rfc768
RFC 768 - User Datagram Protocol (UDP)
- Author: J. Postel (ISI)
- Date: 28 August 1980
Introduction
The User Datagram Protocol (UDP) is defined to provide a datagram mode of packet-switched computer communication in an environment of interconnected computer networks.
This protocol assumes that the Internet Protocol (IP) [1] is used as the underlying protocol.
UDP provides a procedure for application programs to send messages to other programs with a minimum of protocol mechanisms. The protocol is transaction-oriented, and delivery and duplicate protection are not guaranteed.
Applications requiring ordered, reliable delivery of streams of data should use the Transmission Control Protocol (TCP) [2].
Format
Fields
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| Source | Destination |
| Port | Port |
+--------+--------+--------+--------+
| | |
| Length | Checksum |
+--------+--------+--------+--------+
|
| data octets ...
+---------------- ...User Datagram Header Format.
Source Port
This is an optional field. When meaningful, it indicates the port of the sending process and may be assumed to be the port to which a reply should be addressed in the absence of other information.
If not used, a value of zero is inserted.
Destination Port
This field has a meaning within the context of a particular internet destination address.
Length
The length in octets of this user datagram, including header and data.
The minimum value is 8.
Checksum
The 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,
- the data (padded with zero octets at the end, if necessary, to make a multiple of two octets).
The pseudo-header conceptually prefixed to the UDP header contains:
- source address,
- destination address,
- protocol,
- UDP length.
This information provides protection against misrouted datagrams.
This checksum procedure is the same as in TCP.
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| source address |
+--------+--------+--------+--------+
| destination address |
+--------+--------+--------+--------+
| zero |protocol| UDP length |
+--------+--------+--------+--------+If the computed checksum is zero, it is transmitted as all ones (the equivalent in one's complement arithmetic).
An all-zero transmitted checksum value means the transmitter generated no checksum (for debugging or for higher-level protocols that do not care).
User Interface
A user interface should allow:
- creation of new receive ports,
- receive operations on receive ports returning data octets and an indication of source port and source address,
- an operation that allows a datagram to be sent, specifying data, source and destination ports, and addresses.
IP Interface
The UDP module must be able to determine source and destination internet addresses and the protocol field from the internet header.
One possible UDP/IP interface would return the whole internet datagram, including all internet header fields, in response to a receive operation.
Such an interface would also allow UDP to pass a full internet datagram, complete with header, to IP for sending. IP would verify certain fields for consistency and compute the internet header checksum.
Protocol Application
Major uses of this protocol include:
- the Internet Name Server [3],
- the Trivial File Transfer Protocol (TFTP) [4].
Protocol Number
UDP is protocol number 17 (21 octal) when used in IP.
Other protocol numbers are listed in [5].
References
[1] Postel, J., "Internet Protocol," RFC 760, USC/Information Sciences Institute, January 1980.
[2] Postel, J., "Transmission Control Protocol," RFC 761, USC/Information Sciences Institute, January 1980.
[3] Postel, J., "Internet Name Server," USC/Information Sciences Institute, IEN 116, August 1979.
[4] Sollins, K., "The TFTP Protocol," Massachusetts Institute of Technology, IEN 133, January 1980.
[5] Postel, J., "Assigned Numbers," USC/Information Sciences Institute, RFC 762, January 1980.