


In the process of transmitting data using a TCP connection, the client and server need to go through a three-way handshake to establish a connection and a four-way handshake to disconnect.
The details are shown in the figure below
The above figure describes the detailed process of TCP connection from establishment to disconnection. The specific message details will be discussed below.
During the TCP three-way handshake to establish a connection, the client first sends a SYN=1, Sequence=p request packet to the server.
If the server agrees to grant the link to the client, it sends a response packet of ACK=p+1, SYN=1, Sequence=q.
Finally, after the client receives the response packet from the server, it will also send a response packet to the server. The specific content is ACK=q+1, SYN=0, Sequence=p+1.
For TCP disconnection, four handshakes are required. Why not three handshakes? The specific reasons are:
When the server receives the SYN connection request message from the client, it can directly send the SYN+ACK message . The ACK message is used for response, and the SYN message is used for synchronization. But when closing the connection, when the server receives the FIN message, it is likely that the SOCKET will not be closed immediately, so it can only reply with an ACK message first, telling the client, "I received the FIN message you sent." Only when all the messages on my server side have been sent can I send the FIN message, so it cannot be sent together. Therefore, a four-step handshake is required.
The process of disconnection is initiated by the client first. First, the client will send a disconnection request packet with FIN=1 and Sequence=m.
After receiving the request, the server will send a confirmation packetACK=m+1, Sequence=n.
At this time, the unilateral connection between the client and the server has been disconnected, and the client will no longer send data packets to the server. The server can still send data to the client. If the server does not need to send data to the client after sending s data packets, it can send a disconnect request to the client at this time.FIN=1, Sequence=m+s, ACK=m+1.
After the client receives the server’s disconnect request, it will send a confirmation packet.Sequence=m+1,ACK=m+s+1;
Note that after the client sends the last ACK packet, it will still wait for 2MSL (maximum segment survival time), because the packet may will be lost. The client waits for 2MSL because it is worried that the server does not receive the ACK packet and resends the FIN request.

What’s still popular is the ease of use, flexibility and a strong ecosystem. 1) Ease of use and simple syntax make it the first choice for beginners. 2) Closely integrated with web development, excellent interaction with HTTP requests and database. 3) The huge ecosystem provides a wealth of tools and libraries. 4) Active community and open source nature adapts them to new needs and technology trends.

PHP and Python are both high-level programming languages that are widely used in web development, data processing and automation tasks. 1.PHP is often used to build dynamic websites and content management systems, while Python is often used to build web frameworks and data science. 2.PHP uses echo to output content, Python uses print. 3. Both support object-oriented programming, but the syntax and keywords are different. 4. PHP supports weak type conversion, while Python is more stringent. 5. PHP performance optimization includes using OPcache and asynchronous programming, while Python uses cProfile and asynchronous programming.

PHP is mainly procedural programming, but also supports object-oriented programming (OOP); Python supports a variety of paradigms, including OOP, functional and procedural programming. PHP is suitable for web development, and Python is suitable for a variety of applications such as data analysis and machine learning.

PHP originated in 1994 and was developed by RasmusLerdorf. It was originally used to track website visitors and gradually evolved into a server-side scripting language and was widely used in web development. Python was developed by Guidovan Rossum in the late 1980s and was first released in 1991. It emphasizes code readability and simplicity, and is suitable for scientific computing, data analysis and other fields.

PHP is suitable for web development and rapid prototyping, and Python is suitable for data science and machine learning. 1.PHP is used for dynamic web development, with simple syntax and suitable for rapid development. 2. Python has concise syntax, is suitable for multiple fields, and has a strong library ecosystem.

PHP remains important in the modernization process because it supports a large number of websites and applications and adapts to development needs through frameworks. 1.PHP7 improves performance and introduces new features. 2. Modern frameworks such as Laravel, Symfony and CodeIgniter simplify development and improve code quality. 3. Performance optimization and best practices further improve application efficiency.

PHPhassignificantlyimpactedwebdevelopmentandextendsbeyondit.1)ItpowersmajorplatformslikeWordPressandexcelsindatabaseinteractions.2)PHP'sadaptabilityallowsittoscaleforlargeapplicationsusingframeworkslikeLaravel.3)Beyondweb,PHPisusedincommand-linescrip

PHP type prompts to improve code quality and readability. 1) Scalar type tips: Since PHP7.0, basic data types are allowed to be specified in function parameters, such as int, float, etc. 2) Return type prompt: Ensure the consistency of the function return value type. 3) Union type prompt: Since PHP8.0, multiple types are allowed to be specified in function parameters or return values. 4) Nullable type prompt: Allows to include null values and handle functions that may return null values.


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