code: func (s *Stack) JoinGroup(protocol tcpip.NetworkProtocolNumber, nicID tcpip.NICID, multicastAddr tcpip.Address) *tcpip.Error {
  	// TODO: notify network of subscription via igmp protocol.
  	return s.AddAddressWithOptions(nicID, protocol, multicastAddr, NeverPrimaryEndpoint)
  }
docs: // JoinGroup joins the given multicast group on the given NIC.