background.js

var udpSocket = new udp();
udpSocket.localPort = 8943;
udpSocket.receive = receiveMsg;
udpSocket.init(function(){
    udpSocket.joinGroup('224.0.1.100', function(){
        chrome.runtime.onMessage.addListener(function(message, sender, callback){
            if(message.action == 'send'){
                var buf = str2ab(message.msg);
                udpSocket.send('224.0.1.100', udpSocket.localPort, buf, function(){
                    //message is sent
                });
            }
        });
        chrome.app.runtime.onLaunched.addListener(function(){
            chrome.app.window.create('main.html', {
                'id': 'main',
                'bounds': {
                    'width': 400,
                    'height': 600
                }
            });
        });
    });
});

function receiveMsg(info){
    var msg = ab2str(info.data);
    chrome.runtime.sendMessage({action:'receive', msg:msg});
}

function str2ab(str){
    var buf = new ArrayBuffer(str.length*2);
    bufView = new Uint16Array(buf);
    for(var i=0; i<str.length; i++){
        bufView[i] = str.charCodeAt(i);
    }
    return buf;
}

function ab2str(buf){
    return String.fromCharCode.apply(null, new Uint16Array(buf));
}