code: func RegisterLinkEndpoint(linkEP LinkEndpoint) tcpip.LinkEndpointID {
  	linkEPMu.Lock()
  	defer linkEPMu.Unlock()
  
  	v := nextLinkEndpointID
  	nextLinkEndpointID++
  
  	linkEndpoints[v] = linkEP
  
  	return v
  }
docs: // RegisterLinkEndpoint register a link-layer protocol endpoint and returns an
  // ID that can be used to refer to it.