Initial revision
diff --git a/lib/checksum.c b/lib/checksum.c
new file mode 100644
index 0000000..6a29cba
--- /dev/null
+++ b/lib/checksum.c
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+/*
+ * Checksum routine for Internet Protocol family headers (C Version).
+ *
+ * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and
+ * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989,
+ * pp. 86-101, for additional details on computing this checksum.
+ */
+
+#include <zebra.h>
+
+int /* return checksum in low-order 16 bits */
+in_cksum(ptr, nbytes)
+register u_short *ptr;
+register int nbytes;
+{
+ register long sum; /* assumes long == 32 bits */
+ u_short oddbyte;
+ register u_short answer; /* assumes u_short == 16 bits */
+
+ /*
+ * Our algorithm is simple, using a 32-bit accumulator (sum),
+ * we add sequential 16-bit words to it, and at the end, fold back
+ * all the carry bits from the top 16 bits into the lower 16 bits.
+ */
+
+ sum = 0;
+ while (nbytes > 1) {
+ sum += *ptr++;
+ nbytes -= 2;
+ }
+
+ /* mop up an odd byte, if necessary */
+ if (nbytes == 1) {
+ oddbyte = 0; /* make sure top half is zero */
+ *((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */
+ sum += oddbyte;
+ }
+
+ /*
+ * Add back carry outs from top 16 bits to low 16 bits.
+ */
+
+ sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
+ sum += (sum >> 16); /* add carry */
+ answer = ~sum; /* ones-complement, then truncate to 16 bits */
+ return(answer);
+}