12 - Application Protocols

ucla | CS 118 | 2024-11-21

Table of Contents

Client-Server vs. P2P

  • Client-server - server always on with permanent IP, client connects with dynamic IP
    • e.g., HTTP, IMAP, FTP
  • P2P - no such server, all inter client connects, scalable
    • e.g., torrent file sharing - bittorrent

Process Communication

  • processes on the same host comms via Inter-Process Comms (IPC) via OS
  • client-server - processes are either clients or servers and are always listening or trying to connect
  • p2p - procs have both client and server ports
  • comms via sockets (abstraction)
  • procs identified by both IP and Port
    • port e.g., HTTP:80, SMTP:25 (mail)

Application Level Protocols

  • protocols contain/define
  • apps require transport service
  • e.g. proc examples

TCP vs. UDP

  • e.g., app transport protocols
  • TCP security (TLS)

Web and HTTP

  • web page consists of objects stored in a DOM structure
  • object an be HTML file/element, images, applets, auio, etc.
  • web-page consists of base HTML file (e.g. index.html) with several referenced objets addressable via URL e.g.

HTTP

  • HTTP - hypertext transfer protocol
  • HTTP is persistent or non-persistent because storing state is complex
    • e.g., Non-persistent HTTP
    • non-persistent performance
  • persistent (HTTP 1.1) v. non-persistent pro cons
  • messages are request (GET/POST/PUT…) or response (status ode, e.g. 404)
    • request messages
    • request messages

Cookies - State Management

  • HTTP requests are stateless so for multi-step exchange or repeated connections, store cookies to maintain state (stored in the browser)
  • components
    • cookie header line of HTTP response message
    • cookie header line in next HTTP request message
    • cookie kept on user’s host and managed by user’s browser
    • back-end DB at server website containing cookie id val to map state
  • first party cookies - track user behavior for the given website
  • third party cookies - tracks user behavior across multiple websites without visiting third party site
  • GDPR (EU general data protection regulation) requires sites to inform users about 3rd party cookies

Web Cache

  • to inc performance and decrease load on server, initial http get may include data and web cached data
  • may be done via proxy server which acts both as client and server intermediary
  • example
    • og bottleneck
    • sol1: get faster link
    • sol2: web cache, much cheaper than more expensive link

HTTP/2

  • dec delay iin multi obj HTTP requets
  • HTTP1.1 introdued multiple pipelined gets over single TCP connection
  • server responds in order (FCFS) - this may cause head of line (HOL) blocks for small objs behind large objects -> loss recovery stalling transmission
    • e.g, HTTP/1.1 HOL issue
    • e.g., HTTP/2 frame sol

HTTP/3

  • HTTP/2 disadvantages
    • single connection means recovery from packet loss still stalls object transmissions
    • no security over vanilla TCP
  • HTTP/3 adds security and per object error and congestion control via pipelining over UDP

Email, SMTP, IMAP

  • composed of: user agents, mail servers, and SMTP
  • SMTP - simple mail transfer protocol
    • between mail servers on client (sender) - server (receiver) protocol
  • user agent - mail reader, creating editing reading sending mail on various clients - outlook, gmail, iphone, etc,
  • mail server - composed of mailbox (contains incoming messages) and message queue (outgoing mail queue)
  • request and response messages
  • IMAP - internet mail access protocol - provides retrieval and deletion