func: uint16_t net_checksum_tcpudp(uint16_t length, uint16_t proto,
  
                               uint8_t *addrs, uint8_t *buf)
  
  {
  
      uint32_t sum = 0;
  
  
  
      sum += net_checksum_add(length, buf);         // payload
  
      sum += net_checksum_add(8, addrs);            // src + dst address
  
      sum += proto + length;                        // protocol & length
  
      return net_checksum_finish(sum);
  
  }
  