||As more networks deploy software defined networking (SDN) to take advantage of its benefits, including logically centralized, more flexible network-control programming, the inter-connection of such SDN based networks has become a major remaining challenge. Traditional interconnection approaches such as BGP are designed for less flexible networks and hence can have serious efficiency, scalablity and stability issues such as the compact policy program instantiation (CPPI) issue. In this paper, we define the requirements of the inter-connection of SDN networks and propose a novel protocol, called the SDN Federation Protocol (SFP), to achieve scalable, efficient, and stable interconnection of SDN networks. Instead of being a traditional push protocol such as BGP, SFP adopts a novel pub-sub model to substantially increase flexibility, efficiency and scalability. Going beyond only packet handling, SFP introduces flexible network information spaces, such as the packet space and the flowset space, to achieve more efficient, autonomous federation of network resources spanning across multiple networks.