CS 336/621Computer Networks and

Network Programming

Spring 2009

Professor Allan B. Cruse

University of San Francisco

Course synopsis

• Combines a survey of network principles with hands-on exercises and experiments

• Utilizes our classroom and CS laboratory networking hardware infrastructures

• Employs C/C++ programming language and the standard ‘sockets’ API libraries

Course prerequisites

• Open to USF Computer Science students

• Upper-division or graduate-level standing

• Familiarity with Linux (or UNIX) systems

• Programming experience: C/C++/Python

• Prior coursework: CS 110/112 and CS 245 (or the equivalent preparation elsewhere)

methodology

• Lectures

• Readings

• Discussions

• Demonstrations

• Exercises

• Projects

• Quizzes

Course website

• You will find current course-information at this website:

<http://cs.usfca.edu/~cruse/cs336>

• Reading-assignments, exam-dates, class announcements, course-related software, links to other resources and to the ‘signup’ page for our class’s online discussion-list

Textbook

• James F. Kurose and Keith W. Ross • Computer Networking: A Top-Down Approach • 4th Edition• Pearson/Addison-Wesley (2008)

This up-to-date textbook is very widely used in networking courses at colleges and universities in the United States and elsewhere.Lots of material – maybe too much for one semester – the authors recommend we cover at least the first five chapters in sequence, then maybe one of the concluding chapters if time allows for that.

Reference Model

link layer

network layer

transport layer

application layer

The textbook authors have utilized the standard reference model for network-software’s layered architecture as a principle which guides their “Top-Down Approach” to organizing the Chapters

Chapter 2

Chapter 3

Chapter 4

Chapter 5

introduction Chapter 1

Network hardware elements

• NIC (Network Interface Controller)• Cable (e.g., coaxial, fiber optic, twisted pair)• Repeater (boosts signal-strength on long cables)• Hub (promiscuously connects nearby NICs)• Switch (selectively connects nearby NICs)• Bridge (connects multiple network segments)• Router (intelligently forwards network packets)

Network Interface Card (NIC)

Before the internet there was…… sneaker-net!

host

host

Users often would transfer their data from one computer to another by simply copying it onto a floppy diskette, and then carrying that diskette over to their other computer – located, hopefully, not too far away (e.g., just down the hall)

‘crossover’ cable

host

host

An improvement over the ‘sneaker net’ approach to data sharing between two host computers that are a short distance apart is to utilize a special direct cable connection – inexpensive to purchase, easy to connect if NICs are installed, less physical effort is required, no floppy disks are needed, and no risk of any unwanted ‘virus’ software infecting the host machines

NIC

NIC

cable

A ‘Point-to-Point’ connection

host host

repeater

Over a long-distance cable-connection the electrical signals can degrade, but ‘repeaters’ can be inserted to receive and amplify a weakened signal before sending it along (i.e., “repeating it”) to the next connection-point

‘Local’ networks

host

host

host

hosthost

‘ring’ topology

host

host

host

host

HUB

‘star’ topology

Why do you think the star-topology is more widely deployed nowadays?

‘Hub’ versus ‘Switch’

host host host host host host host host

Why are ‘switches’ preferred over ‘hubs’ nowadays?

The ‘Bridge’ device

host

hosthost

switch

host

hosthost

switch

bridge

The ‘Router’ device

host

hosthost

switch

host

hosthost

switch

host

hosthost

switch

router

Network software elements

operating system kernel

networkapplication

program

functionlibrary

NIC

device driver

‘socket’data-structure

User-space Kernel-space

File and I/Osubsystems

(‘protocol stack’)

cable

Demo: iplookup.py

• This application reports the IP- address for a particular network host (if it’s known)

• For example:$ python iplookup.py stargate

The IP-address for ‘stargate’ is 138.202.171.14

$ python iplookup.py pyramid

The IP-address for ‘pyramid’ is 138.202.171.19

$ python iplookup.py neptune

The IP-address for ‘neptune’ is unknown

Source-code (in Python)

#!/usr/bin/python

import systry:

hostname = sys.argv[1]except:

hostname = “localhost”

import sockettry:

hostip = socket.gethostbyname( hostname )except:

hostip = “unknown”

print “The IP-address for \’” + hostname + “\’ is “ + hostip

Demo: getquote.py

• This internet application reports the latest price for a share of Intel Corporation stock

• For example:$ python getquote.py

Intel Stock: $13.53 at “12:05pm” on “1/26/2009”

Internet programming

import socket

try:host = “download.finance.yahoo.com”port = 80sock = socket.socket( socket.AF_INET, socket.SOCK_STREAM )sock.settimeout( 2 )sock.connect( ( host, port) )sock.send( “GET /d/quotes.csv?s=INTC&f=sl1d1t1c1ohgv&e=.csv \r\n” )sock.send( “\r\n” );response_string, server = sock.recvfrom( 4096 )quote = str.split( response_string, ‘,’ )

except socket.error, msg:print “An error occurred:”, msg

else:printprint “Intel Stock: “, ‘$’+quote[1], “at”, quote[3], “on”, quote[2]print

In-class exercise #1

• Can you modify the ‘getquote.py’ program so that it will report the latest stock-price for some other famous technology firms?– Microsoft Corporation– Red Hat Corporation– Oracle Corporation– Siemens Corporation– Sony Corporation