UDP(7)                                              Linux Programmer's Manual                                             UDP(7)



NAME
       udp - User Datagram Protocol for IPv4

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 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 destina-
       tion 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) 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 outgo-
       ing 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 sockets.

              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 pressure.

   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 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:

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

       FIONREAD (SIOCINQ)
              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.  Warning: Using FIONREAD, it is impossible to distinguish the case where no
              datagram is pending from the case where the next pending datagram contains zero bytes of data.  It is safer to use
              select(2), poll(2), or epoll(7) to distinguish these cases.

       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 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2010-06-13                                                     UDP(7)