How data flows across the Internet

Individual Project 1Sarah Pritchard

Fran, a customer of your company, would like to visit your company’s website from her home computer… How does your company’s website show up on her home computer?

First, Fran enters your company’s URL into her browser, presses “enter”, and HTTP sends her request to your company’s server, asking to view your company’s website.

HTTPHypertext Transfer Protocol(Application Layer Protocol)

HTTP is an Application layer protocol that formulates & interprets requests between a web client (Fran) and a server (where your company’s website is located).

Physical Link

Receive Data

• This process is the top (7th) layer of the OSI (Open Systems Interconnection) Model.

• It facilitates communication between software applications and lower-level layers of the OSI Model.

• The network can interpret an application’s request and also interpret data sent from the network (like viewing a website or sending an email to a friend) .

• An application header is added to the data, which contains information about the requirements so Fran can view the webpage properly.

Transmit Data

7 Layers of the OSI Model

The OSI (Open Systems Interconnection) model helps users understand computer-to-computer communication. It divides networking functions into seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Encryption/Decryption

*&kjs^7sff6l10sdk103489l786*^&^$*&#iowu9^&#^k98AS*&^%#&93121i

10100101101010101110101011010100101010010101110101001011

This is a decrypted message that can now be viewed by receiving computer.

• This is the 6th layer of the OSI Model.• The Application layer transfers the request to the

Presentation layer, in the form of a protocol data unit (PDU).

• The Presentation layer determines whether & how it should format or encrypt the data request.

• This layer often serves as a translator between data formats, such as various graphics file formats.

• Responsible for encryption and decryption in network communication (to keep the information secure).

• This information is added to the presentation header.

The webpage that Fran would like to visit must then be formatted by her computer so that she will be able to view it. Your company’s website contains both pictures and formatted text, which requires the process of encryption and decryption for Fran’s computer to be able to view the website securely. This process is especially important if Fran is viewing the webpage on a Macintosh computer, since a PC and a Mac use very different formats (they speak a different language).

7 Layers of the OSI Model

Physical Link

Receive DataTransmit Data

7 Layers of the OSI Model

Physical Link

Receive DataTransmit Data

Fran’s computer must establish and maintain a constant connection with your company’s server so that she is able to view your company’s website. This allows the communication to stay secure, allows the exchange of data, and terminates the communication when the session is complete.

• This represents the 5th layer of the OSI Model

• A session header is added to the PDU, which contains information about how Fran’s computer communicates with the network.

• The Session layer will then pass the PDU (data) to the next Layer.

Fran’s computer and the company’s server perform a “3-step handshake” to establish a connection: 1. Fran’s computer sends a request to the company's server in the form of a packet (a unit of data) to establish a

connection- SYN (synchronization).2. Once the company’s server receives the SYN request, it sends a packet to Fran’s computer stating that it’s willing to

make a connection- SYN-ACK (synchronization- acknowledgment).3. Fran’s computer then sends a packet to the company’s server confirming that it received the packet- ACK

(acknowledgment).

Once Fran’s computer has established a connection with the company’s server, the two devices must “talk” to each other. It’s important for Fran’s computer to make sure the data request arrives at the company’s server and in the correct order.

SYNACK SYN-ACK

ACK

Once this “handshake” is performed and the connection is made, TCP (Transmission Control Protocol) submits HTTP protocol’s request to view the company’s webpage.• Most data is very large, so it gets broken down into smaller pieces of data (called segments) and is given a

sequence number before it is sent to the company’s server. The maximum size of each segment depends on the network transmission being used (like an Ethernet). Once the request to view the webpage gets to the company’s server, the server reassembles the data request and verifies that the data matches the data unit sent by Fran’s computer- Checksum

CHECKSUM

7 Layers of the OSI Model

Physical Link

Receive DataTransmit Data

• This represents the 4th layer of the OSI Model• This layer accepts the data from the Session

layer and manages the delivery of data.• It ensures data arrives reliably in the correct

sequence and without errors.• This information and the segments are added

to the transport header.• Also handles flow control, which estimates the

rate of transmission based on how fast Fran’s computer can receive the information.

• Some Transport layer protocols ensure that data arrives exactly as it was sent (TCP-this is what Fran is using) and others aren’t concerned with reliability (UDP).

• These segments then get passed to the next layer.

Are you getting all

of this?

No, slow down so I can save

accurately!

TCP

Are you getting all

of this?

Who cares, just send it

faster!

UDP• User Datagram Protocol• Transport layer protocol that’s not concerned with

reliability• Useful when data must be transferred quickly• Examples: live streaming of audio or video• Connectionless

• Transmission Control Protocol• Transport layer protocol that reliably

sends the HTTP protocol's request• Connection oriented

Next, each segment must be labeled with logical addressing information so that Fran’s request will be sent to the company’ server through the best possible path (called routing). Once it’s labeled with a unique IP (internet protocol) address, the segments become packets. This states who’s the sender, who the packet is going to, and what route it should take to get there.

Each packet travels through Fran’s router. Her local router will read the IP address on each packet and determines where to send the packets so that they continue in the correct direction (to the company’s server).

Physical Link

Receive DataTransmit Data

7 Layers of the OSI Model

• This represents the 3rd layer of the OSI Model.• This layer accepts the Transport layer segments and adds

logical addressing information (IP address) to the Transport header.

• These segments then become packets.• It also decides how to route the data from the sender

(Fran) to the receiver (the company’s server).• The Network layer then passes the packets to the next

layer.

Physical Link

Receive DataTransmit Data

7 Layers of the OSI Model

Then, a header is added to the front of each packet (data) and a trailer is added to the end of each packet. This is called a frame. Think of a frame as a box that wraps up a package (it just surrounds the data, aka the package).

• This is the 2nd layer of the OSI Model.• This layer packages data into frames.• Frames reduce the possibility of errors or lost

data by using a FCS (Frame Check Sequence). • Each frame consists of the raw data (also

called payload), the sender’s and receiver’s network address (the IP address), and error checking information (FCS).

• Once the data has been verified as “not damaged”, the Data Link layer passes the frames to the next layer.

To access the website, Fran’s computer must be connected to the internet. Her computer is connected to a router, through which she is able to connect to her wireless internet provider (like Cox). The frames then travel through Fran’s router over to her Local Area Network (LAN) until it reaches its destination (the company’s server).

Fran’s computer contains a NIC (Network Interface Card), which allows her computer to connect to a network and communicate with other computers and servers. Each frame then passes through the NIC.

Each NIC contains a Media Access Control (MAC) address, which is a unique identifier. Think of this as your computer’s telephone number.

• This represents the 1st layer of the OSI model.• It consists of all transmission mediums that are required

to move data along a network.

Physical Link

Transmit Data

7 Layers of the OSI Model

Receive Data

Once the frames arrive at your company’s server (all data is transmitted), the server’s physical level accepts the request to view the website. The server then processes your request, reversing the process just described, until it reaches the top of the OSI Model. Fran will then be able to view your company’s website once all data is received by her computer.