请在上面函数的基础上实现gtpu对消息类型和序列号的处理

在上述函数的基础上，我们可以增加处理消息类型和序列号的代码，如下所示：

static_always_inline void
gtpu_echo_request (vlib_main_t * vm, vlib_node_runtime_t * node,
                   gtpu_header_t * gtpu, u32 sw_if_index0)
{
  vnet_main_t *vnm = vnet_get_main ();
  vnet_sw_interface_t *si = vnet_get_sw_interface (vnm, sw_if_index0);

  /* Set MTU of the tunnel interface. This is an estimate since we do not
     know how many labels the packet will have, but it is better than nothing. */
  gtpu4_tunnel_t *t = gtpu4_tunnel_from_gtpu_header (gtpu);
  u32 mtu =
    si->mtu_bytes - sizeof (ip4_header_t) - sizeof (udp_header_t) -
    sizeof (gtpu_header_t);
  t->mtu = clib_min (t->mtu, mtu);

  /* Just swap IP src/dst and send it back */
  ip4_header_t *ip4 = &t->ip4;
  ip_csum_t sum0 = ip4->checksum;
  ip4->checksum = 0;
  ip4->src_address.as_u32 = ip4->dst_address.as_u32;
  ip4->dst_address.as_u32 = gtpu->ip4.src_address.as_u32;
  ip4->checksum = ip4_header_checksum (ip4);

  udp_header_t *udp = &t->udp;
  udp->src_port = udp->dst_port;
  udp->dst_port = clib_host_to_net_u16 (UDP_DST_PORT_GTPU);

  /* Process GTPU message type and sequence number */
  gtpu->flags = GTPU_ECHO_RSP;
  gtpu->message_type = GTPU_ECHO_REQUEST;
  gtpu->sequence_number = gtpu->sequence_number + 1;

  vlib_buffer_t *b = vlib_buffer_from_rte_mbuf (gtpu->buffer);
  vlib_buffer_advance (b, -(word) sizeof (ip4_header_t));
  b->current_data -= sizeof (ip4_header_t);
  b->current_length += sizeof (ip4_header_t);
  b->total_length_not_including_first_buffer += sizeof (ip4_header_t);

  sum0 = ip_csum_update (sum0, 0, 0, ip4->src_address.as_u32);
  sum0 = ip_csum_update (sum0, 0, 0, ip4->dst_address.as_u32);

  udp->length = clib_host_to_net_u16 (b->current_length);
  sum0 = ip_csum_update (sum0, 0, 0, udp->length);

  gtpu->length = clib_host_to_net_u16 (b->current_length - sizeof (*ip4) -
                                       sizeof (*udp));
  sum0 = ip_csum_update (sum0, 0, 0, gtpu->length);

  /* Process GTPU message checksum */
  sum0 = ip_csum_with_carry (sum0, gtpu->flags);
  sum0 = ip_csum_with_carry (sum0, gtpu->message_type);
  sum0 = ip_csum_with_carry (sum0, clib_host_to_net_u16 (gtpu->length));
  sum0 = ip_csum_with_carry (sum0, clib_host_to_net_u32 (gtpu->teid));
  sum0 = ip_csum_with_carry (sum0, clib_host_to_net_u16 (gtpu->sequence_number));

  gtpu->checksum = ~ip_csum_fold (sum0);

  vlib_buffer_enqueue_to_next (vm, node, &b, 1, /* next_index */ 1);
}

在这个函数中，我们首先对消息类型进行处理，将其设置为Echo Request。然后，我们还对序列号进行处理，将其加1后设置为Echo Response的序列号。最后，我们根据GTPU协议的要求，计算出消息的校验和，并将其写入到GTPU头中。这样，我们就完成了对消息类型和序列号的处理。