Back in the early 60s my dad would ride his bike to work. He carried a squeeze-bulb with an ammonia solution for dogs. He never had a repeat 'customer'.

I started riding in the mid '70s, and often waved or struck my frame pump in the doggies direction - result was a bent pump. A good "NO!" works better and is less costly.

Fast forward - I have carried mail for 33+ years and have never been bitten by a dog I have seen first. Halt spray? forgetaboutit!!! The loud primeval yell works best - it doesn't matter what word/sound you use. Act bigger/tougher than the dog. Keep eye contact. Watch for secondary doggies. MAYBE I would carry one of those collapsible coiled-spring-steel batons.