Add QoS beacon

src/Prospect.Server.Api/Services/Qos/QosServer.cs

@@ -0,0 +1,79 @@
+using System.Net;
+using System.Net.Sockets;
+
+namespace Prospect.Server.Api.Services.Qos;
+
+public class QosServer : IAsyncDisposable
+{
+    private readonly ILogger<QosServer> _logger;
+    private readonly UdpClient _client;
+
+    private bool _isStopping;
+    private Task? _runTask;
+    
+    public QosServer(ILogger<QosServer> logger)
+    {
+        _logger = logger;
+        _client = new UdpClient(new IPEndPoint(IPAddress.Any, 3075));
+    }
+
+    public void Start()
+    {
+        _runTask = RunAsync();
+    }
+
+    public async Task StopAsync()
+    {
+        _isStopping = true;
+        _client.Dispose();
+
+        if (_runTask != null)
+        {
+            await _runTask;
+        }
+    }
+
+    private async Task RunAsync()
+    {
+        try
+        {
+            while (true)
+            {
+                var result = await _client.ReceiveAsync();
+            
+                _logger.LogInformation("Received {Data} from {Address}", result.Buffer, result.RemoteEndPoint);
+
+                if (result.Buffer.Length == 22)
+                {
+                    // The server will reply with a single datagram, with the message contents having the
+                    // first 2 bytes "flipped" to 0x0000 (0000 0000 0000 0000).
+                    // The rest of the datagram contents will be copied from the initial ping.
+                    result.Buffer[0] = 0x00;
+                    result.Buffer[1] = 0x00;
+                    result.Buffer[2] = 0x00;
+                    result.Buffer[3] = 0x00;
+
+                    await _client.SendAsync(result.Buffer, result.RemoteEndPoint);
+                }
+            }
+        }
+        catch (SocketException e) when (e.SocketErrorCode == SocketError.OperationAborted || 
+                                        e.SocketErrorCode == SocketError.Interrupted ||
+                                        e.SocketErrorCode == SocketError.InvalidArgument && !OperatingSystem.IsWindows())
+        {
+            if (!_isStopping)
+            {
+                _logger.LogError(e, "SocketException caught in QosServer");
+            }
+        }
+        catch (Exception e)
+        {
+            _logger.LogError(e, "Exception caught in QosServer");
+        }
+    }
+    
+    public async ValueTask DisposeAsync()
+    {
+        await StopAsync();
+    }
+}

src/Prospect.Server.Api/Services/Qos/QosService.cs

@@ -0,0 +1,30 @@
+namespace Prospect.Server.Api.Services.Qos;
+
+/// <summary>
+///     Implementation of a QoS beacon.
+///     https://docs.microsoft.com/en-us/gaming/playfab/features/multiplayer/servers/using-qos-beacons-to-measure-player-latency-to-azure#quality-of-service-beacons
+/// </summary>
+public class QosService : IHostedService
+{
+    private readonly ILogger<QosService> _logger;
+    private readonly QosServer _server;
+
+    public QosService(ILogger<QosService> logger, QosServer server)
+    {
+        _logger = logger;
+        _server = server;
+    }
+    
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        _logger.LogInformation("Starting QoS beacon");
+        _server.Start();
+        return Task.CompletedTask;
+    }
+
+    public async Task StopAsync(CancellationToken cancellationToken)
+    {
+        _logger.LogInformation("Stopping QoS beacon");
+        await _server.StopAsync();
+    }
+}

src/Prospect.Server.Api/Startup.cs

@@ -3,6 +3,7 @@ using Prospect.Server.Api.Converters;
 using Prospect.Server.Api.Middleware;
 using Prospect.Server.Api.Services.Auth;
 using Prospect.Server.Api.Services.Database;
+using Prospect.Server.Api.Services.Qos;
 using Prospect.Server.Api.Services.UserData;
 using Serilog;
 
@@ -32,6 +33,9 @@ public class Startup
             
         services.AddSingleton<DbUserDataService>();
 
+        services.AddHostedService<QosService>();
+        services.AddSingleton<QosServer>();
+        
         services.AddAuthentication(_ =>
             {