homework and exercises - When is tension constant in a rope?

Suppose we have a massless rope with pulling forces applied at each end. In which scenarios is the tension in the rope constant throughout? For example if there is a knot in the rope the tension is not constant throughout (why?. Similarly if the rope is hung over a cylindrical pulley of non-neglible radius, the tension is not constant (why?). But if there isn't anything touching the rope, for example 2 people tugging on the rope at each end, the tension is constant (why?).