RFC 768
J. Postel
ISI
28 August 1980

User Datagram Protocol
----------------------

Introduction

------------
 

This User Datagram Protocol (UDP) is defined to make available a

datagram mode of packet-switched computer communication in the

environment of an interconnected set of computer networks. This

protocol assumes that the Internet Protocol (IP) [1] is used as the

underlying protocol.

This protocol provides a procedure for application programs to send

messages to other programs with a minimum of protocol mechanism. 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

------
 
                                    
                  0      7 8     15 16    23 24    31  
                 +--------+--------+--------+--------+ 
                 |     Source      |   Destination   | 
                 |      Port       |      Port       | 
                 +--------+--------+--------+--------+ 
                 |                 |                 | 
                 |     Length      |    Checksum     | 
                 +--------+--------+--------+--------+ 
                 |                                     
                 |          data octets ...            
                 +---------------- ...                 
 
                      User Datagram Header Format
 

Fields

------
 

Source Port 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 any other information. If

not used, a value of zero is inserted.

Postel [page 1]

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28 Aug 1980

User Datagram Protocol RFC 768

Fields

Destination Port has a meaning within the context of a particular

internet destination address.

Length is the length in octets of this user datagram including this

header and the data. (This means the minimum value of the length is

eight.)

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.

The pseudo header conceptually prefixed to the UDP header contains the

source address, the destination address, the protocol, and the UDP

length. This information gives protection against misrouted datagrams.

This checksum procedure is the same as is used 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 that the transmitter generated no checksum (for

debugging or for higher level protocols that don't care).

User Interface

--------------
 

A user interface should allow

the creation of new receive ports,

receive operations on the receive ports that return the data octets and an indication of source port and source address,

and an operation that allows a datagram to be sent, specifying the data, source and destination ports and addresses to be sent.

[page 2]                                                          Postel

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28 Aug 1980

RFC 768                                           User Datagram Protocol
                                                            IP Interface
 
 
 

IP Interface

-------------
 

The UDP module must be able to determine the 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 of the internet header in response to a receive operation.

Such an interface would also allow the UDP to pass a full internet

datagram complete with header to the IP to send. The IP would verify

certain fields for consistency and compute the internet header checksum.

Protocol Application

--------------------
 

The major uses of this protocol is the Internet Name Server [3], and the

Trivial File Transfer [4].

Protocol Number

---------------
 

This is protocol 17 (21 octal) when used in the Internet Protocol.

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.
 
 
 
 
 
 
 
 
 
Postel                                                          [page 3]

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