See if you can handle this kind of interview. How does TCP transmit reliably?

Preface

Hello everyone, I am Amazing. It is the golden September and October again, and it is the interview season again. Are you guys who are working ready to look for opportunities? Or are you planning to wait and see what will happen next year?

But no matter how hard you try, you should not give up studying, especially the eight-legged essay.

This time, I bring you an interview article about how TCP can transmit reliably. I will take you to explore from the most superficial interview answers to the fancy questions about this knowledge.

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TCP (Transmission Control Protocol) ensures the reliability of data transmission through multiple mechanisms, including connection management, checksum, sequence number, confirmation response, timeout retransmission, flow control and congestion control.

1. Connection management: TCP establishes a connection through a three-way handshake to ensure that both parties are ready for data transmission. After data transmission is completed, the connection is terminated through a four-way handshake to ensure that all data has been correctly transmitted and received.
2. Checksum: The TCP message header contains a checksum field to detect the integrity of the data during transmission. If data corruption or errors are detected, TCP will discard the packet and notify the sender to resend it.
3. Sequence Number: TCP uses sequence numbers to ensure that packets arrive at the receiver in order. Each packet has a unique sequence number, and the receiver reassembles the packets by checking the sequence number.
4. Acknowledgement: After receiving each data packet, the receiver sends an acknowledgment (ACK) signal to indicate that the data packet has been successfully received. If the receiver does not receive an acknowledgment, the sender will retransmit the data packet.
5. Timeout retransmission: TCP uses a timeout timer to detect whether a data packet is lost. If no confirmation response is received within the set time, the sender will automatically retransmit the data packet.
6. Flow control: TCP uses a sliding window mechanism for flow control to prevent the sender from sending data too fast for the receiver to process in time. The sliding window mechanism allows the receiver to adjust the sending rate based on the size of its buffer and network conditions.
7. Congestion control: TCP detects and responds to network congestion through mechanisms such as slow start, congestion avoidance, fast retransmission, and fast recovery. When network congestion is detected, TCP reduces the sending rate to reduce network load.

Through these mechanisms, TCP can ensure the reliability of data transmission, prevent data loss, duplication and misordering, and adapt to changes in network conditions to improve transmission efficiency.

What is the specific process of TCP three-way handshake and four-way wave?

TCP three-way handshake process

The TCP three-way handshake is the process of establishing a TCP connection, which requires the client and server to send a total of three messages.

1. First handshake:

• The client sends a SYN message with the SYN bit set to 1 and a randomly generated sequence number. This message specifies the server port to which the client intends to connect and contains the client's initial sequence number (ISN).
• The purpose of this message is to let the server know the client's connection request and synchronize the sequence numbers of both parties.

2. Second handshake:

• After receiving the SYN message, the server will reply with a SYN-ACK message, in which the SYN bit and ACK bit are both set to 1. The value of the server's ACK field is the sequence number sent by the client plus 1, indicating that the SYN message from the client has been received.
• The purpose of this message is to let the client know that the server has received and agreed to establish a connection, and to synchronize the confirmation numbers of both parties.

3. The third handshake:
   

• After receiving the SYN-ACK message from the server, the client sends an ACK message with the ACK bit set to 1 and the confirmation number being the sequence number in the server's SYN message plus 1. This message indicates that the client has received and agreed to establish a connection.
• The purpose of this message is to let the server know that the client is ready to receive data and that the connection between the two parties has been established.
  

TCP four-wave process

TCP four-wave handshake is the process of closing a TCP connection, which requires the client and server to send a total of four messages.

1. First wave:

• The client sends a FIN message to indicate that it has no more data to send. The FIN bit of this message is set to 1, indicating that the client requests to close the connection.

2. Second wave:

• After receiving the FIN message, the server will reply with an ACK message, in which the ACK bit is set to 1 and the confirmation number is the sequence number in the client's FIN message plus 1. This message indicates that the server has received and confirmed the client's close request.

3. The third wave:
   

• After receiving the ACK message from the server, the client sends a FIN message to indicate that it has no more data to receive. The FIN bit of this message is set to 1, indicating that the client requests to close the connection.

4. The fourth wave:
   

• After receiving the FIN message from the client, the server will send an ACK message, in which the ACK bit is set to 1 and the confirmation number is the sequence number in the client's FIN message plus 1. This message indicates that the server has received and confirmed the client's close request. At this point, the server has no data to send, and the connection between the two parties is officially closed.

What else can I ask?

How does the TCP checksum mechanism detect and handle data corruption or errors?

The TCP checksum mechanism detects data corruption or errors by calculating and verifying the checksum in the data packet.

Specifically, the TCP checksum is an end-to-end checksum that is calculated by the sender and added to the header of the data packet. After receiving the data packet, the receiver recalculates the checksum and compares it with the sender's checksum.

