Re: [lwip-users] seq number & ACK issue

[Kieran Mansley]

On Wed, Mar 03, 2010 at 10:36:09AM +0700, PHAM ANH THIEN wrote:
> "Your sender would therefore send
> bursts of 5 packets less often, and the speed would be about the same"
> 
> Why my sender only can send bursts of 5 packets less often??? i can send
> more than 5 packets, right? i don't understand, could you please explain?


I'll do my best, but there are lots of guides and books to the way that
TCP operates, and it's often easier to explain this sort of thing with a
diagram or in person.

If the receiver is sending an ACK every 5th packet it receives, it will
on average increase the window by 5 packets (assuming it has reached a
steady state).  This would therefore allow the sender to send a burst of
5 packets before it would have to wait for the receiver to send another
ACK to give it more window space to use.  This is the case where the
receiver is slower than the sender, or the TCP window configured on the
receiver is small.

If the receiver is faster and the window on the receiver is made large
enough that the sender hasn't used it all when the ACKs with more window
gets back to the sender then the sender will never run out of window and
will be able to send as fast as it can.  This case is less common though
particularly in systems using lwIP where memory for window and CPU to
make things fast are not available.

Kieran