diff --git a/Assets/Mirror/Core/AckDeltaCompression/AckDeltaCompression.cs b/Assets/Mirror/Core/AckDeltaCompression/AckDeltaCompression.cs
index 0af7d5a4f..c66834878 100644
--- a/Assets/Mirror/Core/AckDeltaCompression/AckDeltaCompression.cs
+++ b/Assets/Mirror/Core/AckDeltaCompression/AckDeltaCompression.cs
@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Linq;
+using UnityEngine;
 
 namespace Mirror
 {
@@ -71,5 +72,30 @@ internal static void TrackIdentityAtTick(
             // add the netid to the hashset
             netIds.Add(netId);
         }
+
+        // when receiving an ack from a connection, update latest ack for
+        // all networkidentities that were in the acked batch.
+        internal static void UpdateIdentityAcks(
+            double timestamp,
+            SortedList<double, HashSet<uint>> identityTicks,
+            Dictionary<uint, double> identityAcks)
+        {
+            // find the identities that were in the acked batch
+            if (!identityTicks.TryGetValue(timestamp, out HashSet<uint> identities))
+            {
+                // for now, at least log a message so we know this happened.
+                Debug.Log($"UpdateLatestAck: batch @ {timestamp} was not in history anymore. This can happen if the other end was too far behind.");
+                return;
+            }
+
+            // update latest acks for all identities that were in the batch
+            foreach (uint netId in identities)
+            {
+                // unreliable messages may arrive out of order.
+                // only update if newer.
+                if (!identityAcks.TryGetValue(netId, out double ackTimestamp) || timestamp > ackTimestamp)
+                    identityAcks[netId] = timestamp;
+            }
+        }
     }
 }
diff --git a/Assets/Mirror/Core/NetworkConnection.cs b/Assets/Mirror/Core/NetworkConnection.cs
index 4c1a77505..e5036dbbf 100644
--- a/Assets/Mirror/Core/NetworkConnection.cs
+++ b/Assets/Mirror/Core/NetworkConnection.cs
@@ -70,7 +70,7 @@ public abstract class NetworkConnection
         // 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 readonly Dictionary<uint, double> identityAcks = new Dictionary<uint, double>();
 
         //
 
diff --git a/Assets/Mirror/Core/NetworkServer.cs b/Assets/Mirror/Core/NetworkServer.cs
index 4df7e7545..22b443b58 100644
--- a/Assets/Mirror/Core/NetworkServer.cs
+++ b/Assets/Mirror/Core/NetworkServer.cs
@@ -497,7 +497,10 @@ static void OnTimeSnapshotMessage(NetworkConnectionToClient connection, TimeSnap
         // ack delta compression ///////////////////////////////////////////////
         static void OnAckMessage(NetworkConnectionToClient connection, AckMessage message)
         {
-            Debug.Log($"NetworkServer received acknowledgement: {message.batchTimestamp} for connId={connection.connectionId}");
+            // for the acknowledged batch's timestamp:
+            // update last ack for all NetworkIdentities that were in the batch.
+            // Debug.Log($"NetworkServer received acknowledgement: {message.batchTimestamp} for connId={connection.connectionId}");
+            AckDeltaCompression.UpdateIdentityAcks(message.batchTimestamp, connection.identityTicks, connection.identityAcks);
         }
 
         // connections /////////////////////////////////////////////////////////
diff --git a/Assets/Mirror/Tests/Editor/AckDeltaCompression/AckDeltaCompressionTests.cs b/Assets/Mirror/Tests/Editor/AckDeltaCompression/AckDeltaCompressionTests.cs
index 7f13633ff..aa4a97d90 100644
--- a/Assets/Mirror/Tests/Editor/AckDeltaCompression/AckDeltaCompressionTests.cs
+++ b/Assets/Mirror/Tests/Editor/AckDeltaCompression/AckDeltaCompressionTests.cs
@@ -90,5 +90,48 @@ public void TrackIdentityAtTick()
             Assert.That(identityTicks[4.0].Contains(1337));
             Assert.That(identityTicks[4.0].Contains(101));
         }
+
+        [Test]
+        public void UpdateIdentityAcks()
+        {
+            // prepare a few batches that were sent, with NetworkIdentities included
+            SortedList<double, HashSet<uint>> identityTicks = new SortedList<double, HashSet<uint>>();
+            int MaxCount = 3;
+
+            // insert t = 1 with a few netids
+            AckDeltaCompression.TrackIdentityAtTick(1.0, 42, identityTicks, MaxCount);
+            AckDeltaCompression.TrackIdentityAtTick(1.0, 1337, identityTicks, MaxCount);
+
+            // insert t = 2 with a the same netIds and one new
+            AckDeltaCompression.TrackIdentityAtTick(2.0, 42, identityTicks, MaxCount);
+            AckDeltaCompression.TrackIdentityAtTick(2.0, 1337, identityTicks, MaxCount);
+            AckDeltaCompression.TrackIdentityAtTick(2.0, 101, identityTicks, MaxCount);
+
+            // insert t = 3 without one of the previous netIds
+            AckDeltaCompression.TrackIdentityAtTick(3.0, 1337, identityTicks, MaxCount);
+            AckDeltaCompression.TrackIdentityAtTick(3.0, 101, identityTicks, MaxCount);
+
+            // remote acks t = 1: 42, 1337
+            Dictionary<uint, double> identityAcks = new Dictionary<uint, double>();
+            AckDeltaCompression.UpdateIdentityAcks(1.0, identityTicks, identityAcks);
+            Assert.That(identityAcks.Count, Is.EqualTo(2));
+            Assert.That(identityAcks[42], Is.EqualTo(1.0));
+            Assert.That(identityAcks[1337], Is.EqualTo(1.0));
+
+            // remote acks t = 3 before t = 2: 1337, 101
+            AckDeltaCompression.UpdateIdentityAcks(3.0, identityTicks, identityAcks);
+            Assert.That(identityAcks.Count, Is.EqualTo(3));
+            Assert.That(identityAcks[42], Is.EqualTo(1.0));   // still from the first ack
+            Assert.That(identityAcks[1337], Is.EqualTo(3.0)); // acked
+            Assert.That(identityAcks[101], Is.EqualTo(3.0));  // acked
+
+            // remote acks t = 2: 42, 1337, 101.
+            // only 42 is should be updated since 1337 and 101 are already newer
+            AckDeltaCompression.UpdateIdentityAcks(2.0, identityTicks, identityAcks);
+            Assert.That(identityAcks.Count, Is.EqualTo(3));
+            Assert.That(identityAcks[42], Is.EqualTo(2.0));   // updated
+            Assert.That(identityAcks[1337], Is.EqualTo(3.0)); // already newer
+            Assert.That(identityAcks[101], Is.EqualTo(3.0));  // already newer
+        }
     }
 }