doc/api/dgram.markdown - external/github.com/v8/node - Git at Google

 # UDP / Datagram Sockets

     Stability: 2 - Stable

 <!-- name=dgram -->

 The `dgram` module provides an implementation of UDP Datagram sockets.

     const dgram = require('dgram');
     const server = dgram.createSocket('udp4');

     server.on('error', (err) => {
       console.log(`server error:\n${err.stack}`);
       server.close();
     });

     server.on('message', (msg, rinfo) => {
       console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
     });

     server.on('listening', () => {
       var address = server.address();
       console.log(`server listening ${address.address}:${address.port}`);
     });

     server.bind(41234);
     // server listening 0.0.0.0:41234

 ## Class: dgram.Socket

 The `dgram.Socket` object is an [`EventEmitter`][] that encapsulates the
 datagram functionality.

 New instances of `dgram.Socket` are created using [`dgram.createSocket()`][].
 The `new` keyword is not to be used to create `dgram.Socket` instances.

 ### Event: 'close'

 The `'close'` event is emitted after a socket is closed with [`close()`][].
 Once triggered, no new `'message'` events will be emitted on this socket.

 ### Event: 'error'

 * `exception` Error object

 The `'error'` event is emitted whenever any error occurs. The event handler
 function is passed a single Error object.

 ### Event: 'listening'

 The `'listening'` event is emitted whenever a socket begins listening for
 datagram messages. This occurs as soon as UDP sockets are created.

 ### Event: 'message'

 * `msg` Buffer object. The message
 * `rinfo` Object. Remote address information

 The `'message'` event is emitted when a new datagram is available on a socket.
 The event handler function is passed two arguments: `msg` and `rinfo`. The
 `msg` argument is a [`Buffer`][] and `rinfo` is an object with the sender's
 address information provided by the `address`, `family` and `port` properties:

     socket.on('message', (msg, rinfo) => {
       console.log('Received %d bytes from %s:%d\n',
                   msg.length, rinfo.address, rinfo.port);
     });

 ### socket.addMembership(multicastAddress[, multicastInterface])

 * `multicastAddress` String
 * `multicastInterface` String, Optional

 Tells the kernel to join a multicast group at the given `multicastAddress`
 using the `IP_ADD_MEMBERSHIP` socket option. If the `multicastInterface`
 argument is not specified, the operating system will try to add membership to
 all valid networking interfaces.

 ### socket.address()

 Returns an object containing the address information for a socket.
 For UDP sockets, this object will contain `address`, `family` and `port`
 properties.

 ### [socket.bind([port][, address][, callback])]

 * `port` Integer, Optional
 * `address` String, Optional
 * `callback` Function with no parameters, Optional. Called when
   binding is complete.

 For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
 named `port` and optional `address`. If `port` is not specified, the operating
 system will attempt to bind to a random port. If `address` is not specified,
 the operating system will attempt to listen on all addresses.  Once binding is
 complete, a `'listening'` event is emitted and the optional `callback` function
 is called.

 Note that specifying both a `'listening'` event listener and passing a
 `callback` to the `socket.bind()` method is not harmful but not very
 useful.

 A bound datagram socket keeps the Node.js process running to receive
 datagram messages.

 If binding fails, an `'error'` event is generated. In rare case (e.g.
 attempting to bind with a closed socket), an [`Error`][] may be thrown.

 Example of a UDP server listening on port 41234:

     const dgram = require('dgram');
     const server = dgram.createSocket('udp4');

     server.on('error', (err) => {
       console.log(`server error:\n${err.stack}`);
       server.close();
     });

     server.on('message', (msg, rinfo) => {
       console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
     });

     server.on('listening', () => {
       var address = server.address();
       console.log(`server listening ${address.address}:${address.port}`);
     });

     server.bind(41234);
     // server listening 0.0.0.0:41234

 ### socket.bind(options[, callback])

 * `options` {Object} - Required. Supports the following properties:
   * `port` {Number} - Required.
   * `address` {String} - Optional.
   * `exclusive` {Boolean} - Optional.
 * `callback` {Function} - Optional.

