diff --git a/binaries/data/mods/public/gui/gamesettings/GameSettings.js b/binaries/data/mods/public/gui/gamesettings/GameSettings.js
index 2d3572dc90..5d4ba8a30e 100644
--- a/binaries/data/mods/public/gui/gamesettings/GameSettings.js
+++ b/binaries/data/mods/public/gui/gamesettings/GameSettings.js
@@ -28,10 +28,6 @@ class GameSettings
 			"value": Engine.HasNetClient(),
 		});
 
-		Object.defineProperty(this, "isController", {
-			"value": !this.isNetworked || Engine.IsNetController(),
-		});
-
 		// Load attributes as regular enumerable (i.e. iterable) properties.
 		for (let comp in GameSettings.prototype.Attributes)
 		{
@@ -86,15 +82,6 @@ class GameSettings
 				this[comp].fromInitAttributes(attribs);
 	}
 
-	/**
-	 * Send the game settings to the server.
-	 */
-	setNetworkInitAttributes()
-	{
-		if (this.isNetworked && this.isController)
-			Engine.SetNetworkInitAttributes(this.toInitAttributes());
-	}
-
 	/**
 	 * Change "random" settings into their proper settings.
 	 */
@@ -128,7 +115,6 @@ class GameSettings
 		this.pickRandomItems();
 
 		Engine.SetRankedGame(this.rating.enabled);
-		this.setNetworkInitAttributes();
 
 		// Replace player names with the real players.
 		for (let guid in playerAssignments)
@@ -137,7 +123,7 @@ class GameSettings
 
 		// NB: for multiplayer support, the clients must be listening to "start" net messages.
 		if (this.isNetworked)
-			Engine.StartNetworkGame();
+			Engine.StartNetworkGame(this.toInitAttributes());
 		else
 			Engine.StartGame(this.toInitAttributes(), playerAssignments.local.player);
 	}
diff --git a/binaries/data/mods/public/gui/gamesetup/Controls/GameSettingsControl.js b/binaries/data/mods/public/gui/gamesetup/Controls/GameSettingsControl.js
index ea8a0cf7c5..f6052a414c 100644
--- a/binaries/data/mods/public/gui/gamesetup/Controls/GameSettingsControl.js
+++ b/binaries/data/mods/public/gui/gamesetup/Controls/GameSettingsControl.js
@@ -3,16 +3,22 @@
  */
 class GameSettingsControl
 {
-	constructor(setupWindow, netMessages, startGameControl, mapCache)
+	constructor(setupWindow, netMessages, startGameControl, playerAssignmentsControl, mapCache)
 	{
+		this.setupWindow = setupWindow;
 		this.startGameControl = startGameControl;
 		this.mapCache = mapCache;
 		this.gameSettingsFile = new GameSettingsFile(this);
 
 		this.guiData = new GameSettingsGuiData();
 
+		// When joining a game, the complete set of attributes
+		// may not have been received yet.
+		this.loading = true;
+
 		this.updateLayoutHandlers = new Set();
 		this.settingsChangeHandlers = new Set();
+		this.loadingChangeHandlers = new Set();
 
 		setupWindow.registerLoadHandler(this.onLoad.bind(this));
 		setupWindow.registerGetHotloadDataHandler(this.onGetHotloadData.bind(this));
@@ -21,10 +27,16 @@ class GameSettingsControl
 
 		setupWindow.registerClosePageHandler(this.onClose.bind(this));
 
+		if (g_IsController && g_IsNetworked)
+			playerAssignmentsControl.registerClientJoinHandler(this.onClientJoin.bind(this));
+
 		if (g_IsNetworked)
 			netMessages.registerNetMessageHandler("gamesetup", this.onGamesetupMessage.bind(this));
 	}
 
+	/**
+	 * @param handler will be called when the layout needs to be updated.
+	 */
 	registerUpdateLayoutHandler(handler)
 	{
 		this.updateLayoutHandlers.add(handler);
@@ -39,6 +51,14 @@ class GameSettingsControl
 		this.settingsChangeHandlers.add(handler);
 	}
 
+	/**
+	 * @param handler will be called when the 'loading' state change.
+	 */
+	registerLoadingChangeHandler(handler)
+	{
+		this.loadingChangeHandlers.add(handler);
+	}
+
 	onLoad(initData, hotloadData)
 	{
 		if (hotloadData)
@@ -52,6 +72,10 @@ class GameSettingsControl
 
 		this.updateLayout();
 		this.setNetworkInitAttributes();
+
+		// If we are the controller, we are done loading.
+		if (hotloadData || !g_IsNetworked || g_IsController)
+			this.setLoading(false);
 	}
 
 	onClose()
@@ -59,6 +83,37 @@ class GameSettingsControl
 		this.gameSettingsFile.saveFile();
 	}
 
