diff --git a/include/server/state/GameStateHandler.h b/include/server/state/GameStateHandler.h
index afc202f..011caa4 100644
--- a/include/server/state/GameStateHandler.h
+++ b/include/server/state/GameStateHandler.h
@@ -19,6 +19,7 @@ class GameStateHandler : public protocol::PacketHandler {
 
 	virtual void Handle(const protocol::packets::SpawnTroopPacket& a_Packet) override;
 	virtual void Handle(const protocol::packets::PlaceTowerPacket& a_Packet) override;
+	virtual void Handle(const protocol::packets::LockStepRequestPacket& a_Packet) override;
 };
 
 }  // namespace server
diff --git a/include/td/protocol/packet/PacketData.h b/include/td/protocol/packet/PacketData.h
index 4e3ac86..b461f83 100644
--- a/include/td/protocol/packet/PacketData.h
+++ b/include/td/protocol/packet/PacketData.h
@@ -110,6 +110,14 @@ struct PlaceTower {
 	TowerCoords m_Position;
 };
 
+struct LockStepRequest {
+	std::vector<StepTime> m_Missing;
+};
+
+struct LockStepResponse {
+	std::map<StepTime, LockStep> m_Steps;
+};
+
 }  // namespace pdata
 }  // namespace protocol
 }  // namespace td
diff --git a/include/td/protocol/packet/Packets.h b/include/td/protocol/packet/Packets.h
index 0fce78f..e4ecb34 100644
--- a/include/td/protocol/packet/Packets.h
+++ b/include/td/protocol/packet/Packets.h
@@ -22,6 +22,8 @@ enum class PacketID : std::uint8_t {
 	BeginGame,
 	Disconnect,
 	KeepAlive,
+	LockStepRequest,
+	LockStepResponse,
 	LockSteps,
 	LoggingSuccess,
 	PlaceTower,
@@ -49,6 +51,8 @@ using BeginGamePacket = PacketMessage<pdata::BeginGame, PacketID::BeginGame>;
 using ChatMessagePacket = PacketMessage<pdata::ChatMessage, PacketID::ChatMessage>;
 using DisconnectPacket = PacketMessage<pdata::Disconnect, PacketID::Disconnect>;
 using KeepAlivePacket = PacketMessage<pdata::KeepAlive, PacketID::KeepAlive>;
+using LockStepRequestPacket = PacketMessage<pdata::LockStepRequest, PacketID::LockStepRequest>;
+using LockStepResponsePacket = PacketMessage<pdata::LockStepResponse, PacketID::LockStepResponse>;
 using LockStepsPacket = PacketMessage<pdata::LockSteps, PacketID::LockSteps>;
 using LoggingSuccessPacket = PacketMessage<pdata::LoggingSuccess, PacketID::LoggingSuccess>;
 using PlaceTowerPacket = PacketMessage<pdata::PlaceTower, PacketID::PlaceTower>;
@@ -63,7 +67,7 @@ using WorldDataPacket = PacketMessage<pdata::WorldData, PacketID::WorldData>;
 }  // namespace packets
 
 using AllPackets = std::tuple<packets::BeginGamePacket, packets::ChatMessagePacket, packets::DisconnectPacket,
-	packets::KeepAlivePacket, packets::LockStepsPacket, packets::LoggingSuccessPacket, packets::PlaceTowerPacket,
+	packets::KeepAlivePacket, packets::LockStepRequestPacket, packets::LockStepResponsePacket, packets::LockStepsPacket, packets::LoggingSuccessPacket, packets::PlaceTowerPacket,
 	packets::PlayerJoinPacket, packets::PlayerLeavePacket, packets::PlayerLoginPacket, packets::PredictCommandPacket,
 	packets::SpawnTroopPacket, packets::WorldHeaderPacket, packets::WorldDataPacket>;
 
diff --git a/include/td/simulation/ClientSimulation.h b/include/td/simulation/ClientSimulation.h
index d445ab2..3f84b53 100644
--- a/include/td/simulation/ClientSimulation.h
+++ b/include/td/simulation/ClientSimulation.h
@@ -2,6 +2,7 @@
 
