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

title	aliases	tags	sr-due	sr-interval	sr-ease
17-app-layer		cosc203 lecture	2022-10-20	3	250

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

• hierarchical, domain-based naming scheme implemented using a distributed databse system

• core internet function, but implemented as an application layer protocol

• tree structure

• domain name space

  • each node has a label : the root label is a null string
  • each node has a domain name: a sequence of labels separated by dots, reading from the node up to the root.
  • 

• name servers

  • name space is divided into non-ovelapping zones
  • each zone had name servers to hold information
  • top level dns servers are responsible for domains such as .com .org .net and country domains
  • authoritative DNS servers: maintained by organization or service providers
  • 

DNS caching

• once (any) name server learns mapping, it caches mapping, and immediately returns a cached mapping in response to a query
  • caching improves response time
  • cache entries timeout (disappear) after some time (TTL)
  • top-level domain names typically cached in local name servers
• cached entries may be out-of-date
  • if named host changes IP address, may not be known Internet-wide until all TTLs expire!
  • best-effort name-to-address translation!

DNS name resolution: recursive query

• e.g., host at cs.otago.ac.nz wants IP address for ai.cs.waikato.ac.nz
• 

Socket programming

• socket
  • software component that has
    • local/remote socket address: IP + port no.
    • transport layer protocol: TCP or UDP
• socket api
  • application programming interface usually provided by the operating system
  •