Guaranteed delivery

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Quick Questions

Shade in the A or B circle for each question.

1. What does guaranteed delivery promise about the packets of a message?

A Every packet arrives as fast as possible, even if some are lost

B Every packet is confirmed as received, so no data is lost

2. How does the sender find out that a packet arrived safely?

A The sender assumes it arrived if no error message appears

B The receiver sends back an acknowledgement (ACK)

3. A packet goes missing and no acknowledgement arrives. What happens under guaranteed delivery?

A The sender resends the packet

B The receiver skips it and carries on without it

4. Which cost comes with guaranteed delivery?

A Extra delay from waiting for acknowledgements and resending packets

B Packets are shortened so some data is removed

5. Which situation needs guaranteed delivery most?

A Downloading a assignment file where every byte must be correct

B Watching a live video call where a late frame is not worth waiting for

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1ACK 1042		Aif no ACK before timeout: resend(packet)
2resend(packet 7)		BThe receiver confirms that packet 1042 arrived safely, so the sender can move on
3Guaranteed delivery: the sender waits for an acknowledgement and resends the packet if none arrives in time		CA received ACK means the packet arrived safely, so it should not be resent. Resending should happen when NO ACK arrives.
4The cost of guaranteed delivery is extra delay, because the sender must wait for confirmations and resend packets		DThe sender sends packet 7 again because it was lost or damaged on the way
5if ACK received: resend(packet)		EtotalTime = sendTime + waitForACK + resendTime
6Guaranteed delivery = fastest possible transfer, no waiting		FGuaranteed delivery is not faster. Waiting for acknowledgements and resending packets adds extra delay.

Definitions

guaranteed delivery

Fill in the Blanks

Complete each line of code by filling in the missing parts.

SEND packet WAIT FOR ___ IF ackReceived THEN SEND nextPacket

IF ___ ackReceived THEN RESEND packet ENDIF

IF timer > timeout THEN ______ packet ENDIF

_____ ackReceived = FALSE SEND packet ENDWHILE

resends = 3 extraDelay = resends _ roundTrip

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A SEND packet 1

B SKIP waiting for ACK

C IF ACK received THEN

D SEND packet 2

E ENDIF

Question 2

A SEND packet 5 then forget it

B WAIT for ACK up to 2 secs

C IF no ACK THEN

D RESEND packet 5

E ENDIF

Question 3

A acked ← TRUE

B WHILE acked = FALSE

C SEND packet 3

D acked ← ACK received

E ENDWHILE

Question 4

A RECEIVE packet 4

B CHECK checksum

C IF checksum differs THEN

D SEND ACK 4

E ELSE SEND resend request

Question 5

A SEND packets 1 to 3

B RECEIVE ACK 1, 2 and 3

C ACK 2 is missing

D RESEND packet 2

Answer Key

Guaranteed delivery

Section A: Quick Questions

B — Every packet is confirmed as received, so no data is lost

B — The receiver sends back an acknowledgement (ACK)

A — The sender resends the packet

A — Extra delay from waiting for acknowledgements and resending packets

A — Downloading a assignment file where every byte must be correct

Section B: Match the Pairs

ACK 1042 → The receiver confirms that packet 1042 arrived safely, so the sender can move on

resend(packet 7) → The sender sends packet 7 again because it was lost or damaged on the way

Guaranteed delivery: the sender waits for an acknowledgement and resends the packet if none arrives in time → if no ACK before timeout: resend(packet)

The cost of guaranteed delivery is extra delay, because the sender must wait for confirmations and resend packets → totalTime = sendTime + waitForACK + resendTime

if ACK received: resend(packet) → A received ACK means the packet arrived safely, so it should not be resent. Resending should happen when NO ACK arrives.

Guaranteed delivery = fastest possible transfer, no waiting → Guaranteed delivery is not faster. Waiting for acknowledgements and resending packets adds extra delay.

Section C: Definitions

guaranteed delivery — Every packet is confirmed as received (acknowledged). Missing or damaged packets are resent so no data is lost, but waiting and resending adds extra delay.

Section D: Fill in the Blanks

SEND packet
WAIT FOR ACK
IF ackReceived THEN SEND nextPacket

IF NOT ackReceived THEN
    RESEND packet
ENDIF

IF timer > timeout THEN
    RESEND packet
ENDIF

WHILE ackReceived = FALSE
    SEND packet
ENDWHILE

resends = 3
extraDelay = resends * roundTrip

Section E: Spot the Error

B — Guaranteed delivery needs the sender to wait for an acknowledgement (ACK) before moving on. Line 2 should be WAIT for ACK.

A — With guaranteed delivery the sender cannot forget the packet. It must keep it, wait for an ACK and resend if needed. Line 1 should be SEND packet 5.

A — Starting with acked as TRUE means the loop never runs, so the packet is never sent or confirmed. It should start as FALSE until an ACK arrives.

C — The receiver should only send an ACK when the checksum matches, meaning the packet is undamaged. Line 3 should be IF checksum matches THEN.

B — ACK 2 is missing, so only ACK 1 and ACK 3 arrived. Line 2 should be RECEIVE ACK 1 and ACK 3, which is why packet 2 is resent.