Key Points

Key Points

• Internet Origins revisited

• The Client-Server Protocol

• Structure of the Internet

• Moving Data: Where

• Moving Data: How

• Moving Data: Transfer Modes

• Basic File-types

Internet Origins Revisited

• 1966 ARPA Experiments 0 People Online

• 1969 1ST ARPA Nodes • 1972 Email Invented• 1980 TCP Experiments• 1986 NSF-net Backbone• 1990 ARPANET Retired 0.2M Online• 1992 Mosaic Introduced 0.7M Online• 1993 1.5M Online• 1994 2.1M Online• 1995 5.0M Online• 1996 9.5M Online• 1997 16.0M Online

Single Computer Operation

• A computer is basically composed of memory, a processor, and hardware to interface with humans (monitor, keyboard, mouse).

MEMORY

Programs

Data

PROCESSOR

Single Computer Operation

• The processor retrieves programs and data from the memory and displays program results to the monitor.

MEMORY

Program1Program2Program3

Data1Data2Data3

PROCESSOR

Program1Data1

Data1

Effects of the Internet

• You have essentially infinite memory storage. Your browser has access to data (html pages typically) stored on other machines.

MEMORY

MS IENetscape

FTP

index.htmlcgs_home.html

links.html

PROCESSOR

Netscapehttp://w.x.y.z

http://w.x.y.z/

INTERNETPrograms And Data:

http://w.x.y.z/


http://w.x.y.z/


• You also MAY have essentially infinite processing power via access to many other processors to help out with large tasks (e.g. SETI Screensaver).

MEMORY

Programs

Data

PROCESSOR

INTERNETPrograms And Data

INTERNETPrograms And Data


• The two big features of being on the internet are– Essentially infinite memory.– Essentially infinite processing power.

• How is it done?– The most common (and in some ways limited) method is by using

browsers.– Browsers, ( MS IE, Netscape, Lynx, Mozilla ) can read and display

information formatted in particular ways.– Browsers allow files to be transferred to your machine but have a

limited capability of transferring files from your machine.

Accessing Remote Storage

• The most common method is to use a browser:– Browsers typically transfer and then display files.– Browsers typically read and display HTML file formats, but also can

display various types of image formats (gif and jpg are common).– Browser capabilities vary greatly from one to another.

• File Transfer Protocol (FTP)– A generic method of transferring files from one machine to another.– File contents are not displayed, just transferred from one machine to

another.– There are some good stand-alone FTP software available for free.– Sophisticated browsers typically support FTP these days.


• Programs (like FTP) can be used to store your data on other machines.

MEMORY

MS IENetscape

FTP

index.htmlcgs_home.htmlmylinks.html

PROCESSOR

FTPmylinks.html

http://w.x.y.z/


mylinks.html


mylinks.html

Effect of the Internet

• Similarly, programs like FTP can be used to store get programs from the internet to your machine.

MEMORY

MS IENetscape

FTP

Game.exe

PROCESSOR

FTPGame.exe

Game.exe

INTERNETPrograms And Data:Game.exe

INTERNETPrograms And Data:Game.exe

Accessing Remote Processors• By accessing remote processors you can operate a remote

computer as if you were sitting right next to it.– You will need access to the computer (like your Pegasus account).– You can use many processors in parallel to do a single job.

• Browsers are limited for security reasons– Although browsers can be used to run programs on remote machines,

its carefully controlled by the owner of the machine.– We will explore this more later in the course

• Telnet: Telnet is very useful and for a long time was a standard method of logging in to remote machines

– Limited by text only.– Slowly being phased out due to security issues.

• Other programs: Remote Shell (rsh), Secure Remote Shell (ssh), putty, exe.

– Some are part of operating systems, some are available on the net.

Client – Server Protocol

• The basic paradigm for all internet communication.

• Most basic terms: the client requires a service, and a server provides it.– Client can be thought of a customer, and the server a seller.

– Diner is a client; Waiters are servers.

– Bank customer is a client; Bank teller is a server.

– All computers online act as clients or servers (or both).

– Typically a many-to-one relationship: Many clients to one server.

