CS244a: An Introduction to Computer Networks Handout #2: Introduction Nick McKeown Professor of Electrical Engineering and Computer Science, Stanford University nickm@stanford.edu http://www.stanford.edu/~nickm Winter 2002 CS244a Handout #2

Outline Introduction Logistics What the class is about. Introduction to the TAs. Logistics Handout #1: Logistics and Text Books. Some ground rules. Winter 2002 CS244a Handout #2

Some Ground Rules Let’s make this educational and enjoyable. It’s a big class, and it’s easy for things to get out of hand, so please… Let me orchestrate the questions. Listen to other people’s questions. Be here. Be here on time. Winter 2002 CS244a Handout #2

Let’s begin Demos An Introduction to the mail system Internet telephony #1 Internet telephony #2 An Introduction to the mail system An Introduction to the Internet [Three Key Enablers of Networking] Winter 2002 CS244a Handout #2

An Introduction to the mail system MIT Stanford Nick Dave Admin Winter 2002 CS244a Handout #2

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? Winter 2002 CS244a Handout #2

An Introduction to the mail system MIT Stanford Application Layer Nick Dave Admin Transport Layer Network Layer Link Layer Winter 2002 CS244a Handout #2

An Introduction to the Internet Athena.MIT.edu Leland.Stanford.edu Application Layer Nick Dave Transport Layer O.S. Header Data Network Layer Link Layer H D H D H D H D H D H D Winter 2002 CS244a Handout #2

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? Winter 2002 CS244a Handout #2

Characteristics of the Internet (2) No guarantee of integrity of data. Packets can be fragmented. Packets may be duplicated. Winter 2002 CS244a Handout #2

Layering in the Internet Transport Layer Provides reliable, in-sequence delivery of data from end-to-end on behalf of 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. Winter 2002 CS244a Handout #2

An Introduction to the mail system MIT Stanford Application Layer Nick Dave Admin Transport Layer Network Layer Link Layer Winter 2002 CS244a Handout #2

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? Winter 2002 CS244a Handout #2

Outline Demos An Introduction to the mail system Internet telephony #1 Internet telephony #2 An Introduction to the mail system An Introduction to the Internet [Three Key Enablers of Networking] Winter 2002 CS244a Handout #2

Three Key Enablers of Networks Digitization of Signals Economies of Scale Network Externalities Winter 2002 CS244a Handout #2

Digitization of Signals Any information bearing signal can be represented by a binary string with arbitrarily high accuracy. Sample + quantize a Digital stream. Digital data is cheaper to store, manipulate and, most importantly, reliably communicate. Winter 2002 CS244a Handout #2

Economies of Scale Cost of capacity grows slowly with increased capacity. A network has fixed costs only: (almost) zero marginal costs. Cost Capacity or # users Winter 2002 CS244a Handout #2

Network Externalities Value of the network increases as the number of users increases, which in turn attracts more users. Boom! Benefit Cost # Users Winter 2002 CS244a Handout #2