Individual Project 1
Sarah 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.
HTTP
Hypertext 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).
7 Layers of the OSI Model
Transmit Data
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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.
Receive Data
Physical Link
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.
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).
Encryption/Decryption
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•
7 Layers of the OSI Model
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Transmit Data
Receive Data
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Physical Link
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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.
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.
7 Layers of the OSI Model
Transmit Data
Receive Data
Physical Link
• 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.
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.
ACK
SYN
ACK
SYN-ACK
CHECKSUM
Fran’s computer and the company’s server perform a “3-step handshake” to establish a connection:
1.
2.
3.
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).
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).
Fran’s computer then sends a packet to the company’s server confirming that it received the packet- ACK
(acknowledgment).
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
7 Layers of the OSI Model
Transmit Data
Who cares,
just send it
faster!
Receive Data
Are you
getting all
of this?
Physical Link
• 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.
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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
Are you
getting all
of this?
•
•
•
No, slow
down so I
can save
accurately!
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).
7 Layers of the OSI Model
Transmit Data
Receive Data
Physical Link
•
•
•
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•
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.
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.
7 Layers of the OSI Model
Receive Data
Transmit Data
Physical Link
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.
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).
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This represents the 1st layer of the OSI model.
It consists of all transmission mediums that are required
to move data along a network.
7 Layers of the OSI Model
Transmit Data
Receive Data
Physical Link
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.