# On the Potential of Enhancing Delay-Tolerant Routing Protocols via Age of Information
## 3 Delay-Tolerant Protocols Enhanced via Age of Information
### Fresh Protocol
It should be emphasized that the selection of the appropriate near anchor node is independent of the direction and is affected only by the distance of the source node from the candidate near anchor node. This procedure is depicted in Figure 4.  
Figure 4: The procedure of formation of the rings for the finding of the “near anchor nodes”. The source node is coloured brown, while the initial radius of the ring is the yellow one, the second one is green and the biggest of all is red. In the first ring, the source node has no near anchor nodes at all, while in the second and third ring, it has 3 and 13 anchor nodes, respectively. ```
Procedure FRESH(\(D\)): ifthisnode.ID = D then  replyToSource() else \(T\gets prevEncounterAge(D)\) \(A\gets findNextAnchor(D,T)\) ifA!=D then \(notifyNextAnchor(A,D)\) endif endif
```  
**Algorithm 1** Pseudocode of FRESH Protocol [16]. In the above, \(D\) is the destination node, and the pseudo-command call _prevEncounterAge(D)_ returns the encounter time of the node to which it was called with the node with an ID equal to \(D\), i.e., how much time has elapsed since the last time the node with an ID equal to \(D\) was a direct neighbor of the node to which the **FRESH(\(D\))** function is called. The _findNextAnchor(D, T)_ pseudo-command then returns the node with an ID equal to \(A\) that has most recently met with destination \(D\) relative to the value \(T\), which is the encounter time of the source with the destination. This search will be conducted as described above. Finally, calling _notifyNextAnchor(A, D)_ will trigger the **FRESH(\(D\))** function on the node with an ID equal to \(A\). The _notifyNextAnchor(A, D)_ pseudo-command does not require the T parameter, as the node with an ID equal to A only needs its own encounter time with destination \(D\). In addition to the above original functionality of the FRESH routing protocol, in this paper, we suggest some variations, depending on the choice of the so-called \"near anchor nodes\" on a practical level:  
**FRESH-near-v1**  
An alternative design implementation has the source node to choose to deliver its packet to its \"optimal\" close neighbor, that is, the one within the specific radius ring that would have formed around the source node and have the smallest encounter time with the destination from all near anchor nodes of the source (and of course less than the encounter time of the source with the destination). **FRESH-near-v2**  
Another alternative is to perform the exact same procedure as before for ring formation but to deliver the packet to its first near anchor node that satisfies the above property and not necessarily to its optimal near anchor node. **FRESH-neigh**  
In addition, a different variation is the following: the selection of the next hop node to be based on the current neighbors of the source (based on the encounter times at the link level) and not based on the process of forming the circular rings, as explained above. Naturally, tradeoffs are expected to be found between the above design implementations, as observed in the Simulation section, where the above variants are compared, as well as the modified version of the FRESH protocol, in which the concept of AoI is utilized. At this point, we need to make an important note regarding the energy consumption of the studied protocols. Energy is a key (enabling) factor for mobile networks and DTNs in particular. Energy consumption depends on the transmission range of each device. The larger the transmission range, the greater the associated energy consumption. Thus, protocols such as FRESH-near-v2 will suffer from increased energy consumption, since the increase in the search ring diameter will lead to more energy consumption. This is something that should be taken into account explicitly, and will be deferred for an appropriate extension of the this work (where AoI can be exploited for jointly improving energy and delay performance), as it is a problem of its own merit.