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-.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
-.\"
-.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
-.\" Permission is granted to distribute possibly modified copies
-.\" of this page provided the header is included verbatim,
-.\" and in case of nontrivial modification author and date
-.\" of the modification is added to the header.
-.\" %%%LICENSE_END
-.\"
-.\" $Id: udp.7,v 1.7 2000/01/22 01:55:05 freitag Exp $
-.\"
-.TH UDP 7 2013-07-31 "Linux" "Linux Programmer's Manual"
-.SH NAME
-udp \- User Datagram Protocol for IPv4
-.SH SYNOPSIS
-.B #include <sys/socket.h>
-.br
-.B #include <netinet/in.h>
-.br
-.B #include <netinet/udp.h>
-.sp
-.B udp_socket = socket(AF_INET, SOCK_DGRAM, 0);
-.SH 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
-.BR sendto (2)
-or
-.BR sendmsg (2)
-with a valid destination address as an argument.
-When
-.BR connect (2)
-is called on the socket, the default destination address is set and
-datagrams can now be sent using
-.BR send (2)
-or
-.BR write (2)
-without specifying a destination address.
-It is still possible to send to other destinations by passing an
-address to
-.BR sendto (2)
-or
-.BR sendmsg (2).
-In order to receive packets, the socket can be bound to a local
-address first by using
-.BR bind (2).
-Otherwise, the socket layer will automatically assign
-a free local port out of the range defined by
-.I /proc/sys/net/ipv4/ip_local_port_range
-and bind the socket to
-.BR 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
-.B MSG_TRUNC
-flag is set.
-.B MSG_WAITALL
-is not supported.
-
-IP options may be sent or received using the socket options described in
-.BR ip (7).
-They are processed by the kernel only when the appropriate
-.I /proc
-parameter
-is enabled (but still passed to the user even when it is turned off).
-See
-.BR ip (7).
-
-When the
-.B MSG_DONTROUTE
-flag is set on sending, the destination address must refer to a local
-interface address and the packet is sent only 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
-.B 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
-.B IP_MTU_DISCOVER
-socket option or the
-.I /proc/sys/net/ipv4/ip_no_pmtu_disc
-file; see
-.BR 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.
-.SS Address format
-UDP uses the IPv4
-.I sockaddr_in
-address format described in
-.BR ip (7).
-.SS 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
-.BR RFC\ 1122 .
-
-For compatibility with legacy code, in Linux 2.0 and 2.2
-it was possible to set the
-.B SO_BSDCOMPAT
-.B SOL_SOCKET
-option to receive remote errors only when the socket has been
-connected (except for
-.B EPROTO
-and
-.BR EMSGSIZE ).
-Locally generated errors are always passed.
-Support for this socket option was removed in later kernels; see
-.BR socket (7)
-for further information.
-
-When the
-.B IP_RECVERR
-option is enabled, all errors are stored in the socket error queue,
-and can be received by
-.BR recvmsg (2)
-with the
-.B MSG_ERRQUEUE
-flag set.
-.SS /proc interfaces
-System-wide UDP parameter settings can be accessed by files in the directory
-.IR /proc/sys/net/ipv4/ .
-.TP
-.IR 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.
-.RS
-.TP 10
-.I 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.
-.TP
-.I pressure
-This value was introduced to follow the format of
-.IR tcp_mem
-(see
-.BR tcp (7)).
-.TP
-.I max
-Number of pages allowed for queueing by all UDP sockets.
-.RE
-.IP
-Defaults values for these three items are
-calculated at boot time from the amount of available memory.
-.TP
-.IR 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
-.I udp_mem
-pressure.
-.TP
-.IR 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
-.I udp_mem
-pressure.
-.SS Socket options
-To set or get a UDP socket option, call
-.BR getsockopt (2)
-to read or
-.BR setsockopt (2)
-to write the option with the option level argument set to
-.BR IPPROTO_UDP .
-Unless otherwise noted,
-.I optval
-is a pointer to an
-.IR int .
-.TP
-.BR 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.
-.\" FIXME document UDP_ENCAP (new in kernel 2.5.67)
-.\" From include/linux/udp.h:
-.\" /* UDP encapsulation types */
-.\" #define UDP_ENCAP_ESPINUDP_NON_IKE 1 /* draft-ietf-ipsec-nat-t-ike-00/01 */
-.\" #define UDP_ENCAP_ESPINUDP 2 /* draft-ietf-ipsec-udp-encaps-06 */
-.\" #define UDP_ENCAP_L2TPINUDP 3 /* rfc2661 */
-.SS Ioctls
-These ioctls can be accessed using
-.BR ioctl (2).
-The correct syntax is:
-.PP
-.RS
-.nf
-.BI int " value";
-.IB error " = ioctl(" udp_socket ", " ioctl_type ", &" value ");"
-.fi
-.RE
-.TP
-.BR 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.
-.B Warning:
-Using
-.BR 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
-.BR select (2),
-.BR poll (2),
-or
-.BR epoll (7)
-to distinguish these cases.
-.\" See http://www.securiteam.com/unixfocus/5KP0I15IKO.html
-.\" "GNUnet DoS (UDP Socket Unreachable)", 14 May 2006
-.TP
-.BR TIOCOUTQ " (" SIOCOUTQ )
-Returns the number of data bytes in the local send queue.
-Only supported with Linux 2.4 and above.
-.PP
-In addition, all ioctls documented in
-.BR ip (7)
-and
-.BR socket (7)
-are supported.
-.SH ERRORS
-All errors documented for
-.BR socket (7)
-or
-.BR ip (7)
-may be returned by a send or receive on a UDP socket.
-.TP
-.B ECONNREFUSED
-No receiver was associated with the destination address.
-This might be caused by a previous packet sent over the socket.
-.SH VERSIONS
-.B IP_RECVERR
-is a new feature in Linux 2.2.
-.\" .SH CREDITS
-.\" This man page was written by Andi Kleen.
-.SH SEE ALSO
-.BR ip (7),
-.BR raw (7),
-.BR socket (7),
-.BR udplite (7)
-
-RFC\ 768 for the User Datagram Protocol.
-.br
-RFC\ 1122 for the host requirements.
-.br
-RFC\ 1191 for a description of path MTU discovery.
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I man-pages
-project.
-A description of the project,
-information about reporting bugs,
-and the latest version of this page,
-can be found at
-\%http://www.kernel.org/doc/man\-pages/.
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