Applications use "datagram sockets to establish host-to-host communications. An application binds a socket to its endpoint of data transmission, which is a combination of an "IP address and a service port. A port is a software structure that is identified by the "port number, a 16 "bit integer value, allowing for port numbers between 0 and 65535. Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.
UDP provides application "multiplexing (via "port numbers) and integrity verification (via "checksum) of the header and payload. If transmission reliability is desired, it must be implemented in the user's application.
The "Internet Assigned Numbers Authority (IANA) has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services. On "Unix-like "operating systems, using one of these ports requires "superuser operating permission. Port numbers 1024 through 49151 are the "registered ports used for IANA-registered services. Ports 49152 through 65535 are dynamic ports that are not officially designated for any specific service, and may be used for any purpose. They also are used as "ephemeral ports, from which software running on the host may randomly choose a port in order to define itself. In effect, they are used as temporary ports primarily by "clients when communicating with "servers.
|0||0||Source port||Destination port|
The UDP header consists of 4 fields, each of which is 2 bytes (16 bits). The use of the fields "Checksum" and "Source port" is optional in IPv4 (pink background in table). In IPv6 only the source port is optional (see below).
The method used to compute the checksum is defined in RFC 768:
In other words, all 16-bit words are summed using one's complement arithmetic. Add the 16-bit values up. Each time a carry-out (17th bit) is produced, swing that bit around and add it back into the least significant bit. The sum is then one's complemented to yield the value of the UDP checksum field.
If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the one's complement (all 1s).
The difference between "IPv4 and "IPv6 is in the data used to compute the checksum.
When UDP runs over IPv4, the checksum is computed using a "pseudo header" that contains some of the same information from the real IPv4 header. The pseudo header is not the real IPv4 header used to send an IP packet, it is used only for the checksum calculation.
|0||0||Source IPv4 Address|
|4||32||Destination IPv4 Address|
|12||96||Source Port||Destination Port|
The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see "List of IP protocol numbers): 17 (0x11). The UDP length field is the length of the UDP header and data. The field data stands for the transmitted data.
UDP checksum computation is optional for IPv4. If a checksum is not used it should be set to the value zero.
When UDP runs over IPv6, the checksum is mandatory. The method used to compute it is changed as documented in RFC 2460:
When computing the checksum, again a pseudo header is used that mimics the real IPv6 header:
|0||0||Source IPv6 Address|
|16||128||Destination IPv6 Address|
|40||320||Source Port||Destination Port|
The source address is the one in the IPv6 header. The destination address is the final destination; if the IPv6 packet does not contain a Routing header, that will be the destination address in the IPv6 header; otherwise, at the originating node, it will be the address in the last element of the Routing header, and, at the receiving node, it will be the destination address in the IPv6 header. The value of the Next Header field is the protocol value for UDP: 17. The UDP length field is the length of the UDP header and data.
Lacking reliability, UDP applications must generally be willing to accept some loss, errors or duplication. Some applications, such as "TFTP, may add rudimentary reliability mechanisms into the application layer as needed.
Most often, UDP applications do not employ reliability mechanisms and may even be hindered by them. "Streaming media, real-time multiplayer games and "voice over IP (VoIP) are examples of applications that often use UDP. In these particular applications, loss of packets is not usually a fatal problem. If an application requires a high degree of reliability, a protocol such as the "Transmission Control Protocol may be used instead.
In VoIP, for example, latency and jitter are the primary concerns. The use of TCP would cause jitter if any packets were lost as TCP does not provide subsequent data to the application while it is requesting re-sending of the missing data. If using UDP the end user applications must provide any necessary handshaking such as real time confirmation that the message has been received.
Numerous key Internet applications use UDP, including: the "Domain Name System (DNS), where queries must be fast and only consist of a single request followed by a single reply packet, the "Simple Network Management Protocol (SNMP), the "Routing Information Protocol (RIP) and the "Dynamic Host Configuration Protocol (DHCP).
Voice and video traffic is generally transmitted using UDP. Real-time video and audio streaming protocols are designed to handle occasional lost packets, so only slight degradation in quality occurs, rather than large delays if lost packets were retransmitted. Because both TCP and UDP run over the same network, many businesses are finding that a recent increase in UDP traffic from these real-time applications is hindering the performance of applications using TCP, such as "point of sale, "accounting, and "database systems. When TCP detects packet loss, it will throttle back its data rate usage. Since both real-time and business applications are important to businesses, developing "quality of service solutions is seen as crucial by some.
Some "VPN systems such as "OpenVPN may use UDP while implementing reliable connections and error checking at the application level.
"Transmission Control Protocol is a connection-oriented protocol, which means that it requires handshaking to set up end-to-end communications. Once a connection is set up, user data may be sent bi-directionally over the connection.
User Datagram Protocol is a simpler message-based "connectionless protocol. Connectionless protocols do not set up a dedicated end-to-end connection. Communication is achieved by transmitting information in one direction from source to destination without verifying the readiness or state of the receiver.
|""||Wikiversity has learning resources about User Datagram Protocol|