Computer network review (2) application layer

(1) Communication process

For Web applications, Hypertext Transfer Protocol (HTTP) is usually used to control communication rules, which depends on the services provided by TCP

According to the standards, HTTP protocols can be divided into HTTP/1.0 and HTTP/1.1, which is an improvement of HTTP/1.0

In HTTP/1.0, non-persistent Connection is used. For CS architecture, the communication process is as follows:

• The client requests to establish a TCP connection with the server
• The server agrees to establish a TCP connection
• The client sends a request specifying the resource to be obtained in the request information
• The server returns a response, including the resource entity in the response information, and then closes the TCP connection
• The client receives the response and retrieves the resource entity
• Repeat the above steps if you find that the resource entity also references other resources

However, in HTTP/1.1, Persistent Connection is used. For CS architecture, the communication process is as follows:

• The client requests to establish a TCP connection with the server
• The server agrees to establish a TCP connection
• The client sends a request specifying the resource to be obtained in the request information
• The server returns the response, including the resource entity in the response information, but does not close the TCP connection
• The client receives the response and retrieves the resource entity
• Repeat steps 3 to 5 if you find a resource entity that also references other resources

Suppose we need to transfer an HTML file, reference ten JEPG images in the file, and calculate the transfer time?

• For non-continuous connection: T = (2RTT + Ttransfer) + (2RTT + Ttransfer) * 10
• If continuous connection is used: T = (2RTT + Ttransfer) + (RTT + Ttransfer) * 10

(2) Message format

HTTP adopts the Request/Response format. The client submits a Request and the server returns a Response.

The format of the request message is shown below. Common request methods are as follows:

• GET: Requests the specified resource and returns the resource entity
• POST: Submits data to a specified resource, which may result in the creation of new resources or the modification of old resources
• HEAD: Similar to a GET request, but returns a response with no concrete content. It is often used to retrieve headers
• DELETE: deletes the specified resource
• PUT: Replaces a specified resource with submitted data
• TRACE: Returns the request received by the server, often used for testing
• OPTIONS: Returns the request methods supported by the server for a specific resource. It can also be used to view the performance of the server

What’s the difference between GET and POST?

1. Different functions of conventions. GET is used to query data from the server, and POST is used to modify data on the server

2, the location of parameters is different (browser implementation), GET request parameters in the URL, POST request parameters in the Body

The size of parameters for GET requests may be limited because browsers may have restrictions on the length of urls

The parameters of a GET request are retained in the browser’s history, but the parameters of a POST request are not

GET requests are idempotent and have no side effects, whereas POST requests are not idempotent and have side effects

When you hit the back button or refresh the page, GET doesn’t have any effect; POST resubmits the request

The result of a GET request can be used as a browser bookmark, but a POST request cannot

The results of GET requests are actively cached by the browser, but POST requests are not

The format of the response message is shown in the picture below, where the common status code and status information are as follows:

• 1** : message. The server receives a request asking the client to continue the operation

• 2 * * : success

  • 200: OK, the server successfully processed the request and returned the requested resource

• 3** : redirects

  • 301: Moved Permanently, Permanently redirected, the requested resource Permanently Moved to a new location
  • 302: Moved Temporarily, redirect the requested resource to a new location

• 4** : Client error

  • 400: Bad Request: Syntax errors exist in the Request packet
  • 403: Forbidden, access to requested resources is denied by the server
  • 404: Not Found, requested resource Not Found on server

• 5** : The server is faulty

  • 500: Internal Server Error. An Error occurs when the Server executes a request
  • 502: Bad Gateway, the server acting as the Gateway received an invalid response from the remote server while executing the request
  • 503: Service Unavailable indicates that the server is temporarily unable to process requests from clients due to server overload or downtime for maintenance

What’s the difference between 301 and 302?

