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About TCP Analysis & How to Use Wireshark

TCP (Transmission Control Protocol) is one of the most important protocols in the Internet, as it provides reliable, ordered, and error-checked delivery of data between applications. TCP analysis is the process of examining TCP packets to detect and troubleshoot various network issues, such as packet loss, retransmission, latency, window size, connection state, etc. TCP analysis can help us improve network performance, security, and efficiency.


Wireshark is a powerful and popular open-source network analysis tool that can capture and display packets of various protocols, and provide rich filtering and statistical features. Wireshark can help us quickly locate and solve network communication problems, and understand the TCP protocol better.


In this blog, I will share some tips and tricks on how to use Wireshark for TCP analysis, based on some common scenarios and tasks.


Tip 1: How to capture TCP packets


The first step of TCP analysis is to capture the TCP packets that we are interested in. Wireshark provides several options to do this, such as:


- Using the Capture Filter to specify the criteria for capturing packets, such as the source or destination IP address, port number, protocol type, etc. For example, if we want to capture only the TCP packets from or to the IP address 192.168.1.100, we can use the capture filter `tcp and host 192.168.1.100`.

- Using the Display Filter to filter the packets that are already captured and displayed, based on the packet fields or expressions. For example, if we want to filter only the TCP packets that have the SYN flag set, we can use the display filter `tcp.flags.syn == 1`.

- Using the Follow TCP Stream feature to display the entire TCP conversation between two endpoints, in a separate window. This can help us see the data exchanged in the TCP session, and the sequence and acknowledgment numbers of each packet. We can access this feature by right-clicking on any TCP packet and selecting **Follow > TCP Stream**.


Tip 2: How to analyze TCP packets


The second step of TCP analysis is to analyze the TCP packets that we have captured and filtered, to identify and diagnose the network issues. Wireshark provides several tools and functions to do this, such as:


- Using the Packet Details pane to view the detailed information of each packet, such as the TCP header fields, flags, options, checksum, etc. We can also expand or collapse the different layers of the packet, such as the Ethernet, IP, TCP, and application layers, to see the relevant information.

- Using the Packet List pane to view the summary information of each packet, such as the packet number, time, source and destination addresses, protocol, length, and info. We can also sort, reorder, or hide the columns of the packet list, to customize the display.

- Using the Expert Info feature to view the diagnostic messages and severity levels of the packets, such as errors, warnings, notes, or chats. This can help us spot the potential problems or anomalies in the TCP communication, such as out-of-order, duplicate, or retransmitted packets, window size issues, connection resets, etc. We can access this feature by selecting **Analyze > Expert Info**.


Tip 3: How to use Wireshark filters for TCP analysis


One of the most powerful and useful features of Wireshark is the filtering capability, which can help us focus on the packets that are relevant to our TCP analysis. Wireshark provides two types of filters: capture filters and display filters, as mentioned in Tip 1. Wireshark filters use a syntax that is similar to the Berkeley Packet Filter (BPF) syntax, which is widely used in other network tools, such as tcpdump.


Wireshark filters can be applied in various ways, such as:


- Using the Filter Toolbar to enter and apply the filter expression, and to save or load the filter expressions. The filter toolbar also provides a color-coded feedback on the validity and syntax of the filter expression, and a list of suggested or completed filter expressions.

- Using the Expression button to construct the filter expression, by selecting the packet fields, operators, and values from the drop-down menus. This can help us avoid typing errors and find the correct packet fields and values.

- Using the Packet Details pane to apply the filter expression, by right-clicking on any packet field and selecting Apply as Filter > Selected. This can help us quickly filter the packets based on the value of the selected field.


Some examples of Wireshark filters for TCP analysis are:


- `tcp.port == 80` : This filter matches the TCP packets that have the source or destination port number equal to 80, which is the default port for HTTP protocol.

- `tcp.flags.fin == 1 and tcp.flags.ack == 1` : This filter matches the TCP packets that have both the FIN and ACK flags set, which indicate the end of a TCP connection.

- `tcp.analysis.retransmission` : This filter matches the TCP packets that are retransmitted, which indicate the packet loss or network congestion.


Conclusion

TCP analysis is an essential skill for network professionals and enthusiasts, as it can help us understand, troubleshoot, and optimize the network communication. Wireshark is a powerful and popular tool for TCP analysis, as it can capture and display TCP packets, and provide rich filtering and statistical features. In this blog, I have shared some tips and tricks on how to use Wireshark for TCP analysis, based on some common scenarios and tasks. I hope you find them useful and interesting. If you have any questions or feedback, please feel free to leave a comment below. Thank you for reading!


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