## 2 Related Work  
As is known to all, TCP is the most widely used reliable transport protocol. But it is proved to be inefficient in ONs, since it misinterprets losses due to route failures and losses due to network congestion [25; 26]. This is even worse in the case of intermittently connected ONs. Transport protocols for MANETs are mainly based on cross-layer signaling between routing and transport so as to notify the transport layer about route failures [27; 28]. They are not suitable for ONs because only opportunistic routing can be performed. A number of reliable transport schemes in ONs are designed for deep space communication, where the traffic is generally light, high data delivery rates are emphasized, and long delays are tolerant. These schemes aim at extending the TCP mode of operation to cope with the unique characteristics of deep-space links. For example, literatures' [12; 13; 14] protocols use retransmission mechanism to achieve transmission reliability, which means that the recovery delay of a lost packet needs to wait at least one supplementary round-trip time (RTT), and the end-to-end delay will be large. This is not problematic for deep space opportunistic networks. However, human ONs have a relatively larger number and higher density of nodes compared to deep-space ON, with more urgent demand for data exchange and lower tolerance for transmission delay; these transport protocols will incur network congestion and unacceptable latency when a huge traffic burst emerges in human ONs with resource constraints. Other solutions such as Licklider transmission protocol (LTP) [29] and those offered by Saratoga [30] make improvements on the basis of the Buddle Protocol (the IETF RFC 5050) [31] proposed by the DTN research group. Specifically, using automatic repeat reQuest (ARQ) on a hop-by-hop basis, they are able to deliver only a per-hop reliability. In addition, the end-to-end delay still depends on the RTT and may exceed the range in human ONs. For terrestrial ONs, to reduce delay and improve the storage efficiency, rateless coding has been increasingly applied in transport schemes for ONs [16; 17; 32]. With coding, the successful reception of a file no longer depends on receiving specific packets, but on receiving a sufficient number of individual packets. Thus, the coding-based protocols can offer resilience to link failures, as well as increase throughput and reduce the delivery delay. Several coding-based schemes have shown that reliable transmission in ON can be allowed by integrating receivers' acknowledgements (ACKs). [PERSON] et al. proposed a reliable transmission mechanism for streaming data by using acknowledgements and network coding [33]. Their pipeline-based stop-and-wait transmission mechanism makes full use of the waiting time for ACK, significantly reduces the data transmission delay, and improves the decoding efficiency. [PERSON] et al. analyze re-encoding strategies at intermediate nodes using an on-the-fly coding paradigm, and propose an adaptive mechanism based on acknowledgements and coding to enable end-to-end reliable communication [34]. Simulation results show that their adaptive mechanism can ensure end-to-end reliability and allow a significant reduction in terms of network overhead. However, they inject considerable redundancy into the network to obtain acceptable performance at the cost of energy consumption at the relays, and they ignore the fact that excessive energy consumption of relays will influence the relays' behavior. In real-life human ONs, consuming a lot of energy will lead to the selfish behavior of relays, and then the reliability of file transfer will be affected. Therefore, to encourage more relays to participate in the transfer process, a reliable transport scheme in real-life human ONs has to take the energy consumption of relay nodes into consideration. Although there are several energy-efficient communication protocols for ONs [35], a few transport protocols considering energy consumption have been proposed in recent years. [PERSON] et al.