Datagram sockets are available through require('dgram'). Datagrams are most commonly
handled as IP/UDP messages but they can also be used over Unix domain sockets.
要使用数据报套接字模块需要调用require('dgram')。数据报通常作为IP/UDP消息来处理，但它也可用于Unix域套接字。
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.
当套接字接收到一个新的数据报的时候触发此事件。msg是一个Buffer缓冲器，rinfo是一个包含了发送方地址信息以及数据报字节长度的对象。
function () { }
Emitted when a socket starts listening for datagrams. This happens as soon as UDP sockets
are created. Unix domain sockets do not start listening until calling bind() on them.
当一个套接字开始监听数据报的时候触发。一旦UDP套接字建立后就会触发这个事件。而Unix域套接字直到调用了bind()方法才会开始监听。
function () { }
Emitted when a socket is closed with close(). No new message events will be emitted
on this socket.
当调用close()方法关闭一个套接字时触发此事件。此后不会再有新的message事件在此套接字上发生。
Creates a datagram socket of the specified types. Valid types are:
udp4, udp6, and unix_dgram.
建立一个指定类型的数据报套接字，有效类型有：udp4，udp6，以及unix_dgram。
Takes an optional callback which is added as a listener for message events.
可选参数callback指定了message事件的监听器回调函数。
For Unix domain datagram sockets, the destination address is a pathname in the filesystem.
An optional callback may be supplied that is invoked after the sendto call is completed
by the OS. It is not safe to re-use buf until the callback is invoked. Note that
unless the socket is bound to a pathname with bind() there is no way to receive messages
on this socket.
对于Unix域数据报套接字而言，目标地址是文件系统中的一个路径。可选参数callback指定了系统调用sendto完成后的回调函数。在回调函数callback执行前重复使用buf是很不安全的。要注意除非这个套接字已经使用bind()方法绑定到一个路径上，否则这个套接字无法接收到任何信息。
Example of sending a message to syslogd on OSX via Unix domain socket /var/run/syslog:
一个在OSX系统上，通过Unix域套接字/var/run/syslog向syslogd发送消息的例子：
var dgram = require('dgram');
var message = new Buffer("A message to log.");
var client = dgram.createSocket("unix_dgram");
client.send(message, 0, message.length, "/var/run/syslog",
function (err, bytes) {
if (err) {
throw err;
}
console.log("Wrote " + bytes + " bytes to socket.");
});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.
对于UDP套接字来说，目标端口和IP地址是必须要指定的。可以用字符串来指定address参数，这个参数通过DNS进行解析。可选参数callback指定一个回调函数，用于检测DNS解析错误以及什么时候buf可被重用。注意DNS查询将会使发送动作最少延迟到下一个时间片执行，如果你想确定发送动作是否发生，使用回调将是唯一的办法。
Example of sending a UDP packet to a random port on localhost;
下面是一个发送UDP数据包到localhost（本机）一个随机端口的例子：
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();For Unix domain datagram sockets, start listening for incoming datagrams on a
socket specified by path. Note that clients may send() without bind(),
but no datagrams will be received without a bind().
对Unix域数据报套接字来说，通过指定一个path（路径）开始在套接字上监听数据报。注意客户端可以无需调用bind()直接使用send()方法发送数据报，但是不使用bind()方法将无法接收到任何数据报。
Example of a Unix domain datagram server that echoes back all messages it receives:
下面是一个Unix域数据报服务器的例子，此服务器将接收到的所有消息原样返回：
var dgram = require("dgram");
var serverPath = "/tmp/dgram_server_sock";
var server = dgram.createSocket("unix_dgram");
server.on("message", function (msg, rinfo) {
console.log("got: " + msg + " from " + rinfo.address);
server.send(msg, 0, msg.length, rinfo.address);
});
server.on("listening", function () {
console.log("server listening " + server.address().address);
})
server.bind(serverPath);Example of a Unix domain datagram client that talks to this server:
下面是一个与上述服务器通信的Unix域数据报客户端的例子：
var dgram = require("dgram");
var serverPath = "/tmp/dgram_server_sock";
var clientPath = "/tmp/dgram_client_sock";
var message = new Buffer("A message at " + (new Date()));
var client = dgram.createSocket("unix_dgram");
client.on("message", function (msg, rinfo) {
console.log("got: " + msg + " from " + rinfo.address);
});
client.on("listening", function () {
console.log("client listening " + client.address().address);
client.send(message, 0, message.length, serverPath);
});
client.bind(clientPath);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.
对于UDP套接字而言，该方法会在port指定的端口和可选地址 address上监听数据报。如果address没有指定，则操作系统会监听所有有效地址。
Example of a UDP server listening on port 41234:
下面是一个监听在41234端口的UDP服务器的例子：
var dgram = require("dgram");
var server = dgram.createSocket("udp4");
var messageToSend = new Buffer("A message to send");
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 0.0.0.0:41234Close the underlying socket and stop listening for data on it. UDP sockets
automatically listen for messages, even if they did not call bind().
该函数关闭底层的套接字并停止在其上监听数据。UDP套接字在没有调用bing()方法的情况下也自动监听消息。
Returns an object containing the address information for a socket. For UDP sockets,
this object will contain address and port. For Unix domain sockets, it will contain
only address.
返回包含套接字地址信息的对象。对于UDP套接字来说，这个对象将包含address（地址）和port（端口）。对于UNIX域套接字来说，这个对象仅包含address（地址）。
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.
设置或者清除套接字的SO_BROADCAST（广播）选项。当该设置生效时，UDP数据包将被发送至本地网络接口的广播地址。
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.
设置套接字的IP_TTL选项，TTL表示“存活时间”，但是在这个上下文环境中，特指一个数据包允许经过的IP跳数。每经过一个路由器或者网关TTL的值都会减一，如果TTL被一个路由器减少到0，这个数据包将不会继续转发。在网络探针或组播应用中会需要修改TTL数值。
The argument to setTTL() is a number of hops between 1 and 255. The default on most
systems is 64.
在setTTL()调用中设置的跳数介于1到255之间，大多数系统缺省会设置为64。
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.
设置套接字的IP_MULTICAST_TTL选项。TTL全称"Time to Live"，原指存活时间，但在这里特指在组播通信中数据包允许经过的IP跳数。每经过一个路由器或者网关TTL的值都会减一。如果TTL被一个路由器减少到0，那么该数据包将不会继续传播。
The argument to setMulticastTTL() is a number of hops between 0 and 255. The default on most
systems is 64.
setMulticastTTL()的参数为1至255之间的跳数，大部分操作系统的默认值为64。
Sets or clears the IP_MULTICAST_LOOP socket option. When this option is set, multicast
packets will also be received on the local interface.
设置或清除套接字的IP_MULTICAST_LOOP选项。当该选项生效时，多路传播的数据包也会被本地网络接口接收到。
Tells the kernel to join a multicast group with IP_ADD_MEMBERSHIP socket option.
该方法通知系统内核使用IP_ADD_MEMBERSHIP套接字选项将套接字加入一个组播组。
If multicastAddress is not specified, the OS will try to add membership to all valid
interfaces.
如果没有指定multicastInterface参数，操作系统将尝试把所有有效的网络接口加入组播组。
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.
与addMembership相反，该方法通知系统内核使用IP_DROP_MEMBERSHIP选项使套接字脱离组播组。由于当套接字关闭或进程终止时该操作会自动执行，所以大部分的应用不需要手动调用该方法。
If multicastAddress is not specified, the OS will try to drop membership to all valid
interfaces.
如果multicastInterface没有指定，操作系统会尝试将所有可用网络接口从组播组里脱离。