diff --git a/checksum.c b/checksum.c
index a4dc9b8..099be6a 100644
--- a/checksum.c
+++ b/checksum.c
@@ -49,19 +49,27 @@
   return checksum;
 }
 
-/* function: ip_checksum_finish
- * close the checksum
+/* function: ip_checksum_fold
+ * folds a 32-bit partial checksum into 16 bits
  * temp_sum - sum from ip_checksum_add
+ * returns: the folded checksum in network byte order
  */
-uint16_t ip_checksum_finish(uint32_t temp_sum) {
+uint16_t ip_checksum_fold(uint32_t temp_sum) {
   while(temp_sum > 0xffff)
     temp_sum = (temp_sum >> 16) + (temp_sum & 0xFFFF);
 
-  temp_sum = (~temp_sum) & 0xffff;
-
   return temp_sum;
 }
 
+/* function: ip_checksum_finish
+ * folds and closes the checksum
+ * temp_sum - sum from ip_checksum_add
+ * returns: a header checksum value in network byte order
+ */
+uint16_t ip_checksum_finish(uint32_t temp_sum) {
+  return ~ip_checksum_fold(temp_sum);
+}
+
 /* function: ip_checksum
  * combined ip_checksum_add and ip_checksum_finish
  * data - data to checksum
@@ -113,3 +121,23 @@
 
   return current;
 }
+
+/* function: ip_checksum_adjust
+ * calculates a new checksum given a previous checksum and the old and new pseudo-header checksums
+ * checksum    - the header checksum in the original packet in network byte order
+ * old_hdr_sum - the pseudo-header checksum of the original packet
+ * new_hdr_sum - the pseudo-header checksum of the translated packet
+ * returns: the new header checksum in network byte order
+ */
+uint16_t ip_checksum_adjust(uint16_t checksum, uint32_t old_hdr_sum, uint32_t new_hdr_sum) {
+  // Algorithm suggested in RFC 1624.
+  // http://tools.ietf.org/html/rfc1624#section-3
+  checksum = ~checksum;
+  uint16_t folded_sum = ip_checksum_fold(checksum + new_hdr_sum);
+  uint16_t folded_old = ip_checksum_fold(old_hdr_sum);
+  if (folded_sum > folded_old) {
+    return ~(folded_sum - folded_old);
+  } else {
+    return ~(folded_sum - folded_old - 1);  // end-around borrow
+  }
+}