It's already known in physical communities that the velocity of light always constant and not depend on motion of observer or source. But it is also teached to us that there are aslo Doppler shift experienced by light wave like that when measuring CMB radiation. But for me it is makes a non-sense. If I approaching the light source then the frequence of light I receive become increased, but when receding the light source the frequency of light I receive decreased. But in this two case the wavelength remain constant (or experienced length contraction in the same amount without seeing whether it is receding or approaching). Then the velocity of light not constant for this two case, becase velocity = wavelength x frequency.