This paper identifies all factors impairing TCP performance based on network protocol hierarchy: Lossy wireless channel at the physical layer; excessive contention and unfair access at the MAC layer; frequent routing changes due to node mobility at the network layer; the fundamentally inappropriate mechanisms of TCP at the transport layer, including window-based congestion control, loss-based congestion detection, slow-start and AIMD (additive increase/multiplicative-decrease) of congestion window; reliance on ACK-clocked characteristics. Then, it designs a novel cross-layer optimal congestion control (CCOC) protocol tailored toward the characteristics of ad hoc networks. Cross-Layer design framework is applied in CCOC to improve fair access at MAC layer, to detect false link failure, to reduce the number of route failures, to quick-start during route changes, to transmit reliably based on SACK, and to implement the adaptive optimization strategy guided by the nonlinear optimization theory. Then, this paper outlines the protocol framework of CCOC. Finally, the extensive packet-level simulations is implemented in NS2 environment. The simulation results show that CCOC significantly outperforms TCP and ATCP in many important performances such as throughput and fairness, under a variety of scenario and mobility conditions.