Unfortunately aggressive dogs will always appear now and again. There is no real answer but I find my chunky Gaerne ankle length SPD boots to be quite useful. Recently I suddenly found myself pursued along a cuclepath (no owner in sight) by an enraged bull terrier that caught up (they are surprisingly quick) and started lunging to bite my calf muscle on my left leg. A few kicks to the head with my size 12 boots and it suddenly lost interest. If it hadn't I would have quite happily kicked it to death because bites from such a dog are no trivial matter, can cause to nerve damage, need for skin grafts, muscle loss etc. By the way, the Gaerne boots ride great as well - quite light but with a really stiff sole!
Cheers,
Adam