SERVER, CLIENT, and HTTP

Client-server architecture

• World Wide Web (WWW) uses the client-server architecture
  • Clients obtain service from a centralized server
  • Server waits for client requests and make response

The OSI network model

• communication between clients and servers can be seen in multiple layers
  • Abstraction è reducing complexity of problems to smaller ones
  • Division of labour

THE OSI NETWORK MODEL

 

Communication protocols

• Clear definition of steps is needed for two computers to communicate
  • rules
  • syntax
  • semantics
  • synchronization of communication
  • error recovery methods
• There are protocols for every layer in the networking model
• The internet engineering Task force (IETF) develops and promotes voluntary internet standards

MAC and IP addresses

• Mac address (layer2): locating a piece of communication device on a local network
• IP address(layer3): identifying a network interface in networks
  • IPv4 (32-bit) vs IPv6 (128-bit)
  • Public addresses vs private addresses
    • Private addresses limit datagrams to be sent within local network only
    • e.g., 192.168.1.123 is only meaningful within a local network
• The ARP table on a device maintains the correspondence between MAC addresses and IP addresses within a local network

TCP VS UDP

• Layer 4 protocols: ensuring reliability of data transmission
• Transmission control protocols (TCP)
  • Handshakes to establish a connection
  • Built in system to check for errors
  • Guarantee for data order and completeness
  • Good for HTTPSs, HTTP etc. 
• User Datagram Protocols (UDP_
  • connectionless -> reducing the overhead of computation (time)
  • Extensive error checking and recovery not required
  • Good for: video conferencing, media streaming etc

Communicating over ports 

• In networking, connections are made on ports of a network device
• Each port is "listen to" served by one piece of software server
• Well known ports: 0-1023 (Http:80, Https:443)
• Registered ports: 1024-49151
• Private ports:49152-65535

Socket

• In network programming, a network socket is an endpoint for communication
• socket=transport protocol+IP address+port number
• Implementation depends on the programming language/environment

Localhost

• It is also possible to have both ends of communication only on one computer
  • Although it is in the same computer, this way allows communication to still go through network layers in the OS
• Usually identified in the computer as "localhost" or 127.0.0.1

Hypertext transfer protocol

• HTTP - hypertext transfer protocol
• Communication is initiated by a client 
  • a client sends a HTTP request to a server
  • the server returns a HTTP response to the client 
• It is stateless
  • every request is treated as an independent request
  • the protocol itself does not offer any means to relate two separate request
• HTTP/1.1 — 1997, HTTP/2 — 2015, HTTP/3 — expired draft (2020)

 

HTTP clients 

• Web client (“user-agent”): a software communicating with the web server over HTTP or HTTPS
  • web browsers: for end-user experience
    • html/css rendering
    • js execution
    • web technologies
  • web crawlers/ search engines
  • programmatic interfaces

HTTP servers

• Web server: handling requests for web documents with http/https, supporting server-side scripts
  • apache (was still the first last yr!)
  • nginx
  • Microsoft ISS
  • node.js
• Web caching
  • forward/reverse proxies
  • Content delivery networks(CDN)