data. The header format closely follows that of UDP. The Datagram Congestion Control Protocol (DCCP) provides a more sophisticated datagram service, particularly suited to unicast streaming services.

UDP packet has very simple structure:

15	31
Source Port	Destination Port
Length	Checksum
Data

Source Port. 16 bits. The port number of the sender. Cleared to zero if not used. Like in TCP, this field indicates which application sent the data contained within the IP packet.
Destination Port. 16 bits. Again, indicates which application is to receive the data contained within the IP packet.
Length. 16 bits. The length in bytes of the UDP header and the encapsulated data. The minimum value for this field is 8. Since all UDP headers are 8 bytes long, if you subtract 8 from this number you'll find out the size of the data being sent.
Checksum. 16 bits. Since UDP is not responsible for ensuring that the data actually arrives, a checksum is optional. If computed it is set to 16-bit one's complement of the one's complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded as needed with zero bytes at the end to make a multiple of two bytes. If the checksum is cleared to zero, then checksuming is disabled. If the computed checksum is zero, then this field must be set to 0xFFFF.
Data. Again, this will be the actual data being sent and will not include any additional headers.

These fields are decoded by all sniffers including tcpdump and Solaris snoop.

In order to use UDP, the application must supply the IP address and port number of the destination application. A port is an abstraction (application number that is somewhat similar to UID) to allow transport protocols like UDP and TCP the capability of handling communications between multiple hosts. It allows a communication to be uniquely identified. Like UID port number is always a positive integer. Among the most commonly used UDP ports are:

53 DOMAIN Domain Name Server
43 NICNAME Whois
69 TFTP Trivial File Transfer
137 NETBIOS-NS NetBIOS Name Service
138 NETBIOS-DGM NetBIOS DataGrams
139 NETBIOS-SSN NetBIOS Session
514 Syslog The BSD Syslog Protocol. See RFC 3164 (rfc3164)

UDP demultiplexes the ports, keeping track of which data is available for or sent to a port.

User Datagram Protocol (UDP)

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