Landmark Routing for Large Ad Hoc Wireless Networks

Globecom 2000San Francisco, Nov 30, 2000

Mario Gerla, Xiaoyan Hong and Gary Pei

Computer Science Department

University of California, Los Angeles

http://www.cs.ucla.edu/NRL/wireless/

Ad Hoc vs Cellular Wireless Nets

Multihop (Ad Hoc)

Single Hop (Cellular)

Base BaseBaseBase

Scalability in ad hoc wireless routing

• Scalability to network size– Potentially, thousands of nodes (e.g., battlefield, sensor networks)

• Scalability to mobility– mobility critical in battlefield and vehicular applications

Do Existing Routing Protocols Scale?

• Proactive routing:– Distance Vector based: DBF, DSDV, WIRP

– Link State

Main limitations: routing table O/H; control traffic O/H

• On-demand, reactive routing:– AODV, TORA, DSR, ABR etc

Main limitations: search-flood O/H with high mobility and many short lived flows

Distance Vector

0

5

1

2

4

3

Destination Next Hop Distance

0 2 31 2 2… … …

Routing table at node 5 :

Tables grow linearly with # nodes

Control O/H grows with mobility and size

Link State Routing

• At node 5, based on the link state packet, topology table is constructed:

• Dijkstra’s Algorithm can then be used for the shortest path

0

5

1

2

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3

{1}

{0,2,3}

{1,4}

{2,4}

{2,3,5}

{1,4,5}

0 1 2 3 4 50 1 1 0 0 0 01 1 1 1 1 0 02 0 1 1 0 1 13 0 1 0 1 1 0

4 0 0 1 1 1 15 0 0 1 0 1 1

0

5

1

2

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3

query(0)

query(0)

query(0)

query(0)

query(0)

query(0)

query(0)

reply(0)

reply(0)

reply(0)

On-demand Routing

Advantages:– on-demand request & reply

eliminates periodic update O/H (channel O/H)

– routing table size is reduced (it includes only routes in use) (storage O/H)

Limitations:– not scalable with traffic load– mobility may trigger frequent

flood-searches

Hierarchical Routing

• Traditional solution in large scale networks (eg, Internet):

hierarchical routing

• Unfortunately, hierarchical routing implementation problematic in ad hoc nets

• In a mobile ad hoc network the hierarchical addresses must be continuously changed to reflect movements

• Some ad hoc routing schemes recently proposed use an “implicit” hierarchy (eg, Fisheye, Zone routing, etc)

Wireless Hierarchical Routing(addresses change with motion)

5

1

7

6

11

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23

10

98

Level = 0

(1,1)

(1,2)(1,3)

(1,4) Level = 1

(2,1) (2,3)Level = 2

DestID

1

6

7

(1,2)

(1,4)

(2,3)

Path

5-1

5-1-6

5-7

5-1-6-(1,2)

5-7-(1,4)

5-7-(1,4)-(2,3)

HSR table at node 5

Implicit hierarchical routing: Fisheye State Routing

11

1

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5

67

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1012

14 1516 17

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2425

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3234

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Hop=1

Hop=2

Hop>2

13

Fisheye Routing

• In Fisheye routing, routing table entries for a given destination are updated (ie, exchanged with the neighbors) with progressively lower frequency as distance to destination increases

• Property 1: the further away the destination, the less accurate the route

• Property 2: as a packet approaches destination, the route becomes progressively more accurate

• Major “scalability” benefit: control traffic O/H is manageable even for very large network size

• Unsolved problems: route table size still grows linearly with network size; out of date routes to remote destinations

Update O/H Reduction in FSR (optional)

0

5

1

2

4

3

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

101122

LST HOP

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

212012

LST HOP

0:{1}1:{0,2,3}2:{5,1,4}3:{1,4}4:{5,2,3}5:{2,4}

221101

LST HOP

Ad Hoc “Group” Hierarchical Solution: Landmark Routing

• Main assumption: nodes move in groups• Three components in LANMAR:• (1) a “local ” proactive routing algorithm that

keeps accurate routes from a source to all destinations within scope N (e.g., Fisheye alg truncated to scope N, Bellman Ford, DSDV, etc)