+	onClientJoin()
+	{
+		/**
+		 * A note on network synchronization:
+		 * The net server does not keep the current state of attributes,
+		 * nor does it act like a message queue, so a new client
+		 * will only receive updates after they've joined.
+		 * In particular, new joiners start with no information,
+		 * so the controller must first send them a complete copy of the settings.
+		 * However, messages could be in-flight towards the controller,
+		 * but the new client may never receive these or have already received them,
+		 * leading to an ordering issue that might desync the new client.
+		 *
+		 * The simplest solution is to have the (single) controller
+		 * act as the single source of truth. Any other message must
+		 * first go through the controller, which will send updates.
+		 * This enforces the ordering of the controller.
+		 * In practical terms, if e.g. players controlling their own civ is implemented,
+		 * the message will need to be ignored by everyone but the controller,
+		 * and the controller will need to send an update once it rejects/accepts the changes,
+		 * which will then update the other clients.
+		 * Of course, the original client GUI may want to temporarily show a different state.
+		 * Note that the final attributes are sent on game start anyways, so any
+		 * synchronization issue that might happen at that point can be resolved.
+		 */
+		Engine.SendGameSetupMessage({
+			"type": "initial-update",
+			"initAttribs": this.getSettings()
+		});
+	}
+
 	onGetHotloadData(object)
 	{
 		object.initAttributes = this.getSettings();
@@ -66,10 +121,26 @@ class GameSettingsControl
 
 	onGamesetupMessage(message)
 	{
+		// For now, the controller only can send updates, so no need to listen to messages.
 		if (!message.data || g_IsController)
 			return;
 
-		this.parseSettings(message.data);
+		if (message.data.type !== "update" &&
+			message.data.type !== "initial-update")
+		{
+			error("Unknown message type " + message.data.type);
+			return;
+		}
+
+		if (message.data.type === "initial-update")
+		{
+			// Ignore initial updates if we've already received settings.
+			if (!this.loading)
+				return;
+			this.setLoading(false);
+		}
+
+		this.parseSettings(message.data.initAttribs);
 
 		// This assumes that messages aren't sent spuriously without changes
 		// (which is generally fair), but technically it would be good
@@ -98,6 +169,15 @@ class GameSettingsControl
 		g_GameSettings.fromInitAttributes(settings);
 	}
 
+	setLoading(loading)
+	{
+		if (this.loading === loading)
+			return;
+		this.loading = loading;
+		for (let handler of this.loadingChangeHandlers)
+			handler();
+	}
+
 	/**
 	 * This should be called whenever the GUI layout needs to be updated.
 	 * Triggers on the next GUI tick to avoid un-necessary layout.
@@ -138,7 +218,12 @@ class GameSettingsControl
 			clearTimeout(this.timer);
 			delete this.timer;
 		}
-		g_GameSettings.setNetworkInitAttributes();
+		// See note in onClientJoin on network synchronization.
+		if (g_IsController)
+			Engine.SendGameSetupMessage({
+				"type": "update",
+				"initAttribs": this.getSettings()
+			});
 	}
 
 	onLaunchGame()
diff --git a/binaries/data/mods/public/gui/gamesetup/NetMessages/GameRegisterStanza.js b/binaries/data/mods/public/gui/gamesetup/NetMessages/GameRegisterStanza.js
index 0e8be21c79..136a48f34e 100644
--- a/binaries/data/mods/public/gui/gamesetup/NetMessages/GameRegisterStanza.js
+++ b/binaries/data/mods/public/gui/gamesetup/NetMessages/GameRegisterStanza.js
@@ -5,7 +5,7 @@
  */
 class GameRegisterStanza
 {
-	constructor(initData, setupWindow, netMessages, gameSettingsControl, mapCache)
+	constructor(initData, setupWindow, netMessages, mapCache)
 	{
 		this.mapCache = mapCache;
 
diff --git a/binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/GameSettings/Single/Dropdowns/MapFilter.js b/binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/GameSettings/Single/Dropdowns/MapFilter.js
index 25b8a82d7e..1712bc5fe6 100644
--- a/binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/GameSettings/Single/Dropdowns/MapFilter.js
+++ b/binaries/data/mods/public/gui/gamesetup/Pages/GameSetupPage/GameSettings/Single/Dropdowns/MapFilter.js
@@ -6,6 +6,7 @@ GameSettingControls.MapFilter = class MapFilter extends GameSettingControlDropdo
 
 		this.values = undefined;
 
+		this.gameSettingsControl.guiData.mapFilter.watch(() => this.render(), ["filter"]);
 		g_GameSettings.map.watch(() => this.checkMapTypeChange(), ["type"]);
 	}
 
@@ -37,9 +38,13 @@ GameSettingControls.MapFilter = class MapFilter extends GameSettingControlDropdo
 			this.gameSettingsControl.guiData.mapFilter.filter = this.values.Name[this.values.Default];
 			this.gameSettingsControl.setNetworkInitAttributes();
 		}
+		this.render();
+	}
+
+	render()
+	{
 		// Index may have changed, reset.
 		this.setSelectedValue(this.gameSettingsControl.guiData.mapFilter.filter);
-
 		this.setHidden(!this.values);
 	}
 
diff --git a/binaries/data/mods/public/gui/gamesetup/Pages/LoadingPage/LoadingPage.js b/binaries/data/mods/public/gui/gamesetup/Pages/LoadingPage/LoadingPage.js
index 173373d54a..f28744fb23 100644
--- a/binaries/data/mods/public/gui/gamesetup/Pages/LoadingPage/LoadingPage.js
+++ b/binaries/data/mods/public/gui/gamesetup/Pages/LoadingPage/LoadingPage.js
@@ -7,19 +7,16 @@ SetupWindowPages.LoadingPage = class
 {
 	constructor(setupWindow)
 	{
-		if (g_IsNetworked)
-			setupWindow.controls.netMessages.registerNetMessageHandler("gamesetup", this.hideLoadingPage.bind(this));
-		else
-			this.hideLoadingPage();
+		setupWindow.controls.gameSettingsControl.registerLoadingChangeHandler((loading) => this.onLoadingChange(loading));
 	}
 
