1. CS244a: An Introduction to Computer Networks
Handout #2: Introduction
Nick McKeown
Professor of Electrical Engineering
and Computer Science, Stanford University
Winter 2002
CS244a Handout #2

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

3. 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

4. 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

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

6. 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

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

8. 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

9. 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

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

11. 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

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

13. 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

14. 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

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

16. 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

17. 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

18. 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