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UDP(7P) UDP(7P)

NAME

UDP - Internet User Datagram Protocol

SYNOPSIS

#include <sys/types.h>

#include <sys/socket.h>

#include <netinet/in.h>

s = socket(AF_INET, SOCK_DGRAM, 0);

s = socket(AF_INET6, SOCK_DGRAM, 0);

DESCRIPTION

UDP is a simple, unreliable datagram protocol used to support the

SOCK_DGRAM socket type for the inter-

net protocol family. UDP sockets are connectionless, and are normally used with the

sendto() and

recvfrom() calls (see send(2) and recv(2). The

connect() call can also be used to simulate a connec-

tion (see connect(2). When used in this manner, it ﬁxes the destination for future transmitted packets (in

which case the

send() or write() system calls can be used), as well as designating the source from

which packets are received. The recv() and

read() calls can be used at any time if the source of the

message is unimportant.

UDP address formats are identical to those used by TCP. In particular, UDP requires a port identiﬁer in

addition to the normal Internet address format. Note that the UDP port domain is separate from the TCP

port domain (in other words, a UDP port cannot be connected to a TCP port).

The default send buffer size for UDP sockets is 65535 bytes. The default receive buffer size for UDP sock-

ets is 2147483647 bytes. The send and receive buffer sizes for UDP sockets can be set by using the

SO_SNDBUF and SO_RCVBUF options of the setsockopt() system call or the XTI_SNDBUF and

XTI_RCVBUF options of the t_optmgmt() system call. The maximum size for these buffers is

2147483647 bytes. The maximum receive buffer size may be lowered using the ndd parameter

udp_recv_hiwater_max

The maximum message size for a UDP datagram socket is limited by the lesser of the maximum size of an

IP datagram and the size of the UDP datagram socket buffer. The maximum size of an IP datagram limits

the maximum message size of a UDP message to 65507 bytes. Therefore, using the maximum socket buffer

size will allow multiple maximum-sized messages to be placed on the send queue. The default inbound and

outbound message size limit for a UDP datagram socket is 65535 bytes.

The maximum message size for a UDP broadcast is limited by the MTU size of the underlying link.

ERRORS

One of the following errors may be returned in

errno if a socket operation fails. For a more detailed list

of errors, see the man pages for speciﬁc system calls.

[EISCONN] Attempt to send a datagram with the destination address speciﬁed, when the

socket is already connected.

[ENOBUFS] No buffer space is available for an internal data structure.

[EADDRINUSE] Attempt to create a socket with a port which has already been allocated.

[EADDRNOTAVAIL] Attempt to create a socket with a network address for which no network inter-

face exists.

AUTHOR

The socket interfaces to UDP were developed by the University of California, Berkeley.