function DirectoryManager:_registerProtocol()
  -- build list of services expected by the directory_proxy
  local services = {}
  for i, iservice in ipairs(self._iservices) do
    services[iservice:getName()] = iservice:getPath()
  end

  -- async call
  self._directory_proxy:addProtocol(self._protocol_name, services, function(status, error)
      if not status then
        self._log:warning('[DirectoryManager]: failure during protocol registration, '..error)
        self._log:debug('[DirectoryManager]: retry in 30 secs')
        return false
      else
        self._log:debug('[DirectoryManager]: protocol successfully registered')
        return true
      end
  end)
end