aggressive toddlers?

Parents, would like to hear your views on this. How do you teach your kids to deal with other aggressive toddlers? My 17mo had 2 such encounters over the weekend. At this age, he won't know fully understand but I want to explain that it's not ok for other kids to push him (especially if it was from deliberate aggression) but at the same time tell him he shouldn't retaliate by doing like wise. He hasn't done the latter btw, he just cries and he's gentle by nature, wouldn't hurt a fly.