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Aisalkyn Aidarova
Aisalkyn Aidarova

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OSI model

#networking #architecture #beginners #devops

🌍 How the Internet Works (and Who Controls It)

1. No one person or company “owns” the internet.

The internet is a global network of networks — millions of private, public, and government networks connected together.
But some organizations coordinate and standardize how everything works so all devices and software speak the same “language.”

🧭 Key Organizations that Control and Standardize It

Organization Full Name Role
ISO International Organization for Standardization Created the OSI model itself to define how network communication should work.
IEEE Institute of Electrical and Electronics Engineers Controls Layer 1 & 2 standards — Ethernet (802.3), Wi-Fi (802.11), Bluetooth, etc.
IETF Internet Engineering Task Force Creates and updates internet protocols like TCP, IP, HTTP, DNS — the “rules” of the internet.
ICANN Internet Corporation for Assigned Names and Numbers Manages domain names and IP addresses (like www.google.com → 142.250.190.46).
IANA Internet Assigned Numbers Authority (part of ICANN) Assigns port numbers, IP ranges, and protocol numbers so they don’t conflict.
W3C World Wide Web Consortium Develops web standards — HTML, CSS, and HTTP behavior for browsers.
Regional Internet Registries (RIRs) ARIN (North America), RIPE (Europe), APNIC (Asia-Pacific), etc. Distribute and manage IP addresses in different world regions.

So, ISO defines the layers,
IEEE builds the hardware standards,
IETF/ICANN/IANA make sure all data travels and resolves correctly,
and W3C ensures web apps display correctly in browsers.

⚙️ How It All Works Together on the Internet

Let’s trace what happens when you open https://www.youtube.com:

  1. Browser (Application Layer)
    Sends a request to find “www.youtube.com”.

  2. DNS (Domain Name System)
    Managed globally by ICANN/IANA — converts domain name → IP address.

  3. Routing (Network Layer)
    Routers (following IETF standards) forward your data across multiple networks using BGP (Border Gateway Protocol).

  4. Transport Layer
    Uses TCP port 443 (HTTPS). These ports are assigned by IANA so all systems agree which service runs where.

  5. Data Link & Physical Layers
    Your Wi-Fi router (using IEEE 802.11) transmits signals, then fiber cables (also IEEE 802.3 Ethernet) carry them to your ISP.

  6. ISP → Internet Backbone → Google Servers
    Your data crosses through many autonomous systems (ASNs) connected globally using IETF-defined protocols.

  7. Server Response
    Google’s server sends back encrypted data via TLS/SSL → decrypted by your browser → shown on your screen.

🧩 Summary – How It’s All Connected

Layer Example Technology Who Sets the Rules
7–5 Application–Session HTTP, HTTPS, DNS, SMTP IETF, W3C
4 Transport TCP, UDP, Port Numbers IETF, IANA
3 Network IP, BGP, ICMP IETF, ICANN, RIRs
2 Data Link Ethernet, Wi-Fi, MAC addresses IEEE
1 Physical Cables, radio, fiber IEEE

🧠 Step-by-Step Simplified OSI Flow

  1. Application Layer:
    You type your message or perform an action in an app (like WhatsApp, email, or a browser).
    → This is the layer you directly interact with as a user.

  2. Presentation Layer:
    Your message is translated into a format suitable for sending and often encrypted (secured).
    → Think of it as converting and locking your message.

  3. Session Layer:
    It keeps your session alive while you’re connected — for example, during your chat or video call.
    → It opens, maintains, and closes the “conversation channel.”

  4. Transport Layer:
    When you hit send, this layer decides how to transport your data:

  • TCP – reliable (for web pages, emails).
  • UDP – fast but less reliable (for videos, gaming). → It also makes sure packets arrive in order.

  1. Network Layer:
    Finds the best path for your data to reach the destination using IP addresses.
    → Like a GPS for your data packets.

  2. Data Link Layer:
    Works inside your local network (like your Wi-Fi).
    It uses MAC addresses to make sure the data goes to the correct device through your router or switch.
    → Like matching a house number on the right street.