-	hideLoadingPage()
+	onLoadingChange(loading)
 	{
 		let loadingPage = Engine.GetGUIObjectByName("loadingPage");
-		if (loadingPage.hidden)
+		if (loadingPage.hidden === !loading)
 			return;
 
-		loadingPage.hidden = true;
-		Engine.GetGUIObjectByName("setupWindow").hidden = false;
+		loadingPage.hidden = !loading;
+		Engine.GetGUIObjectByName("setupWindow").hidden = loading;
 	}
-}
+};
diff --git a/binaries/data/mods/public/gui/gamesetup/SetupWindow.js b/binaries/data/mods/public/gui/gamesetup/SetupWindow.js
index a0070463a5..838b7df4ce 100644
--- a/binaries/data/mods/public/gui/gamesetup/SetupWindow.js
+++ b/binaries/data/mods/public/gui/gamesetup/SetupWindow.js
@@ -29,10 +29,10 @@ class SetupWindow
 		let netMessages = new NetMessages(this);
 		let startGameControl = new StartGameControl(netMessages);
 		let mapFilters = new MapFilters(mapCache);
-		let gameSettingsControl = new GameSettingsControl(this, netMessages, startGameControl, mapCache);
 		let gameRegisterStanza = Engine.HasXmppClient() &&
-			new GameRegisterStanza(initData, this, netMessages, gameSettingsControl, mapCache);
+			new GameRegisterStanza(initData, this, netMessages, mapCache);
 		let playerAssignmentsControl = new PlayerAssignmentsControl(this, netMessages, gameRegisterStanza);
+		let gameSettingsControl = new GameSettingsControl(this, netMessages, startGameControl, playerAssignmentsControl, mapCache);
 		let readyControl = new ReadyControl(netMessages, gameSettingsControl, startGameControl, playerAssignmentsControl);
 
 		// These class instances control central data and do not manage any GUI Object.
diff --git a/binaries/data/mods/public/gui/gamesetup_mp/gamesetup_mp.js b/binaries/data/mods/public/gui/gamesetup_mp/gamesetup_mp.js
index b6b0e31a98..f269191c2b 100644
--- a/binaries/data/mods/public/gui/gamesetup_mp/gamesetup_mp.js
+++ b/binaries/data/mods/public/gui/gamesetup_mp/gamesetup_mp.js
@@ -21,7 +21,6 @@ var g_ServerHasPassword = false;
 var g_ServerId;
 
 var g_IsRejoining = false;
-var g_InitAttributes; // used when rejoining
 var g_PlayerAssignments; // used when rejoining
 var g_UserRating;
 
@@ -226,25 +225,13 @@ function pollAndHandleNetworkClient()
 				}
 				break;
 
-			case "gamesetup":
-				g_InitAttributes = message.data;
-				break;
-
 			case "players":
 				g_PlayerAssignments = message.newAssignments;
 				break;
 
 			case "start":
-				// Copy playernames from initial player assignment to the settings
-				for (let guid in g_PlayerAssignments)
-				{
-					let player = g_PlayerAssignments[guid];
-					if (player.player > 0)	// not observer or GAIA
-						g_InitAttributes.settings.PlayerData[player.player - 1].Name = player.name;
-				}
-
 				Engine.SwitchGuiPage("page_loading.xml", {
-					"attribs": g_InitAttributes,
+					"attribs": message.initAttributes,
 					"isRejoining": g_IsRejoining,
 					"playerAssignments": g_PlayerAssignments
 				});
diff --git a/source/network/NetClient.cpp b/source/network/NetClient.cpp
index de263f986f..3e4e6d7500 100644
--- a/source/network/NetClient.cpp
+++ b/source/network/NetClient.cpp
@@ -58,8 +58,8 @@ class CNetFileReceiveTask_ClientRejoin : public CNetFileReceiveTask
 {
 	NONCOPYABLE(CNetFileReceiveTask_ClientRejoin);
 public:
-	CNetFileReceiveTask_ClientRejoin(CNetClient& client)
-		: m_Client(client)
+	CNetFileReceiveTask_ClientRejoin(CNetClient& client, const CStr& initAttribs)
+		: m_Client(client), m_InitAttributes(initAttribs)
 	{
 	}
 
@@ -72,18 +72,19 @@ public:
 
 		// Pretend the server told us to start the game
 		CGameStartMessage start;
+		start.m_InitAttributes = m_InitAttributes;
 		m_Client.HandleMessage(&start);
 	}
 
 private:
 	CNetClient& m_Client;
+	CStr m_InitAttributes;
 };
 
 CNetClient::CNetClient(CGame* game) :
 	m_Session(NULL),
 	m_UserName(L"anonymous"),
 	m_HostID((u32)-1), m_ClientTurnManager(NULL), m_Game(game),
-	m_GameAttributes(game->GetSimulation2()->GetScriptInterface().GetGeneralJSContext()),
 	m_LastConnectionCheck(0),
 	m_ServerAddress(),
 	m_ServerPort(0),
@@ -103,9 +104,7 @@ CNetClient::CNetClient(CGame* game) :
 	AddTransition(NCS_HANDSHAKE, (uint)NMT_SERVER_HANDSHAKE_RESPONSE, NCS_AUTHENTICATE, (void*)&OnHandshakeResponse, context);
 
