diff --git a/doc/plan.org b/doc/plan.org
index 5218118..b1b2588 100644
--- a/doc/plan.org
+++ b/doc/plan.org
@@ -5266,6 +5266,15 @@ we get
 
 sum 0x48ad (incorrect -> 0x85eb)
 
+Still using scapy + adjusted diff:
+
+    /* here is also a possible overflow in both directions */
+    hdr.tcp.checksum = hdr.tcp.checksum + meta.v6sum - meta.v4sum;
+
+gets
+
+sum 0xc5f2 (incorrect -> 0xe7cf)
+
 ** The NetPFGA saga
 Problems encountered:
 - The logfile for a compile run is 10k+ lines
diff --git a/p4src/actions_delta_checksum.p4 b/p4src/actions_delta_checksum.p4
index 42681e5..1bce770 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -15,9 +15,10 @@ action v4sum() {
 
     /* filtering code copied from scapy */
     tmp = (tmp >> 16) + (tmp & 0xffff);
-    tmp = tmp + (tmp >> 16);
+    tmp = (tmp >> 16) + (tmp & 0xffff);
     tmp = ~tmp;
-    meta.v4sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff);
+//    meta.v4sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff);
+    meta.v4sum = (bit<16>) tmp;
 
 }
 
@@ -48,9 +49,11 @@ action v6sum() {
 
     /* filtering code copied from scapy */
     tmp = (tmp >> 16) + (tmp & 0xffff);
-    tmp = tmp + (tmp >> 16);
+    tmp = (tmp >> 16) + (tmp & 0xffff);
+//    tmp = tmp + (tmp >> 16);
     tmp = ~tmp;
-    meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
+//    meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
+    meta.v6sum = (bit<16>) tmp;
 }