week 1 - webcms3 · week 1 comp 3331/comp 9331 reading guide: chapter 1, sections 1.1 - 1.4 ....
TRANSCRIPT
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Introduction
Introduction to Computer Networks (chapter 1 of text)
Computer Networks and Applications
Week 1
COMP 3331/COMP 9331
Reading Guide: Chapter 1, Sections 1.1 - 1.4
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Introduction
Acknowledgment
v Majority of lecture slides are from the author’s lecture slide set § Several Enhancements + additional material
1-2
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Introduction
1. Introduction Goals: v get “feel” and terminology v defer depth and detail to later in course v understand concepts using the Internet as example
1-3
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Introduction
1. Introduction: roadmap 1.1 what is the Internet? 1.2 network edge
§ end systems, access networks, links
1.3 network core § packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks 1.5 protocol layers 1.6 networks under attack: security 1.7 history
1-4
Hobbe’s Internet Timeline - http://www.zakon.org/robert/internet/timeline/
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Introduction 1-5
Quiz: What is the Internet?
A. One single homogenous network B. An interconnection of different computer networks C. An infrastructure that provides services to networked applications D. Something else (be prepared to discuss)
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Introduction 1-6
Computer)Networks,)Fall)2015 2
end$system
Windows-PCLinux-server MAC-laptop
car-navigator
heart-pacemaker
smartphone
iPad
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Introduction 1-7
Computer)Networks,)Fall)2015 3
end$system switch (routers)
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Introduction 1-8 Computer)Networks,)Fall)2015
phone-lines
4
end$system switch
link
fibers
cable-TV-lines
wireless-links
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Introduction 1-9
Computer)Networks,)Fall)2015 5
end$system switch
link
Internet-Service-Provider
phone-company
cable-companyuniversity-net
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Introduction 1-10
Computer)Networks,)Fall)2015 6
end$system switch
link
packet
path
Internet-Service-Provider
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Introduction 1-11
Computer)Networks,)Fall)2015 7
instant-messaging
instant-messaging
facebook-server
firefox-accessing-facebook
world-of-warcraE-client
world-of-warcraE-server
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Introduction 1-12
Computer)Networks,)Fall)2015 8
while'(...)'{''''message'='...;''''send'('message,'...');'}
while'(...)'{''''message'='receive'('...');'}
Alice
Bob
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Introduction 1-13
Computer)Networks,)Fall)2015
while'(...)'{''''message'='receive'('...');'}
9
ApplicaGon-Programming-InterfaceAlice
while'(...)'{''''message'='...;''''send'('message,'...');'}
Bob
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Introduction 1-14
Computer)Networks,)Fall)2015 10
Alice Bob
hello
hello
give-me-hHp://nal.epfl.ch
here:-...
Protocols – Language of Communication
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Introduction
What’s the Internet: “nuts and bolts” view
v millions of connected computing devices: § hosts = end systems § running network apps
v communication links § fiber, copper, radio,
satellite § transmission rate:
bandwidth
v Packet switches: forward packets (chunks of data) § routers and switches
wired links
wireless links
router
mobile network
global ISP
regional ISP
home network
institutional network
smartphone
PC
server
wireless laptop
1-15
Self-study
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Introduction
“Fun” internet appliances
IP picture frame http://www.ceiva.com/
Web-enabled toaster + weather forecaster
Smart Lightbulbs Internet refrigerator
Networked TV Set top Boxes
1-16
Tweet-a-watt: monitor energy use
Self-study
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Introduction 1-17
http://thenextweb.com/insider/2012/12/09/the-future-of-the-internet-of-things/ http://gigaom.com/2013/03/05/the-future-of-the-internet-is-avatars-and-connected-services-video/ http://share.cisco.com/assets/images/Internet_of_Things_Infographic.jpg
Source: cisco Source: www.beyondplm.com
Self-study
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Introduction
v Internet: “network of networks” § Interconnected ISPs
v protocols control sending, receiving of msgs § e.g., TCP, IP, HTTP, Skype, 802.11
v Internet standards § RFC: Request for comments § IETF: Internet Engineering Task
Force
What’s the Internet: “nuts and bolts” view mobile network
global ISP
regional ISP
home network
institutional network
1-18
Self-study
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What’s the Internet: a service view
v Infrastructure that provides services to applications: § Web, VoIP, email, games, e-
commerce, social nets, … v provides programming
interface to apps § hooks that allow sending
and receiving app programs to “connect” to Internet
§ provides service options, analogous to postal service
mobile network
global ISP
regional ISP
home network
institutional network
Introduction 1-19
Self-study
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Introduction
What’s a protocol?
