Re: [lwip-users] httpd slow response

[Rejean Groleau]

If your HTTP server is only sending every 200ms,
it is probably because your client browser is waiting
for at least 4380 bytes before ACKnowledging
(this value is based on your 1500 bytes MSS).

Since your send buffer is only 3000 bytes long,
you will never send those 4380 byte at once,
hence always waiting for the delayed ACK,
coming only after 200ms.

Like I said in a previous post:


Rejean Groleau wrote:
> 
> I had the same problem on my platform (a Luminary Stellaris
> microcontroller with 64KB of RAM). 
> To speed things up (from a typical 200ms to a less-than-10ms delay), I had
> to do the following: 
> 
> 1. Use a TCP_SND_BUF value four times greater than the TCP_MSS (according
> to RFC 3390). 
> 2. Always fill completely the TCP_SND_BUF (unless no more data is to be
> sent). 
> 3. Set a callback through tcp_poll() to attempt resending data (after an
> arbitrary amount of time). 
> 4. In my case (an HTTP server), use the HTTP 1.1 specification (to reuse
> connections). 
> 
> I did this according to RFC 3390 (available at
> http://tools.ietf.org/html/rfc3390), which states: 
> 
> If (MSS <= 1095 bytes) 
>     then win <= 4 * MSS; 
> If (1095 bytes < MSS < 2190 bytes) 
>     then win <= 4380; 
> If (2190 bytes <= MSS) 
>     then win <= 2 * MSS; 
> 
> I interpreted (read between the lines) that when this "upper-bound" limit
> is reached, an ACK is sent back. 
> 
> Note that although my MSS is 1472 bytes, I still had to go under the first
> condition 
> (4 * MSS instead or 4380 bytes). It seems the web browsers (or windows XP) 
> do not exactly follow this scheme. 
> 
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