OSI model

[1] The Open Systems Interconnection (OSI) model is a reference model developed by the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." It should be noted that it isn't a protocol that represents how the actual internet works and simply a reference model that many use to explain how networks work. Althought the OSI model is more detailed, the internet instead uses TCP/IP.

reference model (systems, enterprise, and software engineering) abstract framework defining a domain's key functions

[3]

[9]

Its name refers to that it's designed to create an "open" standard where various computer systems (systems) could talk to each other (interconnection), no matter who built them.

[1] In the OSI reference model, a communication system's components are distinguished in 7 abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. The model describes communications from the physical implementation to transmit bits across a transmission medium to the highest-level representation of data of a distributed application. Each layer has functions and semantics and serves a class of functionality to the layer above it and is served by the layer below it. Established, well-known communication protocols are decomposed in software development into the model's hierarchy of function calls.

The Internet protocol suite as defined in RFC 1122 and RFC 1123 is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for the development of the Internet. It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model.

Sending and receiving operations

When a data/frame is sent, it goes through the highest to the lowest layers by being encapsulated (wrapped) by having each layer adding informations to it til it reaches the lowest layer, the physical, where it's converted into bits (electrical signals/radiowaves/light pulses) before being transmitted by the physical layer as either:

as electric signals, for Ethernet cables (1s/0s).
as light pulses, for fiber optics.
as radio waves, for Wi-Fi.

Afterward, the receiving device's physical layer detects those signals and turns them back into bits, after which data is through the sending process in reverse by now going through the lowest level, the physical layer, to the highest layer, the application layer, while having its information being de-encapsulated (unwrapped) throughout each layer.

Mnemonics

The following mnemonics help memorize the orders of the OSI model's 7 layers, either from the layer 1 to 7 or vice versa:

Layer 1 to Layer 7:

"Please Do Not Throw Sausage Pizza Away."
"Please Do Not Touch Steve's Pet Alligator."

Layer 7 to Layer 1:

"All people seem to need data processing."

The 7 layers

The 7 layers explain how data encapsulating and de-encapsulating works throughout each layer. Each layer has its own role, using a specific address, and uses different protocols.

The top layers (7-5) represent what user/applications want. The mid layers (4-3) represent how data is received reliably. The lower layers (2-1) represent how actual hardwares send data.

Most layers add a header to the message, with layer 2 often adding a trailer for error checking.

trailer: extra control data added at end of a data unit, often used for error checking and marking the end of a frame

The headers and other information encapsulated to the message being sent is later de-encapsulated by their respective layers.

The OSI model doesn't limit the number of protocols per layer; most can have dozens or even hundreds of protocols, depending on the network needs.

[D1] Depicting the networking protocols in layers allows the layer to be changed without affecting other layers. E.g., when using a different physical connections (e.g. fibre IO copper), different signals must be sent over the cable, but as long as it interacts in the same way with the layers above it then it can still function.

[2.2] Layer 1 - The physical layer is responsible for transmitting raw data bits across physical connections.
Layer 2 - The data Link layer takes the raw bit from the physical layer and organizes them into frames and ensures they're delivered to the right destinations. It's where the Ethernet mainly occupies.
Layer 3 - The network layer routes and transfers data packets across networks. TCP/IP's IP part is a well-known example of this layer.
Layer 4 - The transport layer handles end-to-end communications between 2 nodes, being the layer where TCP/UDP occupies, which provides end-to-end communication between devices, by dividing datas into small manageable segments and sending them individually, each with a sequence number that's used by the receiver end to reassemble the data in the right order.

Higher layers are said to be where the OSI model loses its practice as they don't really reflect reality. It's sufficient to merge them as 1 layer and consider application protocol, like HTTP, as Layer 7

Layer 5 - The session layer
Layer 6 - The presentation layer
Layer 7 - The application layer

Physical layer

[D1] The physical layer is the lowest and 1st layer, providing mechanical, electrical and other functional aids available to enable or disable, they maintain and transmit bits about physical connections, like electric signals, optical signals (optical fiber, laser), electromagnetic waves (wireless networks) or sound. The techniques used are called technical transmission process. Devices and network components associated to the physical layer, e.g., the antenna and the amplifier, plug and socket for the network cable, the repeater, the stroke, the transceiver, the T-bar and the bus terminator.

This layer defines the means to transmit raw bits IO logical data packets over a physical link connecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physical signal transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. [4] Examples of Layer technologies are ethernet cables, hubs.

The electrical connectors' shapes and properties, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, are specified here.

[2.3]

The physical layer digital bit transfer is accomplished on a wireline or cable-free transmission path. A transmission medium's sharing can be carried out on this layer by static multiplexing and dynamic multiplexing. This needs not only the specifications of some transmission media (e.g., copper cable, fiber optic cable, power grid) and the definition of connectors further elements. Furthermore, it must be resolved at this level, in what way a single bit to be transmitted.

Data link layer

[D1] The data link layer is the 2nd layer, being responsible node-to-node data transfer (moving data frames between directly connected adjacent network nodes of the same LAN/WAN segment), error detection/correction accurate data delivery. It carries out node-to-node data transfer via protocols and techs like Ethernet for LANs and Point-to-Point protocol (PPP) for direct point-to-point connections.

