Download Part I: Introduction - Bad Request

Wireless Internet:
Protocols and Performance
Carey Williamson
iCORE Professor
Department of Computer Science
University of Calgary
C. Williamson
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Tutorial Outline
r 1. Introduction/Motivation (10 min)
r 2. Networking Terminology (10 min)
r 3. IEEE 802.11b WLANs (30 min)
r 4. TCP (20 min)
r 5. HTTP (15 min)
r
r
r
r
---- Coffee Break Here ---- (10:30-10:45am)
6. Wireless TCP Performance: Part 1 (15 min)
7. Wireless TCP Performance: Part 2 (15 min)
8. Wireless Web Performance (30 min)
9. Summary, Questions, Discussion (10 min)
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1. Wireless Internet:
Introduction
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What Is Wireless Networking?
r The use of infra-red (IR) or radio frequency
(RF) signals to share information and
resources between devices
r A hot computer industry buzzword:
m
m
Lots of advertising by companies and media
Wireless Broadband, 3G wireless, 4G, WAP,
iMode, Bluetooth, WiFi
r Mobile Internet, Pervasive Computing,
Nomadic Computing, M-commerce
m
Ubiquitous; Global; Revolutionary
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Two Popular 2.4 GHz Standards:
r IEEE 802.11
m
m
m
m
m
m
Fast (11b)
High Power
Long range
Single-purpose
Ethernet
replacement
Easily Available
r Bluetooth
m
m
m
m
m
m
Slow
Low Power
Short range
Flexible
Cable replacement
“Vapourware” (?)
• Apple Airport,
iBook, G4
• Cisco Aironet 350
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IEEE 802.11 Organization Tree:
IEEE 802.11
Working Group
PHYS Layer
Infra-Red (IR)
802.11 IR
1 / 2 Mbit/s
2.4 GHz (FHSS)
802.11 FHSS
1 / 2 Mbit/s
MAC Layer
2.4 GHz (DSSS)
5 GHz (OFDM)
802.11 DSSS
1 / 2 Mbit/s
802.11b
High Data Rate Extension
5.5 / 11 Mbit/s
802.11 MAC
802.11a
6 / 12 / 24 Mbit/s
Optional 9/18/36/54 Mbit/s
802.11e
MAC Enhancements
Security
QOS
802.11g
Data Rates > 20 Mbit/s
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Pros and Cons of 802.11:
r Pro:
m High bandwidth (up to 11 Mbps)
m Two modes of operation: infrastructure vs. ad hoc
r Con:
m Incompatibility between old and new cards
m Signal blocked by reinforced concrete or tinted
glass
m High channel BER can degrade performance (lots!)
m No standard for hand-off between base stations
m Some channel numbers overlap spectrum
m High power consumption in laptops
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Bluetooth
Think USB, not Ethernet
r Cable replacement technology
r Created by Ericsson
r PAN - Personal Area Network
r
m
m
m
m
1-2 Mbps connections
1600 hops per second FHSS
Includes synchronous, asynchronous, voice connections
Piconet routing
Small, low-power, short-range, cheap, versatile
radios
r Used as Internet connection, phone, or headset
r Master/slave configuration and scheduling
r
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Security
r Wireless sniffers, “war driving”
r IEEE 802.11:
m ESSID – Extended Services Set ID
m WEP – Wired Equivalent Privacy (useless)
m NAT - Network Address Translation (firewall)
r Bluetooth Security
m FHSS, with rapid hop sequence
m Short range
m Encrypted transmissions (optional)
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Guerrilla.net
An underground alternative to the wired Internet
r A grassroots movement established in 1996
r
m
m
m
802.11 Wireless LAN cards
Roof mounted antennae
Free software (FreeBSD)
Multi-hop routing, Internet connectivity
r About $500 per node (and dropping)
r Other networks popping up in SF, Seattle, London
r
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Future of Wireless
r
r
r
r
r
r
r
r
r
Higher data rates
Better security
Wider selection of products
Lower prices
Zero configuration networking
More end-user focus
Better software
Less visible
More popular
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2. Background:
Networking Terminology
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Wireless Internet Technologies
r Mobile devices (e.g., notebooks, laptops,
PDAs, cell phones, wearable computers)
r Wireless network access
m
m
m
Bluetooth (1 Mbps, up to 3 meters)
IEEE 802.11b (11 Mbps, up to 100 meters)
IEEE 802.11a (55 Mbps, up to 20 meters)
r Operating modes:
m Infrastructure mode (access point)
m Ad hoc mode
r Wireless Web, WiFi “hot spots”
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Internet Protocol Stack
r
Application: supporting network
applications and end-user services
m
r
Transport: end to end data transfer
m
r
IPv4, IPv6, BGP, RIP, routing protocols
Data Link: hop by hop frames,
channel access, flow/error control
m
r
TCP, UDP
Network: routing of datagrams
from source to destination
m
r
FTP, SMTP, HTTP, DNS, NTP
PPP, Ethernet, IEEE 802.11b
Application
Transport
Network
Data Link
Physical
001101011...
