1.Intro_Networks
Introduction to Computer Networks
Computer Networks and Applications
Week 1
COMP 3331/COMP 9331
Reading Guide: Chapter 1, Sections 1.1 – 1.4
1
Acknowledgment
v Majority of lecture slides are from the author’s
lecture slide set
§ Enhancements + additional material
2
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
3
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
4
Hobbe’s Internet Timeline – http://www.zakon.org/robert/internet/timeline/
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 (answer in comments on Zeeting)
Open a browser and type: www.zeetings.com/salil
Answers: B and C are valid answers
What’s the Internet: “nuts and bolts” view
vmillions of connected
computing devices:
§ hosts = end systems
§ running network apps
vcommunication links
§ fiber, copper, radio,
satellite
§ transmission rate:
bandwidth
vPacket switches: forward
packets (chunks of data)
§ routers and link layer
switches
wired
links
wireless
links
router
mobile network
global ISP
regional ISP
home
network
institutional
network
smartphone
PC
server
wireless
laptop
6
“Fun” Internet appliances
Picture frame
Web-enabled toaster +
weather forecaster
Smart Lightbulbs
Internet
refrigerator
Networked TV Set top Boxes
7
Tweet-a-watt:
monitor energy use
sensorized,
bed
mattress
car
pacemaker
8
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
9
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
10
11
Computer)Networks,)Fall)2015 8
while'(…)'{‘
”’message’=’…;’
”’send'(‘message,’…’);’
}
while'(…)'{‘
”’message’=’receive'(‘…’);’
}
Alice
Bob
12
Computer)Networks,)Fall)2015
while'(…)'{‘
”’message’=’receive'(‘…’);’
}
9
ApplicaGon-
Programming-
InterfaceAlice
while'(…)'{‘
”’message’=’…;’
”’send'(‘message,’…’);’
}
Bob
13
Computer)Networks,)Fall)2015 7
instant-messaging
instant-messaging
facebook-
server
firefox-accessing-
facebook
world-of-warcraE-
client
world-of-warcraE-
server
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
14
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
time
TCP connection
request
What’s a protocol?
15
16
Quiz: Internet of Things
How many Internet-connected devices do you have in your
home (include your computers, phones, tablets)?
A. Less than 10
B. Between 10 to 20
C. Between 20 to 50
D. Between 50 to 100
E. More than 100
Open a browser and type: www.zeetings.com/salil
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
17
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
18
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?
19
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
DSL
modem
splitter
DSL access
multiplexer
20
Access net: digital subscriber line (DSL)
ADSL over POTS
voice, data transmitted
at different frequencies over
dedicated line to central office
Ø Different data rates for upload and download (ADSL)
• < 2.5 Mbps upstream transmission rate (typically < 1 Mbps)
• < 24 Mbps downstream transmission rate (typically < 10
Mbps)
DSL
modem
splitter
21
Low
-pa
ss f
ilter
for
voic
e
High-pass filter for
data
Access net: digital subscriber line (DSL)
22
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
23
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
24
Fiber to the home/premise/curb
v Fully optical fiber path all the way to the home
(or premise or curb)
§ e.g., NBN, Google, Verizon FIOS
§ ~30 Mbps to 1Gbps
25
Access net: home network
to/from headend or
central office
cable or DSL or Fiber modem
router, firewall, NAT
wired Ethernet (1 Gbps)
wireless access
point (54 Mbps)
wireless
devices
often combined
in single box
26
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)
27
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/n (WiFi): 11, 54, 300
Mbps transmission rate
§ 802.11ac: 1 Gpbs(2.4GHz) +
4.34Gbps (5GHz)
§ 802.11ax: WiFi 6
wide-area wireless access
§ provided by telco (cellular)
operator, 10’s km
§ between 10 and 100 Mbps
§ 4G, 5G
to Internet to Internet
28
29
Sample results
FTTC + Cable + WiFi @ my home
Wired Network @ CSEUniwide
4G Network
Can you explain the differences?
30
Quiz: Your access network
Your residential ISP provides connectivity using the following
technology:
A. DSL
B. Cable
C. Fiber to the home/premise/curb
D. Mobile (3G/4G/5G)
E. Satellite
F. Something Else (type in Zeetings comment)
Open a browser and type: www.zeetings.com/salil
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
31
Self Study
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
32
twisted pair (TP)
v two insulated copper
wires
§ Category 5: 100 Mbps, 1
Gpbs Ethernet
§ Category 6: 10Gbps
Self Study
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, 450 Mbps, Gbps
v wide-area (e.g., cellular)
§ 4G cellular: ~ 10 Mbps
v satellite
§ Kbps to 45Mbps channel (or
multiple smaller channels)
§ 270 msec end-end delay
§ geosynchronous versus low
earth-orbiting (LEO)
33
Self Study
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
34
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
35
Circuit Switching
end-end resources allocated
to, reserved for “call”
between source & dest:
§ in diagram, each link has four
circuits.
