udp(7P) 							     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);

       s = socket(AF_INET6, SOCK_DGRAM, 0);

       t = t_open("/dev/udp", O_RDWR);

       t = t_open("/dev/udp6", O_RDWR);

DESCRIPTION

       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 destination 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 unspeci-
       fied 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  appli-
       cation  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)):

       UDP_NAT_T_ENDPOINT

	   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.
       Arriving datagrams which will not fit within its high-water capacity are silently discarded.

       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 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 pack-
       ets 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" notifications are similarly passed upstream. All other ICMP6 messages
       are ignored.

SEE ALSO

       read(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 Center, 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,
       2005.

DIAGNOSTICS

       A socket operation may fail if:

       EISCONN		A connect() operation was attempted on a socket on which a connect() operation had already been performed, and the  socket
			could not be successfully disconnected before making the new connection.

       EISCONN		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.

       ENOTCONN 	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 performed.

       EADDRINUSE	A  bind()  operation  was  attempted  on  a socket with a network address/port pair that has already been bound to another
			socket.

       EADDRNOTAVAIL	A bind() operation was attempted on a socket with a network address for which no network interface exists.

       EINVAL		A sendmsg() operation with a non-NULL msg_accrights was attempted.

       EACCES		A bind() operation was attempted with a "reserved" port number and the effective user ID of the process was not the privi-
			leged user.

       ENOBUFS		The system ran out of memory for internal data structures.

SunOS 5.11							    4 July 2006 							   udp(7P)