The IC-MANET is very useful during wartime as the soldiers and combat vehicles are always equipped with sensors and mobile communications. By using IC-MANET, information is gathered and communicated from a territory when conventional communication method is either destroyed or under an electronic jamming attack. Thus, command, control and communication (C3) are carried out by using IC-MANET.
2.1.3 Major Challenges
While taking into consideration of the above characteristics, some major challenges are experienced are:
Routing: The main objective of routing in IC-MANET is to maximize the probability of message delivery and minimize the resource consumption (i.e., buffer space, network bandwidth and battery energy). Although IC-MANET applications are expected to be tolerant of delay, does not mean that they would not benefit from decreased delay, but it is still meaningful to minimize the delivery latency.
Resource Allocation: In general, the routing protocol should balance the goals of maximizing message delivery and minimizing the resource consumption which are in conflict with each other. For example, if it is to ensure that data is successfully delivered to the destination, then it is unnecessary to store the copies of the data in all hosts in the network, on the other hand, for maximizing the likelihood that a particular message is eventually delivered, the best way is to increase the copies of message at multiple hosts.
Buffer Space: To cope with a long time of disconnection, messages must be buffered for along long period of time. The intermediate router should require buffer space enough to store all the messages to be transmitted. Furthermore, there is a relationship between the buffer space and the number of pending messages (which has not been delivered yet to its destination host). More number of pending messages needs more available buffer space. In IC-MANET since most of the nodes are mobile, a strong vigilance from any unauthorized use is a necessity.
Reliability: For reliable delivery of data in IC-MANET, the routing protocol should have some acknowledgement, which ensures successful and stable delivery of data. For example, when a message correctly reaches to a destination, some acknowledgement is received at source.
Energy: Because of mobility of nodes it consumes considerable power due to absent of easy connectivity to power stations; as the nodes are always want of energy, during routing, lots of energy is consumed for sending, receiving and storing the message, as well as performing computations. Therefore, designing energy-efficient routing protocols is equally important.
Security: Security is always an important issue not only in IC-MANET but in all traditional networks. A message may traverse an arbitrary path of hosts before reaching its ultimate destination. Depending on the sensitivity and security requirements of applications, users may require certain guarantees about the authenticity of a message. At a time cryptographic techniques are beneficial for securing end-to-end routing, because, in this, the receiver establishes by exposing un-trusted hosts. Therefore, the security in routing protocol is still an issue wide open for further research.
2.2 Routing in IC-MANET
Let's classify IC-MANET routing protocols into predictable routing and unpredictable categories. In predictable routing the forwarding message is obtained by directly employing specific routing metrics. Based on the estimation of meeting delay between nodes, the cost of each edge in network graph, is a time-varying function. So, several existing routing algorithms such as Dijkstra can take advantage of the graph to obtain the optimal routing. In unpredictable routing it is hop by hop, and covers active scheme, forwarding and replication.
2.2.1 Predictable Routing
Given the amount of knowledge of a mobile network such as nodes' contacts history, queuing and traffic demand, a graph including edges with time varying capacity and propagation delay can be obtained. By taking advantage of the model derived from the graph, Jain et al. [10] proposed a framework of routing to minimize the delay of message delivery in which the simulations showed that the more knowledge acquired by routing means the better performance. Meanwhile, routing using future knowledge can be avoided by exchanging the generation and propagation of meta-data; however, acquiring the oracle is another challenging work. The authors[36] treated the message routing as a resource allocation problem in which each packet in the buffer of node is decided to be replicated or not. This is to optimize a specific routing metric. A per-packet utility function is derived from the routing metric as designed by administrator. After the information for utility is received from control channel of node, the inference algorithm in the protocol estimates the utility for each packet and the result is employed to pick up the corresponding packet to be replicated and sent. The distributed estimation procedure are found[36], which assumes the inter-meeting time between nodes is exponentially distributed. In encounter based routing[37] future rate of node encounter is predicted by past encounter value per node and based on that number of message copies is exchanged between nodes.
Unpredictable Routing
Active Scheme: Under the active scheme, a set of particular nodes called message ferries (MF) play the role of relaying message. These trajectories of message ferries are controlled in order to maximize the chance of message delivery in sparse mobile network. By employing the Levy Walk which is known as the optimal searching efficiency for sparsely and randomly distributed targets the scheme of Levy message ferries have been discussed [38]. The author analyses that in their active scheme, message ferries move using the Truncated Levy Walk mobility model with smaller value of α that induces higher diffusivity. In paper[13] two schemes are discussed: Node-Initiated MF and Ferry-Initiated MF scheme. The former takes advantage of the fixed routes of ferries to collect data from mobile nodes, and the nodes moves periodically closer to send the message, and the deviation of original path degrades performance on the tasks they need to finish. So there has be a balance between performances gained in data delivery and performance degradation in assigned tasks. The later scheme is initiated by ferry in which it takes proactive movement to contact nodes for communication. In this scheme, each node is equipped with a long range which is used for contact control and a short range radios for message exchange. Also the trajectory control of ferry is to minimize the message drops. The possible extension of the work is to employ multiple ferries to improve the data delivery performance.
In Throwboxes[14] stationary devices are deployed between mobile nodes to facilitate message exchange. Based on the degree of available information, three modes for deployments are presented: contact and traffic based mode, contact based mode and oblivious mode. Intensive simulations suggest that throwboxes are effective in improving throughput and delay when multi-path routing and regular movement are employed. The main difference between throwboxes and message ferry is that former is static and the later can move.
