If met by an aggressive dog, how to kill it and prevent self-damage?

I would like your ideas of how to kill the predators called Dogs the easiest way if met by a stray one in, per example, a dark alley.
Is carrying a Swiss army knife sufficient to put a dog down without hurting oneself? if that's the case, which method do you suggest using and where to put the knife.
If you are not carrying anything but your own skin and get attacked by a large dog, i.e Pitbull, German Shepherd, Rottweiler, or at least notice it's aggression and would like to protect yourself, what methods do you use?