[Networks] 3주 1차 : Application Layer

- Principle of Network Applications

• Some network apps
  • E-mail
  • Web
  • Text massaging
  • Remote login
  • P2P file sharing
  • Multi-user network games
  • …
• Creating a network app
  
  • Write programs that :
    
    • Run on (different) end systems
    • Communicate over network
    • E.g. Web server software communicates with browser software
  • No need to write software for network-core devices
    
    • Network-core devices do not run user applications
    • Applications on end systems allows for rapid app development & propagation
• Application architecture
  
  • Client-server architecture
    
    • Server
      
      • Always-on host
      • Permanent IP address
      • Data center for scaling
    • Clients
      
      • Communicate with server
      • May be intermittently (간헐적으로) connected
      • May have dynamic IP addresses
      • Do not communicate directly with each other

    

    Client-server architecture

  • P2P architecture
    
    • No always-on server
    • Arbitrary end systems directly communicate
    • Peers request service from other peers, provide service in return to other peers
      
      • Self-scalability : New peers bring new service capability, as well as new service demands
    • Peers are intermittently connected and change IP addresses
      
      • Complex management

    

    P2P architecture

• Process communicating
  
  • Process : Program running within a host
    
    • Within same host, two processes communicate using inter-process communication (defined by OS)
    • Processes in different hosts communicate by exchanging messages
    • Client process : Process that initiates communication
    • Server process : Process that waits to be contacted
    • Aside : application with P2P architecture have client processes & server processes
• Sockets
  
  • 프로세스가 네트워크를 통해 데이터를 내보내거나 받는 창구 역할
  • Protocol, IP address, port number로 구성
  • Process sends / receives massages to / from its socket
  • Socket analogous to (유사한) door
    
    • Sending process shoves (밀치다) message out door
    • Sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process

  

  Sockets

• Addressing processes
  
  • To receive messages, process must have identifier
  • Host device has unique 32-bit IP address
  • Identifier includes both IP addresses and port numbers associated with process on host
  • Example
    • Example port numbers
      
      • HTTP server : 80
      • Mail server : 25
    • To send HTTP message to ajou.ac.kr web server
      
      • IP address : 128.xxx.xxx.xx
      • Port number : 80
• App-layer protocol (규칙 체계) defines
  
  • Types of messages exchanged
    
    • E.g. Request, respond
  • Message syntax
    
    • What fields in messages & how fields are delineated?
  • Message semantics (의미)
    
    • Meaning of information in fields
  • Rules for when and how processes send & respond to messages
  • Open protocols
    
    • Defined in RFCs
    • Allows for interoperability (상호 운용 가능)
    • E.g. HTTP, SMTP
  • Proprietary (소유권이 따로 있는) protocols
    
    • E.g. Skype
• What transport service does an app need?
  
  • Data integrity
    
    • Some apps (e.g. File transfer, web transactions) require 100% reliable data transfer
    • Other apps (e.g. Audio) can tolerate some loss
  • Timing
    
    • Some apps (e.g. Internet telephony, interactive games) require low delay to be “effective”
  • Throughput (처리율)
    
    • Some apps (e.g. Multimedia) require minimum amount of throughput to be “effective”
    • Other apps (“elastic apps”) make use of whatever throughput they get
  • Security
  • Transport service requirements : common apps

    

• Internet transport protocols services
  
  • TCP service
    
    • Reliable transport between sending and receiving process
    • Flow control : Sender won’t overwhelm receiver
    • Congestion control : Throttle sender when network overloaded
    • Does not provide : Timing, minimum throughput guarantee, security
    • Connection-oriented : Setup required between client and server processes
  • UDP service
    
    • Unreliable data transfer between sending and receiving process
    • Does not provide : Reliability, flow control, congestion control, timing, throughput guarantees, security or connection setup
    • 그럼에도 UDP를 사용하는 이유는 데이터 처리의 신속성 때문

  

  TCP vs UDP

• Securing TCP
  
  • TCP & UDP
    
    • No encryption
    • Cleartext (암호문이 아닌 평문) passwords sent into socket traverse (횡단하다) internet in cleartext
  • SSL
    
    • Provides encrypted TCP connection
    • Data integrity
    • End-point authentication
  • SSL is at app layer
    
    • Apps use SSL libraries, that “talk” to TCP
  • SSL socket API
    
    • Cleartext passwords sent into socker traverse internet encrypted