The problem of overbuffering in the current Internet (termed as bufferbloat) has drawn the attention of the research community in recent years. Cellular networks keep large buffers at base stations to smooth out the bursty data traffic over the time-varying channels and are hence apt to bufferbloat. However, despite their growing importance due to the boom of smart phones, we still lack a comprehensive study of bufferbloat in cellular networks and its impact on TCP performance. In this paper, we conducted extensive measurement of the 3G/4G networks of the four major U.S. carriers and the largest carrier in Korea. We revealed the severity of bufferbloat in current cellular networks and discovered some ad-hoc tricks adopted by smart phone vendors, which mitigate the impact of bufferbloat but result in performance degradation under various practical scenarios. To address the problem, we propose dynamic receiver window adjustment (DRWA) that requires slight TCP modification only in smart phones, thus guarantees quick deployment via over-the-air updates. Our extensive real-world tests confirm that DRWA reduces the latency of TCP flows by 25-49 percent and increase TCP throughput by up to 51 percent in certain scenarios. It is further verified that DRWA has significant effect on the latency of Voice-over-IP traffic and on the traffic going through TCP split scenarios with performance enhancing proxy.