update tcp checksumming

doc/plan.org

@@ -5255,6 +5255,17 @@ we get on the wire:
 
 sum 0x324a (incorrect -> 0x5429),
 
+Using
+    /* filtering code copied from scapy */
+    tmp = (tmp >> 16) + (tmp & 0xffff);
+    tmp = tmp + (tmp >> 16);
+    tmp = ~tmp;
+    meta.v6sum = (bit<16>) ((((tmp>>8) & 0xff)|tmp<<8) & 0xffff) ;
+
+we get
+
+sum 0x48ad (incorrect -> 0x85eb)
+
 ** The NetPFGA saga
 Problems encountered:
 - The logfile for a compile run is 10k+ lines

p4src/actions_delta_checksum.p4

@@ -73,14 +73,18 @@ action delta_udp_from_v4_to_v6()
 
 action delta_tcp_from_v4_to_v6()
 {
-    delta_prepare();
+    v4sum();
-    hdr.tcp.checksum = hdr.tcp.checksum + meta.headerdiff;
+    v6sum();
+
+    /* here is also a possible overflow in both directions */
+    hdr.tcp.checksum = hdr.tcp.checksum + meta.v6sum - meta.v4sum;
 }
 
 action delta_ipv4_from_v6_to_v4()
 {
     delta_prepare();
-    hdr.tcp.checksum = hdr.tcp.checksum + ~meta.headerdiff;
+    /* TO BE DONE! */
+
 }
 
 #endif