## 1 Introduction  
Recently, the rapid proliferation of intelligent wireless devices (e.g., mobile sensors, smartphones) provides opportunities for peer-to-peer communication. In practical applications, a contemporaneous end-to-end path does not always exist due to limited wireless radio range, noise, or lack of stable electrical power supply. Under such circumstances, communication protocols of mobile ad hoc networks (MANETs) are not very efficient [1]. As an important evolution of MANETs, opportunistic networks (ONs) aim at making communication between the source and destination in an intermittently connected environment, thus becoming an attractive research area [2; 3]. In ONs, a source relies on the mobility of other nodes which act as relays, and takes advantage of the opportunity of contacts which occur when the mobile relays come into the radio range of each other. Data are delivered to the destination in a \"store-carry-forward\" manner, where a message could be sent over an existing link, getting buffered at the current node until the next link in the path comes up, until it arrives at its destination [4]. Although ONs are not competitors for the fastest or most reliable network, they have some advantages. First, they are second to none in terms of delay and interruption tolerance. In addition, they operate independently of infrastructures, can extend the cellular networks' coverage, and offload part of the wireless traffic at a low cost. These features make them very useful in many applications such as the internet of things, field military networks, interplanetary networks, and underwater sensor networks. Opportunistic networks can be formed by intelligent wireless devices carried by human beings, such as mobile phones and laptops, to enable individuals to exchange information with each other, using this peer-to-peer opportunistic communication [5]. This kind of ON has been applied in several scenarios, for example, temporary community [6], the post-disaster system [7], vehicle ad hoc networks [8], etc. In a temporary community, people with the same purpose or interest likely encounter each other frequently; they will gather in conferences, squares, office buildings, or campuses, to construct an ON. The Wi-Fi and Bluetooth interfaces of their automobiles or iPads are utilized to share pictures, audio files, video clips, or advertisements among interested mobile users. In a post-disaster system where cellular and other traditional communication infrastructures are overused or considerably destroyed by natural disasters, rescue teams and survivors can form an ON for key information exchange through their active physical movement. In these human ONs, most of the devices are hand-held and run primarily on alternative power sources (battery) [9], so their network resources such as cache and energy are limited. In addition, the traffic is generally large: People in a temporary community tend to share files frequently with others [10]; rescue workers in post-disaster scenario would try to send reports about the affected area such as temperatures, pollution levels, and triage information of the victims to the accessible control center or the internet, and the survivors tend to connect to their relatives and friends to enquire their safety. It is of great importance to realize reliable data transport, especially in the post-disaster system, ensuring that critical information about the victims and rescue do reach the destination, which can significantly improve the success rate of rescue. However, the exchange of messages is done based on the transient and opportunistic contacts between moving users, such that best effort networks are prone to unbounded delivery delays and poor user experience [11]. A number of reliable transport schemes for ONs have been designed for deep space communication [12; 13; 14]. These schemes make use of retransmission to cope with the unique characteristics of deep-space links, where the traffic is generally light, high data delivery rates are emphasized, and long delays are acceptable.