Wireless Ad hoc networks– Routing

Proposed ad hoc Routing Approaches• Conventional wired-type schemes (global

routing, proactive):– Distance Vector; Link State

• Proactive ad hoc routing:– OLSR, TBRPF

• On- Demand, reactive routing:– DSR (Source routing), MSR, BSR – AODV (Backward learning)

• Scalable routing :– Hierarchical routing: HSR, Fisheye– OLSR + Fisheye– LANMAR (for teams/swarms)

• Geo-routing: GPSR, GeRaF, etc– Motion assisted routing– Direction Forwarding

Wireless multihop routing challenges

• mobility• need to scale to large numbers (100’s to

1000's)• need to support multimedia applications

(QoS)

• unreliable radio channel (fading, external interference, mobility, etc)

• limited bandwidth• limited power

Conventional wired routing limitations• Distance Vector (eg, Bellman-Ford, BGP):

– Tables grow linearly with # nodes– routing control O/H linearly increasing with

network size– convergence problems (count to infinity);

potential loops (mobility?)• Link State (eg, OSPF):

– link update flooding O/H caused by network size and frequent topology changes

CONVENTIONAL ROUTING DOES NOT SCALE TO SIZE AND MOBILITY

DV LSIntra-AS RIP OSPFInter-AS BGP

Proactive ad hoc schemes– OLSR and TBRPF

• Link State explodes because of Link State update overhead

• Question: how can we reduce the O/H?• Answer: Link State with “Topology reduction”

– (1) if the network is “dense”, use fewer forwarding nodes

– (2) if the network is dense, advertise only a subset of the links

• Two leading IETF Link State schemes enhance scalability in large scale networks: – OLSR : Optimal Link State Routing– TBRPF: Topology Broadcast Reverse Path

Routing

LSR (Link State Routing)

• In LSR protocol a lot of control msg unnecessary duplicated

24 retransmissions to diffuse a message up to 3 hops

Retransmission node

OLSR (Optimal Link State Routing)

• In OLSR only a subset of neighbors (MPR-Multipoint Relay Selectors) retransmit control messages:– Reduce size of

control message;– Minimize flooding 11 retransmission to diffuse

a message up to 3 hops

Retransmission node

OLSR Overview

• RFC 3626, October 2003• In LSR protocol a lot of control messages

unnecessarily duplicated• In OLSR only a subset of neighbors (MPR-Multipoint

Relay Selectors) retransmit control messages– Reduce flooding overhead– Adapted for dense network

• OLSR retains all the advantages of LSR:– stable;– Does not depend upon any central entity;– Tolerates loss of control messages;– Supports nodes mobility

On-Demand Routing Protocols

• Routes are established “on demand” as requested by the source

• Only the active routes are maintained by each node

• Channel/Memory overhead is minimized

• Two leading methods for route discovery: source routing and backward learning (similar to LAN interconnection routing)

Existing On-Demand Protocols

• Dynamic Source Routing (DSR) -- CMU• Multipath Source Routing (MSR) – TJU• Backup Source Routing (BSR) – UofO+TJU• Ad-hoc On-demand Distance Vector (AODV)• Associativity-Based Routing (ABR)• Temporarily Ordered Routing Algorithm (TORA)• Zone Routing Protocol (ZRP)• Location assisted routing (LAR, DREAM)• Signal Stability Based Adaptive Routing (SSA)• On Demand Multicast Routing Protocol

(ODMRP) – UCLA

Dynamic Source Routing (DSR)

• RFC 4728 – February 2007• Forwarding: source route driven instead of

hop-by-hop route table driven– Mobility ?

• No periodic routing update message is sent• The first path discovered is selected as the

route• Two main phases

– Route DiscoveryRoute Discovery – Route MaintenanceRoute Maintenance

DSR - Route Discovery

• To establish a route, the source floods a Route Route RequestRequest message with a unique request ID

• The Route Request packet “picks up” the node ID numbers

• Route ReplyRoute Reply message containing path information is sent back to the source either by– the destination, or– intermediate nodes that have a route to the

destination• Each node maintains a Route CacheRoute Cache which

records routes it has learned and overheard over time

DSR - Route Maintenance

• Route maintenance performed only while route is in use

• Monitors the validity of existing routes by passively listening to acknowledgments of data packets transmitted to neighboring nodes

