[컴퓨터네트워크]4. Application Layer

Application architectures

• peer-to-peer(P2P)
• client-server

Client-server architecture

• server
  • always-on host
  • permanent IP address
  • data centers for scaling
• clients
  • communicate with server
  • do not communicate directly with each other
  • may be intermittently connected
  • may have dynamic IP address

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 capacity, as well as new service demands
• peers are intermittently connected and change IP address
  • complex management

Processes communicating

• process : program running within a host
  • within same host
  • different hosts by messages
• server-client
  • client process : initiates communication
  • server process: waits to be contacted
• P2P
  • can be client or server each peers

Sockets

• process sends/receives msg to/from its socket
• analogous to door

Addressing processes

• process must have identifier
• identifier includes both IP address and port numbers(host device has unique 32bit IP address)
• well known port
  • HTTP server: 80
  • mail server: 25

what transport service does an app need?

• data integrity
• timing : require low delay to be effective
• throughput :  require minimum amount of throughput to be effective 

Internet transport protocols services

• transport 계층에서 패킷을 갖고 있으면 minimum throughput 보장 못함

Internet apps : application, layer protocol, transport protocol

App-layer protocol defines

Web and HTTP