diff --git CMakeLists.txt CMakeLists.txt
index ccb567433..4de2adff6 100644
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -280,6 +280,11 @@ if (SIMUTRANS_USE_IP4_ONLY)
 	target_compile_definitions(simutrans PRIVATE USE_IP4_ONLY=1)
 endif ()
 
+if (SIMUTRANS_HEAVY_MODE)
+	target_compile_definitions(simutrans PRIVATE HEAVY_MODE=${SIMUTRANS_HEAVY_MODE})
+	target_sources(simutrans PRIVATE io/rdwr/adler32_stream.cc)
+endif ()
+
 if (SIMUTRANS_STEAM_BUILT)
 	target_compile_definitions(simutrans PRIVATE STEAM_BUILT=1)
 endif ()
diff --git Makefile Makefile
index a7862d834..8fd3e4a1c 100644
--- Makefile
+++ Makefile
@@ -222,6 +222,13 @@ ifdef PROFILE
   endif
 endif
 
+ifdef HEAVY_MODE
+  CFLAGS += -DHEAVY_MODE=$(HEAVY_MODE)
+  ifeq ($(shell expr $(HEAVY_MODE) \>= 1), 1)
+    SOURCES += io/rdwr/adler32_stream.cc
+  endif
+endif
+
 ifdef MULTI_THREAD
   ifeq ($(shell expr $(MULTI_THREAD) \>= 1), 1)
     CFLAGS += -DMULTI_THREAD
diff --git cmake/SimutransCompileOptions.cmake cmake/SimutransCompileOptions.cmake
index 10b7cd6fb..e4676f8f1 100644
--- cmake/SimutransCompileOptions.cmake
+++ cmake/SimutransCompileOptions.cmake
@@ -57,6 +57,14 @@ if(NOT SIMUTRANS_MSG_LEVEL)
 endif ()
 set_property(CACHE SIMUTRANS_MSG_LEVEL PROPERTY STRINGS 0 1 2 3 4)
 
+set(SIMUTRANS_HEAVY_MODE 0 CACHE STRING "\
+	Network desync debugging mode. Stores game state hash of client and server for every sync_step for comparison.\
+	VERY SLOW! Do not enable for normal builds.\
+	0 = disabled (default)\
+	1 = enabled\
+	2 = enabled; Additionally saves the game every sync_step to save/heavy/ and aborts on connection loss/desync.\
+					Using a ramdisk is recommended.")
+
 
 include(CheckCXXCompilerFlag)
 
diff --git cmake/SimutransSourceList.cmake cmake/SimutransSourceList.cmake
index 726627f2c..31ce70db7 100644
--- cmake/SimutransSourceList.cmake
+++ cmake/SimutransSourceList.cmake
@@ -1,4 +1,4 @@
-target_sources(simutrans PRIVATE 
+target_sources(simutrans PRIVATE
 		${CMAKE_SOURCE_DIR}/revision.h
 		bauer/brueckenbauer.cc
 		bauer/fabrikbauer.cc
diff --git config.template config.template
index e29b2cab8..0047816d5 100644
--- config.template
+++ config.template
@@ -36,6 +36,14 @@
 
 #WIN32_CONSOLE := 1 # adds a console for windows debugging
 
+# Network desync debugging mode. Stores game state hash of client and server for every sync_step for comparison.
+# VERY SLOW! Do not enable for normal builds.
+# 0 = disabled (default)
+# 1 = enabled
+# 2 = enabled; Additionally saves the game every sync_step to save/heavy/ and aborts on connection loss/desync.
+#              Using a ramdisk is recommended.
+#HEAVY_MODE := 0
+
 #MULTI_THREAD := 1 # Enable multithreading
 
 # using freetype for Truetype font support
diff --git dataobj/loadsave.cc dataobj/loadsave.cc
index 6e26935f1..b5ce62a08 100644
--- dataobj/loadsave.cc
+++ dataobj/loadsave.cc
@@ -1359,3 +1359,12 @@ uint32 loadsave_t::int_version(const char *version_text, char *pak_extension_str
 
 	return version;
 }
+
+
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+stream_loadsave_t::stream_loadsave_t(rdwr_stream_t *stream)
+{
+	this->stream = stream;
+	finfo.version = int_version(VERSION_NUMBER, NULL);
+}
+#endif
diff --git dataobj/loadsave.h dataobj/loadsave.h
index bfc2cda4b..bc1bad5af 100644
--- dataobj/loadsave.h
+++ dataobj/loadsave.h
@@ -199,11 +199,21 @@ public:
 };
 
