Review computer networks (Application Layer)

2.1 application layer protocol principles

Client Server Architecture

The application layer protocol defines how application processes running on different end systems transmit packets to each other.

Switch packet type, packet type syntax, field meaning, process time, how to send packets and packet response rules

2.2 web applications and HTTP protocol

The Web application layer protocol is hypertext transfer protocol, which is the core of the web.

A web page consists of objects, which are simply files.

The Web browser implements the HTTP client

The Web server is used to store web objects. Each object has URL addressing.

HTTP defines how the Web Client Requests web pages from the server and how the server sends web pages to the client.

HTTP uses TCP as its transport layer transport protocol

HTTP is a stateless protocol

The Web Server Always Opens and has a fixed IP address.

Non-persistent connection and persistent connection: Determine whether each request response is sent through a single TCP connection or all requests and responses are sent through the same TCP connection, previous non-persistent connection and later persistent connection

HTTP message format

1. HTTP Request Message

GET/somedir/page.html HTTP/1.1 (request line)

HOST: www.somescholl.edu is followed by the first line

Connection: Close (using a non-persistent connection)

User-Agent: Mozilla/4.0 (client type)

Accept-language: FR

2. Http response packets

HTTP/1.1 200 OK status line

Connection: first line after close

Date: Thu, 03 Jul 2003 GMT

Server: Apache/1.3.0 (UNIX)

Last-modified: Sun, 6 May 2007 GMT

Content-Length: 6821

Content-Type: text/html

(Data data...) Entity

Status Code: 200 OK. The request is successful. The information is contained in the returned Response Message.

301 moved permanently object has been permanently transferred

400 bad request common error code, the request is not understood by the server

404 Not found the requested document is not on the server

505 HTTP Version Not Supported

User-server interaction: cookie

COOKIE: 1. the HTTP response message contains a cookie header line.

2. the HTTP request message contains a cookie header line.

3. A cookie file is retained by the user's browser.

4. The web site has a backend database

Suan accesses Amazon through a browser. After the request message arrives at Amazon, the Web site generates a unique ID code and uses this as an index to generate a table item in its backend database. Next, Amazon

The server uses a set-COOKIE: the first line of HTTP response packet to respond to Susan's browser. Susan's browser

When an HTTP response packet is received, it will see the set-COOKIE: Header

Web Cache

The Web Cache is also called a proxy server.

Condition get

File Transfer Protocol FTP

Text transfer protocol. Running on TCP, FTP uses two parallel TCP connections to transmit files

One Control connection 21, one data connection 20, and the FTP control information is transmitted out of band.

FTP commands and answers

The user sends a user ID to the server.

Pass User Password

List all files in the current directory of the Request Server

RETR retrieves files from the current directory

Stor current directory stores files

Status code 331 username OK

DNS Internet Directory Service

DNS 1 a distributed database implemented by a layered DNS Server

2. An application layer protocol that allows hosts to query distributed databases

The DNS protocol runs on UDP

Request URL Process

1 client with DNS applications running on the same host

2. the browser extracts the Host Name and sends it to the DNS Client.

3. the DNS client sends a request containing the host name to the DNS server.

4. the DNS client receives an answer packet to address the IP address of the host name.

5. When the browser receives the dns ip address, the HTTP server located by the IP address can send a TCP connection.