Osi Model

The OSI model is a framework for understanding how communications work in a computing system. It is an abstract representation, since no attention is paid to the implementation details of each layer. Instead, each layer simply describes its function and purpose. Put another way, it defines what input it expects, and what output it gives.

Since each layer interfaces directly with the layers above and below it, the consistence of the input and output offered by each layer is the only thing that matters (ex. as long as L1 receives 0's and 1's and delivers frames, all other details are inconsequential)

Crucial to understanding how the OSI Model works is the fact that each layer only communicates with that same layer on the other end of the interaction. Layer 7 data is only interpreted by layer 7 on the receiving end of the communication; the other layers on the receiving end merely pass the data up to layer 7. Similarly, the IP header data that is appended to data packets in layer 3 on one side is only read and interpreted by layer 3 on the other side.

Analogy: Trophic Levels

The OSI model can be related to the food chain.

At trophic level 1 we have the producers (the autotrophs), such called because they produce their own energy. In other words, they turn inorganic matter into organic matter. Many different animals count themselves as part of this group, and they serve the layer above them (ie. the heterotrophs that consume autotrophs). The point to note here, is that it doesn’t matter if any one species performs the role of the autotroph. As long as it gets performed, the whole system works. Likewise for all other trophic levels in the chain.

Protocol Data Unit (PDU)

each layer of the OSI model has the concept of a Protocol Data Unit, which is the format that the data exists in within the current Layer. In other words, it is what an atomic unit of data is called at each layer.

All PDUs are composed of a header and payload

The Protocol Layers

L7 Application

consists of network applications and their application-layer protocols
although known as the application layer, it is not the user interface of the applications themselves. Rather, layer 7 provides functionalities and services that user-facing software applications use to present data.
- If an application is like a house, then layer 7 is the foundation, not the house itself.
primary user interface with communication system.
PDU - messages
HTTP, FTP, DNS, SMTP, POP3, SSH, IRC, TLS/SSL, NFS (network FS)

L6 Presentation

Supports the functionality of the application layer by providing services such as formatting and translation of data.
provide data encryption and data compression.
Data representation (compression, decompression) and encryption
ex. SSL, SSH, IMAP, FTP, TLS, MPEG, JPEG

L5 Session

Maintains the transmission path by synchronizing packets and controlling access to the medium by the Application layer.
controls the connections between computers
provides for delimiting and synchronizing of data exchange, including the means to build
ex. API, sockets, HTTP sessions

L4 Transport

Ensures the quality of transmission and determines the best route for transmission of data using the Network layer below.
concerned with providing reliable communication over an unsecured network
PDU - segments (TCP), datagrams (UDP)
goal: deliver the data to the right software application
ex. TCP, UDP

L3 Network

Finds a route for transmission of data (packets) between 2 routers/hosts, and establishes and maintains the connection between two connected nodes.
goal: pass data chunks over multiple connected networks
PDU - packet
ex. IPv4, IPv6, ICMP, ARP, NAT

L2 Data Link

Creates, transmits, and receives packets. Controls the Physical layer.
Concerned with sharing multiple access channels
They are used to deliver frames on a LAN
PDU - frames
goal: organize the 1s and 0s into chunks of data, and get them to the right place on the wire.
ex. wifi, ethernet, bluetooth, VLAN, port forwarding procol

L1 Physical

Converts data into bits for transmission and converts received bits into usable data for the layers above it.
PDU - bits
goal is to send 0s and 1s across a wire
ex. fiber optic, copper wire, coaxial cable, wireless, modem, repeaters, ethernet (physical portion), USB

The Internet Protocol

The IP stack consists of L1, L2, L3, L4, L7
When an HTTP request is sent, the protocol is established by piggybacking on the TCP connection that had already been made. This TCP connection is enabled by following the internet protocol. This is the point at which the internet protocol determines which routes datagrams
The fact that there are 2 layers that are openly missing from the internet stack poses an interesting question: why are they not there? The reason is that the internet leaves these layers up to the application developer. The application developer can use any implementation of L5 and L6 that they choose in order to achieve their goals.

Routers

As data is sent upstream from a networked-device, it starts at L7 and makes its way down to L1. At L1, it is connected to the link-layer switch and goes up to L2, before going back down to L1. Then it reaches the router, which goes up to L3, then back down to L1, to be repeated depending on the number of subsequent routers. At the final router, the the router's L1 communicates with the L1 of the destination host, as it makes its way back up to L7.

Airplane trip analogy

As we look at the process of planning and taking an airplane, it becomes apparent that there are different layers to the entire process. In fact, each layer appears two times in the whole process— in reverse order:

buy ticket
check baggage
load at gate
takeoff on runway
airplane routing (travel)
land of runway
unload at gate
pickup baggage
complain about ticket.

each layer implements some functionality, and we can see that the opposite action was performed in reverse order.

We can also see that each layer provides service to the layer below it.
- ex. the act of checking baggage only makes sense to a ticketed person.
- ex. the idea of unloading at a gate only makes sense to a person on a landed plane.
we notice that we can replace any layer in the model, as long as the functionality remains the same. In this way, layers provide consistent interfaces to each other, and don't care about each other's implementation— only the outcome (ie. output) it provides.

Encapsulation

as data travels each layer from L7 down to L1, additional information is added. As we pass the information encapsulated in the HTTP request down to L4, the transport layer takes the information of L7 and adds its own information to it. This information that was added is then used by the transport layer of the next node in the chain (likely the destination host). This process of encapsulation continues on down layer by layer until L1.

This idea of encapsulation thus demonstrates what is fundamentally different between each PDU: a message (L7) is an encapsulated datagram (L4). In other words, the datagram encapsulates the message. A datagram is fundamentally a message, plus some other information (provided by the layer). Therefore, at each layer, the PDU has 2 types of fields: header fields, and payload fields (the payload is just a packet from the layer above).

Miscelaneous

the very fact that there are layers means that we can treat it as a modular chain, and swap out one L3 implementation for another (such as wifi for ethernet)
- therefore L2 doesn't care if we are using IP or IPX on L3, just like L3 doesn't care if L2 uses wifi or ethernet
The price we pay by layering is that we have to map between 32 bit IP addresses (L3) and 48 bit MAC addresses (L2).
- The ARP protocol exists to solve this very problem
The OS may be a participant in any or all of the layers
- ex. at L1, signal processing can be offloaded to a host CPU and that requires a driver which interfaces with the operating system