[lwip-users] TCP ACK not received - Probably due to seq no.

[Gaurav Jain]

Hi,
I have tried implementing a lwip server which connects to a client in
a windows machine. My lwip server is running on Virtex 7. Now what I
need to do is send 100MB data to the server, do some operations and
then write back to the client. I did as required. However I have the
following observations:

1. The client successfully sends the data to the server (whatever size).
2. What I had implemented was, that at the end. the client would send
a particular string, upon which the server would start processing the
data. After that the server would writeback to the client.

3. I used wireshark to analyze the packet flow. What is happening is,
that packets sent by the server start with the sequence number
received as ACK seq no from client, instead of starting from 1.
However, the client on the other hand after sending has entered a
blocking receive state, where it expects the data to be with sequence
number 1.

Due to this, the client does not send back any acknowledgement to the
server, where the snd_buf limit then reaches and the sending further
waits/fails.

Is it that once the server receives all the packets from the client,
do I have to again initiate the three-way handshake before it starts
sending to client. (By three way handshake I mean do I need to again
set up the pcb and set up a new connection? Or is there a way to
modify the ACK start number? Or I am somewhere wrong in all this?

Thank You
Gaurav