 For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
 named `port` and optional `address` that are passed as properties of an
 `options` object passed as the first argument. If `port` is not specified, the
 operating system will attempt to bind to a random port. If `address` is not
 specified, the operating system will attempt to listen on all addresses.  Once
 binding is complete, a `'listening'` event is emitted and the optional
 `callback` function is called.

 The `options` object may contain an additional `exclusive` property that is
 use when using `dgram.Socket` objects with the [`cluster`] module. When
 `exclusive` is set to `false` (the default), cluster workers will use the same
 underlying socket handle allowing connection handling duties to be shared.
 When `exclusive` is `true`, however, the handle is not shared and attempted
 port sharing results in an error.

 An example socket listening on an exclusive port is shown below.

     socket.bind({
       address: 'localhost',
       port: 8000,
       exclusive: true
     });

 ### socket.close([callback])

 Close the underlying socket and stop listening for data on it. If a callback is
 provided, it is added as a listener for the [`'close'`][] event.

 ### socket.dropMembership(multicastAddress[, multicastInterface])

 * `multicastAddress` String
 * `multicastInterface` String, Optional

 Instructs the kernel to leave a multicast group at `multicastAddress` using the
 `IP_DROP_MEMBERSHIP` socket option. This method is automatically called by the
 kernel when the socket is closed or the process terminates, so most apps will
 never have reason to call this.

 If `multicastInterface` is not specified, the operating system will attempt to
 drop membership on all valid interfaces.

 ### socket.send(buf, offset, length, port, address[, callback])

 * `buf` Buffer object or string.  Message to be sent
 * `offset` Integer. Offset in the buffer where the message starts.
 * `length` Integer. Number of bytes in the message.
 * `port` Integer. Destination port.
 * `address` String. Destination hostname or IP address.
 * `callback` Function. Called when the message has been sent. Optional.

 Broadcasts a datagram on the socket. The destination `port` and `address` must
 be specified.

 The `buf` argument is a [`Buffer`] object containing the message. The `offset`
 and `length` specify the offset within the `Buffer` where the message begins
 and the number of bytes in the message, respectively. With messages that
 contain  multi-byte characters, `offset` and `length` will be calculated with
 respect to [byte length][] and not the character position.

 The `address` argument is a string. If the value of `address` is a host name,
 DNS will be used to resolve the address of the host. If the `address` is not
 specified or is an empty string, `'0.0.0.0'` or `'::0'` will be used instead.
 It is possible, depending on the network configuration, that these defaults
 may not work; accordingly, it is best to be explicit about the destination
 address.

 If the socket has not been previously bound with a call to `bind`, the socket
 is assigned a random port number and is bound to the "all interfaces" address
 (`'0.0.0.0'` for `udp4` sockets, `'::0'` for `udp6` sockets.)

 An optional `callback` function  may be specified to as a way of reporting
 DNS errors or for determining when it is safe to reuse the `buf` object.
 Note that DNS lookups delay the time to send for at least one tick of the
 Node.js event loop.

 The only way to know for sure that the datagram has been sent is by using a
 `callback`. If an error occurs and a `callback` is given, the error will be
 passed as the first argument to the `callback`. If a `callback` is not given,
 the error is emitted as an `'error'` event on the `socket` object.

 Example of sending a UDP packet to a random port on `localhost`;

     const dgram = require('dgram');
     const message = new Buffer('Some bytes');
     const client = dgram.createSocket('udp4');
     client.send(message, 0, message.length, 41234, 'localhost', (err) => {
       client.close();
     });

 **A Note about UDP datagram size**

 The maximum size of an `IPv4/v6` datagram depends on the `MTU`
 (_Maximum Transmission Unit_) and on the `Payload Length` field size.

 - The `Payload Length` field is `16 bits` wide, which means that a normal
   payload exceed 64K octets _including_ the internet header and data
   (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header);
   this is generally true for loopback interfaces, but such long datagram
   messages are impractical for most hosts and networks.

