feature: NetworkConnectionToClient.rtt via Ping & Pong Messages (#3545)

* NetworkConnectionToClient: send Ping message every PingFrequency

* breaking: NetworkPing/PongMessage .clientTime renamed to .localTime because it'll be used in both directions

* Server->Client->Server Ping/Pong messages and rtt

* don't ping in host mode

* adjust tests

* TODO

Assets/Mirror/Core/LocalConnectionToClient.cs

@@ -29,6 +29,9 @@ internal override void Send(ArraySegment<byte> segment, int channelId = Channels
         // true because local connections never timeout
         internal override bool IsAlive(float timeout) => true;
 
+        // don't ping host client in host mode
+        protected override void UpdatePing() {}
+
         internal void DisconnectInternal()
         {
             // set not ready and handle clientscene disconnect in any case

Assets/Mirror/Core/Messages.cs

@@ -111,22 +111,21 @@ public struct EntityStateMessage : NetworkMessage
         public ArraySegment<byte> payload;
     }
 
-    // A client sends this message to the server
+    // whoever wants to measure rtt, sends this to the other end.
-    // to calculate RTT and synchronize time
     public struct NetworkPingMessage : NetworkMessage
     {
-        public double clientTime;
+        public double localTime;
 
         public NetworkPingMessage(double value)
         {
-            clientTime = value;
+            localTime = value;
         }
     }
 
-    // The server responds with this message
+    // the other end responds with this message.
-    // The client can use this to calculate RTT and sync time
+    // we can use this to calculate rtt.
     public struct NetworkPongMessage : NetworkMessage
     {
-        public double clientTime;
+        public double localTime;
     }
 }

Assets/Mirror/Core/NetworkClient.cs

@@ -471,6 +471,7 @@ internal static void RegisterMessageHandlers(bool hostMode)
                 RegisterHandler<ObjectDestroyMessage>(OnObjectDestroy);
                 RegisterHandler<ObjectHideMessage>(OnObjectHide);
                 RegisterHandler<NetworkPongMessage>(NetworkTime.OnClientPong, false);
+                RegisterHandler<NetworkPingMessage>(NetworkTime.OnClientPing, false);
                 RegisterHandler<SpawnMessage>(OnSpawn);
                 RegisterHandler<ObjectSpawnStartedMessage>(OnObjectSpawnStarted);
                 RegisterHandler<ObjectSpawnFinishedMessage>(OnObjectSpawnFinished);

Assets/Mirror/Core/NetworkConnectionToClient.cs

@@ -46,6 +46,14 @@ public class NetworkConnectionToClient : NetworkConnection
         // Snapshot Buffer size limit to avoid ever growing list memory consumption attacks from clients.
         public int snapshotBufferSizeLimit = 64;
 
+        // ping for rtt (round trip time)
+        // useful for statistics, lag compensation, etc.
+        double lastPingTime = 0;
+        internal ExponentialMovingAverage _rtt = new ExponentialMovingAverage(NetworkTime.PingWindowSize);
+
+        /// <summary>Round trip time (in seconds) that it takes a message to go server->client->server.</summary>
+        public double rtt => _rtt.Value;
+
         public NetworkConnectionToClient(int networkConnectionId)
             : base(networkConnectionId)
         {
@@ -175,12 +183,29 @@ internal void BufferRpc(RpcMessage message, int channelId)
             }
         }
 
+        protected virtual void UpdatePing()
+        {
+            // localTime (double) instead of Time.time for accuracy over days
+            if (NetworkTime.localTime >= lastPingTime + NetworkTime.PingInterval)
+            {
+                // TODO it would be safer for the server to store the last N
+                // messages' timestamp and only send a message number.
+                // This way client's can't just modify the timestamp.
+                NetworkPingMessage pingMessage = new NetworkPingMessage(NetworkTime.localTime);
+                Send(pingMessage, Channels.Unreliable);
+                lastPingTime = NetworkTime.localTime;
+            }
+        }
+
         internal override void Update()
         {
             // send rpc buffers
             FlushRpcs(reliableRpcs, Channels.Reliable);
             FlushRpcs(unreliableRpcs, Channels.Unreliable);
 