  3. Physical Layer:
    Converts your data into electrical signals, light, or radio waves that actually travel through cables or air.
    → The real movement of bits (1s and 0s).

🏠 OSI Model Like Sending a Letter

We’ll match each OSI layer with a real-world mail step, plus the protocols, ports, and controllers in the network world.

Layer 7 – Application

💬 Real-world: You write the letter — your actual message.
📦 Purpose: This is where apps talk to the network.
🧩 Protocols: HTTP, HTTPS, SMTP, FTP, DNS, SSH, Telnet, SNMP
📫 Port Numbers:

  • HTTP → 80
  • HTTPS → 443
  • SMTP → 25
  • DNS → 53
  • SSH → 22
  • FTP → 20/21 ⚙️ Who Controls: The software application (e.g., browser, email app, Jenkins, or curl). 🔗 Connection: It hands your request to the Presentation Layer for formatting or encryption.

Layer 6 – Presentation

💬 Real-world: You translate your letter into another language or encrypt it before sending.
📦 Purpose: Converts, compresses, and encrypts data.
🧩 Protocols: SSL/TLS, ASCII, JPEG, MPEG, JSON, XML
⚙️ Who Controls: The operating system libraries (OpenSSL, GnuTLS) and app frameworks.
🔗 Connection: Feeds ready-to-send data to the Session Layer.

Layer 5 – Session

💬 Real-world: You start a phone call or meeting to coordinate how long you’ll exchange letters.
📦 Purpose: Establishes and maintains communication sessions between two systems.
🧩 Protocols: NetBIOS, RPC, PPTP, SIP, NFS session handling
⚙️ Who Controls: The OS or network software (e.g., managing TCP socket sessions).
🔗 Connection: Works with Transport Layer to ensure an open, valid connection.

Layer 4 – Transport

💬 Real-world: The postal service decides how to deliver (standard mail or express).
📦 Purpose: Handles reliable or fast delivery of data.
🧩 Protocols: TCP (reliable), UDP (fast, connectionless)
📫 Port Numbers:

  • TCP → HTTP (80), HTTPS (443), SSH (22), SMTP (25)
  • UDP → DNS (53), DHCP (67/68), NTP (123) ⚙️ Who Controls: The operating system kernel (Windows TCP/IP stack, Linux networking stack). 🔗 Connection: Gives each app a port number to send and receive data streams.

Layer 3 – Network

💬 Real-world: The postal system figures out which route to take (city, state, country).
📦 Purpose: Routes data between networks.
🧩 Protocols: IP, ICMP, ARP, RIP, OSPF, BGP
⚙️ Who Controls: Routers and Layer 3 firewalls.
🔗 Connection: Sends packets (with source and destination IP addresses) to the next network.

Layer 2 – Data Link

💬 Real-world: The delivery truck moves letters from your street to the local post office.
📦 Purpose: Moves data within your local area (Wi-Fi or Ethernet).
🧩 Protocols: Ethernet, ARP, PPP, VLAN (802.1Q)
⚙️ Who Controls: Switches and NIC cards (Network Interface Cards).
🔗 Connection: Uses MAC addresses to deliver frames to the right local device.

Layer 1 – Physical

💬 Real-world: The roads, mailboxes, and postmen who physically move your letter.
📦 Purpose: Transmits bits (1s and 0s) as signals — electrical, light, or radio waves.
🧩 Examples: Ethernet cables, fiber optics, Wi-Fi, Bluetooth, 4G/5G
⚙️ Who Controls: Hardware manufacturers and drivers.
🔗 Connection: Sends raw signals to another device’s physical port.

🔌 How It All Connects (Simplified Flow)

When you send a web request:

Your browser (App Layer)
↓
Encrypts via TLS (Presentation)
↓
Maintains session (Session)
↓
Chooses TCP port 443 (Transport)
↓
Uses your IP to route (Network)
↓
Encapsulates with MAC address (Data Link)
↓
Sends over Wi-Fi or cable (Physical)
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Each layer adds its own header to the packet (encapsulation),
and the receiver removes them one by one (decapsulation).

