[lwip-users] TCP behaviour with Zero Copy API using Raw API interface

[Sathya Thammanur]

Hi all,
I have a client program that connects to a server on PC. The client is
a simple program that sends data to PC. What I notice is that when I
call  tcp_write() with "copy" argument 1, then my program works
without any issues. However, if I change "copy" to 0 then I notice
that the TCP connection is not established completely. The following
happens :

Client                            Server
SYN ---------->
       <---------             SYN, ACK

Junk packet sent from client. The Server resends the SYN, ACK and then
closes the connection. TCP client sends a lost ACK with incorrect
sequence number after couple of junk packets.

I do notice that my application call back that is registered with
tcp_connected() is called. Here is where I start to send data. I am
using lwip version 0.7.2.  Am I using the tcp_write() in the right way
as intended ? Any correct usage of this function will be very useful.

Any help would be greatly appreciated.

Thanks,
Sathya