The User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. Using UDP, programs on networked computers can send short messages sometimes known as datagrams to one another.
UDP does not provide the reliability and ordering guarantees that TCP does. Datagrams may arrive out of order or go missing without notice. Without the overhead of checking if every packet actually arrived, UDP is faster and more efficient for many lightweight or time-sensitive purposes. Also, its stateless nature is useful for servers that answer small queries from huge numbers of clients. Compared to TCP, UDP is required for broadcast (send to all on local network) and multicast (send to all subscribers).
Common network applications that use UDP include the Domain Name System (DNS), streaming media applications, Voice over IP, Trivial File Transfer Protocol (TFTP), BitTorrent and online games.
UDP utilizes ports to allow application-to-application communication. The port field is 16-bits so the valid range is 0 to 65,535. Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.
Ports 1 through 1023 are named "well-known" ports and on Unix-derived operating systems binding to one of these ports requires root access.
Ports 1024 through 49,151 are registered ports.
Ports 49,152 through 65,535 are ephemeral ports and are used as temporary ports primarily by clients when communicating to servers.
UDP is a minimal message-orientated transport layer protocol that is currently documented in IETF RFC 768.
In the Internet protocol suite, UDP provides a very simple interface between a network layer below (e.g., IPv4) and a session layer or application layer above.
UDP provides no guarantees to the upper layer protocol for message delivery and a UDP sender retains no state on UDP messages once sent (for this reason UDP is sometimes called the Unreliable Datagram Protocol). UDP adds only application multiplexing and checksumming of the header and payload. If any kind of reliability for the information transmited is needed, it must be implemented in upper layers.
The UDP header consists of only 4 fields of which two are optional (red background in table).
This field identifies the sending port when meaningful and should be assumed to be the port to reply to if needed. If not used then it should be zero.
This field identifies the destination port and is required.
A 16-bit field that specifies the length of the entire datagram: header and data. The minimum length is 8 bytes since that's the length of the header.
The 16-bit checksum field is used for error-checking of the header and data. The method used to compute the checksum is defined within RFC 768:
Checksum is the 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 with zero octets at the end (if necessary) to make a multiple of two octets.
In other words, all 16-bit words are summed together using one's complement (with the checksum field set to zero). The sum is then one's complemented. This final value is then inserted as the checksum field. Algorithmically speaking, this is the same as for IPv4.
The difference is in the data used to make the checksum. Included is a pseudo-header that mimics the IP header:
The source and destination addresses are those in the IPv4 header. The protocol is that for UDP (see List of IPv4 protocol numbers): 17. The UDP length field is the length of the UDP header and data.
If the checksum is calculated to be zero (all 0's) it should be sent as negative zero (all 1's). If a checksum is not used it should be sent as zero (all 0's) as zero indicates an unused checksum.