Selective Acknowledgment (SACKs)

What is SACK?

SACK is a Transmission Control Protocol (TCP) strategy that selectively acknowledges packets received and helps handle dropped packet segments. When SACK finds a missing packet, it ensures that the receiver notifies the sender immediately about the missing data. In this situation, the sender will transport only the missing data instead of sending the received ones. SACK helps improve the network's quality and efficiency, especially in times of congestion.

How does SACKs Work?

By appending an additional field in the TCP's header for detailed feedback about segment reception, SACKs allow the receiver to inform the sender about successful and missing data segments. The diagram below shows how packets can be dropped when sending from one device to another in a network. When data is sent from a source device to a destination device in a None-SACK  network, it can get lost without the destination device being notified.

Aside from causing information to get lost at the destination in a None-SACK network, there can also be data duplicates on the receiver as demonstrated below.

What are the Benefits of SACKs?

• Reliability: SACK helps to ensure that any data transmitted or received through the network is reliable. 
• Identification of missing packets: It checks and notifies the sender about packet loss. This helps the sender to identify missing data more effectively.
• Selective Acknowledgment: It helps the receiver to selectively acknowledge any data packet delivered successfully in case some packet segments get lost during transmission.
• Reduction of retransmission: SACK has a mechanism to provide detailed feedback on missing and successful data segments. The feedback helps minimize unnecessary data retransmission between the sender and receiver.

Frequently Asked Questions