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

[Kieran Mansley]

On Tue, 2010-03-02 at 17:19 +0700, PHAM ANH THIEN wrote:
> hi Kieran,
> 
> So you mean the current lwip stack configured sliding window 2 packets
> in window and fixed it & can not change it?

No.  There are two parts of TCP that you are putting together and
confusing:

1) A sliding window where the sender can send to use up all the window,
and the receiver sends ACKs to update it bit by bit as more buffer space
is available.  The size of this window is configured by setting TCP_WND
on the receiver, but the sender can also be limited by other things
(e.g. congestion, its send buffer, the application, and so on) and so
not send immediately there is space in the receiver's window

2) The delayed ACK mechanism.  This makes TCP more efficient by not
sending as many ACKs, at the expense of not updating the sender as
often.  It sends at least one ACK for every second packet.  Not all ACKs
will update the window and allow it to slide so the sender can send some
more: this depends on whether the application has read the packets when
the ACK is sent.

Kieran