+/*
+ * Translate IPv6 UDP/TCP packet to IPv4.
+ */
+always_inline int
+map_ip6_to_ip4_tcp_udp (vlib_buffer_t * p, bool udp_checksum)
+{
+ map_main_t *mm = &map_main;
+ ip6_header_t *ip6;
+ u16 *checksum;
+ ip_csum_t csum = 0;
+ ip4_header_t *ip4;
+ u16 fragment_id;
+ u16 flags;
+ u16 frag_offset;
+ u8 l4_protocol;
+ u16 l4_offset;
+ ip6_address_t old_src, old_dst;
+
+ ip6 = vlib_buffer_get_current (p);
+
+ if (ip6_parse
+ (ip6, p->current_length, &l4_protocol, &l4_offset, &frag_offset))
+ return -1;
+
+ if (l4_protocol == IP_PROTOCOL_TCP)
+ {
+ tcp_header_t *tcp = ip6_next_header (ip6);
+ if (mm->tcp_mss > 0)
+ {
+ csum = tcp->checksum;
+ map_mss_clamping (tcp, &csum, mm->tcp_mss);
+ tcp->checksum = ip_csum_fold (csum);
+ }
+ checksum = &tcp->checksum;
+ }
+ else
+ {
+ udp_header_t *udp = ip6_next_header (ip6);
+ checksum = &udp->checksum;
+ }
+
+ old_src.as_u64[0] = ip6->src_address.as_u64[0];
+ old_src.as_u64[1] = ip6->src_address.as_u64[1];
+ old_dst.as_u64[0] = ip6->dst_address.as_u64[0];
+ old_dst.as_u64[1] = ip6->dst_address.as_u64[1];
+
+ ip4 = (ip4_header_t *) u8_ptr_add (ip6, l4_offset - sizeof (*ip4));
+
+ vlib_buffer_advance (p, l4_offset - sizeof (*ip4));
+
+ if (PREDICT_FALSE (frag_offset))
+ {
+ // Only the first fragment
+ ip6_frag_hdr_t *hdr = (ip6_frag_hdr_t *) u8_ptr_add (ip6, frag_offset);
+ fragment_id = frag_id_6to4 (hdr->identification);
+ flags = clib_host_to_net_u16 (IP4_HEADER_FLAG_MORE_FRAGMENTS);
+ }
+ else
+ {
+ fragment_id = 0;
+ flags = 0;
+ }
+
+ ip4->dst_address.as_u32 = vnet_buffer (p)->map_t.v6.daddr;
+ ip4->src_address.as_u32 = vnet_buffer (p)->map_t.v6.saddr;
+
+ ip4->ip_version_and_header_length =
+ IP4_VERSION_AND_HEADER_LENGTH_NO_OPTIONS;
+ ip4->tos = ip6_translate_tos (ip6);
+ ip4->length =
+ u16_net_add (ip6->payload_length,
+ sizeof (*ip4) + sizeof (*ip6) - l4_offset);
+ ip4->fragment_id = fragment_id;
+ ip4->flags_and_fragment_offset = flags;
+ ip4->ttl = ip6->hop_limit;
+ ip4->protocol = l4_protocol;
+ ip4->checksum = ip4_header_checksum (ip4);
+
+ // UDP checksum is optional over IPv4
+ if (!udp_checksum && l4_protocol == IP_PROTOCOL_UDP)
+ {
+ *checksum = 0;
+ }
+ else
+ {
+ csum = ip_csum_sub_even (*checksum, old_src.as_u64[0]);
+ csum = ip_csum_sub_even (csum, old_src.as_u64[1]);
+ csum = ip_csum_sub_even (csum, old_dst.as_u64[0]);
+ csum = ip_csum_sub_even (csum, old_dst.as_u64[1]);
+ csum = ip_csum_add_even (csum, ip4->dst_address.as_u32);
+ csum = ip_csum_add_even (csum, ip4->src_address.as_u32);
+ *checksum = ip_csum_fold (csum);
+ }
+
+ return 0;
+}
+