  Cosmological observations are becoming increasingly sensitive to the effects
of light particles in the form of dark radiation (DR) at the time of
recombination. The conventional observable of effective neutrino number,
$N_{\rm eff}$, is insufficient for probing generic, interacting models of DR.
In this work, we perform likelihood analyses which allow both free-streaming
effective neutrinos (parametrized by $N_{\rm eff}$) and interacting effective
neutrinos (parametrized by $N_{\rm fld}$). We motivate an alternative
parametrization of DR in terms of $N_{\rm tot}$ (total effective number of
neutrinos) and $f_{\rm fs}$ (the fraction of effective neutrinos which are
free-streaming), which is less degenerate than using $N_{\rm eff}$ and $N_{\rm
fld}$. Using the Planck 2015 likelihoods in conjunction with measurements of
baryon acoustic oscillations (BAO), we find constraints on the total amount of
beyond the Standard Model effective neutrinos (both free-streaming and
interacting) of $\Delta N_{\rm tot} < 0.39$ at 2$\sigma$. In addition, we
consider the possibility that this scenario alleviates the tensions between
early-time and late-time cosmological observations, in particular the
measurements of $\sigma_8$ (the amplitude of matter power fluctuations at
8$h^{-1}$ Mpc), finding a mild preference for interactions among light species.
We further forecast the sensitivities of a variety of future experiments,
including Advanced ACTPol (a representative CMB Stage-III experiment), CMB
Stage-IV, and the Euclid satellite. This study is relevant for probing
non-standard neutrino physics as well as a wide variety of new particle physics
models beyond the Standard Model that involve dark radiation.
