Error correction for network packets (proof of concept)

code/qcommon/msg.c

@@ -21,6 +21,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 #include "q_shared.h"
 #include "qcommon.h"
+#include "../rscode/ecc.h"
 
 static huffman_t		msgHuff;
 
@@ -262,6 +263,31 @@ void MSG_WriteData( msg_t *buf, const void *data, int length ) {
 	}
 }
 
+void MSG_WriteDataWithECC( msg_t *buf, const void *data, int length ) {
+	int i;
+    char buf_copy[28];
+    char data_and_ecc[32];
+	for(i=0;i<length;i++) {
+		MSG_WriteByte(buf, ((byte *)data)[i]);
+        // copy the current byte in a buffer
+        buf_copy[i % 28] = ((byte *)data)[i];
+        // reached the size to generate an ecc, or we are at the end of the message
+        if i == 28 || i == (length - 1) {
+            // if end of message but the buffer is not full, we pad with 0s
+            if i == (length - 1) && (i % length) < 27 {
+                for(j=(i+1) % length;j<28;j++) {
+                    buf_copy[j] = 0;
+                }
+            }
+            // generate ecc symbols (will be appended to the message)
+            encode_data(buf_copy, sizeof(buf_ecc), data_and_ecc);
+            for(j=28;j<sizeof(data_and_ecc);j++) {
+                MSG_WriteByte(buf, ((byte *)data_and_ecc)[i]);
+            }
+        }
+	}
+}
+
 void MSG_WriteShort( msg_t *sb, int c ) {
 #ifdef PARANOID
 	if (c < ((short)0x8000) || c > (short)0x7fff)
@@ -510,6 +536,25 @@ void MSG_ReadData( msg_t *msg, void *data, int len ) {
 	}
 }
 
+void MSG_ReadDataWithECC( msg_t *msg, void *data, int length ) {
+	int i;
+    char data_and_ecc[32];
+    // Decode each codeword blocks
+	for(i=0;i<(int)(length/32);i++) {
+        // Read codeword (data + ecc symbols)
+		MSG_ReadData(msg, data_and_ecc, 32);
+        // Decode the syndromes
+        decode_data(data_and_ecc, sizeof(data_and_ecc);
+        // Check the syndrome, if not null, then there are errors
+        if (check_syndrome () != 0) {
+            // Correct errors
+            correct_errors_erasures (data_and_ecc, sizeof(data_and_ecc), 0, 0); // disable erasures detection because we have no idea where the erasures might be, so just do error correction
+        // Copy the corrected message to data (ie, the first 28 bytes)
+        for(j=0;j<28;j++)
+            ((byte *)data)[i*28+j] = data_and_ecc[j];
+	}
+}
+
 // a string hasher which gives the same hash value even if the
 // string is later modified via the legacy MSG read/write code
 int MSG_HashKey(const char *string, int maxlen) {

code/qcommon/net_chan.c

@@ -74,6 +74,7 @@ void Netchan_Init( int port ) {
 	showpackets = Cvar_Get ("showpackets", "0", CVAR_TEMP );
 	showdrop = Cvar_Get ("showdrop", "0", CVAR_TEMP );
 	qport = Cvar_Get ("net_qport", va("%i", port), CVAR_INIT );
+	initialize_ecc(); // initialize error correction lib
 }
 
 /*
@@ -211,7 +212,9 @@ void Netchan_Transmit( netchan_t *chan, int length, const byte *data ) {
 
 	chan->outgoingSequence++;
 
-	MSG_WriteData( &send, data, length );
+	// Write data
+	//MSG_WriteData( &send, data, length );
+	MSG_WriteDataWithECC( &send, data, length );
 
 	// send the datagram
 	NET_SendPacket( chan->sock, send.cursize, send.data, chan->remoteAddress );
@@ -245,6 +248,10 @@ qboolean Netchan_Process( netchan_t *chan, msg_t *msg ) {
 	int			sequence;
 	int			fragmentStart, fragmentLength;
 	qboolean	fragmented;
+	// for error correction
+	msg_t corrected_msg;
+	int bak_readcount = 0;
+	int bak_bit = 0;
 
