This paper addresses the limitations of conventional sliding mode control (SMC) for managing transmission control protocol (TCP) network congestion under mismatched disturbances. Existing methods lack simplicity and efficiency. To overcome this, a new SMC approach based on a non-smooth integral sliding manifold is proposed. This manifold provides two main advantages; it functions as a reduced-order disturbance observer, enabling the autonomous estimation of mismatched disturbances without the need for an additional observer, and it eliminates reaching phase dynamics by ensuring the system starts directly on the sliding surface. Consequently, the system trajectory remains disturbance-insensitive from the outset. All theoretical results are validated through Lyapunov analysis. Finally, numerical simulations are presented to illustrate the effectiveness of the proposed approach.