They are redirected, meaning that the browser automatically redirects to a new address when it gets 301 or 302

Then there are the differences:

301 indicates that resources at the old address are removed permanently. When searching for new content, the search engine replaces the old address with the new address

302 indicates that the resource of the old address is still in fact, but only temporarily jumps to the new address. The search engine will grab the new content, but keep the old address

(3) the Cookies

Since the HTTP protocol is stateless, the server can use Cookies to track the user’s state

A Cookie is simply a special value that is created and maintained by the server and sent to the client

After that, the client brings a Cookie with each request, and the server can judge the user’s status through the Cookie

(4) Caches

Caches technology reduces the time it takes to get a response from the client, and reduces the pressure on the server to handle requests, so you can kill two birds with one stone

To use Caches is to set up one or more proxy servers that help respond to client requests

The client sends the request directly to the proxy server, and if the requested resource is on the proxy server, it is returned directly to the client

If not, the proxy server requests resources from the server, gets them, stores them locally, and returns them to the client

A brief description of the development of the HTTP protocol?

HTTP/0.9 [1991 年]

• The simplest version can only access resources in HTML format, with no headers and only GET requests

HTTP/1.0 [1996 年]

• Example Add the HTTP version number
• Add status code and status information
• Add request methods, including POST and HEAD
• Add headers for both requests and responses
• Other types of files can be transferred by setting the content-type in the header

HTTP/1.1 [1997 年]

• Adding persistent connections enables HTTP requests to reuse TCP connections
• Pipelined transport is supported, allowing multiple requests to be sent simultaneously in a SINGLE TCP connection
• Add request methods, including DELETE, PUT, OPTIONS, and so on

HTTP/2.0 [2015 年]

• Binary framing: The partitioning of transmitted data into smaller frames and binary encoding of them
• Server push: Allows the client to send resources to the client without request
• Compression header: many requests for many headers are actually repeated, it is necessary to deal with the header is to use compression tools to compress the header and then send, the second is to repeat the field can use the index number to replace

(1) Communication process

For Mail transmission, Simple Mail Transfer Protocol (SMTP) is used to control communication rules, which depends on the services provided by TCP

There are two important components of Mail transmission, namely User Agent and Mail Server.

• User agent: Used to edit and read mail
• Mail server: Used to send and receive mails

For CS architecture, the communication process is as follows:

• The sender edits the message through its own user agent
• The sender’s user agent sends messages to its own mail server
• The sender’s mail server receives the Message, places it in a Message Queue, and waits for it to be sent
• The sender’s mail server sends mail messages to the receiver’s mail server
• The recipient’s Mail server receives the message, puts it in a Mail Box, and waits for it to be read
• The recipient reads the message through its own user agent

(2) Message format

In the beginning, mail could only transmit text, so simple ASCII was used to define the mail format

The regular mail format consists of two parts, Header and Body.

Later, Mail can also transmit multimedia resources, and a new format MIME (Multiperpose Internet Mail Extension) was defined.

Just add a new line in the header specifying that the MIME mail format is used

SMTP also uses the command/response format. The commands sent by the client and the responses returned by the server are 7 bits ASCII

The client sends commands to authorize and request resources. The common commands are as follows:

• HELO: Establish a connection
• AUTH LOGIN: Identity authentication
• MAIL FROM address: Specifies the mailbox from which mails are sent
• RCPT TO address: Specifies the mailbox to receive the mail
• DATA: Specifies the email body
• QUIT: Close the connection

After receiving the command, the server sends a Response to return the result. A Response includes a status code and status information. Common responses are as follows:

• 220: Service ready
• 250: Required operation completed
• 354: Starts to enter the message body to.The end of the
• 221: Service shutdown

(3) Other mail protocols

SMTP is the mail sending protocol. POP3 and IMAP are the mail receiving protocols

• Post Office Protocol 3 (POP3) : allows users to download mails from the mail server
• Internet Mail Access Protocol (IMAP) : allows users to directly manage mails on the Mail server, including Mail movement and Mail classification