Physical: raw transmission of bits
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Internet Protocol Stack
r
Application:
m
r
Transport:
m
m
r
Internet Protocol (IP, IPv4, IPv6)
Data Link:
m
r
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Network:
m
r
e.g., Hyper-Text Transfer Protocol (HTTP)
e.g., IEEE 802.3 Ethernet, IEEE 802.11b
Physical:
m
Device specific, network specific
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Application
Transport
Network
Data Link
Physical
001101011...
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Multi-Hop Wireless Ad Hoc Networks
r Routing protocols used to improve wireless
connections
r Infrastructure-free, dynamic
r True Peer-to-Peer routing
r Fault tolerant
r Examples: AODV, DSDV, TORA, DSR, ...
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Example:
r Multi-hop “ad hoc” networking
Kelly
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Carey
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3. IEEE 802.11b
Wireless LANs
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The Basics
r In several respects, the IEEE 802.11b
wireless LAN (WLAN) standard is similar
to that for IEEE 802.3 (Ethernet) LANs
r Similarities:
m
LAN; limited geographic coverage; multiple
stations; shared transmission medium; CSMAbased Medium Access Control protocol;
48-bit MAC addresses; comparable data rates
(11 Mbps vs 10 Mbps)
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The Basics (Cont’d)
r But there are also distinct differences:
m wireless (air interface) versus wired (coax)
m wireless propagation environment (multipath)
m higher error rate due to interference, etc.
m successful frames are ACKed by receiver
m mobile stations; “hidden node” problem;
potential asymmetries
m CSMA/CA versus CSMA/CD
m multiple data transmission rates (1, 2, 5.5, 11)
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Some Features
r Infrastructure mode vs “ad hoc” mode
r Access Point (AP) sends “beacon frames”
m Mobiles choose AP based on signal strength
r Multiple channel access protocols
supported
m
CSMA/CA (DCF); PCF; RTS/CTS
r MAC-layer can provide error control,
retransmission, rate adaptation, etc.
r Direct Sequence Spread Spectrum (DSSS)
m
signal spread across 14 22-MHz channels
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Where Does Wireless RF Live?
ISM Band: Industrial, Scientific, Medical
902-928 MHz
2400-2483.5 MHz
Old Wireless
802.11/802.11b
5725-5850 MHz
802.11a
Bluetooth
Cordless Phones
Home RF
Baby Monitors
Microwave Ovens
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Where does 802.11 live in the OSI?
Application
Telnet, FTP, Email, Web, etc.
Presentation
Session
TCP, UDP
Transport
IP, ICMP, IPX
Network
Logical Link Control - 802.2
(Interface to the upper layer protocols)
Data Link
MAC
Wireless lives at
Layers 1 & 2
only!
802.3, 802.5, 802.11
Physical Layer Convergence Protocol
LAN: 10BaseT, 10Base2, 10BaseFL
WLAN: FHSS, DSSS, IR
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Physical
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11 Mbps bandwidth
“shared” by all devices
in the Cell!
Wireless Cells
Access Point
coverage
area is called a
“Cell”
Access Point
Channel 6
ESSID: NAI
Range per Access
Point is 100m
• In Canada/US, there are eleven 802.11 channels
• Only channels 1, 6 and 11 are non-overlapping
• Each Access Point coverage area is called a “Cell”
• Computers can roam between cells
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Wireless Cells
1
1
6
11
11
1
Computers can roam between cells
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CSMA-CA + Acknowledgement
Carrier Sense Multiple Access with Collision Avoidance
How CSMA-CA works:
• Device wanting to transmit senses the medium (Air)
• If medium is busy - defers
• If medium is free for certain period (DIFS) - transmits frame
Latency can increase if “air” is very busy! Device
has hard time finding “open air” to send frame!
