Recently, software-defined networking (SDN) has been expected as an efficient technology to realize flexible resource management and system performance control by separating resource management from geo-distributed resources, especially for heterogeneous ultra-dense networks (HetUDNs). This work establishes an SDN based architecture for mobile traffic offloading in HetUDNs, which consist of densely deployed macro-cell base stations (MBSs) and small-cell base stations (SBSs). Additional, we explore a scenario with information asymmetric, specifically, the capacity of the SBSs can be accessible, but their performance for offloading cannot be obtained by the controller of SDN. To address such asymmetry, we propose a bundle of traffic offloading contracts, which are capable of encouraging each SBS to select the right contract that designed personally to it by promising its maximum utility. Moreover, by designing the contracts which offer rationality and incentive compatibility to different SBS types, the characteristics of a large number of SBSs are aggregated to support the efficient selection on SBSs to provide traffic offloading. Then a closed-form expression for SBS types is proposed, and we prove the monotonicity and incentive compatibility of the resulting contracts. Furthermore, simulation results validate the system performance, and the effectiveness and efficiency of the proposed contract-based traffic offloading mechanism.