2024 What is TCP Selective Acknowledgement? The Powerful SACK

We all know that we need TCP (Transmission Control Protocol) for our segments to reach the destination in a correct and orderly way and TCP indeed plays a crucial role in ensuring reliable data transfer. Acknowledgments are fundamental to this process, and TCP Selective Acknowledgment (SACK) takes this concept a step further. In this article, we will understand What is TCP Selective Acknowledgement.

TCP, as a connection-oriented protocol, manages the reliable delivery of data between devices. It breaks down information into packets, sends them, and awaits acknowledgements.

The Need for Acknowledgments:

Acks are very much needed to confirm the delivery. For example, how would you feel if you say ‘Hi’ to someone and she doesn’t reply? To avoid that feeling, we have Acks. Acknowledgements serve as virtual handshakes, confirming that data reached its destination. They are vital for maintaining the integrity of the communication process.


What is TCP selective acknowledgement


What is TCP Selective Acknowledgement (SACK)?


TCP SACK is an extension to the acknowledgement mechanism, enabling the receiver to inform the sender about specific segments that were received successfully.

SACK does not replace ACK, but reports additional information to the sender. A SACK reports a block of data that is out of order, and also a block of segments that is duplicated, i.e. received more than once. However, since there is no provision in the TCP header for adding this type of information, SACK is implemented as an option at the end of the TCP header.

An optional feature called selective acknowledgments allows noncontiguous blocks of data to be acknowledged

Let’s go through a simplified example to illustrate the working of TCP Selective Acknowledgment (SACK):

TCP SACK Example: Let’s take a Scenario to understand it better.

  1. The sender (S) wants to transmit a series of TCP segments to the receiver (R).
  2. The data is divided into segments, each with a unique sequence number.

Data Transmission:

  1. Sender sends three segments: Seg1 (sequence numbers 1-100), Seg2 (sequence numbers 101-200), and Seg3 (sequence numbers 201-300).
  2. Receiver successfully receives Seg1 and Seg3 but experiences packet loss for Seg2.

Selective Acknowledgment:

  1. The Receiver, using SACK, sends an acknowledgement back to the sender, indicating the successful receipt of Seg1 and Seg3 and the missing portion (sequence numbers 101-200).

TCP Header:
ACK: 201 (acknowledging up to sequence number 201)
SACK: [101-200] (indicating missing sequence numbers)

2. The SACK option specifies the range of missing sequence numbers (101-200).

Sender’s Response:

  1. Upon receiving the SACK information, the sender knows that Seg2 (sequence numbers 101-200) was not successfully received.
  2. Instead of retransmitting the entire range of unacknowledged data, as traditional TCP might do, the sender retransmits only the missing portion (Seg2).

Reduced Redundancy:

  1. The sender retransmits Seg2 (sequence numbers 101-200).
  2. The receiver acknowledges the retransmitted segment and the communication continues without unnecessary redundancy.

TCP Header:
ACK: 201 (acknowledging up to sequence number 201)

Adaptability to Network Conditions:

  1. If further packet loss occurs, the selective acknowledgement mechanism continues to work by informing the sender about the missing segments.
  2. SACK allows the sender to adapt to varying network conditions by retransmitting only the necessary segments, reducing congestion and optimizing performance.

SACK proves its effectiveness in this example by selectively acknowledging received data and providing specific information about missing segments. This targeted approach minimizes redundant retransmissions and contributes to improved efficiency and adaptability in the TCP connection.

You can refer RFC also.

How SACK Differs from Regular Acknowledgments


As we know regular acknowledgments confirm the receipt of sequential packets and retransmit entire ranges of data. SACK provides detailed information about which segments were successfully received and handles packet loss by acknowledging individual, non-contiguous segments. This contributes to improved performance and adaptability in TCP connections.


