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Portable Networks:Prototype and Performance

Carey WilliamsonGuangwei Bai and Kehinde

OladosuDept of Computer Science

University of Calgary

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Introduction Wireless technologies are prevalent

today; continued growth in popularity Example: IEEE 802.11b WLAN (“WiFi”) Economical, convenient, flexible

solution for tetherless network access (11 Mbps)

Enabler for mobile computing Two possible modes of usage:

Infrastructure mode Ad hoc mode

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Example: Infrastructure Mode

Carey

Internet

Access Point (AP)

cnn.com

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Example: Ad Hoc Mode

Multi-hop “ad hoc” networking

Carey

Mike

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Example: Ad Hoc Mode

Multi-hop “ad hoc” networking

Carey

Mike

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Example: Ad Hoc Mode

Multi-hop “ad hoc” networking

Carey

Mike

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Example: Ad Hoc Mode

Multi-hop “ad hoc” networking

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Mike

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Motivation Observation: The same wireless

technology that allows clients to be mobile also allows servers to be mobile

Hybrid networking paradigm, combining client-server and ad hoc networking, without general Internet infrastructure

Portable, short-lived, ad hoc networks “Portable networks” Is this useful? How well does it work?

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Portable Network (1 of 2)

CareyAccess Point (AP)

mystuff.com

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Portable Network (2 of 2)

Carey

mystuff.com

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Portable Networks Concept Set up when needed, tear down after Typically needed for minutes or

hours When and where not known a priori No existing network infrastructure General Internet access not

available, but not required either Pre-defined content; target audience Modest number of users; mobile too

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Example Usage Scenarios #1

Classroom area network (e.g. “legacy classroom”)

Press conferences, media events Conventions and trade shows Disaster recovery sites Recruiting events Schools Voting...

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#2 Wireless Media Servers

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#3 Wireless Gaming

Multi-player gaming over an ad hoc wireless network

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#4 Home Networking Even for homes without Internet

access, wireless media servers and ad hoc networks could be quite useful

Possible usage scenarios: use PDA to read recipe while in the

kitchen page your kids for supper time work while on your back porch (backups) music in any room of the house portable media player for parties family gaming fun

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Research Objectives Assess feasibility of portable networks Benchmark the capabilities and

limitations of wireless Web servers in an IEEE 802.11b Wireless LAN

Identify performance bottlenecks Understand sensitivity of performance

to different workload assumptions Understand impacts of wireless

network channel quality and error rates

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Experimental Setupwireless sniffer

clients server

...

SnifferPro 4.6

Apache1.3.23

httperf

Empirical measurement methodology Network: In-building 11 Mbps IEEE 802.11b wireless LAN,

operating in ad hoc mode (single hop; no mobility) Hardware: Compaq Evo N600c notebooks (1.2 GHz

Pentium III, 128 MB RAM, 512 KB L2 Cache, Cisco Aironet 350 network cards

Software: Redhat Linux 7.3 (kernel 2.4.18-3), Apache, SnifferPro, httperf for Web workload generation

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Tutorial: HTTP and TCP

TCP is a connection-oriented protocol

SYN

SYN/ACK

ACKGET URL

YOUR DATA HERE

FINFIN/ACK

ACK

Web Client Web Server

Example Web Page

Harry Potter Movies

As you all know,the new HP bookwill be out in Juneand then there willbe a new movieshortly after that…

“Harry Potter andthe Bathtub Ring”

page.html

hpface.jpg

castle.gif

Client Server

The “classic” approachin HTTP/1.0 is to use oneHTTP request per TCPconnection, serially.

TCP SYN

TCP FIN

page.htmlGet

TCP SYN

TCP FIN

hpface.jpgGet

TCP SYN

TCP FIN

castle.gifGet

Client Server Concurrent (parallel) TCPconnections can be usedto make things faster.TCP SYN

TCP FIN

page.htmlGet

castle.gif

Get

F

S

Get

hpface.jpg

S

F

C S C S

Client Server

The “persistent HTTP”approach can re-use thesame TCP connection forMultiple HTTP transfers,one after another, serially.Amortizes TCP overhead,but maintains TCP statelonger at server.

TCP FIN

Timeout

TCP SYN

page.html

Get

hpface.jpg

Get

castle.gif

Get

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Benchmarking Experiments

Factor Levels Number of Clients 1, 2, 4

HTTP Transaction Rate (per-client) 10, 20, 30, …, 160

HTTP Transfer Size (KB) 1, 2, 4, 8, …, 100

Persistent Connections no, yes

HTTP Requests per Connection 1, 5, 10, 15, …, 60

Transmit Power (mW) 1, 5, 20, 30, 50, 100

Client-Server Distance (m) 1, 10, 100

Experimental Factors and Levels

Performance Metrics: HTTP response time, network throughput

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What is the range of sustainable load?

