Optimized Link State Routing Protocol for Ad Hoc Networks

Qamar A Tarar OLSR Protocol 1

Optimized Link State Routing Protocol for Ad Hoc Networks

Qamar Abbas Tarar

“Mobile ad-hoc networks based on wireless LAN”


Problems in MANETs

Scalability

QoS

Security

Interoperation with the Internet

Limited Battery Life

Node Mobility

Unreliable radio channel Hidden terminal problem

Route maintenace

Unpredictable link properties


Unicast-Routing Protocol for MANET (Topology-based)

Table-Driven/Proactive

Hybrid On-Demand-driven/Reactive

Clusterbased/Hierarchical

Distance-Vector

Link-State

ZRP DSRAODVTORA

LANMARCEDAR

DSDV OLSRTBRPFFSRSTAR

MANET: Mobile Ad hoc Network

(IETF working group)

Classification of Routing Protocols for MANETS

CBRP


Proactive vs Reactive Routing Protocols

Proactive Routing Protocols (DSDV, OLSR)

+ Routes to all reachable nodes in the network available.

+ Minimal initial delay for application.

- Larger signalling traffic and power consumption.

Reactive Routing Protocols (DSR, CBR etc)

+ Smaller signalling traffic and power consumption.

- A long delay for application when no route to the destination available


Structure

OLSR Overview

Multipoint relays

Neighbor sensing

MPR selection

MPR information declaration

Routing table calculation

Extensions in OLSR

Conclusions


Overview

OLSR Developed by IETF Table driven Inherits Stability of

Link-state protocol Selective Flooding Periodic Link State

Information generated only by MPR MPRs employed for optimization


Link State Routing (eg, OSPF)

Each node periodically floods status of its links

Each node re-broadcasts link state information received from its neighbour

Each node keeps track of link state information received from other nodes

Each node uses above information to determine next hope to each destination

24 retransmissions to diffuse a message up to 3 hops

Retransmission node


OLSR Overview In LSR

protocol a lot of control messages unnecessary duplicated

In OLSR only MPR retransmit control messages:

Reduce size of control message; Minimize flooding

Other advantages (the same as for LSR): As stable as LSR protocol; Proactive protocol(routes already known); Does not depend upon any central entity; Tolerates loss of control messages; Supports nodes mobility. Good for dense network


Optimized Link state routing (OLSR)

24 retransmissions to diffuse a message up to 3 hops

Retransmission node

11 retransmission to diffuse a message up to 3 hops

Retransmission node


Description of OLSR

MPR (Multipoint relays)

MPR selector

Symmetric 1-hop

neighbours

Symmetric strict 2-hop

neighbours D

S

B

M

X YZ

P

A


Neighbor sensing

Each node periodically broadcasts Hello message:

List of neighbors with bi-directional link List of other known neighbors.

Hello messages permit each node to learn topology up to 2 hops

Based on Hello messages each node selects its set of MPR’s


Example of neighbor table

One-hop neighbors

Neighbor’s id State of Link

B Bidirectional

G Unidirectional

C MPR

… …

Two-hop neighbors

Neighbor’s id Access though

E C

D C

… …

Also every entry in the table has a timestamp, after which the entry in not valid


Multipoint Relays (MPR)

N

Reduce re-transmission in the same region

Each node select a set of MPR Selectors

MPR Selectors of node N - MPR(N)

- one-hop neighbors of N



N

Reduce re-transmission in the same region

Each node select a set of MPR Selectors

MPR Selectors of node N - MPR(N)

- one-hop neighbors of N

MPR set of Node N

Set of MPR’s is able to transmit to all two-hop neighbors

Link between node and it’s MPR is bidirectional.


Every node keeps a table of routes to all known destination through its MPR nodes

Every node periodically broadcasts list of its MPR Selectors (instead of the whole list of neighbors).