 	// XOR unscramble all data in the packet after the header
 //	Netchan_UnScramblePacket( msg );
@@ -277,6 +284,20 @@ qboolean Netchan_Process( netchan_t *chan, msg_t *msg ) {
 			return qfalse;
 	}
 
+    // Apply error correction on message data if not fragmented (FIXME: implement also for fragmented packets)
+	if ( !fragmented ) {
+		// Store current cursor position
+		bak_readcount = msg->readcount;
+		bak_bit = msg->bit;
+		// Read and ecc decode message content
+		MSG_ReadDataWithECC( msg, corrected_msg, sizeof(msg));
+		// Replace current message with the corrected one, and restore cursor position
+		Q_strncpyz(msg, corrected_msg, MAX_PACKETLEN);
+		MSG_BeginReadingOOB( msg );
+		msg->readcount = bak_readcount;
+		msg->bit = bak_bit;
+	}
+
 	// read the fragment information
 	if ( fragmented ) {
 		fragmentStart = MSG_ReadShort( msg );

code/rscode/LICENSE

@@ -0,0 +1,10 @@
+ * (C) Henry Minsky (hqm@alum.mit.edu) 1991-2009
+ *
+ * This software library is licensed under terms of the GNU GENERAL
+ * PUBLIC LICENSE. [See file gpl.txt]
+ * 
+ * Commercial licensing is available under a separate license, please
+ * contact author for details.
+ *
+ * Latest source code and other info at http://rscode.sourceforge.net
+

code/rscode/Makefile

@@ -0,0 +1,50 @@
+# Makefile for Cross Interleaved Reed Solomon encoder/decoder
+#
+# (c) Henry Minsky, Universal Access 1991-1996
+#
+
+RANLIB = ranlib
+AR = ar
+
+
+VERSION = 1.0
+DIRNAME= rscode-$(VERSION)
+
+
+CC = gcc
+# OPTIMIZE_FLAGS = -O69
+DEBUG_FLAGS = -g
+CFLAGS = -Wall -Wstrict-prototypes  $(OPTIMIZE_FLAGS) $(DEBUG_FLAGS) -I..
+LDFLAGS = $(OPTIMIZE_FLAGS) $(DEBUG_FLAGS)
+
+LIB_CSRC = rs.c galois.c berlekamp.c crcgen.c 
+LIB_HSRC = ecc.h
+LIB_OBJS = rs.o galois.o berlekamp.o crcgen.o 
+
+TARGET_LIB = libecc.a
+TEST_PROGS = example
+
+TARGETS = $(TARGET_LIB) $(TEST_PROGS)
+
+all: $(TARGETS)
+
+$(TARGET_LIB): $(LIB_OBJS)
+	$(RM) $@
+	$(AR) cq $@ $(LIB_OBJS)
+	if [ "$(RANLIB)" ]; then $(RANLIB) $@; fi
+
+example: example.o galois.o berlekamp.o crcgen.o rs.o
+	gcc -o example example.o -L. -lecc
+
+clean:
+	rm -f *.o example libecc.a
+	rm -f *~
+
+dist:
+	(cd ..; tar -cvf rscode-$(VERSION).tar $(DIRNAME))
+
+depend:
+	makedepend $(SRCS)
+
+# DO NOT DELETE THIS LINE -- make depend depends on it.
+

code/rscode/README

@@ -0,0 +1,27 @@
+RSCODE version 1.3
+
+See the files
+
+config.doc 	documentation of some compile time parameters
+rs.doc 		overview of the Reed-Solomon coding program
+rs.man		a man page, slightly outdated at this point
+example.c	a simple example of encoding,decoding, and error correction
+
+Makefile	should work on a Sun system, may require GNU make.
+
+
+Henry Minsky
+hqm@alum.mit.edu
+
+
+ * (c) Henry Minsky (hqm@alum.mit.edu) 1991-2009
+ *
+ * This software library is licensed under terms of the GNU GENERAL
+ * PUBLIC LICENSE. (See gpl.txt)
+ * 
+ * Commercial licensing is available under a separate license, please
+ * contact author for details.
+ *
+ * Source code is available at http://rscode.sourceforge.net
+
+