  1. Acknowledgment Mechanism:
    • Regular ACKs: In traditional TCP, acknowledgements are cumulative. A single ACK indicates the successful receipt of all packets up to a specific sequence number. If there are gaps or missing packets, the sender assumes they are lost and retransmits the entire range.
    • SACK: Selective Acknowledgment allows the receiver to acknowledge the successful receipt of individual, non-contiguous packets. Instead of acknowledging a continuous range, SACK specifies which specific packets have been received successfully.
  2. Handling Packet Loss:
    • Regular ACKs: When a packet is lost, regular ACKs trigger the sender to retransmit the entire range of unacknowledged packets. This can be inefficient, especially if only a few packets are missing.
    • SACK: SACK enables more granular recovery. If packets are lost, the receiver can explicitly inform the sender about the missing packets, and the sender retransmits only those specific segments, minimising redundancy and improving overall efficiency.
  3. Efficiency and Throughput:
    • Regular ACKs: The cumulative acknowledgement mechanism may lead to unnecessary retransmissions of packets that were already successfully received. This can impact overall throughput and network efficiency.
    • SACK: By selectively acknowledging individual packets, SACK reduces redundant retransmissions. This results in improved throughput and more efficient use of network resources.
  4. Adaptability to Network Conditions:
    • Regular ACKs: Traditional TCP may respond to packet loss by retransmitting large segments, leading to potential network congestion.
    • SACK: The selective acknowledgement mechanism allows TCP to adapt more gracefully to varying network conditions. It provides better feedback to the sender, allowing it to respond more intelligently to packet loss without causing unnecessary congestion.

Benefits of TCP SACK

SACK minimises unnecessary retransmissions, enhancing overall network efficiency. This leads to improved performance, especially in scenarios with packet loss or network congestion.

  1. We get Enhanced Reliability:
    • SACK helps improve the reliability of data transmission in TCP by allowing the receiver to acknowledge multiple non-contiguous packets successfully received. This selective acknowledgement enables the sender to identify and retransmit only the missing or lost segments, reducing unnecessary retransmissions and improving overall efficiency.
  2. We get Improved Throughput:
    • By reducing the need for retransmitting entire data segments, SACK can enhance the throughput of TCP connections. This is particularly beneficial in scenarios where network conditions may lead to packet loss.
  3. Helps in Mitigating Congestion:
    • SACK can contribute to congestion control mechanisms by providing more accurate information about the status of received packets. This helps TCP adapt to varying network conditions and prevents unnecessary congestion, leading to a more stable and efficient data transfer.
  4. Helps in Latency Reduction:
    • The selective acknowledgement mechanism in SACK can help reduce latency in data transfer, as it enables faster recovery from packet loss. This is crucial in applications where low latency is a priority, such as real-time communication and streaming.
  5. Optimizes Network Performance:
    • SACK is valuable in optimizing overall network performance by allowing TCP to adapt to different types of network environments. This adaptability ensures that TCP connections remain robust and efficient across diverse conditions.

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In summary, SACK is a crucial component in TCP that enhances the reliability, throughput, and adaptability of data transfer, making it well-suited for various real-world applications in networking and communication. Its benefits in reducing retransmissions and improving network performance make it a valuable asset in modern communication systems. We have well written about What is TCP Selective Acknowledgement?



  1. Is TCP SACK compatible with all devices?
    • Yes, TCP SACK is designed to be compatible with most modern devices and network configurations.
  2. Does SACK increase network complexity?
    • No, when implemented correctly, SACK enhances performance without introducing significant complexity.
  3. Can SACK be used in wireless networks?
    • Absolutely, SACK is versatile and can be implemented in both wired and wireless network setups.
  4. What are the potential downsides of using TCP SACK?
    • While rare, improper implementation or outdated equipment can lead to compatibility issues.
  5. How does SACK contribute to faster data transfer?
    • SACK reduces unnecessary retransmissions, allowing for faster and more efficient data transfer.
  6. What is a sack in TCP? A sack means Selective Ack in TCP meaning a range of values.

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