—This article tackles the issue of reliable transport in Delay-Tolerant mobile ad hoc Networks (DTN), that are operated by some opportunistic routing algorithm. We propose a reliable transport mechanism that relies on ACKnowledgements (ACK) and coding at the source. The various versions of the problem depending on buffer management policies are formulated, and a fluid model based on mean-field approximation is derived for the designed reliable transport mechanism. This model allows to express both the mean file completion time and the energy consumption up to the delivery of the last ACK at the source. The accuracy of this model is assessed through numerical simulations, and a detailed investigation of the impact of the system parameters on the performance is conducted. We eventually present a joint optimization of the mean completion delay with or without an energy constraint, so as to identify the optimal set of parameters to use.