OSI model(Networking series 1)
Getting started with Networking
I believe that the OSI model is explained by so many and to a great length that it will be unwise to cover the same. I would rather like to cover in the context of basic understanding and interview questions surrounding it. There are very few complete implementations of this model, the most widely used models are rather TCP/IP and UDP. So, it brings the question; why bother learning it?…. Anser is that it is still widely used for network references and troubleshooting.
So; to get started, what is the OSI model…..
“The OSI Model (Open Systems Interconnection Model) is a conceptual framework used to describe the functions of a networking system. The OSI model characterizes computing functions into a universal set of rules and requirements in order to support interoperability between different products and software. In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
Created at a time when network computing was in its infancy, the OSI was published in 1984 by the International Organization for Standardization (ISO). Though it does not always map directly to specific systems, the OSI Model is still used today as a means to describe Network Architecture.” * (credit to forcepoint )…
The image here depicts how the model transitions on different abstraction layers. Each layer handles certain tasks and forwards that information to the layer below it. Eventually, the data is pushed from the physical interface to the next hop.
Let’s go quickly over the layers to follow the usage of each of them…
Application layer
The application layer directly interacts with software applications and responsible for protocol negotiation. Protocols such as Http, SNMP, and FTP reside in this layer.
Presentation layer
The presentation layer is responsible for compression and encryption, protocols just as WMV, PNG, etc… reside on this layer.
Session layer
The session layer controls conversations between different computers. A session or connection between machines is set up, managed, and terminated at layer 5. Session layer services also include authentication and reconnections.
Transport layer
The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing, and ultimately the transfer of data between systems and hosts. One of the most common examples of the transport layer is TCP or the Transmission Control Protocol.
Network layer
The network layer is responsible for receiving data path management. The network layer finds the destination by using logical addresses, such as IP (internet protocol). At this layer, routers are a crucial component used to quite literally route information where it needs to go between networks.
Data-link layer
At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.
The data link layer encompasses two sub-layers of its own. The first, media access control (MAC), provides flow control and multiplexing for device transmissions over a network. The second, the logical link control (LLC), provides flow and error control over the physical medium as well as identifies line protocols.
Physical layer
The lowest layer of the OSI Model is concerned with electrically or optically transmitting raw unstructured data bits across the network from the physical layer of the sending device to the physical layer of the receiving device. It can include specifications such as voltages, pin layout, cabling, and radio frequencies. At the physical layer, one might find “physical” resources such as network hubs, cabling, repeaters, network adapters, or modems.
Table format explanation
Wireshark data
This is only HTTP capture as you can see from the filters. It does not capture the TCP/IP communication in detail. However, the unfiltered version may contain information about DNS, TCP/IP handshake also.
