An important challenge for modern data centers is to provide users with low-latency, high-quality, and interactive services. To improve effective application throughput, much work has been done to take into account application deadline in the design of the network flow schedule for transmission of the queried data. This raises a critical question: does application-level throughput necessarily translate to better quality of service for users? We note that queries responses typically include a set of semantically related documents (within a flow), and that while some documents are highly related to that query, others are not. This motivates us to consider the degree of a document's semantic relatedness in the design of an efficient data transmission scheme for data centers. To understand the performance upper bound, we derive a mixed integer linear programming (MILP) model for the above problem, which can be regarded as the schedule of a network subflow problem and is hence complicated. Thus, we propose the importance-aware transmission control protocol (ITCP), a distributed event-driven rate-based delivery control protocol, for server-centric data center networks. ITCP jointly considers the subflow importance, size, and deadline, as well as the characteristics of the underlying infrastructure to maximize the goodput of the most relevant responses of a query. The results of both real and synthetic data simulations show that ITCP outperforms recent works in terms of mean average precision of top k data and the sum of application-level importance.