# On the Potential of Enhancing Delay-Tolerant Routing Protocols via Age of Information
## 3 Delay-Tolerant Protocols Enhanced via Age of Information  
### Fresh Protocol  
The FRESH protocol is a routing protocol widely used in DTNs. More specifically, it is a protocol belonging to the broader class of Mobile Ad-hoc Networks (MANET) [34] protocols, and due to the fact that it relies primarily on encounter times, it belongs to the Encounter-based routing protocol category mentioned above. Its operation is relatively simple and is briefly described in the following. Figure 3 summarizes its operation with a flowchart. Each node of the DTN network maintains a file (vector), which contains the most recent encounter times of this node with all other nodes in the network. In each step, instead of searching only for the destination node, the source node searches for \"near anchor nodes\" (the meaning of \"near anchor nodes\" will be better explained below) that had contact with the destination node more recently than the corresponding contact of the source node with the destination node. This step is then repeated until the destination successfully receives the message. [PERSON] et al. [16] have shown that the total path that each packet will follow does not contain a loop (loop-free), as at each step of the algorithm, the encounter age decreases monotonically until the destination node is found in a finite number of steps, with the sole condition that the source node and the destination node belong to a connected subset of nodes. In addition, it is obvious that the algorithm does not require any general knowledge of the network, so it can easily be implemented as a distributed system, because at each step, each node is only interested in its own local encounter table. Figure 3: Flowchart of FRESH protocol. An encounter between two nodes occurs when these nodes are one-hop neighbors. Since the one-hop neighborhood depends on the link layer, the exact condition in order for an encounter to take place varies, as it depends on the respective wireless technology employed. For example, the 802.11b [35] connectivity range can be over 250 m, while Bluetooth allows only a few meters transmission range. The encounter age between 2 nodes is equal to the time that has elapsed since the most recent meeting of these nodes. The way to specify neighbors at any given time at a practical level varies again, depending on the choice of link layer, e.g., sending normal or Hello packets between neighbors (A Hello packet is a special control packet (message) periodically sent by a router in order to establish and verify network proximity/connectivity relationships with other routers. Hello packets are used primarily in the implementation of the OSPF (Open Shortest Path First) routing protocol, and more generally where there is a need to confirm proximity relationships between nodes of a network). An interesting aspect of FRESH is the definition of the term \"near anchor nodes\" in each step of the algorithm. This term does not necessarily indicate that we are dealing only with neighboring nodes. More specifically, the algorithm implements the following operation: when a source node searches for a near anchor node that should have a smaller encounter time with the destination node than the corresponding encounter time of the source node with the destination node, it forms a ring of a specific radius centered on the source node and those nodes inside this ring are examined by the algorithm. If the radius of the ring is small enough so that the source node has no near anchor nodes or in the general case, no near anchor node satisfies the property of having an encounter time with the destination less than that of the source with the destination, then the search continues, appropriately increasing the radius of the ring, until the node that satisfies the above property is found. The way in which the ring radius grows, as well as the choice of the initial ring radius, vary depending on the occasion and the characteristics of the DTN network.