code: func (n *npmp) AddMapping(protocol string, extPort, intPort int,
  	name string, lifetime time.Duration) error {
  	if lifetime <= 0 {
  		return ErrTooShortLifetime
  	}
  	_, err := n.c.AddPortMapping(strings.ToLower(protocol),
  		intPort, extPort, int(lifetime/time.Second))
  	return err
  }
docs: // AddMapping maps an external port to a local port for a specific
  // service to NAT-PMP interface.