TCP Sequence and Acknowledgment Numbers

Summary

This video explains how TCP sequence and acknowledgment numbers work during a connection between a client and server. It covers the three-way handshake, data exchange, and four-way connection termination. The process demonstrates how sequence numbers track each byte sent, while acknowledgment numbers confirm the next expected byte. Key concepts such as phantom bytes (from SYN/FIN bits) and bidirectional communication are explored, providing viewers with a clear understanding of reliable data transfer and connection management in TCP. Whether looking from the client’s or server’s perspective, the flow of data is consistently acknowledged and sequenced.

Takeaways

• 😀 The client initiates a connection with the server by performing a three-way handshake, which is connection-oriented and bidirectional.
• 😀 TCP sequence numbers and acknowledgment numbers play a crucial role in tracking data sent and received during communication.
• 😀 Sequence numbers and acknowledgment numbers increase even when no actual data is sent, especially when the SYN or FIN bit is set in the TCP header, resulting in a 'phantom byte'.
• 😀 The first sequence number sent by the client is typically set to 0 for simplicity, and the counters increase from there as data or acknowledgments are exchanged.
• 😀 Acknowledgment numbers indicate the next byte the receiver expects, allowing both parties to track the flow of data accurately.
• 😀 The client sends a GET request with a payload of 376 bytes, and the server acknowledges it with the next sequence number, which is 377.
• 😀 Sequence numbers on the server’s side are also tracked, with the acknowledgment number in each packet indicating the expected byte in the next packet.
• 😀 The server responds with HTTP data, sending 270 bytes, and the client acknowledges receipt, expecting the 377th byte to come back from the server.
• 😀 After data is transmitted, the server initiates the connection termination with a FIN bit, which also causes the sequence and acknowledgment numbers to increment by one, even if no payload is sent.
• 😀 The TCP connection ends with the client and server exchanging FIN packets, signaling the end of the communication, with each side acknowledging the other's termination request.

Q & A

What is the first step a client takes when trying to access a website according to the transcript?

The client first performs a DNS lookup to obtain the IP address of the website before setting up a TCP connection with the server.

What is the purpose of the TCP three-way handshake?

The three-way handshake establishes a connection between the client and server, ensuring it is connection-oriented and bidirectional before data transmission begins.

How are sequence numbers used in TCP communication?

Sequence numbers track the order of bytes sent from one side to another, ensuring data is transmitted and received correctly. They increment with each byte of data sent or when control bits like SYN or FIN are used.

What is a 'phantom byte' in the context of TCP?

A 'phantom byte' refers to the increment in sequence or acknowledgment numbers caused by the SYN or FIN bit in the TCP header, even when no actual payload data is transmitted.

How does the client acknowledge data received from the server?

The client sends an acknowledgment number indicating the next byte it expects to receive, effectively confirming receipt of the previous data.

In the transcript example, what happens when the client sends a GET request?

The client sends a GET request with a payload (376 bytes in the example), starting with the next sequence number after the initial SYN phantom byte, and the server acknowledges receipt of this data.

How are sequence numbers counted when data is sent in multiple segments?

Sequence numbers increment cumulatively based on the number of bytes sent. For example, if 376 bytes are sent starting with sequence number 1, the next expected byte would be sequence number 377.

What role does the FIN bit play in closing a TCP connection?

The FIN bit signals that a side has finished sending data. Setting the FIN bit increments the sequence number as a phantom byte, and it initiates the four-way termination process.

How does the server respond to a client’s FIN request?

The server acknowledges the FIN by sending back an acknowledgment number indicating the next expected byte, and eventually sends its own FIN when it is ready to close its side of the connection.

Why is it important to track both sequence and acknowledgment numbers from client and server perspectives?

Tracking sequence and acknowledgment numbers from both perspectives ensures data integrity, correct ordering, and proper flow control, allowing both sides to know which bytes have been sent, received, or expected next.

How does Wireshark help in understanding TCP sequence and acknowledgment numbers?

Wireshark allows you to observe TCP traffic and visualize sequence and acknowledgment numbers, including phantom bytes caused by SYN and FIN bits, making it easier to understand the flow of data between client and server.

Can acknowledgment numbers increase even if no data payload is sent?

Yes, acknowledgment numbers can increase due to SYN or FIN bits being set, which are treated as phantom bytes, even if there is no actual payload transmitted.