udp - User Datagram Protocol for IPv4
udp_socket = socket(PF_INET, SOCK_DGRAM, 0);
This is an implemention of the User Datagram Protocol described in
RFC768. 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 specifying
an 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 an 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
sysctl 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 an local interface address and the packet is only sent to
UDP fragments a packet when its total length exceeds the interface MTU
(Maximum Transmission Unit). A more network friendly alternative is to
use path MTU discovery as described in the IP_PMTU_DISCOVER section of
UDP uses the IPv4 sockaddr_in address format described in ip(7).
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 behaviour differs from many
other BSD socket implementations which don't pass any errors unless the
socket is connected. Linux's behaviour is mandated by RFC1122.
For compatibility with legacy code it is possible to set the SO_BSDCOM-
PAT SOL_SOCKET option to receive remote errors only when the socket has
been connected (except for EPROTO and EMSGSIZE). It is better to fix
the code to handle errors properly than to enable this option. Locally
generated errors are always passed.
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.
These ioctls can be accessed using ioctl(2). The correct syntax is:
error = ioctl(tcp_socket, ioctl_type, &value);
SIOCINQ [Toc] [Back]
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.
SIOCOUTQ [Toc] [Back]
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.
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.
IP_RECVERR is a new feature in Linux 2.2
This man page was written by Andi Kleen.
ip(7), socket(7), raw(7)
RFC768 for the User Datagram protocol.
RFC1122 for the host requirements.
RFC1191 for a description of path MTU discovery.
Linux Man Page 1998-10-02 UDP(7)
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