Which two TCP header fields are used to confirm receipt of data?

Introduction

The Transmission Control Protocol (TCP) is a core protocol within the TCP/IP suite, ensuring reliable, ordered, and error-checked data delivery between applications. For those preparing for the CompTIA Network+ (N10-008) certification, understanding TCP's functionality—especially how it confirms data receipt—is crucial.

Two key TCP header fields play a vital role in acknowledging data receipt:

1. Acknowledgment Number (ACK)
2. Sequence Number

This article explores these fields in detail, their role in the TCP three-way handshake, and how they ensure reliable communication. Additionally, we’ll discuss why Study4Pass is an excellent resource for CompTIA Network+ exam preparation.

The Importance of TCP in Network Communication

TCP is a connection-oriented protocol, meaning it establishes a connection before transmitting data. It ensures:

• Reliable delivery (via acknowledgments)
• Ordered data transmission (via sequencing)
• Error detection and recovery (via checksums and retransmissions)

To achieve this, TCP uses various header fields, with Sequence Number and Acknowledgment Number being essential for confirming data receipt.

TCP Header Structure Overview

The TCP header consists of multiple fields, each serving a specific purpose:

Among these, Sequence Number and Acknowledgment Number are critical for data receipt confirmation.

The Role of the Sequence Number

What is the Sequence Number?

The Sequence Number is a 32-bit field that identifies the first byte of data in a segment. It ensures:

• Ordered data transmission (bytes are sequenced correctly).
• Retransmission tracking (if data is lost, the sequence helps identify missing segments).

How Does It Work?

1. Initial Sequence Number (ISN):

• Generated randomly during the TCP three-way handshake.
• Prevents replay attacks (using old sequence numbers).

• Each byte sent is assigned a sequence number.
• The receiver uses this to reassemble data in order.

Example:

• If a sender transmits 1000 bytes with a Sequence Number = 5000, the next segment will start at 6000 (5000 + 1000).

The Role of the Acknowledgment Number (ACK)

What is the Acknowledgment Number?

The Acknowledgment Number is a 32-bit field that confirms received data. It:

• Specifies the next expected byte (indicating all prior bytes were received).
• Triggers retransmission if an expected ACK is missing.

How Does It Work?

1. During the Three-Way Handshake:

• The client sends a SYN (Sequence Number = X).
• The server responds with SYN-ACK (Acknowledgment Number = X+1).
• The client sends an ACK (Acknowledgment Number = Y+1, if server’s SYN was Y).

• If a sender transmits bytes 5000–5999, the receiver acknowledges 6000 (next expected byte).

Example:

• Sender: Sends data with Sequence Number = 1000.
• Receiver: Responds with ACK = 2000 (if 1000 bytes were received).

TCP Three-Way Handshake and Data Confirmation

The three-way handshake establishes a TCP connection using Sequence and Acknowledgment Numbers:

1. SYN (Synchronize):

• Client sends a SYN packet with a random Sequence Number (e.g., ISN = 1000).

• Server responds with:
• SYN flag + its own Sequence Number (e.g., ISN = 5000).
• ACK flag + Acknowledgment Number = 1001 (client’s ISN + 1).

• Client sends:
• ACK flag + Acknowledgment Number = 5001 (server’s ISN + 1).

Once established, data transmission begins, with each ACK confirming received bytes.

How Retransmission Works (Handling Lost Data)?

If an ACK is not received, TCP assumes data was lost and retransmits:

• Timeout-Based Retransmission: Waits for an ACK; if none arrives, resends.
• Fast Retransmit: If three duplicate ACKs are received, retransmits immediately.

Example Scenario:

1. Sender transmits segments with Seq=1000, 2000, 3000.
2. Receiver gets 1000, 3000 (2000 is lost).
3. Receiver sends ACK=2000 (expecting 2000 next).
4. Sender retransmits Seq=2000 after timeout or duplicate ACKs.

Why Understanding TCP is Crucial for CompTIA Network+ (N10-008)?

The CompTIA Network+ (N10-008) exam tests knowledge of:

• TCP vs. UDP differences (reliability, connection-oriented vs. connectionless).
• Three-way handshake and four-way termination.
• Sequence and Acknowledgment Numbers' role in data integrity.

Mastering these concepts ensures success in:

• Troubleshooting network issues (e.g., connection drops, retransmissions).
• Optimizing network performance (e.g., adjusting window size for throughput).

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Conclusion

The Sequence Number and Acknowledgment Number are critical TCP header fields that ensure reliable data delivery by:

• Sequencing data for correct reassembly.
• Acknowledging received bytes and triggering retransmissions if needed.

For CompTIA Network+ (N10-008) aspirants, mastering these concepts is essential. Leveraging Study4Pass resources can significantly enhance preparation, ensuring exam success and career advancement in networking.

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Actual Exam Questions For CompTIA's N10-008 Certification

Sample Questions For CompTIA N10-008 Exam Success

1. Which two fields in the TCP header are responsible for acknowledging the receipt of data?

A) Sequence Number & Checksum

B) Acknowledgement Number & Flags (ACK)

C) Source Port & Destination Port

D) Window Size & Urgent Pointer

2. In the TCP header, which field indicates the next expected sequence number from the sender?

A) Sequence Number

B) Acknowledgement Number

C) Window Size

D) Data Offset

3. Which TCP flag must be set along with the Acknowledgement Number to confirm data receipt?

A) SYN

B) FIN

C) ACK

D) RST

4. The two TCP header fields that ensure reliable data delivery by confirming received packets are:

A) Source Port & Destination Port

B) Sequence Number & Window Size

C) Acknowledgement Number & ACK Flag

D) Checksum & Urgent Pointer

5. Which mechanism in TCP uses both the ACK flag and Acknowledgement Number to confirm data reception?

A) Flow Control

B) Error Detection

C) Connection Termination

D) Acknowledgement Mechanism