def anchor_stream(self, stream_id, converter="rtc"):
        """Mark a stream as containing anchor points."""

        if isinstance(converter, str):
            converter = self._known_converters.get(converter)

            if converter is None:
                raise ArgumentError("Unknown anchor converter string: %s" % converter,
                                    known_converters=list(self._known_converters))

        self._anchor_streams[stream_id] = converter