 
-// this produced semicautomatic hierarchies
-class xml_tag_t {
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+class stream_loadsave_t : public loadsave_t
+{
+public:
+	stream_loadsave_t(rdwr_stream_t *stream);
+};
+#endif
+
+// this produces semi-automatic hierarchies
+class xml_tag_t
+{
 private:
 	loadsave_t *file;
 	const char *tag;
+
 public:
 	xml_tag_t( loadsave_t *f, const char *t ) : file(f), tag(t) { file->start_tag(tag); }
 	~xml_tag_t() { file->end_tag(tag); }
diff --git io/rdwr/adler32_stream.cc io/rdwr/adler32_stream.cc
new file mode 100644
index 000000000..6aa948343
--- /dev/null
+++ io/rdwr/adler32_stream.cc
@@ -0,0 +1,40 @@
+/*
+ * This file is part of the Simutrans project under the Artistic License.
+ * (see LICENSE.txt)
+ */
+
+#include "adler32_stream.h"
+
+#include "../../simdebug.h"
+
+#include <zlib.h>
+
+
+adler32_stream_t::adler32_stream_t() :
+	rdwr_stream_t(true)
+{
+	adler_checksum = adler32(0, NULL, 0);
+	status = STATUS_OK;
+}
+
+
+size_t adler32_stream_t::read(void *, size_t)
+{
+	dbg->fatal("adler32_stream_t::read", "Cannot reconstruct original message from checksum!");
+	return 0;
+}
+
+
+size_t adler32_stream_t::write(const void *buf, size_t len)
+{
+	adler_checksum = adler32(adler_checksum, static_cast<const uint8 *>(buf), len);
+	return len;
+}
+
+
+uint32 adler32_stream_t::get_hash()
+{
+	const uint32 result = adler_checksum;
+	adler_checksum = adler32(0, NULL, 0);
+	return result;
+}
diff --git io/rdwr/adler32_stream.h io/rdwr/adler32_stream.h
new file mode 100644
index 000000000..79f3663f5
--- /dev/null
+++ io/rdwr/adler32_stream.h
@@ -0,0 +1,35 @@
+/*
+ * This file is part of the Simutrans project under the Artistic License.
+ * (see LICENSE.txt)
+ */
+
+#ifndef IO_RDWR_ADLER32_STREAM_H
+#define IO_RDWR_ADLER32_STREAM_H
+
+
+#include "rdwr_stream.h"
+
+
+/// Computes the adler32 checksum over some data
+class adler32_stream_t : public rdwr_stream_t
+{
+public:
+	adler32_stream_t();
+
+public:
+	/// @copydoc rdwr_stream_t::write
+	size_t write(const void *buf, size_t len) OVERRIDE;
+
+	/// @returns the adler32 checksum of all the data written since the last get_hash()
+	uint32 get_hash();
+
+private:
+	/// DO NOT USE!
+	size_t read(void *buf, size_t len) OVERRIDE;
+
+private:
+	uint32 adler_checksum;
+};
+
+
+#endif
diff --git network/network.cc network/network.cc
index 564b7740f..a3a819345 100644
--- network/network.cc
+++ network/network.cc
@@ -755,7 +755,12 @@ bool network_receive_data( SOCKET sender, void *dest, const uint16 len, uint16 &
 		}
 		if (res == 0) {
 			// connection closed
-			dbg->warning("network_receive_data", "connection [%d] already closed", sender);
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 2
+			dbg->fatal(
+#else
+			dbg->warning(
+#endif
+				"network_receive_data", "Connection [%d] already closed", sender);
 			return false;
 		}
 		received += res;
diff --git simworld.cc simworld.cc
index f88e6e0dc..b305c21e1 100644
--- simworld.cc
+++ simworld.cc
@@ -105,6 +105,11 @@
 #include "player/ai_goods.h"
 #include "player/ai_scripted.h"
 
+#include "io/rdwr/adler32_stream.h"
+
+#include "pathes.h"
+
+
 // forward declaration - management of rotation for scripting
 namespace script_api
 {
@@ -7183,9 +7188,16 @@ void karte_t::do_network_world_command(network_world_command_t *nwc)
 		const int offset = server_checklist.print(buf, "server");
 		LCHKLST(server_sync_step).print(buf + offset, "client");
 		dbg->warning("karte_t:::do_network_world_command", "sync_step=%u  %s", server_sync_step, buf);
+
 		if(  LCHKLST(server_sync_step)!=server_checklist  ) {
-			dbg->warning("karte_t:::do_network_world_command", "disconnecting due to checklist mismatch" );
 			network_disconnect();
+
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 2
+			dbg->fatal(
+#else
+			dbg->warning(
+#endif
+				"karte_t:::do_network_world_command", "Disconnected due to checklist mismatch" );
 		}
 	}
 	else {
@@ -7243,6 +7255,23 @@ sint16 karte_t::get_sound_id(grund_t *gr)
 }
 
 
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 2
+static void heavy_rotate_saves(const char *prefix, uint32 sync_steps, uint32 num_to_keep)
+{
+	dr_mkdir( SAVE_PATH_X "heavy");
+
+	char name[128];
+	sprintf(name, SAVE_PATH_X "heavy/heavy-%s-%04d.sve", prefix, sync_steps);
+	world()->save(name, false, SERVER_SAVEGAME_VER_NR, true);
+
+	if (sync_steps >= num_to_keep) {
+		sprintf(name, SAVE_PATH_X "heavy/heavy-%s-%04d.sve", prefix, sync_steps - num_to_keep);
+		dr_remove(name);
+	}
+}
+#endif
+
+
 bool karte_t::interactive(uint32 quit_month)
 {
 
