Group 3Sandeep Chinni

Arif KhanVenkat Rajiv

Delay Tolerant NetworksPath from source to destination is not present

at any single point in time.Combining snapshots of the network at

different times may result in the formation of a source-destination path.

Protocols for DTNPrioritized Epidemic Routing for

Opportunistic Networks Ram Ramanathan, Regina Rosales-hain

ACM MobiOpp 2007

Oracle Based RoutingS. Jain, K. Fall, and R. Patra. Routing in a Delay

Tolerant Network. In Proc. ACM Sigcomm, pages 145–158, 2004

Epidemic RoutingGoal is to deliver messages with high

probability even when there is never a fully connected path.

Epidemic RoutingGoal is to deliver messages with high

probability even when there is never a fully connected path.- Can we do better?

The overall goal of Epidemic Routing is to maximize message delivery rate minimize message delivery latencyminimizing the aggregate system resources

consumed in message delivery

Epidemic Routing ProtocolAnti-Entropy sessions

Message InformationMessage ID – a unique ID for all the

messages that will be transmitted.Hop Count – The maximum hops that a

message can take before reaching the destination.

Optional Ack request

Hosts/NodesNodes set a maximum buffer size to aid

epidemic routing.This setting will limit memory and network

usage.There is a trade off between resource

consumption and message delivery rate/latency.

Simple buffer management strategies like FIFO can be used when there is contention for resources - not the best though.

Prioritized Epidemic Routing(PREP)Prioritizes the messages for transmission and

deletion using a priority function.Priority function is based on

Current cost to destinationCurrent cost from sourceExpiry timeGeneration time

Inter-node costs are computed with a metric called average availability.

Features of PREPPREP has two modules:

Topology awareness Helps in calculating routing costs from a node to a

destination.Message drop and Transmit property

A priority scheme for deleting and transmitting message packets.

Topology AwarenessEach node runs a neighbor discovery algorithm

to find out its neighbors.Each link between two nodes has a metric called

the Average Availability(AA).The average availability is calculated based on a

short history of node link availability information.If a link is not available for a configured time,

then it is forgotten.Periodically or whenever sufficient new link

information is available Link State Advertisements (LSA) are exchanged between nodes.

Topology AwarenessThis LSA exchange is called Topology Sync as

the nodes learn from each other.LSA exchange gives the nodes the knowledge of

the network topology during the recent time period.

This “best effort” topology awareness is used to compute routing costs.

Formula : (1-AA)+0.01AA-Average Availability

Dijkstra’s algorithm is used for lowest cost route.

Message Drop & Transmit PriorityEach message has a drop priority(Pd) and

transmit priority(Pt).Pd of a packet is the lowest cost path from the

current node to the destination.Pt of a packet is based on the cost to the

destination and time-to-expire of the packet.When the buffer of a node crosses a

threshold, it starts to drop packets based on Pd and stops only after a lower threshold is crossed.

SimulationPREP compared with Epidemic routing and

AODV and simulation done in NS-2.Simulation Parameters

Advantages of PREPSuccessful, as long as the resources are not

overloaded.Does not rely on extrapolating previous

contact information.Improves performance of Epidemic routing at

high loads.

DisadvantagesVery high resource utilization even when less

number of messages are being transmitted.Route cost calculation is not possible in all

cases and Pd cannot be computed.

Oracle Based RoutingKnowledge centers (Oracles) are used to

make routing decisions.Based on the amount of information and

network resources available suitable Routing protocols can be used.

OraclesContacts Summary Oracle

can answer questions about time-invariant aggregate statistics or summary characteristics about contacts.

Contacts Oraclecan answer any question regarding contacts between two

nodes at any point in time.Can be used for admission control.

Queuing Oraclegives information about instantaneous buffer occupancies

(queuing) at any node at any time.can be used to route around congested nodes.

Traffic Demand OracleCan answer any questions regarding present or future traffic

demand.

Components for Path CalculationQueuing time:

Time until a contact becomes available.Transmission delay:

Time to inject a message completely into an edge.Propagation delay:

Time to deliver the message (includes any intermediate queuing delay).

Storage Capacity.

Routing Algorithm ClassesNo knowledge

They do not use any oracles and hence perform badly.

Complete KnowledgeThey utilize contacts, traffic and queuing

oracles.Partial Knowledge

They find routes in the absence of traffic demand oracle and use other oracles.

Oracle Based Routing AlgorithmsSchemes:

Simulation with Bus Routes

Average Delay

Delivery Ratio

Bandwidth Variation

Advantages & DrawbacksAdvantages

Based on the oracles available we can choose an appropriate algorithm for route calculation.

DrawbacksCreating and maintaining oracles is a

significant distributed systems problem.

What have we taken out of these papers?Prioritized epidemic routing might be of

interest in worst case scenarios for our DTN protocol.

References[1] A. Vahdat and D. Becker. Epidemic routing

for partially connected ad hoc networks, 2000.