Is it an obligation to leave tips at a restaurant??

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I always thought that a tip was something extra you left for the service provided in a restaurant, for example. I was a waitress too at some point and I used to give an extra in order to get better tips, becuase I knew it depended on me. But nowadays I don´t know. I was recently confronted by the waiter at a restaurant because he considered I didn´t give him the tip he expected. First of all the service was awful! He didn´t provide a good service, I had to constantly remind him everything i needed, and he made remarks at my comments. Second, it´s up to me if and how much I should tip. But he said that he provided a service, good or bad,but he should be rewarded for it,period. So I was wondering if you agree with him. I don´t consider myself a bad tipper but I don´t reward a bad service either. And once in a restaurant, the tip was even included in the bill,so I´m guessing it´s now an obligation or what??