human protocols: v “what’s the time?” v “I have a question” v introductions
… specific msgs sent … specific actions taken
when msgs received, or other events
network protocols: v machines rather than
humans v all communication activity
in Internet governed by protocols
protocols define format, order of msgs sent and received among network entities,
and actions taken on msg transmission, receipt
1-20
Self-study
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Introduction
a human protocol and a computer network protocol:
Q: other human protocols?
Hi
Hi
Got the time? 2:00
TCP connection response
Get http://www.awl.com/kurose-ross
<file> time
TCP connection request
What’s a protocol?
1-21
Self-study
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Introduction
Silly example: Clay Tablet Carrier Pigeon Protocol (CTCP) v Inscribe message on clay tablet v Let dry in the sun v Tie to leg of carrier pigeon v Let pigeon go v Viola – your message is sent !! v Recipient catches pigeon and reads tablet Questions for later:
- Is it connectionless? - Is it reliable?
IP over avian carriers – RFC1149 http://en.wikipedia.org/wiki/IP_over_Avian_Carriers
What’s a protocol?
1-22
Self-study
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Introduction
1. Introduction: roadmap 1.1 what is the Internet? 1.2 network edge
§ end systems, access networks, links
1.3 network core § packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history
1-23
Self-study
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Introduction
A closer look at network structure:
v network edge: § hosts: clients and servers § servers often in data
centers
v access networks, physical media: wired, wireless communication links
v network core: § interconnected routers § network of networks
mobile network
global ISP
regional ISP
home network
institutional network
1-24
Self-study
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Introduction
Access networks and physical media
Q: How to connect end systems to edge router?
v residential access nets v institutional access
networks (school, company)
v mobile access networks
keep in mind: v bandwidth (bits per second)
of access network? v shared or dedicated?
1-25
Self-study
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Introduction
Access net: digital subscriber line (DSL)
central office
ISP
telephone network
DSLAM
voice, data transmitted at different frequencies over
dedicated line to central office
v use existing telephone line to central office DSLAM
§ data over DSL phone line goes to Internet § voice over DSL phone line goes to telephone net
v < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) v < 24 Mbps downstream transmission rate (typically < 10 Mbps)
DSL modem
splitter
DSL access multiplexer
1-26
Self-study
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Introduction
Access net: cable network
cable modem
splitter
… cable headend
Channels
V I D E O
V I D E O
V I D E O
V I D E O
V I D E O
V I D E O
D A T A
D A T A
C O N T R O L
1 2 3 4 5 6 7 8 9
frequency division multiplexing: different channels transmitted in different frequency bands
1-27
Self-study
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Introduction
data, TV transmitted at different frequencies over shared cable
distribution network
cable modem
splitter
… cable headend
CMTS
ISP
cable modem termination system
v HFC: hybrid fiber coax § asymmetric: up to 30Mbps downstream transmission rate, 2
Mbps upstream transmission rate v network of cable, fiber attaches homes to ISP router
§ homes share access network to cable headend § unlike DSL, which has dedicated access to central office
Access net: cable network
1-28
Self-study
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Fiber to the home (FTTH)
v Fully optical fiber path all the way to the home § e.g., Verizon FIOS, Google, NBN § ~30 Mbps to 1Gbps
v Active (like switched Ethernet) or passive optical networking as shown below
Introduction 1-29
Self-study
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Introduction
Access net: home network
to/from headend or central office
cable or DSL modem
router, firewall, NAT
wired Ethernet (100 Mbps)
wireless access point (54 Mbps)
wireless devices
often combined in single box
1-30
Self-study
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Introduction
Enterprise access networks (Ethernet)
v typically used in companies, universities, etc v 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates v today, end systems typically connect into Ethernet switch
