Network

HTTP

Method	Description
`GET`	Used to request data.
`HEAD`	Requests the headers that would be returned.
`POST`	Send data to the server.
`PUT`	Creates a new resource or replaces target resource with the request payload.

Method	Description
`2xx`	Success Code
`200 OK`	Success
`204 No Content`	The request was successfully processed, but there is no content. The headers may be useful.
`3xx`	Redirection code
`301 Moved Permanently`	Resource yas moved to new URL.
`304 Not Modified`	Used for caching purposes. The response hasn’t been modified.
`4xx`	Client error code
`400 Bad Request`	Server won't process due to client error.
`401 Unauthorized`	The user doesn’t have valid authentication credentials to get the requested resource.
`403 Forbidden`	The client doesn’t have access rights to the content.
`404 Not found`	Cannot find the resource.
`405 Method Not Allowed`	Target resource doesn't support this method.
`429 Too Many Request`	Rate limit error
`5xx`	Server error code
`500 Internal Server Error`	The server has encountered an unexpected error and cannot complete the request.
`502 Bad Gateway`	The server acts as a gateway and gets an invalid response from an inbound host.
`503 Service Unavailable`	The server is unable to process the request. This often occurs when a server is overloaded or down for maintenance.
`504 Gateway Timeout`	he server was acting as a gateway or proxy and timed out, waiting for a response.

Image proxy

(Forward) Proxy	Reversed Proxy
Sit between user and Internet. Forward request on behalf of user	Sit between Internet and server. Receive request on behalf of server
Avoid browsing restriction. Block certain content. Protect user identify online	Load balancing. Protect DDOS attack. Cache static content. Encrpyt and decrypt SSL communication

OSI 7 layers

Physical Layer
- Function: Transmits raw bit streams over a physical medium.
- Examples: Cables, switches, hubs, and other hardware elements.
Data Link Layer
- Function: Provides node-to-node data transfer, error detection and correction, and frame synchronization.
- Examples: Ethernet, PPP (Point-to-Point Protocol), MAC addresses.
Network Layer
- Function: Manages device addressing, tracks the location of devices on the network, and determines the best way to move data.
- Examples: IP (Internet Protocol), routers.
Transport Layer
- Function: Ensures complete data transfer, error recovery, and flow control. It segments and reassembles data for communications between end-to-end nodes.
- Examples: TCP (Transmission Control Protocol), UDP (User Datagram Protocol).
Session Layer
- Function: Manages sessions between applications, establishing, maintaining, and terminating connections.
- Examples: NetBIOS, RPC (Remote Procedure Call).
Presentation Layer
- Function: Translates, encrypts, and compresses data. It ensures that data is in a usable format and is presented to the application layer correctly.
- Examples: SSL/TLS, JPEG, GIF.
Application Layer
- Function: Provides network services directly to applications. It interfaces with software applications and provides services such as email, file transfer, and web browsing.
- Examples: HTTP, FTP, SMTP.
Mnemonics: Please Do Not Throw Sausage Pizza Away

User enter website URL.
Browser resolve DNS domain name to retrieve IP address.
Browser initialise TCP connection. Establish 3-way handshake.
Perform SSL/TLS handshake for HTTPS. Have certification verification, key exchange.
Browser send HTTP GET request to fetch homepage.
Browser rendering web page. Loading html, javascript, css, images.
User interact with the web page. Browsing product, cart which may trigger additional HTTPS request to server and restart process.
If there is require user login and authentication. User enter credential and send via HTTPS POST request.
Server then validate and establish session for user.

How HTTPS works

Feature	TCP	UDP
Connection	Connection-oriented Perform 3-way handshake	Connectionless
Reliability	Reliable Use acknowledgement, retransmission	Unreliable no acknowledgement
Header size	Bigger	Smaller
Speed	Slower due overhead	Faster due minimal overhead
Use case	Web browsing, email, file transfer	Streaming, gaming, broadcast