Software Architecture Design

We will start a new project and need to make architecture design decision. The architecture is basically Domain driven architecture. Our layers : App.Entities, App.Common, App.UI, App.BLL, App.DAL. We have our interfaces in BLL layer. We need to know what is the optimal way to locate interfaces in application. I mean, is it ideal interface contracts to be in App.BLL layer with implementations? Or is it better to be in seperate App.BLLContract layer. Do you have any suggestions and what is the reason for it?