← ACARA Digital Technologies Years 9–10

Transmission Control Protocol (TCP)

Transmission Control Protocol. It sets up a connection, numbers packets, checks each one arrived and resends any missing, so data arrives complete and in order. Web pages and email use it.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 52, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 91, WA Computer Science ATAR Year 11 — Network Communications — Module 52, WA Computer Science ATAR Year 11 — Cyber Security — Module 21

Understanding Transmission Control Protocol (TCP) in ACARA Digital Technologies Years 9–10

What it does: Sequence numbers label every piece of data, so even if packets arrive out of order, TCP can reorder them. HTTP (web pages) and SMTP (email) both run on top of TCP, which resends lost packets so nothing is missing. TCP (Transmission Control Protocol) is reliable. It checks that all the data arrives. UDP just sends data and does not check. The handshake is how two devices agree to talk. It happens first, before any web page or email data is sent. It does not encrypt anything. An ACK is a receipt. If the sender does not get one in time, it sends the packet again.

Common mistakes: In the three-way handshake the client starts by sending SYN to the server. It should be 'SYN: client -> server'. The server must answer the client's SYN with a SYN-ACK to carry on the handshake. It should be 'server replies SYN-ACK'.

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Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 41–55) and word search (Modules 12–55).

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Learning

Duo

Which protocol sets up a connection, numbers the data and checks every packet arrived?

TCPUDP

Matches

TCP sends data straight away without any handshake

↓

TCP always sets up a connection first with the three-way handshake (SYN, SYN-ACK, ACK) before sending data.

Applying

Meanings

Term

TCP

Meaning

Transmission Control Protocol. It sets up a connection, numbers packets, checks each one arrived and resends any missing, so data arrives complete and in order. Web pages and email use it.

Practice

client sends SYN

Testing

Gaps

client -> server:

Check Answer

Spots

1SYN: server -> client

2SYN-ACK: server -> client

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Quick Questions

1. Which protocol sets up a connection, numbers the data and checks every packet arrived?

A. UDP

B. TCP ✓

TCP (Transmission Control Protocol) is reliable. It checks that all the data arrives. UDP just sends data and does not check.

2. What is the three-way handshake used for?

A. Setting up a TCP connection before data is sent ✓

B. Encrypting data so hackers cannot read it

The handshake is how two devices agree to talk. It happens first, before any web page or email data is sent. It does not encrypt anything.

3. Why does TCP give each packet a sequence number?

A. So the packets travel faster across the network

B. So the packets can be put back in the right order ✓

Packets can take different routes and arrive out of order. The sequence numbers let the destination reorder them. They do not make packets faster.

4. A receiver gets packets 1, 2 and 4, but packet 3 never arrives. What does TCP do?

A. The missing packet 3 is sent again ✓

B. The data is used as it is, with packet 3 missing

TCP notices the gap in the sequence numbers and the sender resends packet 3. That way the data arrives complete.

5. What does an acknowledgement (ACK) tell the sender?

A. The packet arrived safely ✓

B. The packet must be deleted

An ACK is a receipt. If the sender does not get one in time, it sends the packet again.

Match the Pairs

SYN, SYN-ACK, ACK → The three-way handshake: the two devices agree to connect before any data is sent

ACK 5 → An acknowledgement telling the sender that the data up to packet 4 arrived and packet 5 is expected next

TCP numbers each packet so the receiver can put the data back in the correct order → seq = 1, 2, 3, 4

TCP is used by web pages and email because they need all the data to arrive complete → HTTP -> TCP, SMTP -> TCP

TCP sends data straight away without any handshake → TCP always sets up a connection first with the three-way handshake (SYN, SYN-ACK, ACK) before sending data.

A packet is lost, so TCP ignores it and carries on → TCP notices the missing packet because no ACK comes back, then the sender resends it so the data is complete.

Key Term

TCP

Transmission Control Protocol. It sets up a connection, numbers packets, checks each one arrived and resends any missing, so data arrives complete and in order. Web pages and email use it.

Fill in the Blanks

1. client -> server: SYN server -> client: SYN-ACK client -> server: ACK

Hint: Type the name of the first handshake message, which asks the server to open a connection.

2. send(packet, seq=1) send(packet, seq=2) send(packet, seq=3)

Hint: Type the name of the setting that numbers each packet so the data can be put back in order.

3. if noAck: resend(packet)

Hint: Type the function name for what TCP does when a packet is not acknowledged.

4. webPage.protocol = TCP email.protocol = TCP

Hint: Type the acronym of the protocol that web pages and email use for complete, ordered delivery.

5. seq = 5 ack = seq + 1

Hint: Type the operator that works out the number of the next packet the receiver expects.

Spot the Error

Question 1

A SYN: server -> client

B SYN-ACK: server -> client

C ACK: client -> server

Line A — In the three-way handshake the client starts by sending SYN to the server. It should be 'SYN: client -> server'.

Question 2

A client sends SYN

B server ignores the SYN

C client sends ACK

D data transfer begins

Line B — The server must answer the client's SYN with a SYN-ACK to carry on the handshake. It should be 'server replies SYN-ACK'.

Question 3

A packet 1 = seq 1

B packet 2 = seq 1

C packet 3 = seq 3

Line B — TCP gives each packet its own sequence number, counting up. Packet 2 should have seq 2, not a repeated seq 1.

Question 4

A send packet 4

B start timer

C if ACK arrives before timeout:

D resend packet 4

Line C — TCP only resends a packet when no acknowledgement arrives before the timeout. It should be 'if no ACK before timeout:'.

Question 5

A TCP: send without a connection

B number every packet

C resend any missing packet

Line A — TCP is connection-oriented, so it sets up a connection with the three-way handshake before any data is sent.

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