## 5 Discussion  
In this section, we discuss on the benefits brought by the adoption of a RD/SD mechanism exploiting the normal behavior of RPL, such as DiRPL, rather than using a RD/SD protocol which is operating at upper ISO/OSI layers. With reference to the architecture proposed in Section 3, it is clear that, by exploiting the basic operations of the RPL protocol (namely, transmission of control messages for DODAG maintenance), the RD/SD approach embedded in DiRPL does not insert, by default, any additional operation and, thus, any additional delay. More precisely, DiRPL does not introduce any additional overhead in the DIO/DAO messages exchange among entering nodes and the RPL Border Router during the DODAG topology construction. This can be inherently considered as an energy efficient approach, since the energy consumption for the constrained nodes joining the DODAG is unavoidable, while the energy consumption of the Border Router is likely not a problem, since the Border Router is expected to be plugged in into an electrical network. Hence, the constrained nodes do not incur any supplementary energy and time consumption beyond those required by the RPL protocol during its normal operations. Moreover, if compared with other RD/SD mechanisms (e.g., DNS-SD, mDNS [30; 31; 32], Bonjour [33], HyperCat [34], AllJoyn [35], DLNA [36], SSDP [37], Physical Web [38], NetBIOS [39], WS-Discovery [40]), the RD/SD approach proposed in DiRPL is less time-consuming, due to the fact that DiRPL works at the ISO/OSI L3 layer, while other approaches operate at higher layers (e.g., L4, L5, and L7). This can be denoted in Figure 12, in which some RD/SD mechanisms are highlighted with respect to their proper ISO/OSI layer, and in which is possible to note that, in a RPL-based network, all these protocols should work on top of RPL itself, it this way increasing the time required to complete the RD/SD procedure. On the contrary, adhering to a RD/SD paradigm such as the one in DiRPL, it is possible to restrain the time required to complete these operations, as well as to avoid both time and energy consumption caused by packets having to go up to upper ISO/OSI layers, as shown in Figure 12. Hence, as depicted in Figure 13, the DiRPL approach allows to perform RD/SD only exploiting the normal operation of RPL, with packets flowing, into each DODAG-handled node, till L3 of the ISO/OSI protocol stack. Figure 12: Comparison among different RD/SD protocols operating at various ISO/OSI layers. An illustrative multi-hop path is shown: intermediate nodes use their protocol stacks up to L3. For what concerns the performance evaluation, the choice of evaluating the performance of DiRPL on a linear network topology is due to the simulation implementation efficiency of this type of topology: the simulation time required to generate the proposed results is approximately 240 h on a commercial PC (as shown in Section 4, equipped with Ubuntu 12.04, equipped with a 2.2 GHz Intel i7-3632 QM processor and with 4 GB RAM). Moreover, the performance of a linear network can be considered as a performance benchmark for a multi-hop path in a more general (e.g., tree-like) network topology. The application of DiRPL to other scenarios is an interesting research direction, possibly comparing the obtained performance with various simulated constrained devices (e.g., TelosB, Tmote Sky, Z1, all characterized by different RAM/ROM availability). We also remark that, regardless of the complete network topology, each multi-hop path (e.g., toward the BR) of the proposed IoT scenarios can be interpreted as a linear \"subnetwork,\" making the obtained results applicable (as a theoretical performance benchmark) to a general case. Nevertheless, we plan to move to a more efficient simulation platform, such as a cluster farm composed by more powerful servers, specifically dedicated to simulation purposes. This will allow us to simulate DODAGs with a larger number of constrained nodes in an acceptable time.