Day 3 OSI Mpdel & TCP/IP Suite

1. Networking Models

• Provide structure and categorization for networking protocols and standards

• Define rules for how network devices and software should work

• Enable interoperability between different vendors and systems

2. OSI Model (7 Layers)

• Conceptual framework created by ISO

• Divides network functions into 7 distinct layers

• Each layer serves specific functions and communicates with adjacent layers

3. TCP/IP Suite (4 Layers)

• Practical model used in real-world networks

• Developed by US Department of Defense (DARPA)

• Fewer layers than OSI but serves same purpose

OSI MODEL LAYERS (Top to Bottom)

Layer 7 - Application

• Closest to end user

• Interacts with software applications

• Protocols: HTTP, HTTPS, FTP, SMTP, DNS

• Functions: Identifying communication partners, synchronizing communication

• Who uses it: Application developers

Layer 6 - Presentation

• Translates data between application and network formats

• Functions: Encryption/decryption, compression, format translation

• Example: Converting JPEG to network-transmittable format

Layer 5 - Session

• Controls dialogues (sessions) between hosts

• Functions: Establishes, manages, and terminates connections

• Example: Managing connection between web browser and YouTube server

Layer 4 - Transport

• PDU: Segment

• Segments and reassembles data

• Provides host-to-host communication

• Protocols: TCP, UDP

• Breaks large data into smaller segments

• Header added: L4 header

Layer 3 - Network

• PDU: Packet

• Provides connectivity between different networks

• Logical addressing: IP addresses

• Path selection between source and destination

• Devices: Routers operate here

• Header added: L3 header (added to segment)

Layer 2 - Data Link

• PDU: Frame

• Provides node-to-node connectivity

• Defines data formatting for physical transmission

• Detects and possibly corrects Physical Layer errors

• Physical addressing: MAC addresses (separate from Layer 3)

• Devices: Switches operate here

• Headers/Trailers added: L2 header + L2 trailer

Layer 1 - Physical

• PDU: Bit

• Defines physical characteristics of transmission medium

• Specifications: Voltage levels, maximum distances, connectors, cable specs

• Converts digital bits to electrical or radio signals

• Example: Cable types, pin layouts, RJ-45 connectors

  
  

OSI Model Layers (NUMBER + NAME)

1. Physical (Layer 1)

2. Data Link (Layer 2)

3. Network (Layer 3)

4. Transport (Layer 4)

5. Session (Layer 5)

6. Presentation (Layer 6)

7. Application (Layer 7)

TCP/IP Model Layers

1. Link (equivalent to OSI Layers 1+2)

2. Internet (equivalent to OSI Layer 3)

3. Transport (equivalent to OSI Layer 4)

4. Application (equivalent to OSI Layers 5+6+7)

Key Protocols by Layer

• Layer 7: HTTP, HTTPS, FTP, SMTP, DNS, DHCP

• Layer 4: TCP, UDP

• Layer 3: IP, ICMP, OSPF, EIGRP

• Layer 2: Ethernet, PPP, Frame Relay

Devices by Layer

• Layer 1: Hubs, Repeaters, Cables

• Layer 2: Switches, Bridges

• Layer 3: Routers, Layer 3 Switches

• Layer 7: Firewalls (application-layer), Load Balancers

Pitfall #1: Confusing Interaction Types

• ❌ Wrong: Thinking a web browser talking to a web server is adjacent-layer interaction

• ✅ Correct: This is same-layer interaction (both at Layer 7)

• Tip: Adjacent = different layers SAME device; Same-layer = same layer DIFFERENT devices

Pitfall #2: PDU Identification

• ❌ Wrong: Counting headers incorrectly for PDU identification

• ✅ Correct:

  • 1 header (L4) = Segment

  • 2 headers (L4+L3) = Packet

  • 3 headers + 1 trailer (L4+L3+L2+trailer) = Frame

• Tip: Count the layers involved in encapsulation

Pitfall #3: TCP/IP vs OSI Mapping

• ❌ Wrong: Thinking TCP/IP Link layer = only OSI Data Link layer

• ✅ Correct: TCP/IP Link = OSI Data Link + Physical (both layers)

