There are plenty of instructions on what to do if approached by a dog, for example this one : http://www.wikihow.com/Handle-a-Dog-Attack . I don't see anything new there, have been using similar approach, despite never reading any instructions or guides on how to handle dog attacks.
But how does this change if I have my dog with me? And he is the target of the other dog?
I have an English springer spaniel and when we walk, sometimes we come across unleashed dogs who approach and start fighting, and don't follow their owner's commands. German shepherd, staffordshire terrier and boxer, those are the breeds.
I can tell my dog to stand still, I can put the leash on him. But trying to not let a fight happen can be difficult at times. And breaking one up as well.
Shouting angrily in a deep voice works most of the time. But sometimes dogs just ignore me, and try to ran past me to attack my dog.
Another question, related to first one, what should I do if some dog is biting mine, and not letting go? I have not had this happen to me, but a neighbor had his collie badly hurt by a boxer. He was unable to get him off until boxer's owner came.
There is plenty of information on these questions on the internet, but it is hard for me to understand the quality of information in this case. And I don't want to do something stupid if the described situation happens. I hope that here I will get answers from knowledgeable people, typical to StackExchange communities.