diff --git a/doc/plan.org b/doc/plan.org
index e3aa2f7..100ebd9 100644
--- a/doc/plan.org
+++ b/doc/plan.org
@@ -5659,6 +5659,20 @@ Vivado%
 Vivado%
 #+END_CENTER
 *** TODO 2019-07-21: whether or not to handle icmp(6) checksums
+*** TODO 2019-07-21: implement v6->v4 delta based
+#+BEGIN_CENTER
+while true; do mx h3 "echo V4-OK | socat - TCP-LISTEN:2343"; sleep 2;
+done
+
+while true; do mx h1 "echo V6-OK | socat -
+TCP6:[2001:db8:1::a00:1]:2343"; sleep 2; done
+
+ mx h1 "echo V6-OK | socat - TCP6:[2001:db8:1::a00:1]:2343"
+#+END_CENTER
+
+Result: no reply, but translated packets seen on h3; wrong IPv4
+checksum; tcp checksum is ok
+-> doing manual calculation to see where diff comes from
 ** 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 0e6fcc7..8997ba2 100644
--- a/p4src/actions_delta_checksum.p4
+++ b/p4src/actions_delta_checksum.p4
@@ -125,22 +125,43 @@ action delta_tcp_from_v6_to_v4()
 action delta_ipv4_from_v6_to_v4()
 {
     bit<16> tmp = 0;
+    bit<16> shift_tmp = 0;
 
    /* we don't have ANY checksum, but tcp or udp: we can
        base on that ones for calculating the diff for IPv4
 
     Does NOT contain payload! -> can be done manually */
 
-    tmp = tmp + (bit<16>) hdr.ipv4.version;          /* 4 bit */
-	tmp = tmp + (bit<16>) hdr.ipv4.ihl;              /* 4 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.diff_serv;        /* 6 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.ecn;              /* 2 bit */
+    shift_tmp = ((bit<16>) hdr.ipv4.version)   << 12 +
+                ((bit<16>) hdr.ipv4.ihl)       << 8  +
+                ((bit<16>) hdr.ipv4.diff_serv) << 2  +
+                ((bit<16>) hdr.ipv4.ecn);
+    tmp = tmp + shift_tmp;
+
+    // tmp = tmp + (bit<16>) hdr.ipv4.version;          /* 4 bit */
+	// tmp = tmp + (bit<16>) hdr.ipv4.ihl;              /* 4 bit */
+    // tmp = tmp + (bit<16>) hdr.ipv4.diff_serv;        /* 6 bit */
+    // tmp = tmp + (bit<16>) hdr.ipv4.ecn;              /* 2 bit */
+
+
     tmp = tmp + (bit<16>) hdr.ipv4.totalLen;         /* 16 bit */
+
     tmp = tmp + (bit<16>) hdr.ipv4.identification;   /* 16 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.flags;            /* 3 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.fragOffset;       /* 13 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.ttl;              /* 8 bit */
-    tmp = tmp + (bit<16>) hdr.ipv4.protocol;         /* 8 bit */
+
+    shift_tmp = ((bit<16>) hdr.ipv4.flags) << 13 +
+                ((bit<16>) hdr.ipv4.fragOffset);
+    tmp = tmp + shift_tmp;
+
+    // tmp = tmp + (bit<16>) hdr.ipv4.flags;            /* 3 bit */
+    // tmp = tmp + (bit<16>) hdr.ipv4.fragOffset;       /* 13 bit */
+
+    shift_tmp = ((bit<16>) hdr.ipv4.ttl) << 8 +
+                ((bit<16>) hdr.ipv4.protocol);
+    tmp = tmp + shift_tmp;
+
+    // tmp = tmp + (bit<16>) hdr.ipv4.ttl;              /* 8 bit */
+    // tmp = tmp + (bit<16>) hdr.ipv4.protocol;         /* 8 bit */
+
     tmp = tmp + (bit<16>) hdr.ipv4.src_addr[15:0];   /* 16 bit */
     tmp = tmp + (bit<16>) hdr.ipv4.src_addr[31:16];  /* 16 bit */
     tmp = tmp + (bit<16>) hdr.ipv4.dst_addr[15:0];   /* 16 bit */