+            // ping client for rtt
+            UpdatePing();
+
             // call base update to flush out batched messages
             base.Update();
         }

Assets/Mirror/Core/NetworkServer.cs

@@ -266,6 +266,7 @@ internal static void RegisterMessageHandlers()
             RegisterHandler<ReadyMessage>(OnClientReadyMessage);
             RegisterHandler<CommandMessage>(OnCommandMessage);
             RegisterHandler<NetworkPingMessage>(NetworkTime.OnServerPing, false);
+            RegisterHandler<NetworkPongMessage>(NetworkTime.OnServerPong, false);
             RegisterHandler<EntityStateMessage>(OnEntityStateMessage, true);
             RegisterHandler<TimeSnapshotMessage>(OnTimeSnapshotMessage, true);
         }

Assets/Mirror/Core/NetworkTime.cs

@@ -109,15 +109,16 @@ internal static void UpdateClient()
             }
         }
 
+        // client rtt calculation //////////////////////////////////////////////
         // executed at the server when we receive a ping message
         // reply with a pong containing the time from the client
         // and time from the server
         internal static void OnServerPing(NetworkConnectionToClient conn, NetworkPingMessage message)
         {
-            // Debug.Log($"OnPingServerMessage conn:{conn}");
+            // Debug.Log($"OnServerPing conn:{conn}");
             NetworkPongMessage pongMessage = new NetworkPongMessage
             {
-                clientTime = message.clientTime,
+                localTime = message.localTime,
             };
             conn.Send(pongMessage, Channels.Unreliable);
         }
@@ -128,8 +129,32 @@ internal static void OnServerPing(NetworkConnectionToClient conn, NetworkPingMes
         internal static void OnClientPong(NetworkPongMessage message)
         {
             // how long did this message take to come back
-            double newRtt = localTime - message.clientTime;
+            double newRtt = localTime - message.localTime;
             _rtt.Add(newRtt);
         }
+
+        // server rtt calculation //////////////////////////////////////////////
+        // Executed at the client when we receive a ping message from the server.
+        // in other words, this is for server sided ping + rtt calculation.
+        // reply with a pong containing the time from the server
+        internal static void OnClientPing(NetworkPingMessage message)
+        {
+            // Debug.Log($"OnClientPing conn:{conn}");
+            NetworkPongMessage pongMessage = new NetworkPongMessage
+            {
+                localTime = message.localTime,
+            };
+            NetworkClient.Send(pongMessage, Channels.Unreliable);
+        }
+
+        // Executed at the server when we receive a Pong message back.
+        // find out how long it took since we sent the Ping
+        // and update time offset
+        internal static void OnServerPong(NetworkConnectionToClient conn, NetworkPongMessage message)
+        {
+            // how long did this message take to come back
+            double newRtt = localTime - message.localTime;
+            conn._rtt.Add(newRtt);
+        }
     }
 }

Assets/Mirror/Tests/Editor/NetworkConnection/NetworkConnectionToClientTests.cs

@@ -35,6 +35,7 @@ public void Send_BatchesUntilUpdate()
         {
             // create connection and send
             NetworkConnectionToClient connection = new NetworkConnectionToClient(42);
+            NetworkTime.PingInterval = float.MaxValue; // disable ping for this test
             byte[] message = {0x01, 0x02};
             connection.Send(new ArraySegment<byte>(message));
 
@@ -63,6 +64,7 @@ public void SendBatchingResetsPreviousWriter()
 
             // create connection
             NetworkConnectionToClient connection = new NetworkConnectionToClient(42);
+            NetworkTime.PingInterval = float.MaxValue; // disable ping for this test
 
             // send and update big message
             byte[] message = {0x01, 0x02};