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## UDP / Datagram Sockets
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Datagram sockets are available through `require('dgram')`.
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### Event: 'message'
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`function (msg, rinfo) { }`
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Emitted when a new datagram is available on a socket. `msg` is a `Buffer` and `rinfo` is
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an object with the sender's address information and the number of bytes in the datagram.
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### Event: 'listening'
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`function () { }`
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Emitted when a socket starts listening for datagrams. This happens as soon as UDP sockets
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are created.
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### Event: 'close'
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`function () { }`
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Emitted when a socket is closed with `close()`. No new `message` events will be emitted
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on this socket.
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### Event: 'error'
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`function (exception) {}`
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Emitted when an error occurs.
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---
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### dgram.createSocket(type, [callback])
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Creates a datagram socket of the specified types. Valid types are `udp4`
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and `udp6`.
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Takes an optional callback which is added as a listener for `message` events.
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Call `socket.bind` if you want to receive datagrams. `socket.bind()` will bind
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to the "all interfaces" address on a random port (it does the right thing for
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both `udp4` and `udp6` sockets). You can then retrieve the address and port
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with `socket.address().address` and `socket.address().port`.
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### dgram.send(buf, offset, length, port, address, [callback])
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For UDP sockets, the destination port and IP address must be specified. A string
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may be supplied for the `address` parameter, and it will be resolved with DNS. An
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optional callback may be specified to detect any DNS errors and when `buf` may be
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re-used. Note that DNS lookups will delay the time that a send takes place, at
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least until the next tick. The only way to know for sure that a send has taken place
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is to use the callback.
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If the socket has not been previously bound with a call to `bind`, it's
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assigned a random port number and bound to the "all interfaces" address
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(0.0.0.0 for `udp4` sockets, ::0 for `udp6` sockets).
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Example of sending a UDP packet to a random port on `localhost`;
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var dgram = require('dgram');
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var message = new Buffer("Some bytes");
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var client = dgram.createSocket("udp4");
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client.send(message, 0, message.length, 41234, "localhost", function(err, bytes) {
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client.close();
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});
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**A Note about UDP datagram size**
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The maximum size of an `IPv4/v6` datagram depends on the `MTU` (_Maximum Transmission Unit_)
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and on the `Payload Length` field size.
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- The `Payload Length` field is `16 bits` wide, which means that a normal payload
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cannot be larger than 64K octets including internet header and data
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(65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header);
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this is generally true for loopback interfaces, but such long datagrams
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are impractical for most hosts and networks.
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- The `MTU` is the largest size a given link layer technology can support for datagrams.
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For any link, `IPv4` mandates a minimum `MTU` of `68` octets, while the recommended `MTU`
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for IPv4 is `576` (typically recommended as the `MTU` for dial-up type applications),
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whether they arrive whole or in fragments.
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For `IPv6`, the minimum `MTU` is `1280` octets, however, the mandatory minimum
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fragment reassembly buffer size is `1500` octets.
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The value of `68` octets is very small, since most current link layer technologies have
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a minimum `MTU` of `1500` (like Ethernet).
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Note that it's impossible to know in advance the MTU of each link through which
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a packet might travel, and that generally sending a datagram greater than
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the (receiver) `MTU` won't work (the packet gets silently dropped, without
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informing the source that the data did not reach its intended recipient).
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### dgram.bind(port, [address])
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For UDP sockets, listen for datagrams on a named `port` and optional `address`. If
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`address` is not specified, the OS will try to listen on all addresses.
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Example of a UDP server listening on port 41234:
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var dgram = require("dgram");
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var server = dgram.createSocket("udp4");
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server.on("message", function (msg, rinfo) {
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console.log("server got: " + msg + " from " +
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rinfo.address + ":" + rinfo.port);
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});
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server.on("listening", function () {
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var address = server.address();
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console.log("server listening " +
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address.address + ":" + address.port);
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});
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server.bind(41234);
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// server listening 0.0.0.0:41234
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### dgram.close()
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Close the underlying socket and stop listening for data on it.
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### dgram.address()
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Returns an object containing the address information for a socket. For UDP sockets,
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this object will contain `address` and `port`.
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### dgram.setBroadcast(flag)
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Sets or clears the `SO_BROADCAST` socket option. When this option is set, UDP packets
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may be sent to a local interface's broadcast address.
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### dgram.setTTL(ttl)
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Sets the `IP_TTL` socket option. TTL stands for "Time to Live," but in this context it
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specifies the number of IP hops that a packet is allowed to go through. Each router or
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gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a
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router, it will not be forwarded. Changing TTL values is typically done for network
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probes or when multicasting.
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The argument to `setTTL()` is a number of hops between 1 and 255. The default on most
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systems is 64.
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### dgram.setMulticastTTL(ttl)
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Sets the `IP_MULTICAST_TTL` socket option. TTL stands for "Time to Live," but in this
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context it specifies the number of IP hops that a packet is allowed to go through,
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specifically for multicast traffic. Each router or gateway that forwards a packet
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decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.
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The argument to `setMulticastTTL()` is a number of hops between 0 and 255. The default on most
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systems is 64.
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### dgram.setMulticastLoopback(flag)
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Sets or clears the `IP_MULTICAST_LOOP` socket option. When this option is set, multicast
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packets will also be received on the local interface.
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### dgram.addMembership(multicastAddress, [multicastInterface])
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Tells the kernel to join a multicast group with `IP_ADD_MEMBERSHIP` socket option.
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If `multicastInterface` is not specified, the OS will try to add membership to all valid
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interfaces.
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### dgram.dropMembership(multicastAddress, [multicastInterface])
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Opposite of `addMembership` - tells the kernel to leave a multicast group with
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`IP_DROP_MEMBERSHIP` socket option. This is automatically called by the kernel
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when the socket is closed or process terminates, so most apps will never need to call
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this.
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If `multicastInterface` is not specified, the OS will try to drop membership to all valid
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interfaces.
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