Ethernet switch
institutional mail, web servers
institutional router
institutional link to ISP (Internet)
1-31
Self-study
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Introduction
Wireless access networks v shared wireless access network connects end system to router
§ via base station aka “access point”
wireless LANs: § within building (100 ft) § 802.11b/g (WiFi): 11, 54 Mbps
transmission rate
wide-area wireless access § provided by telco (cellular)
operator, 10’s km § between 1 and 10 Mbps § 3G, 4G: LTE
to Internet
to Internet
1-32
Self-study
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Host: sends packets of data
host sending function: v takes application message v breaks into smaller
chunks, known as packets, of length L bits
v transmits packet into access network at transmission rate R § link transmission rate,
aka link capacity, aka link bandwidth
R: link transmission rate host
1 2
two packets, L bits each
packet transmission
delay
time needed to transmit L-bit
packet into link
L (bits) R (bits/sec) = =
1-33
Self-study
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Introduction
Physical media
v bit: propagates between transmitter/receiver pairs
v physical link: what lies between transmitter & receiver
v guided media: § signals propagate in solid
media: copper, fiber, coax v unguided media:
§ signals propagate freely, e.g., radio
1-34
Self-study
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Introduction
Physical media: twisted pair, coax, fiber
coaxial cable: v two concentric copper
conductors v broadband:
§ multiple channels on cable § HFC
fiber optic cable: v glass fiber carrying light
pulses, each pulse a bit v high-speed operation:
§ high-speed point-to-point transmission (e.g., 10’s-100’s Gpbs transmission rate)
v low error rate: § repeaters spaced far apart § immune to electromagnetic
noise
1-35
twisted pair (TP) v two insulated copper
wires § Category 5: 100 Mbps, 1
Gpbs Ethernet § Category 6: 10Gbps
Self-study
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Introduction
Physical media: radio
v signal carried in electromagnetic spectrum, i.e., no physical “wire”
v propagation environment
effects: § reflection § obstruction by objects § interference
radio link types: v terrestrial microwave
§ e.g. up to 45 Mbps channels
v LAN (e.g., WiFi) § 11Mbps, 54 Mbps
v wide-area (e.g., cellular) § 3G cellular: ~ few Mbps
v satellite § Kbps to 45Mbps channel (or
multiple smaller channels) § 270 msec end-end delay § geosynchronous versus low
earth-orbitting (LEO)
1-36
Self-study
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Introduction
1. Introduction: roadmap 1.1 what is the Internet? 1.2 network edge
§ end systems, access networks, links
1.3 network core § packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history
1-37
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Introduction
v mesh of interconnected routers/switches
v Two forms of switched networks: § Circuit switching: used in
the legacy telephone networks
§ Packet switching: used in the Internet
The network core
1-38
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Circuit switching
(1) Node A sends a reservation request (2) Interior switches establish a connection -- i.e., “circuit” (3) A starts sending data (4) A sends a “teardown circuit” message
Idea: source reserves network capacity along a path
A B 10Mb/s?
10Mb/s?
10Mb/s?
1-39 Introduction
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Old Time Multiplexing
1-40 Introduction
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Introduction
Circuit switching: FDM versus TDM
FDM
frequency
time TDM
frequency
time
4 users Example:
1-41
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Information time
Timing in Circuit Switching
Circuit Establish
ment
Transfer
Circuit Tear-down 1-42 Introduction
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Numerical example
v How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? § All links are 1.536 Mbps § Each link uses TDM with 24 slots/sec § 500 msec to establish end-to-end circuit
Let’s work it out!
1/24*1.536Mb/s=64kb/s640,000/64kb/s=10s10s+500ms=10.5s
1-43 Introduction
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Consider a circuit-switched network with N=100 users where each user is active with probability p=0.2 and when active, sends data at a rate R=1Mbps. How much capacity must the network be provisioned with?