– Servers typically provide a very specific service.

Client – Server Example

• Consider checking your bank account balance online.

Client“FrontDoor”Server

PasswordServer

AccountServer

Client – Server Example (cont)

• Notice that each server performed a very specific task.– Sometimes this is for security reasons.

• Passwords, personal information, account numbers

– Sometimes this is so the server (or system) can handle lots of requests

• Retrieving big data structures, providing very specific pieces of information

• What happened to many servers to one client?– Look at it from the point of view of the servers.– You are one person, one request.– Each server sees thousands of people like you.

Internet Structure

• The internet is a disorganized collection of computers

– Heterarchical structure as opposed to hierarchical structure

• Networks (and the Internet) are typically composed of servers, gateways, routers, and bridges.

• Networks are commonly classified as Local Area Networks (LAN) or Wide Area Networks (WAN).

• A Backbone is a network connecting LAN’s and WAN’s to other LAN’s and WAN’s

How the Internet Looks to You

Gateway

You

CHAOS

LAN

Internet Structure

• Servers: basically any computer on the internet that has the ability to provide a service.

• Gateways: Connect local LAN’s and WAN’s to the internet.

• Bridges: Connect two LAN’s using similar communication protocols.

• Routers: Connect two networks that may not use similar communication protocols.

Moving Data: Where

• In order to send data to its appropriate place we must have some scheme of designating machines in this disorganized structure.

• Internet addresses (IP address) provide the basic information for message routing.

– IP = Internet Protocol

• An IP address is composed of four numbers, each between 0 and 255, separated by periods.

– 132.170.108.2 is the UCF Computer Science server.

– You can run ‘winipcfg’ on Windows machnies to find your IP address.

Moving Data: Where

• How many IP addresses do you know? How many could you remember if you had to?

– For the convenience of users, machines on the internet are assigned names as well as numbers:

– www.cs.ucf.edu is the name for the UCF Computer Science server.

– People use machine names, computers routing the data use numbers: A translation has to occur.

– Dynamic Name Servers (DNS) translate machine names to IP addresses.

Dynamic Name Servers (DNS)

• There are lots and lots of DNS, but your computer probably only talks to one of them.

• Each DNS has a list of machine names and IP addresses.– This list is dynamic and so changes all the time.

• If you request a machine name on the list, the DNS returns the IP address.

• If you request a machine name not on the list, the DNS sends out a request to other DNS asking for the IP address.

• Eventually, some DNS somewhere returns the IP address to your DNS. Your DNS passes the address on to you and adds the name-IP address pair to its list.

Moving Data: How

• Example: We want to send the following message to another machine:– “Send lawyers, guns, and money.”

• Assume the DNS has returned IP address w.x.y.z.

• The complete message is usually too large to send all at once, so we divide it into “packets.”– Each packet typically contains routing information, a packet

number, and its part of the overall message.

w.x.y.z 1 Send

w.x.y.z 2 lawyers, w.x.y.z 4 money.

w.x.y.z 3 guns, and

Example: Viewing a Web Page

Gateway

You

Gateway

DNS

Host

You request a web page via your gateway.


Gateway

You

Gateway

DNS

Host

Your gateway asks the DNS for the IP address.


Gateway

You

Gateway

DNS

Host

The remote host is contacted and asked for the page.


Gateway

You

Gateway

DNS

Host

The remote host verifies the web page exists and returns it.

Failures and Messages

• Cannot find remote host:– Either your gateway or DNS is dead.

– You mistyped something.

• Contacting remote host:– Your gateway is ok

– DNS has returned an IP address for the host.

– The host is probably dead or very busy.

• The 404 Error:– The most common error.

– The page was not found.

– Notorious on the internet due to an inability to keep up with changes to web sites.

• Downloading is slow:– One or more elements of your internet connection is busy or crippled.

Key Points – 8/22

• Internet Origins revisited

• The Client-Server Protocol

• Structure of the Internet

• Moving Data: Where

• Moving Data: How

• Moving Data: Transfer Modes

• Basic File-types

Documents

Key Points