* DIFS - Distributed Inter-Frame Space (approx 128 µs)
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CSMA-CA + Acknowledgement
Carrier Sense Multiple Access with Collision Avoidance
send frame
DIFS
data
SIFS
Receive ACK back
that frame was
received intact!
ack
others
NAV: defer access
“Air” is free
for DIFS time
period
source destination
All other devices
must defer while
“air” is busy
• Every frame is ack’ed - except broadcast and multicast!
* SIFS - Short Inter-Frame Space (approx 28 µs)
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MAC-Layer Retransmission
r If no ACK received “right away”, then the
sender retransmits the frame again at the
MAC layer
m
m
m
indicates frame (or ACK) was lost/corrupted
very short timeout (e.g., 1 msec)
exponential backoff (doubling) if repeated loss
r Typically recovers before TCP would notice
r Max retransmission limit (e.g., 8)
r May do MAC-layer rate adaptation or frame
fragmentation if channel error rate is high
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Other MAC Protocols Supported
r Point Coordination Function (PCF)
m
m
AP polls stations in turn to see if frames to send
useful for real-time traffic
r Request-To-Send/Clear-To-Send
(RTS/CTS)
m
m
m
m
m
reservation-based approach (ask permission)
useful for very large frames
useful for solving the “hidden node” problem
request asks for clearance (permission) to send
request also indicates time required for transmit
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Frame Formats
r Two frame formats available:
m
m
long preamble
short preamble
r Configuration option for NIC and AP
r Variable-size frames (max 2312 data bytes)
r 16-bit Cyclic Redundancy Code (CRC)
for error checking of frames
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Long Preamble
Long Preamble = 144 bits
• Interoperable with older 802.11 devices
• Entire Preamble and 48 bit PLCP Header sent at 1 Mbps
Transmitted at 1 Mbps
Signal
128 bit Preamble
(Long)
16 bit
Length 16 bit
Service
Speed
Start
of
CRC
(unused)
1,2,5.5,
Frame
Payload
11
Delimiter
Payload
0-2312 bytes
Mbps
Transmitted at X Mbps
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Short Preamble
Short Preamble = 72 bits
• Preamble transmitted at 1 Mbps
• PLCP Header transmitted at 2 Mbps
• more efficient than long preamble
Transmitted
at
2 Mbps
Transmitted
at
1 Mbps
Signal
56 bit
Preamble
16 bit
Speed Service
Start
1,2,5.5, (unused)
Frame
11
Delimiter
Length 16 bit
of
CRC
Payload
Transmitted
at
X Mbps
Payload
0-2312 bytes
Mbps
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Even More Features
r Power Management
m
m
m
mobile nodes can “sleep” to save power
AP will buffer frames until client requests them
AP can use virtual bitmap field in beacons to
indicate which stations have data waiting
r Security
m Wired Equivalent Privacy (WEP)
m not secure at all!
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Summary
r IEEE 802.11b (WiFi) is a wireless LAN
technology that is rapidly growing in
popularity
r Convenient, inexpensive, easy to use
r Growing number of “hot spots” everywhere
m
airports, hotels, bookstores, Starbucks, etc
r Estimates: 70% of WLANs are insecure!
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4. TCP 101:
Transmission Control Protocol
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Tutorial: TCP 101
r The Transmission Control Protocol (TCP) is
r
r
r
r
the protocol that sends your data reliably
Used for email, Web, ftp, telnet, p2p,…
Makes sure that data is received correctly:
right data, right order, exactly once
Detects and recovers from any problems
that occur at the IP network layer
Mechanisms for reliable data transfer:
sequence numbers, acknowledgements,
timers, retransmissions, flow control...
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TCP 101 (Cont’d)
r TCP is a connection-oriented protocol
SYN
SYN/ACK
GET URL
ACK
Web Client
Web Server
YOUR DATA HERE
FIN
ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r TCP slow-start and congestion avoidance
ACK
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TCP 101 (Cont’d)
r This (exponential growth) “slow start”
process continues until either:
m
m
m
packet loss: after a brief recovery phase, you
enter a (linear growth) “congestion avoidance”
phase based on slow-start threshold found
limit reached: slow-start threshold, or
maximum advertised receive window size
all done: terminate connection and go home
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Tutorial: TCP 201
r There is a beautiful way to plot and
visualize the dynamics of TCP behaviour
r Called a “TCP Sequence Number Plot”
r Plot packet events (data and acks) as
points in 2-D space, with time on the
horizontal axis, and sequence number on
the vertical axis
r Example: Consider a 14-packet transfer
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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So What?
r What can it tell you?
r Everything!!!
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
RTT
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X
X
+
+
+
+
+
+
Time
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SeqNum
TCP
Seg.
