diff --git a/p4src/actions_delta_checksum.p4 b/p4src/actions_delta_checksum.p4
index dce3b58..958dc18 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -16,9 +16,10 @@ action v4sum() {
     carryover = tmp >> 16;            /* maximum is 6*(2**16)  >> 16 == 6 */
     tmp = (tmp & 0xffff) + carryover; /* Now tmp contains at maximum 65541 */
     carryover = tmp >> 16;            /* Now carryover contains at maximum 1 */
-    tmp = (tmp & 0xffff) + carryover; /* No overrun possible anymore */
+    tmp = tmp + carryover;            /* No overrun possible anymore */
 
-    meta.v4sum = (bit<16>) tmp;
+    /* filtering code copied from scapy */
+    meta.v4sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
 }
 
 action v6sum() {
@@ -47,11 +48,12 @@ action v6sum() {
     tmp = tmp + (bit<32>) hdr.ipv6.next_header;                 // 8 bit
 
     carryover = tmp >> 16;            /* maximum is 18*(2**16)  >> 16 == 18 */
-    tmp = (tmp & 0xffff) + carryover; /* Now tmp contains at maximum 65554*/
+ffff) + carryover; /* Now tmp contains at maximum 65554*/
     carryover = tmp >> 16;            /* Now carryover contains at maximum 1 */
-    tmp = (tmp & 0xffff) + carryover; /* No overrun possible anymore */
+    tmp = tmp + carryover;            /* No overrun possible anymore */
 
-    meta.v6sum = (bit<16>) tmp;
+    /* filtering code copied from scapy */
+    meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
 }
 
 action delta_prepare()
@@ -68,13 +70,13 @@ action delta_prepare()
 action delta_udp_from_v4_to_v6()
 {
     delta_prepare();
-    hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff +1;
+    hdr.udp.checksum = hdr.udp.checksum + ~meta.headerdiff;
 }
 
 action delta_tcp_from_v4_to_v6()
 {
     delta_prepare();
-    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff +1;
+    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
 }
 
 action delta_ipv4_from_v6_to_v4()