Which request headers can be used for a browser/client fingerprint?

For added security our server keeps track of the browser fingerprint. At the moment we use the following headers:

• HTTP_CLIENT_IP, HTTP_X_FORWARDED_FOR, HTTP_X_FORWARDED, HTTP_X_CLUSTER_CLIENT_IP, HTTP_FORWARDED_FOR, HTTP_FORWARDED, REMOTE_ADDR (take the first non-empty as the client-IP)
• HTTP_ACCEPT_* headers
• HTTP_USER_AGENT

Are there any more (optional) headers that can be used?

What in general is the best 'algorithm' to calculate the client fingerprint?

you can use a unique browser fingerprint (user agent, web browser, canvas, etc) and after get the hash.

/* Generate a fingerprint string for the browser */
function generateFingerprint(){
//Generate a string based on "stable" information taken from the browser
//We call here "stable information", information that normally don't   change during the user
//browse the application just after authentication
var fingerprint = [];

//Take plugins
for(var i = 0; i < navigator.plugins.length; i++){
   fingerprint.push(navigator.plugins[i].name);
   fingerprint.push(navigator.plugins[i].filename);
   fingerprint.push(navigator.plugins[i].description);
   fingerprint.push(navigator.plugins[i].version);
}

//Take User Agent
fingerprint.push(navigator.userAgent);

//Take Screen resolution
fingerprint.push(screen.availHeight);
fingerprint.push(screen.availWidth);
fingerprint.push(screen.colorDepth);
fingerprint.push(screen.height);
fingerprint.push(screen.pixelDepth);
fingerprint.push(screen.width);

//Take Graphical card info
//See http://output.jsbin.com/ovekor/3/
try {
    //Add a Canvas element if the body do not contains one
    if ( $("#glcanvas").length == 0 ){
        $(document.body).append("<canvas id='glcanvas'></canvas>");
    }
    //Get ref on Canvas
    var canvas = document.getElementById("glcanvas");
    //Retrieve Canvas properties
    gl = canvas.getContext("experimental-webgl");
    gl.viewportWidth = canvas.width;
    gl.viewportHeight = canvas.height;
    fingerprint.push(gl.getParameter(gl.VERSION));
    fingerprint.push(gl.getParameter(gl.SHADING_LANGUAGE_VERSION));
    fingerprint.push(gl.getParameter(gl.VENDOR));
    fingerprint.push(gl.getParameter(gl.RENDERER));
    fingerprint.push(gl.getSupportedExtensions().join());
} catch (e) {
    //Get also error because it's will be stable too..
    fingerprint.push(e);
}

//Last and, in order to made this browser unique, generate a random ID that we will store
//in local storage (in order to be persistent after browser close/reopen)
//Add this ID because, in Enterprise, most of the time browser have the same configuration
var browserUniqueID = localStorage.getItem("browserUniqueID");
if (browserUniqueID === null) {
  localStorage.setItem("browserUniqueID", CryptoJS.lib.WordArray.random(80));
  browserUniqueID = localStorage.getItem("browserUniqueID");
}
fingerprint.push(browserUniqueID);

return fingerprint.join();
}

And finally get the hash and sent to the server.

//Call the fingerprint dedicated function
var fingerprint = generateFingerprint();
//Use CryptoJS library ot generate a hex encoded string of the hash of the fingerprint
var fingerprintHash = CryptoJS.SHA256(fingerprint);

Source: https://www.owasp.org/index.php/JSON_Web_Token_(JWT)_Cheat_Sheet_for_Java#Token_sidejacking https://browserleaks.com/canvas