Chapter 2 Application Layer

1. Application layer protocol principle:

C/S
P2P
C/S and p2p hybrid

2. Two major services at the application layer:

TCP service: connection-oriented and reliable data transmission
UDP service: connectionless, unreliable transmission

3. Application layer protocol classification: (the following three are all TCP connections)

Web's HTTP protocol:
HTTP's two types of connections (persistent and non-persistent),
HTTP's two types of messages (request messages, response messages),
Cookies technology,
web caching technology
FTP protocol for file transfer: FTP process
E-mail protocol: mail sending protocol (SMTP protocol-MIME protocol) and mail access protocol (POP3 protocol)

4. DNS introduction: two kinds of queries (recursive, iterative)
5. P2P application

1. Application layer protocol principle

1. The application layer is the application you touch most frequently, such as: email, Web, P2P, remote login, real-time video conferencing, etc.

2. The core of developing web applications: write programs that can run on different end systems and communicate with each other through the network.
For example, if you want to develop a web service, you need web server software and browser software to communicate

3. No application software runs on the core equipment of the network
(that is, there is no application layer on routers and switches. This design method promotes the development of applications, because you only need to consider the same layer of communication. )

4. Network application program architecture:

Client/server architecture
P2P architecture
Client/server and P2P hybrid architecture

5. Client/Server Architecture
Server:

Always open host
Have a fixed, well-known IP address
Host clusters are often used to create powerful virtual servers

Client computer:

Communicate with the server
Can be connected to the server intermittently and can have a dynamic IP address
Clients do not communicate directly with each other

6. Pure P2P architecture

There is no always open server
Any pair of hosts communicate directly with each other
The peer is connected intermittently and can change the IP address, for example: Gnutella
Advantages: self-scalability
Disadvantages: difficult to manage

7. Client/server and P2P hybrid architecture:
instant messaging:

P2P between two chat users
Register and query through the server
Register on the central server when users go online
The user contacts the central server to find an online partner

Napster.

Files are exchanged directly between peers
File search through the server
Central server records peer content
The peer queries the central server to determine the required file location

8. Process communication

Process: A program running in the end system
Two processes on the same host communicate through the internal process communication mechanism
Processes on different hosts communicate with each other by exchanging messages

Client process: the process that initiates communication; server process: the process waiting for contact

Note: The application with P2P architecture has both client process and server process.

9. Application layer protocol

10. Two major services:

2. HTTP overview

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3. Non-persistent connection and persistent connection

1. Two HTTP connections
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2. Non-persistent connection

3. Response time model

3. Persistent HTTP connection

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4. HTTP message format

1. HTTP two types of messages
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2. HTTP request message

3. Request method type

4. HTTP response message format

5. Interaction between user and server: cookies

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Traditional HTTP cannot satisfy high-speed interaction, so cookies are used

6.web cache

1. The concept of
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web caching 2. Examples of web caching

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Solution 1:

Solution 2:

3. Conditional get method

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7. File transfer protocol

1. Process
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2. Data establishment mode

3. Data transmission mode

8. SMTP protocol

1. Three major components
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2. Mail sending protocol SMTP

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9. Mail message format and MIME

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The above is how you actually use SMTP to send mail

10. Mail access agreement

The mail sending protocol is sending mail, and the mail access protocol is receiving mail.
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Operation example:

11. DNS system introduction

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Each host needs to be configured with a local DNS server in order to access the Internet normally
When the host sends a DNS request, the request is sent to the local DNS server