func: putsum(uint8_t *data, uint32_t n, uint32_t sloc, uint32_t css, uint32_t cse)
  
  {
  
      uint32_t sum;
  
  
  
      if (cse && cse < n)
  
          n = cse + 1;
  
      if (sloc < n-1) {
  
          sum = net_checksum_add(n-css, data+css);
  
          stw_be_p(data + sloc, net_checksum_finish(sum));
  
      }
  
  }
  