 - The `MTU` is the largest size a given link layer technology can support for
   datagram messages. For any link, `IPv4` mandates a minimum `MTU` of `68`
   octets, while the recommended `MTU` for IPv4 is `576` (typically recommended
   as the `MTU` for dial-up type applications), whether they arrive whole or in
   fragments.

   For `IPv6`, the minimum `MTU` is `1280` octets, however, the mandatory minimum
   fragment reassembly buffer size is `1500` octets. The value of `68` octets is
   very small, since most current link layer technologies, like Ethernet, have a
   minimum `MTU` of `1500`.

 It is impossible to know in advance the MTU of each link through which
 a packet might travel. Sending a datagram greater than the receiver `MTU` will
 not work because the packet will get silently dropped without informing the
 source that the data did not reach its intended recipient.

 ### socket.setBroadcast(flag)

 * `flag` Boolean

 Sets or clears the `SO_BROADCAST` socket option.  When set to `true`, UDP
 packets may be sent to a local interface's broadcast address.

 ### socket.setMulticastLoopback(flag)

 * `flag` Boolean

 Sets or clears the `IP_MULTICAST_LOOP` socket option.  When set to `true`,
 multicast packets will also be received on the local interface.

 ### socket.setMulticastTTL(ttl)

 * `ttl` Integer

 Sets the `IP_MULTICAST_TTL` socket option.  While TTL generally stands for
 "Time to Live", in this context it specifies the number of IP hops that a
 packet is allowed to travel through, specifically for multicast traffic.  Each
 router or gateway that forwards a packet decrements the TTL. If the TTL is
 decremented to 0 by a router, it will not be forwarded.

 The argument passed to to `socket.setMulticastTTL()` is a number of hops
 between 0 and 255. The default on most systems is `1` but can vary.

 ### socket.setTTL(ttl)

 * `ttl` Integer

 Sets the `IP_TTL` socket option. While TTL generally stands for "Time to Live",
 in this context it specifies the number of IP hops that a packet is allowed to
 travel through.  Each router or gateway that forwards a packet decrements the
 TTL.  If the TTL is decremented to 0 by a router, it will not be forwarded.
 Changing TTL values is typically done for network probes or when multicasting.

 The argument to `socket.setTTL()` is a number of hops between 1 and 255.
 The default on most systems is 64 but can vary.

 ### socket.ref()

 By default, binding a socket will cause it to block the Node.js process from
 exiting as long as the socket is open. The `socket.unref()` method can be used
 to exclude the socket from the reference counting that keeps the Node.js
 process active. The `socket.ref()` method adds the socket back to the reference
 counting and restores the default behavior.

 Calling `socket.ref()` multiples times will have no additional effect.

 The `socket.ref()` method returns a reference to the socket so calls can be
 chained.

 ### socket.unref()

 By default, binding a socket will cause it to block the Node.js process from
 exiting as long as the socket is open. The `socket.unref()` method can be used
 to exclude the socket from the reference counting that keeps the Node.js
 process active, allowing the process to exit even if the socket is still
 listening.

 Calling `socket.unref()` multiple times will have no addition effect.

 The `socket.unref()` method returns a reference to the socket so calls can be
 chained.

 ### Change to asynchronous `socket.bind()` behavior

 As of Node.js v0.10, [`dgram.Socket#bind()`][] changed to an asynchronous
 execution model. Legacy code that assumes synchronous behavior, as in the
 following example:

     const s = dgram.createSocket('udp4');
     s.bind(1234);
     s.addMembership('224.0.0.114');

 Must be changed to pass a callback function to the [`dgram.Socket#bind()`][]
 function:

     const s = dgram.createSocket('udp4');
     s.bind(1234, () => {
       s.addMembership('224.0.0.114');
     });

 ## `dgram` module functions

 ### dgram.createSocket(options[, callback])
 * `options` Object
 * `callback` Function. Attached as a listener to `'message'` events.
 * Returns: Socket object

 Creates a `dgram.Socket` object. The `options` argument is an object that
 should contain a `type` field of either `udp4` or `udp6` and an optional
 boolean `reuseAddr` field.

