Why TCP provides reliable delivery in Computer Networks - Visual Breakdown

Concept Flow - Why TCP provides reliable delivery

Data to send

↓

Segment data into packets

↓

Send packet with sequence number

↓

Wait for ACK from receiver

↓

Timeout or NACK

↓

Retransmit lost packet

↓

All packets acknowledged?

No→Wait for ACK

Yes↓

Data delivered reliably

TCP breaks data into packets, sends each with a number, waits for confirmation, and resends if needed to ensure reliable delivery.

Execution Sample

Computer Networks

Send packet #1
Wait for ACK #1
If no ACK, resend packet #1
Send packet #2
Wait for ACK #2
...

This shows TCP sending packets one by one, waiting for confirmation before moving on.

Analysis Table

Step	Action	Packet Number	ACK Received?	Next Step
1	Send packet	1	No	Wait for ACK #1
2	Timeout - no ACK	1	No	Resend packet #1
3	Resend packet	1	Yes	Send packet #2
4	Send packet	2	Yes	Send packet #3
5	Send packet	3	No	Wait for ACK #3
6	ACK received	3	Yes	All packets delivered

💡 All packets acknowledged, so TCP confirms reliable delivery.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Packet Sent	None	1	1	1	2	3	3
ACK Received	None	No	No	Yes	Yes	No	Yes
Next Packet to Send	1	1	1	2	3	3	Done

Key Insights - 2 Insights

Why does TCP resend a packet when no ACK is received?

Because the execution_table shows at Step 2 that no ACK was received for packet 1, TCP resends it to ensure the receiver gets the data.

How does TCP know when all data is delivered?

At Step 6, the table shows all packets have been acknowledged, so TCP confirms delivery is complete.

Visual Quiz - 3 Questions

Look at the execution_table at Step 3, what action does TCP take?

A Resend packet #1 after no ACK

B Send packet #2 immediately

C Wait longer for ACK #1

D Close the connection

💡 Hint

Check Step 3 row in execution_table where ACK Received? is Yes after resending.

At which step does TCP receive ACK for packet #2?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at Step 4 in execution_table where packet 2 is sent and ACK is Yes.

If TCP never received ACK for packet #3, what would happen next?

A TCP would close connection immediately

B TCP would send packet #4 anyway

C TCP would resend packet #3 after timeout

D TCP would ignore the missing ACK

💡 Hint

Refer to Steps 1 and 2 where no ACK causes resending.

Concept Snapshot

TCP ensures reliable delivery by:
- Breaking data into numbered packets
- Sending each packet and waiting for an ACK
- Resending packets if ACK not received
- Confirming all packets acknowledged before finishing
This process prevents data loss and ensures order.

Full Transcript

TCP provides reliable delivery by dividing data into packets, each with a sequence number. It sends a packet and waits for an acknowledgment (ACK) from the receiver. If the ACK is not received within a timeout, TCP resends the packet. This continues until the receiver confirms all packets are received. This method ensures no data is lost or received out of order, making TCP reliable.