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udp - User Datagram Protocol for IPv4
!!SYNOPSIS
#include <sys/socket.h>
#include <netinet/in.h>

udp_socket = socket(PF_INET, SOCK_DGRAM, 0);

!!DESCRIPTION
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 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 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 that interface.

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 ip(7).

!!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 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 RFC:1122 .

For compatibility with legacy code it is 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__). 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.

!!IOCTLS


These ioctls can be accessed using ioctl(2). The
correct syntax is:


 __int__ ''value''__;
 __''error'' __= ioctl(__''tcp_socket''__,__ ''ioctl_type''__, __''value''__);__


;__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.

*** NOTE: 2003/07/03 SIOCINQ is undefined in Linux 2.4.20 / glibc 2.3.1 (Kit Peters) ***

;__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

!!CREDITS
This man page was written by Andi Kleen.

!!SEE ALSO
ip(7), socket(7), raw(7), [UDP], tcp(7)


RFC:768 for the User Datagram protocol. RFC:1122 for the host requirements. RFC:1191 for a description of path [MTU]
discovery.
8 pages link to udp(7):
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Last edited on Monday, February 10, 2003 10:16:41 pm by PerryLorier
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