## UDP / Datagram Sockets Datagram sockets are available through `require('dgram')`. ### Event: 'message' `function (msg, rinfo) { }` Emitted when a new datagram is available on a socket. `msg` is a `Buffer` and `rinfo` is an object with the sender's address information and the number of bytes in the datagram. ### Event: 'listening' `function () { }` Emitted when a socket starts listening for datagrams. This happens as soon as UDP sockets are created. ### Event: 'close' `function () { }` Emitted when a socket is closed with `close()`. No new `message` events will be emitted on this socket. ### dgram.createSocket(type, [callback]) Creates a datagram socket of the specified types. Valid types are `udp4` and `udp6`. Takes an optional callback which is added as a listener for `message` events. Call `socket.bind` if you want to receive datagrams. `socket.bind()` will bind to the "all interfaces" address on a random port (it does the right thing for both `udp4` and `udp6` sockets). You can then retrieve the address and port with `socket.address().address` and `socket.address().port`. ### dgram.send(buf, offset, length, port, address, [callback]) For UDP sockets, the destination port and IP address must be specified. A string may be supplied for the `address` parameter, and it will be resolved with DNS. An optional callback may be specified to detect any DNS errors and when `buf` may be re-used. Note that DNS lookups will delay the time that a send takes place, at least until the next tick. The only way to know for sure that a send has taken place is to use the callback. If the socket has not been previously bound with a call to `bind`, it's assigned a random port number and bound to the "all interfaces" address (0.0.0.0 for `udp4` sockets, ::0 for `udp6` sockets). Example of sending a UDP packet to a random port on `localhost`; var dgram = require('dgram'); var message = new Buffer("Some bytes"); var client = dgram.createSocket("udp4"); client.send(message, 0, message.length, 41234, "localhost"); 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 cannot be larger than 64K octets including 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 datagrams are impractical for most hosts and networks. - The `MTU` is the largest size a given link layer technology can support for datagrams. 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 have a minimum `MTU` of `1500` (like Ethernet). Note that it's impossible to know in advance the MTU of each link through which a packet might travel, and that generally sending a datagram greater than the (receiver) `MTU` won't work (the packet gets silently dropped, without informing the source that the data did not reach its intended recipient). ### dgram.bind(port, [address]) For UDP sockets, listen for datagrams on a named `port` and optional `address`. If `address` is not specified, the OS will try to listen on all addresses. Example of a UDP server listening on port 41234: var dgram = require("dgram"); var server = dgram.createSocket("udp4"); server.on("message", function (msg, rinfo) { console.log("server got: " + msg + " from " + rinfo.address + ":" + rinfo.port); }); server.on("listening", function () { var address = server.address(); console.log("server listening " + address.address + ":" + address.port); }); server.bind(41234); // server listening 0.0.0.0:41234 ### dgram.close() Close the underlying socket and stop listening for data on it. ### dgram.address() Returns an object containing the address information for a socket. For UDP sockets, this object will contain `address` and `port`. ### dgram.setBroadcast(flag) Sets or clears the `SO_BROADCAST` socket option. When this option is set, UDP packets may be sent to a local interface's broadcast address. ### dgram.setTTL(ttl) Sets the `IP_TTL` socket option. TTL stands for "Time to Live," but in this context it specifies the number of IP hops that a packet is allowed to go 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 `setTTL()` is a number of hops between 1 and 255. The default on most systems is 64. ### dgram.setMulticastTTL(ttl) Sets the `IP_MULTICAST_TTL` socket option. TTL stands for "Time to Live," but in this context it specifies the number of IP hops that a packet is allowed to go 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 to `setMulticastTTL()` is a number of hops between 0 and 255. The default on most systems is 64. ### dgram.setMulticastLoopback(flag) Sets or clears the `IP_MULTICAST_LOOP` socket option. When this option is set, multicast packets will also be received on the local interface. ### dgram.addMembership(multicastAddress, [multicastInterface]) Tells the kernel to join a multicast group with `IP_ADD_MEMBERSHIP` socket option. If `multicastInterface` is not specified, the OS will try to add membership to all valid interfaces. ### dgram.dropMembership(multicastAddress, [multicastInterface]) Opposite of `addMembership` - tells the kernel to leave a multicast group with `IP_DROP_MEMBERSHIP` socket option. This is automatically called by the kernel when the socket is closed or process terminates, so most apps will never need to call this. If `multicastInterface` is not specified, the OS will try to drop membership to all valid interfaces.