Information Flow through the Internet

Tommy Wei

First Step: Connecting to the Internet

• In order to connect your LAN to the Internet, you need a GATEWAY

• Gateway- device that connects the LAN to the Internet, passes messages between computers on the LAN and on the Internet

• Gateways can• translate IP addresses on the LAN and IP addresses on the Internet

• Control the types of information that flow

• Stores information so it can be accessed again

Router

• There are many devices, but the most common device used for a gateway is a router

• It is the most intelligent of all the connectivity devices

• One of the simplest and most effective ways to connect your LAN to the Internet

• All you do is cable the router to your LAN, connect your phone line or cable modem, plug it into power and you are on the Internet

The OSI Model

• In order the understand how information flows through the internet and how you get to certain webpages (www.company.com), you need to learn about the OSI Model

• It is a model describing how information is transferred from computer to computer, going through 7 layers in total

OSI Model

• Here are the 7 layers

• The main idea is that information gets sent down the layers on the left side (your computer) and is received by going back up on the right side (receiving computer)

My Computer Receiving Computer

OSI Model - The 7 Layers

• I will now simply explain each of the OSI layers, helping you understand the main function of each

• Application Layer- this layer is application specific. Think of it as the browser application or email client used for surfing the web or sending email. Provides application services and defines protocols

• Presentation Layer- basically changes the data into the correct “form” or syntax to fix compatibility problems

• Session Layer- establishes, manages and terminates connections between applications, deals with coordination

OSI Model - The 7 Layers

• Transport Layer – this layer handles messaging. Responsible for getting the entire message, so it has to keep track of fragments and packets, responsible for transfer of data between systems

• Network Layer – in this layer, routing and addressing information is the main function. You deal with IP addresses and packets in this layer

• Data Link Layer– the main idea you need to know is that layer 2 translates data frames into bits, deals with the physical address

• Physical Layer – it is simply hardware that is used for sending and receiving data (cables, cards and other physical aspects)

Putting all the Layers Together(1)

• Now that you know the basics of the OSI Model, we will combine this with visiting a web page when the user types in the URL

• Every step from typing the URL to receiving the Web page can be associated with a layer in the OSI Model

• Let’s start with typing in the URL!

Putting all the Layers Together(2)

• When you type in the url, the user is in the application layer of the OSI model, the user provides raw data input

• much more information is generated by the browser application• The type of browser

• The type of operating system on the user’s computer

• Language

• These pieces of information are within the application, presentation and session layers of the OSI model

• The information is needed by the application to ensure successful communication with the end system, formats the data correctly

Putting all the Layers Together (3)

• Once this application information has been generated, it is packaged and moved to the transport layer

• The packets of information are sent from the Session layer to the Transport Layer

• The protocol TCP is used in the transport layer to ensure successful packet delivery

• Information about the port number, sequence numbers and checksums are also included to provide reliable transport

• All this information in the transport layer is added to the heads of the packets of information

Putting all the Layers Together (4)

• Now the packets are passed to the Network Layer, because new information is going to be added

• the information added will be information on the IP address of the client and the end system

• This information is used to ensure the correct delivery path from the user’s computer to the end system

• The router(found in this layer) receives an incoming packet and reads the logical addressing information and then determines which network the packet must be delivered

Putting all the Layers Together (5)

• Now the packets are moving down to the Data link layer, where they are now called frames

• In the data link layer you add the physical address, also known as the MAC address or Ethernet address

• This address is different from the IP address found in layer 3 because the physical address is unique to the entire world

• The physical address will be used by switches (found in the data link layer) to find other PCs on the same network.

Putting all the Layers Together (6)

• At last, once all the information is added to the header of the packet, it is moved to the physical layer where the data is turned into 1s and 0s and transmitted onto the physical medium (wire or cable)

• the physical layer primarily concerns itself with the transmission and reception of that raw data over a physical medium

• The data is sent to the receiving computer and has to work its way back up the OSI model, starting from the physical layer to the application layer, breaking apart the large piece of data at the bottom of this slide

Data traveling from one computer to another

Packets of Data

Getting the Webpage

• In order to fetch a web page, your web browser (chrome, firefox) must “talk” to a web server and vice versa; the language they speak in is known as HTTP

• They go through this whole OSI process to send messages from one computer to another

Getting the Webpage (2)

• Picking up from slide 8…..

• Once you type in the URL, your web browser will contact what is called a DNS (domain name system) that will translate the human readable website (facebook.com) into a numeric IP address

• If the current DNS doesn’t know a particular IP address, then it will ask another server “higher up” in the hierarchy of domain name systems

IP Address

Getting the Webpage (3)

• Once a DNS server finds the IP address of the website you are looking for, that IP address is returned to your browser

• Your browser will now use the IP address returned by DNS to communicate with the web server and send an HTTP request for the website that you want to visit (facebook).

• To communicate it uses a protocol called TCP to establish a connection and allow for exchanges of data

HTTP Address

IP Address

Getting the Webpage (3)

• Now that your browser has a connection with the website’s web server, your browser will retrieve the html code (used to design and create the page) of the specific page that is requested

• Once your browser receives the HTML code from the web server, it will display that HTML code to you in the browser window

• The browser then parses through the page and looks for other page elements it needs to complete the page

• For more needed elements, the browser makes additional connections and HTTP (the language used to communicate) requests to the server

HTML Code