In this paper, we provide a holistic picture of the research efforts towards the design and development of transport protocols for DTN environments. In the first part, we provide an exhaustive and insightful survey of the literature on transport protocols and proposals aimed at DTNs. In the second part, we describe a new reliable transport protocol based on coding. Our proposed protocol is targeted at terrestrial DTN environments consisting of a large number of highly mobile nodes with random mobility. The key idea behind our proposal is that the average dynamics under such a network setting can be captured by a fluid-limit model and the protocol parameters can be optimized based on the fluid-limit model. Through simplified versions of our proposal, we guide the readers in a step-by-step manner through the intricacies of obtaining deterministic fluid-limit models for networks where the dynamics can be stochastically modeled by a continuous time Markov chain with a large state space. We also provide the relevant background material so as to help the readers clearly understand the methodology and enable him/her to apply the technique to their own research problems.