Size
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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TCP Connection Duration
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
Num
Bytes
Sent
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Bytes
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Sec
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
Access
Network
Bandwidth
(Bytes/Sec)
X
+
X
X
X
X
X
X
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Sender’s
Flow Control
Window Size
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
TCP
Slow
Start
X
+
X
X
X
X
X
X
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
X
+
X
X
+
X
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
Delayed ACK
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
Packet
Loss
X
X
X
X
X
X
X
X
X
+
X
X
+
+
+
+
++
+
+
+
Duplicate
ACK
+
+
Time
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SeqNum
Cumulative ACK
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
+
X
X
+
X
++
+
+
+
X
Retransmit
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
+
X
X
++
+
+
+
X
+
+
+
+
+
+
Time
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X
RTO
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TCP 201 (Cont’d)
r What happens when a packet loss occurs?
r Quiz Time...
m Consider a 14-packet Web document
m For simplicity, consider only a single packet loss
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
?
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
X
+
X
+
X
+
X
+
X
+
X
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
X
+
X
+
X
+
X
+
X
+
X
X
+
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
X
?
+
X
+
X
+
X
+
X
+
X
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
X
++
X
+
X
+
X
+
X
+
X
+
X
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
?
X
X
X
X
X
X
X
X
+
X
X
+
+
+
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
X
X
X
X
X
+
X
X
+
X
+++
+
+
+
+
+
+
+
+
+
Time
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SeqNum
+
X
+
X
+
X
+
X
+
X
+
X
+
X
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
+
X
X
+
+
+
+
+
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
?
X
+
X
+
Time
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SeqNum
Key: X Data Packet
+ Ack Packet
X
X
X
X
X
X
+
+
+
+
+
X
X
X
+
+
+
++
X
Time
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TCP 201 (Cont’d)
r Main observation:
m
“Not all packet losses are created equal”
r Losses early in the transfer have a huge
adverse impact on the transfer latency
r Losses near the end of the transfer always
cost at least a retransmit timeout
r Losses in the middle may or may not hurt,
depending on congestion window size at the
time of the loss
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Congratulations!
r You are now a TCP expert!
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5. HTTP:
HyperText Transfer Protocol
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Introduction to HTTP
Laptop w/
Netscape
r
m
http response
http response
Server w/ Apache
Desktop w/
Explorer
Communication protocol between clients and servers
Application layer protocol for WWW
Client/Server model:
m
m
r
http request
HTTP: HyperText Transfer Protocol
m
r
http request
Client: browser that requests, receives, displays object
Server: receives requests and responds to them
Protocol consists of various operations
m
m
C. Williamson
Few for HTTP 1.0 (RFC 1945, 1996)
Many more in HTTP 1.1 (RFC 2616, 1999)
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Request Generation
User clicks on something
r Uniform Resource Locator (URL):
r
m
m
m
m
http://www.cnn.com
http://www.cpsc.ucalgary.ca
https://www.paymybills.com
ftp://ftp.kernel.org
Different URL schemes map to different services
r Hostname is converted from a name to a 32-bit IP
address (DNS lookup, if needed)
r Connection is established to server (TCP)
r
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What Happens Next?
r
Client downloads HTML document
m
m
m
Sometimes called “container page”
Typically in text format (ASCII)
Contains instructions for rendering
(e.g., background color, frames)
m
r
Links to other pages
Many have embedded objects:
m
m
Images: GIF, JPG (logos, banner ads)
Usually automatically retrieved
• I.e., without user involvement
• can control sometimes
(e.g. browser options, junkbusters)
C. Williamson
<html>
<head>
<meta
name=“Author”
content=“Erich Nahum”>
<title> Linux Web
Server Performance
</title>
</head>
<body text=“#00000”>
<img width=31
height=11
src=“ibmlogo.gif”>
<img
src=“images/new.gif>
<h1>Hi There!</h1>
Here’s lots of cool
linux stuff!
<a href=“more.html”>
Click here</a>
for more!
</body>
</html>
sample html file
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Web Server Role
r
Respond to client requests, typically a browser
m
m
r
Can be a proxy, which aggregates client requests (e.g., AOL)
Could be search engine spider or robot (e.g., Keynote)
May have work to do on client’s behalf:
m
m
Is the client’s cached copy still good?
Is client authorized to get this document?
