Allow the server to ack no or multiple packets in the same frame.

OpenRA.Game/Network/Connection.cs

@@ -312,16 +312,26 @@ namespace OpenRA.Network
 					Send(ping);
 					record = false;
 				}
-				else if (OrderIO.TryParseAck(p, out var ackFrame))
+				else if (OrderIO.TryParseAck(p, out var ackFrame, out var ackCount))
 				{
-					if (!sentOrders.TryDequeue(out var q))
-						throw new InvalidOperationException("Received Ack with empty send queue");
+					if (ackCount > sentOrders.Count)
+						throw new InvalidOperationException($"Received Ack for {ackCount} > {sentOrders.Count} frames.");
 
 					// The Acknowledgement packet is a placeholder that tells us to process the first packet in our
 					// local sent buffer and the frame at which it should be applied. This is an optimization to avoid having
 					// to send the (much larger than 5 byte) packet back to us over the network.
-					orderManager.ReceiveOrders(clientId, (ackFrame, q.Orders));
-					Recorder?.Receive(clientId, q.Orders.Serialize(ackFrame));
+					OrderPacket packet;
+					if (ackCount != 1)
+					{
+						var orders = Enumerable.Range(0, ackCount)
+							.Select(i => sentOrders.Dequeue().Orders);
+						packet = OrderPacket.Combine(orders);
+					}
+					else
+						packet = sentOrders.Dequeue().Orders;
+
+					orderManager.ReceiveOrders(clientId, (ackFrame, packet));
+					Recorder?.Receive(clientId, packet.Serialize(ackFrame));
 					record = false;
 				}
 				else if (OrderIO.TryParseOrderPacket(p.Data, out var orders))

OpenRA.Game/Network/OrderIO.cs

@@ -63,6 +63,20 @@ namespace OpenRA.Network
 				ms.WriteArray(o.Serialize());
 			return ms.ToArray();
 		}
+
+		public static OrderPacket Combine(IEnumerable<OrderPacket> packets)
+		{
+			var orders = new List<Order>();
+			foreach (var packet in packets)
+			{
+				if (packet.orders == null)
+					throw new InvalidOperationException("OrderPacket.Combine can only be used with locally generated OrderPackets.");
+
+				orders.AddRange(packet.orders);
+			}
+
+			return new OrderPacket(orders.ToArray());
+		}
 	}
 
 	public static class OrderIO
@@ -130,16 +144,17 @@ namespace OpenRA.Network
 			return true;
 		}
 
-		public static bool TryParseAck((int FromClient, byte[] Data) packet, out int frame)
+		public static bool TryParseAck((int FromClient, byte[] Data) packet, out int frame, out byte count)
 		{
 			// Ack packets are only accepted from the server
-			if (packet.FromClient != 0 || packet.Data.Length != 5 || packet.Data[4] != (byte)OrderType.Ack)
+			if (packet.FromClient != 0 || packet.Data.Length != 6 || packet.Data[4] != (byte)OrderType.Ack)
 			{
-				frame = 0;
+				frame = count = 0;
 				return false;
 			}
 
 			frame = BitConverter.ToInt32(packet.Data, 0);
+			count = packet.Data[5];
 			return true;
 		}