UDP(7P)UDP(7P)NAMEUDP - Internet User Datagram Protocol
SYNOPSISDESCRIPTIONUDP is a simple, unreliable datagram protocol used to support the
socket type for the internet protocol family. UDP sockets are connec‐
tionless, and are normally used with the and calls (see send(2) and
recv(2). The call can also be used to simulate a connection (see con‐
nect(2). When used in this manner, it fixes the destination for future
transmitted packets (in which case the or system calls can be used), as
well as designating the source from which packets are received. The
and calls can be used at any time if the source of the message is unim‐
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
The default send buffer size for UDP sockets is 65535 bytes. The
default receive buffer size for UDP sockets is 2147483647 bytes. The
send and receive buffer sizes for UDP sockets can be set by using the
and options of the system call or the and options of the system call.
The maximum size for these buffers is 2147483647 bytes. The maximum
receive buffer size may be lowered using the parameter
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 mes‐
sages 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.
One of the following errors may be returned in if a socket operation
fails. For a more detailed list of errors, see the man pages for spe‐
cific system calls.
[EISCONN] Attempt to send a datagram with the desti‐
nation address specified, when the socket
is already connected.
[ENOBUFS] No buffer space is available for an inter‐
nal 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 interface
The socket interfaces to UDP were developed by the University of Cali‐
SEE ALSOndd(1M). getsockopt(2), recv(2), send(2), socket(2), t_open(3), t_opt‐
RFC 768 User Datagram Protocol
RFC 1122 Requirements for Internet hosts