  We consider the definition of unpolarized transverse-momentum-dependent
parton distribution functions while staying on-the-light-cone. By imposing a
requirement of identical treatment of two collinear sectors, our approach,
compatible with a generic factorization theorem with the soft function
included, is valid for all non-ultra-violet regulators (as it should), an issue
which causes much confusion in the whole field. We explain how large logarithms
can be resummed in a way which can be considered as an alternative to the use
of Collins-Soper evolution equation. The evolution properties are also
discussed and the gauge-invariance, in both classes of gauges, regular and
singular, is emphasized.