 	AddTransition(NCS_AUTHENTICATE, (uint)NMT_AUTHENTICATE, NCS_AUTHENTICATE, (void*)&OnAuthenticateRequest, context);
-	AddTransition(NCS_AUTHENTICATE, (uint)NMT_AUTHENTICATE_RESULT, NCS_INITIAL_GAMESETUP, (void*)&OnAuthenticate, context);
-
-	AddTransition(NCS_INITIAL_GAMESETUP, (uint)NMT_GAME_SETUP, NCS_PREGAME, (void*)&OnGameSetup, context);
+	AddTransition(NCS_AUTHENTICATE, (uint)NMT_AUTHENTICATE_RESULT, NCS_PREGAME, (void*)&OnAuthenticate, context);
 
 	AddTransition(NCS_PREGAME, (uint)NMT_CHAT, NCS_PREGAME, (void*)&OnChat, context);
 	AddTransition(NCS_PREGAME, (uint)NMT_READY, NCS_PREGAME, (void*)&OnReady, context);
@@ -474,9 +473,10 @@ void CNetClient::SendClearAllReadyMessage()
 	SendMessage(&clearAllReady);
 }
 
-void CNetClient::SendStartGameMessage()
+void CNetClient::SendStartGameMessage(const CStr& initAttribs)
 {
 	CGameStartMessage gameStart;
+	gameStart.m_InitAttributes = initAttribs;
 	SendMessage(&gameStart);
 }
 
@@ -717,8 +717,6 @@ bool CNetClient::OnGameSetup(void* context, CFsmEvent* event)
 	CNetClient* client = static_cast<CNetClient*>(context);
 	CGameSetupMessage* message = static_cast<CGameSetupMessage*>(event->GetParamRef());
 
-	client->m_GameAttributes = message->m_Data;
-
 	client->PushGuiMessage(
 		"type", "gamesetup",
 		"data", message->m_Data);
@@ -759,6 +757,7 @@ bool CNetClient::OnGameStart(void* context, CFsmEvent* event)
 	ENSURE(event->GetType() == (uint)NMT_GAME_START);
 
 	CNetClient* client = static_cast<CNetClient*>(context);
+	CGameStartMessage* message = static_cast<CGameStartMessage*>(event->GetParamRef());
 
 	// Find the player assigned to our GUID
 	int player = -1;
@@ -768,10 +767,17 @@ bool CNetClient::OnGameStart(void* context, CFsmEvent* event)
 	client->m_ClientTurnManager = new CNetClientTurnManager(
 			*client->m_Game->GetSimulation2(), *client, client->m_HostID, client->m_Game->GetReplayLogger());
 
-	client->m_Game->SetPlayerID(player);
-	client->m_Game->StartGame(&client->m_GameAttributes, "");
+	// Parse init attributes.
+	const ScriptInterface& scriptInterface = client->m_Game->GetSimulation2()->GetScriptInterface();
+	ScriptRequest rq(scriptInterface);
+	JS::RootedValue initAttribs(rq.cx);
+	scriptInterface.ParseJSON(message->m_InitAttributes, &initAttribs);
 
-	client->PushGuiMessage("type", "start");
+	client->m_Game->SetPlayerID(player);
+	client->m_Game->StartGame(&initAttribs, "");
+
+	client->PushGuiMessage("type", "start",
+						   "initAttributes", initAttribs);
 
 	return true;
 }
@@ -782,9 +788,11 @@ bool CNetClient::OnJoinSyncStart(void* context, CFsmEvent* event)
 
 	CNetClient* client = static_cast<CNetClient*>(context);
 
+	CJoinSyncStartMessage* joinSyncStartMessage = (CJoinSyncStartMessage*)event->GetParamRef();
+
 	// The server wants us to start downloading the game state from it, so do so
 	client->m_Session->GetFileTransferer().StartTask(
-		shared_ptr<CNetFileReceiveTask>(new CNetFileReceiveTask_ClientRejoin(*client))
+		shared_ptr<CNetFileReceiveTask>(new CNetFileReceiveTask_ClientRejoin(*client, joinSyncStartMessage->m_InitAttributes))
 	);
 
 	return true;
diff --git a/source/network/NetClient.h b/source/network/NetClient.h
index dbd36dd0e3..a8f7b8e86c 100644
--- a/source/network/NetClient.h
+++ b/source/network/NetClient.h
@@ -44,7 +44,6 @@ enum
 	NCS_CONNECT,
 	NCS_HANDSHAKE,
 	NCS_AUTHENTICATE,
-	NCS_INITIAL_GAMESETUP,
 	NCS_PREGAME,
 	NCS_LOADING,
 	NCS_JOIN_SYNCING,
@@ -231,7 +230,7 @@ public:
 
 	void SendClearAllReadyMessage();
 
-	void SendStartGameMessage();
+	void SendStartGameMessage(const CStr& initAttribs);
 
 	/**
 	 * Call when the client has rejoined a running match and finished
@@ -326,9 +325,6 @@ private:
 	/// True if the player is currently rejoining or has already rejoined the game.
 	bool m_Rejoin;
 
