Reliable data transfer mechanisms in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the protocol that uses acknowledgments to ensure reliable data transfer.

Computer Networks

The protocol that uses [1] to confirm receipt of data is called Stop-and-Wait ARQ.

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

Achecksums

Btimeouts

Cacknowledgments

Dsequence numbers

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing acknowledgments with timeouts.

Thinking checksums are used to confirm receipt instead of detect errors.

Stop-and-Wait ARQ uses acknowledgments to confirm that data has been received correctly before sending the next packet.

2fill in blank

medium

Complete the code to describe the purpose of sequence numbers in reliable data transfer.

Computer Networks

Sequence numbers are used to [1] packets and detect duplicates in data transfer protocols.

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

Acompress

Border

Cencrypt

Ddrop

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming sequence numbers are for encryption.

Confusing sequence numbers with compression techniques.

Sequence numbers help keep track of the order of packets and detect if any duplicates arrive.

3fill in blank

hard

Fix the error in the description of Go-Back-N ARQ protocol.

Computer Networks

In Go-Back-N ARQ, the sender can send multiple packets without waiting for [1] for each one, but must retransmit from the first unacknowledged packet if an error occurs.

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

Aacknowledgment

Bchecksum

Ctimeout

Dsequence number

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing acknowledgment with checksum or timeout.

Thinking the sender waits for a timeout before sending next packets.

Go-Back-N ARQ allows sending multiple packets before receiving acknowledgments, but retransmits starting from the first missing acknowledgment.

4fill in blank

hard

Fill both blanks to complete the description of Selective Repeat ARQ.

Computer Networks

Selective Repeat ARQ allows the sender to transmit multiple packets and only retransmits those packets that are [1] or [2].

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

Alost

Bdelayed

Ccorrupted

Dduplicated

Attempts:

3 left

💡 Hint

Common Mistakes

Including delayed or duplicated packets as reasons for retransmission.

Confusing lost packets with delayed packets.

Selective Repeat ARQ retransmits only the packets that are lost or corrupted, improving efficiency.

5fill in blank

hard

Fill all three blanks to complete the explanation of how reliable data transfer handles errors.

Computer Networks

Reliable data transfer protocols use [1] to detect errors, [2] to confirm correct receipt, and [3] to resend lost or corrupted packets.

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

Achecksums

Backnowledgments

Cretransmissions

Dtimeouts

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing timeouts with error detection.

Mixing up acknowledgments and retransmissions.

Checksums detect errors, acknowledgments confirm receipt, and retransmissions resend lost or corrupted packets to ensure reliability.

Practice

1. What is the main purpose of reliable data transfer in computer networks?

easy

A. To reduce the size of data packets

B. To ensure data arrives complete and without errors

C. To encrypt data for security

D. To increase the speed of data transmission

Solution

1. Step 1: Understand the goal of reliable data transfer

   Reliable data transfer aims to make sure data sent from one point reaches the other point correctly and fully.

2. Step 2: Compare options with this goal

   Only To ensure data arrives complete and without errors matches this goal by focusing on completeness and error-free delivery.

3. Final Answer:

   To ensure data arrives complete and without errors -> Option B

4. Quick Check:

   Reliable data transfer = complete, error-free data [OK]

Hint: Reliable means complete and correct data delivery [OK]

Common Mistakes:

• Confusing reliability with speed
• Thinking encryption is part of reliability
• Assuming data size reduction is reliability

2. Which of the following is a key feature of the Stop-and-Wait protocol?

easy

A. Sender waits for acknowledgment before sending next packet

B. Sender sends all packets without waiting

C. Receiver sends data back to sender

D. Packets are sent in random order

Solution

1. Step 1: Recall Stop-and-Wait protocol behavior

   In Stop-and-Wait, the sender sends one packet and waits for an acknowledgment before sending the next.

2. Step 2: Match this behavior to options

   Sender waits for acknowledgment before sending next packet correctly describes this waiting for acknowledgment step.

3. Final Answer:

   Sender waits for acknowledgment before sending next packet -> Option A

4. Quick Check:

   Stop-and-Wait = wait for ACK before next send [OK]

Hint: Stop-and-Wait means wait for ACK before next send [OK]

Common Mistakes:

• Thinking sender sends all packets at once
• Confusing receiver's role with sender's
• Assuming packets are sent randomly

3. Consider a Sliding Window protocol with window size 3. If the sender has sent packets 1, 2, and 3, and receives an acknowledgment for packet 1, which packets can it send next?

medium

A. Packets 4 and 5

B. Packets 4 only

C. Packets 2 and 3 again

D. Packets 5 and 6

Solution

1. Step 1: Understand Sliding Window with size 3

   The sender can have up to 3 unacknowledged packets sent at once.

2. Step 2: Analyze acknowledgment for packet 1

   Receiving ACK for packet 1 means the window slides forward by one, freeing space to send one more packet. Since packets 2 and 3 are still unacknowledged, the sender can send packets 4 and 5 to fill the window.

3. Final Answer:

   Packets 4 and 5 -> Option A

4. Quick Check:

   Sliding window size 3 allows 3 unacknowledged packets; after ACK 1, packets 2 and 3 remain unacknowledged, so 2 new packets (4 and 5) can be sent [OK]

Hint: ACK moves window; send as many as window size allows [OK]

Common Mistakes:

• Sending two packets after one ACK
• Resending already acknowledged packets
• Sending packets beyond window size

4. A sender using Stop-and-Wait protocol never receives an acknowledgment. What is the most likely problem?

medium

A. The receiver is sending acknowledgments for wrong packets

B. The sender is sending packets too fast

C. The acknowledgment packets are lost or delayed

D. The sender window size is too large

Solution

1. Step 1: Understand Stop-and-Wait behavior on lost ACKs

   If acknowledgments are lost or delayed, the sender waits or resends the same packet repeatedly.

2. Step 2: Evaluate options for no ACK received

   The acknowledgment packets are lost or delayed correctly identifies lost or delayed ACKs as the cause; other options do not fit Stop-and-Wait protocol behavior.

3. Final Answer:

   The acknowledgment packets are lost or delayed -> Option C

4. Quick Check:

   No ACK = lost/delayed ACK packets [OK]

Hint: No ACK means lost or delayed acknowledgment packets [OK]

Common Mistakes:

• Blaming sender speed in Stop-and-Wait
• Confusing window size with Stop-and-Wait
• Assuming receiver sends wrong ACKs

5. In a network using Sliding Window protocol, if the window size is 4 and the sender receives duplicate acknowledgments for packet 3, what should the sender do to ensure reliable data transfer?

hard

A. Stop sending packets until timeout occurs

B. Ignore duplicates and continue sending new packets

C. Increase the window size to 6

D. Resend packet 4 immediately to recover from possible loss

Solution

1. Step 1: Understand duplicate ACKs in Sliding Window

   Duplicate ACKs usually indicate a lost packet after the acknowledged one.

2. Step 2: Identify sender's response to duplicate ACKs

   The sender should resend the next expected packet (packet 4) immediately to recover lost data.

3. Step 3: Evaluate other options

   Ignoring duplicates risks data loss; increasing window size or stopping sending are not correct immediate responses.

4. Final Answer:

   Resend packet 4 immediately to recover from possible loss -> Option D

5. Quick Check:

   Duplicate ACKs = resend missing packet [OK]

Hint: Duplicate ACKs signal lost packet; resend it immediately [OK]

Common Mistakes:

• Ignoring duplicate acknowledgments
• Changing window size unnecessarily
• Waiting for timeout instead of fast resend