The majority of Internet traffic is transferred by transport protocols. The correctness of these transport protocol implementations is hard to validate as their behaviors depend not only on their protocols but also on their network environments that can introduce dynamic packet delay and loss. Random testing, widely used in industry due to its simplicity and low cost, struggles to detect packet delay related faults which occur with low probability. Symbolic execution based testing is promising at detecting such low probability faults, but it requires large testing budgets as it attempts to cover a prohibitively large input space of packet dynamics. To improve its cost-effectiveness, we propose two domain-specific heuristic techniques, called packet retransmission based priority and network state based priority, which are motivated by two common transport protocol properties. In our experiments using the Linux TFTP programs, our techniques improve the cost-effectiveness of symbolic execution based testing for transport protocols, detecting three times as many faults when the budget is in the range of minutes and hours.