Currently, there are several attempts to connect ad hoc network with legacy infrastructure network like 802.11 WLAN. This paper is initial work about how we can efficiently use ad hoc network with infrastructure at the view of ad hoc TCP. To design efficient infrastructure support for ad hoc network, we propose mathematical models of TCP over ad hoc network and over hybrid network. Unlikely to previous TCP models for wired network, our models for ad hoc network mainly depends on hop-count, not congestion window size. Based on the new TCP models, we propose a cost-efficient position plan for base stations (BSs) and a route selection algorithm. The proposed position plan of BSs promises cost reduction with small performance trade-off. The route selection algorithm makes the decision on whether we should use infrastructure support or not. Roughly, a n-hop route using infrastructure would be better than \frac{n}{2}-hop route only using pure ad hoc network. We have evaluated the TCP models and the proposed infrastructure design principals using the Network Simulator2(ns-2).