computer network

1. What is TCP Congestion Control? What is its purpose?

TCP congestion control is a mechanism used to control data traffic 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, including 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 the server receives the SYN packet, it sends an ACK (confirmation) packet in response, sets the confirmation sequence number to the client's initial sequence number plus 1, and sets the ACK flag bit to 1. At the same time, it sends its own SYN packet and randomizes the initial sequence number. generate.

5. After receiving the ACK packet and SYN packet from the server, the client sends an ACK packet as a response, sets the confirmation sequence number to the server's initial sequence number plus 1, and sets the ACK flag bit to 1.

6. What is the hand-waving process of TCP? Please explain the purpose of each step?

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

• The TCP waving process is the process of closing the TCP connection, including the following steps:

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

8. Step 2: After the other party receives the FIN packet, it 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 to express its agreement 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 response: Each TCP segment has a sequence number, which is used to identify the data in the segment. The receiver acknowledges 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 receiver. The size of the sliding window determines the amount of data that the sender can send, and the receiver informs the sender of the window size by acknowledging the response.

• Flow Control: TCP uses flow control mechanisms to ensure that the sender does not send too much data that the receiver can handle. The receiver informs the sender of the amount of data that can be received by sending the window size.

12. What is the OSI model? Please briefly describe the functionality of each tier.

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

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

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

• Network Layer (Network Layer): responsible for the routing and forwarding of data packets to achieve communication between different networks.

• Transport Layer (Transport Layer): Provide end-to-end reliable data transmission, and realize communication between processes through port numbers and protocols.

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

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

• Application Layer (Application Layer): Provides the interface between network services and applications, including HTTP, FTP, SMTP, etc.

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

• TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are two common transport layer protocols, and 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 need to establish a connection.

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

• Orderliness: TCP guarantees the orderliness of data and ensures 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 to avoid network congestion by dynamically adjusting the sending rate, while UDP has no congestion control mechanism.

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

14. What is the HTTP protocol? How does it work?

• HTTP (Hypertext Transfer Protocol) is an application layer protocol used to transmit data on the Web. Here's how it works:

• 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, etc.

• The server responds to the request: After the server receives the request, it 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 response status code, response header and response body, etc.

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

• Stateless: 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?

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

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

• IPv6: IPv6 uses 128-bit addresses, usually expressed as eight groups of hexadecimal numbers, each group of numbers ranges from 0 to FFFF, such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334. The IPv6 address space is huge, with approximately 340 trillion trillion trillions of 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 readiness for data transfer.

• The TCP three-way handshake is the process of establishing a TCP connection, including 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 the server receives the SYN packet, it sends an ACK (confirmation) packet in response, sets the confirmation sequence number to the client's initial sequence number plus 1, and sets the ACK flag bit to 1. At the same time, it sends its own SYN packet and randomizes the initial sequence number. generate.

19. After receiving the ACK packet and SYN packet from the server, the client sends an ACK packet as a response, sets the confirmation sequence number to the server's initial sequence number plus 1, and sets the ACK flag bit to 1.

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

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

• No connectivity: UDP does not need to establish a connection and sends data packets directly, without guaranteeing the reliability and order 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 will send data at a fixed rate and will not adjust according to network conditions.

• Applicable scenarios: UDP is suitable for application scenarios that require high real-time performance 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's the difference between them?

A process is an instance of an executing program and has independent memory space and system resources.

Threads are execution units within a process and share the memory space and resources of the process.

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

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

• A 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 under which deadlock occurs include mutual exclusion, possession and waiting, non-preemption and circular waiting.

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

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

• Its functions include expanding 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?

• A 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 and has high performance and reliability.

• ext3: The predecessor of ext4, it 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 snapshot, compression and verification functions.

• 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 for 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 the specified process. The process to be terminated can be specified using the process ID (PID) or process name.

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

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

6. What is a Linux pipeline (Pipeline)? How to connect commands using pipes?

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

• Multiple commands can be connected using the pipe symbol (|). For example, command1 | command2 takes the output of command1 as the input of command2.

• The function of the pipeline is to realize data transfer and processing between commands, and can combine multiple simple commands to complete complex tasks.

7. What are Linux soft links and hard links? What's 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 shortcuts in Windows. Soft links can span file systems and can be linked 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 does not affect the hard links since they share the same inode and only when all links are deleted the storage space for the file is freed.

8. What is Linux Interprocess 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 (Named Pipe): Similar to pipes, but can communicate between unrelated processes.

• Signal: Used to deliver 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 transfer complex messages and data blocks between processes.

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