OSI Model#

Note: You can view the packet capture in my github repository. This is meant to be supplementary material for my presentation, so don’t worry if it’s not clear. I will update it later with the full explanation.

The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that “provides a common basis for the coordination of standards development for the purpose of systems interconnection.

The TCP/IP Model is what is widely used in practical networking and forms the foundation of the internet.

For the actual specifications, we can view the RFC’s:

These documents give a structured specification for implementation.

The seven layers of the OSI model#

S.No	Layer	Role	Example
7	Application Layer	interacts directly with the user data, think of it as an interface for the applications to use.	HTTP,FTP,SMTP,SSH
6	Presentation Layer	makes the data presentable for the application layer, responsible for translation,encryption or compression	TLS
5	Session Layer	opening, maintaing and closing communication between the two devices. It synchornizes the data transfer and keeps the connection alive	TCP
4	Transport Layer	responsible for end-to-end communication. This includes taking data from the session layer and breaking it up into chunks called segments before sending it to layer 3.	TCP
3	Network Layer	facilitates data transfer between two different networks, breaks up segments into smaller units called packets. Finds the best physical path(routing)	IP,ICMP
2	Data Link layer	physical addressing on the local network segment, finding where to deliver the data	MAC addresses, ethernet frames
1	Physical Layer	The physical medium/connection between the devices such as cables and switches

The TCP/IP model#

S.No	Layer	Role	Example
4	Application Layer	OSI model’s Application, Presentation, and Session layers.	HTTP, FTP, SMTP
3	Transport Layer	provides reliable communication between devices	UDP,TCP
2	Internet Layer	manages routing, ensuring data packets reach the correct destination.	IP,ICMP
1	Network Access Layer	Also known as the Link layer, it covers physical transmission and access to network media.	Ethernet, Wi-Fi

The TCP/IP model is practical, straightforward, and essential for modern networking, while the OSI model is valued for its clarity and is a powerful tool for diagnosing network issues.

Understanding how protocols work#

They are a standard way for computers to communicate with each other. Just like languages for us, computers have protocols.

They define the “grammar” and structure of the message, such as:

what does this part of the packet signify?
in what structure should I send the message?

We will be directly communicating with the protocol using netcat netcat is a simple unix utility which reads and writes data across network connections, using TCP or UDP protocol. https://www.commandlinux.com/man-page/man1/nc.1.html

The two generals’ problem#

TCP cannot guarantee state consistency between endpoints

A Practical example with Wireshark#

I will be sending a test email to myself on localhost using the SMTP protocol. SMTP works on port 25 by default, lets connect to it using netcat

nc localhost 25

[quix@quixel osi-model]$ nc localhost 25
220 quixel.localdomain ESMTP Postfix
HELO localhost
250 quixel.localdomain
MAIL FROM:<me@localhost> 
250 2.1.0 Ok
RCPT TO:<quix@localhost>
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
subject: test mail

This is a test mail to demonstrate the OSI model
.
250 2.0.0 Ok: queued as 406BF140A8C
QUIT
221 2.0.0 Bye

HELO/EHLO - initiates the SMTP session.
MAIL FROM - senders email address
RCPT TO - receivers mail
DATA - start of the email message body, end with a .
QUIT - end the SMTP session

Mails are usually stored in /var/spool/mail/<username>