OSI Model

Tags

security

Provides the physical description of transferring data between two hosts in a network. When we refer to a layer in discussions about the internet, TCP, UDP, etc. we mean a layer as described in the OSI model.

OSI Layers

The model divides the transference of data into several layers.

Physical Layer

Is the lowest layer of the model in charge of transferring and receiving unstructured raw data between the machine and a physical transmission medium. This layer concerns itself with voltage levels, timing of voltage changes, and other physical qualities related to the transmission of data across a wire.

Data Link Layer

Provides a link between two directly connected nodes. Also correcting errors found in the physical layer. This layer establishes a protocol for forming and ending a connection between two physically connected devices and also specifies a protocol for flow control between them.

This layer is where Ethernet and WiFi live.

Medium Access Control

Is a sub-layer responsible for controlling how devices in a network gain access to a medium and the permission to transmit data.

Logical Link Control

Is a sub-layer responsible for identifying and encapsulating network layer protocols and controlling error checking and frame synchronisation.

Network Layer

Specifies a means to transfer packets from one node/endpoint to another connected in different networks given just a destination address and the message. If the message is too large to be transferred through the data-link layer the network may split the message into several smaller fragments to be sent independently and re-assembled at the destination.

Warn: This layer doesn't guarantee reliability. Packets can be lost or re-assembled out of order. A protocol may provide reliability but it's not a requirement.

The is the layer where the internet protocol is defined.

Transport Layer

Provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host, while maintaining the quality of service functions.

The transport layer creates segments out of the messages received from the application layer. It uses this to either determine which packets failed to transmit and then resend them, or send acknowledgement responses back to the source to clarify the packet was received.

This is the layer where TCP and UDP is defined.

Session Layer

Controls the dialogues between hosts by establishing, managing and terminating the connections between the local and remote applications. This layer is also responsible for check-pointing and recovery but this is commonly implemented explicitly in the RPC interface of an application.

Presentation Layer (AKA Syntax Layer)

Converts data between the application and network formats. This can include compression, decryption or decoding depending on how the data was transferred.

Application Layer

Is the OSI layer closest to the end user. This layer generally involves identifying communication partners, determining resource availability and synchronising communication between them.

This is the layer where the HTTP, FTP, SMTP, SSH, Bitcoin, BitTorrent, and NTP protocols are defined.