Introduction to The OSI Model

15 Feb

Summary

If you just want a very short summary of the different OSI models, this is for you! Below will show you each layer of the OSI model and a short description of what is important to know.

Application Layer (Layer 7) - Delivers appropriate formatted payloads to a correct instance of an application. It also includes protocols like HTTP, SNMP, DNS. Information travelling through this layer is referred to as data.
Presentation Layer (Layer 6) - Converts data into different formats. Compression and encryption take place at this layer. Includes formats such as MP3, JPEG, GIF. Information travelling through this layer is referred to as data.
Session Layer (Layer 5) - Establishes and maintains communications. Includes protocols such as PAP and RPC. It uses requests and responses. Information travelling through this layer is referred to as data.
Transport Layer (Layer 4) - Applies flow control and error detection and sequencing. Includes protocols such as TCP and UDP. Information travelling through this layer is referred to as segments.
Network Layer (Layer 3) - Responsible for logical addressing and routing. Devices such as routers and Layer 3 switches operate here. Includes protocols such as IPv4, IPv6, IPX, OSPF, EIGRP. Information travelling through this layer is referred to as packets.
Data Link Layer (Layer 2) - Defines how devices communicate. Also responsible for managing physical addressing and switching (MAC addresses). Devices such as switches and bridges. Includes protocols such as Ethernet, Frame Relay, Token Ring, PPP, and CDP. Information travelling through this layer is referred to as frames.
Physical Layer (Layer 1) - Defines how bits are passed over a medium. They can be passed electronically, mechanically, optically or by radio signals. Media includes coaxial cables, twisted-pair copper cables and fibre-optic cables. This layer also includes the connectors used to connect cables to the devices. Devices include NICs, hubs and repeaters. This layer forwards bits to the next hop in the network. Protocols such as Ethernet, USB and ADSL are used. Information travelling through this layer is referred to as bits.

Application Layer

This layer determines whether adequate resources exist for communication. It also manages communications between apps and directs data to the correct program. It is responsible for converting data into a format usable by applications and directing that data to the proper application window.

If multiple instances of an application exist, this layer ensures that data is delivered to the correct instance. Some protocols used by this layer include:

HyperText Transfer Protocol (HTTP) - Transfer web pages over the internet
File Transfer Protocol (FTP) - Transfer files over a network
Trivial File Transfer Protocol (TFTP) - transfer files over a network (simpler than FTP)
Dynamic Host Configuration Protocol (DHCP) - assigns IP addressing information to clients
Domain Name System (DNS) - translates hostnames to IPs (eg google.com to 142.250.187.238)
Simple Mail Transfer Protocol (SMTP) - Send email messages
Post Office Protocol (POP3) - Receive email messages
Telnet - Creates terminal connection to remote devices (no encryption)
Secure Shell (SSH) - Creates secure terminal connection to networked devices

Presentation Layer

This layer is responsible for converting and representing the payload in different formats. For example, data-based, character-based, image-based, audio-based, video-based etc.

Compression and encryption are often handled by this layer. Some formats used by this layer include:

Graphics Interchange Format
Joint Photographic Experts Group
Motion Picture Experts Group
QuickTime

Session Layer

This layer is responsible for establishing, maintaining and terminating data communications between applications or devices.

Sessions are made up of requests and responses. It identifies the data as belonging to a particular session and ensures that the requests and responses are sent back and forth between the two parties.

Protocols that operate at this layer include:

Password Authentication Protocol (PAP) - Authentication protocol that uses a simple user and password pair
Remote Procedure Call - Allows clients to initiate a process that is executed on a remote server

Transport Layer

This layer is responsible for error-free delivery of information between devices. It is also responsible for flow control and sequencing. The information traversing this layer is called a segment.

Protocols that operate at the transport layer include:

User Datagram Protocol (UDP) - provides connectionless, unreliable data transfer between networked computers

Transmission Control Protocol (TCP) - provides connection-oriented, reliable data transfer between networked computers

Network Layer

This layer is responsible for logical addressing and routing on a network. Logical addressing methods include those defined by IPv4 and IPv6. The information traversing this layer is called a packet.

Examples of protocols that are used at this layer include:

IPv4 - Used to uniquely identify devices on a network

IPv6 - Used to uniquely identify devices on a network

Open Shortest Path First (OSPF) - Link State-Routing protocol

Enhanced Interior Gateway Routing Protocol (EIGRP) - Cisco created hybrid routing protocol

Data Link Layer

This layer defines how devices communicate over a network. It is responsible for managing physical addressing and switching on a network. Physical (MAC) addresses are handled here. Information traversing this layer is called a frame.

Data Link layer devices include switches and bridges. Switching is handled at this layer because switches use physical addresses to forward packets to the correct port.

Some portions of the 802.11 wireless standard function at this layer and some portions at the physical layer.

Protocols that operate at the Data Link layer include:

Ethernet
Frame Relay
Point-to-Point Protocol (PPP)
Cisco Discovery Protocol (CDP)

Physical Layer

This layer defines how bits are passed over a medium. They can be passed electronically, mechanically, optically or by radio signals. Media can include:

Coaxial cable
Twisted-pair copper cable
Fibre-optic cable

This layer includes the connectors used to connect the cables to the devices that operate here. This layer also passes bits between the Data Link and Physical devices on a network.

Examples of devices that operate here are:

Network Interface Cards (NIC)
Hubs
Repeaters

The devices that operate at this layer receive and forward bits to other devices without making any path determination about the bits. They simply forward the bits to the next hop in the network.