 When `reuseAddr` is `true` [`socket.bind()`][] will reuse the address, even if
 another process has already bound a socket on it. `reuseAddr` defaults to
 `false`. An optional `callback` function can be passed specified which is added
 as a listener for `'message'` events.

 Once the socket is created, calling [`socket.bind()`][] will instruct the
 socket to begin listening for datagram messages. When `address` and `port` are
 not passed to  [`socket.bind()`][] the method will bind the socket to the "all
 interfaces" address on a random port (it does the right thing for both `udp4`
 and `udp6` sockets). The bound address and port can be retrieved using
 [`socket.address().address`][] and [`socket.address().port`][].

 ## dgram.createSocket(type[, callback])

 * `type` String. Either 'udp4' or 'udp6'
 * `callback` Function. Attached as a listener to `'message'` events.
   Optional
 * Returns: Socket object

 Creates a `dgram.Socket` object of the specified `type`. The `type` argument
 can be either `udp4` or `udp6`. An optional `callback` function can be passed
 which is added as a listener for `'message'` events.

 Once the socket is created, calling [`socket.bind()`][] will instruct the
 socket to begin listening for datagram messages. When `address` and `port` are
 not passed to  [`socket.bind()`][] the method will bind the socket to the "all
 interfaces" address on a random port (it does the right thing for both `udp4`
 and `udp6` sockets). The bound address and port can be retrieved using
 [`socket.address().address`][] and [`socket.address().port`][].

 [`EventEmitter`]: events.html
 [`Buffer`]: buffer.html
 [`'close'`]: #dgram_event_close
 [`addMembership()`]: #dgram_socket_addmembership_multicastaddress_multicastinterface
 [`close()`]: #dgram_socket_close_callback
 [`dgram.createSocket()`]: #dgram_dgram_createsocket_options_callback
 [`dgram.Socket#bind()`]: #dgram_socket_bind_options_callback
 [`Error`]: errors.html#errors_class_error
 [`socket.address().address`]: #dgram_socket_address
 [`socket.address().port`]: #dgram_socket_address
 [`socket.bind()`]: #dgram_socket_bind_port_address_callback
 [byte length]: buffer.html#buffer_class_method_buffer_bytelength_string_encoding
	# UDP / Datagram Sockets

	Stability: 2 - Stable

	<!-- name=dgram -->

	The `dgram` module provides an implementation of UDP Datagram sockets.

	const dgram = require('dgram');
	const server = dgram.createSocket('udp4');

	server.on('error', (err) => {
	console.log(`server error:\n${err.stack}`);
	server.close();
	});

	server.on('message', (msg, rinfo) => {
	console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
	});

	server.on('listening', () => {
	var address = server.address();
	console.log(`server listening ${address.address}:${address.port}`);
	});

	server.bind(41234);
	// server listening 0.0.0.0:41234

	## Class: dgram.Socket

	The `dgram.Socket` object is an [`EventEmitter`][] that encapsulates the
	datagram functionality.

	New instances of `dgram.Socket` are created using [`dgram.createSocket()`][].
	The `new` keyword is not to be used to create `dgram.Socket` instances.

	### Event: 'close'

	The `'close'` event is emitted after a socket is closed with [`close()`][].
	Once triggered, no new `'message'` events will be emitted on this socket.

	### Event: 'error'

	* `exception` Error object

	The `'error'` event is emitted whenever any error occurs. The event handler
	function is passed a single Error object.

	### Event: 'listening'

	The `'listening'` event is emitted whenever a socket begins listening for
	datagram messages. This occurs as soon as UDP sockets are created.

	### Event: 'message'

	* `msg` Buffer object. The message
	* `rinfo` Object. Remote address information

	The `'message'` event is emitted when a new datagram is available on a socket.
	The event handler function is passed two arguments: `msg` and `rinfo`. The
	`msg` argument is a [`Buffer`][] and `rinfo` is an object with the sender's
	address information provided by the `address`, `family` and `port` properties:

	socket.on('message', (msg, rinfo) => {
	console.log('Received %d bytes from %s:%d\n',
	msg.length, rinfo.address, rinfo.port);
	});

	### socket.addMembership(multicastAddress[, multicastInterface])

	* `multicastAddress` String
	* `multicastInterface` String, Optional

	Tells the kernel to join a multicast group at the given `multicastAddress`
	using the `IP_ADD_MEMBERSHIP` socket option. If the `multicastInterface`
	argument is not specified, the operating system will try to add membership to
	all valid networking interfaces.

