udp, UDP - Internet User Datagram Protocol
s = socket(AF_INET, SOCK_DGRAM, 0);
s = socket(AF_INET6, SOCK_DGRAM, 0);
t = t_open("/dev/udp", O_RDWR);
t = t_open("/dev/udp6", O_RDWR);
UDP is a simple datagram protocol which is layered directly above the
Internet Protocol ("IP") or the Internet Protocol Version 6 ("IPv6").
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 supports the connectionless (T_CLTS) service type.
Within the socket interface, UDP is normally used with the sendto(),
sendmsg(), recvfrom(), and recvmsg() calls (see send(3SOCKET) and
recv(3SOCKET)). If the connect(3SOCKET) call is used to fix the desti‐
nation for future packets, then the recv(3SOCKET) or read(2) and
send(3SOCKET) 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 IPor IPv6 address to identify the endpoint 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 a TCP port. The bind(3SOCKET) call
can be used to set the local address and port number of a UDP socket.
The local IP or IPv6 address may be left unspecified in the bind() call
by using the special value INADDR_ANY for IP, or the unspecified
address (all zeroes) for IPv6. If the bind() call is not done, a local
IP or IPv6 address and port number will be assigned to the endpoint
when the first packet is sent. Broadcast packets may be sent, assuming
the underlying network supports this, by using a reserved "broadcast
address" This address is network interface dependent. Broadcasts may
only be sent by the privileged user.
Note that no two UDP sockets can be bound to the same port unless the
bound IP addresses are different. IPv4 INADDR_ANY and IPv6 unspecified
addresses compare as equal to any IPv4 or IPv6 address. For example, if
a socket is bound to INADDR_ANY or unspecified address and port X, no
other socket can bind to port X, regardless of the binding address.
This special consideration of INADDR_ANY and unspecified address can be
changed using the SO_REUSEADDR socket option. If SO_REUSEADDR is set
on a socket doing a bind, IPv4 INADDR_ANY and IPv6 unspecified address
do not compare as equal to any IP address. This means that as long as
the two sockets are not both bound to INADDR_ANY/unspecified address or
the same IP address, the two sockets can be bound to the same port.
If an application does not want to allow another socket using the
SO_REUSEADDR option to bind to a port its socket is bound to, the
application can set the socket level option SO_EXCLBIND on a socket.
The option values of 0 and 1 represent enabling and disabling the
option, respectively. Once this option is enabled on a socket, no
other socket can be bound to the same port.
IPv6 does not support broadcast addresses; their function is supported
by IPv6 multicast addresses.
Options at the IP level may be used with UDP. See ip(7P) or ip6(7P).
Additionally, there is one UDP-level option of interest to IPsec Key
Management applications (see ipsec(7P)and pf_key(7P)):
If this boolean option is set, datagrams sent via this socket will
have a non-ESP marker inserted between the UDP header and the data.
Likewise, inbound packets that match the endpoint's local-port will
be demultiplexed between ESP or the endpoint itself if a non-ESP
marker is present. This option is only available on IPv4 sockets
(AF_INET), and the application must have sufficient privilege to
use PF_KEY sockets to also enable this option.
There are a variety of ways that a UDP 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
provided for each UDP socket. This queue has a limited capacity. Arriv‐
ing datagrams which will not fit within its high-water capacity are
UDP processes Internet Control Message Protocol ("ICMP") and Internet
Control Message Protocol Version 6 ("ICMP6") error messages received in
response to UDP packets it has sent. See icmp(7P) and icmp6(7P).
ICMP "source quench" messages are ignored. ICMP "destination unreach‐
able," "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 communication process.
ICMP6 "destination unreachable" packets are ignored unless the enclosed
code indicates that the port is not in use on the target host, in which
case, the application is notified. ICMP6 "parameter problem" notifica‐
tions are similarly passed upstream. All other ICMP6 messages are
SEE ALSOread(2), write(2), bind(3SOCKET), connect(3SOCKET), recv(3SOCKET),
send(3SOCKET), icmp(7P), icmp6(7P), inet(7P), inet6(7P), ip(7P),
ipsec(7P), ip6(7P), pf_key(7P), tcp(7P)
Postel, Jon, RFC 768, User Datagram Protocol, Network Information Cen‐
ter, SRI International, Menlo Park, Calif., August 1980
Huttunen, A., Swander, B., Volpe, V., DiBurro, L., Stenberg, M., RFC
3948, UDP Encapsulation of IPsec ESP Packets, The Internet Society,
A socket operation may fail if:
A connect() operation was attempted on a socket on
which a connect() operation had already been per‐
formed, and the socket could not be successfully dis‐
connected before making the new connection.
A sendto() or sendmsg() operation specifying an
address to which the message should be sent was
attempted on a socket on which a connect() operation
had already been performed.
A send() or write() operation, or a sendto() or
sendmsg() operation not specifying an address to which
the message should be sent, was attempted on a socket
on which a connect() operation had not already been
A bind() operation was attempted on a socket with a
network address/port pair that has already been bound
to another socket.
A bind() operation was attempted on a socket with a
network address for which no network interface exists.
A sendmsg() operation with a non-NULL msg_accrights
A bind() operation was attempted with a "reserved"
port number and the effective user ID of the process
was not the privileged user.
The system ran out of memory for internal data struc‐
Jul 4, 2006 UDP(7P)