Summary

Web and HTTP

• HTTP: The Web’s application layer protocol.
  • Client/Server Model: Client (browser) requests objects; Server sends objects.
  • Client initiates TCP connection to server port 80, HTTP messages exchanged then TCP connection closed.
  • Stateless: The server maintains no information about past client requests.
  • Web page consists of base HTML-file which includes several referenced objects,each addressable by a URL, e.g.
    • http://www.siit.tu.ac.th/index.html:
    • siit.tu.ac.th is host name
    • /index.html is path name of object on that server
• HTTP Connections:
  • Non-persistent HTTP (1.0): At most one object sent over a TCP connection. Connection then closed. Downloading multiple objects requires multiple connections (high overhead).
    • Response Time = 2 Round-trip time (RTT) + file transmission time
    • 
  • Persistent HTTP (1.1): Server leaves connection open after sending response. Multiple objects can be sent over a single TCP connection between client and server.
    • Response Time = RTT + file transmission time
    • 
  • HTTP/2: decreased delay in multi-object HTTP requests
    • transmission order of requested objects based on client-specified object priority
    • Divide objects into frames, schedule frames to mitigate Head-of-line blocking
  • HTTP 1.1:
  • HTTP 2:
• HTTP/3: adds security, per object error- and congestion-control ovre UDP
• HTTP Message Format:
  • Request Message:
    • 
    • GET: get user data
    • POST: form input
    • HEAD : get header information only
    • PUT : upload file
  • Response Message:
    • 
    • Status codes (e.g., 200 OK, 404 Not Found, 301 Moved Permanently).
• Cookies: Used to maintain state (user history, shopping cart) on top of the stateless HTTP.
  • Four components:
    • Cookie Header line in response
    • Cookie Header line in next request
    • Cookie file kept on user’s host, managed by user’s browser
    • back-end database at website.
  • 
  • Privacy Concerns: Cookies can be used to track user behavior across websites, leading to privacy issues. Users can manage cookie settings in their browsers to mitigate this.
• Web Caches (Proxy Server):
  • Satisfy client request without involving the origin server.
  • User sets browser to access the Web via cache.
  • Benefit: Reduces response time for client, reduces traffic on institution’s access link.