[lwip-devel] [bug #24493] Dropped incoming packets cause a burst of duplicate ACKs

[Kieran Mansley]

Follow-up Comment #1, bug #24493 (project lwip):

>From the trace you've shown the behaviour of lwIP looks, at first glace, to
be correct.

The problem in the stream is that there seems to be a lot of missing packets
between sequence 8892 and 39552.  Unfortunately this part of the connection
appears to be before the start of the capture, or possibly they got lost in
the second of inactivity (and an ARP request, which is a little suspicious in
the middle of a connection) near frames 9 - 10.

As a result of these missing packets lwIP sees a large number of packets that
are out of order (frames 13 - 25 for example) - it's expecting 8892 and sees
39552 and so on.  For each packet it receives out of order it sends a
duplicate ACK, hence the large number of duplicate ACKs in a short space of
time.  These duplicate ACKs should trigger fast retransmissions by the sender,
which we see in the following frames (frame 30, and a long sequence from frame
48 onwards).

The series of duplicate ACKs from 97-107 are due to the unnecessary
retransmission of some of the frames in the earlier sequence - the sender
doesn't know that we've buffered some of the frames after the original loss,
and so sends them all again.  The stack has now received these twice.  I'd
need to read up to check on whether sending duplicate ACKs in this case is
absolutely correct, but I've a feeling it is.

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