TCP segment structure in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the purpose of the Sequence Number field in a TCP segment

What is the main role of the Sequence Number field in a TCP segment?

A It indicates the order of bytes sent from sender to receiver to ensure data is reassembled correctly.

B It contains the checksum value used for error detection in the segment.

C It specifies the maximum size of the TCP segment that can be received.

D It signals the type of service requested for the segment in the network.

💡 Hint

Think about how TCP ensures data arrives in the right order.

The Sequence Number field helps the receiver put the data back in the correct order by numbering each byte sent.

📋 Factual

intermediate

Size of the TCP Header without options

What is the size in bytes of a standard TCP header without any options?

A 40 bytes

B 8 bytes

C 20 bytes

D 32 bytes

💡 Hint

Recall the fixed length of the TCP header before options are added.

The standard TCP header is 20 bytes long when no options are included.

🔍 Analysis

advanced

Determine the effect of the Window Size field in TCP flow control

How does the Window Size field in a TCP segment affect data transmission?

A It contains the port number of the destination application.

B It specifies the total size of the TCP segment including header and data.

C It indicates the priority level of the segment in the network queue.

D It controls how many bytes the sender can transmit before waiting for an acknowledgment, managing flow control.

💡 Hint

Consider how TCP prevents overwhelming the receiver with too much data at once.

The Window Size tells the sender how much data it can send before needing an acknowledgment, helping to avoid congestion.

❓ Comparison

advanced

Compare the roles of the Acknowledgment Number and Sequence Number fields

Which statement correctly compares the Acknowledgment Number and Sequence Number fields in a TCP segment?

A Sequence Number is used only in connection setup; Acknowledgment Number is used only in connection teardown.

B Sequence Number indicates the byte number sent; Acknowledgment Number indicates the next expected byte from the other side.

C Sequence Number shows the total bytes received; Acknowledgment Number shows the total bytes sent.

D Sequence Number and Acknowledgment Number both indicate the same byte number for error checking.

💡 Hint

Think about how TCP tracks sent data and confirms receipt.

The Sequence Number marks the byte position sent; the Acknowledgment Number tells the sender which byte the receiver expects next.

❓ Reasoning

expert

Identify the error caused by an incorrect Checksum field in a TCP segment

What happens if the Checksum field in a received TCP segment does not match the computed checksum?

A The segment is discarded because it is considered corrupted during transmission.

B The segment is accepted but flagged for retransmission later.

C The segment is forwarded to the application layer without error checking.

D The segment triggers a connection reset immediately.

💡 Hint

Consider how TCP ensures data integrity before processing.

If the checksum does not match, TCP treats the segment as corrupted and discards it to avoid passing bad data.

Practice

1. Which part of a TCP segment is responsible for identifying the sending and receiving applications?

easy

A. Sequence number

B. Urgent pointer

C. Checksum

D. Source and destination port numbers

Solution

1. Step 1: Understand the role of ports in TCP

   Ports identify specific applications or services on the sending and receiving devices.

2. Step 2: Identify the fields related to application identification

   The source and destination port numbers in the TCP header specify which applications are communicating.

3. Final Answer:

   Source and destination port numbers -> Option D

4. Quick Check:

   Ports identify apps = Source and destination port numbers [OK]

Hint: Ports identify apps communicating [OK]

Common Mistakes:

• Confusing sequence number with port numbers
• Thinking checksum identifies applications
• Mixing urgent pointer with port functions

2. Which of the following is the correct order of fields in the TCP header starting from the beginning?

easy

A. Destination port, Source port, Urgent pointer, Sequence number

B. Sequence number, Source port, Destination port, Flags

C. Source port, Destination port, Sequence number, Acknowledgment number

D. Checksum, Source port, Destination port, Sequence number

Solution

1. Step 1: Recall TCP header field order

   The TCP header starts with source port, then destination port, followed by sequence number and acknowledgment number.

2. Step 2: Match the correct sequence

   Source port, Destination port, Sequence number, Acknowledgment number correctly lists the first four fields in order.

3. Final Answer:

   Source port, Destination port, Sequence number, Acknowledgment number -> Option C

4. Quick Check:

   TCP header starts with ports then sequence numbers [OK]

Hint: Ports come first in TCP header [OK]

Common Mistakes:

• Mixing sequence number before ports
• Placing checksum at the start
• Confusing urgent pointer position

3. Given a TCP segment with sequence number 1000 and acknowledgment number 2000, which field indicates the next expected byte from the sender?

medium

A. Acknowledgment number

B. Sequence number

C. Checksum

D. Window size

Solution

1. Step 1: Understand the meaning of sequence and acknowledgment numbers

   The sequence number shows the byte number of the first byte in this segment. The acknowledgment number indicates the next byte expected from the other side.

2. Step 2: Identify the field indicating the next expected byte

   The acknowledgment number tells the sender which byte the receiver expects next.

3. Final Answer:

   Acknowledgment number -> Option A

4. Quick Check:

   Next expected byte = Acknowledgment number [OK]

Hint: Ack number shows next expected byte [OK]

Common Mistakes:

• Confusing sequence number with acknowledgment number
• Thinking checksum indicates byte order
• Mixing window size with byte tracking

4. A TCP segment has the flags field set to 0x12 (binary 00010010). Which flags are set in this segment?

medium

A. RST and URG flags

B. SYN and ACK flags

C. FIN and PSH flags

D. ACK and URG flags

Solution

1. Step 1: Convert 0x12 to binary and identify flags

   0x12 in binary is 00010010. The TCP flags bits from left to right are: URG, ACK, PSH, RST, SYN, FIN. Here, bits for ACK (bit 4) and SYN (bit 1) are set.

2. Step 2: Match flags to bits set

   Bits 4 and 1 correspond to ACK and SYN flags respectively, so these two flags are set.

3. Final Answer:

   SYN and ACK flags -> Option B

4. Quick Check:

   0x12 = SYN + ACK flags set [OK]

Hint: 0x12 means SYN and ACK flags set [OK]

Common Mistakes:

• Misreading binary flag bits
• Confusing PSH with SYN
• Mixing URG with FIN flags

5. You want to ensure data integrity in a TCP segment. Which field should you check and why?

hard

A. Checksum, because it verifies data correctness

B. Sequence number, because it orders data

C. Source port, because it identifies sender

D. Urgent pointer, because it marks urgent data

Solution

1. Step 1: Understand the purpose of the checksum field

   The checksum is used to detect errors in the TCP segment's header and data during transmission.

2. Step 2: Identify which field ensures data integrity

   Only the checksum verifies that the data has not been corrupted or altered.

3. Final Answer:

   Checksum, because it verifies data correctness -> Option A

4. Quick Check:

   Data integrity = Checksum verification [OK]

Hint: Checksum checks data errors [OK]

Common Mistakes:

• Confusing sequence number with error checking
• Thinking source port verifies data
• Misunderstanding urgent pointer's role