As the former owner of a rottie (he's now in doggie heaven after 16 or so faithful years) I have a beef with the advice that you should attack it's throat.
Don't try attacking a Rottweiler in the throat. To be in a position to do so means that you'll have to be down low where the Rotti's superiour center of balance, weight, and strength will leave you at a severe disadvantage.
Like pitt bulls, rottweiler's can lock thier jaws. Once he clamps down, he's not coming off w/out a crowbar and/or a saw, even if he's dead.
Basically, what everyone else said is true. Make yourself bigger. Make agressive noises and gesture (a quick stomp forward accompanied by a yell (not a scared yell, an angry yell)) works very well on all species of dogs. BUT, for goodness sake, don't do this in the dog's home territory. The last thing you want it thinking is that you want to take it's territory away from it. Them's fightin' words!
I wouldn't recommend this to everyone ('cause any hesitation could cost you a limb), but canines will NOT bite off thier own tongues. If there's an angry rottie (this works, I've had to do it) reach in there and grab it's tongue. DO NOT LET GO!!! THis leaves you pretty much at an impasse. And if there are other dogs nearby, you've left yourself open for attack. Like I said, not recommended, but it works as a last ditch option.
Also, if you want a chance of backing out of a dogs territory, Move slowly, telegraph your movements (don't want to surprise the dogs), and DON'T make eye-contact. You want to get away, not challenge for pack leadership.