[lwip-users] Understanding retransmission

[Jan Menzel]

Hi All!
        May I ask for your comments on the attached Wireshark capture?
        192.168.5.108 is LwIPs Git master on a LPC1768. 162.242.228.73 is some
Linux based server. The LPC is just flushing some larger buffer (a few
mb) to the server. In line 21 the shark records a duplicated ACK, which
I've seen a few times already (there is one in line 7 for example.). As
result LwIP sends a packet, that the Shark flagged as "new fragment
overlaps old data". Looking into the details, this seems to me just a
retransmission of the data that follows the ones that have been just
acknowledged again. As you can see from the following lines, LwIp
retransmits the retransmission a few times but the server does not
respond anymore. The next reaction of the server is in line 29 a
"Encryption Alert" which to my understanding is the request to terminate
the SSL layer on top of the TCP connection.
        Comparing the duplicated acknowledge from line 21 with the one in line
7 one can see, that in line 8 the server has immediately send an other
acknowledge, which keeps the data flowing: LwIp is sending the next
bunch of data in line 9.
        From my limited understanding the flow in line 21 and onwards is ok
from LwIps point of view... However its not ok that the server refuses
to continue after the retransmission.
        Is there anything I can do in LwIp to fix this?

        Jan

DB_150119_1.pcapng
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