Professor Yashar Ganjali
Department of Computer Science
University of Toronto
yganjali@cs.toronto.edu
http://www.cs.toronto.edu/~yganjali
Announcements
 Piazza: if you are not registered yet, please register.
 Check out class web page for slides, and lecture
notes.
 Volunteer for lecture notes?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
2
The Story So Far …
• Computer networks and society
• Overview, science of networks
• Life areas that computer networks have changed
• Healthcare, business, entertainment, cloud computing,
cyber security, privacy
• This Week: Introduction to Computer Networks
• Basics concepts and components
• An introduction to the mail system
• An introduction to the Internet
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
3
Connecting to the Network
 What do we need to connect to the Internet?
 A computer
 A link


Wired
Wireless
 A device which is connected to the network
Router/
Switch
Reproduced with permission of Bill Cheswick, Lumeta Inc
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
4
What is a Link?
 Different media (light, electric signal, …)
 Connects two nodes
 Normally!
 Used to transfer a sequence of 0s and 1s
 Rate: number of bits it can transfer per second
 Example: 1 mega bit per second = 106 bits every second
 Delay: time it takes for a bit to traverse the link
 Example: 1 millisecond = 0.001 second
1
0
Reproduced with permission of Bill Cheswick, Lumeta Inc
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
5
Link Delay
 Total Delay = Propagation Delay + Transmission Delay
 Propagation Delay: time it takes for bits to traverse
the link
 Transmission Delay: time it takes to inject the bits to
the link
 Example: think of students leaving this room towards
Sidney Smith Hall
 Propagation Delay: time for each student to walk to
Sidney Smith Hall – 5min
 Transmission Delay: 24 students one leaving every 5
seconds – 24x5 sec = 120 sec = 2min
 Total Delay = 5min + 2min = 7min
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
6
Packets
 Data is encapsulated in “packets”.
 Why?
 Think of envelopes in mail
 Each packet has
 Payload: the actual data
 Header: destination address, …
001001101001….01010111
Header
Reproduced with permission of Bill Cheswick, Lumeta Inc
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
7
Connecting to the Internet
Links to other
routers
Internet
routers
???
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
8
What Does a Router Do?
 Each router receives packets
on input lines
 Looks up the destination
?
?
 Address in header
 Sends it out on the
appropriate line
 Question. How does the
router know which output to
send the packet to?
 Question. What if there is a
loop?
?
 Time-To-Live or TTL
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
9
The Internet – Collection of Nodes
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
10
The Internet – Collection of Nodes
Reproduced with permission of Bill Cheswick, Lumeta Inc
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
11
The Internet Cloud
Reproduced with permission of Bill Cheswick, Lumeta Inc
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
12
History of the Internet
 1966: First two machines connect
 1968: ARPAnet created by DARPA (Defense Advanced
Research Projects Agency)
 1970: First five nodes:
 UCLA
 Stanford
 UC Santa Barbara
 U of Utah, and
 BBN
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
13
History of the Internet – Cont’d
 1972: First e-mail program, first FTP (?!)
 ARPAnet has 15 nodes
 1974: Architecture for interconnecting networks
 1979: ARPAnet has 200 nodes
 1984: 1000 hosts, called “Internet”
 Early 1990’s: ARPAnet decommissioned
 Early 1990s: Web
 1990’s: commercialization of the Web
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
14
Number of Internet Hosts
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
15
Brief Introduction to Computer Networks
 We’ll go through two examples
 Mail system
 File transfer in the Internet
 Consider similarities
 And differences
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
16
An Introduction to the Mail System
U of T
Stanford
Yashar
Nick
Admin
Admin
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
17
Characteristics of the Mail System
 Each envelope is individually routed.
 No time guarantee for delivery.
 No guarantee of delivery in sequence.
 No guarantee of delivery at all!
 Things get lost
 How can we acknowledge delivery?
 Retransmission
 How to determine when to retransmit? Timeout?
 Need local copies of contents of each envelope.
 How long to keep each copy.
 What if an acknowledgement is lost?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
18
An Introduction to the Mail System
U of T
Stanford
Application Layer
Yashar
Nick
Transport Layer
Admin
Admin
Network Layer
Link Layer
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
19
An Introduction to the Internet
leland.stanford.edu
cs.toronto.edu
Application Layer
Nick
Yashar
Transport Layer
O.S.
Datagram
Data
Header
Data
Header
O.S.
Network Layer
Link Layer
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
20
Characteristics of the Internet
 Each packet is individually routed.
 No time guarantee for delivery.
 No guarantee of delivery in sequence.
 No guarantee of delivery at all!
 Things get lost
 Acknowledgements
 Retransmission
 How to determine when to retransmit? Timeout?
 Need local copies of contents of each packet.
 How long to keep each copy?
 What if an acknowledgement is lost?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
21
Characteristics of the Internet – Cont’d
 No guarantee of integrity of data
 Content of a packet can change
 Why?
 Packets can be fragmented (split) into pieces
 Why?
 Packets may be duplicated
 How?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
22
An Introduction to the Mail System
U of T
Stanford
Application Layer
Nick
Yashar
Transport Layer
Admin
Admin
Network Layer
Link Layer
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
23
Some Questions About the Mail System
 How many sorting offices are needed and where
should they be located?
 How much sorting capacity is needed?
 Should we allocate for Mother’s Day?
 How can we guarantee timely delivery?
 What prevents delay guarantees?
 Or delay variation guarantees?
 How do we protect against fraudulent mail deliverers,
or fraudulent senders?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
24
Internet Principles
 Original principles designing the Internet
 Autonomy

No internal changes required to interconnect networks
 Best effort service model

No guarantees
 Stateless routers


Routers don’t remember which packets have been sent
before
Unlike phone network
 No centralized control
 All have major impact on Internet’s fast growth.
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
25
Final Comments, Discussion
 Computer networks have and continue to change our
lives
 For better?
 Internet’s original design principles have had a major
impact on its growth.
 Will it continue to grow at the same rate?
 Some revolutionary applications expedite this impact
 What else can we expect?
SII 199 - Computer Networks and Society
University of Toronto – Fall 2015
26