Article directory

introduction
- The importance and widespread application of TCP
- 2The importance and role of MSL
TCP connection closing process
2What is MSL
2MSL’s functions and application scenarios
2MSL specific calculation method
2MSL influencing factors and tuning strategies
2MSL limitations and considerations
- 2MSL limitations and possible problems
- Things to note and common misunderstandings
in conclusion
references
appendix

introduction

The importance and widespread application of TCP

TCP (Transmission Control Protocol) is a reliable, connection-oriented network protocol that plays an important role in the Internet. The TCP protocol is responsible for ensuring the reliable transmission of data in the network. By using sequence numbers, confirmation and retransmission mechanisms, it ensures that data can reach the destination in the correct order.

TCP protocol is widely used in various network communication scenarios, such as web browsing, file transfer, email, etc. Its reliability and stability make it the protocol of choice for Internet communications.

2The importance and role of MSL

2MSL (Two Maximum Segment Lifetime) is an important time parameter in the TCP connection closing process. It refers to waiting for twice the maximum segment lifetime time after the party actively closing the connection sends the last ACK (confirmation) message before the connection is considered to have been completely closed.

The role of 2MSL is to ensure that all segments of the connection can completely disappear from the network to prevent old segments from interfering with new connections. In network communication, correct understanding and reasonable setting of 2MSL are crucial to ensure the reliability and stability of the connection.

TCP connection closing process

TCP connection establishment and closing

Before understanding 2MSL, we first need to understand the establishment and closing process of TCP connections.

The establishment of a TCP connection is completed through a three-way handshake, namely:

The client sends a SYN (synchronization) message to the server, requesting to establish a connection.
After receiving the SYN message, the server replies with a SYN+ACK (synchronization + confirmation) message to indicate that it agrees to establish a connection.
After receiving the SYN+ACK message from the server, the client replies with an ACK (confirmation) message, indicating that the connection is successfully established.

The closing of the TCP connection is completed through Four-way Handshake, namely:

The party that actively closes the connection sends a FIN (End) message requesting that the connection be closed.
After receiving the FIN message, the other party replies with an ACK message, indicating that the shutdown request has been received.
After the other party completes sending its own data, it sends a FIN message requesting to close the connection.
After the party actively closing the connection receives the FIN message, it replies with an ACK message, indicating that the connection has been completely closed.

Why you need to close the connection

In network communication, it is important to close the connection in time when a communication ends. Closing the connection can release system resources and avoid wasting resources. At the same time, closing the connection in time can also ensure the reliability and stability of the connection.

TCP’s four wave processes

TCP's four-wave process ensures that both parties to the connection can properly close the connection to avoid data loss and incomplete transmission.

The specific four-wave waving process is as follows:

The party that actively closes the connection sends a FIN message requesting that the connection be closed.
After receiving the FIN message, the other party replies with an ACK message, indicating that the shutdown request has been received.
After the other party completes sending its own data, it sends a FIN message requesting to close the connection.
After the party actively closing the connection receives the FIN message, it replies with an ACK message, indicating that the connection has been completely closed.

2What is MSL

MSL (Maximum Segment Lifetime) is a parameter in the TCP protocol, which refers to the longest survival time of a segment in the network. The specific value of MSL is usually determined by the operating system, and usually ranges from tens of seconds to several minutes.

2MSL is twice the MSL, which is 2 times the maximum segment lifetime time. During the TCP connection closing process, after the party actively closing the connection sends the last ACK message, it needs to wait 2MSL before the connection is considered to have been completely closed.

The meaning of 2MSL is to ensure that all segments of the connection can completely disappear from the network. Consider this situation: after the party that actively closes the connection sends the last ACK message, there may still be some delayed arrival segments in the network. If the party that actively closed the connection immediately re-establishes a new connection, these old fragments may interfere with the new connection. Waiting for 2MSL can ensure that old segments disappear from the network and avoid interference with new connections.

2MSL’s functions and application scenarios

Why do you need to wait 2MSL?

