# On the Potential of Enhancing Delay-Tolerant Routing Protocols via Age of Information
## 2 Related Work
### Delay Tolerant Networks  
A special category of networks, referred to as Delay Tolerant Networks (DTNs), describes networks capable of managing long end-to-end delays and potential data losses more effectively in a variety of disrupted scenarios and environments [1; 2; 18]. Such envi  
Figure 2: An indicative sample path of the process \(\Delta(t)\) showing how AoI grows and drops upon receipt of an update. This would be the AoI measured by a node for packets from a specific source. Multiple such samples would be obtained at the same node for packets belonging to other flows from different sources (or the same source). ronments could be emergency communication infrastructures following a disaster, space communications, battlefield communication networks, earthquake and extreme terrestrial environments, etc. The alternative term disruption-tolerant network is also occasionally used. The main feature of such networks is that they may not have permanent connectivity between any two nodes. DTN differs from the traditional Internet in terms of the transmission of the information. More specifically, in the traditional Internet a store-and-forward mechanism [5; 6] is used in order to transmit the desired information, in which packets are forwarded to an intermediate node where they are stored and sent at a later time either to the final destination or to another intermediate node. At the intermediate stop, the packet integrity is verified before further forwarding (by analyzing packet headers). On the other hand, the mobility in DTN essentially imposes a different approach called store-carry-forward mechanism [3; 4], in which a node sending a packet stores it in its buffer. The packet is forwarded to another node when the two nodes are in communication proximity, and if a pre-defined forwarding condition, e.g., distance to final destination is reduced, is met. Through opportunistic contacts and a series of intermediate forwarding, the packet can be delivered to its final destination. Some of the key features of DTN are the following [1; 18]:  
1. **Disconnection and variety in mobility patterns**. In the DTN setting, it is frequently impossible to establish an end-to-end connection or observe deterministic mobility patterns. Stochastic node mobility means that any two nodes may not eventually meet for a long time. 2. **Long delay in queues**. Due to the fact that link disruption is very common in DTN, nodes must temporarily store messages, and queuing time can vary from seconds to minutes, even days. 3. **Limited resources**. DTN nodes typically have limited memory, as well as limited processing power. This results in the use of special routing protocols, which are significantly different from the routing protocols used in the traditional Internet. 4. **High rate of data transmission errors**. The fact that the environment in DTN is changing so dynamically as well as the unknown mobility of DTN nodes results in a higher probability of packet loss than in traditional communication networks. Routing can be a major challenge in DTNs due to the first feature mentioned above. Various works proposing routing solutions have been proposed to date. Considering them, DTN routing protocols can be segregated into four main categories, on the basis of the employed knowledge to determine the next hop node: _encounter-based, time-based, infrastructure-based_ and _hybrid and other types of routing_[1]. They are explained briefly in the following:  
* **Encounter-based routing:** This category includes protocols that exploit the meeting history between nodes for making packet forwarding decisions. When two nodes are in communication range, they are considered to have an \"encounter\". In the protocols of this category, each node maintains the history of such encounters. Indicative examples are the Epidemic Routing Protocol [19; 20; 21], PROPHET [22; 23] and FRESH [16].