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RE: [lwip-users] TCP not sending initial ACK

From: Chuck Kuecker
Subject: RE: [lwip-users] TCP not sending initial ACK
Date: Fri, 28 Aug 2009 07:17:31 -0500

Hi, Simon,

I attempted to send the two Wireshark captures as attachments, but the
email bounced. So I will try again with a zip file.

Here's two captures - one with the 2500-count loop delay just before
sending the ACK for the first packet (only - all other packets have the
call to tcp_recved() immediately after processing the packet), and one
with the delay commented out. Note the three-second delay between
packets 8 and 9 with the delay commented out.

I've got TCP_WND and TCP_MSS set to 1200. This seems to give the best
communications. Setting TCP_WND 2048 and TCP_MSS to 1500 worked for
other communications, but caused this transfer to have a bunch of
repeated packets near the end. Other choices for these parameters caused
patterns of repeated packets in the body of the transfer.

Speed and throughput are secondary to accuracy for this product. It only
communicates very occasionally, but must do so reliably when it does
check in.


-----Original Message-----
From: address@hidden
[mailto:address@hidden On
Behalf Of address@hidden
Sent: Friday, August 28, 2009 4:53 AM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] TCP not sending initial ACK

Chuck Kuecker wrote:
> For what it's worth, I've got TCP_WND and TCP_MSS set to 1200. Other 
> settings seem to cause resends in the body or at the end of the file 
> transfer.
Setting TCP_WND and TCP_MSS to the same value is not a good idea if you 
want to achieve high throughput as you effectively disable the sliding 
window algorithm.
> The timers are all running with the default values set up by the 
> Luminary library. All other communications, ARP, UDP and TCP, work 
> fine - it's just this file transfer that is strange.
Could you maybe get a wireshark capture and attach it here (pcap format)

to try to trace the packets in the stack? Maybe the delayed ACK 
mechanism interferes with the used protocol...


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