code: func (l *Listener) SetDSCP(dscp int) error {
  	if nc, ok := l.conn.(net.Conn); ok {
  		addr, _ := net.ResolveUDPAddr("udp", nc.LocalAddr().String())
  		if addr.IP.To4() != nil {
  			return ipv4.NewConn(nc).SetTOS(dscp << 2)
  		} else {
  			return ipv6.NewConn(nc).SetTrafficClass(dscp)
  		}
  	}
  	return errors.New(errInvalidOperation)
  }
docs: // SetDSCP sets the 6bit DSCP field of IP header