Lab 4 Wireshark Exploring TCP Part 1 Capturing A Bulk TCP Tr

Lab 4 Wireshark Exploring Tcppart 1 Capturing A Bulk Tcp Transfer

Analyze a captured Wireshark trace from a TCP file transfer to understand TCP behavior, including connection initiation, data transfer, retransmissions, and congestion control mechanisms. The assignment requires observing packet details, extracting IP and TCP header information, sequence numbers, timing data, and assessing TCP flow characteristics including slow start and congestion avoidance phases.

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The exploration of TCP behavior through Wireshark captures provides vital insights into the mechanisms that underpin reliable data transmission over the internet. This analysis emphasizes not only the fundamental aspects of TCP connection initiation and data transfer but also illustrates advanced concepts such as retransmission, flow control, and congestion management, which are critical to understanding TCP's robustness and efficiency.

Initially, capturing a bulk TCP transfer begins with the user initiating a file upload via a web browser. The process starts by navigating to the web form and selecting a local file—in this case, an ASCII text copy of "Alice in Wonderland." The user then triggers Wireshark to begin capturing packets (by selecting Capture -> Start), ensuring all subsequent network activities are logged. Upon uploading the file to a remote server (gaia.cs.umass.edu), the transfer generates a series of TCP packets reflective of the connection’s lifecycle. Wireshark then allows detailed inspection of these packets to analyze individual headers, sequence numbers, timing, and flow control data.

The first analytical step involves identifying the client’s IP address and TCP port number used during the transfer. This is accomplished by selecting an HTTP packet, examining its TCP segment header, and decoding the transfer details. The server’s IP address is found through its hostname resolution, with its TCP port numbers typically being standard HTTP port 80 or dynamically assigned ephemeral ports. The source IP and port, along with the destination IP and port, provide a precise snapshot of the client-server exchange. These header fields are crucial for understanding how TCP manages multiple concurrent connections and distinguishes distinct streams.

Next, the TCP handshake provides a foundation for analyzing TCP's connection establishment procedures. The initial SYN segment, marked by the SYN flag in the TCP header, initiates the three-way handshake. Its sequence number, selected by the client, is essential for subsequent acknowledgment and data ordering. The server responds with a SYN-ACK segment, which reciprocates with its own sequence number and acknowledges the client’s initial sequence number (reflected in the Acknowledgment field). The sequence numbers and acknowledgment values can be parsed directly from the TCP headers, revealing the synchronization process and initial flow parameters. These handshake parameters also offer insights into TCP’s capacity for synchronization and initial flow control.

Following connection setup, the transfer of data is primarily captured by the segment containing the HTTP POST request. By isolating that packet—specifically looking for the segment with the "POST" keyword within the segment data—the sequence number of this segment indicates where data transmission commences within the TCP stream. By examining subsequent segments, their sequence numbers, and timestamps, we can analyze the flow of data packets. The sequence flow, along with the time each segment was issued and acknowledged, allows computation of round-trip times (RTT), a key metric for measuring network latency and understanding TCP’s dynamic adjustments during transfer.

The detailed inspection of the first six segments reveals the pattern of TCP transmission. Each segment’s sequence number, combined with acknowledgment timestamps, facilitates calculating RTTs for each segment, shedding light on network performance and congestion response. The length of each TCP segment indicates how much data is transmitted per packet, influencing throughput and overall transfer efficiency. By analyzing the buffer advertisement field (window size) in TCP headers, one can determine the minimum available buffer space and monitor whether flow control mechanisms throttled the sender during the transfer.

Scrutiny of the capture also involves identifying retransmitted segments, which indicate packet loss or network congestion. These retransmissions are detected by observing duplicate sequence numbers or repeated segments within the capture timeframe—significant for diagnosing network reliability issues. Moreover, the typical acknowledgment size—that is, how much data the receiver generally acknowledges—provides insight into TCP’s flow control and window scaling features. The overall transfer throughput, calculated as the total data transmitted divided by the transfer duration, offers a performance metric crucial for network analysis and optimization.

Lastly, the TCP congestion control dynamics are vividly illustrated through the TCP stream graph, particularly the time-sequence plot. By selecting a segment and generating the Stevens time-sequence graph, the phase transitions between TCP’s slow start, congestion avoidance, and potential congestion events become evident. The initial slow start phase starts with exponential growth of the congestion window, which eventually levels off as TCP enters congestion avoidance mode. Comparing the plotted data to idealized TCP behavior reveals real-world deviations caused by network variability, packet loss, RTT fluctuations, and other factors affecting throughput and stability.

In conclusion, analyzing Wireshark captures of TCP file transfers offers a comprehensive understanding of fundamental TCP operations, from connection setup to flow management and congestion control. Such insights are vital for network administrators, developers, and researchers working to optimize data transmission and diagnose network issues, ultimately contributing to improved network performance and reliability.

References

  • Stevens, W. R. (1994). TCP/IP Illustrated, Volume 1: The Protocols. Addison-Wesley.
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  • Jacobson, V. (1988). Congestion avoidance and control. ACM SIGCOMM Computer Communications Review, 18(4), 314-329.
  • Abbasi, V., & Sari, S. (2021). Evaluating TCP congestion control mechanisms. IEEE Transactions on Network and Service Management, 18(1), 59-70.
  • He, J., et al. (2017). High-fidelity TCP analysis with Wireshark. International Journal of Communication Systems, 30(9), e3213.
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