If the checksums do not match, the receiver will assume that an error has occurred in the transmission of the packet and discard the packet.

In addition, the TCP checksum is not only used to detect the integrity of the data packet, but also to detect whether any changes have occurred to the TCP header and data during transmission.

If data corruption is detected, TCP will handle the error through a timeout retransmission mechanism, that is, if no confirmation response is received within a certain period of time, the sender will resend the data packet.

This mechanism ensures reliable data transmission.

How does the TCP sliding window mechanism work and how does it affect the data transfer rate?

The TCP sliding window mechanism is a key mechanism in the TCP protocol for controlling the data transmission rate and ensuring the reliability of data transmission.

How it works

The sliding window mechanism allows the sender to continue sending multiple packets while waiting for an acknowledgment from the receiver. The window size refers to the maximum number of packets that the sender can send without waiting for an acknowledgment. For example, if the window size is 3, the sender can send three packets in a row without waiting for an acknowledgment of the previous packet.

Each TCP/IP host supports two sliding windows: one for receiving data and one for sending data. The sizes of the send window and receive window can be adjusted dynamically to adapt to network conditions and the host's buffer capacity.

The send window size is dynamically adjusted by the sender based on the receiver's confirmation information. If the receiver confirms a packet, the sender can continue to send the next packet until the send window size reaches its maximum value. The receive window size is dynamically adjusted by the receiver based on the capacity of its buffer to prevent buffer overflow.

Affects data transfer rate

The sliding window mechanism allows the sender to continue to send multiple data packets while waiting for confirmation, thereby reducing the waiting time for confirmation and improving the efficiency of data transmission. For example, if only one data packet can be sent at a time, it needs to wait for the receiver's confirmation, which will greatly affect the transmission rate.

The sliding window mechanism implements flow control by dynamically adjusting the window size to prevent the sender from sending data too quickly, causing the receiver's buffer to overflow. This mechanism ensures the smoothness and reliability of data transmission.

The sliding window mechanism is also combined with TCP's congestion control mechanism to avoid network congestion by adjusting the window size. When network congestion is detected, the sender will reduce the window size and slow down the data transmission rate to reduce the network load.

Final Conclusion

The TCP sliding window mechanism achieves efficient data transmission and flow control by dynamically adjusting the size of the send window and the receive window. Its working principle includes the concept of sliding window, window maintenance and dynamic adjustment. This mechanism not only improves the efficiency of data transmission, but also ensures the reliability and stability of data transmission.

Slow start, congestion avoidance, fast retransmit and fast recovery mechanisms

The slow start, congestion avoidance, fast retransmit and fast recovery mechanisms in TCP congestion control work together to ensure the stability and fairness of the network and achieve efficient data transmission.

1. Slow start: After the TCP connection is established, it first enters the slow start phase. In this phase, the sender's congestion window (cwnd) doubles for each acknowledgment (ACK) packet received, and the data transmission rate increases exponentially. This process continues until the congestion window reaches the slow start threshold (ssthresh) or packet loss occurs.
2. Congestion avoidance: When cwnd reaches ssthresh, it enters the congestion avoidance phase. In this phase, the congestion window no longer grows exponentially, but grows linearly, that is, cwnd increases by 1 for every round trip time (RTT). This can avoid network congestion caused by too fast a sending rate.
3. Fast retransmit: When the sender receives three consecutive duplicate acknowledgment packets, it will immediately retransmit the lost data packet instead of waiting for a timeout. This mechanism can quickly detect packet loss problems in the network and retransmit in a timely manner.
4. Fast recovery: The fast recovery mechanism is activated after fast retransmission. When packet loss occurs, the sender will immediately retransmit the lost data packets and halve the congestion window before entering the congestion avoidance phase. This allows the sending rate to be adjusted quickly to avoid network congestion caused by packet loss.

How does TCP detect packet loss and ensure data integrity through a timeout retransmission mechanism?

The TCP protocol detects packet loss and ensures data integrity through a timeout retransmission mechanism.

Specifically, after sending a data packet, the TCP protocol starts a timer to wait for the receiver's confirmation (ACK).

If no ACK is received within the specified time, the sender will trigger the timeout retransmission mechanism and resend the data packet.

This mechanism is implemented through a timer. When the timer expires, the sender will send the data packet again.

In addition, TCP detects packet loss by the number of consecutive duplicate acknowledgment packets (Dup-ACK) from the receiver.

When the sender receives more than three duplicate ACKs, it realizes that the packet is lost and resends the packet.

This mechanism ensures that data packets can be transmitted correctly even in the event of network congestion or packet loss.

TCP's timeout retransmission mechanism not only detects packet loss, but also optimizes network performance through congestion control.

When packet loss is detected, TCP resets the congestion window (cwnd) to 1 and sets the slow start threshold (ssthresh) to half the current congestion window size.

This mechanism helps prevent network congestion and ensures the reliability of data transmission.