Handout # 3: Introduction to Computer Networks

1. Handout # 3:Introduction to Computer Networks SII 199 – Computer Networks and Society Professor Yashar Ganjali Department of Computer Science University of Toronto yganjali@cs.toronto.edu http://www.cs.toronto.edu/~yganjali

2. Announcements • Class mailing list: if you haven’t received an e-mail from me, please let me know. • Check out class web page for slides, and lecture notes. • Volunteer for lecture notes? University of Toronto – Fall 2012

3. 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 University of Toronto – Fall 2012

4. 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 University of Toronto – Fall 2012

5. 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 0 1 1 0 0 1 1 1 Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2012

6. 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 University of Toronto – Fall 2012

7. Packets • Data is encapsulated in “packets”. • Why? • Think of envelopes in mail • Each packet has • Payload: the actual data • Header: destination address, … Header 001001101001….01010111 Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2012

8. Connecting to the Internet Links to other routers Internet routers ??? University of Toronto – Fall 2012

9. 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 ? ? ? University of Toronto – Fall 2012

10. The Internet – Collection of Nodes University of Toronto – Fall 2012

11. The Internet – Collection of Nodes Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2012

12. The Internet Cloud Reproduced with permission of Bill Cheswick, Lumeta Inc University of Toronto – Fall 2012

13. 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 University of Toronto – Fall 2012

14. 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 University of Toronto – Fall 2012

15. Number of Internet Hosts University of Toronto – Fall 2012

16. Brief Introduction to Computer Networks • We’ll go through two examples • Mail system • File transfer in the Internet • Consider similarities • And differences University of Toronto – Fall 2012

17. Admin Admin An Introduction to the Mail System Stanford U of T Yashar Nick University of Toronto – Fall 2012

18. 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? University of Toronto – Fall 2012

19. Admin Admin Network Layer Link Layer An Introduction to the Mail System Stanford U of T Application Layer Yashar Nick Transport Layer University of Toronto – Fall 2012

20. Application Layer Transport Layer O.S. O.S. Network Layer Link Layer Data Data Header Header An Introduction to the Internet leland.stanford.edu cs.toronto.edu Nick Yashar Datagram University of Toronto – Fall 2012

21. 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? University of Toronto – Fall 2012

22. 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? University of Toronto – Fall 2012

23. Admin Admin Network Layer Link Layer An Introduction to the Mail System Stanford U of T Application Layer Nick Yashar Transport Layer University of Toronto – Fall 2012

24. 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? University of Toronto – Fall 2012

25. 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. University of Toronto – Fall 2012

26. 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? University of Toronto – Fall 2012