feature: Ping message includes timestamp for RTT

kcp2k/kcp2k.Tests/ClientServerTests.cs

@@ -952,6 +952,24 @@ public void TimeoutIsResetByPing()
             Assert.That(server.connections.Count, Is.EqualTo(1));
         }
 
+        [Test]
+        public void PingUpdatesRtt()
+        {
+            Assert.That(client.rttInMilliseconds, Is.EqualTo(0));
+
+            server.Start(Port);
+            ConnectClientBlocking();
+            int connectionId = ServerFirstConnectionId();
+
+            // update a few times, let at least PING_INTERVAL elapse, update again
+            UpdateSeveralTimes(10);
+            Thread.Sleep(KcpPeer.PING_INTERVAL);
+            UpdateSeveralTimes(10);
+
+            Assert.That(client.rttInMilliseconds, Is.GreaterThan(0));
+            Assert.That(server.connections[connectionId].rttInMilliseconds, Is.GreaterThan(0));
+        }
+
         // fake a kcp dead_link by setting state = -1.
         // KcpConnection should detect it and disconnect.
         [Test]

kcp2k/kcp2k/highlevel/KcpHeader.cs

@@ -15,6 +15,7 @@ public enum KcpHeaderReliable : byte
         // we already have a KcpHeader for reliable messages.
         // ping is only used to keep it alive, so latency doesn't matter.
         Ping       = 2,
+        Pong       = 4, // '4' not '3' in order to keep backwards compatibility
         Data       = 3,
     }
 

kcp2k/kcp2k/highlevel/KcpPeer.cs

@@ -62,6 +62,7 @@ public abstract class KcpPeer
         // same goes for slow paced card games etc.
         public const int PING_INTERVAL = 1000;
         uint lastPingTime;
+        uint lastPongTime;
 
         // if we send more than kcp can handle, we will get ever growing
         // send/recv buffers and queues and minutes of latency.
@@ -144,6 +145,10 @@ public static int UnreliableMaxMessageSize(int mtu) =>
         public readonly int unreliableMax;
         public readonly int reliableMax;
 
+        // round trip time (RTT) for convenience.
+        // this is the time that it takes for a reliable message to travel to remote and back.
+        public uint rttInMilliseconds { get; private set; }
+
         // SetupKcp creates and configures a new KCP instance.
         // => useful to start from a fresh state every time the client connects
         // => NoDelay, interval, wnd size are the most important configurations.
@@ -360,6 +365,13 @@ void TickIncoming_Connected(uint time)
                     case KcpHeaderReliable.Ping:
                     {
                         // ping keeps kcp from timing out. do nothing.
+                        // safety: don't reply with pong message before authenticated.
+                        break;
+                    }
+                    case KcpHeaderReliable.Pong:
+                    {
+                        // ping keeps kcp from timing out. do nothing.
+                        // safety: don't handle pong message before authenticated.
                         break;
                     }
                     case KcpHeaderReliable.Data:
@@ -417,7 +429,34 @@ void TickIncoming_Authenticated(uint time)
                     }
                     case KcpHeaderReliable.Ping:
                     {
-                        // ping keeps kcp from timing out. do nothing.
+                        // ping includes the sender's local time for RTT calculation.
+                        // simply send it back to the sender.
+                        // for safety, we only reply every PING_INTERVAL at max.
+                        // so attackers can't force us to reply a PONG every time.
+                        if (message.Count == 4)
+                        {
+                            if (time >= lastPongTime + PING_INTERVAL)
+                            {
+                                Utils.Decode32U(message.Array, message.Offset, out uint pingTimestamp);
+                                SendPong(pingTimestamp);
+                                lastPongTime = time;
+                            }
+                        }
+                        break;
+                    }
+                    // ping keeps kcp from timing out, and is used for RTT calcualtion
+                    case KcpHeaderReliable.Pong:
+                    {
+                        if (message.Count == 4)
+                        {
+                            Utils.Decode32U(message.Array, message.Offset, out uint originalTimestamp);
+                            if (time >= originalTimestamp)
+                            {
+                                rttInMilliseconds = time - originalTimestamp;
+                                // Log.Info($"[KCP] {GetType()}: RTT={rttInMilliseconds}ms");
+                            }
+                        }
+
                         break;
                     }
                 }
@@ -736,7 +775,22 @@ public void SendData(ArraySegment<byte> data, KcpChannel channel)
 
         // ping goes through kcp to keep it from timing out, so it goes over the
         // reliable channel.
-        void SendPing() => SendReliable(KcpHeaderReliable.Ping, default);
+        readonly byte[] pingData = new byte[4]; // 4 bytes timestamp
+        void SendPing()
+        {
+            // when sending ping, include the local timestamp so we can
+            // calculate RTT from the pong.
+            Utils.Encode32U(pingData, 0, time);
+            SendReliable(KcpHeaderReliable.Ping, pingData);
+        }
+
+        void SendPong(uint pingTimestamp)
+        {
+            // when sending ping, include the local timestamp so we can
+            // calculate RTT from the pong.
+            Utils.Encode32U(pingData, 0, pingTimestamp);
+            SendReliable(KcpHeaderReliable.Pong, pingData);
+        }
 
         // send disconnect message
         void SendDisconnect()