[네트워크] 응용 계층 : Application layer

Introduction

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응용 계층(Application layer)은 프로토콜 스택 가장 위에 있는 계층으로, 네트워크를 사용하는 응용 프로그램 간 소통을 위해 제작되었다.

 

 

Network Structure

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1. Client-Server Architecture

• Server
  • Always-on host
  • Permanent IP address
  • Examples: Web servers, mail servers, DNS servers
• Client
  • Dynamically connected
  • May have dynamic IP addresses
  • Communicate with servers
  • Examples: Web browsers, email clients
• Pros : stability
• Cons : scaling cost

2. Peer-to-Peer (P2P) Architecture

• Features:
  • No dedicated servers
  • Direct communication between pairs of connected hosts (peers)
  • Self-scaling
• Examples:
  • BitTorrent, early versions of Skype
• Pros:
  • scaling cost
• Cons:
  • stability
• Challenges:
  • Security, performance, reliability

 

Application Layer Protocols

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1. HTTP (Hypertext Transfer Protocol)

• Purpose: Web objects transfer
• Features:
  • Stateless protocol
  • Uses TCP on port 80 (HTTP) or 443 (HTTPS)
  • Methods: GET, POST, PUT, DELETE, etc.
  • Status codes (200 OK, 404 Not Found, etc.)
• Challenges:
  • latency: persistent HTTP, non-persistent HTTP, proxy Server
  • stateless to stateful: session and cookies

2. SMTP (Simple Mail Transfer Protocol)

• Purpose: Email transfer
• Features:
  • Uses TCP on port 25
  • Push protocol
  • Three phases: handshaking, transfer, closure
• Related Protocols:
  • POP3 (Post Office Protocol)
  • IMAP (Internet Message Access Protocol)
  • HTTP

3. DNS (Domain Name System)

• Purpose: translate domain name to IP address
• Features:
  • Name resolution
  • load balancing
• Hierarchy Structure:
  • 1 : Root DNS servers
  • 2 : TLD(Top-level domain) servers (.com, .org, etc.)
  • 3 : Authoritative DNS servers
  • 4 : Local DNS servers
  • query methods : iterated query, recursive query

4. FTP (File Transfer Protocol)

• Purpose: File transfer
• Features:
  • Separate control and data connections
  • Authentication required
  • Supports binary and ASCII transfers

 

Conclusion

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• 응용 계층(Application layer)은 다른 네트워크 어플리케이션과 소통하기 위해 필요한 계층이다.
• 응용 계층의 관점에서 네트워크 구조는 (1)서버-클라이언트 구조와 (2)P2P 구조가 있다.
• HTTP, SMTP, DNS, FTP 등 공개된 프로토콜 외에도 다양한 프로토콜들이 존재한다.

 

Practice Questions

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1. DNS 서버의 계층 구조에 대해 설명하고, 왜 그러한 구조를 가지게 되었는지 설명하시오.
2. 클라이언트-서버 구조와 P2P 구조 간의 차이점을 서술하시오.
3. Proxy server가 무엇인지 설명하고 link를 증설하는 것보다 얼마나 효율적인지 서술하시오.