A. 100 Mbps B. 20 Mbps C. 200 Mbps D. 50 Mbps E. 500 Mbps
Introduction 1-44
Quiz: Circuit Switching Capacity
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v Pros:
v Cons:
1-45 Introduction
Quiz: What are the pros and cons of circuit switching?
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”*
01000111100010101001110100011001
1. Internet Address 2. Age (TTL) 3. Checksum to protect header
Header Data header
payload 1-46 Introduction
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload”*
§ payload is the data being carried § header holds instructions to the network for how to
handle packet (think of the header as an API)
1-47 Introduction
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers
1-48 Introduction
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Switches forward packets
EDINBURGH
OXFORD
GLASGOW
UCL
Destination Next Hop
GLASGOW 4
OXFORD 5
EDIN 2
UCL 3
Forwarding Table 111010010 EDI
N
switch#2
switch#5
switch#3
switch#4
1-49 Introduction
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time
Timing in Packet Switching
payload hdr
What about the time to process the packet at the switch? • We’ll assume it’s relatively negligible (mostly true)
1-50 Introduction
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time
Timing in Packet Switching payloa
d hdr
Could the switch start transmitting as soon as it has processed the
header?
Introduction 1-51
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time
Timing in Packet Switching payloa
d hdr
Could the switch start transmitting as soon as it has processed the
header?
Yes! This would be called a “cut through” switch
Introduction 1-52
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time
Timing in Packet Switching
payload hdr
We will always assume a switch processes/forwards a packet after it has received it entirely.
This is called “store and forward” switching 1-53 Introduction
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers
1-54 Introduction
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers v Each packet travels independently
§ no notion of packets belonging to a “circuit”
1-55 Introduction
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Packet Switching
v Data is sent as chunks of formatted bits (Packets) v Packets consist of a “header” and “payload” v Switches “forward” packets based on their headers v Each packet travels independently v No link resources are reserved in advance. Instead
packet switching leverages statistical multiplexing
1-56 Introduction
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Multiplexing
Sharing makes things efficient (cost less) v One airplane/train for 100 people v One telephone for many calls v One lecture theatre for many classes v One computer for many tasks v One network for many computers v One datacenter many applications
1-57 Introduction
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Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time Capacity
1-58 Introduction
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Data Rate 1
Data Rate 2
Data Rate 3
When Each Flow Gets 1/3rd of Capacity
Time
Time
Time
Frequent Overloading
1-59 Introduction
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When Flows Share Total Capacity
Time
Time
Time
No Overloading
Statistical multiplexing relies on the assumption that not all flows burst at the same time.
Very similar to insurance, and has same failure case
1-60 Introduction
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Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time Capacity
1-61 Introduction
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Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time Capacity
1-62 Introduction
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Data Rate 1+2+3 >> Capacity
Three Flows with Bursty Traffic
Time
Time Capacity
What do we do under overload? 1-63 Introduction
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time à
BW à
pkt tx time
1-64 Introduction
Statistical multiplexing: pipe view
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1-65 Introduction
Statistical multiplexing: pipe view
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No Overload
1-66 Introduction
Statistical multiplexing: pipe view
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Transient Overload Not such a rare event
Queue overload into Buffer
1-67 Introduction
Statistical multiplexing: pipe view
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Transient Overload Not such a rare event
Queue overload into Buffer
1-68 Introduction
Statistical multiplexing: pipe view
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Statistical multiplexing: pipe view
Transient Overload Not such a rare event
Queue overload into Buffer
1-69 Introduction
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Transient Overload Not such a rare event
Queue overload into Buffer
1-70 Introduction
Statistical multiplexing: pipe view
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Transient Overload Not such a rare event
Queue overload into Buffer
1-71 Introduction
Statistical multiplexing: pipe view
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Transient Overload Not a rare event! Buffer absorbs transient bursts
Queue overload into Buffer
1-72 Introduction
Statistical multiplexing: pipe view
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What about persistent overload? Will eventually drop packets
Queue overload into Buffer
1-73 Introduction
Statistical multiplexing: pipe view
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Introduction
Packet Switching: queueing delay, loss
A
B
C R = 100 Mb/s
R = 1.5 Mb/s D
E queue of packets waiting for output link
1-74
queuing and loss: v If arrival rate (in bits) to link exceeds transmission rate of
link for a period of time: § packets will queue, wait to be transmitted on link § packets can be dropped (lost) if memory (buffer) fills up
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v Pros:
v Cons:
1-75 Introduction
Quiz: What are the pros and cons of packet switching?
