diff -Naur nat46-6.old/nat46/modules/nat46-core.c nat46-6.new/nat46/modules/nat46-core.c
--- nat46-6.old/nat46/modules/nat46-core.c	2017-04-17 09:59:50.025700000 +0530
+++ nat46-6.new/nat46/modules/nat46-core.c	2017-04-17 10:11:16.107523000 +0530
@@ -807,11 +807,12 @@
 }
 
 void fill_v4hdr_from_v6hdr(struct iphdr * iph, struct ipv6hdr *ip6h, __u32 v4saddr, __u32 v4daddr, __u16 id, __u16 frag_off, __u16 proto, int l3_payload_len) {
+  uint32_t ver_class_flow = ntohl(*(__be32 *)ip6h);
   iph->ttl = ip6h->hop_limit;
   iph->saddr = v4saddr;
   iph->daddr = v4daddr;
   iph->protocol = proto;
-  *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | (0x00/*tos*/ & 0xff));
+  *((__be16 *)iph) = htons((4 << 12) | (5 << 8) | ((ver_class_flow >> 20) & 0xff));
   iph->frag_off = frag_off;
   iph->id = id;
   iph->tot_len = htons( l3_payload_len + IPV4HDRSIZE );
@@ -1743,8 +1744,8 @@
   nat46_instance_t *nat46 = get_nat46_instance(old_skb);
   struct sk_buff *new_skb;
   uint16_t sport = 0, dport = 0;
+  uint8_t tclass;
 
-  int tclass = 0;
   int flowlabel = 0;
   int do_l4_translate = 0;
 
@@ -1752,6 +1753,7 @@
 
   struct ipv6hdr * hdr6;
   struct iphdr * hdr4 = ip_hdr(old_skb);
+  tclass = hdr4->tos;
 
   char v6saddr[16], v6daddr[16];
 
@@ -1829,7 +1831,6 @@
   memset(hdr6, 0, sizeof(*hdr6) + (add_frag_header?8:0));
 
   /* build IPv6 header */
-  tclass = 0; /* traffic class */
   *(__be32 *)hdr6 = htonl(0x60000000 | (tclass << 20)) | flowlabel; /* version, priority, flowlabel */
 
   /* IPv6 length is a payload length, IPv4 is hdr+payload */