code: func (s *Stack) EnableNIC(id tcpip.NICID) *tcpip.Error {
  	s.mu.RLock()
  	defer s.mu.RUnlock()
  
  	nic := s.nics[id]
  	if nic == nil {
  		return tcpip.ErrUnknownNICID
  	}
  
  	nic.attachLinkEndpoint()
  
  	return nil
  }
docs: // EnableNIC enables the given NIC so that the link-layer endpoint can start
  // delivering packets to it.