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76
Packet switching versus circuit switching Packet-switching Circuit-switching
v Resources sharing dedicated
v Congestion may lead to it admission control
v Overhead less overhead; more overhead; no connection setup reserve resources 1st
v Guarantee Best-effort provide guarantee
no guarantee good for multimedia
Introduction
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Introduction
v great for bursty data § resource sharing § simpler, no call setup
v excessive congestion possible: packet delay and loss § protocols needed for reliable data transfer, congestion
control v Q: How to provide circuit-like behavior?
§ bandwidth guarantees needed for audio/video apps § still an unsolved problem (chapter 7)
is packet switching a “slam dunk winner?”
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Packet switching versus circuit switching
1-77
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Introduction
Packet switching versus circuit switching
example: § 1 Mb/s link § each user:
• 100 kb/s when “active” • active 10% of time
v circuit-switching: § 10 users
v packet switching: § with 35 users, probability >
10 active at same time is less than .0004 *
packet switching allows more users to use network!
N users
1 Mbps link
Q: how did we get value 0.0004?
Q: what happens if > 35 users ?
…..
1-78
Further Resources: Problem 8 & 9, Chapter 1, Kurose and Ross, 6th Edition http://gaia.cs.umass.edu/kurose_ross/interactive/ps_versus_cs.php
Self-study
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Introduction 79
v Probability Basics: Bernoulli Trials § Let p be the probability of some event A
• Examples of A – When you toss a coin, A is the event that the outcome is tails – When you throw a dice, A is the event that the dice shows 5
§ Assume that you conduct n independent experiments • Examples of these are
– When you toss a coin n times in succession – When you throw a dice n times in succession
§ Probability that the event A will occur x times • E.g: Probability that x out of n tosses result in a tail/head
• This is known as the Binomial Distribution
€
nx"
# $ %
& ' px 1− p( )n−x
How? Self-study
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Introduction 80
Revisiting the Problem v P (more than 10 out of n users transmits) = 0.0004 v In our case, A is the event that a user is transmitting v Hence, p=0.1 and x=10 v Now, P (x> 10) = 0.0004 v Hence, P (x<= 10) = 0.9996 v P(x=0) + P( x=1) + P(x=2) …. = 0.9996
v Solving this, we get n = 35 €
n0"
# $ %
& ' 0.10 1− 0.1( )n−0 +
n1"
# $ %
& ' 0.11 1− 0.1( )n−1 + ...= 0.9996
Self-study
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Internet structure: network of networks
v End systems connect to Internet via access ISPs (Internet Service Providers) § Residential, company and university ISPs
v Access ISPs in turn must be interconnected. v So that any two hosts can send packets to each other
v Resulting network of networks is very complex v Evolution was driven by economics and national policies
v Let’s take a stepwise approach to describe current Internet structure
1-81 Introduction
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Internet structure: network of networks Question: given millions of access ISPs, how to connect them together?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
1-82 Introduction
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Internet structure: network of networks Option: connect each access ISP to every other access ISP?
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
…
…
connecting each access ISP to each other directly doesn’t
scale: O(N2) connections.
1-83 Introduction
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
Option: connect each access ISP to a global transit ISP? Customer and provider ISPs have economic agreement.
global ISP
1-84 Introduction
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
But if one global ISP is viable business, there will be competitors ….
