Node.js Outgoing tcp connection limit (cannot make connections more than five)

I'm building a data transfer proxy server using node.js. It pipes client's request to swift object storage server using http(s) REST API.

It works fine for the individual request but when the outgoing ESTABLISHED tcp connection for the same destination and port(443) reaches five, it cannot create any new connection.

It does not seem to be a problem of O/S, because I've tried to create more than 10 connections using java servlet and it works fine.

I've tried to set maximum sockets for globalAgent like below, but it does not change anything.

http.globalAgent.maxSockets = 500;
https.globalAgent.maxSockets = 500;

Here is a part of my source code.

app.post('/download*', function(req, res){

    /***********************************************************
     * Some codes here to create new request option
     ***********************************************************/

    var client = https.request(reqOptions, function(swiftRes) {
        var buffers = [];
        res.header('Content-Length', swiftRes.headers['content-length']);
        res.header('Content-Type', swiftRes.headers['content-type']);
        res.header('range', swiftRes.headers['range']);
        res.header('connection', swiftRes.headers['connection']);

        swiftRes.pipe(res);

        swiftRes.on('end', function(err){
            res.end();
        });
    });

    client.on('error', function(err) {
        callback && callback(err);
        client.end(err);
        clog.error('######### Swift Client Error event occurred. Process EXIT ');
    });

    client.end();   
});

I hope I can get the solution for this problem.

Thanks in advance.