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-rw-r--r--lib/checksum.c65
1 files changed, 57 insertions, 8 deletions
diff --git a/lib/checksum.c b/lib/checksum.c
index 4dfa252..94cf71e 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -11,6 +11,62 @@
#include "nest/bird.h"
#include "checksum.h"
+static u16 /* One-complement addition */
+add16(u16 sum, u16 x)
+{
+ u16 z = sum + x;
+ return z + (z < sum);
+}
+
+static u32
+add32(u32 sum, u32 x)
+{
+ u32 z = sum + x;
+ return z + (z < sum);
+}
+
+static u16
+ipsum_calc_block(u16 *x, unsigned len, u16 sum)
+{
+ int rest;
+ u32 tmp, *xx;
+
+ /*
+ * A few simple facts about the IP checksum (see RFC 1071 for detailed
+ * discussion):
+ *
+ * o It's associative and commutative.
+ * o It's byte order independent.
+ * o It's word size independent.
+ *
+ * This gives us a neat 32-bits-at-a-time algorithm which respects
+ * usual alignment requirements and is reasonably fast.
+ */
+
+ ASSERT(!(len % 2));
+ if (!len)
+ return sum;
+ len >>= 1;
+ if ((unsigned long) x & 2) /* Align to 32-bit boundary */
+ {
+ sum = add16(sum, *x++);
+ len--;
+ }
+ rest = len & 1;
+ len >>= 1;
+ tmp = 0;
+ xx = (u32 *) x;
+ while (len)
+ {
+ tmp = add32(tmp, *xx++);
+ len--;
+ }
+ sum = add16(sum, add16(tmp & 0xffff, tmp >> 16U));
+ if (rest)
+ sum = add16(sum, *(u16 *) xx);
+ return sum;
+}
+
static u16
ipsum_calc(void *frag, unsigned len, va_list args)
{
@@ -18,14 +74,7 @@ ipsum_calc(void *frag, unsigned len, va_list args)
for(;;)
{
- u16 *x = frag;
- ASSERT(!(len % 2));
- while (len)
- {
- u16 z = sum + *x++;
- sum = z + (z < sum);
- len -= 2;
- }
+ sum = ipsum_calc_block(frag, len, sum);
frag = va_arg(args, void *);
if (!frag)
break;