 #include <td/game/World.h>
 #include <optional>
+#include <td/misc/Signal.h>
 
 namespace td {
 namespace sim {
@@ -17,16 +18,18 @@ class ClientSimulation : public protocol::PacketHandler {
 	GameBuffer m_History;
 	std::uint64_t m_CurrentTime;
 	std::uint64_t m_LastTime;
-	std::size_t m_CurrentStep;
+	StepTime m_CurrentStep;
 
 	std::shared_ptr<WorldSnapshot> m_LastSnapshot;
-	std::uint64_t m_LastValidStep;
+	StepTime m_LastValidStep;
 
 	static const protocol::LockStep EMPTY_LOCKSTEP;
 
 	using protocol::PacketHandler::Handle;
 
   public:
+	utils::Signal<const std::vector<StepTime>&> OnMissingLockSteps;
+
 	/**
 	 * \brief Replay constructor
 	 * \param a_StepTime in ms
@@ -45,15 +48,18 @@ class ClientSimulation : public protocol::PacketHandler {
 	float Update();
 
 	virtual void Handle(const protocol::packets::LockStepsPacket& a_LockSteps) override;
-	virtual void Handle(const protocol::packets::PredictCommandPacket& a_Predict) override;
+	virtual void Handle(const protocol::packets::LockStepResponsePacket& a_LockSteps) override;
 
   private:
-	void Step();
+	/**
+	 * \returns false if the empty lockstep was used
+	 */
+	bool Step();
 
 	/**
 	 * \brief Ticks a_Count times
 	 */
-	void FastForward(std::size_t a_Count);
+	std::size_t FastForward(std::size_t a_Count);
 
 	/**
 	 * \brief Tries to recompute simulation if needed (for example in late command receival)
diff --git a/include/td/simulation/ServerSimulation.h b/include/td/simulation/ServerSimulation.h
index 599041f..61af852 100644
--- a/include/td/simulation/ServerSimulation.h
+++ b/include/td/simulation/ServerSimulation.h
@@ -31,6 +31,8 @@ class ServerSimulation : public protocol::CommandHandler {
 
 	virtual void Handle(const protocol::commands::SpawnTroopCommand& a_SpawnTroop) override;
 	virtual void Handle(const protocol::commands::PlaceTowerCommand& a_PlaceTower) override;
+
+	protocol::packets::LockStepResponsePacket GetResponse(const protocol::packets::LockStepRequestPacket& a_Missing) const;
 };
 
 }  // namespace sim
diff --git a/src/main.cpp b/src/main.cpp
index 72ad7fb..6bc672f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -49,8 +49,8 @@ class ClientHandler : public td::protocol::PacketHandler {
 		m_Simulation.Handle(a_LockStep);
 	}
 
-	void Handle(const td::protocol::packets::PredictCommandPacket& a_Predict) {
-		m_Simulation.Handle(a_Predict);
+	void Handle(const td::protocol::packets::LockStepResponsePacket& a_LockStep) {
+		m_Simulation.Handle(a_LockStep);
 	}
 };
 
@@ -125,6 +125,10 @@ int main(int argc, char** argv) {
 	td::sim::ClientSimulation simulation(*clientWorld, td::STEP_TIME);
 	ClientHandler clientHandler(simulation);
 
+	simulation.OnMissingLockSteps.Connect([&fakeSocket](const std::vector<td::StepTime>& a_MissingSteps){
+		fakeSocket->OnReceive(0, td::protocol::packets::LockStepRequestPacket(a_MissingSteps));
+	});
+
 	// temporary tests
 	display.OnKeyDown.Connect([&fakeSocket](SDL_Keycode key) {
 		if (key == SDLK_A) {
@@ -134,6 +138,7 @@ int main(int argc, char** argv) {
 		}
 	});
 
+	// make a fake player join
 	fakeSocket->ConnectFakePeer(0);
 
 	// packets from the server to the client
diff --git a/src/server/state/GameStateHandler.cpp b/src/server/state/GameStateHandler.cpp
index 3b0e9fc..525a728 100644
--- a/src/server/state/GameStateHandler.cpp
+++ b/src/server/state/GameStateHandler.cpp
@@ -20,5 +20,9 @@ void GameStateHandler::Handle(const protocol::packets::PlaceTowerPacket& a_Packe
 	m_GameState.m_Simulation.Handle(place);
 }
 
+void GameStateHandler::Handle(const protocol::packets::LockStepRequestPacket& a_Packet) {
+	m_GameState.SendPacket(m_PlayerId, m_GameState.m_Simulation.GetResponse(a_Packet));
+}
+
 }  // namespace server
 }  // namespace td
diff --git a/src/td/simulation/ClientSimulation.cpp b/src/td/simulation/ClientSimulation.cpp
index e8045e6..8075355 100644
--- a/src/td/simulation/ClientSimulation.cpp
+++ b/src/td/simulation/ClientSimulation.cpp
@@ -4,6 +4,8 @@
 
 #include <iostream>
 
+// TODO: mutex this bad boy
+
 namespace td {
 namespace sim {
 
@@ -37,9 +39,7 @@ ClientSimulation::ClientSimulation(game::World& a_World, std::uint64_t a_StepTim
 	m_LastTime(GetTime()),
 	m_CurrentStep(0),
 	m_LastSnapshot(std::make_shared<WorldSnapshot>()),
-	m_LastValidStep(0) {
-	// Step();
-}
+	m_LastValidStep(0) {}
 
 float ClientSimulation::Update() {
 	// TODO: handle freezes (m_CurrentTime > 2 * m_StepTime)
@@ -52,15 +52,20 @@ float ClientSimulation::Update() {
 	return (float)m_CurrentTime / (float)m_StepTime;
 }
 
