Re: [lwip-users] TCP Out-Of-Order/Retransmission/Connection closed issue

[David Belohrad]

Hi Axel,

I experience the same issues(lwip1.4.1 from git). I have found that mine were 
caused by improper calling of tcp_tmr. In lwipopts.h one declares a variable 
(TCP_TMR_INTERVAL), which tells in what milliseconds 'distance' you call 
tcp_tmr. If this constant does not reflect the real situation, you'll get lots 
of disconnects and retransmissions. In my case i have set TCP_TMR_INTERVAL to 
100ms, but I was calling tcp_tmr() each 200ms, this causes the algorithms in 
tcp_tmr not to work correctly and causes for example retransmission, because 
tcp_tmr 'thinks' that the timer for retransmission has already expired. And 
then you find in captured packets weird thing like 'tcp previous segment not 
captured', but if you follow the seq/ack numbers, you'll find that it was 
indeed captured correctly. Another example of retransmission is of course lossy 
line, but I guess this is unlikely to happen. These I can see as well in my 
capture list, but they rarely finish by closing the line, and their occurence 
is not as high as occurence of retransmissions caused by improper call to 
tcp_tmr.

note: of course it depends on your system. I call tcp_tmr manually from my IRQ 
on embedded system (NO_SYS=1). I believe if you have some sort of RTOS, it is 
up to scheduler to call tcp_tmr in a correct time....

I still did not resolve my retransmission issue on 100% (last count i have is 
18 connections closed because of the retransmission fail happening during last 
weekend)


if you find some other reasons for this, i'd be glad to hear them as i might 
encounter them as well. I use beaglebone baremetal app, no operating system....

cheers
.d.


----------------------------------------------
Dr. David Belohrad, Div. BE-BI
C.E.R.N. Site de Prevessin, F-01631 CERN CEDEX
http://www.cern.ch
address@hidden
Tel +41.22.76.76318  Fax +41.22.76.69056
GSM +41.75.411.3455
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Axel Lin <address@hidden> writes:

> Hi,
> I'm using lwIP git tree (47f55b02bf9f889eb38379a68de9d765c163b89a).
> Sometimes I found the connection will be reset, and I found a couple
> of TCP Out-Of-Order/Retransmission before connection reset.
>
> Many dup-ack e.g. No110, 112, 190, 191, 330,...
>
> Below is the log can be opened by wireshark.
> https://www.dropbox.com/s/55cnyapc0l7oy6p/sim-pppos.pcapng?dl=0
>
> Is there anything wrong in the log? (lwip+pppos)
>
> Thanks.
>
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