• When problem detected, send Route ErrorRoute Error packet to original sender to perform new route discovery

MSR - Multipath Source Routing

• Direct Descendant of DSR • On-demand + Source Routing + Multipath • Probing-based adaptive load balancing among

multiple paths• Motivation of MSR

– Efficiently using the network resource– Alleviate the oscillation in adaptive single

path routing– Fast re-routing– Reducing computing & storage requirement– Exploiting computing power of host instead

of link capacity

Distributing Traffic among Multiple Paths

• Quantities: A heuristic equation• Probing-based adaptive control

– Decoupling between transport layer and network layer: SRPing

– Cost effective • Scheduling: Packet Weighted Round Robin• TCP out-of-order (re-sequencing) problem

Distributing Traffic among Multiple Paths

• Heuristic equation– Rationale: Autonomous system, homogeneous

assumption, bandwidth-delay product constant

where , is the delay of route with index i,

is the maximum delay of all the routes to the same destination, R is a factor to control the switching frequency between routes. U is a bound value to insure that should not to be too large.

jdmax

maxmin ,j

j

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MSR Summary

• Reduce network congestion • Improve throughput, delay, mobility, fault

tolerance (CBR & FTP)• Acceptable routing overhead?

– Little more than that of DSR – Route discovery – Route maintenance

• Probing (unicast) add little O/H• Good candidate for QoS support

– QoS-MSR, reliable-MSR• Acceptable packet out-of-order level ?

Backup Source Routing (BSR)• Establish and maintain backup routes that

can be utilized after the primary path breaks• Define a new routing metric - route

reliability, and use it to provide the basis for the backup path selection

• Reduce the frequency of route discovery flooding, which is a major overhead in on-demand protocols

• Can improve the performance significantly in more challenging situations of high mobility

Simulation Methodology

• ns – Wireless extensions by CMU• Adopt methods used in [Broch98, Johnson99]• Two major files:

– Movement pattern file– Communication pattern file

• 50 mobile hosts placed randomly within a 1500m×300m area

• 20 connections• Different traffic types: CBR & FTP• Two set of simulations: Max speed 20m/s &

1m/s

Performance Evaluation

• MSR vs. DSR vs. BSR

• Performance Metrics

– Packet delivery ratio

– Data throughput

– End-to-end delay

– Packet drop probability

– Queue size

Simulation Results with UDP Traffic

0 100 200 300 400 500 6000.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Pause time (s)

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ket

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DSRMSRBSR

-- Packet delivery ratio for 20 sources

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Connection No.

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ughp

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ackets

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Simulation Results – CBR• End-to-end throughput

Simulation Results with UDP Traffic

0 100 200 300 400 500 6000

0.05

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Pause time (s)

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rage

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)

DSRMSRBSR

-- Average end-to-end delay for 20 sources

11

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Node No.

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DSRMSR

Simulation Results - CBR

• Packets dropped at each node

Previous Work on Using Multiple Paths

• Alternate use (primary and backup)– It works OK for CBR traffic (BSR, Bypass -

DSR, Node Disjoint M-path AODV, etc)– TCP does not get much benefit. Backup path

is used only after timeout; not efficient in mobility/errors.?

• Concurrent use (ie, packet scattering)– MSR– TCP does well in a static, error free net with

long paths (up to 50% improvement)– With mobility & errors, TCP suffers out-of-

order problems because of RTT difference on the two paths

“TCP Performance on multiple paths in ad hoc nets..” Liaw et al ICC 2004

Static net, no errors, opt W: max improvement 50%; typical improvement between 8% and 18%

Multiple Path TCP with Packet Replicas

• TCP data packet duplication on multiple paths– May introduce less O/H than repeated end to

end retransmissions• Improve end-to-end route robustness when

single route is not stable:– Replicate packet on multiple paths– Combat random, non correlated link losses– Combat path breakage

Variable Loss Rate [ 0.05; 0.1; 0.15; 0.2] T

otal

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ut(

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s/s)

Original TCP Multipath TCP

Mobility(m/s)

Where do we stand?

• OLSR and TBRPF can dramatically reduce the “state” sent out on update messages

• They are very effective in “dense” networks.

• However, the state still grows with O(N)• Neither of the above schemes can handle

large scale nets from 10’s to thousands of nodes

• What to do?

