enter image description hereWhile calculating RTT for each packet in Wireshark, it occurs to me that several packets sent may have less corresponding respond packets. For example, as shown in the figure attached to this question, the client sent packet #10 and #11, but will only receive packet #14 as a response. The server returns the ACK# of #11, so how can I determine the RTT for #10? Will this be regarded as one transmission round or two?

  • Here's a reference, in case your question was specific to wireshark: ask.wireshark.org/questions/34017/when-is-rtt-calculated You can search for "delayed ack" in context to your question, which is the term for an ack that is sent later than immediately when the packet is processed.
    – Jeff Meden
    Apr 5, 2016 at 20:56
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 13, 2017 at 19:23

1 Answer 1


I'm not sure this really answers your question, but keep in mind that TCP spends its time trying to determine the RTT of the network, not that of each packet. The packet RTTs are a means to end.

TCP implementations have all kinds of special cases to handle RTT estimations in the presence of different events : retransmissions, multiple retransmissions, etc... the difficulty being to ignore things that would give you a poor RTT estimation, but not ignore events that would signal an actual substantial change in the network RTT.

So in the case of cumulative ACKs, most likely TCP will just ignore packet #10 and can determine the RTT from packets #11 and #14.

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