Hundreds or thousands of simultaneous clients
r Hard to predict how many will show up on some day
(e.g., “flash crowds”, diurnal cycle, global presence)
r Many requests are in progress concurrently
r
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HTTP Request Format
GET /images/penguin.gif HTTP/1.0
User-Agent: Mozilla/0.9.4 (Linux 2.2.19)
Host: www.kernel.org
Accept: text/html, image/gif, image/jpeg
Accept-Encoding: gzip
Accept-Language: en
Accept-Charset: iso-8859-1,*,utf-8
Cookie: B=xh203jfsf; Y=3sdkfjej
<cr><lf>
• Messages are in ASCII (human-readable)
• Carriage-return and line-feed indicate end of headers
• Headers may communicate private information
(browser, OS, cookie information, etc.)
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Request Types
Called Methods:
r GET: retrieve a file (95% of requests)
r HEAD: just get meta-data (e.g., mod time)
r POST: submitting a form to a server
r PUT: store enclosed document as URI
r DELETE: removed named resource
r LINK/UNLINK: in 1.0, gone in 1.1
r TRACE: http “echo” for debugging (added in 1.1)
r CONNECT: used by proxies for tunneling (1.1)
r OPTIONS: request for server/proxy options (1.1)
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Response Format
HTTP/1.0 200 OK
Server: Tux 2.0
Content-Type: image/gif
Content-Length: 43
Last-Modified: Fri, 15 Apr 1994 02:36:21 GMT
Expires: Wed, 20 Feb 2002 18:54:46 GMT
Date: Mon, 12 Nov 2001 14:29:48 GMT
Cache-Control: no-cache
Pragma: no-cache
Connection: close
Set-Cookie: PA=wefj2we0-jfjf
<cr><lf>
<data follows…>
• Similar format to requests (i.e., ASCII)
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Response Types
r 1XX: Informational (def’d in 1.0, used in 1.1)
100 Continue, 101 Switching Protocols
r 2XX: Success
200 OK, 206 Partial Content
r 3XX: Redirection
301 Moved Permanently, 304 Not Modified
r 4XX: Client error
400 Bad Request, 403 Forbidden, 404 Not Found
r 5XX: Server error
500 Internal Server Error, 503 Service
Unavailable, 505 HTTP Version Not Supported
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Outline of an HTTP Transaction
r This section describes the
basics of servicing an HTTP
GET request from user space
r Assume a single process
running in user space, similar
to Apache 1.3
r We’ll mention relevant socket
operations along the way
C. Williamson
initialize;
forever do {
get request;
process;
send response;
log request;
}
server in
a nutshell
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Readying a Server
s = socket();
bind(s, 80);
listen(s);
while (1) {
newconn =
r
r
r
r
r
/* allocate listen socket */
/* bind to TCP port 80
*/
/* indicate willingness to accept */
accept(s); /* accept new connection */b
First thing a server does is notify the OS it is interested in
WWW server requests; these are typically on TCP port 80.
Other services use different ports (e.g., SSL is on 443)
Allocate a socket and bind()'s it to the address (port 80)
Server calls listen() on the socket to indicate willingness to
receive requests
Calls accept() to wait for a request to come in (and blocks)
When the accept() returns, we have a new socket which
represents a new connection to a client
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Processing a Request
remoteIP = getsockname(newconn);
remoteHost = gethostbyname(remoteIP);
gettimeofday(currentTime);
read(newconn, reqBuffer, sizeof(reqBuffer));
reqInfo = serverParse(reqBuffer);
r
r
r
r
r
getsockname() called to get the remote host name
m
for logging purposes (optional, but done by most)
m
again for logging purposes
m
both for Date header and for logging
m
“GET /images/jul4/flag.gif”
gethostbyname() called to get name of other end
gettimeofday() is called to get time of request
read() is called on new socket to retrieve request
request is determined by parsing the data
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Processing a Request (cont)
fileName = parseOutFileName(requestBuffer);
fileAttr = stat(fileName);
serverCheckFileStuff(fileName, fileAttr);
open(fileName);
r
stat() called to test file path
m
m
m
r
to see if file exists/is accessible
may not be there, may only be available to certain people
"/microsoft/top-secret/plans-for-world-domination.html"
stat() also used for file meta-data
m
m
e.g., size of file, last modified time
"Has file changed since last time I checked?“
might have to stat() multiple files and directories
r assuming all is OK, open() called to open the file
r
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Responding to a Request
read(fileName, fileBuffer);
headerBuffer = serverFigureHeaders(fileName, reqInfo);
write(newSock, headerBuffer);
write(newSock, fileBuffer);
close(newSock);
close(fileName);
write(logFile, requestInfo);
r
r
read() called to read the file into user space
write() is called to send HTTP headers on socket
(early servers called write() for each header!)
