How to make a infinitely scrolling background?

I’m trying to make the background of a game vertically scroll forever. I’m not sure how to go about it.
This is what I have so far:

void ofApp::setup(){
	ofSetFrameRate(30);
	background.load("bg1.png");
}

//--------------------------------------------------------------
void ofApp::update(){
}

//--------------------------------------------------------------
void ofApp::draw(){
	background.draw(0, 0);

}

Thank you.