@@ -7381,7 +7410,16 @@ bool karte_t::interactive(uint32 quit_month)
 						network_frame_count = 0;
 					}
 					sync_steps = steps * settings.get_frames_per_step() + network_frame_count;
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+					LCHKLST(sync_steps) = checklist_t(get_gamestate_hash());
+
+#if HEAVY_MODE >= 2
+					heavy_rotate_saves(env_t::server ? "server" : "client", sync_steps, 10);
+#endif
+
+#else
 					LCHKLST(sync_steps) = checklist_t(get_random_seed(), halthandle_t::get_next_check(), linehandle_t::get_next_check(), convoihandle_t::get_next_check());
+#endif
 					// some server side tasks
 					if(  env_t::networkmode  &&  env_t::server  ) {
 						// broadcast sync info regularly and when lagged
@@ -7620,3 +7658,15 @@ player_t *karte_t::get_public_player() const
 {
 	return get_player(1);
 }
+
+
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+uint32 karte_t::get_gamestate_hash()
+{
+	adler32_stream_t *stream = new adler32_stream_t;
+	stream_loadsave_t ls(stream);
+
+	rdwr_gamestate(&ls, NULL);
+	return stream->get_hash();
+}
+#endif
diff --git simworld.h simworld.h
index 00f3d23c6..9bec5d42e 100644
--- simworld.h
+++ simworld.h
@@ -1800,6 +1800,10 @@ public:
 	 */
 	uint32 generate_new_map_counter() const;
 
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	uint32 get_gamestate_hash();
+#endif
+
 private:
 	void process_network_commands(sint32* ms_difference);
 	void do_network_world_command(network_world_command_t *nwc);
diff --git utils/checklist.cc utils/checklist.cc
index e497fa996..c180c6869 100644
--- utils/checklist.cc
+++ utils/checklist.cc
@@ -13,30 +13,43 @@
 
 
 checklist_t::checklist_t() :
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	hash(0)
+#else
 	random_seed(0),
 	halt_entry(0),
 	line_entry(0),
 	convoy_entry(0)
+#endif
 {
 }
 
 
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+checklist_t::checklist_t(const uint32 &hash) :
+	hash(hash)
+#else
 checklist_t::checklist_t(uint32 _random_seed, uint16 _halt_entry, uint16 _line_entry, uint16 _convoy_entry) :
 	random_seed(_random_seed),
 	halt_entry(_halt_entry),
 	line_entry(_line_entry),
 	convoy_entry(_convoy_entry)
+#endif
 {
 }
 
 
 bool checklist_t::operator==(const checklist_t& other) const
 {
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	return hash == other.hash;
+#else
 	return
 		random_seed==other.random_seed &&
 		halt_entry==other.halt_entry &&
 		line_entry==other.line_entry &&
 		convoy_entry==other.convoy_entry;
+#endif
 }
 
 
@@ -48,16 +61,24 @@ bool checklist_t::operator!=(const checklist_t& other) const
 
 void checklist_t::rdwr(memory_rw_t *buffer)
 {
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	buffer->rdwr_long(hash);
+#else
 	buffer->rdwr_long(random_seed);
 	buffer->rdwr_short(halt_entry);
 	buffer->rdwr_short(line_entry);
 	buffer->rdwr_short(convoy_entry);
+#endif
 }
 
 
 int checklist_t::print(char *buffer, const char *entity) const
 {
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	return sprintf(buffer, "%s=[adler32=%08x] ", entity, hash);
+#else
 	return sprintf(buffer, "%s=[rand=%u halt=%u line=%u cnvy=%u] ",
 		entity, random_seed, halt_entry, line_entry, convoy_entry);
+#endif
 }
 
diff --git utils/checklist.h utils/checklist.h
index b9b0cd897..2b6761872 100644
--- utils/checklist.h
+++ utils/checklist.h
@@ -17,7 +17,11 @@ struct checklist_t
 {
 public:
 	checklist_t();
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+	explicit checklist_t(const uint32 &hash);
+#else
 	checklist_t(uint32 _random_seed, uint16 _halt_entry, uint16 _line_entry, uint16 _convoy_entry);
+#endif
 
 	bool operator==(const checklist_t &other) const;
 	bool operator!=(const checklist_t &other) const;
@@ -25,11 +29,16 @@ public:
 	void rdwr(memory_rw_t *buffer);
 	int print(char *buffer, const char *entity) const;
 
+#if defined(HEAVY_MODE) && HEAVY_MODE >= 1
+private:
+	uint32 hash;
+#else
 public:
 	uint32 random_seed;
 	uint16 halt_entry;
 	uint16 line_entry;
 	uint16 convoy_entry;
+#endif
 };
 
 #endif