noble (7) udp.7.gz

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NAME

       udp - User Datagram Protocol for IPv4

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/udp.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 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  processed  by
       the  kernel  only 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 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 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 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.  Unless otherwise noted, optval is a pointer to an int.

       Following is a list of UDP-specific socket options.  For details of some other socket  options  that  are
       also applicable for UDP sockets, see socket(7).

       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.

       UDP_SEGMENT (since Linux 4.18)
              Enables  UDP  segmentation  offload.   Segmentation  offload  reduces send(2) cost by transferring
              multiple datagrams worth of data as a single large packet through the kernel transmit  path,  even
              when  that  exceeds  MTU.   As  late as possible, the large packet is split by segment size into a
              series of datagrams.  This segmentation offload step is deferred to hardware  if  supported,  else
              performed  in  software.   This  option  takes  a  value in the range [0, USHRT_MAX] that sets the
              segment size: the size of datagram payload, excluding the UDP header.  The segment  size  must  be
              chosen  such  that  at  most  64  datagrams are sent in a single call and that the datagrams after
              segmentation meet  the  same  MTU  rules  that  apply  to  datagrams  sent  without  this  option.
              Segmentation  offload  depends  on  checksum  offload,  as  datagram  checksums are computed after
              segmentation.  The option may also be set for individual sendmsg(2)  calls  by  passing  it  as  a
              cmsg(3).   A  value of zero disables the feature.  This option should not be used in code intended
              to be portable.

       UDP_GRO (since Linux 5.0)
              Enables UDP receive offload.  If enabled, the socket may receive multiple datagrams worth of  data
              as a single large buffer, together with a cmsg(3) that holds the segment size.  This option is the
              inverse of segmentation offload.  It reduces receive cost by handling multiple datagrams worth  of
              data as a single large packet in the kernel receive path, even when that exceeds MTU.  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.   Supported  only  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)

       The kernel source file Documentation/networking/ip-sysctl.txt.

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