ROUTING STRATEGIES IN DTN

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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.

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Encounter Schedule

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Network Capacity

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Storage

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Energy

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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.

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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.

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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.

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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.

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EPIDEMIC ROUING SCHEMES
DIRECT CONTACT SCHEMES
ONE HOP RELAY SCHEMES
ROUTING BASED ON KNOWLEDGE
ORACLES
LOCATION BASED SCHEMES
GRADIENT BASED SCHEMES
CONTROLLED RELICATION SCHEMES

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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…)

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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.

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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.

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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.

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.
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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.

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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.

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.
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(contd.)

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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.
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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 .

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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 only
within a specific zone.
> This scheme can bring benefits in terms of
enhanced message delivery and reduced
communication costs.
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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.

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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.)

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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.)

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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.

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.)
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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:
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Spray and Focus provides further
improvements by taking advantage of the
mobility information in the wait-phase.
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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.

DoS Attacks
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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.

ROUTING STRATEGIES IN DTN

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