On Tue, Dec 5, 2017 at 12:22 AM, Steve Ibanez <siba...@stanford.edu> wrote: > Hi Neal, > > Happy to help out :) And thanks for the tip! > > I was able to track down where the missing bytes that you pointed out > are being lost. It turns out the destination host seems to be > misbehaving. I performed a packet capture at the destination host > interface (a snapshot of the trace is attached). I see the following > sequence of events when a timeout occurs (note that I have NIC > offloading enabled so wireshark captures packets larger than the MTU): > > 1. The destination receives a data packet of length X with seqNo = Y > from the src with the CWR bit set and does not send back a > corresponding ACK. > 2. The source times out and sends a retransmission packet of length Z > (where Z < X) with seqNo = Y > 3. The destination sends back an ACK with AckNo = Y + X > > So in other words, the packet which the destination host does not > initially ACK (causing the timeout) does not actually get lost because > after receiving the retransmission the AckNo moves forward all the way > past the bytes in the initial unACKed CWR packet. In the attached > screenshot, I've marked the unACKed CWR packet with a red box. > > Have you seen this behavior before? And do you know what might be > causing the destination host not to ACK the CWR packet? In most cases > the CWR marked packets are ACKed properly, it's just occasionally they > are not.
Thanks for the detailed report! I have not heard of an incoming CWR causing the receiver to fail to ACK. And in re-reading the code, I don't see an obvious way in which a CWR bit should cause the receiver to fail to ACK. That screen shot is a bit hard to parse. Would you be able to post a tcpdump .pcap of that particular section, or post a screen shot of a time-sequence plot of that section? To extract that segment and take screen shot, you could use something like: editcap -A "2017-12-04 11:22:27" -B "2017-12-04 11:22:30" all.pcap slice.pcap tcptrace -S -xy -zy slice.pcap xplot.org a2b_tsg.xpl & # take screenshot Or, alternatively, would you be able to post the slice.pcap on a web server or public drive? thanks, neal
