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The User Datagram Protocol
The User Datagram Protocol (UDP) is defined in :rfc:`768`. It provides an unreliable connectionless transport service on top of the unreliable network layer connectionless service. The main characteristics of the UDP service are :
the UDP service cannot deliver SDUs that are larger than 65467 bytes [#fmtuudp]_
the UDP service does not guarantee the delivery of SDUs (losses can occur and SDUs can arrive out-of-sequence)
the UDP service will not deliver a corrupted SDU to the destination
Compared to the connectionless network layer service, the main advantage of the UDP service is that it allows several applications running on a host to exchange SDUs with several other applications running on remote hosts. Let us consider two hosts, e.g. a client and a server. The network layer service allows the client to send information to the server, but if an application running on the client wants to contact a particular application running on the server, then an additional addressing mechanism is required other than the IP address that identifies a host, in order to differentiate the application running on a host. This additional addressing is provided by `port numbers`. When a server using UDP is enabled on a host, this server registers a `port number`. This `port number` will be used by the clients to contact the server process via UDP.
The figure below shows a typical usage of the UDP port numbers. The client process uses port number `1234` while the server process uses port number `5678`. When the client sends a request, it is identified as originating from port number `1234` on the client host and destined to port number `5678` on the server host. When the server process replies to this request, the server's UDP implementation will send the reply as originating from port `5678` on the server host and destined to port `1234` on the client host.
UDP uses a single segment format shown in the figure below.
UDP Header Format
The UDP header contains four fields :
a 16 bits source port
a 16 bits destination port
a 16 bits length field
a 16 bits checksum
As the port numbers are encoded as a 16 bits field, there can be up to only 65535 different server processes that are bound to a different UDP port at the same time on a given server. In practice, this limit is never reached. However, it is worth noticing that most implementations divide the range of allowed UDP port numbers into three different ranges :
the privileged port numbers (1 < port < 1024 )

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locale/pot/protocols/udp.pot, string 1