Re: [lwip-users] TCP [TCP Window Update]

[address@hidden]

Unfortunately, I think there is currently no way to change this 
behaviour when using the socket API. The reason is this:
1) the ACK packet gets sent from the tcpip_thread, when the received 
data is put into the mbox.
2) Then there's a thread change to your application thread. After 
reading the data from that thread,
3) there's again a thread change back to tcpip_thread to tell the stack 
the data is actually processed.

Step three results in the window being changed. However, the window 
update is not sent right away but after 250 ms (delayed ACK mechanism): 
with full duplex application-protocols, the window update would just be 
sent with the next packet. Since you don't send a packet in this 
timespan, an extra packet is generated for the window update.

Looking at the steps above, it would not even matter if your application 
was faster than 1ms, it's just the way lwIP is designed currently.

You *could* change the behaviour if you would use the raw API and tell 
the stack about the window update right away - but I guess that would be 
a pretty big change to your application. Another way to solve it would 
be to set a new flag in the pcb and update the window when putting the 
data into the mbox (when that flag is set) - however, that would mean 
you have to change the lwIP core code - or you'd have to wait until such 
a behaviour is implemented in CVS.

Simon


Alain Mouette wrote:
> Hi, I am getting quite a lot of these in WireShark:
>
> TCP [TCP Window Update]
>
> 0) I am using the socket interface :)
> 1) lwip sends a 15 byte packet to the server
> 2) the server responds an 11 byte packet, with ACK
> 3) lwip sends an ACK with a reduced window, from 2048 to 2037, delay is 
> between 5 and 66ms
> 4) lwip sends an extra ACK with the full window size, delay is around 250ms.
> 5) next packet is sent usualy many minutes later...
>
> What I can understand that is happening is this: I take a little too 
> long to read the received data. I am using select to poll for data, with 
> 1ms delay, but it is with FreeRTOS and so it might take a little longer 
> sometimes...
>
> My question is: is there a way to delay that ack so that it only goes 
> after I read the data? Or store it somewhere (like buffers)?
>
> This is to be with GPRS and I am getting over 20% extra trafic just for 
> that :(
>
> Thanks,
> Alain
>
>
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