3. OSI MODEL & TCP/IP SUITE

What is a networking model?

Networking models categorize and provide a structure for networking protocols and standards.

(Protocols are a set of logical rules defining how network devices and software should work)

OSI MODEL

• Open Systems Interconnection Model
• Conceptual model that categorizes and standardizes the different functions in a network.
• Created by the "International Organization for Standardization" (ISO)
• Functions are divided into 7 "Layers"
• These layers work together to make the network work.

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As data moves from the top layer, downward, the process is called “encapsulation”

As data moves from the bottom layer, upward, the process is called “de-encapsulation”

When interactions occur on the same layer, it’s called “same-layer interaction”

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Mnemonic to help remember the Data Layer Names / Order

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The layers are :

7 - APPLICATION

• This Layer is closest to end user.
• Interacts with software applications.
• HTTP and HTTPS are Layer 7 protocols

Functions of Layer 7 include:

• Identifying communication partners
• Synchronizing communication

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6 - PRESENTATION

• Translates data to the appropriate format (between Application and Network formats) to be sent over the network.

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5 - SESSION

• Controls dialogues (sessions) between communicating hosts.
• Establishes, manages, and terminates connections between local application and the remote application.

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Network engineers don't usually work with the top 3 layers. Application developers work with the top layers of the OSI model to connect their applications over networks.

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4 - TRANSPORT

• Segments and reassembles data for communication between end hosts.
• Breaks large pieces of data into smaller segments which can be more easily sent over the network and are less likely to cause transmission problems if errors occur.
• Provides HOST-TO-HOST (end to end) communication

When Data from Layer 7-5 arrives, it receives a Layer 4 Header in the Transport layer.

<< DATA + L4 Header >>

This is called a SEGMENT.

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3 - NETWORK

• Provides connectivity between end hosts on different networks (ie: outside of the LAN).
• Provides logical addressing (IP Addresses).
• Provides path selection between source and destination
• ROUTERS operate at Layer 3.

When Data and the Layer 4 Header arrive in the Network Layer, it receives a Layer 3 Header.

<< DATA + L4 Header + L3 Header >>

This is called a PACKET.

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2 - DATA LINK

• Provides NODE-TO-NODE connectivity and data transfer (for example, PC to Switch, Switch to Router, Router to Router)
• Defines how data is formatted for transmission over physical medium (for example, copper UTP cables)
• Detects and (possibly) corrects Physical (Layer 1) errors.
• Uses Layer 2 addressing, separate from Layer 3 addressing.
• SWITCHES operate at Layer 2

When the Layer 3 Packet arrives, a Layer 2 Trailer and Header are added.

<< L2 Trailer + DATA + L4 Header + L3 Header + L2 Header >>

This is called a FRAME.

All the steps leading up to transmission is called ENCAPSULATION. When the frame is sent to the receiver, it then goes through the reverse process, DE-ENCAPSULATION, stripping off layers while travelling from OSI Layer 1 to Layer 7.

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1 - PHYSICAL

• Defines physical characteristics of the medium used to transfer data between devices. For example : voltage levels, maximum transmission distances, physical connectors, cable specs.
• Digital bits are converted into electrical (for wired connections) or radio (for wireless connections) signals.
• All of the information in SECTION 2 (NETWORKING DEVICES) is related to the Physical Layer

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OSI MODEL - PDU's

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A PDU is a Protocol Data Unit. Each step of the process is a PDU.

<< L2 Trailer + DATA + L4 Header + L3 Header + L2 Header >>

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TCP/IP Suite

• Conceptual model and set of communications protocols used in the Internet and other networks.
• Known as TCP/IP because those are two of the foundational protocols in the suite.
• Developed by the US Dept. of Defense through DARPA (Defense Advanced Research Projects Agency).
• Similar structure to the OSI Model, but fewer layers.
• THIS is the model actually in use in modern networks.
• • Note : The OSI Model still influences how network engineers think and talk about networks.

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Layer Interactions

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Adjacent-Layer Interactions:

• Interactions between different layers of the OSI Model on same host.

Example:

Layers 5-7 sending Data to Layer 4, which then adds a Layer 4 header (creating a SEGMENT).

Same-Layer Interactions:

• Interactions between the same Layer on different hosts.
• The concept of Same-Layer interaction allows you to ignore the other layers involved and focus on the interactions between a single layer on different devices.

Example:

The Application Layer of YouTube's web server and your PC's browser.