Added packet checksumming code. Watch the comments for an explanation.

lib/Modules

@@ -27,3 +27,5 @@ slists.c
 slists.h
 event.c
 event.h
+checksum.c
+checksum.h

lib/checksum.c

@@ -0,0 +1,59 @@
+/*
+ *	BIRD Library -- IP One-Complement Checksum
+ *
+ *	(c) 1999 Martin Mares <mj@ucw.cz>
+ *
+ *	Can be freely distributed and used under the terms of the GNU GPL.
+ */
+
+#include <stdarg.h>
+
+#include "nest/bird.h"
+#include "checksum.h"
+
+static u16
+ipsum_calc(void *frag, unsigned len, va_list args)
+{
+  u16 sum = 0;
+
+  for(;;)
+    {
+      u16 *x = frag;
+      ASSERT(!(len % 2));
+      while (len)
+	{
+	  u16 z = sum + *x++;
+	  sum = z + (z < sum);
+	  len -= 2;
+	}
+      frag = va_arg(args, void *);
+      if (!frag)
+	break;
+      len = va_arg(args, unsigned);
+    }
+  return sum;
+}
+
+int
+ipsum_verify(void *frag, unsigned len, ...)
+{
+  va_list args;
+  u16 sum;
+
+  va_start(args, len);
+  sum = ipsum_calc(frag, len, args);
+  va_end(args);
+  return sum == 0xffff;
+}
+
+u16
+ipsum_calculate(void *frag, unsigned len, ...)
+{
+  va_list args;
+  u16 sum;
+
+  va_start(args, len);
+  sum = ipsum_calc(frag, len, args);
+  va_end(args);
+  return 0xffff - sum;
+}

lib/checksum.h

@@ -0,0 +1,20 @@
+/*
+ *	BIRD Library -- IP One-Complement Checksum
+ *
+ *	(c) 1999 Martin Mares <mj@ucw.cz>
+ *
+ *	Can be freely distributed and used under the terms of the GNU GPL.
+ */
+
+#ifndef _BIRD_CHECKSUM_H_
+#define _BIRD_CHECKSUM_H_
+
+/*
+ *	Both checksumming functions accept a vararg list of packet
+ *	fragments finished by NULL pointer.
+ */
+
+int ipsum_verify(void *frag, unsigned len, ...);
+u16 ipsum_calculate(void *frag, unsigned len, ...);
+
+#endif