• Tip: TCP/IP combines the bottom two OSI layers

Pitfall #4: Layer Responsibilities

• ❌ Wrong: Thinking Layer 3 provides host-to-host communication

• ✅ Correct: Layer 4 (Transport) provides host-to-host; Layer 3 provides network-to-network

• Tip: Layer 3 = logical addressing and routing; Layer 4 = reliable delivery

Pitfall #5: Network Engineer Focus

• ❌ Wrong: Thinking network engineers work with all 7 layers equally

• ✅ Correct: Network engineers focus on Layers 1-4 (especially 2-3)

• Tip: Top 3 layers (5-7) are for application developers

Pitfall #6: Encapsulation Direction

• ❌ Wrong: Confusing encapsulation with de-encapsulation

• ✅ Correct:

  • Encapsulation = Adding headers going DOWN (Layer 7→1)

  • De-encapsulation = Removing headers going UP (Layer 1→7)

OSI Layer Mnemonics

Top-Down (7→1):

• "All People Seem To Need Data Processing"

• "Please Do Not Teach Students Pointless Acronyms" (reverse)

Bottom-Up (1→7):

• "Please Do Not Throw Sausage Pizza Away"

Layer Functions - Quick Reference

Layer 7 (Application) → User interface, HTTP/HTTPS

Layer 6 (Presentation) → Data translation, encryption

Layer 5 (Session) → Establishes/manages sessions

Layer 4 (Transport) → Segments data, TCP/UDP, HOST-TO-HOST

Layer 3 (Network) → IP addressing, routing, ROUTERS

Layer 2 (Data Link) → MAC addressing, SWITCHES, frames

Layer 1 (Physical) → Cables, signals, bits

"Some People Fear Birthdays"

• Segment (Layer 4)

• Packet (Layer 3)

• Frame (Layer 2)

• Bits (Layer 1)

Headers & Trailers Rule

• Only Layer 2 adds a TRAILER (all others only add headers)

• Trailer is needed for error detection at physical layer

Device Layer Association

• "Really Sounds Heavy" = Routers (L3), Switches (L2), Hubs (L1)

Example 1: Web Browsing (YouTube)

YOU → Web Browser → YouTube Server

Layer 7: Your browser (HTTP) ←same-layer→ YouTube server (HTTP)

Layer 4: Your computer's TCP ←same-layer→ YouTube's TCP

Layer 3: Your IP address ←routing→ YouTube's IP address

Layer 2: Your MAC ←node-to-node→ Router MAC → ... → YouTube MAC

Layer 1: Electrical signals on your cable

Example 2: Encapsulation Process

1. Application creates DATA

2. Transport adds L4 header → SEGMENT

3. Network adds L3 header → PACKET

4. Data Link adds L2 header + L2 trailer → FRAME

5. Physical converts to BITS (electrical/radio signals)

Example 3: Network Engineer vs Application Developer

Network Engineer focuses on:

- Layer 4: TCP/UDP port configurations

- Layer 3: IP addressing, routing protocols (OSPF, EIGRP)

- Layer 2: VLAN configuration, switch settings

- Layer 1: Cable types, physical connections

Application Developer focuses on:

- Layer 7: HTTP, APIs, user interfaces

- Layer 6: Data formatting, encryption

- Layer 5: Session management

A set of rules that categorizes and provides structure for networking protocols and standards, defining how network devices and software should work.

7 layers (Application, Presentation, Session, Transport, Network, Data Link, Physical)

4 layers (Application, Transport, Internet, Link)

Open Systems Interconnection (created by ISO - International Organization for Standardization)

Transmission Control Protocol/Internet Protocol, developed by US Department of Defense through DARPA

Closest to end user; interacts with software applications; identifies communication partners and synchronizes communication

HTTP, HTTPS, FTP, SMTP, DNS (any 3)

No, application developers work with Layer 7. Network engineers focus on Layers 1-4.

