​In-depth analysis of computer networks and operating systems: essential knowledge analysis for interviews

Computer Network

1. What is TCP congestion control? What is its purpose?

TCP congestion control is a mechanism used to control the flow of data in a network to avoid network congestion.

Its purpose is to ensure that each node in the network can process data at a reasonable rate, thereby improving the performance and stability of the network.

2. What is the TCP handshake process? Please explain the purpose of each step.

• The TCP handshake process is the process of establishing a TCP connection, which includes the following steps:

3. The client sends a SYN (synchronization) packet, randomly generates an initial sequence number, and sets the SYN flag to 1.

4. After receiving the SYN packet, the server sends an ACK (acknowledgement) packet as a response, sets the confirmation sequence number to the client's initial sequence number plus 1, and sets the ACK flag to 1. At the same time, it sends its own SYN packet and generates a randomly generated initial sequence number.

5. After the client receives the ACK packet and SYN packet from the server, it sends an ACK packet in response, sets the confirmation sequence number to the server's initial sequence number plus 1, and sets the ACK flag to 1.

6. What is the TCP handshake process? Please explain the purpose of each step?

• The purpose of the handshake process is to safely close the TCP connection, ensure that both parties have completed data transmission, and release the resources occupied by the connection.

• The TCP handshake process is the process of closing a TCP connection, which includes the following steps:

7. Step 1: One party sends a FIN (end) packet, indicating that it will no longer send data but can still receive data.

8. Step 2: After receiving the FIN packet, the other party sends an ACK packet as a response to confirm receipt of the FIN packet.

9. Step 3: The other party sends its own FIN packet, indicating that it agrees to close the connection.

10. Step 4: After receiving the FIN packet, one party sends an ACK packet as a response to confirm receipt of the FIN packet.

11. How does TCP achieve stable and orderly data transmission?

• TCP achieves stable and orderly data transmission through the following mechanisms:

• Sequence number and acknowledgment: Each TCP segment has a sequence number that identifies the data in the segment. The receiver confirms the received data by sending an acknowledgment (ACK) segment.

• Timeout retransmission: The sender will start a timer after sending data. If no confirmation response is received within a certain period of time, the data will be resent.

• Sliding Window: TCP uses a sliding window mechanism to control the data flow between the sender and the receiver. The sliding window size determines the amount of data that the sender can send, and the receiver informs the sender of the window size through a confirmation reply.

• Flow Control: TCP uses flow control mechanisms to ensure that the sender does not send too much data, exceeding the receiver's processing capacity. The receiver tells the sender how much data it can receive by sending a window size.

12. What is the OSI model? Briefly describe the function of each layer.

• The OSI (Open Systems Interconnection) model is a reference model for understanding and describing computer network functions. It consists of seven layers:

• Physical Layer: Responsible for transmitting bit streams and defining physical media and electrical signal specifications.

• Data Link Layer: Provides reliable data transmission, data grouping and error detection through frames.

• Network Layer: Responsible for the routing and forwarding of data packets, enabling communication between different networks.

• Transport Layer: Provides end-to-end reliable data transmission and implements communication between processes through port numbers and protocols.

• Session Layer: manages sessions and connections between different applications.

• Presentation Layer: handles data representation and conversion to ensure data format compatibility between different systems.

• Application Layer: Provides interfaces between network services and applications, including HTTP, FTP, SMTP, etc.

13. What is the difference between TCP and UDP? What are the application scenarios they are suitable for?

• TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are two common transport layer protocols. They have the following differences:

• Connectivity: TCP is a connection-oriented protocol that establishes a reliable connection through a three-way handshake, while UDP is a connectionless protocol that does not require a connection to be established.

• Reliability: TCP provides reliable data transmission and ensures data reliability through sequence numbers, acknowledgment and retransmission mechanisms, while UDP does not provide reliability guarantees.

• Orderliness: TCP ensures the orderliness of data and guarantees the order of data packets through sequence numbers and confirmation responses, while UDP does not guarantee the orderliness of data.

• Congestion control: TCP has a congestion control mechanism that avoids network congestion by dynamically adjusting the sending rate, while UDP does not have a congestion control mechanism.

• Applicable scenarios: TCP is suitable for application scenarios with high requirements for data reliability, such as file transfer and web browsing; UDP is suitable for application scenarios with high requirements for real-time performance, such as audio and video transmission and real-time games.

14. What is HTTP? How does it work?

• HTTP (Hypertext Transfer Protocol) is an application layer protocol for transmitting data on the Web. It works as follows:

• The client initiates a request: The client sends an HTTP request to the server. The request includes the request method (such as GET, POST), URL, request header, and request body.

• Server responds to the request: After receiving the request, the server processes it according to the requested URL and method and generates an HTTP response.

• Data transmission: The server sends the generated HTTP response back to the client. The response includes the response status code, response header, and response body.

• Connection management: The HTTP protocol uses TCP as the transmission protocol and transmits data by establishing and managing TCP connections.

• Statelessness: The HTTP protocol is stateless, that is, the server does not retain the client's state information. Each request is independent and the server does not remember previous requests.

15. What is an IP address? What is the difference between IPv4 and IPv6?

• An IP address (Internet Protocol Address) is a digital identifier used to uniquely identify a device on a network. IPv4 and IPv6 are two common versions of IP addresses, and they have the following differences:

• IPv4: IPv4 uses 32-bit addresses, usually represented as four decimal numbers, each ranging from 0 to 255, such as 192.168.0.1. The IPv4 address space is limited, with approximately 4.2 billion available addresses.

