diff --git a/Assets/Mirror/Core/NetworkConnection.cs b/Assets/Mirror/Core/NetworkConnection.cs
index 851beaea0..4c1a77505 100644
--- a/Assets/Mirror/Core/NetworkConnection.cs
+++ b/Assets/Mirror/Core/NetworkConnection.cs
@@ -61,6 +61,19 @@ public abstract class NetworkConnection
         //       different connections.
         public double remoteTimeStamp { get; internal set; }
 
+        // unreliable delta compression: we need to keep track of which identities were in which sent batch @ timestamp.
+        // so when the ack comes back, we know which identities were acked.
+        // this is necessary since we sync per-identity not per-worldstate.
+        internal readonly SortedList<double, HashSet<uint>> identityTicks = new SortedList<double, HashSet<uint>>();
+
+        // unreliable delta compression: we need to know the last acked state for each connection.
+        // currently we sync one entity at a time in batches, so we also need to store acks for one at a time.
+        // <netId, timestamp>
+        // TODO remove old when unspawned
+        readonly Dictionary<uint, double> identityAcks = new Dictionary<uint, double>();
+
+        //
+
         internal NetworkConnection()
         {
             // set lastTime to current time when creating connection to make
diff --git a/Assets/Mirror/Core/NetworkServer.cs b/Assets/Mirror/Core/NetworkServer.cs
index e6bcb1cfd..e49157fbc 100644
--- a/Assets/Mirror/Core/NetworkServer.cs
+++ b/Assets/Mirror/Core/NetworkServer.cs
@@ -1986,6 +1986,10 @@ static void BroadcastToConnection(NetworkConnectionToClient connection)
                             payload = serialization.ToArraySegment()
                         };
                         connection.Send(message, Channels.Unreliable);
+
+                        // keep track of which entities we sent to the connection on this tick.
+                        // so when the connection sends back an ack, we know which entities were acked.
+                        AckDeltaCompression.TrackIdentityAtTick(NetworkTime.localTime, identity.netId, connection.identityTicks, tickHistorySize);
                     }
                 }
                 // spawned list should have no null entries because we