06. Application Layer
Application Layer
1. Network Application Models
- Client/Server (C/S) model: Client actively requests, server passively serves
- P2P model: Peer nodes act as both client and server
2. DNS System
- Hierarchical domain name space: root, top-level, second-level, host name
- Domain name servers: root servers, top-level domain servers, authoritative servers, cache servers
- Domain name resolution process: recursive query, iterative query
3. FTP Protocol
- Working principle: separation of control connection and data connection
- Control connection: commands and responses
- Data connection: file transfer
4. Email
- System structure: user agent, mail server, mailbox
- Email format and MIME: supports multimedia content
- SMTP protocol: sending emails
- POP3 protocol: receiving emails
5. WWW and HTTP
- WWW concept and structure: hypertext, browser, server
- HTTP protocol: connectionless, stateless, request-response based
- Common HTTP methods: GET, POST, HEAD, PUT, DELETE
Exercises
- Briefly describe the difference between the C/S model and the P2P model.
- What are the main steps in the DNS domain name resolution process?
- Why does FTP distinguish between control connection and data connection?
- What are the roles of SMTP and POP3 in the email system?
- What are the main features of the HTTP protocol? What are the common HTTP methods?
Reference Answers
1. C/S vs. P2P Difference
C/S: Client initiates, server responds, clear structure; P2P: Peers are equal, resource sharing, strong scalability
2. DNS Resolution Process
Recursive query, iterative query, step-by-step resolution of domain name to IP address
3. FTP Connection Distinction
Control connection transmits commands, data connection transmits files, they do not interfere with each other
4. SMTP and POP3
SMTP is used for sending emails, POP3 is used for receiving emails
5. HTTP Features and Methods
Connectionless, stateless, request-response; common methods: GET, POST, HEAD, PUT, DELETE