Re: [lwip-users] TCP speed drops to a crawl.

[Adrian Godwin]

Michael Williamson wrote:
> Hi,
>
>   I am fairly new to lwip.  We are using lwip 1.1.0 with a port to TI's 
> Code Composer BIOS for a 6711 processor.  The stack seems to be working, 
> but we are seeing a strange behaviour with a simple TCP packet 
> throughput test that smells like a configuration problem.  We are using 
> the BSD sockets.c style API and have code that looks something like that 
> below.
> If we set TCP_NODELAY, we get about 3100 packets very quickly (about 
> 1000 per second) then the rate drops substantially and it looks like 
> (using ethereal) the lwip stack is waiting for an ack sequence from the 
> client before sending any new messages.  Our client (telnet on a PC 
> running Windows 2000) is ack'ing packets on a 200 ms latency, so we are 
> seeing packet messages at about a 5 Hz rate instead of the 1000 Hz rate 
> we were expecting.  No packets are dropped or timed out in the 
> transaction sequence, just stopping until an ack is received.
>
> We're wondering if there is something like a water mark or some other 
> configuration in the lwipopts.h that might present such a behaviour.   
> I've included our settings after the pseudo code below.
>
> Any insight would be greatly appreciated.


I found a PC client acking slowly because the TCP window wasn't
filled : it was expecting further frames because they were
invited, but lwip didn't send them.

The amount of data lwip is prepared to push into the window is
conditioned in the BSD sockets API primarily with logic
related to TCP_SND_BUF. The PC is inclined to offer quite a
large window (64k).

-adrian