ISP B
ISP A
ISP C
1-85 Introduction
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
But if one global ISP is viable business, there will be competitors …. which must be interconnected
ISP B
ISP A
ISP C
IXP
IXP
peering link
Internet exchange point
1-86 Introduction
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
… and regional networks may arise to connect access nets to ISPS
ISP B
ISP A
ISP C
IXP
IXP
regional net
1-87 Introduction
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Internet structure: network of networks
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net
access net access
net
access net
…
… … …
…
… and content provider networks (e.g., Google, Microsoft, Akamai ) may run their own network, to bring services, content close to end users
ISP B
ISP A
ISP B
IXP
IXP
regional net
Content provider network
1-88 Introduction
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Introduction
Internet structure: network of networks
v at center: small # of well-connected large networks § “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national &
international coverage § content provider network (e.g, Google): private network that connects
it data centers to Internet, often bypassing tier-1, regional ISPs
1-89
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
access ISP
Regional ISP Regional ISP
IXP IXP
Tier 1 ISP Tier 1 ISP Google
IXP
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Introduction
Tier-1 ISP: e.g., Sprint
…
to/from customers
peering
to/from backbone
…
………
POP: point-of-presence
1-90
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Introduction
1. Introduction: roadmap 1.1 what is the Internet? 1.2 network edge
§ end systems, access networks, links
1.3 network core § packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history
1-91
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Nodes and Links and Delays
A B
Introduction 1-92
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Properties of Links
v Bandwidth (capacity): “width” of the link § number of bits sent (or received) per unit time (bits/sec or bps)
v Propagation delay: “length” of the link § propagation time for data to travel along the link (seconds)
v Bandwidth-Delay Product (BDP): “volume” of the link § amount of data that can be “in flight” at any time § propagation delay × bits/time = total bits in link
bandwidth
Prop. Delay
delay x bandwidth
Introduction 1-93
Implications will become apparent when we study reliable data delivery and TCP
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Examples of Bandwidth-Delay v Same city over a slow link:
§ BW~100Mbps § Propagation delay~0.1msec § BDP ~ 10,000bits ~ 1.25KBytes
v Cross-country over fast link:
§ BW~10Gbps § Propagation delay~10msec § BDP ~ 108bits ~ 12.5GBytes
v LFN: long fat network (typically BDP > 105 bits)
NOTE: For ease of math, you should assume 1MB = 1000KB, 1KB = 1000B… Introduction 1-94
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time=0
Packet Delay Sending a 100B packet from A to B?
A B
Time
1Mbps, 1ms
Time to transmit one bit = 1/106s
Time when that bit reaches B
= 1/106+1/103s
Introduction 1-95
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time=0
Packet Delay Sending a 100B packet from A to B?
A B
100Byte packet
Time
1Mbps, 1ms
Time to transmit 800 bits=800x1/106s
The last bit reaches B at
(800x1/106)+1/103s = 1.8ms
Introduction 1-96
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time=0
Packet Delay Sending a 100B packet from A to B?
A B
100Byte packet
Time
1Mbps, 1ms
Time to transmit 800 bits=800x1/106s
Time when that bit reaches B
= 1/106+1/103s
The last bit reaches B at
(800x1/106)+1/103s = 1.8ms
Packet Delay = Transmission Delay + Propagation Delay Packet Delay = (Packet Size ÷ Link Bandwidth) + Propagation Delay
Introduction 1-97
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Packet Delay Sending a 100B packet from A to B?
A B
Time
1Gbps, 1ms?
The last bit reaches B at
(800x1/109)+1/103s = 1.0008ms
Introduction 1-98
100Byte packet
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Packet Delay Sending a 100B packet from A to B?
A B
100Byte packet
Time
1Gbps, 1ms?
1GB file in 100B packets
The last bit reaches B at
(800x1/109)+1/103s = 1.0008ms
The last bit in the file reaches B at
(107x800x1/109)+1/103s = 8001ms
107 x 100B packets
Introduction 1-99
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Introduction
Four sources of packet delay
dproc: nodal processing § check bit errors § determine output link § typically < msec
A
B
propagation
transmission
nodal processing queueing
dqueue: queueing delay § time waiting at output link
for transmission § depends on congestion
level of router
dnodal = dproc + dqueue + dtrans + dprop
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Introduction
dtrans: transmission delay: § L: packet length (bits) § R: link bandwidth (bps) § dtrans = L/R
dprop: propagation delay: § d: length of physical link § s: propagation speed in medium
(~2x108 m/sec) § dprop = d/s dtrans and dprop
very different
Four sources of packet delay
propagation
nodal processing queueing
dnodal = dproc + dqueue + dtrans + dprop
1-101
A
B
transmission
Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/transmission/delay.html
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Introduction
Caravan analogy
v cars “propagate” at 100 km/hr
v toll booth takes 12 sec to service car (bit transmission time)
v car~bit; caravan ~ packet v Q: How long until caravan is
lined up before 2nd toll booth?