Design:• Number of Client: 1• HTTP request rate (req/sec): 10, 20, …, 160• HTTP transfer size: 1 KB• Non-persistent HTTP• Client-server distance: 1 meter (same desk)

Research Question

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Request Rate Results

Tra

nsm

it Q

ueu

e L

engt

h

Transmitted Packet ID

(b) Client Transmit Queue

Tra

nsacti

on

Rate

(re

q/s

ec)

Request Rate (req/sec)

(a) Successful HTTP Transaction Rate

- Maximum sustainable 1 KB HTTP transaction rate for 1 client is about 85 reqs/sec (throughput about 0.9 Mbps)- Beyond this request rate, the client link-layer transmit queue builds up and overflows, losing packets even before they get onto the wireless LAN!- Wireless LAN is bottleneck (802.11b channel access protocol)

CSMA-CA + AcknowledgementCarrier Sense Multiple Access with Collision Avoidance

* SIFS - Short Inter-Frame Space (approx 28 µs)

• Every frame is ack’ed - except broadcast and multicast!

“Air” is freefor DIFS time

period(128 usec)

Receive ACK backthat frame was received intact!

send frame

source destination others

DIFS

SIFS

All other devicesmust defer while

“air” is busy

data

ack

NA

V:

defe

r access

IEEE 802.11b Frame Format

Long Preamble = 144 bits

• Interoperable with older 802.11 devices

• Entire Preamble and 48 bit PLCP Header sent at 1 Mbps

128-bit Preamble

(Long)

16 bitStart ofFrame

Delimiter

Signal Speed

1,2,5.5,11

Mbps

Lengthof

Payload

16 bitCRC

Payload0-2312 bytes

Transmitted at 1 Mbps Transmitted at X Mbps

DLL HDR (IP TCP HTTP Data)

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Research Question

Does persistent HTTP help?

Design:• Number of Clients: 1, 2• HTTP request rate: 10 req/sec• HTTP transfer size: 1 KB• Persistent HTTP• Client-server distance: 1 m (same desk)

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Results for Persistent Connections

Peak throughput: 3.2 Mbps, 3.5x improvement over non-persistent connection case (0.9 Mbps) for 1 KB transfers

Typically 2 TCP packets per HTTP transaction (vs 10)

Th

rou

ghp

ut

(Mb

ps)

HTTP Req/Connection

YES!

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Research Question

What is maximum throughput achievable?

Design• Number of Client: 1• HTTP request rate: 10 req/sec• HTTP transfer size (KB): 1, 2, 4, 8, … • Non-persistent HTTP• Client-server distance: 1 m (same desk)

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Transfer Size ResultsT

CP

Con

nec

tion

Du

rati

on (

sec)

Connection ID

Fre

qu

ency

(%

)

Duration (Sec)

48 KB:Mean Duration: 97 msThroughput: 4.1 Mbps

bottleneck shifts to the server’s link layer transmit queue

8 KB:

Mean Duration: 24 ms

Throughput: 2.9 Mbps

1 KB:

Mean Duration: 9.7 ms

Throughput: 0.9 Mbps

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Classroom Experiments (Feb’03)

Aggregate Packets Transmitted on WLAN

0

100

200

300

400

500

600

700

800

Time in Seconds

Pa

cke

ts p

er

1 se

c In

terv

al

0 1400

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Packets Transmitted by Client IP 200

0

100

200

300

400

500

600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

Packets Transmitted by Client IP 204

0

100

200

300

400

500

600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

Packets Transmitted by Client IP 207

0

100

200

300

400

500

600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

Packets Transmitted by Client IP 212

0

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200

300

400

500

600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

Packets Transmitted by Client IP 220

0

100

200

300

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600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

Packets Transmitted by Client IP 208

0

100

200

300

400

500

600

700

Time in Seconds

Pa

cke

ts p

er

1 s

ec I

nte

rva

l

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Summary of Results Wireless Web servers can work! Wireless LAN is the bottleneck Bottleneck manifests itself differently,

depending on the Web workload client side, for small HTTP transfers server side, for large HTTP transfers unfairness amongst clients if TCP SYN

losses network thrashing in some scenarios

Persistent HTTP helps a lot!

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Conclusions Portable networks: a novel paradigm

for the use of ad hoc networks Reasonable performance with

existing off-the-shelf hardware and software

Performance bottleneck at the WLAN manifests itself in interesting ways

IEEE 802.11a (55 Mbps) may help Need to explore novel scenarios for

the use of this networking paradigm

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The End

Thanks for your attention! Credits:

Guangwei Bai Kehinde (Kenny) Oladosu

More info: carey@cpsc.ucalgary.ca

Questions?