## 4 Partition Aware-RPL Protocol Overview
### Protocol Description  
In order to be in conformity with the recommendations of RPL [5] standard, any customization (or improvement) of the initial routing protocol should be limited to the objective function (OF). In fact, the user can define his own OF depending on the application context and constraints. Note that the OF is the RPL mechanism that decides about the best parent selection for any given node. Other RPL mechanisms, like loop checking and avoidance, local and global repair, etc., should be maintained in order to ensure compatibility with IoT standards.  
As such, our main purpose here is to define an objective function (OF) that enables to build a Partition Aware-DODAG reflecting the physical distribution of crops into parcels in our Farmland. In Figure 4, we show an example of a built DODAG respecting such a property. Each parcel can be seen as a separate land covered by a dedicated sub-DODAG, i.e., the nodes of a parcel belong to a unique sub-DODAG: A parcel respecting this property is said to be duly covered [14]. The complete set of sub-DODAGs compose the overall farmland Partition Aware-DODAG.  
Figure 3: DODAG resulting from standard RPL deployed in a sample farmland.  
By definition in [14], every node in a parcel P that is duly covered will be attached to the same branch or sub-DODAG of the already built DODAG. This sub-DODAG, by itself, forms a destination oriented directed acyclic graph, rooted at a particular and single node \(i^{*}\). It is straightforward that all transmitted information from the parcel nodes to the sink transits through this particular node \(i^{*}\). In the following, we refer to this particular node (sub-DODAG root) as the Parcel Head (or Partition Head: PH). Moreover, we refer to a Bridge and edge \(e\) that leaves (considering the \(UP\) direction: from leafs towards the DODAG root) a given parcel \(P\) of the land. Straightforwardly, a farmland parcel \(P\) is duly covered, if and only if, it contains exactly one single and unique bridge. Both definitions about Parcel Head and bridges are graphically illustrated in Figure 4.  
In order to derive this particular structure of DODAG, our proposed algorithm assumes that every node knows the the parcel to which it belongs, i.e., its color, either by knowing its location to derive the color, by hard programming it into the nodes, or by imagining a running initialization phase, where this information is flooded from the sink node towards all the nodes of the network. The proposed improved RPL is based on a novel OF which compares (one by one) the capabilities of the current best parent to all retained parents (i.e., neighbor nodes with higher rank). This function should define a set of tests in order to reach the target objective. For the conventional/standard RPL, the best parent selection decision relies on several common routing metrics usually used for LLNs [24].  
These metrics include the latency, the number of hops, and more significantly the Expected Transmission Count (ETX) [24]. This latter is being used in various implementations of RPL [7], where the selection of parents in the DODAG, is made according to the ETX metric, advertised/dispatched through the DIO messages.  
The parent selection process, in the standard RPL [5], is triggered every time a potential next hop information is updated. This possibly occurs either upon the reception of a DIO message, or after a timer elapses, or also when all stored DODAG parents are no longer available. The next subsection is devoted to present our parent selection algorithm directing the behavior of our OF.