quartz/content/notes/17-app-layer.md

title	aliases	tags
17-app-layer		cosc203 lecture

Client-server vs peer to peer

• social network
• email
• multi player games
• video streaming
• P2P file sharing
• voice over IP e.g., skype
• remote ogin

Principles

• run of different end systems
• communicate over network
• no need to run application specific software on netwrk-core devices

client-server

sercer

• provide services to clients
• always on
• often in data centers, for scaling
• needs to powerful
  • multiple client requesting info at the same time
  • have specialzed software

client

• request services by contacting and communicating with server
• may be intermittently connected
• do not communicate directly with each other

peer 2 peer

• no alwasy on server
• decentralize resource on a network
• end systems are both client and servers
• end systems communcatae directly
• take advantage of distrivuted shared resources (bandwidth, CPU storage)
• operate in dynamic environment with frequent join and leave
• e.g., bitTorrent, blockchain

communication

• process: network application running with a host
  • client proces and server process
  • processes in different hosts communicate by exhanging messages
• process must have identifiers
  • ip address to differentiate processes in different hosts
    • IPv4: 10.96.35.21 • IPv6: fe80::44b:184c:7ebb:6524
  • port number to differentiate processes in same host
    • HTTP server (80), mail server (25)

functions of application protocols

protocols define:

• types of messages
  • e.e.g, request, response
• message syntax
  • what fields in messages and how field are delineated
• semantics
  • meaning on information in fields
• communcation rules
  • when and how proceses send and response to messages

WEB http

• client server model
  • browser requests, recieves and displays web objects
  • server sends objects in response to requests
• HTTP
  • communcation betwen browsers and servers
• https
  • extensioin of http that uses transport layer security for security

methods

• get, head, post, put (request to store a webpage), delete, link (connect two existing resources), unlink

web cache

• store data to be used later
• makes loading faster
• browser sends all request to cache
  • if object is not in cache, the cache requests from the server
  • files are stored as representation (a snapshot)
  • files expire after their time to live is finished
• we need to check if cache is up to date
  • conditional get: If-modified-since:
  • if up to date then: HTTP/1.0 304 Not Modified

Email

three components

• user agents
  • software to compose, read, reply etc email
  • handle local mailboxes
  • e.g., outlook, thunderbird
• mail servers
  • mailbox contains emails for user
  • each box has a unique address
  • localpart@domain
  • message queue of outgoing to send messages
• protocols
  • 
  • SMTP to send (simple mail transfer protocol)
    • send from user agent to mail server and between mail servers
    • simple ascii protocol
  • POP (post office protocol)
    • fetch email from mail server
    • simple but limited
    • delete and keep mode (delete from server of keep)
  • IMAP (internet mail access protocol)
    • more functions
    • e.g., check email header before downloading, partially download email
    • allow users to organize mails on server
  • MIME (multi prpose internet mail extensions)
    • supplementary protocol to allow non-ASCII dat to be send through emails
    • WAN connect LANS together
• email format
  • 
  • envelope
    • contains address of sender and reciever
    • used by smtp to transport email
  • message
    • header
      • defines sender, reciever, subject etc
    • body
      • contains actial information
  • email spoofing can make the addresses in header and in envelope different

DNS

• ip adresses are hard to remember
• need to map betwen host name and ip address

Socket programming