🧭 Who Controls What

Layer Controlled By Example
7–5 (App–Session) Software & protocols Browser, email app, API client
4 (Transport) Operating system TCP/UDP sockets
3 (Network) Routers & OS IP routing tables
2 (Data Link) Switches, NIC MAC tables, ARP cache
1 (Physical) Hardware Cables, antennas, fiber

💬 Simple OSI Model Explanation

# Layer Name What It Does Real-Life Example
7 Application This is what you actually use — your app. You type a message in WhatsApp or open a website.
6 Presentation Makes data readable and secure. Your message gets encrypted so no one can spy on it.
5 Session Keeps the connection alive while you chat. The app keeps your chat open with your friend.
4 Transport Delivers your message correctly and in order. If one message fails, it resends it. (TCP/UDP)
3 Network Finds the best route to send it. Like Google Maps finding the fastest road for your data. (IP address)
2 Data Link Sends the message inside your local area (home Wi-Fi). Uses your device’s MAC address to reach the right laptop or phone.
1 Physical The actual wires or signals carrying your data. Wi-Fi waves, fiber cables, or your phone’s 4G/5G signal.

💡 In One Sentence:

When you send a message,

  • The Application creates it,
  • Presentation secures it,
  • Session keeps the chat open,
  • Transport guarantees delivery,
  • Network finds the route,
  • Data Link delivers it locally,
  • Physical carries it through wires or air.

🌐 OSI Model Overview

The OSI (Open Systems Interconnection) model standardizes how network communication happens between devices.
It divides data transmission into 7 layers, each responsible for specific tasks — from sending electrical signals to displaying web pages.

🧩 The Seven Layers (Simplified with DevOps Focus)

Layer Name Key Function DevOps Perspective
1 Physical Sends raw bits over cables, fiber, or radio signals. When setting up servers (on-prem or cloud), DevOps ensures proper networking hardware or virtual network interfaces (ENIs, VPCs).
2 Data Link Organizes bits into frames; uses MAC addresses for local delivery. When configuring EC2 security groups, VPC subnets, or Docker bridge networks, you indirectly work with this layer.
3 Network Routes packets across networks using IP addresses. DevOps configures VPCs, subnets, CIDR blocks, and route tables — all belong here. Tools like ping, traceroute, and ip route operate at this layer.
4 Transport Ensures reliable delivery with TCP or fast, connectionless transfer with UDP. Understanding ports (HTTP-80, HTTPS-443) and load balancers (ALB/NLB) is critical. Monitoring latency or dropped packets (e.g., netstat, ss, curl -v) happens here.
5 Session Manages and maintains communication sessions between applications. In APIs, SSH, or web sockets, DevOps ensures stable sessions using reverse proxies (Nginx) or session persistence in load balancers.
6 Presentation Translates, encrypts, or compresses data (e.g., SSL/TLS, JSON, JPEG). SSL certificates, HTTPS termination, and data encoding in pipelines (base64, gzip) relate to this layer.
7 Application Provides services directly to users (HTTP, FTP, SMTP, DNS). DevOps deploys and monitors applications at this layer—web servers (Nginx, Apache), DNS setups, CI/CD delivery of app code.

🧠 Practical Example — Visiting YouTube

  1. Application: Browser uses HTTP/HTTPS to request the YouTube page.
  2. Presentation: Data encrypted via TLS.
  3. Session: Connection maintained between your browser and YouTube servers.
  4. Transport: TCP ensures reliable delivery of video data.
  5. Network: IP routing directs packets globally.
  6. Data Link: Frames delivered via MAC addresses in your LAN or Wi-Fi.
  7. Physical: Data turned into electrical or radio signals over Ethernet or Wi-Fi.

🧰 Why OSI Matters for DevOps

  • Troubleshooting: Knowing which layer fails helps isolate issues — e.g., DNS (Layer 7) vs. routing (Layer 3).
  • Infrastructure setup: VPCs, subnets, and gateways map to layers 2–3.
  • Security: Firewalls, SSL certificates, and IAM policies relate to Layers 3–7.
  • Monitoring: Tools like Wireshark, tcpdump, netstat, and Prometheus metrics target different OSI layers.

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