Cloud federation paradigm can help cloud providers (CPs) to overcome resource limitation during spikes in demand, by outsourcing requests to other CPs with idle resources in federation. In this paper, we consider a broker-based market where multiple CPs compete to lease multiple CPs with idle resources for request outsourcing. We propose an iterative double auction mechanism that ensures the efficient operation of the market by maximizing the social welfare of the federation. The proposed scheme takes into account the particular characteristics of request outsourcing in cloud federation, such as the average response time requirement of CPs and the coupling of CPs' outsourcing decisions. Moreover, this scheme does not require the full information of CPs, and achieves a non-negative revenue for the broker.