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Handout # 3: Introduction to Computer Networks
SII 199 – Computer Networks and Society Handout # 3: Introduction to Computer Networks Professor Yashar Ganjali Department of Computer Science University of Toronto
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University of Toronto – Fall 2015
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 Computer Networks and Society University of Toronto – Fall 2015
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University of Toronto – Fall 2015
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 Computer Networks and Society University of Toronto – Fall 2015
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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 SII Computer Networks and Society Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2015
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University of Toronto – Fall 2015
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 = second 1 1 1 1 1 SII Computer Networks and Society Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2015
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University of Toronto – Fall 2015
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 Computer Networks and Society University of Toronto – Fall 2015
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University of Toronto – Fall 2015
Packets Data is encapsulated in “packets”. Why? Think of envelopes in mail Each packet has Payload: the actual data Header: destination address, … Header … SII Computer Networks and Society Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2015
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Connecting to the Internet
Links to other routers Internet routers ??? SII Computer Networks and Society University of Toronto – Fall 2015
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University of Toronto – Fall 2015
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 Computer Networks and Society University of Toronto – Fall 2015
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The Internet – Collection of Nodes
SII Computer Networks and Society University of Toronto – Fall 2015
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The Internet – Collection of Nodes
SII Computer Networks and Society Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2015
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University of Toronto – Fall 2015
The Internet Cloud SII Computer Networks and Society Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2015
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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 Computer Networks and Society University of Toronto – Fall 2015
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History of the Internet – Cont’d
1972: First 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 Computer Networks and Society University of Toronto – Fall 2015
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Number of Internet Hosts
SII Computer Networks and Society University of Toronto – Fall 2015
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Brief Introduction to Computer Networks
We’ll go through two examples Mail system File transfer in the Internet Consider similarities And differences SII Computer Networks and Society University of Toronto – Fall 2015
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An Introduction to the Mail System
Stanford U of T Yashar Nick Admin SII Computer Networks and Society University of Toronto – Fall 2015
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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 Computer Networks and Society University of Toronto – Fall 2015
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An Introduction to the Mail System
Stanford U of T Application Layer Yashar Nick Admin Transport Layer Network Layer Link Layer SII Computer Networks and Society University of Toronto – Fall 2015
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An Introduction to the Internet
leland.stanford.edu cs.toronto.edu Application Layer Yashar Nick Transport Layer O.S. Header Data Network Layer Link Layer Datagram SII Computer Networks and Society University of Toronto – Fall 2015
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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 Computer Networks and Society University of Toronto – Fall 2015
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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 Packets may be duplicated How? SII Computer Networks and Society University of Toronto – Fall 2015
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An Introduction to the Mail System
Stanford U of T Application Layer Yashar Nick Admin Transport Layer Network Layer Link Layer SII Computer Networks and Society University of Toronto – Fall 2015
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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 Computer Networks and Society University of Toronto – Fall 2015
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University of Toronto – Fall 2015
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 Computer Networks and Society University of Toronto – Fall 2015
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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 Computer Networks and Society University of Toronto – Fall 2015
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