diff --git a/OpenRA.Server/Server.cs b/OpenRA.Server/Server.cs
index ee8757171c..df2665c07f 100644
--- a/OpenRA.Server/Server.cs
+++ b/OpenRA.Server/Server.cs
@@ -5,8 +5,9 @@ using System.IO;
 using System.Linq;
 using System.Net;
 using System.Net.Sockets;
-using OpenRa.FileFormats;
 using System.Security.Cryptography;
+using OpenRa;
+using OpenRa.FileFormats;
 
 namespace OpenRA.Server
 {
diff --git a/OpenRa.FileFormats/Exts.cs b/OpenRa.FileFormats/Exts.cs
index a2781f0318..34a5cb27c3 100644
--- a/OpenRa.FileFormats/Exts.cs
+++ b/OpenRa.FileFormats/Exts.cs
@@ -1,5 +1,5 @@
 
-namespace OpenRa.FileFormats
+namespace OpenRa
 {
 	public static class Exts
 	{
diff --git a/OpenRa.Game/Network/OrderIO.cs b/OpenRa.Game/Network/OrderIO.cs
index 2e12b09be4..46155d5d56 100755
--- a/OpenRa.Game/Network/OrderIO.cs
+++ b/OpenRa.Game/Network/OrderIO.cs
@@ -41,5 +41,18 @@ namespace OpenRa.Network
 			}
 			return ret;
 		}
+
+		public static byte[] SerializeSync( this List<int> sync, int frameNumber )
+		{
+			var ms = new MemoryStream();
+			using( var writer = new BinaryWriter( ms ) )
+			{
+				writer.Write( frameNumber );
+				writer.Write( (byte)0x65 );
+				foreach( var s in sync )
+					writer.Write( s );
+			}
+			return ms.ToArray();
+		}
 	}
 }
diff --git a/OpenRa.Game/Network/OrderManager.cs b/OpenRa.Game/Network/OrderManager.cs
index 4414a65f5e..5ec36c4081 100755
--- a/OpenRa.Game/Network/OrderManager.cs
+++ b/OpenRa.Game/Network/OrderManager.cs
@@ -67,6 +67,8 @@ namespace OpenRa.Network
 					var frame = BitConverter.ToInt32( packet, 0 );
 					if( packet.Length == 5 && packet[ 4 ] == 0xEF )
 						readyForFrames.Add( frame );
+					else if( packet.Length >= 5 && packet[ 4 ] == 0x65 )
+						CheckSync( packet );
 					else if( frame == 0 )
 						immediatePackets.Add( packet );
 					else
@@ -78,6 +80,30 @@ namespace OpenRa.Network
 					UnitOrders.ProcessOrder( o );
 		}
 
+		Dictionary<int, byte[]> syncForFrame = new Dictionary<int, byte[]>();
+
+		void CheckSync( byte[] packet )
+		{
+			var frame = BitConverter.ToInt32( packet, 0 );
+			byte[] existingSync;
+			if( syncForFrame.TryGetValue( frame, out existingSync ) )
+			{
+				if( packet.Length != existingSync.Length )
+					OutOfSync( frame );
+				else
+					for( int i = 0 ; i < packet.Length ; i++ )
+						if( packet[ i ] != existingSync[ i ] )
+							OutOfSync( frame );
+			}
+			else
+				syncForFrame.Add( frame, packet );
+		}
+
+		void OutOfSync( int frame )
+		{
+			throw new InvalidOperationException( "out of sync in frame {0}".F( frame ) );
+		}
+
 		public bool IsReadyForNextFrame
 		{
 			get { return readyForFrames.Contains( FrameNumber ); }
@@ -93,8 +119,18 @@ namespace OpenRa.Network
 			localOrders.Clear();
 
 			var frameData = frameClientData[ FrameNumber ];
+			var sync = new List<int>();
+			sync.Add( world.SyncHash() );
+
 			foreach( var order in frameData.OrderBy( p => p.Key ).SelectMany( o => o.Value.ToOrderList( world ) ) )
+			{
 				UnitOrders.ProcessOrder( order );
+				sync.Add( world.SyncHash() );
+			}
+
+			var ss = sync.SerializeSync( FrameNumber );
+			CheckSync( ss );
+			Connection.Send( ss );
 
 			++frameNumber;
 		}