Upon receipt of MPR information each node recalculates and updates routes to each known destination



MRP selection in OLSR

Node 1 Hop Neighbors 2 Hop Neighbors MPR(s)

B A,C,F,G D,E C

Available BW

OLSR: node B will select C as its MPR So all the other nodes know that they can reach B via C

30

10050110

25

60

10

40

5 10

D->B route is D-C-B, whose bottleneck BW is 3

3


MRP selection in OLSR

Node 1 Hop Neighbors 2 Hop Neighbors MPR(s)

B A,C,F,G D,E C

Available BW

OLSR: node B will select C as its MPR So all the other nodes know that they can reach B via C

30

10050110

25

60

10

40

5 10

D->B route is D-C-B, whose bottleneck BW is 3

3

Optimal route (i.e., path with maximum bottleneck bandwidth: D-F-B (bottleneck bandwidth of 10)


Multi-Point Relays/routers

Passes Topology Information

Acts as router between hosts

Minimizes information retransmission

Forms a routing backbone


Structure of an OLSR Network

MPRs form routing backboneOther nodes act as “hosts”




As devices move




As devices moveTopological relationships changeRoutes changeBackbone shape and composition changes


MPR information declaration

TC – Topology control message:

Sent periodically. Message might not be sent if there are no updates and sent earlier if there are updates

Contains: MPR Selector Table Sequence number

Each node maintains a Topology Table based on TC messages

Routing Tables are calculated based on Topology tables


Topology Table

Destination address

Destination’s MPR

MPR Selector sequence number

Holding time

MPR Selector in the received TC message

Last-hop node to the destination.

Originator of TC message


Topology Table (cont) Upon receipt of TC message:

If there exist some entry to the same destination with higher Sequence Number, the TC message is ignored

If there exist some entry to the same destination with lower Sequence Number, the topology entry is removed and the new one is recorded

If the entry is the same as in TC message, the holding time of this entry is refreshed

If there are no corresponding entry – the new entry is recorded


Routing Table

Each node maintains a routing table to all known destinations in the network Routing table is calculated from Topological Table, taking the connected pairs Routing table:

Destination address Next Hop address Distance

Routing Table is recalculated after every change in neighborhood table or in topological table


Extensions in OLSR

Qos OLSR

Fast OLSR

Towards IPv6 OLSR

Power saver mode

Change in the contents of TC packet


QoS Routing: Difficulties in QoS routing

Due to mobility Availability and manageability of Link state metrics Link quality changes quickly and continuously

Computational cost and protocol overhead affect

the performance of the QoS routing protocol

Protocol performance evaluation is complex


Proactive QoS Routing

Advantages suitable for the unpredictable nature of Ad-Hoc networks suitable for the requirement of quick reaction to QoS demands makes call admission control possible avoids the waste of network resources

Disadvantages introduces additional protocol overhead trade-off between the QoS performance and traditional protocol

performance

But.. Little work has been done to analyse the impact of the additional

overhead on pro-active QoS routing


QoS Versions of OLSR

30

10050110

25

60

10

40

510

OLSR protocol does not guarantee

to find the best bandwidth route

3 heuristics are proposed to enhance

OLSR in bandwidth aspect

The heuristics select good bandwidth

neighbour as MPR

3


QoS Versions of OLSR OLSR_R1: similar to OLSR (i.e., choose 1-hop neighbours that cover max. number of 2-hop neighbours), tie-breaker now max BW

Node 1 Hop Neighbors 2 Hop Neighbors MPR(s) B A,C,F,G D,E C

OLSR_R2: select the best BW neighbors as MPRs until all the 2-hop neighbors are covered.

Node 1 Hop Neighbors 2 Hop Neighbors MPR(s) B A,C,F,G D,E F

OLSR_R3: selects the MPRs in a way such that all the 2-hop neighbors have the max. bottleneck BW path through the MPRs to the current node.

