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GENERAL KNOWLEDGE /
The connection between computers is built up by protocols. A protocol defines rules. If a device follows these rules, can freely distribute or receive data from the internet.
The internet protocols form a hiearchical system of layers.
FIGURE OF PROTOCOL LAYERS:
Upon realizing communication, a stack of these layers are used by a device.
FIGURE OF AN EXAMPLE FOR A PROTOCOL STACK:
The internet contains all the devices, connections and services as a whole.
The World Wide Web (WWW) is a part of it.
1. PHYSICAL LAYER
The physical layer is formed by physical connections and the connected devices.
Devices can be rooters, switches, computing devices (PCs, laptops, smartphones etc.).
The physical connection between the devices can be hard (wired with cables) and radiowave-based (wireless, e.g. wifi).
Cables can be LAN (or UTP, Ethernet) or optical (fiber).
A local network is called LAN (Local Area Network, e.g. in a building) and WAN (Wide Area Network, the outer internet).
When a network element is connected to the network, the nearest rooter will detect it and record it in its a look-up table. When a message arrives to the rooter, targeted this device, the rooter will send the data directly to the right direction.
The proper internet is redundant: data can reach the destination if a connection is cut on alternative routs.
2. NETWORK LAYER (IP)
This layer is gives two things:
gives an identifier (IP address) to a device (IPv4 or IPv6)
defines the Internet Protocol (IP)
The IPv4 contains four four-digit numbers like 326.65.226.45. Usually the first two tags defines the domain to which the LAN is connected (so the internet service provider's or ISP's identifier in the WAN). The latter parts defines the device within the LAN, these numbers are called subnet masks.
Behind a rooter it is possible to define a whole new subnet with unique starting numbers like 192.168.0.1.
The IPv6 contains hexadecimal numbers like 32A6:07B2:3265:45F1:6545:0548:1202:3214.
The IP protocol defines that the additionally to the data metadata must be attached which includes the source and destination IP numbers.
3. TRANSPORT LAYER (TCP, UDP)
This layer is above the IP layer.
These protocols demands that the data must be sliced to packages and these packages must be sent away.
The two main types:
User Datagram Protocol (UDP): is for fast data transfer but sensitive for data loss
Transmission Control Protocol (TCP): slower but not sensitive for data loss (makes the lost package re-sendable)
They both contain the sender's and the receiver's IP numbers plus a checksum and the package number.
The UDP is used mostly for streaming where data loss is not an issue.
The TCP contains flags like send, acknowledge, received, lost, finish, stop, plus length of package and package number. With these, upon sending, a 3-way handshake happens first (send, acknowledge, acknowledge) with empty data in the package, then the packages are sent away, and when finished, a closing communication comes (finish, acknowledge-finish, acknowledge).
The packages can be merged by following the package numbers.
If a package is lost, the receiver send a package without data to the sender with the missing package number, which will be re-sent.
The receiver also communicates if a package arrives.
If the TCP is above the IP, is marks as TCP/IP.
4. APPLICATION LAYER (HTTP, FTP, SMTP, POP, IMAP, TLS, DNS)
These protocols define how the purpose and processing mechanism of the data:
Hypertext Transfer Protocol (HTTP) is for websites
File Transfer Protocol (FTP) for file transfer
SMTP, POP, IMAP for email services
Transport Layer Security (TLS) adds a security layer (HTTPs)
Domain Name System (DNS) connects the user-friendly URL with IP numbers; represented by dedicated servers
The HTTP operates with requests (GET, POST, PUT etc.).
5. SCALABILITY
Scalability means that a systems is prepared to handle increasing data traffic. The trend can be step-by-step or a peak, the amount of communicated data can increase by thousandfold or more in some cases.
A web application can be scalable only if the network under it is scalable as well.
When a system is built, for scalability, the engineers launch a test phase in which the system is put under pressure from a network of controlled computers which monitoring the network.
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