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Mobile Ad-hoc Networks
Dr. Muazzam A. Khan
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The manet problem
• Mobile
Random and perhaps constantly changing
• Ad-hoc
Not engineered
• Networks
Elastic data applications which use networks to communicate
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Examples of such networks
• Sensor networks
• Automotive networks
• Military applications
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Sensor networks
• Networks deployed in random distribution
• Low power
• Delivering sensor data to a central site for some purpose
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Traffic networks
• “Smart cars” and “smart roads”
• Onboard systems “talk” to the “road”:
Map obstacles and delays
Obtain maps
Inform the road of its actions
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Military applications
• Combat regiment in the field
Perhaps 4000-8000 objects in constant unpredictable motion…
• Intercommunication of forces
Proximity, function, plan of battle
• Special issues
Low probability of detection
Random association and topology
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Basis of routing:
• Bellman-Ford-Moore proof
In any graph there exists a spanning tree
Ford (1956), Bellman (1958), Moore (1958)
• Spanning tree:
A set of arcs which visits every node in a graph exactly once
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Proof by construction
• Select one node in graph
• Place all arcs connected to that node in set of “available choices”
• Place all other nodes and arcs in set of “unavailable choices”
• Repeatedly select an arc from “available choices”
If it leads to an “unavailable” node,
Add that node and arc to the graph,
Add all other arcs connecting to the new node to “available choices”
Otherwise, discard the arc
• All nodes get included in the graph
Some arcs may be left over
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Issues in routing
• Selection of arcs (metrics and objectives)
• Characteristics of various interfaces
• Convergence
• Algorithms
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Selection of arcs (metrics)
• Available bandwidth (traffic engineering)
• Link speed (shortest path)
Often measured as duration of a bit, or duration of an operation
• Hop count or other administrative distance
Might be power or S/N ratio for radio
• General policy (BGP)
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Characteristics of various interfaces
• LAN:
Many to many
• Frame Relay/ATM/MPLS:
1:many circuits
• Serial Lines:
1:1
• Radio:
1:many with inconsistent sets
Often unidirectional
S/N ratio and distance affect connectivity
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Convergence goals
• Ensuring routing consistency
Algorithms vary by type of properties required
• Managing inconsistency
The backbone is said to never actually converge
Manet networks probably don’t either
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Common algorithms
• ExplorationNetwork is explored to find possible routes
Of which one is selected
Example: IEEE 802.5 Source Routing
• Bellman-Ford algorithm does this in a distributed mannerCalled "Distance Vector" these days
Generates routes to a node from somewhere else
• Shortest Path First (Ford, CACM, 1968) does this in a databaseDjikstra applied to networks, 1972
Generates routes from a node to somewhere else
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Specific issues in manet networks
• Dynamic Connections
Constant change (motion)
Random interconnection
• Radio Characteristics
Unidirectional Links
Varying S/N Ratio
Overlapping connectivity
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Overview of Proposals
• Reactive (on-demand) Protocols
Overview of DSR
Overview of AODV
• Proactive (pre-calculated) Protocols
Overview of TBRPF
Overview of OLSR
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Dynamic Source Routing
“IEEE 802.5 Source Routing” in wireless networks
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Route Discovery
• A route between two nodes is found by sending an Route Request
• Route Request builds a source route on every path through the network
• First Route Request to arrive is accepted; target responds on that path and tells initiator what the source route is
• Source route is used on subsequent data traffic
B
H I J
GFE
D
A
C
K
Detected Source Route:(A, B, F, J, K)
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Link reliability
• Passing source routes cached
To know how to send routing error messages
• Network layer acknowledgements watched
To ensure delivery
To detect failed routing nodes
• After several retransmissions,
Neighboring routing node declared dead
Route Error sent to senders using the node
Packets flushed
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Route Maintenance
• When routing node/routing node link changes, existing source routes no longer work
• routing nodes respond to source routes with a Route Error, triggering a new route discovery
• routing nodes may attempt to change source route and re-forward on a cached route
B
H I J
GFE
D
A
C
K
RouteError
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Status of testing
• Simulations and some operational testing
• Testing shows some documented features in use of route cache don’t work properly
• Ongoing research, seeking experimental status
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Ad-hoc On-Demand Distance Vector