-	/// Latest copy of game setup attributes heard from the server
-	JS::PersistentRootedValue m_GameAttributes;
-
 	/// Latest copy of player assignments heard from the server
 	PlayerAssignmentMap m_PlayerAssignments;
 
diff --git a/source/network/NetMessages.h b/source/network/NetMessages.h
index ff65319547..f2259d0a7e 100644
--- a/source/network/NetMessages.h
+++ b/source/network/NetMessages.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2020 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -28,7 +28,7 @@
 
 #define PS_PROTOCOL_MAGIC                         0x5073013f	// 'P', 's', 0x01, '?'
 #define PS_PROTOCOL_MAGIC_RESPONSE                0x50630121	// 'P', 'c', 0x01, '!'
-#define PS_PROTOCOL_VERSION                       0x01010017	// Arbitrary protocol
+#define PS_PROTOCOL_VERSION                       0x01010018	// Arbitrary protocol
 #define PS_DEFAULT_PORT                           0x5073		// 'P', 's'
 
 // Set when lobby authentication is required. Used in the SrvHandshakeResponseMessage.
@@ -174,6 +174,7 @@ START_NMT_CLASS_(FileTransferAck, NMT_FILE_TRANSFER_ACK)
 END_NMT_CLASS()
 
 START_NMT_CLASS_(JoinSyncStart, NMT_JOIN_SYNC_START)
+	NMT_FIELD(CStr, m_InitAttributes)
 END_NMT_CLASS()
 
 START_NMT_CLASS_(Rejoined, NMT_REJOINED)
@@ -213,6 +214,7 @@ START_NMT_CLASS_(LoadedGame, NMT_LOADED_GAME)
 END_NMT_CLASS()
 
 START_NMT_CLASS_(GameStart, NMT_GAME_START)
+	NMT_FIELD(CStr, m_InitAttributes)
 END_NMT_CLASS()
 
 START_NMT_CLASS_(EndCommandBatch, NMT_END_COMMAND_BATCH)
diff --git a/source/network/NetServer.cpp b/source/network/NetServer.cpp
index d6546990d1..8f4114a6bf 100644
--- a/source/network/NetServer.cpp
+++ b/source/network/NetServer.cpp
@@ -126,7 +126,10 @@ public:
 		// they'll race and get whichever happens to be the latest received by the server,
 		// which should still work but isn't great
 		m_Server.m_JoinSyncFile = m_Buffer;
+
+		// Send the init attributes alongside - these should be correct since the game should be started.
 		CJoinSyncStartMessage message;
+		message.m_InitAttributes = m_Server.GetScriptInterface().StringifyJSON(&m_Server.m_InitAttributes);
 		session->SendMessage(&message);
 	}
 
@@ -411,7 +414,7 @@ void CNetServerWorker::Run()
 	// We create a new ScriptContext for this network thread, with a single ScriptInterface.
 	shared_ptr<ScriptContext> netServerContext = ScriptContext::CreateContext();
 	m_ScriptInterface = new ScriptInterface("Engine", "Net server", netServerContext);
-	m_GameAttributes.init(m_ScriptInterface->GetGeneralJSContext(), JS::UndefinedValue());
+	m_InitAttributes.init(m_ScriptInterface->GetGeneralJSContext(), JS::UndefinedValue());
 
 	while (true)
 	{
@@ -420,7 +423,7 @@ void CNetServerWorker::Run()
 
 		// Implement autostart mode
 		if (m_State == SERVER_STATE_PREGAME && (int)m_PlayerAssignments.size() == m_AutostartPlayers)
-			StartGame();
+			StartGame(m_ScriptInterface->StringifyJSON(&m_InitAttributes));
 
 		// Update profiler stats
 		m_Stats->LatchHostState(m_Host);
@@ -454,25 +457,26 @@ bool CNetServerWorker::RunStep()
 			return false;
 
 		newStartGame.swap(m_StartGameQueue);
-		newGameAttributes.swap(m_GameAttributesQueue);
+		newGameAttributes.swap(m_InitAttributesQueue);
 		newLobbyAuths.swap(m_LobbyAuthQueue);
 		newTurnLength.swap(m_TurnLengthQueue);
 	}
 
 	if (!newGameAttributes.empty())
 	{
-		JS::RootedValue gameAttributesVal(rq.cx);
-		GetScriptInterface().ParseJSON(newGameAttributes.back(), &gameAttributesVal);
-		UpdateGameAttributes(&gameAttributesVal);
+		if (m_State != SERVER_STATE_UNCONNECTED && m_State != SERVER_STATE_PREGAME)
+			LOGERROR("NetServer: Init Attributes cannot be changed after the server starts loading.");
+		else
+		{
+			JS::RootedValue gameAttributesVal(rq.cx);
+			GetScriptInterface().ParseJSON(newGameAttributes.back(), &gameAttributesVal);
+			m_InitAttributes = gameAttributesVal;
+		}
 	}
 
 	if (!newTurnLength.empty())
 		SetTurnLength(newTurnLength.back());
 
