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       udp - User Datagram Protocol for IPv4


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

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);


       This  is  an  implementation of the User Datagram Protocol described in
       RFC 768.  It implements a connectionless,  unreliable  datagram  packet
       service.   Packets  may  be reordered or duplicated before they arrive.
       UDP generates and checks checksums to catch transmission errors.

       When a UDP socket is  created,  its  local  and  remote  addresses  are
       unspecified.   Datagrams  can  be  sent  immediately using sendto(2) or
       sendmsg(2) with a valid  destination  address  as  an  argument.   When
       connect(2)  is called on the socket, the default destination address is
       set and datagrams can now be sent using  send(2)  or  write(2)  without
       specifying  a  destination  address.   It  is still possible to send to
       other destinations by passing an address to  sendto(2)  or  sendmsg(2).
       In order to receive packets, the socket can be bound to a local address
       first by using bind(2).  Otherwise the socket layer will  automatically
       assign   a   free   local   port   out   of   the   range   defined  by
       net.ipv4.ip_local_port_range and bind the socket to INADDR_ANY.

       All receive operations return only one  packet.   When  the  packet  is
       smaller  than  the passed buffer, only that much data is returned; when
       it is bigger, the packet is truncated and the MSG_TRUNC  flag  is  set.
       MSG_WAITALL is not supported.

       IP  options  may be sent or received using the socket options described
       in ip(7).  They are only processed by the kernel when  the  appropriate
       /proc  parameter  is enabled (but still passed to the user even when it
       is turned off).  See ip(7).

       When the MSG_DONTROUTE flag is set on sending, the destination  address
       must  refer to a local interface address and the packet is only sent to
       that interface.

       By default,  Linux  UDP  does  path  MTU  (Maximum  Transmission  Unit)
       discovery.   This  means  the  kernel  will  keep track of the MTU to a
       specific target IP address and return EMSGSIZE when a UDP packet  write
       exceeds  it.   When  this  happens, the application should decrease the
       packet size.  Path MTU discovery can  be  also  turned  off  using  the
       IP_MTU_DISCOVER socket option or the /proc/sys/net/ipv4/ip_no_pmtu_disc
       file; see ip(7) for  details.   When  turned  off,  UDP  will  fragment
       outgoing UDP packets that exceed the interface MTU.  However, disabling
       it is not recommended for performance and reliability reasons.

   Address Format
       UDP uses the IPv4 sockaddr_in address format described in ip(7).

   Error Handling
       All fatal errors will be passed to the user as  an  error  return  even
       when  the  socket  is not connected.  This includes asynchronous errors
       received from the network.  You may get an error for an earlier  packet
       that  was  sent  on  the  same socket.  This behavior differs from many
       other BSD socket implementations which don’t pass any errors unless the
       socket is connected.  Linux’s behavior is mandated by RFC 1122.

       For  compatibility  with  legacy  code,  in  Linux  2.0  and 2.2 it was
       possible to set the SO_BSDCOMPAT SOL_SOCKET option  to  receive  remote
       errors  only  when the socket has been connected (except for EPROTO and
       EMSGSIZE).  Locally generated errors are always  passed.   Support  for
       this  socket  option  was  removed  in later kernels; see socket(7) for
       further information.

       When the IP_RECVERR option is enabled, all errors  are  stored  in  the
       socket  error  queue,  and  can  be  received  by  recvmsg(2)  with the
       MSG_ERRQUEUE flag set.

   /proc interfaces
       System-wide UDP parameter settings can be  accessed  by  files  in  the
       directory /proc/sys/net/ipv4/.

       udp_mem (since Linux 2.6.25)
              This is a vector of three integers governing the number of pages
              allowed for queueing by all UDP sockets.

              min       Below this number of pages, UDP is not bothered  about
                        its  memory  appetite.   When  the  amount  of  memory
                        allocated by UDP exceeds this number,  UDP  starts  to
                        moderate memory usage.

              pressure  This  value  was  introduced  to  follow the format of
                        tcp_mem (see tcp(7)).

              max       Number of  pages  allowed  for  queueing  by  all  UDP

              Defaults  values  for  these  three items are calculated at boot
              time from the amount of available memory.

       udp_rmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
              Minimal size, in bites, of receive buffer used by UDP sockets in
              moderation.   Each  UDP  socket  is  able  to  use  the size for
              receiving data, even  if  total  pages  of  UDP  sockets  exceed
              udp_mem pressure.

       udp_wmem_min (integer; default value: PAGE_SIZE; since Linux 2.6.25)
              Minimal  size,  in  bytes, of send buffer used by UDP sockets in
              moderation.  Each UDP socket is able to use the size for sending
              data,  even  if  total  pages  of  UDP  sockets  exceed  udp_mem

   Socket Options
       To set or get a UDP  socket  option,  call  getsockopt(2)  to  read  or
       setsockopt(2) to write the option with the option level argument set to

       UDP_CORK (since Linux 2.5.44)
              If this option is enabled, then all data output on  this  socket
              is  accumulated  into a single datagram that is transmitted when
              the option is disabled.  This option should not be used in  code
              intended to be portable.

       These ioctls can be accessed using ioctl(2).  The correct syntax is:

              int value;
              error = ioctl(udp_socket, ioctl_type, &value);

              Gets  a  pointer to an integer as argument.  Returns the size of
              the next pending datagram in the integer in bytes, or 0 when  no
              datagram is pending.

              Returns  the number of data bytes in the local send queue.  Only
              supported with Linux 2.4 and above.

       In addition all ioctls documented in ip(7) and socket(7) are supported.


       All  errors documented for socket(7) or ip(7) may be returned by a send
       or receive on a UDP socket.

              No receiver was associated with the destination  address.   This
              might be caused by a previous packet sent over the socket.


       IP_RECVERR is a new feature in Linux 2.2.


       ip(7), raw(7), socket(7), udplite(7)

       RFC 768 for the User Datagram Protocol.
       RFC 1122 for the host requirements.
       RFC 1191 for a description of path MTU discovery.


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