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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 desti-
       nation 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 appli-
       cation 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 pack-
       ets  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 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 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  accumu-
              lated  into  a  single  datagram that is transmitted when the option is dis-
              abled.  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.

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



Linux                             2008-11-21                            UDP(7)

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