## 4 Procedures for VTL and Platoon Management over the Virtual Nodes
### Platooning Announcement on Virtual Nodes  
It is unlikely that the vehicles that might be interested in joining a certain platoon ( reach some place in an efficient and comfortable way) will just come across it. Therefore, it is important to disseminate information about existing platoons, their destinations, and the expected times of arrival at different locations along their routes, so that any vehicles can make informed decisions on whether to join a platoon, adjusting its own timing and even the routes they planned to follow accordingly. The dissemination of information is also central in our approach, since it affects the timing of the VTLs and, ultimately, proper global behavior depends on having a reasonably cohesive and updated view of the vehicular traffic at the L1 VNs (the virtual nodes at the intersections).  
In our approach, the platoon leaders (the vehicles at the head of the trains) periodically send announcement messages that make their way through the sequences of L1 VNs along the routes they are going to follow. Those L1 VNs thus learn about the current length, position, and route of the platoon. Thereby, they can update the PSI of the corresponding intersection, with entries containing a timestamp, platoon ID, an estimated times of arrival of the platoon to successive intersections. The estimations are made according to an exponentially weighted moving average (EWMA) of the transit times along the road segments, which are readily measured by the VaNetLayer and VNIBR down in the protocol stack, for the two possible directions (If no data is received for some road segment for a relatively long time, the L1 VN assumes that it is empty and the transit time is estimated from the length and a speed close to the maximum allowed). An estimation of the time taken to go through the intersections is added, too, taking into account that platoons will be given priority over individual vehicles by the VTLs.  
Since the accuracy of the estimations inevitably degrade in each hop between L1 CNs, each estimation in the PSI is labeled with a value between 1 and 4 that represents the amount of uncertainty in the estimation. Accordingly, the announcement messages are only forwarded to the next 4 intersections on the route of the platoon, and each L1 VN in turn disseminates the estimations towards nearby intersections (also up to 4 road segments away).  
It is worth noting that, as shown in Figure 3, the route followed in the dissemination of the announcement message (represented by a dotted white line) does not necessarily match the platoon's route (the solid black line). The data packets will be forwarded passing through all the L1 VNs in the platoon's route, but following the routes decided by the VNIBR protocol. In the example, the platoon leader sends the announcement to the next L1 VN in its route (lower left corner). As mandated by the routing protocol, the next hop is \(L_{1}2\); however, since \(L_{1}1\) senses poor QoS over the road segment to \(L_{1}2\) (due to the large gap of the empty L3 VN between \(L_{1}1\) and \(L_{1}2\)), the packets are delivered via a detour through \(L_{1}3\), \(L_{1}4\) and \(L_{1}5\).  
Figure 3: Platoon announcement along the L1 VNs in the platoon’s route.