Forwarding: In forwarding-based routing, one copy of message is transmitted during the communication process, and a single message is forwarded to a reliable relay node. The representative protocols using single copy are Seek and Focus [16][18] and Mobyspace[39]. The hybrid approach of Seek and Focus includes utility-based and randomized routing, which can conquer the slow-start phase and routing jamming by local maximum of utility. The initial step of this protocol is to discover the potential relay neighbor by using the utility-based approach. In order to avoid stuck for a long time at local maximum of utility, the randomized routing is applied in re-seek phase.
Mobyspace [39] is a generic routing scheme using high-dimensional Euclidean space. The main idea of the scheme is forward the message to node whose mobility pattern is similar to that of the destination of the message. Several metrics have been proposed to compute the similarity of mobility models including Euclidean distance, Canberra distance, Cosine angle separation and Matching distance. In the simulation part, the authors assumed that nodes have full knowledge of other's mobility patterns. Therefore, in the extensible part, the procedure of learning mobility patterns can be included.
Replication: Epidemic routing[21] is the basic replication based routing in mobile network. In this, the message called summary vector is exchanged to detect the missing contents in neighboring nodes. Once a node realizes the discrepancy, it requests the unseen message. The epidemic routing floods message in short time with wastage of huge resources such as bandwidth. Based on the epidemic routing, PROPHET[22] employs a probabilistic metric called delivery predictability which indicates how likely the neighboring node is able to deliver the message to destination. Three equations under referred were used to predict the delivery probability. The first is:
= + (1 -x
Where, P(a,b) ϵ [0,1] means the probability at every node 'a' for each destination 'b' and Pinit ϵ[0,1] is an initialization constant. The second equation for aging:
= x
Where, 'γ' is aging constant and 'k' is the number of time units that have been elapsed. The last one is to measure the transitivity, in it, if node 'a' frequently meets node 'b' and node 'b' frequently encounters node 'c', then node 'a' is a good candidate to relay message to c(through b) even if a rarely sees 'c'.
= + (1 - x x x β
Where, β is the scaling constant to decide the impact of transitivity on the delivery predictability. Therefore, the forwarding happens only when the delivery predictability of neighboring node is higher.
Spray and Wait protocol[19] reduces the transmission overhead of flooding-based scheme by spraying only a fixed number of message copies into network, and then wait until the nodes carry these messages encounters the destination node. Thus, based on the estimation of network parameter, the optimal number of copies is obtained for simple and scalable routing scheme. The Spray and Focus [19] attempts to improve the protocol with localized mobility. The difference is that in Spray and Focus, the message carrier forwards the copy to another suitable neighbor if it does not encounter for a long time.
Island Hopping[40] is another mobility assisted routing protocol that relies on the clusters in network. Through the analysis, the authors introduce a novel model with stable Concentration Points (CP) in which the nodes are assumed to communicate only in same CP. In this, the routing algorithm first discovers the whole graph collaboratively to employ a sequence of CPs to forward the message. This discovery of graph consists of two steps: vertex labeling, and edge discovery, the former identifies each CP and the later estimates the edge sets. To estimate the position of destination, Last Encounter Table is used. Then, the next CP is decided by taking advantage of the shortest path between the source and destination. During forwarding of messages, message copies at each CP, and one-hop acknowledgment scheme makes sure the reliability of transmission. At the same time, the suppression mechanism works when an earlier copy appears in the same CP. The whole algorithm relies on the basis of a stable topology of concentration points, so the performance may suffer from unstable topology or group movement of nodes.
Controlled Replication: Compared to traditional epidemic routing based schemes and its variants that rely on reducing the consumption of network resources, Spray and Wait[19] presented a novel way to achieve efficient routing in IC-MANET. Because it reduces the number of copies of a given message, and hence, the number of transmissions for a given message to a fixed value L can be tuned in accordance with the delivery delay requirement. However, it has also been shown that simply generating and handling over a few redundant copies may not often suffice in situations where the mobility or interaction between nodes are highly correlated and follow specific patterns[19][41]. In Multiperiod Spray and Wait[42] it minimizes the average copy count used per message while maintaining the predefined message delivery rate by given deadlines.
Network-coding: As opposed to the traditional model of forwarding in IC-MANET where nodes may forward the entire copy of the message to encountered relays, an alternate approach is to employ network coding based schemes. The authors [43][44] provide an erasure-coding based approach forwarding data in IC-MANET, in which the source node encodes a message and generates a large number of code blocks guided by a replication factor 'r'. The generated code blocks are then equally split among the first 'k* r' relays, for some constant 'k', and those relays are to deliver the coded blocks, directly to the destination. The original message can be decoded once 1/r coded blocks have been received. In other words, the message can be decoded as soon as 'k' relays deliver their data to the destination. This scheme is more robust to failures of a few relays, or some bad forwarding choices.
Summary of Routing Scheme
The brief summary and the comparison of routing schemes are shown in the under referred table. The mobility model for simulation denotes the movement patterns of mobile nodes, designed to approximate the real situations for evaluating the protocol. Moreover, the item of applicable environment expresses the limitations of protocol mentioned as quoted by the authors of each paper. It suggests that several schemes can be applied to general mobile network, and the rest need corresponding assumptions which aim at deriving the optimal routing solution, or employing a particular network topology.