	### socket.address()

	Returns an object containing the address information for a socket.
	For UDP sockets, this object will contain `address`, `family` and `port`
	properties.

	### [socket.bind([port][, address][, callback])]

	* `port` Integer, Optional
	* `address` String, Optional
	* `callback` Function with no parameters, Optional. Called when
	binding is complete.

	For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
	named `port` and optional `address`. If `port` is not specified, the operating
	system will attempt to bind to a random port. If `address` is not specified,
	the operating system will attempt to listen on all addresses. Once binding is
	complete, a `'listening'` event is emitted and the optional `callback` function
	is called.

	Note that specifying both a `'listening'` event listener and passing a
	`callback` to the `socket.bind()` method is not harmful but not very
	useful.

	A bound datagram socket keeps the Node.js process running to receive
	datagram messages.

	If binding fails, an `'error'` event is generated. In rare case (e.g.
	attempting to bind with a closed socket), an [`Error`][] may be thrown.

	Example of a UDP server listening on port 41234:

	const dgram = require('dgram');
	const server = dgram.createSocket('udp4');

	server.on('error', (err) => {
	console.log(`server error:\n${err.stack}`);
	server.close();
	});

	server.on('message', (msg, rinfo) => {
	console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
	});

	server.on('listening', () => {
	var address = server.address();
	console.log(`server listening ${address.address}:${address.port}`);
	});

	server.bind(41234);
	// server listening 0.0.0.0:41234

	### socket.bind(options[, callback])

	* `options` {Object} - Required. Supports the following properties:
	* `port` {Number} - Required.
	* `address` {String} - Optional.
	* `exclusive` {Boolean} - Optional.
	* `callback` {Function} - Optional.

	For UDP sockets, causes the `dgram.Socket` to listen for datagram messages on a
	named `port` and optional `address` that are passed as properties of an
	`options` object passed as the first argument. If `port` is not specified, the
	operating system will attempt to bind to a random port. If `address` is not
	specified, the operating system will attempt to listen on all addresses. Once
	binding is complete, a `'listening'` event is emitted and the optional
	`callback` function is called.

	The `options` object may contain an additional `exclusive` property that is
	use when using `dgram.Socket` objects with the [`cluster`] module. When
	`exclusive` is set to `false` (the default), cluster workers will use the same
	underlying socket handle allowing connection handling duties to be shared.
	When `exclusive` is `true`, however, the handle is not shared and attempted
	port sharing results in an error.

	An example socket listening on an exclusive port is shown below.

	socket.bind({
	address: 'localhost',
	port: 8000,
	exclusive: true
	});

	### socket.close([callback])

	Close the underlying socket and stop listening for data on it. If a callback is
	provided, it is added as a listener for the [`'close'`][] event.

	### socket.dropMembership(multicastAddress[, multicastInterface])

	* `multicastAddress` String
	* `multicastInterface` String, Optional

	Instructs the kernel to leave a multicast group at `multicastAddress` using the
	`IP_DROP_MEMBERSHIP` socket option. This method is automatically called by the
	kernel when the socket is closed or the process terminates, so most apps will
	never have reason to call this.

	If `multicastInterface` is not specified, the operating system will attempt to
	drop membership on all valid interfaces.

	### socket.send(buf, offset, length, port, address[, callback])

	* `buf` Buffer object or string. Message to be sent
	* `offset` Integer. Offset in the buffer where the message starts.
	* `length` Integer. Number of bytes in the message.
	* `port` Integer. Destination port.
	* `address` String. Destination hostname or IP address.
	* `callback` Function. Called when the message has been sent. Optional.

	Broadcasts a datagram on the socket. The destination `port` and `address` must
	be specified.