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Target networks
• Targets networks in which
Routing churn is high enough that maintaining routes is unproductive, and
Can absorb a network wide broadcast rate
• Modified on-demand Distance Vector routing
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Messages
• Route Request: “I need a route”
• Route Response: “Route advertisement”
• Route Error: “Withdraw route”
• Periodic route response to neighbors acts as “hello”, installing and refreshing route
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Route Discovery
• A route between two nodes is found by sending an Route Request to a locality
Initial locality small, grows with failure
After that, a little larger than the locality target last found in
• Route Response sent
By target if necessary
By neighboring routing node if possible to “join” existing route
• Network stores the route
d
j k l
ihg
f
A
e
C
B
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Route Errors
• Routes expire if not refreshed
• routing nodes log recent downstream users of a route
• When routes expire or are flushed, downstream users are notified to flush
• New route request triggered d
j k l
ihg
f
A
e
C
B
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Status of testing
• Simulations and some operational testing
• Not perfect, but few major problems
• Ongoing research, seeking experimental status
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Topology Broadcast Based on Reverse-Path Forwarding
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Target networks
• Targets networks in which
Routing churn is consistent with pre-calculated routes, and
Capable of supporting fairly intelligent device
• Modified Shortest Path First routing
Routing by “need to know”
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Messages
• “Hello”
Neighbor discovery and relationship maintenance
• Topology Update
Distributes subset of interconnectivity information
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Topology Update Distribution
• Therefore every routing node effectively calculates all routes from all nodes for which it has information
• Topology information distributed to the set of systems that are deemed to need it.
You need information about a routing node if your routing would use it…
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Status of testing
• Simulations and some operational testing
Boeing and SRI actively promoting
Not all algorithms publicly documented
• Not perfect
Needs operational experience to debug
• Ongoing research, may seek experimental status
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Optimized Link State Routing
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Target networks
• Targets networks in which
Routing churn is consistent with pre-calculated routes, and
Capable of supporting fairly intelligent device
• Modified Shortest Path First routing
Level 1/Level 2 routing distinction
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Messages
• “Hello”
Neighbor discovery and relationship maintenance
• Topology Update
Distributes subset of interconnectivity information
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Neighbor relationships
• Each device emits a periodic “Hello”
Advertise itself to its neighbors
Determine who else is there
Select some systems to act as Multi-Point Relays
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Multi-Point Relays
• Passes Topology Information
Acts as router between hosts, in wired parlance
Minimizes information retransmission
Forms a routing backbone
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Structure of an OLSR Network
• MPRs form routing backbone
Other nodes act as “hosts”
• As devices move
Topological relationships change
Routes change
Backbone shape and composition changes
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Status of testing
• Simulations and some operational testing
Inria in active research
Not all algorithms publicly documented
• Not perfect
Needs operational experience to debug
• Ongoing research, may seek experimental status
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Status of research
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Open problems
• Behavior of applications in manet networks
TCP likely to not work well if loss leads to congestion avoidance
ECN (RFC 3168) allows separation of the concepts
Various other research proposals
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Open problems in DSR
• Route Maintenance as specified results in problems in routing
Man-in-the-middle issues
• Requires protocol number from IANA
Small number space
We don’t give those to just anybody
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Open problems in TBRPF
• High computational overhead
• High messaging overhead
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Open problems in OLSR
• Selection and dynamic minimization of MPR set
• Operational testing of protocol
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Open problems in AODV
• Protocol needs operational experience to discover further issues
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Directions for manet working group
• Probably will take several protocols to “experimental” status
RFC is an archive, not a standard
• Likely to standardize
One reactive protocol
One proactive protocol
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Mobile Ad-hoc Networks