Duplicate ack why

Before you go through this article, make sure that you have gone through the previous article on Three Way Handshake. In this article, we will discuss how a lost TCP segment is retransmitted. Also Read- Time Out Timer.

The sender sends two segments back to back. The sequence numbers of the first and second segments are and respectively. When the receiving socket detects an incoming segment of data, it uses the acknowledgement number in the TCP header to indicate receipt. After sending a packet of data, the sender will start a retransmission timer of variable length. If it does not receive an acknowledgment before the timer expires, the sender will assume the segment has been lost and will retransmit it.

The TCP retransmission mechanism ensures that data is reliably sent from end to end. If retransmissions are detected in a TCP connection, it is logical to assume that packet loss has occurred on the network somewhere between client and server. Most packet analyzers will indicate a duplicate acknowledgment condition when two ACK packets are detected with the same ACK numbers.

Sending TCP sockets usually transmit data in a series. Rather than sending one segment of data at a time and waiting for an acknowledgement, transmitting stations will send several packets in succession. If one of these packets in the stream goes missing, the receiving socket can indicate which packet was lost using selective acknowledgments. These allow the receiver to continue to acknowledge incoming data while informing the sender of the missing packet s in the stream. As shown above, selective acknowledgements will use the ACK number in the TCP header to indicate which packet was lost.

Most network analyzers will flag these packets as duplicate acknowledgements because the ACK number will stay the same until the missing packet is retransmitted, filling the gap in the sequence.

Typically, duplicate acknowledgements mean that one or more packets have been lost in the stream and the connection is attempting to recover. They are a common symptom of packet loss. In most cases, once the sender receives three duplicate acknowledgments, it will immediately retransmit the missing packet instead of waiting for a timer to expire.

These are called fast retransmissions. Connections with more latency between client and server will typically have more duplicate acknowledgement packets when a segment is lost. To Be Removed Title. URL Name. Article Record Type. Follow Following. View Article. Was this article helpful? Like Dislike.