Technical data

SunOS 5.5 Protocols udp(7P)
NAME udp, UDP − Internet User Datagram Protocol
SYNOPSIS #include <sys/socket.h>
#include <netinet/in.h>
s = socket(AF_INET, SOCK_DGRAM, 0);
t = t_open("/dev/udp", O_RDWR);
DESCRIPTION UDP is a simple datagram protocol which is layered directly above the Internet Protocol
(IP). Programs may access UDP using the socket interface, where it supports the
SOCK_DGRAM socket type, or using the Transport Level Interface (TLI), where it sup-
ports the connectionless (T_CLTS) service type.
Within the socket interface, UDP is normally used with the sendto(), sendmsg(),
recvfrom(), and recvmsg() calls (see send(3N) and recv(3N)). If the connect(3N) call is
used to fix the destinationfor future packets, then the recv(3N) or read(2) and send(3N)
or write(2) calls may be used.
UDP address formats are identical to those used by the Transmission Control Protocol
(TCP). Like TCP,UDP uses a port number along with an IP address to identify the end-
point of communication. The UDP port number space is separate from the TCP port
number space (that is, a UDP port may not be “connected” to aTCP port). The bind(3N)
call can be used to set the local address and port number of a UDP socket. The local IP
address may be left unspecified in the bind() call by using the special value
INADDR_ANY. If the bind() call is not done, a local IP address and port number will be
assigned to the endpoint when the first packet is sent. Broadcast packets may be sent
(assuming the underlyingnetwork supports this) by usinga reserved “broadcast
address”; This address is network interface dependent. Broadcasts may only be sent by
the privileged user.
Options at theIP level may be used with UDP; see ip(7P).
There are a variety of ways that aUDP packet can be lost or corrupted, including a failure
of the underlying communication mechanism. UDP implements a checksum over the
data portion of the packet. If the checksum of a received packet is in error, the packet
will be dropped with no indication given to the user. A queue of received packets is pro-
vided for each UDP socket. This queue has a limited capacity. Arriving datagrams which
will not fit within its high-water capacity are silently discarded.
UDP processes Internet Control Message Protocol (ICMP) error messages received in
response to UDP packets it has sent. See icmp(7P). ICMP “source quench” messages are
ignored. ICMP “destination unreachable,” “time exceeded” and “parameter problem”
messages disconnect the socket from its peer so that subsequent attempts to send packets
using that socket will return an error. UDP will not guarantee that packets are delivered
in the order they were sent. As well, duplicate packets may be generated in the commun-
ication process.
modified 3 Jul 1990 7P-391