§ time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec
§ time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr
§ A: 62 minutes
toll booth
toll booth
ten-car caravan
100 km 100 km
1-102
Self-study
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Introduction
Caravan analogy (more)
v suppose cars now “propagate” at 1000 km/hr v and suppose toll booth now takes one min to service a car v Q: Will cars arrive to 2nd booth before all cars serviced at first
booth?
§ A: Yes! after 7 min, 1st car arrives at second booth; three cars still at 1st booth.
toll booth
toll booth
ten-car caravan
100 km 100 km
1-103
Self-study
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Host A Host B transmission rate R = 1 Mbps
distance = 1 km, speed = 2x108m/s
Packet length = L bits
Question: Which bit is being transmitted at the time the first bit arrives at Host B?
Answer: First bit arrives after 1/R + d/s = 1/106 + 103/(2x108) = 10-6 + 5x10-6 = 6x10-6 = 6 µsec After 6 µsec 6 bits are already transmitted; so 7th bit is being transmitted
1-104 Introduction
Quiz: Delays
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Transmission vs. propagation
Host A Host B transmission rate R = ??
distance = 2 km, speed = 2x108m/s
L=100Bytes
Question: At what rate (bandwidth) R would the propagation delay equal the transmission delay of packet?
Answer: ❒ Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec ❒ Transmission delay = 100x8 (bits)/R ❒ Prop. delay = trans. delay => R=105x100x8 = 80 Mbps
1-105 Introduction
Quiz: Delays
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Queueing delay (more insight)
v Every second: aL bits arrive to queue v Every second: R bits leave the router v Question: what happens if aL > R ? v Answer: queue will fill up, and packets will get dropped!!
aL/R is called traffic intensity
queue Packet arrival rate = a packets/sec
Link bandwidth = R bits/sec
Packet length = L bits
Introduction 1-106
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Queueing delay: illustration
Arrival rate: a = 1/(L/R) = R/L (packet/second) Traffic intensity = aL/R = (R/L) (L/R) = 1 Average queueing delay = 0 (queue is initially empty)
queue Link bandwidth = R bits/sec 1 packet arrives
every L/R seconds
Packet length L bits
Introduction 1-107
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Queueing delay: illustration
Arrival rate: a = N/(LN/R) = R/L packet/second Traffic intensity = aL/R = (R/L) (L/R) = 1 Average queueing delay (queue is empty is time 0) ? {0 + L/R + 2L/R + … + (N-1)L/R}/N = L/(RN){1+2+…+(N-1)} =L(N-1)/(2R) Note: traffic intensity is same as previous scenario, but queueing delay is different
queue Link bandwidth = R bits/sec N packet arrive simultaneously
every LN/R seconds
Packet length L bits
Introduction 1-108
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Queueing delay: behaviour
q La/R ~ 0: avg. queueing delay small q La/R -> 1: delays become large q La/R > 1: more “work” than can be
serviced, average delay infinite! (this is when a is random!)
queue Packet arrival rate = a packets/sec
Link bandwidth = R bits/sec
Packet length = L bits
Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/queuing/queuing.html
Introduction 1-109
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Introduction
“Real” Internet delays and routes v what do “real” Internet delay & loss look like? v traceroute program: provides delay
measurement from source to router along end-end Internet path towards destination. For all i: § sends three packets that will reach router i on path
towards destination § router i will return packets to sender § sender times interval between transmission and reply.
3 probes
3 probes
3 probes
1-110
Your first lab
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Introduction
“Real” Internet delays, routes
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
traceroute: gaia.cs.umass.edu to www.eurecom.fr 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu
* means no response (probe lost, router not replying)
trans-oceanic link
1-111 * Do some traceroutes from exotic countries at www.traceroute.org
Your first lab
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v Propagation delay depends on the size of the packet. True or false?
