[lib] Add fletcher checksum implementation
2008-08-13 Jingjing Duan <Jingjing.Duan@sun.com>
* lib/checksum.?: (fletcher_checksum) implementation of
Fletcher checksum, as per RFC1008.
Signed-off-by: Paul Jakma <paul@quagga.net>
diff --git a/lib/checksum.c b/lib/checksum.c
index 201da59..88ec72a 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -45,3 +45,82 @@
answer = ~sum; /* ones-complement, then truncate to 16 bits */
return(answer);
}
+
+/* Fletcher Checksum -- Refer to RFC1008. */
+#define MODX 4102 /* 5802 should be fine */
+
+/* To be consistent, offset is 0-based index, rather than the 1-based
+ index required in the specification ISO 8473, Annex C.1 */
+u_int16_t
+fletcher_checksum(u_char * buffer, int len, u_int16_t offset)
+{
+ u_int8_t *p;
+ int x;
+ int y;
+ u_int32_t mul;
+ u_int32_t c0;
+ u_int32_t c1;
+ u_int16_t checksum;
+ u_int16_t *csum;
+ int i, init_len, partial_len;
+
+ checksum = 0;
+
+ /*
+ * Zero the csum in the packet.
+ */
+ csum = (u_int16_t *) (buffer + offset);
+ *(csum) = checksum;
+
+ p = buffer;
+ c0 = 0;
+ c1 = 0;
+ init_len = len;
+
+ while (len != 0)
+ {
+ partial_len = MIN(len, MODX);
+
+ for (i = 0; i < partial_len; i++)
+ {
+ c0 = c0 + *(p++);
+ c1 += c0;
+ }
+
+ c0 = c0 % 255;
+ c1 = c1 % 255;
+
+ len -= partial_len;
+ }
+
+ mul = (init_len - offset)*(c0);
+
+ x = mul - c0 - c1;
+ y = c1 - mul - 1;
+
+ if (y > 0)
+ y++;
+ if (x < 0)
+ x--;
+
+ x %= 255;
+ y %= 255;
+
+ if (x == 0)
+ x = 255;
+ if (y == 0)
+ y = 1;
+
+ /*
+ * Now we write this to the packet.
+ * We could skip this step too, since the checksum returned would
+ * be stored into the checksum field by the caller.
+ */
+ buffer[offset] = x;
+ buffer[offset + 1] = y;
+
+ /* Take care of the endian issue */
+ checksum = htons((x << 8) | (y & 0xFF));
+
+ return checksum;
+}
diff --git a/lib/checksum.h b/lib/checksum.h
index ccc4da0..d3ce930 100644
--- a/lib/checksum.h
+++ b/lib/checksum.h
@@ -1 +1,2 @@
extern int in_cksum(void *, int);
+extern u_int16_t fletcher_checksum(u_char * buffer, int len, u_int16_t offset);