TCP Sequence Numbers, ACKs & Retransmission (RTO)

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Steps

setup

Client sends SYN to initiate connection

The client sends a SYN packet to the server to start the TCP three-way handshake. This packet has an initial sequence number (seq=1000).

💡 This step establishes the start of a TCP connection and sets the initial sequence number for tracking data.

Line:

send_packet(src='client', dst='server', seq=1000, flags=['SYN'])

💡 TCP connections begin with a SYN packet to synchronize sequence numbers.

setup

Server receives SYN and replies with SYN-ACK

The server receives the SYN, processes it, and replies with a SYN-ACK packet. The server's sequence number is 3000, and it acknowledges the client's SYN by setting ACK=1001.

💡 The server acknowledges the client's SYN and sends its own SYN to synchronize sequence numbers both ways.

Line:

send_packet(src='server', dst='client', seq=3000, ack=1001, flags=['SYN','ACK'])

💡 TCP handshake requires both sides to synchronize sequence numbers with SYN and ACK flags.

setup

Client receives SYN-ACK and sends ACK

The client receives the SYN-ACK, processes it, and sends an ACK packet back to the server. The ACK acknowledges the server's SYN by setting ACK=3001.

💡 This finalizes the three-way handshake, confirming both sides have synchronized sequence numbers.

Line:

send_packet(src='client', dst='server', seq=1001, ack=3001, flags=['ACK'])

💡 The handshake completes when the client acknowledges the server's SYN.

fill_cells

Client sends first data packet (seq=1001)

The client sends the first data packet with sequence number 1001 and payload 'Hello'.

💡 Data transfer begins after connection setup, with sequence numbers tracking bytes sent.

Line:

send_packet(src='client', dst='server', seq=1001, ack=3001, flags=[], data='Hello')

💡 Sequence numbers track the byte offset of data sent for reliable delivery.

compare

Server receives data and sends ACK (ack=1006)

The server receives the data packet with seq=1001 and payload 'Hello' (5 bytes). It sends an ACK with ack=1006, indicating it expects the next byte at 1006.

💡 ACK numbers confirm receipt of data and indicate the next expected byte.

Line:

send_packet(src='server', dst='client', seq=3001, ack=1006, flags=['ACK'])

💡 ACK numbers advance by the length of received data to confirm delivery.

fill_cells

Client sends second data packet (seq=1006)

The client sends the second data packet with sequence number 1006 and payload 'World'.

💡 Data continues flowing with sequence numbers incremented by previous data length.

Line:

send_packet(src='client', dst='server', seq=1006, ack=3001, flags=[], data='World')

💡 Sequence numbers ensure data ordering and tracking of bytes sent.

prune

Second data packet lost in transit (no ACK received)

The second data packet is lost and does not reach the server. The client waits for an acknowledgment but none arrives before the retransmission timeout (RTO).

💡 Packet loss triggers retransmission mechanisms in TCP to ensure reliability.

Line:

# Packet lost, no ACK received; wait for RTO

💡 TCP detects lost packets by missing ACKs and uses timers to trigger retransmission.

retransmit

Retransmission timeout (RTO) triggers resend of lost packet

The client's retransmission timer expires (RTO). It resends the lost data packet with seq=1006 and payload 'World'.

💡 Retransmission ensures lost data is resent to maintain reliable delivery.

Line:

send_packet(src='client', dst='server', seq=1006, ack=3001, flags=['RTO'], data='World')

💡 TCP uses timers to detect lost packets and trigger retransmission automatically.

compare

Server receives retransmitted packet and sends ACK (ack=1011)

The server receives the retransmitted packet with seq=1006 and payload 'World'. It sends an ACK with ack=1011, confirming receipt of all data up to byte 1010.

💡 ACK confirms cumulative receipt of data, including retransmitted packets.

Line:

send_packet(src='server', dst='client', seq=3001, ack=1011, flags=['ACK'])

💡 TCP acknowledges the highest contiguous byte received, confirming retransmission success.

traverse

Client receives final ACK confirming all data received

The client receives the ACK with ack=1011, confirming all sent data has been received by the server. The connection is now fully synchronized and data transfer is complete.

💡 Final ACK confirms reliable delivery of all data and allows the client to send more data or close connection.

Line:

# Client processes final ACK

💡 TCP uses ACKs to confirm data delivery and maintain flow control.

def send_packet(src, dst, seq, ack=None, flags=None, data=None):
    # STEP 1-10: Simulate sending TCP packets with sequence and acknowledgment numbers
    packet = {'src': src, 'dst': dst, 'seq': seq, 'ack': ack, 'flags': flags or [], 'data': data}
    # Normally send packet over network
    return packet

# STEP 1
send_packet('client', 'server', seq=1000, flags=['SYN'])

# STEP 2
send_packet('server', 'client', seq=3000, ack=1001, flags=['SYN','ACK'])

# STEP 3
send_packet('client', 'server', seq=1001, ack=3001, flags=['ACK'])

# STEP 4
send_packet('client', 'server', seq=1001, ack=3001, data='Hello')

# STEP 5
send_packet('server', 'client', seq=3001, ack=1006, flags=['ACK'])

# STEP 6
send_packet('client', 'server', seq=1006, ack=3001, data='World')

# STEP 7
# Packet lost: no send_packet call

# STEP 8
send_packet('client', 'server', seq=1006, ack=3001, flags=['RTO'], data='World')

# STEP 9
send_packet('server', 'client', seq=3001, ack=1011, flags=['ACK'])

# STEP 10
# Client receives final ACK (no send_packet call needed)

📊

TCP Sequence Numbers, ACKs & Retransmission (RTO) - Watch the Algorithm Execute, Step by Step

Watching this step-by-step visualization helps you understand how TCP manages sequence numbers and acknowledgments to guarantee data delivery, and how retransmission works when packets are lost or delayed.