-void ClientSimulation::Step() {
+bool ClientSimulation::Step() {
 	const auto& step = m_History[m_CurrentStep];
 	if (step.has_value()) {
-		m_LastSnapshot = m_World.Tick(step.value(), FpFloat(m_StepTime) / FpFloat(1000));
-		m_LastValidStep = m_CurrentStep;
+		auto snapshot = m_World.Tick(step.value(), FpFloat(m_StepTime) / FpFloat(1000));
+		if (m_LastValidStep + 1 == m_CurrentStep) {
+			m_LastValidStep = m_CurrentStep;
+			m_LastSnapshot = snapshot;
+		}
 	} else {
 		m_World.Tick(EMPTY_LOCKSTEP, FpFloat(m_StepTime) / FpFloat(1000));
+		std::cout << "Empty tick (" << m_CurrentStep << ") !\n";
 	}
 	m_CurrentStep++;
+	return step.has_value();
 }
 
 void ClientSimulation::Handle(const protocol::packets::LockStepsPacket& a_LockSteps) {
@@ -68,16 +73,32 @@ void ClientSimulation::Handle(const protocol::packets::LockStepsPacket& a_LockSt
 	for (std::size_t i = 0; i < LOCKSTEP_BUFFER_SIZE; i++) {
 		m_History[a_LockSteps->m_FirstFrameNumber + i] = steps[i];
 	}
+
+	// to be in the state [0-1]
+	if (a_LockSteps->m_FirstFrameNumber == 0)
+		Step();
+
 	FastReplay();
 }
 
-void ClientSimulation::Handle(const protocol::packets::PredictCommandPacket& a_Predict) {}
-
-void ClientSimulation::FastForward(std::size_t a_Count) {
-	for (std::size_t i = 0; i < a_Count; i++) {
-		Step();
+void ClientSimulation::Handle(const protocol::packets::LockStepResponsePacket& a_LockSteps) {
+	for (const auto& [stepTime, lockStep] : a_LockSteps->m_Steps) {
+		m_History[stepTime] = lockStep;
 	}
-	std::cout << "Was behind " << a_Count << " ticks !\n";
+	FastReplay();
+}
+
+std::size_t ClientSimulation::FastForward(std::size_t a_Count) {
+	std::size_t stepCount;
+	bool hasMissingStep = false;
+	for (std::size_t i = 0; i < a_Count; i++) {
+		if (!Step() && !hasMissingStep) {
+			hasMissingStep = true;
+			stepCount = i + 1;
+		}
+	}
+	std::cout << "Was behind " << (hasMissingStep ? stepCount : a_Count) << " ticks !\n";
+	return hasMissingStep ? stepCount : a_Count;
 }
 
 void ClientSimulation::FastReplay() {
@@ -86,11 +107,21 @@ void ClientSimulation::FastReplay() {
 
 	m_World.ResetSnapshots(m_LastSnapshot, m_LastSnapshot);
 
-	// TODO: cover holes
 	const std::size_t stepCount = m_CurrentStep - m_LastValidStep;
 	m_CurrentStep = m_LastValidStep;
 
-	FastForward(stepCount);
+	if (FastForward(stepCount) == stepCount)
+		return;
+
+	std::vector<StepTime> missingSteps;
+	missingSteps.reserve(stepCount);
+
+	for (StepTime step = m_LastValidStep + 1; step < m_CurrentStep; step++) {
+		if (!m_History[step].has_value())
+			missingSteps.push_back(step);
+	}
+
+	OnMissingLockSteps(missingSteps);
 }
 
 }  // namespace sim
diff --git a/src/td/simulation/ServerSimulation.cpp b/src/td/simulation/ServerSimulation.cpp
index bde94dd..311dff8 100644
--- a/src/td/simulation/ServerSimulation.cpp
+++ b/src/td/simulation/ServerSimulation.cpp
@@ -36,5 +36,17 @@ void ServerSimulation::Handle(const protocol::commands::PlaceTowerCommand& a_Pla
 	AddToCommandHistory(m_History[m_CurrentTime + LOCKSTEP_BUFFER_SIZE], a_PlaceTower);
 }
 
+protocol::packets::LockStepResponsePacket ServerSimulation::GetResponse(
+	const protocol::packets::LockStepRequestPacket& a_Missing) const {
+	std::map<StepTime, protocol::LockStep> result;
+
+	for (StepTime step : a_Missing->m_Missing) {
+		result.emplace(step, m_History[step]);
+	}
+
+	return {result};
+}
+
+
 }  // namespace sim
 }  // namespace td