Is ripples in frequency response of a filter ,result of periodic extension of a signal in time domain thereby producing discontinuities?
It's mostly related to what's called the uncertainty principle of the Fourier Transform: "the more compact a signal is in one domain the more spread out it is in the either". A sharp transients in the time domain has a wide spectrum. A sharp transition in the frequency domain has a long time domain ringing. See for example http://www.ams.org/samplings/feature-column/fcarc-uncertainty