Скачать или смотреть Understanding the TCP Previous Segment Not Captured Issue: Root Causes and Solutions

Discover the root causes of the TCP `Previous segment not captured` issue and learn how to correct sequence and acknowledgment numbers for smoother packet transmission.
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Understanding the TCP Previous Segment Not Captured Issue: Root Causes and Solutions

When dealing with TCP packet transmission, encountering the message TCP Previous segment not captured in tools like Wireshark can be frustrating. This issue often indicates a problem in communication, particularly with the handling of sequence (Seq) and acknowledgment (Ack) numbers. In this guide, we will explore why this error occurs and how we can fix it.

The Problem Explained

The issue manifests itself when TCP communication involves sending packets after the initial handshake. Wireshark captures packets between two communicating hosts, revealing discrepancies in sequence and acknowledgment numbers, particularly in scenarios involving raw sockets and custom packet manipulation.

For instance, in your scenarios, you're attempting to send a PSH, ACK packet after the TCP handshake, but Wireshark logs indicate a Previous segment not captured. This usually stems from improper handling of sequence and acknowledgment numbers during transmission.

Breakdown of Symptoms

Key observations that suggest a problem include:

The presence of large sequence and acknowledgment numbers in Wireshark logs.

Duplicate acknowledgments (often indicated by TCP Dup ACK messages).

Inaccurate increments in sequence numbers due to incorrect handling in the code.

Identifying the Root Causes

From the provided packet analysis and the code, we can identify several core issues leading to the problem:

Incorrect Handling of Sequence and Acknowledgment Numbers:

You are pulling sequence and ACK numbers from incoming packets and storing them correctly using ntohl. However, the references to these numbers during packet sending are mismatched, leading to confusion in packet processing.

Improper Use of Conversion Functions:

The sequence and acknowledgment numbers are 32-bit integers, but you are using htons (which is for 16-bit integers) instead of htonl. This mismatch causes the higher byte data to be lost, resulting in erroneous sequence values.

Inconsistent Sequence Number Management:

If the data_len variable, which indicates the size of data being sent, is not set to 1, incrementing the sequence number by one will eventually misalign with the data being sent, leading to potential inconsistencies.

Solutions to the Problem

To correct the issues causing the Previous segment not captured, follow these steps:

1. Correct Sequence and Acknowledgment Handling

Make sure you are properly assigning the seq and ack_seq values in your code. Consider redefining how you're managing these numbers:

[[See Video to Reveal this Text or Code Snippet]]

This ensures the sequence numbers are accurately reflected.

2. Use the Correct Conversion Function

Instead of using htons, which drops part of the payload:

[[See Video to Reveal this Text or Code Snippet]]

This will preserve all bytes of the sequence numbers.

3. Align Data Length with Sequence Increment

Make sure that when you send additional data, the sequence numbers reflect the actual data length being sent. For instance, if sending multiple bytes of payload, the sequence number should increment accordingly:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Encountering TCP Previous segment not captured can seem complex at first. However, by unraveling the underlying issues related to sequence and acknowledgment number handling, you can significantly enhance the reliability of your TCP communication. Adjusting your code according to the suggestions outlined above will help in resolving these capture inaccuracies and lead to smoother packet transactions.

Ensuring accurate packet processing and sequence number management is essential for develop