in winter is not that much of a problem since i will have minimal injuries if the dog attacks the arm.
Don't be too sure. When I was an MP, we had one of our dog handlers bring out his female Malanois to do some training with our platoon in the field.
One of our guys put on the sleeve and fired off some blanks from his rifle. The dog bit him hard enough that even with the sleeve on the guy had deep bruises and a little bleeding from the four canine hits.
That dog was all of 55 lbs.
Dogs don't just have their bite strength, they have their entire body weight plus their pretty darn impressive speed added to that.
As to the original question: Pepper spray, or maybe even the smaller sized bear spray is probably the best option. It's easier to make a hit with a good spray, it's very effective as a deterrent, and you have less legal hassles. Of course having a stout stick or a firearm for backup (or both) is not a bad idea either.
I wouldn't engage a dog with a pocket knife, though. You're going to want to keep his mouth full of knives off you, and an enraged dog isn't likely to respond to much less than a lethal knife wound. You're better off using both hands to wrestle the dog, or to yank him off his feet, which eliminates the base that a lot of his power comes from. If you're strong enough to snatch the dog off the ground and repeatedly slam it into something like a car or a telephone pole, you can take a lot of fight out of a dog.