Corrected Cancellation Source for KeepAlive Messages

Source/MQTTnet/MqttClient.cs

@@ -1041,11 +1041,13 @@ async Task TrySendKeepAliveMessages(CancellationToken cancellationToken)
 
                 if (timeWithoutPacketSent > keepAlivePeriod)
                 {
-                    using (var timeoutCancellationTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken))
-                    {
-                        timeoutCancellationTokenSource.CancelAfter(Options.Timeout);
-                        await PingAsync(timeoutCancellationTokenSource.Token).ConfigureAwait(false);
-                    }
+                    using var pingTimeout = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+
+                    // We already reached the keep alive timeout. Due to the RFC part [MQTT-3.1.2-24] the server will wait another
+                    // 1/2 of the keep alive time. So we can also use this value as the timeout.
+                    pingTimeout.CancelAfter(keepAlivePeriod / 2);
+
+                    await PingAsync(pingTimeout.Token).ConfigureAwait(false);
                 }
 
                 // Wait a fixed time in all cases. Calculation of the remaining time is complicated

Source/ReleaseNotes.md

@@ -13,6 +13,7 @@
 * Client: MQTT 5.0.0 is now the default version when connecting with a server **(BREAKING CHANGE)**
 * Client: Fixed enhanced authentication.
 * Client: Exposed WebSocket compression options in MQTT client options (thanks to @victornor, #2127)
+* Client: Fixed wrong timeout for keep alive check (thanks to @Erw1nT, #2129)
 * Server: Fixed enhanced authentication.
 * Server: Set default for "MaxPendingMessagesPerClient" to 1000 **(BREAKING CHANGE)**
 * Server: Set SSL version to "None" which will let the OS choose the version **(BREAKING CHANGE)**