The previous schemes reduce control traffic O/H but do not significantly reduce routing table size

Solution: use hierarchical routing to reduce table size

In the process, reduce also control traffic O/HProposed hierarchical schemes include:

– Hierarchical State Routing (HSR)– Fisheye State Routing (FSR)– Landmark Routing – Zone routing (hybrid scheme)

Hierarchical Routing

Routing

• Current MANET solutions have limitations: – (a) proactive routing solutions (eg, Optimal

Links State -OLSR) do not scale because of table size and control traffic overhead

– (b) on demand routing cannot handle high mobility and dense traffic patterns

– (c) explicit hierarchical routing introduces excessive address maintenance O/H in high mobility

• MANET protocols do not scale• UCLA approach: LANMAR

– Exploit implicit hierarchy induced by group mobility

Solution: Landmark Routing Overlay

• Main assumption: nodes move in groups• Groups are predefined or dynamically

recognized • Node address: < group ID , Host address>• Landmark elected in each group• Landmarks advertisements maintain the

landmark overlay

Logical SubnetLogical Subnet

LandmarkLandmark

LANMAR Overlay Routing (cont)

• Builds upon existing MANET protocols– (1) “local ” routing algorithm that keeps

accurate routes within local scope < k hops (e.g., OLSR)

– (2) Landmark routes advertised to all mobiles using DSDV

Logical SubnetLogical Subnet

LandmarkLandmark

LANMAR Overlay Routing (cont)• Packet Forwarding:

– A packet to “local” destination is routed directly using local tables

– A packet to remote destination is routed to Landmark corresponding to logical addr.

– Once the landmark is “in sight”, the direct route to destination is found in local tables

• Benefits: low storage, low update traffic O/H

Logical SubnetLogical Subnet

LandmarkLandmark

Landmark Routing In action

Logical SubnetLogical Subnet

LandmarkLandmarkLM1 LM2

LM3

sourcesourcedestdest

Long haul routinglocal routing

1. Node address = {subnet ID, Host ID}2. Look up local routing table to locate dest fail3. Look up landmark table to find destination subnet

LM14. Send a packet toward LM1

Link Overhead of LANMAR• Dramatic O/H reduction from linear to O(N) to O (sqrtN)

LANMAR enhances MANET routing schemes

We compare:

(a) MANET routing schemes: DSDV, OLSR and FSR; and

(b) same MANET schemes, BUT with LANMAR overlay on top

Delivery Ratio

• DSDV and FSR decrease quickly when number of nodes increases• OLSR generates excessive control packets, cannot exceed 400 nodes

OLSR

DSDV

FSR

LANMAR-DSDV

LANMAR-OLSR

LANMAR-FSR

Georouting - Key Idea

• Each node knows its geo-coordinates (eg, from GPS or Galileo)

• Source knows destination geo-coordinates; it stamps them in the packet

• Geo-forwarding: at each hop, the packet is forwarded to the neighbor closest to destination

• Options:– Each node keeps track of neighbor

coordinates– Nodes know nothing about neighbor

coordinates

Greedy Perimeter Stateless Routing for Wireless Networks (GPSR)

• Greedy forwarding– Each nodes knows own coordinates– Source knows coordinates of destination– Greedy choice – “select” the most forward

node

Finding the most forward neighbor

• Beaconing: periodically each node broadcasts to neighbors own {MAC ID, IP ID, geo coordinates}

• Each data packet piggybacks sender coordinates

• Alternatively (for low energy, low duty cycle ops) the sender solicits “beacons” with “neighbor request” packets

Greedy Perimeter Forwarding

D is the destination; x is the node where the packet enters perimeter mode; forwarding hops are solid arrows;

> Greedy forwarding failure. x is a local maximum in its geographic proximity to D; w and y are farther from D.> Node x’s void with respect to destination D

Got stuck? Perimeter forwarding

GPSR vs DSR

TCP over GPSR, AODV, DSR and DSDV

Speed(m/s)

Th

rou

gh

pu

t (K

bp

s)

GPSR commentary• Very scalable:

– small per-node routing state – small routing protocol message complexity– robust packet delivery on densely deployed,

mobile wireless networks• TCP is extremely sensitive to path breakage

(timeout) -- It does very well with georouting• Outperforms DSR and AODV• Drawback: it requires knowledge of dest geo

coordinates (explicit forwarding node address)– Beaconing overhead– nodes may go to sleep (on and off)