## 3 ROR: Reliable Opportunistic Routing for CRSN
### VCG Formation  
After the route selection, all the nodes along the chosen route are now referred to as local minimum resolution (LMR) nodes. Accordingly, from the sink, the next node in the route is referred to as \(LMR_{(h-1)}\), \(LMR_{(h-2)}\),..., \(LMR_{0}\). The sink then generates and broadcasts the VCG formation packet via CC along the chosen route to the source node. The important fields of the VCG payloads are as shown in Figure 3. All nodes that receive the VCG packet, which is identifiable by reading the VCG packet identifier, are programmed to read and take proper action, and only the LMR node whose address is identified is allowed to forward the broadcast along the upstream to the source node. Before broadcasting the VCG formation packet, the sink places the next upstream LMR node, \((LMR_{(h-1)})\), address, \(n_{j}\), and its corresponding distance, \(r_{j}\), from the sink into the VCG formation payload. You will recall that this information has already been acquired during the route request phase. Other required information to be stored in the VCG payload are the sending nodes' distance to the sink; in this case, since the sending node is the sink, the value for this field will be zero. As has been earlier discussed in Section 3.3, there is the possibility that a node might have different channels for upstream and downstream communication. At this point, it is important to note that the Cr0 and Cr1 fields will be respectively replaced with the Cr and Cs information acquired during the route request; while the Cs field will retain the value of Cs, as already acquired from the route request. The significance of this configuration is that it expands the possible reception area, which has the potential of reducing the final hop count. This is because, based on geographical forwarding schemes that make use of both distance  
Figure 3: Virtual contention group (VCG) formation operation with payload. LMR, local minimum resolution. and the signal-to-noise reception ratio as forwarding parameters, it has been observed that nodes in the transitional region usually exhibit the highest (joint distance and SNR) forwarding values for lossy links [18], as is the considered case. Finally, the required data rate is also included in the payload, as illustrated in the VCG formation payload in Figure 3. It is important to mention the rationale behind all the identified fields of the VCG payload. Thus, the VCG packet identifier is to differentiate the control packets and help all nodes in the region between consecutive LMR nodes to perform the appropriate VCG formation operation, which shall be discussed shortly. The next upstream LMR node address is to specify the only node with the right to rebroadcast the VCG formation packet. Both the sender node's distance to sink and the LMR node's distance to sink is to enable the other nodes that are not part of the selected route, but exist in the region between two LMR nodes, to determine their position with respect to the sending node and the next upstream LMR node. It is important to note that, although the sink node is not an LMR node, however, with respect to this phase, it can be regarded as an LMR node for simplicity. Afterwards, all nodes that receive the VCG packet determine its position accordingly. If the nodes' locations fulfill the criteria, \(d_{sk}^{lmr}\geq d_{sk}\geq d_{sk}^{snd}\), then the node proceeds with the next operations of the VCG eligibility operation, as discussed below. Where \(d_{sk}^{lmr}\) is the distance of the next upstream LMR node from the sink, \(d_{sk}^{snd}\) is the sending node's distance from the sink and \(d_{sk}\) is the receiving node's distance from the sink.