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The literature classifies existing flooding approaches in four categories, which are: Simple flooding, probability-based flooding, area-based flooding and flood- ing based on the knowledge about neighboring vehicles [136].

Simple flooding. With simple flooding, also referred to as blind flooding, a node simply rebroadcasts a message exactly once. The nodes use e.g., the source ID and packet ID to identify already received messages. This distribution process contin- ues until the message has traversed the network. Hence, every node that receives a message forwards this message to all its neighbors although they may have already received this information. As a result, messages are duplicated, and bandwidth is wasted. However, this algorithm achieves a high probability of reliability, achieved by (redundant) repetitions of messages.

Probabilistic-based flooding. Probabilistic-based flooding, e.g, [99], is similar to simple flooding, except that nodes only rebroadcast a message with a certain probability. In networks with a high density of nodes, a high reception probability can be achieved while saving scarce wireless bandwidth because multiple nodes share similar radio transmission ranges. However, in networks with a low density of nodes, the reception probability decreases, and not all nodes receive the mes- sage.

The counter-based scheme in [99] uses the inverse relation between the number of times a packet is received by a node. The nodes calculate the probability that it can reach additional neighbors with a rebroadcast. Upon reception of a new message, the node initiates a counter and starts a timer which is randomly chosen. As long as the timer continues, the counter is incremented for each redundantly received packet. Upon timer expiration, the packet is only rebroadcast in case the counter is less than a pre-defined threshold. Otherwise, the node drops the message.

Area-based flooding. Area-based flooding algorithms use distance information in the decision process whether a packet should be rebroadcast or not. The node may use geographical position knowledge of a positioning system or it could es- timate the distance via signal strength measurements. When the receiving node is close to the sender, the additional area covered by a retransmission would be small whereas a receiver far away covers more additional area. The distance-based scheme and location-based scheme in [99] estimate the additional coverage area in this way.

4.3. RELATED WORK 103 A node using the distance-based scheme compares the distance between it- self and all neighbors from which it received the message. Upon reception of a previously not received message, the node initiates a timer and caches redundant packets. When the timer expires, all nodes compare their distance to the source to a threshold, and the packet is only rebroadcast in case a node is closer than a pre-defined distance.

Location-based schemes use a more precise estimation of the expected addi- tional coverage area, which relies on geographical positioning. Each node must be able to determine its geographical position. Each packet contains the geographical position of its sender or forwarder. When a node receives the packets, it calcu- lates the physical distance and the additional coverage area it could reach. Again, it compares the result with a pre-defined threshold in order to decide if the node rebroadcasts the packet.

Neighbor knowledge flooding. Using self-pruned flooding [85], a node broad- casts a message on the wireless link that includes a list of its single-hop neighbors. Every node that receives the message compares the list of nodes in the message (except the node itself) with its own neighbor list (NL). In case the node has fur- ther neighbors that are not in the list (i.e., N L(rec) > N L(msg)S(sender)), it

replaces the list of neighbors in the message by its own single-hop neighbors and rebroadcasts the message. While this algorithm achieves a similar level of relia- bility as simple flooding, it reduces the overhead significantly since the approach avoids message duplications to neighbors in overlapping wireless regions.

The time-extended reliable geographical flooding (TERGF) algorithm of this chapter uses the basic idea of flooding with self-pruning, although in a different context. The TERGF algorithm distributes information rather than packets, i.e., the TERGF design assists content-based aggregation of information to reduce band- width consumption. Beyond, TERGF combines self-pruning with GeoCast and extends the algorithm by an acknowledgment scheme in order to achieve full relia- bility. For self-pruned flooding, the node includes only those neighbors into the list in the message that are located inside the geographical target area. If a receiving node compares the list with its own neighbors, it also excludes the neighbors that are not inside the target area. As a result, a node rebroadcasts a message only if it reaches further nodes inside the geographical target area.

The scalable broadcast algorithm (SBA) [105] uses two-hop neighbor knowl- edge. In order to establish this knowledge, the nodes periodically exchange hello messages that contain the neighbor list of its predecessor. When a node receives a packet, the receiver compares its own neighbors with the sender’s neighbors in order to determine if a rebroadcast would reach additional nodes.

The dominant pruning approach [85] also utilizes two-hop neighbor knowl- edge. In this approach, the sender proactively selects the one-hop neighbor(s)

which should forward the message. Only selected nodes are allowed to rebroadcast the message. When a node receives a message, it checks if its address is included. If so, it rebroadcasts the message and uses a modified version of the greedy set cover algorithm [88] in order to select the next level forwarding nodes.

The multipoint relaying mechanism [112], which is part of the optimized link state routing (OLSR) [30] protocol, is similar to dominant pruning. Based on a two-hop neighbor knowledge, the sender determines multipoint relays (MPRs) that are responsible for the redistribution of the message.

The ad hoc broadcast protocol (AHBP) [106], CDS-based broadcast algorithm and the lightweight and efficient network-wide broadcast (LENWB) [124] are sim- ilar to the multipoint relaying approach, but differ in the calculation effort to deter- mine the forwarding nodes. Details are given in [136].