A. True
B. False
Introduction 1-112
Quiz: Delays
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v Which of the following delays is significantly affected by the load in the network?
A. Processing delay
B. Queuing delay
C. Transmission delay
D. Propagation delay
Introduction 1-113
Quiz: Delays
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v Consider a packet that has just arrived at a router. What is the correct order of the delays encountered by the packet until it reaches the next-hop router?
A. Transmission, processing, propagation, queuing
B. Propagation, processing, transmission, queuing
C. Processing, queuing, transmission, propagation
D. Queuing, processing, propagation, transmission
Introduction 1-114
Quiz: Delays
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http://gaia.cs.umass.edu/kurose_ross/interactive/one-hop-delay.php
http://gaia.cs.umass.edu/kurose_ross/interactive/end-end-delay.php
Introduction 1-115
Quiz: Solve on your own
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Packet switching: store and forward
q delay (one packet only) = L/R + d/s
R L d
q distance = d meters; speed of propagation = s m/sec q transmission rate of link = R bits/s
q delay (one packet only) = L/R + ½d/s + L/R + ½d/s = 2L/R + d/s
Example: q d/s = 0.5 sec q L = 10 Mbits q R = 1 Mbps q delay = 10.5 sec
Example: q d/s = 0.5 sec q L = 10 Mbits q R = 1 Mbps q delay = 20.5 sec
R R L
d/2 d/2
1-116 Introduction
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Store-and-forward & queuing delay
q Case 1: Assume R1 < R2
q distance = d meters; speed of propagation = s m/sec q transmission rate of link = R1 and R2 bits/s q Consider sending two packets A and B back to back
R1 R2 L
d
q Case 2: Assume R1 > R2
Q: is there a queuing delay? how much is this delay? Answer (queue is empty initially): Time for last bit of 2nd pkt to arrive at router: d1= L/R1 + L/R1 + d/(2s) Time for last bit of 1st pkt to leave router: d2= L/R1 + d/(2s) + L/R2 Queueing delay = d2 – d1 = L/R2 – L/R1 if positive, otherwise 0. Hence: when R1 < R2, queueing delay = d2 – d1 = 0 when R1 > R2, queueing delay = d2 – d1 = L/R2 – L/R1
1-117 Introduction
d/2 d/2
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Introduction
Packet loss v queue (aka buffer) preceding link in buffer has finite
capacity v packet arriving to full queue dropped (aka lost) v lost packet may be retransmitted by previous node,
by source end system, or not at all
A
B
packet being transmitted
packet arriving to full buffer is lost
buffer (waiting area)
1-118
Interactive Java Applet: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/queuing/queuing.html
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Introduction
Throughput
v throughput: rate (bits/time unit) at which bits transferred between sender/receiver § instantaneous: rate at given point in time § average: rate over longer period of time
server, with file of F bits
to send to client
link capacity Rs bits/sec
link capacity Rc bits/sec
server sends bits (fluid) into pipe
pipe that can carry fluid at rate Rs bits/sec)
pipe that can carry fluid at rate Rc bits/sec)
1-119
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Introduction
Throughput (more)
v Rs < Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
v Rs > Rc What is average end-end throughput?
link on end-end path that constrains end-end throughput bottleneck link
Rs bits/sec Rc bits/sec
1-120
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Introduction
Throughput: Internet scenario
10 connections (fairly) share backbone bottleneck link R bits/sec
Rs
Rs
Rs
Rc
Rc
Rc
R
v per-connection end-end throughput: min(Rc,Rs,R/10)
v in practice: Rc or Rs is often bottleneck
1-121
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Introduction
Introduction: summary
covered a “ton” of material! v Internet overview v what’s a protocol? v network edge, core, access
network § packet-switching versus
circuit-switching § Internet structure
v performance: loss, delay, throughput
v Next Week § Protocol layers, service models § Network security overview
you now have: v context, overview, “feel”
of networking v more depth, detail to
follow!
1-122
Solve all remaining problems Reading Exercise Chapter 1: 1.5 – 1.8 Chapter 2: 2.1 – 2.4