Node 1 Hop Neighbors 2 Hop Neighbors MPR(s) B A,C,F,G D,E A,F

30

10050110

25

60

10

40

5103


Evaluation of QoS OLSR

Simulation: generate networks, run OLSR algorithms, compare results against paths calculated by Link-State algorithm (i.e. complete knowledge, all-pair shortest path)

Network area: 1000 M 1000 M

Number of nodes: 100

Transmission range: 100 M, 200 M, 300 M

Bandwidth: assigned randomly

Results are averaged over 100 randomly generated networks


Performance Metrics

Error rate: percentage of routes with non-optimal bandwidth

Average difference: for routes with non-optimal bandwidth, how far off the optimal bandwidth are we

Overhead: the average number of control messages transmitted per node

MPR count: average number of MPRs in the network


Experimental ResultsAlgorithm Transmissi

on Range

Performace Cost

Error Rate

Average difference Over-head

MPR Count

Standard OLSR

300 M 28% 46% 12 65

200 M 41% 51% 24 68

100 M 12% 45% 5 42

OLSR_R1 300 M 14% 22% 12 65

200 M 21% 26% 24 68

100 M 8% 44% 5 42

OLSR_R2 300 M 0% 0% 18 70

200 M 0% 0% 33 72

100 M 0% 0% 5.7 45

OLSR_R3 300 M 0% 0% 26 71

200 M 0% 0% 38 73

100 M 0% 0% 5.7 44

Pure Link State

Algorithm

300 M 0% 0% 1245 100

200 M 0% 0% 979 100

100 M 0% 0% 28 100


Fast OLSR

Due to Proactive nature,routes available

when needed

However In dense network, due to fast node Mobility, links valid only for short time period.

Hence to minize packet loss,

broken links between node and its

neighbors must be quickly detected.


Neighbor Discovery in Fast OLSR

3-procedures:

Switch to Fast-Moving/Default mode:

In Fast mode,send Fast-Hellos and vice versa.

A Fast-Hello is smaller than a Hello

Establishing fast Links:

A node in Fast-Moving mode sends Fast-Hello

messages at high frequency.

Refresh Fast links & Detect new broken links:

by sending periodic Fast-Hellos


Towards IPv6 OLSR

OLSR operate well with both IPv4 and IPv6

To operate with IPv6, the only required change is to replace the IPv4 addresses with IPv6 address.

The minimum packet and message sizes should be adjusted accordingly, considering the greater size of

IPv6 addresses.


Power saver mode

A node can indicate if it agrees to keep the packets of its neighbors

Any node, who wants to go in sleep mode, will select ONLY that neighbor as MPR who can keep its packets

TC packet will diffuse this info, and all data packets will be routed through that “power saver” node


Change in the contents of TC packet

Instead of advertising its set of MPRs, a node

will list its neighbors who has selected him as

an MPR

Many nodes (loosely connected, or at the boundaries) will not be selected MPR any node. So they will not send any TC (25% less overhead)

Less frequent changes in this set


Advantages

Route immediately available

Reactivity to topological changes can be adjusted by setting the time interval for HELLO messages

Minimize flooding by using MPR

Can be integrated into existing system as it requires no change to IP format

Disadvantages

Bigger overhead

Need more power

Not all allgoritms pubically documented

Needs more operational experience to debug

Conclusions


Readings G. Pei, M. Gerla, and X. Hong, "

LANMAR: Landmark Routing for Large Scale Wireless Ad Hoc Networks with Group Mobility," In Proceedings of IEEE/ACM MobiHOC 2000, Boston, MA, Aug. 2000.

R. Ogier, F. Templin, M. Lewis, " Topology Dissemination Based on Reverse-Path Forwarding (TBRPF) ," IETF Internet Draft , July 28 2003.

Thomas Clausen, Philippe Jacquet, " Optimized Link State Routing Protocol (OLSR) ," IETF Internet Draft , July 3 2003.

X. Hong, K. Xu, and M. Gerla, " Scalable Routing Protocols for Mobile Ad Hoc Networks " IEEE Network Magazine, July-Aug, 2002, pp. 11-21

Thomas Kunz,Ying Ge, Louise Lamont, “ Quality of Service Routing in Ad-Hoc Networks Using OLSR” Carleton University, CRC,2002

M Benzaid, P Minet and K A Agha, “Integrating fast mobility in the

OLSR routing protocol” INRIA, LRI, France,September 2002.


Q & A

Documents

Optimized Link State Routing Protocol for Ad Hoc Networks