Wireshark Lab TCP V70 Supplement To Computer Networking

Wireshark Lab Tcp V70 Supplement To Computer Networking A Top Do

Analyze TCP protocol behavior by examining a packet trace of transferring a 150KB file from your computer to a remote server using Wireshark. Focus on TCP’s use of sequence and acknowledgment numbers, congestion control mechanisms such as slow start and congestion avoidance, receiver-advertised flow control, connection setup, and overall connection performance metrics like throughput and round-trip time. Capture or use a provided trace, filter TCP packets, identify key segments (e.g., SYN, SYNACK, HTTP POST), measure sequence and acknowledgment numbers, analyze TCP windowing and retransmissions, and plot TCP dynamics to observe congestion control phases. Provide detailed observations, with annotated packet prints, supported by credible references for TCP/IP behavior and Wireshark analysis techniques.

Paper For Above instruction

The analysis of TCP behavior through packet trace examination offers critical insights into how reliable data transfer and congestion control are implemented and managed in real networks. This paper discusses probing TCP's operational mechanisms during a file transfer, particularly utilizing Wireshark to reveal the intricacies of sequence numbers, acknowledgment strategies, congestion control phases, and overall connection performance. The study is grounded in a trace capturing the transfer of an ASCII version of "Alice in Wonderland," enabling a detailed exploration of TCP processes in action.

Introduction

Transmission Control Protocol (TCP) is fundamental to the Internet's reliable data transfer paradigm. Understanding TCP's internal mechanics—such as sequence numbers, acknowledgments, flow control, and congestion control—is vital for network diagnostics and performance tuning. This analysis employs a packet trace captured during a file transfer from a client computer to a remote server, providing real-world data to support TCP education and research. Using Wireshark, a leading network protocol analyzer, allows for a detailed dissection of individual TCP segments, enabling an exploration into TCP’s operational phases and issues like retransmissions, window management, and RTT estimation.

Methodology

The primary method involves capturing network traffic during the file upload process, specifically focusing on TCP segments and associated control messages. The capture process was initiated by opening Wireshark before starting the file upload, ensuring all relevant packets are recorded. The trace file was then filtered to display TCP packets, isolating the transfer of a large file via HTTP POST. Critical packets such as the three-way handshake, data segments, acknowledgments, and retransmissions were identified and detailed. Annotating these packets clarified the understanding of each TCP process involved.

Initial TCP Connection Analysis

The initial connection setup begins with a SYN packet from the client, characterized by a specific sequence number. Correct identification of this segment is crucial, as it initiates the TCP handshake. The corresponding SYNACK reply from the server contains its own sequence number and an acknowledgment number, which confirms receipt of the client's SYN. The presence of the SYN and ACK flags in these segments confirms the handshake process. The trace data show that the client’s SYN uses a random sequence number, complying with TCP specifications for connection randomness and security.

Sequence and Acknowledgment Numbers

Once the connection is established, data transfer commences with subsequent segments bearing sequence numbers that are incremented by the number of bytes sent. The segment carrying the HTTP POST command is identified by searching packet contents for "POST." The sequence number of this segment marks the start of data transmission, followed by ACKs from the server that acknowledge the receipt of data up to a certain byte. The acknowledgment number in ACK segments indicates the next expected byte, which exemplifies TCP's cumulative acknowledgment strategy. These values help to demonstrate TCP's reliable transfer function, as acknowledgments confirm receipt with specific sequence number references.

Congestion Control and Flow Control phenomena

Examining the time-sequence-graph plot from Wireshark reveals TCP's congestion control phases. TCP starts in slow start, with window size increasing exponentially during initial data transmission—visible as a rapidly rising sequence number plot. As throughput stabilizes, congestion avoidance mechanisms employ linear window growth, observable by a more gradual slope increase. The trace analysis also reveals instances where TCP halves its congestion window following packet loss or perceived congestion, aligning with TCP's AIMD (Additive Increase/Multiplicative Decrease) algorithm. The receiver's advertised window size appears in the TCP header, limiting sender’s transmission rate, thus demonstrating flow control's role in preventing buffer overflow.

Retransmissions, RTT Measurement, and Throughput

Retransmissions are identifiable in the trace by duplicate sequence numbers within retransmitted segments. Checks for triple duplicate ACKs and retransmission timeouts help understand TCP’s loss recovery mechanisms. The RTT for individual segments is measured as the time difference between segment transmission and acknowledgment receipt, obtainable from Wireshark's timing features. The estimated RTT is then iteratively computed, reflecting TCP's adaptive timeout adjustments. Finally, throughput is calculated by dividing total data transferred by the duration of the transfer, with the trace revealing typical data acknowledgment patterns, window management, and retransmission behaviors that influence overall transfer efficiency. Notably, the trace indicates how TCP dynamically adapts to network conditions to optimize data flow.

Discussion and Conclusion

This real network analysis underpins the theoretical constructs of TCP's operation, such as the significance of the three-way handshake, congestion window adjustments, and retransmission strategies. Observing the phases of slow start transitioning into congestion avoidance underscores TCP's effectiveness in managing network congestion. The detailed examination of sequence and acknowledgment numbers, window advertisements, and retransmissions confirms TCP’s robustness in ensuring data integrity despite packet loss or network variability. These insights affirm TCP’s pivotal role as a reliable, congestion-aware transport protocol in modern networks, with the trace serving as a practical validation of its complex, adaptive behavior.

References

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