Hello All, We are observing multiple TCP Duplicate ACK’s coming from the receiver and after going through the TCP RFC’s and I found the following reasons why our product is replying with multiple TCP DupACK's.
a. [RFC 2001 – Section 3] under Fast Retransmit, TCP may generate an immediate acknowledgment (a duplicate ACK) when an out-of-order segment is received (Section 18.104.22.168 of , with a note that one reason for doing so was for the experimental fast-retransmit algorithm). This duplicate ACK should not be delayed. The purpose of this duplicate ACK is to let the other end know that a segment was received out of order, and to tell it what sequence number is expected.
b. [RFC 2001 – Section 3] Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received. It is assumed that if there is just a reordering of the segments, there will be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost. TCP then performs a retransmission of what appears to be the missing segment, without waiting for a retransmission timer to expire.
So, the receiver is behaving normally as per TCP standards, and is only sending duplicate ACK#1 which is the indication that an out-of-order TCP packet might have been received, but if I am checking the "Expert Info" of Wireshark then there is no indication of out-of-order pkt.
Can anyone help here what might be the other possible reason for the receiver to send duplicate ACK's??
Regards Amitav Nayak
asked 02 Apr '14, 09:56
Duplicated ACKs may also be caused by very high path latency, brief connection outage or actual packet loss.
You can check for out of order packets with the filter "tcp.analysis.out_of_order" and check for retransmissions with this filter: tcp.analysis.retransmission || tcp.analysis.fast_retransmission
answered 11 Sep '15, 06:11