diff --git a/p4src/checksum_diff.p4 b/p4src/checksum_diff.p4
index ee91c41..8c626ad 100644
--- a/p4src/checksum_diff.p4
+++ b/p4src/checksum_diff.p4
@@ -197,7 +197,8 @@ control MyIngress(inout headers hdr,
                     v4sum();
                     v6sum();
                     bit<16> diff = meta.v6sum - meta.v4sum;
-                    hdr.udp.checksum = hdr.udp.checksum + diff;
+                    # hdr.udp.checksum = hdr.udp.checksum + diff;
+                    hdr.udp.checksum = diff;
 
                     #else
                     meta.chk_udp_v6 = 1;