r
r
r
r
write() is called to write the file on the socket
close() is called to close the socket
close() is called to close the open file descriptor
write() is called on the log file
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Network View: HTTP and TCP
r TCP is a connection-oriented protocol
SYN
SYN/ACK
GET URL
ACK
Web Client
Web Server
YOUR DATA HERE
FIN
ACK
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FIN/ACK
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Example Web Page
Harry Potter Movies
page.html
C. Williamson
As you all know,
the new HP book
will be out in June
and then there will
be a new movie
shortly after that…
hpface.jpg
“Harry Potter and
the Bathtub Ring”
castle.gif
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Server
Client
TCP SYN
G
page.html
TCP FIN
TCP SYN
The “classic” approach
in HTTP/1.0 is to use one
HTTP request per TCP
connection, serially.
G
hpface.jpg
TCP FIN
TCP SYN
G
castle.gif
Williamson
TCPC. FIN
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Server
Client
TCP SYN
G
page.html
TCP FIN
C
S
Concurrent (parallel) TCP
connections can be used
to make things faster.
C
S
S
S
G
G
hpface.jpg
F
C. Williamson
castle.gif
F
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Server
Client
TCP SYN
G
page.html
The “persistent HTTP”
approach can re-use the
same TCP connection for
Multiple HTTP transfers,
one after another, serially.
Amortizes TCP overhead,
but maintains TCP state
longer at server.
G
hpface.jpg
G
castle.gif
Timeout
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TCPC. FIN
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Server
Client
TCP SYN
G
page.html
The “pipelining” feature
in HTTP/1.1 allows
requests to be issued
asynchronously on a
persistent connection.
Requests must be
processed in proper order.
Can do clever packaging.
GG
hpface.jpg
castle.gif
Timeout
Williamson
TCPC. FIN
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Summary of Web and HTTP
r The major application on the Internet
m Majority of traffic is HTTP (or HTTP-related)
r Client/server model:
m Clients make requests, servers respond to them
m Done mostly in ASCII text (helps debugging!)
r Various headers and commands
m Too many to go into detail here
m Many web books/tutorials exist
(e.g., Krishnamurthy & Rexford 2001)
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6. Wireless TCP:
Performance Issues (Part 1)
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Example #1
r Wireless TCP Performance Problems
Low capacity,
high error rate
Wireless
Access
C. Williamson
Wired Internet
Hard to distinguish losses
here from losses here
High
capacity,
low
error
rate
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Example #1 (Cont’d)
r Solution: “wireless-aware TCP” (I-TCP,
ProxyTCP, Snoop-TCP, split connections...)
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Example #2
r Wireless TCP Fairness Problems
D
DATA
ACK
Wireless
Bottleneck AP
Wired Internet
U
DATA
C. Williamson
ACK
Loss of ACK = Loss of DATA
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Example #3
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Multi-hop “ad hoc” networking
Kelly
C. Williamson
Carey
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Example #3 (Cont’d)
r Two interesting subproblems:
m Dynamic ad hoc routing: node movement can
disrupt the IP routing path at any time,
disrupting TCP connection; yet another way to
lose packets!!!; possible solution: Explicit Loss
Notification (ELN)? Handoff? Route prediction?
m
TCP flow control: the bursty nature of TCP
packet transmissions can create contention for
the shared wireless channel among forwarding
nodes; collisions between DATA and ACKs
possible solution: rate-based flow control? Burst
mode? Spatial reuse of channels?
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Summary of Wireless TCP
r TCP is the “four wheel drive” of TP’s
r Wireless is a newly emerging technology
with rapidly growing deployment popularity
r “TCP” and “Wireless” don’t fit together all
that well
r Making TCP smarter about wireless helps!
