udp


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

       udp_socket = socket(AF_INET, SOCK_DGRAM, 0);

DESCRIPTION
       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  con-
       nect(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  speci-
       fying  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
       /proc/sys/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) discov-
       ery.  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_DIS-
       COVER 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
       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  allo-
                        cated by UDP exceeds this number, UDP starts to moder-
                        ate 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 sock-
                        ets.

              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 bytes, of receive buffers 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  pres-
              sure.

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

       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.

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

       TIOCOUTQ (SIOCOUTQ)
              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.

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

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

VERSIONS
       IP_RECVERR is a new feature in Linux 2.2.

SEE ALSO
       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.

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-06-13                            UDP(7)
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