Re: [lwip-users] TCP Zero Window

[Jan Menzel]

Hi Norbert!
        The advertised receive window size specifies the amount of memory the
stack can consume without further acknowledgement. Once the application
side has read received data and acknowledged/freed it, the receive
window is enlarged by this amount and - after a certain timeout - a
window update packet is send to the server to inform about the enlarged
receive window. If your application is reading the data very slowly, the
windows size will go down to zero and open up on read.
        Modifying the receive window size can be used to optimize the
throughput. A larger receive window allows more data to be on the fly
(not acknowledge by the client) but requires more work to recover from
lost data, especially if out of order queueing is disabled.
        If your receive windows does not open up again, you likely did not read
all data received.

        Jan

On 09.09.2016 10:18, address@hidden wrote:
> Hi all,
> 
> I have a small question. Under which circumstances does the lwIP stack
> reduce the window size even while the window update is disabled.
> 
> I am sending 16 kB to the microcontroller (lwIP-Stack) copy them to a
> certain address and send them back to the laptop (linux). At the end of
> the 66th receive cycle i receive an unexpected package and with the
> acknowledgement the lwIP stack reduces the window size by the length of
> the last 2 received packages. Afterwards we receive another unexpected
> package that is equal to the length of the window. From this point on
> the stack sends all 16 kB back and advertises the window size 0. The
> stack doesn't recover from that and will only send  zero windows
> afterwards. A change of the amount of buffers and the size of them had
> no influence on this problem.
> 
> greetings,
> 
> Norbert
> 
> 
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