• call gets 2nd circuit in top
link and 1st circuit in right
link.
§ dedicated resources: no sharing
• circuit-like (guaranteed)
performance
§ circuit segment idle if not used
by call (no sharing)
§ commonly used in traditional
telephone networks
36
Circuit switching: FDM versus TDM
FDM
frequency
timeTDM
frequency
time
4 users
Example:
37
Information
time
Timing in Circuit Switching
Circuit
Establish
ment
Transfer
Circuit
Tear-
down 38
Why circuit switching is not feasible?
Ø Inefficient
• Computer communications tends to be very bursty. For
example, viewing a sequence of web pages
• Dedicated circuit cannot be used or shared in periods of
silence
• Cannot adopt to network dynamics
39
Ø Fixed data rate
• Computers communicate at very diverse rates. For
example, viewing a video vs using telnet or web browsing
• Fixed data rate is not useful
Ø Connection state maintenance
• Requires per communication state to be maintained that
is a considerable overhead
• Not scalable
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
HeaderData header
payload
40
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)
41
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
42
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
43
time
Timing in Packet Switching
paylo
ad
h
d
r
What about the time to process the packet at the switch?
• We’ll assume it’s relatively negligible (mostly true)
44
time
Timing in Packet Switching
paylo
ad
h
d
r
Could the switch start transmitting
as soon as it has processed the
header?
45
time
Timing in Packet Switching
paylo
ad
h
d
r
Could the switch start transmit as
soon as it has processed the
header?
Yes! This would be called
a “cut through” switch
46
time
Timing in Packet Switching
paylo
ad
h
d
r
We will always assume a switch processes/forwards
a packet after it has received it entirely.
This is called “store and forward” switching
47
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
48
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”
49
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
50
Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time
Capacity
51
Data Rate 1
Data Rate 2
Data Rate 3
When Each Flow Gets 1/3rd of Capacity
Time
Time
Time
Overloaded
52
like circuit switching
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
53
packet switching
Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time
Capacity
54
Data Rate 1
Data Rate 2
Data Rate 3
Three Flows with Bursty Traffic
Time
Time
Time
Capacity
55
Data Rate 1+2+3 >> Capacity
Three Flows with Bursty Traffic
Time
Time
Capacity
What do we do under overload?
56
time à
B
W
à
pkt tx
time
57
Statistical multiplexing: pipe view
58
Statistical multiplexing: pipe view
No Overload
59
Statistical multiplexing: pipe view
Transient Overload
Not such a rare event
Queue overload
into Buffer
60
Statistical multiplexing: pipe view
Transient Overload
Not such a rare event
Queue overload
into Buffer
61
Statistical multiplexing: pipe view
Statistical multiplexing: pipe view
Transient Overload
Not such a rare event
Queue overload
into Buffer
62
Transient Overload
Not such a rare event
Queue overload
into Buffer
63
Statistical multiplexing: pipe view
Transient Overload
Not such a rare event
Queue overload
into Buffer
64
Statistical multiplexing: pipe view
Transient Overload
Not a rare event!Buffer absorbs transient bursts
Queue overload
into Buffer
65
Statistical multiplexing: pipe view
What about persistent overload?
Will eventually drop packets
Queue overload
into Buffer
66
Statistical multiplexing: pipe view
Packet switching versus circuit switching
example:
§ 1 Mb/s link
§ each user:
• 100 kb/s when “active”
• active 10% of time
vcircuit-switching:
§ 10 users
vpacket 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
say 70?
…
..
67
Hint: Bernoulli Trials and Binomial Distribution
Binomial Probability Distribution
v A fixed number of observations (trials), n
§ E.g., 5 tosses of a coin
v Binary random variable
§ E.g., head or tail in a coin toss
§ Often called as success or failure
§ Probability of success is p and failure is (1-p)
v Constant probability for each observation
68
Binomial Distribution: Example
v Q: What is the probability of observing exactly 3
heads in a sequence of 5 coin tosses
v A:
§ One way to get exactly 3 heads is: HHHTT
§ Probability of this sequence occurring = (1/2) x (1/2) x
(1/2) x (1-1/2) x (1-1/2) = (1/2)5
§ Another way to get exactly 3 heads is: THHHT
§ Probability of this sequence occurring = (1-1/2) x (1/2)
x (1/2) x (1/2) x (1-1/2) = (1/2)5
§ How many such unique combinations exist?
