UDP(7)                           (TCP/IP)                            UDP(7)


   NAME
         UDP - Internet User Datagram Protocol

   SYNOPSIS
         #include <sys/socket.h>
         #include <netinet/in.h>

         s = socket(AFINET, SOCKDGRAM, 0);

         t = topen("/dev/udp", ORDWR);

   DESCRIPTION
         UDP is a simple datagram protocol which is layered directly above the
         Internet Protocol (IP).  Programs may access UDP using the socket
         interface, where it supports the SOCKDGRAM socket type, or using the
         Transport Level Interface (TLI), where it supports the connectionless
         (TCLTS) service type.

         Within the socket interface, UDP is normally used with the sendto(),
         sendmsg(), recvfrom(), and recvmsg() calls [see send(2) and recv(2)].
         If the connect(2) call is used to fix the destination for future
         packets, then the recv(2) or read(2) and send(2) or write(2) calls
         may be used.

         UDP address formats are identical to those used by the Transmission
         Control Protocol (TCP).  Like TCP, UDP uses a port number along with
         an IP address to identify the endpoint of communication.  The UDP
         port number space is separate from the TCP port number space (that
         is, a UDP port may not be connected to a TCP port).  The bind(2) call
         can be used to set the local address and port number of a UDP socket.
         The local IP address may be left unspecified in the bind() call by
         using the special value INADDRANY.  If the bind() call is not done,
         a local IP address and port number will be assigned to the endpoint
         when the first packet is sent.  Broadcast packets may be sent
         (assuming the underlying network supports this) by using a reserved
         broadcast address; This address is network interface dependent.
         Broadcasts may only be sent by the privileged user.

         Options at the IP level may be used with UDP; see ip(7).

         There are a variety of ways that a UDP packet can be lost or
         corrupted, including a failure of the underlying communication
         mechanism.  UDP implements a checksum over the data portion of the
         packet.  If the checksum of a received packet is in error, the packet
         will be dropped with no indication given to the user.  A queue of
         received packets is provided for each UDP socket.  This queue has a
         limited capacity.  Arriving datagrams which will not fit within its
         high-water capacity are silently discarded.





   7/91                                                                 Page 1

   UDP(7)                           (TCP/IP)                            UDP(7)


         UDP processes Internet Control Message Protocol (ICMP) error messages
         received in response to UDP packets it has sent.  See icmp(7).  ICMP
         source quench messages are ignored.  ICMP destination unreachable,
         time exceeded and parameter problem messages disconnect the socket
         from its peer so that subsequent attempts to send packets using that
         socket will return an error.  UDP will not guarantee that packets are
         delivered in the order they were sent.  As well, duplicate packets
         may be generated in the communication process.

   SEE ALSO
         read(2), write(2), bind(3N), connect(3N), recv(3N), send(3N),
         icmp(7), inet(7), ip(7), tcp(7).

         Postel, Jon, User Datagram Protocol, RFC 768, Network Information
         Center, SRI International, Menlo Park, Calif., August 1980.

   DIAGNOSTICS
         A socket operation may fail if:

         EISCONN             A connect() operation was attempted on a socket
                             on which a connect() operation had already been
                             performed, and the socket could not be
                             successfully disconnected before making the new
                             connection.

         EISCONN             A sendto() or sendmsg() operation specifying an
                             address to which the message should be sent was
                             attempted on a socket on which a connect()
                             operation had already been performed.

         ENOTCONN            A send() or write() operation, or a sendto() or
                             sendmsg() operation not specifying an address to
                             which the message should be sent, was attempted
                             on a socket on which a connect() operation had
                             not already been performed.

         EADDRINUSE          A bind() operation was attempted on a socket with
                             a network address/port pair that has already been
                             bound to another socket.

         EADDRNOTAVAIL       A bind() operation was attempted on a socket with
                             a network address for which no network interface
                             exists.

         EINVAL              A sendmsg() operation with a non-NULL
                             msgaccrights was attempted.

         EACCES              A bind() operation was attempted with a reserved
                             port number and the effective user ID of the
                             process was not the privileged user.



   Page 2                                                                 7/91

   UDP(7)                           (TCP/IP)                            UDP(7)


         ENOBUFS             The system ran out of memory for internal data
                             structures.



















































   7/91                                                                 Page 3