DELAY TOLERANT NETWORKS

What Is DTN?

Delay tolerant networks are the Networks which has lack of continuous connectivity.

So,there will be more Delay.Even Delay’s of several days is inevitable in this network.

DTN’s are also called as Intermittently connected Mobile Networks.

Genral challenges In DTN

Encounter Schedule

Network Capacity

Storage

Energy

ENCOUNTER SCHEDULE

Encounter schedule is the term used to denote hoe the nodes in the network is contacted by the other (or) how the nodes in the network contact each other.

The challenge here is that the source node may or may not encounter the destination node.

In case of periodic mobility of the nodes, even if the encounter schedules are known to some extent, there may be some chaos.

In the extreme case of mobility, it leads to memory less encounter schedules (i.e) no assumptions can be made about the node contact pattern.

NETWORK CAPACITY

In general, the duration of the encounter and the link Bandwidth dictates the amount o data that can be transferred among the nodes.

Another factor is that the contention of multiple nodes that tries to send the data to the same destination node at a given encounter.

STORAGE

During an encounter, The node may decide to exchange the entire data, which is not possible in the storage-constrained node.

Hence there may be a loss in data. To overcome this, intelligent routines to

avoid the copies of same data and to delete the stale copies of the data are used.

In an heterogeneous node capacity n/w ’s better delivery strategy has to be applied for efficient data delivery.

ENERGY

Transmission ,reception and computation of data incurs power.

In certain scenarios like battery operated wireless networks ,the residual energy in the source and destination has to be taken into consideration before starting the data transfer.

Data delivery mechanisms should also take this into account and should adapt the wide range of scenarios.

ROUTING PROTOCOL SCHEMES

EPIDEMIC ROUING SCHEMES DIRECT CONTACT SCHEMES ONE HOP RELAY SCHEMES ROUTING BASED ON KNOWLEDGE

ORACLES LOCATION BASED SCHEMES GRADIENT BASED SCHEMES CONTROLLED RELICATION SCHEMES

EPIDEMIC ROUTING SCHEMES

Whenever two nodes contact each other,they will exchange all the data they have currently with them irrespective of the source and destination.

At the end of the encounter, both will have the same set of data with them.

Eventually, data will be flooded in the network.

Data transfer will take place even after the data reaches the destination node. (contd…)

in 2001 epidemic scheme was modified by DAVIS with DROPPING POLICIES.

Here,size of the buffer is resricted according to the dropping policies.

The dropping stratagies are Drop Random (DRA) Drop Least Recently Ussed(DLR) Drop Oldest (DOA) In all the above strategies, DLE and DOA

yields best results. In 2005, Time TO Live (TTL) was

inroduced to avoid flooding.

MERITS

This has no bandwidth or Energy/ Power constraints.

This doesn’t care about the source and destination nodes.

No extra computations must be made on Routing processes.

DEMERITS

As it doesn’t care about the source and the destination, data is FLODDED throghout the network.

Stales copies will be present in each and every node in the network.

Data will be transferred even when the data has already reached the destination.

DIRECT CONTACT SCHEMES

This is the simple scheme which delivers the data to the destination when the source encounters the destination node.

This doesn’t require any additional resources. MERITS: No additional resoureces are required as a result no additional engery and power is neede.DEMERITS: Delivery delay can be extremely large as the source may or may not encounter the destination node.

ONE-HOP RELAY SCHEMES

In this scheme ,source delivers the packet to an intermediate node, which in turn delivers it to the destination.

Compared with the previous direct contact scheme,it only creates an additional copy of the data.

The mobility of the relay node may be controlled or random.

Intermediate mobile nodes that follow a random walk mobility model are used to carry data from static sensors to base-stations.

The individual sensor nodes transfer their data to the intermediate node when it comes in radio range and the collected data is in turn delivered to the sinks.

CONSRAINTS

The data transfer capacity is based on the mobility pattern that the intermediate node follows .

Another dependency is on the buffer that the intermediate mobile node has (i.e) it marks the capacity of the data that an intermediate mobile node can carry to the sink node.

ROUTING BASED ON KNOWLEDGE ORACLES

DTN is modelled as a directed multistage graph with time-varying edge costs, based on propagation delay and edge capacity.

The various knowledge oracles considered provide information about the following

>all future contacts of nodes such as time of contact, duration of contact, bandwidth available for information exchange during contact. (contd.)

The future traffic-demand of the nodes. The instantaneous queue sizes at each node. Using information from one or more oracles, various

algorithms have been designed to route the data. A linear programming formulation that uses all the

oracles to determine the optimal routing for minimizing average delay in the network.

As algorithms are fed more knowledge from the oracles, they provide better performance.

They introduce the concept of per-contact routing where nodes frequently recompute their routing table, similar to a traditional link-state routing protocol, whenever contact is made with another node.

(contd.)

This routing info.then redistributed through the network using an epidemic routing like protocol thereby allowing other n odes to k ow about the neighbors route.

