1. Lect-1: IntroductionComputer Networks1 371-1-0291 : An Introduction to Computer Networks Handout #1: Introduction Additional Reading Text book: Chaps. 1, 9.2.1 Homepage http://www.cse.bgu.ac.il/Courses/course.asp?ID=63

2. Lect-1: IntroductionComputer Networks2  An Introduction to the mail system  An Introduction to the Internet  Important concepts in Networking  Key Enablers of Large Networks Outline

3. Lect-1: IntroductionComputer Networks3 An Introduction to the mail system ShimonReuven Beer Sheva Technion Admin Letter Envelop Delivery System

4. Lect-1: IntroductionComputer Networks4 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?  Each mailing functionality is carried out by a specific network architecture layer

5. Lect-1: IntroductionComputer Networks5 An Introduction to the mail system Admin Application Layer Transport Layer Network Layer Link Layer ShimonReuven Beer Sheva Technion Router HOST

6. Lect-1: IntroductionComputer Networks6 An Introduction to the Internet Network Layer Link Layer Application Layer Transport Layer O.S. HeaderDataHeaderData HD HD HD HDHD HD bgumail.bgu.ac.il mail.technion.ac.il ShimonReuven Router HOST Packet

7. Lect-1: IntroductionComputer Networks7 Characteristics of the Internet Protocol (IP)  Each packet is individually routed.  No time guarantee for delivery.  No guarantee of delivery in sequence.  No guarantee of delivery at all !!!  Packets get lost  Packet acknowledgements  Packet retransmission  How to determine when to retransmit? Timeout?  Need local copies of contents of each packet.  How long to keep each copy?  End-to-end or hop-by-hop?  What if an acknowledgement is lost?

8. Lect-1: IntroductionComputer Networks8 Further Characteristics of the Internet Protocol (IP)  No guarantee of integrity of data.  Packets can be fragmented.  Packets may be duplicated.  These need to be handled by the network  So which layer does what ?

9. Lect-1: IntroductionComputer Networks9 An Introduction to the mail system Admin Application Layer Transport Layer Network Layer Link Layer ShimonReuven Router HOST

10. Lect-1: IntroductionComputer Networks10 Layering in the Internet  Transport Layer  Provides reliable, in-sequence delivery of data from end- to-end on behalf of the application.  Network Layer  Provides “best-effort”, but unreliable, delivery of “datagrams”.  Link Layer  Carries data over (usually) point-to-point links between hosts and routers; or between routers and routers. Three out of seven

11. Lect-1: IntroductionComputer Networks11 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 more for Rosh Hashana?  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?

12. Lect-1: IntroductionComputer Networks12  An Introduction to the mail system  An Introduction to the Internet  Important concepts in Networking  Key Enablers of Large Networks Outline

13. Lect-1: IntroductionComputer Networks13 Important Concepts in Networking  Multiplexing  Buffering  Encapsulation  Socket API  Common Applications

14. Lect-1: IntroductionComputer Networks14 Multiplexing  A method of sharing resources among users  Provides a cost-effective resource sharing  Packetizing messages enables Time Mux  Routers enable Space Mux

15. Lect-1: IntroductionComputer Networks15 Buffering  The process of temporarily storing data until a resource becomes available  Enables to overcome random delays and async transmission  Handles temporarily overflow and reduces loss of data

16. Lect-1: IntroductionComputer Networks16 Encapsulation  The process of embedding “data” in a new “message format”  The new message will consist of header+data  Enables hierarchical layering where one layer places its data into another layer format  The latter is responsible to provide its end-to- end service to the encapsulated data

17. Lect-1: IntroductionComputer Networks17 An Introduction to the mail system Admin Application Layer Transport Layer Network Layer Link Layer ShimonReuven Router HOST

18. Lect-1: IntroductionComputer Networks18 Socket API  API: Application Programming Interface  Socket API is a common interface by which remote application can exchange messages  Enables distributed development of applications that can talk to each other

19. Lect-1: IntroductionComputer Networks19 Common Applications  A key factor for the Internet success  Part of Internet suite  Notable ones are:  Domain Name Server (DNS)  Mail  Network File System (NFS)  HTTP (one of the Web protocols)  RTSP (Streaming – another Web protocol)

20. Lect-1: IntroductionComputer Networks20  An Introduction to the mail system  An Introduction to the Internet  Important concepts in Networking  Key Enablers of Large Networks Outline

21. Lect-1: IntroductionComputer Networks21 Three Key Enablers of Networks  Digitization of Signals  Economies of Scale  Network Externalities

22. Lect-1: IntroductionComputer Networks22 Digitization of Signals  Any information bearing signal can be represented by a binary string with arbitrarily high accuracy.  Sample + quantize  Digital stream.  Digital data is cheaper to store, manipulate and, most importantly, reliably communicate.

23. Lect-1: IntroductionComputer Networks23 Economies of Scale  Cost of capacity grows slowly with increased capacity.  A network has fixed costs only (almost) and zero marginal costs. Capacity or # users Cost

24. Lect-1: IntroductionComputer Networks24 Network Externalities  Value of the network increases as the number of users increases, which in turn attracts more users. Cost # Users tradeoff Benefit

25. Lect-1: IntroductionComputer Networks25 Next Lecture A Detailed FTP Example Layering Packet Switching and Circuit Switching Some terms –Data rate, “Bandwidth” and “throughput” –Propagation delay –Packet, header, address –Bandwidth-delay product, RTT Additional Reading Text book: Chaps. 1.2, 3.1