-	// Do StartGame last, so we have the most up-to-date game attributes when we start
-	if (!newStartGame.empty())
-		StartGame();
-
 	while (!newLobbyAuths.empty())
 	{
 		const std::pair<CStr, CStr>& auth = newLobbyAuths.back();
@@ -690,7 +694,7 @@ void CNetServerWorker::SetupSession(CNetServerSession* session)
 	session->AddTransition(NSS_PREGAME, (uint)NMT_GAME_SETUP, NSS_PREGAME, (void*)&OnGameSetup, context);
 	session->AddTransition(NSS_PREGAME, (uint)NMT_ASSIGN_PLAYER, NSS_PREGAME, (void*)&OnAssignPlayer, context);
 	session->AddTransition(NSS_PREGAME, (uint)NMT_KICKED, NSS_PREGAME, (void*)&OnKickPlayer, context);
-	session->AddTransition(NSS_PREGAME, (uint)NMT_GAME_START, NSS_PREGAME, (void*)&OnStartGame, context);
+	session->AddTransition(NSS_PREGAME, (uint)NMT_GAME_START, NSS_PREGAME, (void*)&OnGameStart, context);
 	session->AddTransition(NSS_PREGAME, (uint)NMT_LOADED_GAME, NSS_INGAME, (void*)&OnLoadedGame, context);
 
 	session->AddTransition(NSS_JOIN_SYNCING, (uint)NMT_KICKED, NSS_JOIN_SYNCING, (void*)&OnKickPlayer, context);
@@ -729,10 +733,6 @@ void CNetServerWorker::OnUserJoin(CNetServerSession* session)
 {
 	AddPlayer(session->GetGUID(), session->GetUserName());
 
-	CGameSetupMessage gameSetupMessage(GetScriptInterface());
-	gameSetupMessage.m_Data = m_GameAttributes;
-	session->SendMessage(&gameSetupMessage);
-
 	CPlayerAssignmentMessage assignMessage;
 	ConstructPlayerAssignmentMessage(assignMessage);
 	session->SendMessage(&assignMessage);
@@ -1145,6 +1145,8 @@ bool CNetServerWorker::OnAuthenticate(void* context, CFsmEvent* event)
 
 	if (isRejoining)
 	{
+		ENSURE(server.m_State != SERVER_STATE_UNCONNECTED && server.m_State != SERVER_STATE_PREGAME);
+
 		// Request a copy of the current game state from an existing player,
 		// so we can send it on to the new player
 
@@ -1176,7 +1178,7 @@ bool CNetServerWorker::OnSimulationCommand(void* context, CFsmEvent* event)
 	const ScriptInterface& scriptInterface = server.GetScriptInterface();
 	ScriptRequest rq(scriptInterface);
 	JS::RootedValue settings(rq.cx);
-	scriptInterface.GetProperty(server.m_GameAttributes, "settings", &settings);
+	scriptInterface.GetProperty(server.m_InitAttributes, "settings", &settings);
 	if (scriptInterface.HasProperty(settings, "CheatsEnabled"))
 		scriptInterface.GetProperty(settings, "CheatsEnabled", cheatsEnabled);
 
@@ -1288,10 +1290,14 @@ bool CNetServerWorker::OnGameSetup(void* context, CFsmEvent* event)
 	if (server.m_State != SERVER_STATE_PREGAME)
 		return true;
 
+	// Only the controller is allowed to send game setup updates.
+	// TODO: it would be good to allow other players to request changes to some settings,
+	// e.g. their civilisation.
+	// Possibly this should use another message, to enforce a single source of truth.
 	if (session->GetGUID() == server.m_ControllerGUID)
 	{
 		CGameSetupMessage* message = (CGameSetupMessage*)event->GetParamRef();
-		server.UpdateGameAttributes(&(message->m_Data));
+		server.Broadcast(message, { NSS_PREGAME });
 	}
 	return true;
 }
@@ -1310,15 +1316,17 @@ bool CNetServerWorker::OnAssignPlayer(void* context, CFsmEvent* event)
 	return true;
 }
 
-bool CNetServerWorker::OnStartGame(void* context, CFsmEvent* event)
+bool CNetServerWorker::OnGameStart(void* context, CFsmEvent* event)
 {
 	ENSURE(event->GetType() == (uint)NMT_GAME_START);
 	CNetServerSession* session = (CNetServerSession*)context;
 	CNetServerWorker& server = session->GetServer();
 
-	if (session->GetGUID() == server.m_ControllerGUID)
-		server.StartGame();
+	if (session->GetGUID() != server.m_ControllerGUID)
+		return true;
 
+	CGameStartMessage* message = (CGameStartMessage*)event->GetParamRef();
+	server.StartGame(message->m_InitAttributes);
 	return true;
 }
 
@@ -1510,7 +1518,7 @@ bool CNetServerWorker::CheckGameLoadStatus(CNetServerSession* changedSession)
 	return true;
 }
 
-void CNetServerWorker::StartGame()
+void CNetServerWorker::StartGame(const CStr& initAttribs)
 {
 	for (std::pair<const CStr, PlayerAssignment>& player : m_PlayerAssignments)
 		if (player.second.m_Enabled && player.second.m_PlayerID != -1 && player.second.m_Status == 0)
@@ -1526,9 +1534,6 @@ void CNetServerWorker::StartGame()
 
 	m_State = SERVER_STATE_LOADING;
 
-	// Send the final setup state to all clients
-	UpdateGameAttributes(&m_GameAttributes);
-
 	// Remove players and observers that are not present when the game starts
 	for (PlayerAssignmentMap::iterator it = m_PlayerAssignments.begin(); it != m_PlayerAssignments.end();)
 		if (it->second.m_Enabled)
@@ -1538,22 +1543,14 @@ void CNetServerWorker::StartGame()
 
