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SACK selective acknowledgment

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Selective Acknowledgment (SACK) is a technique used in computer networks to improve the reliability and efficiency of data transmission over a TCP (Transmission Control Protocol) connection. It is an extension to the standard TCP acknowledgment mechanism and is designed to enhance the handling of lost or out-of-order packets.

In the traditional TCP acknowledgment scheme, the receiver sends a cumulative acknowledgment (ACK) to the sender, indicating the highest sequence number of data received successfully. This means that if a packet is lost or arrives out of order, the receiver cannot explicitly inform the sender about the missing or out-of-order packets, leading to retransmissions of packets that may have already been received.

SACK solves this problem by allowing the receiver to provide more detailed feedback to the sender. Instead of sending a single ACK, the receiver can send a selective acknowledgment that specifies a range or list of sequence numbers of the packets it has received successfully. This way, the receiver can inform the sender about specific packets that have been received out of order or indicate gaps in the received data.

Here's a step-by-step explanation of how SACK works:

  1. Sender sends packets: The sender transmits a series of packets containing data over the TCP connection to the receiver. Each packet is assigned a unique sequence number to help identify and order the data.
  2. Receiver receives packets: The receiver receives the packets sent by the sender. It keeps track of the sequence numbers of the received packets and identifies any missing or out-of-order packets.
  3. Generating SACK information: If the receiver detects missing or out-of-order packets, it generates a SACK option to provide feedback to the sender. The SACK option includes one or more blocks of sequence numbers representing the received data. Each block specifies a range of contiguous sequence numbers that have been successfully received.
  4. SACK transmission: The receiver sends the SACK option to the sender in an acknowledgment packet. The SACK option is added to the TCP header and indicates the specific packets that have been received successfully.
  5. Sender processes SACK: Upon receiving the SACK option, the sender updates its knowledge of which packets have been received successfully. It can now identify the missing or out-of-order packets that need to be retransmitted.
  6. Retransmission: The sender retransmits the missing or out-of-order packets based on the information provided by the SACK option. This helps in minimizing unnecessary retransmissions and reduces network congestion.

By using SACK, TCP connections can recover from packet loss and congestion more efficiently. It allows for faster retransmissions and reduces the need for retransmitting large amounts of data that have already been received successfully. As a result, SACK helps improve the overall performance and reliability of TCP connections, especially in scenarios where packet loss or reordering is common, such as high-speed networks or networks with high latency.