Reliable data transfer mechanisms in Computer Networks - Step-by-Step Execution

Concept Flow - Reliable data transfer mechanisms

Sender prepares data packet

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Send packet over network

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Receiver gets packet

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Receiver sends acknowledgment (ACK)

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Sender waits for ACK

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Send next

This flow shows how data is sent reliably by sending packets, waiting for acknowledgments, and resending if needed.

Execution Sample

Computer Networks

Send packet 1
Wait for ACK
If ACK received, send packet 2
If timeout, resend packet 1

This simple sequence shows sending a packet, waiting for confirmation, and resending if no confirmation arrives.

Analysis Table

Step	Action	Packet Sent	ACK Received	Timeout Occurred	Next Step
1	Send packet 1	Packet 1	No	No	Wait for ACK
2	Wait for ACK	Packet 1	No	Yes	Timeout - resend packet 1
3	Resend packet 1	Packet 1	No	No	Wait for ACK
4	Wait for ACK	Packet 1	Yes	No	Send packet 2
5	Send packet 2	Packet 2	No	No	Wait for ACK
6	Wait for ACK	Packet 2	Yes	No	Send packet 3
7	Send packet 3	Packet 3	No	No	Wait for ACK
8	Wait for ACK	Packet 3	No	Yes	Timeout - resend packet 3
9	Resend packet 3	Packet 3	No	No	Wait for ACK
10	Wait for ACK	Packet 3	Yes	No	Send packet 4
11	Send packet 4	Packet 4	No	No	Wait for ACK
12	Wait for ACK	Packet 4	Yes	No	Transmission complete
13	End	-	-	-	-

💡 Transmission ends after all packets are sent and acknowledged.

State Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	After Step 10	Final
Packet Sent	None	Packet 1	Packet 1	Packet 1	Packet 2	Packet 3	Packet 4
ACK Received	No	No	No	Yes	Yes	Yes	Yes
Timeout Occurred	No	No	Yes	No	No	No	No

Key Insights - 3 Insights

Why does the sender resend a packet even if it was already sent once?

Because the sender did not receive an acknowledgment (ACK) within the expected time (timeout), so it assumes the packet or ACK was lost and resends it. See steps 2 and 3 in the execution_table.

What happens if the ACK is received after the timeout and resend?

The sender treats the ACK as confirmation and moves on to send the next packet. The resend ensures reliability even if the first ACK was lost. Refer to step 4 where ACK is received after resend.

Does the sender send the next packet before receiving ACK for the current one?

No, the sender waits for the ACK of the current packet before sending the next one to ensure reliable delivery. This is shown in the flow and execution_table steps.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What causes the sender to resend packet 1?

A Timeout occurred

B ACK was received

C Packet was corrupted

D Sender decided to send next packet

💡 Hint

Check the 'Timeout Occurred' column at step 2 in the execution_table.

At which step does the sender first receive an ACK for packet 1?

A Step 1

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'ACK Received' column for packet 1 in the execution_table.

If the sender never received any ACKs, what would happen according to the flow?

A Sender would stop after first send

B Sender would send all packets at once

C Sender would keep resending the same packet after timeouts

D Sender would send next packet without waiting

💡 Hint

Refer to the flow diagram where timeout leads to resending the packet.

Concept Snapshot

Reliable data transfer means sending data packets and waiting for acknowledgments (ACKs).
If ACK is not received in time (timeout), the sender resends the packet.
This ensures data is not lost or duplicated.
Sender sends next packet only after ACK for current packet.
Common in protocols like TCP.

Full Transcript

Reliable data transfer mechanisms work by sending data packets from sender to receiver. The receiver sends back an acknowledgment (ACK) when it gets a packet. The sender waits for this ACK before sending the next packet. If the ACK does not arrive within a certain time (timeout), the sender resends the same packet. This process repeats until all packets are sent and acknowledged, ensuring no data is lost. The execution table shows each step: sending packets, waiting for ACKs, handling timeouts, and resending packets. Key points include why resending happens after timeout and that the sender waits for ACK before proceeding. This method is used in network protocols like TCP to guarantee reliable communication.