Application Layers and Contracts

Question

What is the prefered communication: IAggregationRoot marker is stored in Domain Layer and Data Access Layer references it in IRepository contract or vice versa?

EDIT

I looked at Tim Maccharty's (http://www.wrox.com/WileyCDA/WroxTitle/productCd-0470147563,descCd-authorInfo.html) implementation of repository pattern and unit of work pattern. It's very useful for unit-testing purpose to have own/fake implementation of IUnitOfWorkRepository. I got confused a little bit more with where do I store such contracts.

Thanks!