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What is push ACK?

Published in Networking Protocols 3 mins read

Push ACK, or PSH ACK, in the context of TCP communication, refers to a TCP segment that has both the PSH (Push) and ACK (Acknowledgment) flags set. This combination signals the sender to "push" the buffered data to the receiving application immediately, while simultaneously acknowledging that the previous data segment(s) have been received successfully.

Understanding the PSH and ACK Flags

To fully grasp the meaning of Push ACK, let's examine the individual flags:

  • ACK (Acknowledgment) Flag: This flag is used to acknowledge the receipt of data. When set to 1, it indicates that the Acknowledgment Number field in the TCP header is valid, confirming that the sender has received the data up to the specified sequence number. It signifies that the TCP connection is established and communication is ongoing.

  • PSH (Push) Flag: This flag requests the receiving TCP stack to immediately deliver the data in the buffer to the receiving application. Normally, TCP may buffer data to optimize transmission efficiency. However, setting the PSH flag indicates that the data is urgent and should not be buffered. This is often used for interactive applications where timely delivery of data is crucial.

The Significance of Push ACK

The combination of PSH and ACK flags in a TCP segment indicates that the sender is both acknowledging previously received data and requesting the immediate delivery of the current data to the application layer. This is a common occurrence in interactive applications like Telnet or SSH.

Scenario Example

Imagine a Telnet session where a user types a command and presses Enter.

  1. The client's TCP stack packages the command into a TCP segment and sets the PSH and ACK flags.
  2. The PSH flag tells the server to immediately deliver the command to the Telnet application.
  3. The ACK flag acknowledges any previous data received from the server.

The server then processes the command and sends the response back to the client, also often using PSH ACK to ensure prompt delivery.

Why Use Push ACK?

  • Responsiveness: Ensures timely delivery of data to the application, enhancing the user experience.
  • Interactive Applications: Critical for applications where real-time data transfer is essential.
  • Efficiency: Although seemingly contradictory to TCP's buffering nature, PSH ACK optimizes for perceived speed in situations where latency is more critical than raw throughput.

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