// Locking both mu and wmu here allows frame encoding to read settings with only wmu held.
	// Acquiring wmu when f.IsAck() is unnecessary, but convenient and mostly harmless.
	cc.wmu.Lock()
	defer cc.wmu.Unlock()

	if err := rl.processSettingsNoWrite(f); err != nil {
		return err
	}
	if !f.IsAck() {
		cc.fr.WriteSettingsAck()
		cc.bw.Flush()
	}
	return nil
}