diff --git a/doc/plan.org b/doc/plan.org
index dadded5..9f389e2 100644
--- a/doc/plan.org
+++ b/doc/plan.org
@@ -5335,8 +5335,20 @@ Gives off-by-one in udp, sometimes!
 
 10:08:52.626713 IP6 (hlim 64, next-header UDP (17) payload length: 14) 2001:db8:1::a00:1.51345 > 2001:db8::1.2342: [bad udp cks
 
-*** TODO 2019-07-16: get values: v6sum, v4sum
+*** TODO 2019-07-16: get values from P4: v6sum, v4sum and co.
     - v6sum = 0x9a6b
+    - v4sum = 0xeadd
+
+    - 0x4a8a + 0x9a6b = 0xe4f5 (same as python)
+    (code: hdr.udp.checksum = 0x4a8a + 0x9a6b)
+
+    - hdr.udp.checksum = 0x4a8a + 0x9a6b - 0xeadd;
+    ../p4src/actions_delta_checksum.p4(58): warning: -1512: negative
+    value with unsigned type
+    hdr.udp.checksum = 0x4a8a + 0x9a6b - 0xeadd;
+    Result: 0xfa18!
+
+
 
 ** The NetPFGA saga
 Problems encountered:
diff --git a/p4src/actions_delta_checksum.p4 b/p4src/actions_delta_checksum.p4
index aedb110..56dbf55 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -53,16 +53,29 @@ action delta_prepare()
 action delta_udp_from_v4_to_v6()
 {
     delta_prepare();
-//    hdr.udp.checksum = meta.v6sum;
-//    hdr.udp.checksum = meta.v4sum;
     hdr.udp.checksum = hdr.udp.checksum + meta.v6sum;
-    hdr.udp.checksum = hdr.udp.checksum - meta.v4sum;
+
+    /* prevent negative wrap around that loses 1 */
+    if(meta.v4sum > hdr.udp.checksum) {
+        bit<16> tmp = 0xffff - meta.v4sum;
+        hdr.udp.checksum = hdr.udp.checksum + tmp;
+    } else {
+        hdr.udp.checksum = hdr.udp.checksum - meta.v4sum;
+    }
 }
 
 action delta_tcp_from_v4_to_v6()
 {
     delta_prepare();
-    hdr.tcp.checksum = hdr.tcp.checksum + meta.v6sum - meta.v4sum;
+    hdr.tcp.checksum = hdr.tcp.checksum + meta.v6sum;
+
+    /* prevent negative wrap around that loses 1 */
+    if(meta.v4sum > hdr.tcp.checksum) {
+        bit<16> tmp = 0xffff - meta.v4sum;
+        hdr.tcp.checksum = hdr.tcp.checksum + tmp;
+    } else {
+        hdr.tcp.checksum = hdr.tcp.checksum - meta.v4sum;
+    }
 }
 
 action delta_ipv4_from_v6_to_v4()