Alekhine's Defense variation, understanding white's overextended pawns, black's multiple early knight moves I'm a casual, semi-serious chess player and have been an active player for over 10 years. In my time playing, one of the most unusual openings I have had the pleasure of playing against is known as Alekhine's Defense:

[FEN ""]

1.e4 Nf6 2.e5 Nd5 3.d4


In the above line I gave the usual move 3.d4. This seems like a very good position for white, and an underdeveloped position for black. I am, however, more interested in the less common variation 3. c4:

[FEN ""]

1.e4 Nf6 2.e5 Nd5 3.c4 Nb6


which is my usual line of play. From my perspective, the aggressive 3.c4 seems like a better move, buying a pawn development move with a tempo (the pawn at c4 is defended by the white bishop after the response 4...Nb6.) In fact, there are over 300 tournament games along this line of play, indicating better chances for white in the final position above.

I have read in chess literature that if an attacking player is not careful with Alekhine's Defense, he may easily overextend his pawn structure, one of the subtleties of this particular openings. I'd like to know if the last variation I described above is already considered an overextended pawn structure, if so why, and if not, what must white be wary of in order to avoid overextending. 

Furthermore, what does black hope to accomplish by placing his knight in peril so many times? Does the fact that white must overextend himself slightly in order to chase the knight multiple times outweigh the opportunity cost of moving the same knight 4 times in the first 4 moves?
In short, white may well be overextending. 

From black's point of view¹, white is offering many targets for black to attack :


d4 as c3 is not possible anymore, and if e5 is traded for d6.
e5 if d6 gets traded for d4
c4 if white's king bishop moves anywhere useful
Behind the pawns is a lovely place to land knights (d3 via b4 is not uncommon, e3 via f5 seldom happens…)


Once white's pawns are fixed, one can wonder about what advancing them actually achieved : white loses as much time pushing pawns as black moving the knight.

¹ which I adopt as an all-time Alekhine Defense player myself.