vpp插件gtpu中，gtpu处理 Echo Request消息这个函数怎么写（gtpu_handle_echo_request）？

在VPP插件GTPU中，处理Echo Request消息的函数是gtpu_echo_request。该函数的定义如下：

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);

  gtpu->flags = GTPU_ECHO_RSP;
  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);

  gtpu->checksum = ~ip_csum_fold (sum0);

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

这个函数的作用是将Echo Request消息的源地址和目的地址进行交换，然后将其发送回去，以此来测试是否可以进行双向通信。同时，它还会设置隧道接口的MTU，以预估传输时所需的最大传输单元。