• IPv6: IPv6 uses 128-bit addresses, usually represented as eight groups of hexadecimal numbers, each ranging from 0 to FFFF, such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334. The IPv6 address space is huge, with about 340 trillion trillion trillion available addresses.

16. What is the TCP three-way handshake? What is its purpose?

• The purpose of the TCP three-way handshake is to ensure that both the client and the server can send and receive data normally and synchronize the initial sequence numbers of both parties. Through this process, both parties confirm each other's reachability and are ready for data transmission.

• The TCP three-way handshake is the process of establishing a TCP connection, which includes the following steps:

17. The client sends a SYN (synchronization) packet, randomly generates an initial sequence number, and sets the SYN flag to 1.

18. After receiving the SYN packet, the server sends an ACK (acknowledgement) packet as a response, sets the confirmation sequence number to the client's initial sequence number plus 1, and sets the ACK flag to 1. At the same time, it sends its own SYN packet and generates a randomly generated initial sequence number.

19. After the client receives the ACK packet and SYN packet from the server, it sends an ACK packet in response, sets the confirmation sequence number to the server's initial sequence number plus 1, and sets the ACK flag to 1.

20. What is the UDP protocol? What are its characteristics? What application scenarios is it suitable for?

• UDP (User Datagram Protocol) is a connectionless transport layer protocol with the following characteristics:

• No connection: UDP does not need to establish a connection, it sends data packets directly and does not guarantee the reliability and sequence of data.

• Simplicity: UDP has a small header overhead and high transmission efficiency, and is suitable for application scenarios with high real-time requirements.

• No congestion control: UDP does not have a congestion control mechanism. The sender sends data at a fixed rate and does not adjust according to network conditions.

• Applicable scenarios: UDP is suitable for application scenarios with high real-time requirements and acceptable data loss, such as audio and video transmission, real-time games, DNS queries, etc.

operating system

1. What are processes and threads? What is the difference between them?

A process is an instance of an executing program with its own independent memory space and system resources.

A thread is an execution unit within a process, sharing the process's memory space and resources.

The difference is that a process is an independent execution entity, while a thread is a flow of execution within a process.

2. What is a deadlock? What are the conditions for a deadlock?

• Deadlock occurs when two or more processes wait indefinitely for resources held by each other, causing the system to be unable to continue executing.

• The conditions for deadlock to occur include mutual exclusion, possession and waiting, non-preemption, and circular waiting.

3. What is virtual memory and what does it do?

• Virtual memory is an operating system memory management technology that combines physical memory and disk space to provide an independent address space for each process.

• Its functions include expanding the available memory space, implementing memory protection, and achieving isolation between processes.

4. What is a Linux file system? What are the common Linux file systems?

• The Linux file system is a structure used to organize and manage files and directories. Common Linux file systems include:

• ext4: It is the most commonly used file system in Linux, with high performance and reliability.

• ext3: It is the predecessor of ext4 and is also a common Linux file system.

• XFS: is a high-performance log file system suitable for large files and high concurrent access.

• Btrfs: is an advanced replication file system with features such as snapshots, compression, and checksums.

• ZFS: is an advanced file system with advanced data management and data integrity protection capabilities.

5. What is a Linux process? How to view and manage Linux processes?

• A Linux process is an instance of a running program. You can use the following commands to view and manage Linux processes:

• ps command: used to view the list of currently running processes. For example, "ps aux" can display detailed information of all processes.

• top command: displays the processes running in the system and the usage of system resources in real time.

• kill command: used to terminate a specified process. You can use the process ID (PID) or process name to specify the process to be terminated.

• nice and renice commands: used to adjust the priority of the process.

• nohup command: used to run a process in the background and detach it from the terminal. Even if the terminal is closed, the process still runs.

6. What is Linux Pipeline? How to connect commands using pipeline?

• Linux pipes are a mechanism for passing the output of one command as the input of another command.

• You can use the vertical bar symbol (|) to connect multiple commands. For example, command1 | command2 will use the output of command1 as the input of command2.

• The function of a pipeline is to realize data transmission and processing between commands. Multiple simple commands can be combined to complete complex tasks.

7. What are Linux soft links and hard links? What is the difference between them?

• Linux soft links and hard links are two different types of file linking methods.

• Soft link: A soft link is a shortcut to a target file or directory, similar to a shortcut in Windows. Soft links can cross file systems and can link to directories. Deleting the original file will not affect the soft link, but deleting the soft link will make the target file inaccessible.

• Hard link: A hard link is a direct link to the target file, they share the same inode and data blocks. Hard links can only link to files in the same file system, and cannot link to directories. Deleting the original file will not affect the hard link, because they share the same inode, and only when all links are deleted will the storage space of the file be released.

8. What is Linux Inter-Process Communication (IPC)? What are the common IPC mechanisms?

• Linux inter-process communication (IPC) refers to the mechanism for data exchange and communication between different processes. Common IPC mechanisms include:

• Pipe: used for one-way communication between parent and child processes or sibling processes.

• Named Pipe: Similar to a pipe, but can communicate between unrelated processes.

• Signal: Used to pass simple messages and notifications between processes.

• Shared Memory: allows multiple processes to share the same memory area for efficient data exchange.

• Semaphore: used for synchronization and mutual exclusion between processes and to control access to shared resources.

• Message Queue: Used to pass complex messages and data blocks between processes.

• Socket: used for inter-process communication on the network, including TCP and UDP communication.

This article is reproduced from the WeChat public account "The Programmer's Journey of Upgrading and Fighting Monsters", written by " Wang Zhongyang Go ", and can be followed through the following QR code.

To reprint this article, please contact the public account of "Programmer's Upgrade and Monster-Fighting Journey".