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7.3 TCPI / IP Model vs OSI Model

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By So Hee Park 4 min read

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✅ 7.3 TCP/IP Model vs OSI Model

☑️ Models and Protocols

✔️ What is a model?

  • A model is a structure with layers used to explain how the internet works.

✔️ What is a protocol?

  • A protocol is the set of rules followed by each layer.

✔️ Types of models

  • OSI Model (Open Systems Interconnection)

    • Theoretical and more complex model.
    • Not used as much in real internet communication.
    • Structured in 7 layers.

  • TCP/IP Model

    • Most used model in internet connections.
    • Structured in 4 layers.

✅ TCP/IP Model

☑️ Structure of the TCP/IP Model

✔️ 4th Layer — APPLICATION

  • Set of rules explaining how applications work on the internet and how they are designed.
  • Here we have the data.

✔️ 3rd Layer — TRANSPORT

  • Set of rules guaranteeing communication between computers.

  • Here we have the segment:

    • Source port
    • Destination port

✔️ 2nd Layer — INTERNET or NETWORK

  • Set of rules ensuring data pieces find the best path.

  • Here we have the packet:

    • Source IP
    • Destination IP

✔️ 1st Layer — ACCESS LAYER (LINK LAYER)

  • Responsible for transforming information into binary and ensuring bits are sent.

  • Here we have the frame:

    • Source MAC
    • Destination MAC
    • FCS (Final Checksum)

✅ Example with an Email Application

☑️ Data and Segments

  • Suppose the application is an email.
  • The information we want to send is called data.
  • Data is broken into smaller pieces called fragments.
  • When source and destination ports are added, fragments become segments.

✔️ Application Headers

  • Each application is identified with a header attached to its data.

  • Example:

    • Microsoft Office Online → application header.

✅ Ports and Services

☑️ Ports

  • A host (computer connected to the internet) can run multiple applications.
  • Different applications use different portions of the same connection called ports.

✔️ Examples of ports

  • One port for:

    • Email
    • Web pages
    • Files

✔️ Closing ports

  • Closing a port means forbidding one service.

✔️ Firewall

  • The system that opens and closes ports is the firewall.

✅ Packets and IP Addresses

☑️ IP Addresses

  • Many communication portions may use the same port number.
  • To differentiate origin and destination, we use IP addresses.

✔️ Packet

  • When source and destination IP addresses are added to a segment, it becomes a packet.

✔️ IP Characteristics

  • IP is normally related to location, not device.

✅ MAC Address

☑️ MAC Address

  • The MAC address uniquely identifies hardware devices.

✔️ NIC (Network Interface Card)

  • The MAC address is found in the NIC (network card).

✅ Headers

☑️ Purpose of Headers

  • Headers are represented by the baby blue sections in the diagram.
  • Every portion of communication must be identified with a header.

✔️ Types of Headers

  • Header 1: Application header

    • Example: “Microsoft Office Online”

  • Header 2: Service header

    • Indicates source and destination ports.

    • Example:

      • Email service
      • Webpage service

  • Header 3: Network header

    • Contains:

      • Source IP
      • Destination IP

✅ Encapsulation

☑️ Definition

  • Encapsulation is the process of adding headers to each portion of a message.

✔️ Why encapsulation is needed

  • Internet portions do not always follow the same route.
  • Portions need identification to reach the correct destination.

✔️ Routers

  • The router finds the best route for message portions.

✔️ Weak Connections

  • If a portion encounters a weak connection:

    • It may be delayed.
    • It may be lost forever.

✅ FCS (Final Checksum)

☑️ What is FCS?

  • Each portion includes an extra tail called FCS (Final Checksum).

✔️ Purpose

  • Contains extra 0s and 1s for error detection.

✔️ Process

  • Created by the transmitter.
  • Checked by the receiver.

✔️ Controlled Communication

  • FCS values must match.

  • If they do not match:

    • The receiver tries to guess missing information.

✅ Frames

☑️ Frame Definition

  • When source MAC, destination MAC, and FCS are added to a packet, it becomes a frame.

✅ Socket

☑️ Socket Definition

  • A socket is a combination of:

    • IP address
    • Service

✔️ Important Note

  • Intersecting a socket does not mean seeing the message.

  • It means knowing:

    • The person
    • The use/service

✅ De-encapsulation

☑️ Definition

  • De-encapsulation is the process of:

    • Removing headers
    • Removing FCS
    • Rejoining the data to recover the final message

✔️ Important Detail

  • What travels through the wire contains:

    • Headers
    • Portions
    • FCS

✅ Multiplexing

☑️ Definition

  • Multiplexing happens when:

    • Different portions
    • Of different services
    • From different people
    • And different applications
    • Travel together.

✅ Protocols

☑️ Protocol Rules

  • The rules of each layer are divided into different protocols.
  • Protocols act like different books of the same manual.

✅ OSI Model

☑️ General Information

  • The OSI model is more complex than TCP/IP.
  • It contains 7 layers.

✔️ Main Difference

  • In TCP/IP:

    • The Application layer is split into:

      • Application
      • Presentation
      • Session

  • In TCP/IP:

    • The Link layer is split into:

      • Data Link
      • Physical

✅ OSI Model Layers

☑️ 7th Layer — APPLICATION

  • Explains how the application works.

☑️ 6th Layer — PRESENTATION

  • Controls the appearance/aspect of the application.

☑️ 5th Layer — SESSION

  • Differentiates users inside the application.

☑️ 2nd Layer — DATA LINK

  • Responsible for:

    • Adding MAC addresses
    • Adding FCS

☑️ 1st Layer — PHYSICAL

  • Responsible for:
  • Transforming bits into electricity or waves.
DAW bilingual, Computer System
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