DEPENDENCIES: >This scheme basically depends only on the algorithm we use to calculate best route and the type of knowledge oracles we use.

LOCATION B ASED SCHEMES

In certain scenarios, the nodes may be aware of their location which can be used for opportunistic forwarding in DTNs.

The location information may be known in either a physical (for example, from GPS) or a virtual coordinate space (designed to represent network topology taking obstacles into account).

On an encounter, a node forwards data to another node only if it is closer to the destination. Hence, location-based routing is a form of greedy, geographical-based routing .

This minimal information is enough to perform routing and deliver data to the destinations.

Hence, they avoid the need to maintain any routing tables or exchange any additional control information between the nodes.

The MoVe scheme (LeBrun et al. (2005)) employs information about the motion vectors of the mobile nodes in addition to the location information.

With, the location and relative node velocity information, the scheme calculates the closest distance of a mobile node to get to the destination when following its current movement.

So a node only forwards to a neighbor if the neighbor is predicted to be moving toward the destination and getting closer to the destination than itself.

(contd…)

Routing is done by forwarding messages toward nodes that have mobility patterns that are more similar to the mobility pattern of the destination.

LIMITATIONS: > These schemes have a well-known limitation where they suffer from a local minima phenomenon (i.e) the nodes can be tracked onlywithin a specific zone. > This scheme can bring benefits in terms of enhanced message delivery and reduced communication costs.

GRADIENT BASED SCHEMES

In gradient-based routing, the message follows a gradient of improving utility functions toward the destination thereby delivering the packet with a low delay and using minimal system resources

The idea is that each node is associated with a metric that represents its delivery predictability for a given destination.

When a node carrying a message encounters another node with a better metric to the destination, it passes the message to it.

The metrics are positively updated based on recent node encounters and metrics for sparsely encountered nodes are appropriately aged.

The connectivity information is exchanged periodically among the nodes thereby allowing nodes to maintain meaningful metrics.

As nodes run out of memory, the eviction candidate is selected based on intelligent eviction strategies.

The Shortest Expected Path Routing (SEPR) is another scheme based on the link probability calculated from the history of node encounters.

(contd.)

Each message in a nodes cache is assigned an effective path length (EPL) based on the link probabilities along the shortest path to the destination.

When two nodes meet, they first exchange the link probability table and employ Dijkstra algorithm to get expected path length to all other nodes in the network.

This algorithm is similar to a traditional link state routing protocol.Nodes update their local tables on an encounter and in this way connectivity information is maintained in the network in a distributed manner.

(contd.)

LIMITATIONS AND REMEDIES

Gradient-based routing schemes suffer from a slow-start phase. Sufficient number of encounters must happen before the nodes develop meaningful metrics for each destination.

In addition, this information needs to be propagated through the network.

solution to address this shortcoming is the Seek and Focus scheme. This scheme initially forwards the message picking a neighbor at random until the metric utility value reaches a certain threshold.

Thereafter a gradient-based approach may be employed to deliver the message to the destination.

CONTROLLED REPLICATION SCHEMES

This has two variants >SPRAY AND WAIT >SPRAY AND FOCUS SPRAY AND WAIT: > The idea is that 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 that can be tuned in accordance with the delivery delay requirement. (contd.)

The scheme ‘sprays’ a number of copies of a message into the network to L distinct relays and then ‘waits’ till one of these relays meets the destination.

A number of heuristics are presented about how the L copies are sprayed, for example, the source is responsible for spraying all L copies or more optimally.

Each progressive node encountered by a source or relay is handed over the responsibility to distribute half of the remaining copies.

This scheme requires no knowledge of the mobility of the nodes.

This delay is independent of the size of the network and only depends on the number of nodes.

The scheme is shown to posses robust scalability as the node density goes up.

SPRAY AND WAIT: Spray and Focus provides further

improvements by taking advantage of the mobility information in the wait-phase.

The idea is that once the spray phase is over, each relay can then forward the packet further using a single-copy utility based scheme instead of naively waiting to meet the destination.

Security Vulnerabilities in DTN

DoS Attacks

In these attacks, makes the network resurce unavailable by sending bulk requests to the target so that it cannot respond to the legitimate traffic.

In DTN’s latencies are larger while forwarding.

So,attackers have ample amount of time to access the network and create possibility of Dos and makes the resource unavailable.

Two types of DoS is (i) consumes scarce resource (ii) Bundle Flooding

BLACK HOLE RISKS: here the malicious node is called as the

black node because it drops the incoming packets.

GREY HOLE ATTACKS: it is same as the black hole attack .The

difference is that the valid node in a network changes to a blackhole node.

WORMHOLE ATTACKS: Here a malicious node records the

packets and forwards it to the other connive nodes.

SYBIL RISKS: This is the risk where a node creates

pseudonyms to enter into a network for spoofing and the probability of occurrence of this kind of attack is high.

CONFIDNETIALITY RISKS: These are the attacks which targets the

confidential bundles in the network.