	The `buf` argument is a [`Buffer`] object containing the message. The `offset`
	and `length` specify the offset within the `Buffer` where the message begins
	and the number of bytes in the message, respectively. With messages that
	contain multi-byte characters, `offset` and `length` will be calculated with
	respect to [byte length][] and not the character position.

	The `address` argument is a string. If the value of `address` is a host name,
	DNS will be used to resolve the address of the host. If the `address` is not
	specified or is an empty string, `'0.0.0.0'` or `'::0'` will be used instead.
	It is possible, depending on the network configuration, that these defaults
	may not work; accordingly, it is best to be explicit about the destination
	address.

	If the socket has not been previously bound with a call to `bind`, the socket
	is assigned a random port number and is bound to the "all interfaces" address
	(`'0.0.0.0'` for `udp4` sockets, `'::0'` for `udp6` sockets.)

	An optional `callback` function may be specified to as a way of reporting
	DNS errors or for determining when it is safe to reuse the `buf` object.
	Note that DNS lookups delay the time to send for at least one tick of the
	Node.js event loop.

	The only way to know for sure that the datagram has been sent is by using a
	`callback`. If an error occurs and a `callback` is given, the error will be
	passed as the first argument to the `callback`. If a `callback` is not given,
	the error is emitted as an `'error'` event on the `socket` object.

	Example of sending a UDP packet to a random port on `localhost`;

	const dgram = require('dgram');
	const message = new Buffer('Some bytes');
	const client = dgram.createSocket('udp4');
	client.send(message, 0, message.length, 41234, 'localhost', (err) => {
	client.close();
	});

	A Note about UDP datagram size

	The maximum size of an `IPv4/v6` datagram depends on the `MTU`
	(_Maximum Transmission Unit_) and on the `Payload Length` field size.

	- The `Payload Length` field is `16 bits` wide, which means that a normal
	payload exceed 64K octets _including_ the internet header and data
	(65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header);
	this is generally true for loopback interfaces, but such long datagram
	messages are impractical for most hosts and networks.

	- The `MTU` is the largest size a given link layer technology can support for
	datagram messages. For any link, `IPv4` mandates a minimum `MTU` of `68`
	octets, while the recommended `MTU` for IPv4 is `576` (typically recommended
	as the `MTU` for dial-up type applications), whether they arrive whole or in
	fragments.

	For `IPv6`, the minimum `MTU` is `1280` octets, however, the mandatory minimum
	fragment reassembly buffer size is `1500` octets. The value of `68` octets is
	very small, since most current link layer technologies, like Ethernet, have a
	minimum `MTU` of `1500`.

	It is impossible to know in advance the MTU of each link through which
	a packet might travel. Sending a datagram greater than the receiver `MTU` will
	not work because the packet will get silently dropped without informing the
	source that the data did not reach its intended recipient.

	### socket.setBroadcast(flag)

	* `flag` Boolean

	Sets or clears the `SO_BROADCAST` socket option. When set to `true`, UDP
	packets may be sent to a local interface's broadcast address.

	### socket.setMulticastLoopback(flag)

	* `flag` Boolean

	Sets or clears the `IP_MULTICAST_LOOP` socket option. When set to `true`,
	multicast packets will also be received on the local interface.

	### socket.setMulticastTTL(ttl)

	* `ttl` Integer

	Sets the `IP_MULTICAST_TTL` socket option. While TTL generally stands for
	"Time to Live", in this context it specifies the number of IP hops that a
	packet is allowed to travel through, specifically for multicast traffic. Each
	router or gateway that forwards a packet decrements the TTL. If the TTL is
	decremented to 0 by a router, it will not be forwarded.

	The argument passed to to `socket.setMulticastTTL()` is a number of hops
	between 0 and 255. The default on most systems is `1` but can vary.

	### socket.setTTL(ttl)

	* `ttl` Integer

	Sets the `IP_TTL` socket option. While TTL generally stands for "Time to Live",
	in this context it specifies the number of IP hops that a packet is allowed to
	travel through. Each router or gateway that forwards a packet decrements the
	TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
	Changing TTL values is typically done for network probes or when multicasting.

