Reliable data transfer mechanisms in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of reliable data transfer mechanisms in networking?

The main goal is to ensure data is delivered accurately and in order, without loss or duplication, despite errors or network issues.

beginner

What does the 'stop-and-wait' protocol do in reliable data transfer?

It sends one data packet and waits for an acknowledgment before sending the next, ensuring each packet is received correctly before proceeding.

intermediate

How does the 'sliding window' protocol improve data transfer efficiency?

It allows multiple packets to be sent before needing an acknowledgment, increasing throughput by using a window of packets that can be sent and acknowledged in sequence.

beginner

What role do acknowledgments (ACKs) play in reliable data transfer?

ACKs confirm receipt of data packets, helping the sender know which packets arrived successfully and which need retransmission.

beginner

Why is retransmission important in reliable data transfer mechanisms?

Retransmission ensures lost or corrupted packets are sent again, so the receiver eventually gets all data correctly.

Which protocol waits for an acknowledgment after sending each packet before sending the next?

A Sliding window

B Stop-and-wait

C UDP

D Broadcast

Stop-and-wait sends one packet and waits for an ACK before sending the next, ensuring reliability.

What does an acknowledgment (ACK) signal in reliable data transfer?

A Sender should stop sending

B Packet was lost

C Packet was received correctly

D Network is down

An ACK tells the sender that the packet arrived correctly and no retransmission is needed.

How does the sliding window protocol increase data transfer speed?

A By sending multiple packets before waiting for ACKs

B By ignoring lost packets

C By sending packets one at a time

D By using smaller packets

Sliding window allows multiple packets to be sent before waiting for acknowledgments, improving throughput.

What happens if a packet is lost during transmission in a reliable data transfer system?

A It is ignored

B The receiver sends a negative acknowledgment

C The connection closes

D It is retransmitted

Lost packets are retransmitted to ensure all data is received correctly.

Which of the following is NOT a feature of reliable data transfer?

A Data encryption

B Error detection

C Acknowledgments

D Retransmission

Data encryption is about security, not reliability; reliable data transfer focuses on error detection, acknowledgments, and retransmission.

Explain how the stop-and-wait protocol ensures reliable data transfer.

Think about how the sender waits before sending more data.

Describe the advantages of the sliding window protocol over stop-and-wait.

Consider how sending more than one packet at once helps speed.

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