TCP segment structure in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the field that holds the source port number in a TCP segment.

Computer Networks

The first 16 bits of a TCP segment represent the [1].

Drag options to blanks, or click blank then click option'

Aacknowledgment number

Bsource port

Csequence number

Ddestination port

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing source port with destination port.

Thinking sequence number is at the start.

The first 16 bits in a TCP segment header specify the source port number, which identifies the sending application.

2fill in blank

medium

Complete the code to identify the size of the TCP header length field.

Computer Networks

The TCP header length field is [1] bits long.

Drag options to blanks, or click blank then click option'

A4

B8

C16

D32

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing header length field size with other fields like sequence number.

Assuming it is 8 bits or more.

The TCP header length field is 4 bits long and indicates the size of the TCP header in 32-bit words.

3fill in blank

hard

Fix the error in the statement about the TCP flags field size.

Computer Networks

The TCP flags field is [1] bits long.

Drag options to blanks, or click blank then click option'

A6

B8

C12

D9

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming only 6 or 8 bits for flags.

Forgetting the NS flag bit.

The TCP flags field is 9 bits long, including flags like SYN, ACK, FIN, and others.

4fill in blank

hard

Fill both blanks to complete the description of the TCP checksum and urgent pointer fields.

Computer Networks

The TCP checksum field is [1] bits long and is followed by the urgent pointer field which is [2] bits long.

Drag options to blanks, or click blank then click option'

A16

B8

C32

D12

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the sizes of checksum and urgent pointer fields.

Assuming one is 8 bits.

Both the TCP checksum and urgent pointer fields are 16 bits long each.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps TCP header fields to their sizes in bits, filtering only fields larger than 16 bits.

Computer Networks

tcp_fields = {"[1]": [2], "[3]": 32, "window": 16, "checksum": 16}
large_fields = {field: size for field, size in tcp_fields.items() if size > 16}

Drag options to blanks, or click blank then click option'

Asequence_number

B32

Cacknowledgment_number

D16

Attempts:

3 left

💡 Hint

Common Mistakes

Including fields with size 16 bits in the filtered dictionary.

Mixing up field names or sizes.

The sequence number and acknowledgment number fields are 32 bits each, so they are included in the filtered dictionary.

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