## 2 Problem Background
### Existing Solutions for Improving OppNet Routing Fairness  
Solutions addressing the issue of unfairness in SCF-based networks include FairRoute [23], the Context Aware Forwarding Algorithm (CAFE) [60], the Fairness-based Opportunistic Networking (FOG) framework [19] and Congestion Control with Adjustable Fairness (CCAF) [61]. FairRoute'sidea is to allow less popular nodes to find alternative routes by considering long-term interactions, while directing traffic towards nodes with less buffer occupancy. The forwarding utility is based on an aggregated interaction strength between nodes, indicating the frequency of long-term interactions and also accounting for spurious bursts of activity. In order to distribute traffic among nodes, a queue control mechanism forwards messages to nodes with equal or less queue length.  
CAFE favours nodes that are able to receive and retain messages. A congestion mechanism distributes load away from popular nodes by prioritizing nodes with a higher percentage of free buffer space. The forwarding utility combines a smoothed degree of centrality, a filtered common neighbour metric, and the interaction strength between a node pair. The smoothed degree centrality increases the size of centrality subsets, thereby distributing load more evenly. This reduces the likelihood of sending messages to nodes with higher buffer occupancies. On the other hand, message custodians with higher buffer occupancy are allowed a wider range of relays to choose from.  
In order to increase the uniformity of burden distribution, [PERSON] and [PERSON]'s [19] forwarding rule considers nodes with slightly less rank (i.e., forwarding utility). Burden is perceived as the number of messages a node can carry. The authors introduce FOG, a framework to ensure a balanced efficiency-fairness trade-off with low overhead. FOG consists of two algorithms, namely Proximity Fairness Algorithm (PFA) and Message Context Fairness Algorithm (MCFA), and selects a suitable one based on network properties. PFA forwards messages to relays with higher rank if their burden does not exceed the relative average. Since this may overburden popular nodes and cease to utilize them for certain periods, MCFA aims to maximize the overall satisfaction of every user over time. With MCFA, popular nodes are utilized only when less popular nodes estimate that the former are surely needed. A node infers this need from a utility--simply referred to as need-utility here--derived from its rank, its free buffer space and the estimated number of message copies in the network, such that a higher need utility indicates a higher need to engage popular nodes. With this, a node forwards messages to higher-ranked relays only if its need-utility exceeds a threshold.  
CCAF aims to achieve a tuneable efficiency-fairness trade-off by being as fair as possible and utilizing popular nodes wisely. Traffic load is derived from the ratio between remaining storage space and total storage capacity, thereby indicating more burden under higher buffer occupancy. In order to distribute the traffic load fairly, a message is forwarded only if the relay node has a much higher forwarding utility. Messages are forwarded to a relay node with almost equal forwarding utility as the message custodian only when the decision contributes to balancing the traffic load between them.