Re: [lwip-users] LW IP - TCP instantaneous data transmission

[Noam Weissman]

Hi,

 

Before you start re-inventing the wheel why not take a readymade HTTP server ?

 

You have a basic good HTTP server in the lwIP contribution or similar that runs in RAW API.

 

BR,

Noam.

 

From: lwip-users [mailto:lwip-users-bounces+address@hidden On Behalf Of anand arjunan
Sent: Wednesday, August 31, 2016 12:18 PM
To: address@hidden
Subject: [lwip-users] LW IP - TCP instantaneous data transmission

 

Hi,

 

I am using LWIP (Raw TCP mode) (in FreeRTOS) for handling http page requests, construct the web page and send the page to the client. I have set the TCP_SND_BUF as 8*TCP_MSS where TCP_MSS is 1460. There will be only one client / browser connecting to the system at a time.

 

In the receive call back function, I constructed the entire web page and used the tcp_write () (with TCP_WRITE_FLAG_COPY ) to send the data to LWIP buffer. Due to memory limitations, I have to reuse a buffer size of 2.5 K to construct the page before writing it to LWIP using tcp_write () as the actual page size to be sent to client is more than 14 K. I was thinking that as and when I call tcp_output () the data would be sent instantaneously to the client. But, I got to know that tcp_Output() does not work in the callback function and LWIP will start sending the packets automatically once the control is out of the callback function. 

 

Due to this limitation I slightly modified the design. I have made the actual LWIP receive callback to just set another (second) callback function so that the actual LWIP callback would return immediately (so that tcp_output( ) can work). The second callback would actually construct the page and would be invoked by http_poll function or some other timer.

 

Even this does not work. It still accumulates the data worth of of 8*1460 bytes fully and sends only that much to the client. Once the buffer is full, tcp_sndbuf(pcb) returns zero and I cannot write anymore to it. I thought this would return non zero value as I am calling tcp_output ( ) outside the LWIP calling function using a timer function.

 

I tried disabling the nagle's algorithm using tcp_nagle_disbale( ) before transmitting the packets. That did not help either. 

 

I want the data to be sent to the client (without accumulating) as and when I call tcp_output( ) because of the limitation that I can construct only 2.5 K bytes of page at a time and reuse it to construct the next section of the page.

 

Please help. Any help / pointers would be appreciated.

 

Thanks

LWIP_Starter