	The argument to `socket.setTTL()` is a number of hops between 1 and 255.
	The default on most systems is 64 but can vary.

	### socket.ref()

	By default, binding a socket will cause it to block the Node.js process from
	exiting as long as the socket is open. The `socket.unref()` method can be used
	to exclude the socket from the reference counting that keeps the Node.js
	process active. The `socket.ref()` method adds the socket back to the reference
	counting and restores the default behavior.

	Calling `socket.ref()` multiples times will have no additional effect.

	The `socket.ref()` method returns a reference to the socket so calls can be
	chained.

	### socket.unref()

	By default, binding a socket will cause it to block the Node.js process from
	exiting as long as the socket is open. The `socket.unref()` method can be used
	to exclude the socket from the reference counting that keeps the Node.js
	process active, allowing the process to exit even if the socket is still
	listening.

	Calling `socket.unref()` multiple times will have no addition effect.

	The `socket.unref()` method returns a reference to the socket so calls can be
	chained.

	### Change to asynchronous `socket.bind()` behavior

	As of Node.js v0.10, [`dgram.Socket#bind()`][] changed to an asynchronous
	execution model. Legacy code that assumes synchronous behavior, as in the
	following example:

	const s = dgram.createSocket('udp4');
	s.bind(1234);
	s.addMembership('224.0.0.114');

	Must be changed to pass a callback function to the [`dgram.Socket#bind()`][]
	function:

	const s = dgram.createSocket('udp4');
	s.bind(1234, () => {
	s.addMembership('224.0.0.114');
	});

	## `dgram` module functions

	### dgram.createSocket(options[, callback])
	* `options` Object
	* `callback` Function. Attached as a listener to `'message'` events.
	* Returns: Socket object

	Creates a `dgram.Socket` object. The `options` argument is an object that
	should contain a `type` field of either `udp4` or `udp6` and an optional
	boolean `reuseAddr` field.

	When `reuseAddr` is `true` [`socket.bind()`][] will reuse the address, even if
	another process has already bound a socket on it. `reuseAddr` defaults to
	`false`. An optional `callback` function can be passed specified which is added
	as a listener for `'message'` events.

	Once the socket is created, calling [`socket.bind()`][] will instruct the
	socket to begin listening for datagram messages. When `address` and `port` are
	not passed to [`socket.bind()`][] the method will bind the socket to the "all
	interfaces" address on a random port (it does the right thing for both `udp4`
	and `udp6` sockets). The bound address and port can be retrieved using
	[`socket.address().address`][] and [`socket.address().port`][].

	## dgram.createSocket(type[, callback])

	* `type` String. Either 'udp4' or 'udp6'
	* `callback` Function. Attached as a listener to `'message'` events.
	Optional
	* Returns: Socket object

	Creates a `dgram.Socket` object of the specified `type`. The `type` argument
	can be either `udp4` or `udp6`. An optional `callback` function can be passed
	which is added as a listener for `'message'` events.

	Once the socket is created, calling [`socket.bind()`][] will instruct the
	socket to begin listening for datagram messages. When `address` and `port` are
	not passed to [`socket.bind()`][] the method will bind the socket to the "all
	interfaces" address on a random port (it does the right thing for both `udp4`
	and `udp6` sockets). The bound address and port can be retrieved using
	[`socket.address().address`][] and [`socket.address().port`][].

	[`EventEmitter`]: events.html
	[`Buffer`]: buffer.html
	[`'close'`]: #dgram_event_close
	[`addMembership()`]: #dgram_socket_addmembership_multicastaddress_multicastinterface
	[`close()`]: #dgram_socket_close_callback
	[`dgram.createSocket()`]: #dgram_dgram_createsocket_options_callback
	[`dgram.Socket#bind()`]: #dgram_socket_bind_options_callback
	[`Error`]: errors.html#errors_class_error
	[`socket.address().address`]: #dgram_socket_address
	[`socket.address().port`]: #dgram_socket_address
	[`socket.bind()`]: #dgram_socket_bind_port_address_callback
	[byte length]: buffer.html#buffer_class_method_buffer_bytelength_string_encoding