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Cooperative and Adaptive Cruise Control (CACC) is widely focused to enhance driving fuel-efficiency by maintaining a close following gap. The ecology of CACC could be further enhanced by adapting to the rolling terrain. However, current studies cannot ensure both planning optimality and computational efficiency. Firstly, current studies are mostly formulated on the conventional time domain. These time domain based methods cannot ensure planning optimality for space-varying road slopes. Secondly, fuel consumption models are non-linear and hard to solve efficiently. Hence, this paper proposes a space domain based Ecological-CACC (Eco-CACC) controller. It is formulated into a nonlinear optimal control problem with the objective of optimizing global fuel consumptions. Furthermore, a differential dynamic programming-based solving method is developed to ensure real-time computational efficiency. Simulation results have shown that the proposed Eco-CACC controller can improve average fuel saving by 37.67% at collector road and about 17.30% at major arterial. String stability of the proposed method has been theoretically proven and experimentally validated.