Re: [lwip-users] Pulling data from a server using HTTP with lwIP

[Robert Wood]

Hmmmm. I seem to be wrong, It *does* send out the ACK. I took out these 
lines:

strcpy(GetString,"GET /index.html HTTP/1.1\r\n");

netconn_write(xNetConn,GetString,sizeof(GetString),NETCONN_NOCOPY);

Which immediately followed:

rc2 = netconn_connect ( xNetConn, &remote_ip, 80 );

And it now does do the three way handshake.

Previously, what was happening was that it send out the HTTP GET 
request, but the Apache server was sending a timeout message, I assume, 
because it didn't get the third part of the initial handshake.

So, I suppose my question changes to:

Do I have to wait, in my code, for something in the lwIP stack to inform 
me that the ACK went out and I can send data to the established 
connection? If so, how do I do that?

If not, what I am doing wrong?

Thanks again.


On 13/11/14 09:54, Robert Wood wrote:
> I'm getting started with lwIP; I have the SAM7X FreeRTOS demo running
> and it is serving its demo file just fine.
>
> However, what I really need to do is pull a file *from* a server. So,
> I've created a new thread and done the following:
>
> xNetConn = netconn_new ( NETCONN_TCP );
>
> IP4_ADDR(&local_ip,emacIPADDR0,emacIPADDR1,emacIPADDR2,emacIPADDR3);
> rc1 = netconn_bind ( xNetConn, &local_ip, 0 );
> IP4_ADDR(&remote_ip,192,168,0,87);
> rc2 = netconn_connect ( xNetConn, &remote_ip, 80 );
>
> local_ip is the fixed address of my board, 192.168.0.87 is my desktop.
>
> I've hooked up Wireshark to look at what's going on and I get two
> packets (*): the lwIP SAM7X board sending out the initial request for a
> connection with the SYN flag set and my desktop, which is running an
> Apache server, replying with SYN & ACK flags set.
>
> Now, I would expect the lwIP stack to then automatically return the
> third packet in the handshake with just ACK flag set.
>
> When the desktop sends out its SYN/ACK packet, I can see it goes through
> a number of functions. It goes into ip_input(), you can see that the IP
> addresses match and it calls tcp_input() which is happy that the ports
> are correct and match. It clearly sees the flags are set to 0x12, which
> is exactly what Wireshark says it is.
>
> It ends up going into tcp_process(), in the switch statement for
> pcb->state is goes into the SYN_SENT case, seems to increment the
> sequence number correctly (I can see that number in the Wireshark data
> and that gets incremented by one. Eventually, it goes into tcp_ack(),
> which calls tcp_output(). pcb->flags is set to 0x01 which doesn't seem
> right, but what is less right is that no packet gets sent out.
>
> So, what vital step am I missing here? Is this something to do with
> needing to specify a callback function and me sending out the
> appropriate message? It looked to me like there was one set up; maybe it
> was,  just set-up and queued but not sent?
>
> I only have a tenuous grasp callback functions, so it may be that I'm
> missing something vital.
>
> I have plenty of FreeRTOS experience, but this is my first foray into
> lwIP. In truth, a lot of this is new; I talk about these flags like SYN
> and ACK as if I know what I'm on about, but this is all new to me in the
> past few days!
>
> Many thanks. :~)
>
> Rob
>
> * Actually I get a few more - my desktop resending its last packet!
>
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