The waiting time of 2MSL is to ensure the reliability and stability of the connection. In network communication, old segments may stay in the network for a period of time. Waiting for 2MSL can ensure that these old segments disappear completely and avoid interference with new connections.

2The role and significance of MSL

The role of 2MSL is mainly reflected in the following aspects:

Ensure reliable closing of the connection: Waiting for 2MSL can ensure that all segments of the connection completely disappear from the network, preventing old segments from interfering with new connections.
Avoid connection reuse problems: During the waiting time of 2MSL, the port number and IP address of the connection cannot be reused by other connections, ensuring the uniqueness of the connection.
Ensure the consistency of the connection status: Waiting for 2MSL can ensure that the status information in the connection status table has expired, ensuring the consistency of the connection status.

2MSL application scenarios in network communications

2MSL has a variety of application scenarios in network communications, including but not limited to:

Prevent old segments from interfering with new connections: After the old connection is closed, wait 2MSL before establishing a new connection. This can avoid interference from old segments on new connections and ensure the reliability of the connection.
Ensure the uniqueness of the connection: During the waiting time of 2MSL, the port number and IP address of the connection cannot be reused by other connections, ensuring the uniqueness and security of the connection.
Ensure the consistency of the connection status: Waiting for 2MSL can ensure that the status information in the connection status table has expired, ensuring the consistency of the connection status.

2MSL specific calculation method

How to calculate the value of 2MSL

Calculating the value of 2MSL requires knowing the value range of MSL. The MSL is usually determined by the operating system and ranges from tens of seconds to minutes.

The specific way to calculate the value of 2MSL is: 2MSL = 2 * MSL

Common value ranges of MSL

The value of MSL usually ranges from tens of seconds to several minutes. The specific value range may vary depending on the operating system, but in general, the common value range of MSL is 30 seconds to 120 seconds.

The value of MSL depends on the implementation of the operating system and the characteristics of the network environment. Shorter MSL values free up resources and reuse port numbers and IP addresses more quickly, but may increase the risk of old segments interfering with new connections. A longer MSL value better ensures reliable closing of the connection, but increases the connection closing time.

2MSL calculation examples

Assuming the value of MSL is 60 seconds (1 minute), then the value of 2MSL is 120 seconds (2 minutes).

During the TCP connection closing process, after the party actively closing the connection sends the last ACK message, it needs to wait 2MSL before the connection is considered to have been completely closed. In this example, the party that actively closed the connection will wait for 2 minutes before re-establishing a new connection.

This example illustrates the specific calculation method and function of 2MSL, ensuring that all segments of the connection can completely disappear in the network to avoid interference by old segments on new connections.

2MSL influencing factors and tuning strategies

Factors affecting 2MSL

The value of 2MSL is affected by multiple factors, including but not limited to the following:

Implementation of operating system: Different operating systems may have different settings for the values of MSL and 2MSL.
Characteristics of the network environment: Characteristics of the network environment such as network delay and bandwidth will also affect the value of 2MSL.
Application requirements: The application's connection closing requirements and performance requirements will also affect the value of 2MSL.

How to adjust the value of 2MSL

In some cases, the value of 2MSL may need to be adjusted to meet specific needs. Adjusting the value of 2MSL requires caution and the following points need to be considered:

Refer to the operating system's recommendations: First, you should refer to the operating system's documentation or recommendations to understand the operating system's default values and adjustment methods for MSL and 2MSL.
Evaluate the network environment: Evaluate the characteristics of the network environment, including factors such as delay and bandwidth, to determine whether the value of 2MSL needs to be adjusted.
Consider application requirements: Consider the application's connection closing requirements and performance needs to determine whether the value of 2MSL needs to be adjusted.
Conduct experiments and tests: Before making actual adjustments, you should conduct experiments and tests to evaluate the effect and impact of the adjustments to ensure that there will be no negative impact on the stability and reliability of the system and applications.

