diff --git a/lib/checksum.c b/lib/checksum.c
new file mode 100644
index 0000000..6a29cba
--- /dev/null
+++ b/lib/checksum.c
@@ -0,0 +1,47 @@
+/*
+ * 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);
+}