mobile ad-hoc networks

1© 2002, Cisco Systems, Inc. All rights reserved.

ISOC.nlMaster Class

Mobile Ad-hoc Networks

Dr. Muazzam A. Khan


ISOC.nlMaster Class

The manet problem

• Mobile

Random and perhaps constantly changing

• Ad-hoc

Not engineered

• Networks

Elastic data applications which use networks to communicate


ISOC.nlMaster Class

Examples of such networks

• Sensor networks

• Automotive networks

• Military applications


ISOC.nlMaster Class

Sensor networks

• Networks deployed in random distribution

• Low power

• Delivering sensor data to a central site for some purpose


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Issues in routing

• Selection of arcs (metrics and objectives)

• Characteristics of various interfaces

• Convergence

• Algorithms


ISOC.nlMaster Class

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)


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Specific issues in manet networks

• Dynamic Connections

Constant change (motion)

Random interconnection

• Radio Characteristics

Unidirectional Links

Varying S/N Ratio

Overlapping connectivity


ISOC.nlMaster Class

Overview of Proposals

• Reactive (on-demand) Protocols

Overview of DSR

Overview of AODV

• Proactive (pre-calculated) Protocols

Overview of TBRPF

Overview of OLSR


ISOC.nlMaster Class

Dynamic Source Routing

“IEEE 802.5 Source Routing” in wireless networks


ISOC.nlMaster Class

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)


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Ad-hoc On-Demand Distance Vector


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Status of testing


• Not perfect, but few major problems

• Ongoing research, seeking experimental status


ISOC.nlMaster Class

Topology Broadcast Based on Reverse-Path Forwarding


ISOC.nlMaster Class

Target networks


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”


ISOC.nlMaster Class

Messages

• “Hello”

Neighbor discovery and relationship maintenance

• Topology Update

Distributes subset of interconnectivity information


ISOC.nlMaster Class

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…


ISOC.nlMaster Class

Status of testing


Boeing and SRI actively promoting

Not all algorithms publicly documented

• Not perfect

Needs operational experience to debug

• Ongoing research, may seek experimental status


ISOC.nlMaster Class

Optimized Link State Routing


ISOC.nlMaster Class

Target networks


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


ISOC.nlMaster Class

Messages

• “Hello”

Neighbor discovery and relationship maintenance

• Topology Update

Distributes subset of interconnectivity information


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Multi-Point Relays

• Passes Topology Information

Acts as router between hosts, in wired parlance

Minimizes information retransmission

Forms a routing backbone


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Status of testing


Inria in active research

Not all algorithms publicly documented

• Not perfect

Needs operational experience to debug

• Ongoing research, may seek experimental status


ISOC.nlMaster Class

Status of research


ISOC.nlMaster Class

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


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Open problems in TBRPF

• High computational overhead

• High messaging overhead


ISOC.nlMaster Class

Open problems in OLSR

• Selection and dynamic minimization of MPR set

• Operational testing of protocol


ISOC.nlMaster Class

Open problems in AODV

• Protocol needs operational experience to discover further issues


ISOC.nlMaster Class

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


ISOC.nlMaster Class

Mobile Ad-hoc Networks

mobile ad-hoc networks

Documents