 	SendPlayerAssignments();
 
+	// Update init attributes. They should no longer change.
+	m_ScriptInterface->ParseJSON(initAttribs, &m_InitAttributes);
+
 	CGameStartMessage gameStart;
+	gameStart.m_InitAttributes = initAttribs;
 	Broadcast(&gameStart, { NSS_PREGAME });
 }
 
-void CNetServerWorker::UpdateGameAttributes(JS::MutableHandleValue attrs)
-{
-	m_GameAttributes = attrs;
-
-	if (!m_Host)
-		return;
-
-	CGameSetupMessage gameSetupMessage(GetScriptInterface());
-	gameSetupMessage.m_Data = m_GameAttributes;
-	Broadcast(&gameSetupMessage, { NSS_PREGAME });
-}
-
 CStrW CNetServerWorker::SanitisePlayerName(const CStrW& original)
 {
 	const size_t MAX_LENGTH = 32;
@@ -1694,14 +1691,14 @@ void CNetServer::StartGame()
 	m_Worker->m_StartGameQueue.push_back(true);
 }
 
-void CNetServer::UpdateGameAttributes(JS::MutableHandleValue attrs, const ScriptInterface& scriptInterface)
+void CNetServer::UpdateInitAttributes(JS::MutableHandleValue attrs, const ScriptInterface& scriptInterface)
 {
 	// Pass the attributes as JSON, since that's the easiest safe
 	// cross-thread way of passing script data
 	std::string attrsJSON = scriptInterface.StringifyJSON(attrs, false);
 
 	std::lock_guard<std::mutex> lock(m_Worker->m_WorkerMutex);
-	m_Worker->m_GameAttributesQueue.push_back(attrsJSON);
+	m_Worker->m_InitAttributesQueue.push_back(attrsJSON);
 }
 
 void CNetServer::OnLobbyAuth(const CStr& name, const CStr& token)
diff --git a/source/network/NetServer.h b/source/network/NetServer.h
index f2045fc463..cc677b6fe0 100644
--- a/source/network/NetServer.h
+++ b/source/network/NetServer.h
@@ -125,16 +125,16 @@ public:
 
 	/**
 	 * Call from the GUI to asynchronously notify all clients that they should start loading the game.
+	 * UpdateInitAttributes must be called at least once.
 	 */
 	void StartGame();
 
 	/**
 	 * Call from the GUI to update the game setup attributes.
-	 * This must be called at least once before starting the game.
-	 * The changes will be asynchronously propagated to all clients.
-	 * @param attrs game attributes, in the script context of scriptInterface
+	 * The changes won't be propagated to clients until game start.
+	 * @param attrs init attributes, in the script context of scriptInterface
 	 */
-	void UpdateGameAttributes(JS::MutableHandleValue attrs, const ScriptInterface& scriptInterface);
+	void UpdateInitAttributes(JS::MutableHandleValue attrs, const ScriptInterface& scriptInterface);
 
 	/**
 	 * Set the turn length to a fixed value.
@@ -237,25 +237,15 @@ private:
 	bool SetupConnection(const u16 port);
 
 	/**
-	 * Call from the GUI to update the player assignments.
 	 * The given GUID will be (re)assigned to the given player ID.
 	 * Any player currently using that ID will be unassigned.
-	 * The changes will be propagated to all clients.
 	 */
 	void AssignPlayer(int playerID, const CStr& guid);
 
 	/**
-	 * Call from the GUI to notify all clients that they should start loading the game.
+	 * Switch in game mode and notify all clients to start the game.
 	 */
-	void StartGame();
-
-	/**
-	 * Call from the GUI to update the game setup attributes.
-	 * This must be called at least once before starting the game.
-	 * The changes will be propagated to all clients.
-	 * @param attrs game attributes, in the script context of GetScriptInterface()
-	 */
-	void UpdateGameAttributes(JS::MutableHandleValue attrs);
+	void StartGame(const CStr& initAttribs);
 
 	/**
 	 * Make a player name 'nicer' by limiting the length and removing forbidden characters etc.
@@ -268,7 +258,7 @@ private:
 	CStrW DeduplicatePlayerName(const CStrW& original);
 
 	/**
-	 * Get the script context used for game attributes.
+	 * Get the script context used for init attributes.
 	 */
 	const ScriptInterface& GetScriptInterface();
 
@@ -301,7 +291,7 @@ private:
 	static bool OnClearAllReady(void* context, CFsmEvent* event);
 	static bool OnGameSetup(void* context, CFsmEvent* event);
 	static bool OnAssignPlayer(void* context, CFsmEvent* event);
-	static bool OnStartGame(void* context, CFsmEvent* event);
+	static bool OnGameStart(void* context, CFsmEvent* event);
 	static bool OnLoadedGame(void* context, CFsmEvent* event);
 	static bool OnJoinSyncingLoadedGame(void* context, CFsmEvent* event);
 	static bool OnRejoined(void* context, CFsmEvent* event);
@@ -330,7 +320,7 @@ private:
 
 	/**
 	 * Internal script context for (de)serializing script messages,
-	 * and for storing game attributes.
+	 * and for storing init attributes.
 	 * (TODO: we shouldn't bother deserializing (except for debug printing of messages),
 	 * we should just forward messages blindly and efficiently.)
 	 */
@@ -339,9 +329,11 @@ private:
 	PlayerAssignmentMap m_PlayerAssignments;
 