69
Binomial Distribution: Example
70P (3 heads and 2 tails) = 10 x (1/2)5 = 0.3125
71
Binomial Distribution
Packet switching versus circuit switching
v Let’s revisit the earlier problem
v N = 35 users
v Prob (# active users > 10)= 1– Prob (# active = 10)
– Prob (# active = 9)
– Prob (# active = 8)
…
– Prob (# active = 0)
where Prob (# active = 10) = C(35,10) x 0.110 x 0.925
v Prob (# active users > 10) = 0.0004 (approx)
72
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
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
73
74
Quiz: Switching
In ____________ resources are allocated on
demand
A. Packet switching
B. Circuit switching
C. Both
D. None
Open a browser and type: www.zeetings.com/salil
Answer: A
75
Quiz: Switching
A message from device A to B consists of packet X
and packet Y. In a circuit switched network, packet
Y’s path ___________________ packet X’s path
A. is the same
B. is independent
C. is always different from
Open a browser and type: www.zeetings.com/salil
Answer: A
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
76
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
netaccess
net
access
net
…
…
……
…
…
77
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
netaccess
net
access
net
…
…
……
…
…
…
…
…
……
connecting each access ISP
to each other directly doesn’t
scale: O(N2) connections.
78
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
netaccess
net
access
net
…
…
……
…
…
Option: connect each access ISP to a global transit ISP? Customer
and provider ISPs have economic agreement.
global
ISP
79
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
netaccess
net
access
net
…
…
……
…
…
But if one global ISP is viable business, there will be competitors
….
ISP B
ISP A
ISP C
80
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
netaccess
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
81
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
netaccess
net
access
net
…
…
……
…
…
… and regional networks may arise to connect access nets to
ISPS
ISP B
ISP A
ISP C
IXP
IXP
regional net
82
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
netaccess
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
83
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, Orange,
Deutsche Telekom), national & international coverage
§ content provider network (e.g., Google): private network that connects
it data centers to Internet, often bypassing tier-1, regional ISPs 84
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
1-85
AARNET: Australia’s Academic and
Research Network
v https://www.aarnet.edu.au/
v https://www.submarinecablemap.com
https://www.aarnet.edu.au/
https://www.submarinecablemap.com/
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
86
How do loss and delay occur?
Packets queue in router buffers
Ø Packet arrival rate to link (temporarily) exceeds output link
capacity
Ø Packets queue, wait for turn
A
B
packet being transmitted (delay)
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
87
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
88
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/sdtrans and dprop
very different
Four sources of packet delay
propagation
nodal
processing queueing
dnodal = dproc + dqueue + dtrans + dprop
89
A
B
transmission
Caravan analogy
Ø Car ~bit; Caravan ~ packet
Ø Cars “propagate” at
100 km/hr
Ø Toll booth takes 12 sec to
service car (bit transmission
time)
Ø 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
90
Caravan analogy (more)
Ø Suppose cars now “propagate” at 1000 km/hr
Ø And suppose toll booth now takes one min to service a car
Ø 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
91
Interactive Java Applet – Propagation vs transmission delay
https://www2.tkn.tu-berlin.de/teaching/rn/animations/propagation/
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
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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
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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 at 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
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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://computerscience.unicam.it/marcantoni/reti/applet/QueuingAndLossInteractive/1.html
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End to End Delay
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d1,r1 d2,r2 d3,r3
R1 R2Client Server
Client
Server
Time
R1
R2
Packet length = L
Propagation speed = s
d1/s
d2/s
d3/s
L/r1
L/r2
L/r3
Queueing Delay
d3/s
L/r3
In the picture, r1 = r2 = r3, you may wish to consider what happens when this is not the case
“Real” Internet delays and routes
v what do “real” Internet delay & loss look like?
v Traceroute: 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
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“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
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* Do some traceroutes from countries at www.traceroute.org
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“Real” delay variations
dnodal = dproc + dqueue + dtrans + dprop
End-to-end delay = sum of all dnodal along the path
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Quiz: Propagation Delay
Propagation delay depends on the size of the packet
A. True
B. False
Open a browser and type: www.zeetings.com/salil
Answer: B
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Quiz: Oh these delays
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
Open a browser and type: www.zeetings.com/salil
Answer: C
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
A
B
packet being transmitted
packet arriving to
full buffer is lost
buffer
(waiting area)
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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)
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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
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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
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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
§ Application Layer
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