It provides the functional and procedural means to transfer data between network entities.

node-to-node data transfer: data movement between 2 directly connected devices/nodes on same local network segment. Unlike PPP, this is only a networking model/concept
- node: any connected device on a network that can send/receive/forward data. E.g., computer, router, switch, printer
procedural: relating to established set of steps/rules, offical ways to do things
point-to-point protocol (PPP): Layer 2 communication protocol designed to establish direct connection between 2 directly connected network nodes (device). Unlike node-to-node data transfer, this is a layer 2 protocol.

[5] This layer manages MAC (Media Access Control) addresses and is divided into two sublayers: Logical Link Control (LLC) and MAC.

[5]

Network layer

[5] The network (net) layer is the 3rd layer that manages/controls device connections, like maintaining and terminating connections, ensuring data are exchanged efficiently.

[6] The network-to-network connections are done by sending data packets back and forth between various networks. The Internet Protocol (IP) is one of the main protocols here, handling addressing, packaging, and routing data so it moves successfully across networks. [7] This functions like a postal zip code guiding a mail.

[6] Suppose Person A and B connect to the same LAN, and 'A' wants to send 'B' a message. Since 'A" is on the same network as 'B', he could send it directly to 'B''s computer across the network. But if 'B''s is instead on another LAN far away, 'A''s message must be addressed and sent to 'B''s network before it reaches 'B's computer, which is a network layer process.

The following are the net layer's functions:

[8] Logical addressing: Each device on the network must be identified uniquely. So net layer gives an addressing scheme to identify the device. It places each sender and receiver's IP address in the header, which consists of the network and host ID of the network.
Host-to-host data delivery: The nenet twork layer ensures the packet is delivered successfully from the sender to the receiver, ensuring the packet reaches the intended recipient only.
Fragmentation: To transmit the bigger data from sender to receiver, the net layer fragments it into smaller packets. Fragmentation is used as each node has its own fixed capacity for receiving data.
Congestion control: Congestion is defined as a situation where the router can't route the packets properly, causing packets aggregation in the network. Congestion occurs if a big amount of packets flood in a network. Thus net layer controls data packets congestion in it.
- aggregation: accumulation, clustering, gathering together
Routing and forwarding: Routing is the process deciding the path of packets from sender to receiver. It often chooses the shortest path between the sender and the receiver. Routing protocols that are mostly used are path vector, distance vector routing, link state routing, etc.

[5]

Transport layer

[5] The transport layer is the 4th layer provides end-to-end communication services in applications. It ensures full data transfer, error recovery, and flow control between hosts. This layer segments and reassembles data for efficient transmission and provides reliability with error detection and correction mechanisms.

segment (verb; in this case): to divide big data stream into smaller manageable chuncks to be transmitted efficiently across network

Protocols at this layer include TCP and UDP. TCP is connection-oriented and ensures reliable data transfer with error checking and flow control, making it suitable for applications like web browsing and email. UDP is connectionless, offering faster, though less reliable, transmission, suitable for applications like video streaming and online gaming.

[5]

Session layer

[5] The session layer is the 5th layer that manages and controls device connections, like maintaining and terminating connections, ensuring data are exchanged efficiently. Session checkpointing and recovery are also crucial roles.

Protocols operating here are NetBIOS, PPTP, and RPC, which enable reliable communication, especially in complex network environments.

Note: The RPC is actually more in the application layer as it manages the dialogue and connection state between separate programs on various computers, but handles communication during a session.

Its name refers to its job to establish/manage/terminate communication "sessions" between applications/devices, where session is defined as a temporary connection/dialogue between 2 devices/apps to exchange info.

[5]

.Presentation layer

[5] The presentation/syntax layer is the 6th layer that translates data between application layer and the network formats, ensuring datas are readable by other systems.
This layer's key roles are data encryption, formatting, and compression

E.g., it may convert data from ASCII to EBCDIC. Its encryption role ensure data security in transmissions and compression protocols to reduce the amount of data for efficient transmission. This layer's protocols are rules that let software programs talk to each other across a network. E.g., HTTP for web pages, DNS for name lookups, and SMTP for email.

[5]

Application layer

[5] The application (app) layer is the 7th layer, serving as the interface to end-users apps and the underlying network services and provides protocols and services directly that end-user applications use. Its key applications involves resources sharing, remote file access, and network management.

end-user application: software made for final person who really uses it daily
underlying: fundamental, basic, beneath surface

Protocols operating in the application layer are Hypertext Transfer Protocol (HTTP) for web browsing, File Transfer Protocol (FTP) for file transfers, Simple Mail Transfer Protocol (SMTP) for email services, and Domain Name System (DNS) to resolve domain names to IP addresses. The protocols ensure applications effectively communicate to each other and servers over a network.

RPC, here, mainly make remote code execution present remote code execution to feel like local code execution to devs and software programs. IO a programmer manually writing low-level net codes to open a socket, format net packets, handle IP addresses, and parase raw responses, RPC is the middleman handling all that automatically behind the scenes.

References

My documents

[D1] ELE8940: OSI layer overview 2# (Canva)
[D2]

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OSI model

Sending and receiving operations

Mnemonics

The 7 layers

Physical layer

Data link layer

Network layer

Transport layer

Session layer

.Presentation layer

Application layer

References

My documents

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