 	/**
-	 * Stores the most current game attributes.
+	 * Stores the most current init attributes.
+	 * NB: this is not guaranteed to be up-to-date until the server is LOADING or INGAME.
+	 * At that point, the settings are frozen and ought to be identical to the simulation Init Attributes.
 	 */
-	JS::PersistentRootedValue m_GameAttributes;
+	JS::PersistentRootedValue m_InitAttributes;
 
 	int m_AutostartPlayers;
 
@@ -424,7 +416,7 @@ private:
 
 	// Queues for messages sent by the game thread (protected by m_WorkerMutex):
 	std::vector<bool> m_StartGameQueue;
-	std::vector<std::string> m_GameAttributesQueue;
+	std::vector<std::string> m_InitAttributesQueue;
 	std::vector<std::pair<CStr, CStr>> m_LobbyAuthQueue;
 	std::vector<u32> m_TurnLengthQueue;
 };
diff --git a/source/network/scripting/JSInterface_Network.cpp b/source/network/scripting/JSInterface_Network.cpp
index e4d9493a5b..b845609f79 100644
--- a/source/network/scripting/JSInterface_Network.cpp
+++ b/source/network/scripting/JSInterface_Network.cpp
@@ -221,7 +221,7 @@ JS::Value PollNetworkClient(const ScriptInterface& scriptInterface)
 	return scriptInterface.CloneValueFromOtherCompartment(g_NetClient->GetScriptInterface(), pollNet);
 }
 
-void SetNetworkInitAttributes(const ScriptInterface& scriptInterface, JS::HandleValue attribs1)
+void SendGameSetupMessage(const ScriptInterface& scriptInterface, JS::HandleValue attribs1)
 {
 	ENSURE(g_NetClient);
 
@@ -267,10 +267,14 @@ void ClearAllPlayerReady ()
 	g_NetClient->SendClearAllReadyMessage();
 }
 
-void StartNetworkGame()
+void StartNetworkGame(const ScriptInterface& scriptInterface, JS::HandleValue attribs1)
 {
 	ENSURE(g_NetClient);
-	g_NetClient->SendStartGameMessage();
+
+	// TODO: This is a workaround because we need to pass a MutableHandle to a JSAPI functions somewhere (with no obvious reason).
+	ScriptRequest rq(scriptInterface);
+	JS::RootedValue attribs(rq.cx, attribs1);
+	g_NetClient->SendStartGameMessage(scriptInterface.StringifyJSON(&attribs));
 }
 
 void SetTurnLength(int length)
@@ -293,7 +297,7 @@ void RegisterScriptFunctions(const ScriptRequest& rq)
 	ScriptFunction::Register<&DisconnectNetworkGame>(rq, "DisconnectNetworkGame");
 	ScriptFunction::Register<&GetPlayerGUID>(rq, "GetPlayerGUID");
 	ScriptFunction::Register<&PollNetworkClient>(rq, "PollNetworkClient");
-	ScriptFunction::Register<&SetNetworkInitAttributes>(rq, "SetNetworkInitAttributes");
+	ScriptFunction::Register<&SendGameSetupMessage>(rq, "SendGameSetupMessage");
 	ScriptFunction::Register<&AssignNetworkPlayer>(rq, "AssignNetworkPlayer");
 	ScriptFunction::Register<&KickPlayer>(rq, "KickPlayer");
 	ScriptFunction::Register<&SendNetworkChat>(rq, "SendNetworkChat");
diff --git a/source/network/tests/test_Net.h b/source/network/tests/test_Net.h
index 7d1e593289..fb87a5e6af 100644
--- a/source/network/tests/test_Net.h
+++ b/source/network/tests/test_Net.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2020 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -161,7 +161,7 @@ public:
 			"mapPath", "maps/scenarios/",
 			"thing", "example");
 
-		server.UpdateGameAttributes(&attrs, scriptInterface);
+		server.UpdateInitAttributes(&attrs, scriptInterface);
 
 		CNetClient client1(&client1Game);
 		CNetClient client2(&client2Game);
@@ -240,7 +240,7 @@ public:
 			"mapPath", "maps/scenarios/",
 			"thing", "example");
 
-		server.UpdateGameAttributes(&attrs, scriptInterface);
+		server.UpdateInitAttributes(&attrs, scriptInterface);
 
 		CNetClient client1(&client1Game);
 		CNetClient client2(&client2Game);
diff --git a/source/ps/GameSetup/GameSetup.cpp b/source/ps/GameSetup/GameSetup.cpp
index 0997d4f2b9..c79367a7da 100644
--- a/source/ps/GameSetup/GameSetup.cpp
+++ b/source/ps/GameSetup/GameSetup.cpp
@@ -1563,7 +1563,7 @@ bool Autostart(const CmdLineArgs& args)
 
 		g_NetServer = new CNetServer(false, maxPlayers);
 		g_NetServer->SetControllerSecret(secret);
-		g_NetServer->UpdateGameAttributes(&attrs, scriptInterface);
+		g_NetServer->UpdateInitAttributes(&attrs, scriptInterface);
 
 		bool ok = g_NetServer->SetupConnection(PS_DEFAULT_PORT);
 		ENSURE(ok);