  A deep understanding of the Epoch of Reionization is still missing in our
knowledge of the universe. While future probes will allow us to test the
precise evolution of the free electron fraction from redshifts between $z\simeq
6$ and $z\simeq 20$, at present one could ask what kind of reionization
processes are allowed by present Cosmic Microwave Background temperature and
polarization measurements. An early contribution to reionization could imply a
departure from the standard picture where star formation determines the
reionization onset. BBy considering a broad class of possible reionization
parameterizations, we find that current data do not require an early
reionization component in our universe and that only one marginal class of
models, based on a particular realization of reionization, may point to that.
In addition, the frequentist Akaike Information Criterion (AIC) provides strong
evidence against alternative reionization histories, favoring the most simple
reionization scenario, which describes reionization by means of only one
(constant) reionization optical depth $\tau$.
