Re: [lwip-users] Handshake trouble when packet is lost

[Enrico Murador - Research & Development - CET]

Dear Kieran,

Thank you... now (maybe) I got the point and I think I'm a little more confident with TCP.

- But now, why the device repeats (with packet 10687) the data from
byte 27857? Because it has not received an acknowledgment for that
packet

That retransmission is a little confusing.  You are capturing packets at
the PC end, and have to remember that data will take a little time to
get to and from the device end, and so packets may appear to occur in
different orders at the device.  In this case, you see packet 10686 come
before 10687, but the lwIP device probably sees them the other way
around.  I.e. it sends 10687 before it has seen and processed 10686 and
so isn't aware that it doesn't need to send this packet.

Effectively something strange could be suspected by taking a look also at
the timestamps of the "equivalent meaning" packets 11417 and 11418,
that Wireshark places in reverse time order (by wireshark's own deduction?).

So I think to have understood that at (re)transmission of packet 10685
the device's TCP stack had already planned to send all the packets
unacknowledged at the moment, and has the time of (re)transmitting
packet 10687 before the cumulative ACK is received.

Thank you for your support (and patience)...
Enrico