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RFC 768                                                        J. Postel                                                                     ISI                                                          28 August 1980User Datagram Protocol----------------------Introduction------------This User Datagram  Protocol  (UDP)  is  defined  to  make  available  adatagram   mode  of  packet-switched   computer   communication  in  theenvironment  of  an  interconnected  set  of  computer  networks.   Thisprotocol  assumes  that the Internet  Protocol  (IP)  [1] is used as theunderlying protocol.This protocol  provides  a procedure  for application  programs  to sendmessages  to other programs  with a minimum  of protocol mechanism.  Theprotocol  is transaction oriented, and delivery and duplicate protectionare not guaranteed.  Applications requiring ordered reliable delivery ofstreams 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 FormatFields------Source Port is an optional field, when meaningful, it indicates the portof the sending  process,  and may be assumed  to be the port  to which areply should  be addressed  in the absence of any other information.  Ifnot used, a value of zero is inserted.Postel                                                          [page 1]

                                                             28 Aug 1980User Datagram ProtocolRFC 768FieldsDestination  Port has a meaning  within  the  context  of  a  particularinternet destination address.Length  is the length  in octets  of this user datagram  including  thisheader  and the data.   (This  means  the minimum value of the length iseight.)Checksum is the 16-bit one's complement of the one's complement sum of apseudo header of information from the IP header, the UDP header, and thedata,  padded  with zero octets  at the end (if  necessary)  to  make  amultiple of two octets.The pseudo  header  conceptually prefixed to the UDP header contains thesource  address,  the destination  address,  the protocol,  and the  UDPlength.   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 (theequivalent  in one's complement  arithmetic).   An all zero  transmittedchecksum  value means that the transmitter  generated  no checksum  (fordebugging 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

28 Aug 1980RFC 768                                           User Datagram Protocol                                                            IP InterfaceIP Interface-------------The UDP module  must be able to determine  the  source  and  destinationinternet addresses and the protocol field from the internet header.  Onepossible  UDP/IP  interface  would return  the whole  internet  datagramincluding all of the internet header in response to a receive operation.Such an interface  would  also allow  the UDP to pass  a  full  internetdatagram  complete  with header  to the IP to send.  The IP would verifycertain fields for consistency and compute the internet header checksum.Protocol Application--------------------The major uses of this protocol is the Internet Name Server [3], and theTrivial 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]