How do I obtain the latency between server and client in C#?

Question

I'm working on a C# Server application for a game engine I'm writing in ActionScript 3. I'm using an authoritative server model as to prevent cheating and ensure fair game. So far, everything works well:

When the client begins moving, it tells the server and starts rendering locally; the server, then, tells everyone else that client X has began moving, among with details so they can also begin rendering. When the client stops moving, it tells the server, which performs calculations based on the time the client began moving and the client render tick delay and replies to everyone, so they can update with the correct values.

The thing is, when I use the default 20ms tick delay on server calculations, when the client moves for a rather long distance, there's a noticeable tilt forward when it stops. If I increase slightly the delay to 22ms, on my local network everything runs very smoothly, but in other locations, the tilt is still there. After experimenting a little, I noticed that the extra delay needed is pretty much tied to the latency between client and server. I even boiled it down to a formula that would work quite nicely: delay = 20 + (latency / 10).

So, how would I proceed to obtain the latency between a certain client and the server (I'm using asynchronous sockets). The CPU effort can't be too much, as to not have the server run slowly. Also, is this really the best way, or is there a more efficient/easier way to do this?

Answer 1

Sorry that this isn't directly answering your question, but generally speaking you shouldn't rely too heavily on measuring latency because it can be quite variable. Not only that, you don't know if the ping time you measure is even symmetrical, which is important. There's no point applying 10ms of latency correction if it turns out that the ping time of 20ms is actually 19ms from server to client and 1ms from client to server. And latency in application terms is not the same as in networking terms - you may be able to ping a certain machine and get a response in 20ms but if you're contacting a server on that machine that only processes network input 50 times a second then your responses will be delayed by an extra 0 to 20ms, and this will vary rather unpredictably.

That's not to say latency measurement it doesn't have a place in smoothing predictions out, but it's not going to solve your problem, just clean it up a bit.

On the face of it, the problem here seems to be that that you're sent information in the first message which you use to extrapolate data from until the last message is received. If all else stays constant then the movement vector given in the first message multiplied by the time between the messages will give the server the correct end position that the client was in at roughly now-(latency/2). But if the latency changes at all, the time between the messages will grow or shrink. The client may know he's moved 10 units, but the server simulated him moving 9 or 11 units before being told to snap him back to 10 units.

The general solution to this is to not assume that latency will stay constant but to send periodic position updates, which allow the server to verify and correct the client's position. With just 2 messages as you have now, all the error is found and corrected after the 2nd message. With more messages, the error is spread over many more sample points allowing for smoother and less visible correction.

It can never be perfect though: all it takes is a lag spike in the last millisecond of movement and the server's representation will overshoot. You can't get around that if you're predicting future movement based on past events, as there's no real alternative to choosing either correct-but-late or incorrect-but-timely since information takes time to travel. (Blame Einstein.)

Answer 2

One thing to keep in mind when using ICMP based pings is that networking equipment will often give ICMP traffic lower priority than normal packets, especially when the packets cross network boundaries such as WAN links. This can lead to pings being dropped or showing higher latency than traffic is actually experiencing and lends itself to being an indicator of problems rather than a measurement tool.

The increasing use of Quality of Service (QoS) in networks only exacerbates this and as a consequence though ping still remains a useful tool, it needs to be understood that it may not be a true reflection of the network latency for non-ICMP based real traffic.

There is a good post at the Itrinegy blog How do you measure Latency (RTT) in a network these days? about this.

Answer 3

You could use the already available Ping Class. Should be preferred over writing your own IMHO.

Answer 4

Have a "ping" command, where you send a message from the server to the client, then time how long it takes to get a response. Barring CPU overload scenarios, it should be pretty reliable. To get the one-way trip time, just divide the time by 2.

Answer 5

We can measure the round-trip time using the Ping class of the .NET Framework.

Instantiate a Ping and subscribe to the PingCompleted event:

Ping pingSender = new Ping();
pingSender.PingCompleted += PingCompletedCallback;

Add code to configure and action the ping.

Our PingCompleted event handler (PingCompletedEventHandler) has a PingCompletedEventArgs argument. The PingCompletedEventArgs.Reply gets us a PingReply object. PingReply.RoundtripTime returns the round trip time (the "number of milliseconds taken to send an Internet Control Message Protocol (ICMP) echo request and receive the corresponding ICMP echo reply message"):

public static void PingCompletedCallback(object sender, PingCompletedEventArgs e)
{
    ...
    Console.WriteLine($"Roundtrip Time: {e.Reply.RoundtripTime}");
    ...
}

Code-dump of a full working example, based on MSDN's example. I have modified it to write the RTT to the console:

public static void Main(string[] args)
{
    string who = "www.google.com";
    AutoResetEvent waiter = new AutoResetEvent(false);

    Ping pingSender = new Ping();

    // When the PingCompleted event is raised,
    // the PingCompletedCallback method is called.
    pingSender.PingCompleted += PingCompletedCallback;

    // Create a buffer of 32 bytes of data to be transmitted.
    string data = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
    byte[] buffer = Encoding.ASCII.GetBytes(data);

    // Wait 12 seconds for a reply.
    int timeout = 12000;

    // Set options for transmission:
    // The data can go through 64 gateways or routers
    // before it is destroyed, and the data packet
    // cannot be fragmented.
    PingOptions options = new PingOptions(64, true);

    Console.WriteLine("Time to live: {0}", options.Ttl);
    Console.WriteLine("Don't fragment: {0}", options.DontFragment);

    // Send the ping asynchronously.
    // Use the waiter as the user token.
    // When the callback completes, it can wake up this thread.
    pingSender.SendAsync(who, timeout, buffer, options, waiter);

    // Prevent this example application from ending.
    // A real application should do something useful
    // when possible.
    waiter.WaitOne();
    Console.WriteLine("Ping example completed.");
}

public static void PingCompletedCallback(object sender, PingCompletedEventArgs e)
{
    // If the operation was canceled, display a message to the user.
    if (e.Cancelled)
    {
        Console.WriteLine("Ping canceled.");

        // Let the main thread resume. 
        // UserToken is the AutoResetEvent object that the main thread 
        // is waiting for.
        ((AutoResetEvent)e.UserState).Set();
    }

    // If an error occurred, display the exception to the user.
    if (e.Error != null)
    {
        Console.WriteLine("Ping failed:");
        Console.WriteLine(e.Error.ToString());

        // Let the main thread resume. 
        ((AutoResetEvent)e.UserState).Set();
    }

    Console.WriteLine($"Roundtrip Time: {e.Reply.RoundtripTime}");

    // Let the main thread resume.
    ((AutoResetEvent)e.UserState).Set();
}

You might want to perform several pings and then calculate an average, depending on your requirements of course.