• (2) a Landmark selection alg for each logical group

• (3) a routing algorithm that maintains accurate routes to landmarks from all mobiles in the field

Logical SubnetLogical Subnet

• Logical subnet: group of nodes with functional affinity with each other (eg, they move together)

• Node logical address = <subnet, host>

Landmark Routing: the Concept

LandmarkLandmark

• A Landmark is elected in each subnet• Every node keeps Fisheye Link State table/routes

to neighbors up to hop distance N• Every node maintains routes to all Landmarks

Landmark Routing (cont’d)

• A packet to local destination is routed directly using Fisheye table based on MAC address

• A packet to remote destination is routed to corresponding Landmark based on logical addr

• Once the packet gets within Landmark scope, the direct route is found in Fisheye tables

• Benefits: dramatic reduction of both routing overhead and table size; scalable to large networks

LandmarkLandmark

Logical SubnetLogical Subnet

Landmark Routing: Dynamic Election

• Dynamic landmark election a must in a mobile environment and in presence of enemy attacks

• Node with largest number of group members in its scope proclaims itself Landmark for group; ties broken by lowest ID

• “Oscillation” of landmark role is eliminated by hysteresis.

• Multiple landmarks may coexist if group spans several “scopes” (they can be hierarchically organized)

Landmark Election (detail - may skip)

• Landmark election algorithm:– No landmark exists initially, only FSR progresses.

– A node proclaims itself as a landmark when it detects > T number of group members in its FSR scope.

– An election is required to select the winner in the group.

• Simple election winner algorithm– A node with the largest number of group members wins and the

lowest ID breaks a tie.

• Hysteresis election winner algorithm– The current election winner replaces the old landmark when its

number of group members is larger than the old one by an extra fraction.

– Or, the old landmark gives up the landmark role when its number of group members reduces to a value smaller than a threshold T.

Drifting nodes (detail - may skip)

• Drifters are nodes outside of the scope of their landmark

• Drifters periodically “register” with Landmark• Registration message creates reverse path from

Landmark to drifter• A packet directed to a drifter must be first

received by the Landmark and then forwarded to drifter

• Routing table entries to drifters increase routing table OH; however, the extra O/H is low if drifter fraction is low

Illustration by Example

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C D

HI

JK L

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P

LM1

LM2

LM3

LM4

Simulation Environment

• GlomoSim platform• 100 nodes• 1000x1000 square meter simulation area• 150m radio range• UDP sessions between random node pairs• CBR traffic ( one 512 byte pkt every 2.5 sec)• # of logical groups = 4• 2-level Fisheye with radius = 2 hops• IEEE 802.11 MAC layer; 2Mbps link rate• Reference Point Group Mobility model

– random waypoint model is used for both individual and group component of the mobility vector

Throughput and Delay

mobility = 6 m/s

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number of communication pairs

dela

y (m

sec)

AODV

LANMAR

FSR

mobility = 6 m/s

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number of communication pairs

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ughp

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AODV

LANMAR

FSR

Routing Load with and w/o Election

Conclusions

• Accuracy of the route to Landmark nodes proves to be adequate

• LANMAR exhibits good scalability with increasing communication pairs

• LANMAR provides a dramatic reduction in routing table storage overhead with respect to FSR

• Dynamic Landmark Election introduces only a moderate increase in routing O/H (with respect to fixed Landmark)

Work in Progress (optional)

• Independent (instead of group) mobility• Very small groups (in the limit, all isolated

nodes)• “Optimal” scope of local routing• Hierarchical Landmark organization• Membership change from one group to

another • Landmarking in a heterogeneous

structure: directive antennas, UAVs etc

The End

Thank You !

www. cs.ucla.edu/NRL