**5.3 Security and privacy**

464 Wireless Communications and Networks – Recent Advances

needs to forward a packet it includes the identifier of the destination-node and its geographical location into the header of the packet. Each node along the forwarding path consults its list of neighbors and forwards the packet to the neighbor closest to the

Although the geographical forwarding works well for networks where nodes are uniformly distributed, perhaps cannot find a route to a packet's destination when the packet has to travel around a topology "hole" - that is, when an intermediate forwarding node has no

 **Ad Hoc On Demand Distance Vector (AODV)** is a routing algorithm where the nodes of the network upon receiving a broadcast query they record the address of the querying node to their routing table. The process of recording the previous hop is called backward learning. When a packet reaches its destination a reply packet is sent back to the source through the full path retrieved from the process of backward learning. At every node of the path, the previous hop should be recorded, creating this way the forward path from the source. The query together with the response create a complete bidirectional path. After setting the path, it is maintained as long as the source uses it. A failure on a link will be reported recursively to the source and in turn this will trigger another query-response process for finding the new route. More details about AODV

 **Dynamic Source Routing (DSR)** is an algorithm that uses source routing, that is the source indicates to a data packet the sequence of intermediate nodes on the routing path. In DSR, the query packet copies in its header the identities of the intermediate nodes it has already visited. Afterwards, the destination uses the query packet to retrieve the entire path to respond to the source. As a result, the source can establish a path to the destination. If the destination node is allowed to send multiple routes responses, the source node may receive and store these multiple routes. An alternative route can be used in case a link of the current path is broken. In a low mobility network DSR has the advantage over AODV in case the alternative route can be tested before the DSR initiates another query to discover the route. There are two major differences between AODV and DSR. The first is that in AODV data packets carry the destination address, while in DSR data packets carry all the routing information. This means that DSR has probably more routing burden than AODV. Moreover, as the diameter of the network increases, the burden on the data packet will continue growing. The second difference is that in AODV route response packets carry the destination address and the sequence number, while in DSR they carry the address of each node along the route.

The interested reader in DSR can refer to (Johnson & Maltz, 1996). A brief description of some *location-based* routing algorithms is given below:

 **Connectivity-Aware Routing (CAR)** is a routing algorithm which derives from the work performed by the Preferred Group Broadcast (PGB) to reduce the broadcasted packets during the discovery of the AODV route taking also into account the mobility of the nodes. CAR uses the route discovery of AODV to find routes with reduced broadcasting from PGB. However, the nodes forming the route record neither their previous node from the backward learning nor their previous node which forwards

destination in terms of physical location, until it reaches its final destination.

neighbors who are closer than itself to the destination of the packet. An overview of some *topology-based* routing algorithms is given below:

one can find in (Perkins & Royer, 1999).

*Security* in V2V and V2I communications is a prerequisite for future development of cooperative systems and actual deployment in the real world. Co-operative systems have to ensure that data transmission derives from a trusted source and has not been counterfeited. For example, in a red light violation warning application, the in-vehicle system receives data from the equipment which is installed in the traffic light and then decides to issue or not a warning to the driver. An incorrect transmission from a malfunctioning or compromised unit might jeopardize vehicle's safety as well as others' safety in the vicinity. Similarly, the future development of safety applications is jeopardized without securing that transmissions are coming from a trusted source.

*Privacy* and anonymity are primary issues that also have to be addressed. In co-operative applications vehicles are broadcasting messages about their current location, speed and heading. It is desirable for the users to maintain their privacy since they fear that such a system could be used to build tracking mechanisms which would allow harassment, automatic issue of tickets for speeding or otherwise act in an undesirable way for them.

Unfortunately, on the other hand anonymity may be abused. Some examples are sending fake information or spamming. If the system ensures accountability1 then the users know that there will be consequences for others if their data is abused. The challenge here is ensuring anonymity and at the same time accountability, as they seem to be conflicting.

There are many ongoing research activities on security and privacy in co-operative systems. Some ideas that have been proposed for solving such issues include public key certificates or digital signatures. For more information the interested reader can refer to (Fischer et al., 2007; Raya & Hubaux, 2007).

 1Accountability is the ability to attribute actions to the entity that caused those actions.