Client sends SYN to initiate connection and set initial sequence number

Hop: 1

Client

Server

📦Packet

↗ fromclient

↘ toserver

🔌 protocolTCP

🚩 flagsSYN

src→10.0.0.1:50000

dst→10.0.0.2:80

flags→SYN

SYN: client→server (seq=1000)

Server replies with SYN-ACK to acknowledge client's SYN and send its own sequence number

Hop: 2

Client

Server

📦Packet

↗ fromserver

↘ toclient

🔌 protocolTCP

🚩 flagsSYN, ACK

src→10.0.0.2:80

dst→10.0.0.1:50000

flags→SYN, ACK

SYN: client→server (seq=1000)

SYN-ACK: server→client (seq=3000, ack=1001)

Client sends ACK to complete handshake and confirm server's sequence number

Hop: 3

Client

Server

📦Packet

↗ fromclient

↘ toserver

🔌 protocolTCP

🚩 flagsACK

src→10.0.0.1:50000

dst→10.0.0.2:80

flags→ACK

SYN: client→server (seq=1000)

SYN-ACK: server→client (seq=3000, ack=1001)

ACK: client→server (seq=1001, ack=3001)

Client sends first data packet with sequence number 1001

Hop: 4

Client

Server

📦Packet

↗ fromclient

↘ toserver

📄 payloadHello

🔌 protocolTCP

🚩 flags

src→10.0.0.1:50000

dst→10.0.0.2:80

data→Hello

SYN-ACK: server→client (seq=3000, ack=1001)

ACK: client→server (seq=1001, ack=3001)

DATA: client→server (seq=1001, data='Hello')

Server acknowledges receipt of 5 bytes, expecting next byte 1006

Hop: 5

Client

Server

📦Packet

↗ fromserver

↘ toclient

🔌 protocolTCP

🚩 flagsACK

src→10.0.0.2:80

dst→10.0.0.1:50000

flags→ACK

ACK: client→server (seq=1001, ack=3001)

DATA: client→server (seq=1001, data='Hello')

ACK: server→client (seq=3001, ack=1006)

Client sends second data packet with sequence number 1006

Hop: 6

Client

Server

📦Packet

↗ fromclient

↘ toserver

📄 payloadWorld

🔌 protocolTCP

🚩 flags

src→10.0.0.1:50000

dst→10.0.0.2:80

data→World

DATA: client→server (seq=1001, data='Hello')

ACK: server→client (seq=3001, ack=1006)

DATA: client→server (seq=1006, data='World')

Second data packet lost; client waits for ACK but none arrives

Hop: 7

Client

Server

ACK: server→client (seq=3001, ack=1006)

DATA: client→server (seq=1006, data='World')

Packet lost: DATA seq=1006

Client retransmits lost packet after timeout (RTO)

Hop: 8

Client

Server

📦Packet

↗ fromclient

↘ toserver

📄 payloadWorld

🔌 protocolTCP

🚩 flagsRTO

src→10.0.0.1:50000

dst→10.0.0.2:80

data→World

flags→RTO

DATA: client→server (seq=1006, data='World')

Packet lost: DATA seq=1006

RTO Retransmit: client→server (seq=1006, data='World')

Server acknowledges receipt of retransmitted data up to byte 1010

Hop: 9

Client

Server

📦Packet

↗ fromserver

↘ toclient

🔌 protocolTCP

🚩 flagsACK

src→10.0.0.2:80

dst→10.0.0.1:50000

flags→ACK

Packet lost: DATA seq=1006

RTO Retransmit: client→server (seq=1006, data='World')

ACK: server→client (seq=3001, ack=1011)

Client receives final ACK confirming all data received

Hop: 10

Client

Server

📦Packet

↗ fromserver

↘ toclient

🔌 protocolTCP

🚩 flagsACK

src→10.0.0.1:50000

dst→10.0.0.2:80

flags→ACK

RTO Retransmit: client→server (seq=1006, data='World')

ACK: server→client (seq=3001, ack=1011)

ACK received: client confirms all data received

Key Takeaways

✓ TCP uses sequence numbers and ACKs to track data sent and received, ensuring reliable delivery.

This insight is hard to see from code alone because sequence and ACK numbers are abstract counters; visualization shows their flow and updates clearly.

✓ Retransmission timeout (RTO) triggers resending lost packets automatically to recover from packet loss.

Plain English explanation: when an ACK is not received in time, TCP resends the missing data to guarantee delivery.

✓ The three-way handshake synchronizes sequence numbers between client and server before data transfer begins.

Seeing the SYN, SYN-ACK, and ACK packets in sequence clarifies how TCP establishes connection state.