Translates data between application and network formats; handles encryption/decryption and data compression

Encrypting data as it is sent and decrypting data as it is received; translating between different application formats (e.g., JPEG, ASCII)

Controls dialogues (sessions) between communicating hosts; establishes, manages, and terminates connections

Managing the connection between your web browser and a YouTube server; maintaining the session while you watch videos

Segment

Host-to-host (end-to-end) communication

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)

Layer 4 header (creating a segment from the data)

Segments and reassembles data for communications between end hosts; breaks large data into smaller segments

Packet

Logical addressing (IP addresses)

Routers (and Layer 3 switches)

Provides connectivity between end hosts on different networks; provides logical addressing (IP); provides path selection (routing)

Layer 3 header (added to the segment to create a packet)

Frame

Physical addressing (MAC addresses) - separate from Layer 3 addressing

Switches and bridges

Node-to-node connectivity (e.g., PC to switch, switch to router, router to router)

It is the ONLY layer that adds both a header (L2 header) AND a trailer (L2 trailer)

Defines how data is formatted for physical transmission; detects and possibly corrects Physical Layer errors; provides node-to-node connectivity

Bit

Hubs, repeaters, cables, physical connectors

Defines physical characteristics of transmission medium (voltage levels, distances, connectors); converts digital bits into electrical or radio signals

Cable specifications, voltage levels, maximum transmission distances, physical connectors (RJ-45), pin layouts

Protocol Data Unit - the specific name for data at each layer of the OSI model

Data (L7-L5) → Segment (L4) → Packet (L3) → Frame (L2) → Bit (L1)

Segment (Layer 4 PDU)

Packet (Layer 3 PDU)

Frame (Layer 2 PDU)

The process of adding headers (and trailers) to data as it moves DOWN the OSI layers (Layer 7 → Layer 1)

The process of removing headers (and trailers) as data moves UP the OSI layers (Layer 1 → Layer 7)

Data → +L4 header = Segment → +L3 header = Packet → +L2 header & trailer = Frame → converted to Bits

Encapsulate (adding headers as data goes down the layers)

De-encapsulate (removing headers as data goes up the layers)

Communication between different layers of the OSI model on the SAME device

Communication between the same layer on DIFFERENT devices (e.g., Layer 7 on your PC talking to Layer 7 on a web server)

Same-layer interaction (both are operating at Layer 7 using HTTP)

Adjacent-layer interaction (different layers on the same device)

1. Link, 2. Internet, 3. Transport, 4. Application

OSI Data Link (Layer 2) + Physical (Layer 1)

OSI Network layer (Layer 3)

OSI Transport layer (Layer 4)

OSI Application (Layer 7) + Presentation (Layer 6) + Session (Layer 5)

TCP/IP is actually used, but the OSI model influences how network engineers think and talk about networks

Layers 1-4 (Physical, Data Link, Network, Transport) - especially Layers 2 and 3

Layers 5-7 (Session, Presentation, Application) - the upper layers

Layer 3 (Network layer)

Layer 2 (Data Link layer)

Layer 3 (Network layer)

Layer 2 (Data Link layer)

Layer 3 (Network layer)

Layer 4 (Transport layer)

"All People Seem To Need Data Processing" (Application, Presentation, Session, Transport, Network, Data Link, Physical)

"Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application)

"Some People Fear Birthdays" - Segment, Packet, Frame, Bits

Frame (Layer 2 PDU) - it has 3 headers and 1 trailer

Layer 3 (Network layer) - routers operate at Layer 3 and use IP addresses

Layer 6 (Presentation layer) - handles encryption/decryption

Layer 4 (Transport layer) - segments and reassembles data

Layer 2 (Data Link layer) - switches operate at Layer 2 and use MAC addresses

Layer 1 (Physical layer) - converts bits to electrical/radio signals

Layer 5 (Session layer) - manages connections/sessions

Host-to-host (Layer 4) = end-to-end between final devices; Node-to-node (Layer 2) = direct link between adjacent devices

Logical addressing (Layer 3) = IP addresses, used for routing between networks; Physical addressing (Layer 2) = MAC addresses, used for local network communication

The trailer is used for error detection (e.g., Frame Check Sequence/FCS) to verify data integrity at the physical layer

Segment = Layer 4 PDU (data + L4 header); Packet = Layer 3 PDU (segment + L3 header)

No, Layer 2 switches operate at the Data Link layer and only read MAC addresses. Only Layer 3 devices (routers) read IP addresses.