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7. Wireless TCP:
Performance Issues (Part 2)
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Motivation
TCP performance often degrades over wireless
networks; reasons “well-known”
r Solutions to improve TCP performance over
wireless links exist, but how well do they work in a
real wireless LAN environment?
r How do link-layer mechanisms interact with TCP
and affect the overall performance?
r Where is the bottleneck in the network protocol
processing path, and why?
r
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Background - TCP
r Widely used on the Internet (e.g. Web)
r Connection-oriented, reliable byte stream
r Window-based flow control
r Slow start and congestion avoidance
r Fast retransmission, fast recovery
r Other extensions, including TCP SACK
r Many different versions in use
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Background – IEEE 802.11b
r
r
r
r
r
An “Ethernet-like” LAN standard (11 Mbps)
Infrastructure mode and ad hoc mode
Carrier-sense multiple access with collision avoidance
(CSMA/CA) to reduce collisions
MAC-layer: positive acknowledgment and
retransmissions (to recover from channel errors)
Dynamic rate adaptation: can choose data
transmission rate of 1, 2, 5.5, or 11 Mbps
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Background – USB
r
r
r
r
r
r
Widely used industry standard for connecting a
computer to its peripherals (bus topology)
Lots of USB-based (wireless) network cards
Data transfers managed by Host Controller (HC)
Synchronous bus: 1 msec slots for transfers
Transfer requests are handled using vertical and
horizontal linked-list data structures
Two processing modes for HC:
m
r
Breadth-First
or
Depth-First
High Speed Bandwidth Reclamation (HSBR)
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Background – USB (cont’d)
r
r
Queued mode (keep HC busy)
Transfer size: 64 – 1023 bytes each
BF
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DF
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Background – USB (cont’d)
r
r
Queued mode (keep HC busy)
Transfer size: 64 – 1023 bytes each
FSBR
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Experimental Methodology
r
Experimental Setup (HW/SW)
m
m
m
m
m
r
Laptop – Compaq Evo 719c with multiport USB wireless
card (Linux 2.4)
Access point – Lucent RG-1000
Stationary host on Ethernet LAN – SunOS 5.8
Run netperf on laptop and netserver on wired host
SnifferPro 4.6 wireless “sniffer” and tcpdump
Experimental Factors
m
m
USB mode, driver settings
Wireless channel (distance) between laptop and AP
tcpdump netserver
C. Williamson
data
Ethernet
acks
netperf
AP
laptops
Sniffer
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2003 Tutorial
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Initial Result
r
Windows 2000 implementation of TCP is more than
3 times faster than Linux TCP!
OS
Linux
Windows 2000
r
Throughput
1.52 Mbps
5.11 Mbps
Reason: Linux driver bug (2 Mbps vs 11 Mbps)
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Results – USB Experiments
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Results – USB Experiments
With FSBR disabled, USB is the bottleneck
r With FSBR enabled (the default in Linux), the
wireless network is the bottleneck
r Queued mode makes no difference with FSBR on,
but helps when FSBR is turned off
r Queued mode (even with FSBR turned on) may be
very important when higher speed wireless link is
used (e.g. IEEE 802.11a)
r
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Results – Wireless Problems
r
We observed unusually high collision rates on the
wireless channel for TCP transfers, which we call
the TCP data/ACK collision problem
m
m
m
m
m
Scenario: laptop and AP are 1 m apart
For TCP, MAC-layer retransmit rate: 4.58-4.73%
For UDP, MAC-layer retransmit rate: 0.47-0.98%
In general, a retransmission rate of 1.75%-7.2% has been
seen for other vendor HW/SW (N = 1)
For TCP, disabling MAC-layer retransmission degrades
throughput by 23%
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Results – Wireless Problems
r The MAC-layer rate adaptation problem
m
m
m
m
Scenario: laptop and AP are 100 m apart
Lousy TCP throughput, lots of retransmits
Reason: the multiplicative increase and
multiplicative decrease (MIMD) bandwidth
probing mechanism causes network thrashing
and wastes battery power
The small congestion window causes temporary
deadlock if the TCP receiver uses delayed Ack
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Results – Wireless Problems
(MAC-layer rate adaptation)
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Summary of Observations
TCP performance on WLAN can be wacky! (at
least for Compaq Multiport 802.11b USB wireless
card under Linux 2.4)
r Several factors can affect overall performance
r
m
m
m
m
Poorly configured USB bus could be the bottleneck
Linux TCP implementation bug makes TCP unable to
recognize the first spurious timeout
Poor MAC-layer rate adaptation algorithm can cause a
“network thrashing” problem
TCP’s data/ACK structure may induce excessive collisions
at the MAC layer on wireless LANs
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7. Wireless Web:
Performance Issues
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Introduction and Motivation
 Observation: the same wireless technology
that allows a Web client to be mobile also
allows Web servers to be mobile
 Idea: portable, short-lived, ad hoc networks
 Possible applications:
o classroom area networks, seminars
o press conferences, media events
o sporting events, gaming, exhibitions
o conferences and trade shows
o disaster recovery sites, field work, etc.
