Week 9 - Transport Layer Part 2

Question 1

What type of communication does TCP provide at the Transport Layer?

Answer 1

TCP provides point-to-point communication between two devices.

Question 2

How does TCP ensure reliable data transfer?

Answer 2

TCP uses sequence numbers, acknowledgements (ACKs), and timeouts to guarantee reliable data delivery.

Question 3

What does it mean that TCP is “connection-oriented”?

Answer 3

TCP uses a handshaking process to initialise sender and receiver states before any data exchange, establishing a reliable connection.

Question 4

What additional features does TCP provide for managing data transfer?

Answer 4

TCP supports in-order byte-stream delivery, pipelining, and flow and congestion control to prevent the receiver from being overwhelmed.

Question 5

In TCP, what is implicitly numbered?

Answer 5

Each byte of data is assigned a sequence number, not just each segment.

Question 6

If a file size is 500,000 bytes and the Maximum Segment Size (MSS) is 1000 bytes, how many segments will TCP create?

Answer 6

TCP will create 500 segments (500,000 ÷ 1000 = 500).

Question 7

What would be the sequence number for the first few TCP segments?

Answer 7

• Segment 1: Sequence number = 0
• Segment 2: Sequence number = 1000
• Segment 3: Sequence number = 2000
• Segment 500: Sequence number = 500000

Question 8

How are sequence numbers used in TCP segments?

Answer 8

Sequence numbers allow TCP to keep track of where each piece of data fits into the original data stream, ensuring reliable, in-order delivery.

Question 9

What does a TCP segment’s ACK number represent?

Answer 9

It is the sequence number of the next byte the receiver expects from the sender, indicating cumulative acknowledgement.

Question 10

What is a cumulative ACK in TCP?

Answer 10

It means the receiver acknowledges all bytes up to (but not including) the byte indicated by the ACK number.

Question 11

How does TCP handle segments received out-of-order?

Answer 11

The TCP specification doesn’t require a specific behavior — it’s up to the application (but usually, out-of-order segments are kept for reordering).

Question 12

When does TCP start the retransmission timer?

Answer 12

TCP starts the retransmission timer when sending a segment if the timer is not already running.

Question 13

What does the TCP retransmission timer track?

Answer 13

It tracks the oldest unacknowledged (unACK’d) segment.

Question 14

What happens when TCP receives an ACK?

Answer 14

TCP updates the record of what data has been ACK’d and restarts the timer if there are still unACK’d segments.

Question 15

What happens if the TCP retransmission timer times out?

Answer 15

TCP retransmits the segment that caused the timeout and restarts the timer.

Question 16

n a TCP timeout scenario, what happens to a segment that was already acknowledged (e.g., Segment #100)?

Answer 16

A segment that has already been ACK’d (like Segment #100) is not retransmitted, even if a timeout occurs.

Question 17

During a TCP timeout, which segment is retransmitted?

Answer 17

Only the oldest unACK’d segment is retransmitted when the timeout occurs.

Question 18

What ensures that TCP doesn’t unnecessarily retransmit already ACK’d data?

Answer 18

TCP keeps track of ACKs, and only retransmits unACK’d segments, avoiding duplicate data transmission.

Question 19

Why is avoiding unnecessary retransmissions important in TCP?

Answer 19

It improves efficiency, saves bandwidth, and prevents network congestion.

Question 20

What is a cumulative ACK in TCP?

Answer 20

A cumulative ACK acknowledges all bytes up to (but not including) the next expected byte, confirming receipt of multiple segments at once.

Question 21

In a TCP timeout with cumulative ACKs, why might Segment #92 not be retransmitted?

Answer 21

Because Segment #92 was already covered by the cumulative ACK, meaning it was received correctly, even if later segments timeout.

Question 22

How does a longer timeout help in TCP retransmission with cumulative ACKs?

Answer 22

A longer timeout allows TCP to receive and acknowledge multiple segments without retransmitting ones that were already successfully delivered.

Question 23

What is the advantage of using cumulative ACKs during retransmission timeouts?

Answer 23

It reduces unnecessary retransmissions, making TCP more efficient and reducing congestion on the network.

Question 24

Why does TCP require a handshake before data exchange?

Answer 24

Because TCP is connection-oriented; sender and receiver must establish a connection first.

Question 25

What happens during the TCP handshake?

Answer 25

The sender and receiver agree on connection parameters, such as initial sequence numbers.

Question 26

What type of handshake does TCP use to establish a connection?

Answer 26

TCP uses a 3-Way Handshake to establish a connection.

Question 27

What type of handshake does TCP use to close a connection?

Answer 27

TCP uses a 4-Way Handshake to close the connection properly.

Question 28

Why does TCP need flow control?

Answer 28

To prevent the receiver’s buffer from overflowing by limiting how much data the sender transmits.

Question 29

How does the TCP receiver control the flow of data from the sender?

Answer 29

By altering the window size using the window_size (rwnd) parameter in the TCP header.

Question 30

What does the TCP window size represent?

Answer 30

It is the amount of unACK’d (in-flight) data the sender is allowed to send at any time.

Question 31

What happens when the receiver decreases the window size?

Answer 31

It reduces the amount of data the sender can send, limiting incoming traffic to avoid buffer overflow.