2MSL tuning strategies and practices

Here are some common 2MSL tuning strategies and practices:

Increase the value of 2MSL: If the network environment is poor or the application has high reliability requirements for connection closure, you can increase the value of 2MSL appropriately.
Reduce the value of 2MSL: If the network environment is good or the application has high performance requirements for connection closing, you can consider appropriately reducing the value of 2MSL.
Use the TCP Keep-Alive mechanism: The TCP Keep-Alive mechanism can send a probe message after the connection has been idle for a period of time to detect whether the connection is still valid. By using the TCP Keep-Alive mechanism, connection abnormalities can be discovered in time and the connection can be closed faster, thereby reducing the waiting time of 2MSL.
Use a connection pool to manage connections: For applications that need to frequently establish and close connections, consider using a connection pool to manage connections. The connection pool can reuse established connections to avoid frequent establishment and closing of connections, thereby reducing the waiting time of 2MSL.
Monitor and tune the network environment: Regularly monitor the delay, bandwidth and other indicators of the network environment, detect network anomalies in a timely manner, and take corresponding tuning measures to ensure the reliability and stability of the connection.

2MSL limitations and considerations

2MSL limitations and possible problems

Although 2MSL plays an important role in ensuring reliable closing of connections, it also has some limitations and possible problems:

Extend the connection closing time: The waiting time of 2MSL will extend the connection closing time, which may have a certain impact on the performance of the application.
Interference of old segments on new connections: During the waiting time of 2MSL, old segments may interfere with new connections, resulting in instability or delay in the connection.
Limitations of the operating system and network environment: The value of 2MSL is limited by the operating system and network environment, and may not meet the needs of specific application scenarios.

Things to note and common misunderstandings

When using and adjusting 2MSL, you need to pay attention to the following points:

Adjust the value of 2MSL carefully: Adjusting the value of 2MSL requires careful operation. The effect and impact of the adjustment should be fully evaluated and tested to avoid negative impacts on the stability and reliability of the system and applications.
Refer to the operating system's recommendations: When adjusting the value of 2MSL, you should refer to the operating system's documentation or recommendations to understand the operating system's default values and adjustment methods for MSL and 2MSL.
Comprehensive consideration of the network environment and application requirements: When determining the value of 2MSL, it is necessary to comprehensively consider the characteristics of the network environment and the application's need for connection closure to find a suitable balance point.
Regularly monitor and optimize the network environment: Regularly monitor the delay, bandwidth and other indicators of the network environment, detect network anomalies in a timely manner, and take corresponding optimization measures to ensure the reliability and stability of the connection.

in conclusion

TCP's 2MSL is an important time parameter in network communication. It ensures reliable closing of the connection and avoids the interference of old segments on new connections. The value of 2MSL is affected by the operating system, network environment and application requirements, and needs to be adjusted carefully.

In the process of using and adjusting 2MSL, you need to comprehensively consider the network environment and application requirements, refer to the recommendations of the operating system, and conduct experiments and tests to ensure the reliability and performance of the connection.

Proper use and adjustment of 2MSL is crucial to ensure the reliability and stability of the connection. In practical applications, we should evaluate and decide the value of 2MSL based on specific scenarios and needs, while paying attention to monitoring and optimizing the network environment to ensure the normal closing of connections and the stable establishment of new connections.

references

[1] TCP/IP Illustrated, Volume 1: The Protocols, W. Richard Stevens

[2] TCP/IP Detailed Explanation Volume 1: Protocol, W. Richard Stevens, Translator: Li Bo

[3] RFC 793 - Transmission Control Protocol

[4] RFC 1122 - Requirements for Internet Hosts - Communication Layers

[5] TCP Connection Termination, https://www.geeksinphoenix.com/blog/post/2016/05/10/tcp-connection-termination.aspx

[6] Understanding TCP/IP Network Stack & Writing Network Apps, https://www.cubrid.org/blog/understanding-tcp-ip-network-stack-writing-network-apps/

appendix

Appendix A: TCP connection establishment process