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Client-Server Model

  • A client is a thing, a machine, that speaks (sends data to a server or request data from a server) to servers.
  • A server is a thing, a machine, that listens to clients, or listens for clients to speak with, and then it speaks (returns some form of data to the client) back to those clients.
  • The client—server model is a thing made up of a bunch of clients and servers talking to one another.

Question: what happens when you go to a website in your browser?​

  • Your browser (google chrome) is a client, the website is the server (not the actual server)
  • the browser does not actually know what the website is
    • all that it knows is that it can communicate with it (speak to it, request stuff from it)
    • but doesn't actually know what the server represents
    • it will request information from the server
      • and then base on that information (based on the response from the server), it will be able to do stuff
  • when typing the URL bar, at first the browser doesn't even know how to talk to the website (or the server)
    • what it does behind the scenes, actually, is it makes a DNS query to find out what the IP address of the website is
      • only then can the browser really speak directly to the website (the server)
  • once browser knows where the website is with the IP address
    • it sends a HTTP request to the IP address and specify what port to communicate with
      • it basically sends a bunch of bytes or characters that are going to get packed into what we call packets in some special format to the website (servers)
      • the request will also contain the IP address of the browser, or computer
        • this is the called the source address of the request
      • after receiving the IP address information, the server will then know where to send the response to
  • A server usually listens for requests on specific ports
    • any machine that has a distinct IP address has 16,000 ports that programs on the machine can listen to
    • therefore, when communication with another machine, you need to specify what port you want to communicate on
  • most clients actually know the port that they should use, depending on the protocol that they're trying to speak to the server with
    • eg.
      • if the client is trying to speak to a server using the HTTP protocol
        • it is always going to use port 80, which was decided a long time ago
      • if the client is trying to speak to a server using HTTPS
        • it will use port 443
  • after the server receives the request from the browser, the server understands that you are trying to view the HTML of the website
    • so the server will return the HTML of the website in its response to the client
    • the client will receive the response and then renders the HTML on the page for you

DNS query: it is a special request, that goes to a predetermined set of servers to request for the ip address of the URL​

IP address: it is a unique identifier for a machine​

  • all computers connected to the internet, have ways to find public IP addresses, or at least, they have ways to discover routes to those addresses
    • this means that the computers can send data to IP addresses
      • they can send packets of information in the form of bytes, to IP addresses
  • therefore, a website that was granted an IP address could be a cloud provider (such as google cloud platform)
    • so what the website creator did was to go to the cloud platform after creating the website, and requested to be given and to reserve an IP address

HTTP: it is a way to send information, that the server is able to understand​

Terms used​

Client​

  • a machine or process that requests data or service from a server
  • note that a single machine or piece of software can be both a client and a server at the same time
    • for instance, a single machine could act as a server for end users and as a client for a database

Server​

  • a machine or process that provides data or service for a client
    • usually by listening for incoming network calls
  • note that a single machine or piece of software can be both a client and a server at the same time
    • for instance, a single machine could act as a server for end users and as a client for a database

Client-Server Model​

  • the paradigm by which modern systems are designed, which consists of clients requesting data or service from servers and servers providing data or service to clients

IP Address​

  • an address given to each machine connected to the public internet
  • IPv4 addresses consist of 4 numbers separated by dots: a.b.c.d where all 4 numbers are between 0 and 255
    • Special values
      • 127.0.0.1: your own local machine, also referred to as localhost
      • 192.168.x.y: your private network
        • for instance, your machine and all machines on your private wifi network will usually have the 192.168 prefix

Port​

  • in order for multiple programs to listen for new network connections on the same machine without colliding, they pick a port to listen on
  • a port is an integer between 0 and 65, 532 (2^16 ports total)
  • typically, ports 0 - 1023 are reserved for system ports (also well-known ports) and shouldn't be used by user-level processes
    • certain ports have pre-defined uses, and although you usually won't be required to have them memorized, they can sometimes come in handy
      • 22: secure shell
      • 53: DNS lookup
      • 80: HTTP
      • 443: HTTPS

DNS​

  • short for Domain Name System
  • it describes the entities and protocols involved in the translation from domain names to IP addresses
  • typically, machines makes a DNS query to a well known entity which is responsible for returning the IP address (or multiple ones) of the requested domain name in the response