TCP segment structure in Computer Networks - Step-by-Step Execution

Concept Flow - TCP segment structure

Start: Data to send

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Create TCP Header

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Add Source Port

↓

Add Destination Port

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Add Sequence Number

↓

Add Acknowledgment Number

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Set Data Offset & Flags

↓

Add Window Size

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Add Checksum

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Add Urgent Pointer

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Add Options (if any)

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Attach Data

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Send TCP Segment

This flow shows how a TCP segment is built step-by-step by adding header fields and data before sending.

Execution Sample

Computer Networks

Source Port: 1234
Destination Port: 80
Sequence Number: 1001
Flags: SYN
Data: "Hello"

This example builds a TCP segment with ports, sequence number, SYN flag, and some data.

Analysis Table

Step	Field Added	Value	Purpose	Resulting Segment Size (bytes)
1	Source Port	1234	Identifies sender port	2
2	Destination Port	80	Identifies receiver port	4
3	Sequence Number	1001	Tracks byte order	8
4	Acknowledgment Number	0	Next expected byte (0 if none)	12
5	Data Offset & Flags	SYN set	Control flags and header size	14
6	Window Size	65535	Flow control size	16
7	Checksum	calculated	Error checking	18
8	Urgent Pointer	0	Urgent data position	20
9	Options	None	Optional features	20
10	Data	"Hello"	Payload data	25
11	Send Segment	-	Segment ready to send	25

💡 All header fields and data added; segment ready for transmission.

State Tracker

Field	Initial	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 10	Final
Segment Size (bytes)	0	2	4	8	12	14	16	18	20	20	25	25
Flags	None	None	None	None	None	SYN	SYN	SYN	SYN	SYN	SYN	SYN
Data	None	None	None	None	None	None	None	None	None	None	"Hello"	"Hello"

Key Insights - 3 Insights

Why does the segment size increase unevenly after adding different fields?

Each TCP header field has a fixed size in bytes; some fields like ports add 2 bytes each, while sequence numbers add 4 bytes. Data size depends on payload length. See execution_table rows for exact size increments.

What does the Data Offset field represent and why is it important?

Data Offset indicates the size of the TCP header in 32-bit words. It tells where the data begins. This is shown in step 5 where Data Offset & Flags are set together.

Why is the checksum calculated after adding most header fields?

Checksum covers header and data to detect errors. It must be calculated after all fields except checksum itself are set, as shown in step 7.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what flag is set in the TCP segment?

A ACK

B FIN

C SYN

D RST

💡 Hint

Check the 'Flags' column at step 5 in the execution_table.

At which step does the TCP segment size first reach 20 bytes?

A Step 8

B Step 7

C Step 6

D Step 9

💡 Hint

Look at the 'Resulting Segment Size (bytes)' column in execution_table rows for steps 6 to 9.

If the data "Hello" was removed, what would be the final segment size?

A 13 bytes

B 20 bytes

C 24 bytes

D 17 bytes

💡 Hint

Refer to variable_tracker for 'Segment Size' after step 9 (before data is added).

Concept Snapshot

TCP Segment Structure:
- Starts with Source and Destination Ports (2 bytes each)
- Includes Sequence and Acknowledgment Numbers (4 bytes each)
- Contains Data Offset, Flags, Window Size, Checksum, Urgent Pointer
- Options field is optional
- Followed by Data (payload)
- Header size varies; Data Offset indicates header length
- Checksum ensures data integrity

Full Transcript

The TCP segment structure is built step-by-step by adding fields in order: source port, destination port, sequence number, acknowledgment number, data offset and flags, window size, checksum, urgent pointer, options if any, and finally the data payload. Each field has a specific size and purpose, such as ports identifying sender and receiver, sequence numbers tracking byte order, and flags controlling connection state. The data offset field tells where the data begins in the segment. The checksum is calculated after most fields are set to ensure error detection. The segment size grows as fields are added, starting from zero and increasing with each field's byte size. This process prepares the TCP segment for transmission over the network.

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