[lwip-users] Re: [lwip] Some lwIP findings : ACKs and slow start

[Kieran Mansley]

On Tue, 16 Jul 2002, Chris Borrelli wrote:

> Does anyone know why lwIP sends out ACK segs without data in the payload.  I
> am using an echo server for some tests, and lwIP sends 2x the number of
> packets it should because it sends empty ACKs...

When a packet arrives, the need for an ACK is noted.  It is not sent
straight away incase it can piggyback on a data packet and so make more
efficient use of the bandwidth.  If after a short amount of time no data
packet has been transmitted, the ACK has to be sent on its own.  My
guess is that your PC is faster than whatever you have running lwip, so
the echo is returned more quickly, and so the data packet is transmitted
before the timeout for the ACK.  Either that, or the stack on your PC
is happy to delay the ACK for longer.

The other problem sounds like a window issue.  If the advertised window is
<500 bytes the transmitter will not be able to send a full segment.  This
is one reason why delaying ACKs a long time is bad, as it artificially
closes the window.

Kieran


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