Same-layer interaction (both at Layer 7), NOT adjacent-layer interaction

1 header = Segment; 2 headers = Packet; 3 headers + 1 trailer = Frame; Bits = Layer 1

TCP/IP Link layer = OSI Data Link + OSI Physical (both bottom layers)

Layer 4 (Transport) provides host-to-host; Layer 3 provides network-to-network connectivity

No, network engineers focus on Layers 1-4; application developers work with Layers 5-7

TCP and UDP

IP, ICMP, routing protocols (OSPF, EIGRP)

HTTP, HTTPS, FTP, SMTP, DNS, DHCP, SSH, Telnet

HTTPS (HTTP Secure) - operates at Layer 7

The OSI model provides a more detailed conceptual framework that helps network engineers understand and troubleshoot networks, even though TCP/IP is the practical implementation

DOWN the layers (7→1) during encapsulation, then sent across the network

UP the layers (1→7) during de-encapsulation

They are removed/stripped off as data moves up the layers (Layer 1→7)

Smaller segments are easier to send over the network and are less likely to cause transmission problems if errors occur

Error detection - contains Frame Check Sequence (FCS) to verify data wasn't corrupted during transmission

Start with Layer 1 (cables/connections), then Layer 2 (switch issues), Layer 3 (routing/IP), and Layer 4 (TCP/UDP)

Layer 3 (Network layer) - routers operate at Layer 3 and handle packet forwarding

Layer 1 (Physical layer) - handles physical medium and connections

7-Application, 6-Presentation, 5-Session, 4-Transport, 3-Network, 2-Data Link, 1-Physical

Layer 4 = Segment, Layer 3 = Packet, Layer 2 = Frame, Layer 1 = Bit

Routers = Layer 3, Switches = Layer 2, Hubs = Layer 1

IP addresses = Layer 3 (Network); MAC addresses = Layer 2 (Data Link)

Encapsulation = adding headers going DOWN (L7→L1); De-encapsulation = removing headers going UP (L1→L7)

Layers 1-4 (Physical, Data Link, Network, Transport)

TCP/IP Link = OSI 1+2; TCP/IP Internet = OSI 3; TCP/IP Transport = OSI 4; TCP/IP Application = OSI 5+6+7

1. Protocols and Standards

• Protocol: Set of rules defining how data should be communicated between devices

• Standard: Agreed-upon specification describing how a protocol/technology should work

• Key Difference: Standards are vendor-neutral; proprietary protocols are vendor-specific

2. Historical Context

• ARPANET (1969): First packet-switching network

• TCP/IP (1974-1983): Became dominant due to open standards

• Key Date: January 1, 1983 - ARPANET fully switched to TCP/IP

3. Standards Organizations

• IEEE: Local area network technologies (Ethernet 802.3, Wi-Fi 802.11)

• IETF: Internet protocols, publishes RFCs (Requests for Comments)

Layered Model Concepts

Layer 5: Application Layer

• Function: Where applications meet the network

• OSI Equivalent: Layer 7

• Protocols to Memorize:

  • HTTP/HTTPS (Web browsing)

  • FTP (File transfer)

  • SMTP, POP3, IMAP (Email)

  • DNS (Name resolution)

  • TFTP (Trivial File Transfer)

Layer 4: Transport Layer

• Function: End-to-end communication between processes

• Uses: Port numbers

• Key Protocols:

  • TCP: Connection-oriented, reliable

  • UDP: Connectionless, faster, less overhead

• PDU Name:

  • TCP = Segment

  • UDP = Datagram

Layer 3: Network/Internet Layer

• Function: End-to-end delivery across networks (routing)

• Uses: IP addresses

• Key Protocols:

  • IPv4

  • IPv6

  • ICMP

• PDU Name: Packet

• Devices: Routers

Layer 2: Data Link Layer

• Function: Hop-to-hop delivery on local network

• Uses: MAC addresses

• Key Protocols:

  • Ethernet (IEEE 802.3)

  • Wi-Fi (IEEE 802.11)

• PDU Name: Frame

• Devices: Switches

• Unique Feature: Only layer with both header AND trailer

Layer 1: Physical Layer

• Function: Transmit bits as signals

• Types: Electrical, optical, radio signals

• Components: Cables, connectors, NICs

• PDU Name: Bits

1. PDU Name Confusion

❌ WRONG: Calling everything a "packet" ✅ CORRECT:

• Layer 4 = Segment (TCP) or Datagram (UDP)

• Layer 3 = Packet

• Layer 2 = Frame

• Layer 1 = Bits

2. Layer Numbering

❌ WRONG: Thinking Application is always Layer 5 ✅ CORRECT: In OSI terms, Application = Layer 7

3. Encapsulation Direction

❌ WRONG: Thinking encapsulation happens going UP ✅ CORRECT: Encapsulation = DOWN the stack, Decapsulation = UP the stack

4. What Switches Count As

❌ WRONG: Switches create new hops ✅ CORRECT: Switches extend the local network; routers create hops

5. Layer 2 Header/Trailer

❌ WRONG: All layers have trailers ✅ CORRECT: Only Layer 2 has a trailer (for error checking)

6. TCP vs UDP PDU Names

❌ WRONG: Both create segments ✅ CORRECT: TCP = Segment, UDP = Datagram

A set of rules defining how data should be communicated between devices over a network.

An agreed-upon specification that describes how a protocol or technology should work (vendor-neutral).

Transmission Control Protocol/Internet Protocol

The process of adding headers (and trailers) to data as it moves DOWN the protocol stack.

The process of removing headers (and trailers) as data moves UP the protocol stack.

1. Physical

2. Data Link/Local Network

3. Network/Internet

4. Transport

5. Application

Transmits bits as electrical, optical, or radio signals over the physical medium.

Provides hop-to-hop delivery on a local network using MAC addresses.

Provides end-to-end delivery between hosts across networks using IP addresses and routers.

Provides end-to-end communication between application processes using port numbers.

Where network communications meet applications; defines how applications format, send, and interpret data.

Frame

Packet

Segment

Datagram

Bits

Layer 2 (Data Link) - trailer is for error checking

MAC (Media Access Control) address

IP address

Port number

Switch

Router

IEEE 802.3

IEEE 802.11

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)

IETF (Internet Engineering Task Force)

Request for Comments

January 1, 1983

7 layers

ISO (International Organization for Standardization)

1. Physical

2. Data Link

3. Network

4. Transport

5. Session

6. Presentation

7. Application

Because it corresponds to the OSI model's Layer 7 (Application), which is the common reference model.

When each layer provides a service to the layer above it and is serviced by the layer below it (vertical interaction).

When the same layer on different devices communicates with each other (horizontal interaction).

One step along the path between two devices (from one router/host to the next router/host). Switches don't count as hops.

The contents encapsulated by that layer's header/trailer (everything "inside" that layer).

No, routers normally operate based on Layer 3 (IP) information only.

Port 80

HTTPS (uses port 443)

Port 21

SMTP (sending), POP3 (receiving), IMAP (receiving)

Application Layer (Layer 5/7)

TCP is connection-oriented and reliable; UDP is connectionless, simpler, and faster with less overhead.

A protocol is the actual set of rules; a standard is the agreed-upon specification describing how it should work.

It was published as open standards that any vendor could implement and could run over many network types.

Each layer adds its header (L4, L3, L2), and L2 adds a trailer, creating: Data → Segment → Packet → Frame

Source and destination MAC addresses for the next hop

Source and destination IP addresses for end-to-end communication

Source and destination port numbers for process-to-process communication

L2 header first, then L3 header, L4 header, data, and L2 trailer last

1. Master PDU names - This is heavily tested

2. Understand encapsulation flow - Know what happens at each layer

3. Memorize IEEE standards - 802.3 and 802.11 appear frequently

4. Know which devices operate at which layers

5. Understand the difference between hops and segments

6. Remember Layer 2 is the only layer with a trailer