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Objectives
 to assess feasibility of portable networks
 to benchmark the performance capabilities
and limitations of an Apache Web server in
a wireless ad hoc network
 to identify the performance bottlenecks
 to understand impacts of different factors
o number of clients
o Web object size
o persistent connections
o transmit power (energy consumption)
o wireless channel conditions
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Experimental Setup
• Compaq Notebooks (1.2GHz Pentium III, 128MB RAM,
512 KB L2 cache, Cisco Aironet 350 network cards)
• RedHat Linux 7.3, httperf, Apache 1.3.23, SnifferPro 4.6
• Network: 11 Mbps IEEE 802.11b wireless LAN, ad hoc mode
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Experimental Setup (Cont’d)
• IEEE 802.11b: a standard for wireless LANs
Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA), up to 11 Mbps data rate at physical layer
• ad hoc mode
frames are addressed directly from sender to receiver
• httperf
Web benchmarking software tool developed at HP Labs
• Web server: Apache (version 1.3.23)
Process-based, flexible, powerful, HTTP/1.1-compliant
• SnifferPro 4.6
real-time capture, recording all wireless channel activity,
enabling protocol analysis at MAC, IP, TCP and HTTP layers
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Experimental Design
• Impacts of different factors on wireless Web server
performance (one-factor-at-a-time)
Experimental Factors and Levels
Factor
Number of Clients
HTTP Transaction Rate (per-client)
Levels
1, 2, 4
10, 20, 30, …, 160
HTTP Transfer Size (KB)
Persistent Connections
HTTP Requests per Connection
Transmit Power (mW)
1, 2, 4, 8, …, 100
no, yes
1, 5, 10, 15, …, 60
1, 5, 20, 30, 50, 100
Client-Server Distance (m)
1, 10, 100
• Performance metrics
– HTTP transaction rate, throughput, response time, error rate
at Application Layer,
– TCP connection duration at Network Layer
– Transmit queue behaviour at Link Layer,
129
Experiment 1: Request Rate
Purpose: to determine the range of feasible and sustainable
loads for the wireless Web server
Design:
• Number of Clients: 1
• HTTP transaction rate: 10, 20, …, 160 req/sec
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: no
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Wireless Web Performance at Application Layer
Main observation:
• As the offered load increases:
linear increase  instability  lower plateau
• Peak throughput < 1 Mbps for 1 KB transfers
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Experiment 2: Transfer Size
Purpose: to study impact of HTTP response size
Design:
• Number of Clients: 1
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size (KB): 1, 2, 4, 8, …
• Persistent connections: no
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Measurement at Network Layer
Light load: 8 KB
Duration: 24 msec
Throughput: 2.8 Mbps
Medium load: 32 KB
Duration: 67 msec
Throughput: 3.9 Mbps
Overload: 64 KB
Duration: >100 msec
Throughput: 4.1 Mbps
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Experiment 3: Persistent Connections
Persistent Connections:
• Multiple HTTP transactions can be sent on the
same TCP connection.
• amortize overhead of TCP connection processing
• reduce memory consumption for TCP state
Purpose of this experiment: to study impact of
persistent connection on wireless Web performance
Design:
• Number of Clients: 1 and 2
• HTTP transaction rate: 10 req/sec (fixed)
• HTTP transfer size: 1 KB (fixed)
• Persistent connections: yes
• Transmit power: 100 mW
• Client-server distance: 1 meter (on same desk)
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Achieved Throughput for Experiment with Persistent Connections
Main observation:
• Peak throughput: 3.22 Mbps, 3.5x improvement
over non-persistent connections (0.9 Mbps),
• two clients share the server and network resources
equally
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Summary and Conclusions
 What we did: wireless Web server, portable nw
• Application-layer measurements (httperf)
• Network-layer measurements (Wireless Sniffer)
 Our results show:
• Server capability: 100 conn/sec for non-persistent
HTTP with throughputs up to 4 Mbps (adequate?)
• Bottleneck: at wireless network interface
• Some “network thrashing” for large HTTP transfers
when the network utilization is high (aborts, resets)
• Effect of wireless channel on performance at
TCP and HTTP-level (MAC-layer retransmits)
• Power consumption issue for mobile client and server
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8. Summary,
Questions, and Discussion
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Credits
r Thanks to the following people for the use
of their slides and/or knowledge:
m
m
m
m
m
m
David Schwab, U of Saskatchewan
Erich Nahum, IBM
Sniffer Technologies
Network Associates
IEEE
Guangwei Bai and Tianbo Kuang,U of C
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