User datagram protocol





The User Datagram Protocol (UDP) is a minimal message-oriented transport
layer protocol that is currently documented by IETF RFC 768.

In the TCP/IP model, UDP provides a very simple interface between a network
layer below and an application layer above. UDP provides no guarantees for
message delivery and a UDP sender retains no state on UDP messages once sent
onto the network. UDP adds only application multiplexing and data
checksumming on top of an IP datagram.

The UDP header consists of only 4 header fields of which two are optional.
The source and destination port fields are 16 bit fields that identify the
sending and receiving process. Since UDP is stateless and a UDP sender may
not solicit replies, the source port is optional. If not used, the source
port should be set to zero. The port fields are followed by a mandatory
length field specified as bytes of the UDP datagram including the data. The
minimum value of the length field is 8 (octets). The remaining header field
is a 16-bit checksum field covering the header and data. The checksum is
also optional, but almost always used in practice.

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 require reliability mechanisms and may
even be hindered by them. Real-time games and streaming multimedia are
typical examples of applications that often use UDP. If an application
requires a high degree of reliability, a protocol such as the Transmission
Control Protocol may be used.

Lacking any congestion avoidance and control mechanisms, network-based
mechanisms are required to minimize potential congestion collapse effects of
uncontrolled, high rate UDP traffic loads. In other words, since UDP senders
cannot detect congestion, network-based elements such as routers using
packet queueing and dropping techniques will often be the only tool
available to slow down excessive UDP traffic. The Datagram Congestion
Control Protocol (DCCP) is being designed as a partial solution to this
problem by adding end host congestion control behavior to high-rate UDP
streams such as streaming media.

While the total amount of UDP traffic found on a typical network is often on
the order of only a few percent, numerous key applications use UDP. These
include the Domain Name System (DNS), the simple network management protocol
(SNMP), the dynamic host configuration protocol (DHCP) and the Routing
Information Protocol (RIP) to name just a few.