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Stack Gives Back Featured on Meta. New post summary designs on greatest hits now, everywhere else eventually. Related Hot Network Questions. Question feed. Stack Overflow works best with JavaScript enabled. A duplicate acknowledgment is sent when a receiver receives out-of-order packets let say sequence Upon receiving packet 4 the receiver starts sending duplicate acks so the sender would start the fast-retransmit process.
Another situation is packet loss. Keep in mind - packet loss is quite normal in TCP networks. TCP actually regulates itself with packet loss as a feedback mechanism. RDT protocol use to retransmit the packet only when timer expires. TCP now uses duplicate acks as well as timeout to retransmit a packet if lost. Duplicate acks are used as a part of fast retransmission and packet recovery.
Generally, if tcp timer expires , it is assumed that the packet is lost and tcp retransmits the same packet. But it need to wait, until the timer expires. As a part of congestion control techniques, TCP behaves very politely during the times of congestion by increasing the timer interval by 2 times , so that the packet would be retransmitted slowing , thus not contributing to congestion.
However timeout increases exponentially assuming packet is lost every time it sends. Retransmission of the same packet is ,time consuming due to increased timeout and the sender need to wait for longer time thus causing delay. Hence tcp implemented duplicate acks and the sole purpose of it is to intimate the sender before timeout occurs.
If the sender receivers duplicate packets greater than 3 then it will retransmit the packet. Duplicate packets are send immediately by receiver if out of order segments are arrived.
However if no packet loss was found , ack is delayed hoping that to acknowledge the back to back segments and reduce the number of acks in network. This approach is sending cumulative acknowledgements instead of sending ack to every segment. If sender receives the duplicate acks, sender immediately sends the lost packet based on acknowledgement number and doesn't send any application data in the send buffer until the lost packet is sent.
The first segment was lost but the second segment was received correctly by the receiver. Let X be the amount of data carried in the first segment in bytes and Y be the ACK number sent by the receiver. Get more notes and other study material of Computer Networks. Computer Networks.
Three Way Handshake- Before you go through this article, make sure that you have gone through the previous article on Three Way Handshake.
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