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Re: [lwip-users] Sending too slow.

From: Kieran Mansley
Subject: Re: [lwip-users] Sending too slow.
Date: Tue, 12 Sep 2006 09:53:34 +0100

On Tue, 2006-09-12 at 10:44 +0200, Mateusz Plocinski wrote:
> Hi,
> I'm currently working on building audioserver based on AT91SAM7X256 
> microcontroller and freertos+lwip stack.
> I'm new to lwIP and I encountered one problem while sending packets. When I 
> send a frame (for example 32 KB), lwip sends it as 1.5KB packets and thats 
> normal for TCPIP. But every packet it waits for acknowledgement from client 
> (PC, WinXP). 
> And PC sends it after 200ms!
> I have to send about 100KB per second or more and waiting for ack 200ms is 
> not possible. As far as I know, I have a right to send the whole window size 
> (windows XP sends 65KB window requests) without waiting for ack.
> My question: is it possible to make lwIP stack send packets faster without 
> waiting for ack every single packet?

TCP (and in particular lwIP) has two windows to limit the send speed:
the receive window (which is the one you describe) and the congestion
window.  It can only send up to the smaller of these two before
receiving an acknowledgement.  When a connection is first established
the congestion window will be small, which is why you have to wait for
an acknowledgement, but it should grow with each round trip time (as
long as there are no lost packets) until you are able to use the whole
receive window at a time.  However, it sounds like you aren't seeing the
congestion window grow, and I can only assume there is some problem with
your port as this is not normal lwIP behaviour.  Can you provide a
packet trace (for example, using tcpdump or ethereal) to illustrate your



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