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Re: [lwip-users] TCP keeps re-transmitting but no ACK packet after SYN,

From: Mohsin
Subject: Re: [lwip-users] TCP keeps re-transmitting but no ACK packet after SYN, SYN+ACK
Date: Tue, 10 Mar 2015 02:29:43 -0700 (MST)

Sergio R. Caprile wrote
> Sorry, I don't fully agree with your definitions of "known to be good"
> or "known to work". To me those are "supposed to be OK". Your web server
> is not lwIP code, how are you certain it works OK ? 
> ...
> you need a known to work application, and viceversa.

Here is the capture file for the same webserver which is working for my
backup based on lwIP 1.4.1 which may justify what you call and I believe as
"known to be good".


I'm also getting the webpages served by webserver and can see them in my
browser. Below is the screen shot of webserver that is working.


> As Simon pointed out (I missed that, only TCP checksum validation
> activated on my Wireshark...), you are sending IP datagrams with bad
> checksums. That is the reason why your browser does not see the SYN+ACK
> and keeps sending SYNs.

Though IP datagrams checksums are bad for above attached captured file, its
webserver is working fine (as shown in screen shot above) which I'm also
unable to judge.

> PS: how come you are developing networking applications and don't know
> how to do a plain telnet to a port ? you open a command shell or
> whatever M$ calls it and write "telnet" followed by the port number, or
> install PuTTY.

What I actually don't know about telnet is configuring stack/port for telnet
(which I'm not sure about and might not be required at all). Otherwise I
know this basic telnet using command prompt in Windows-7 PC and I also tried
as u suggested to "telnet mybox 80" but connection gets closed and telnet is
also not working. My box sends RST+ACK packets and connection gets
terminated. Below I've also attached capture file of telnet test and screen
shot that shows what command I'm entering to try the telnet.


As per your suggestions, now my next goal is to test raw API based webserver
with minimal application instead of my current webserver which is using
Socket API.

[Note: I guess there is some problem at TCP layer in stack bcoz of which no
ACK is received in response to SYN+ACK. If TCP connection will only not be
established then how those TCP segments will be further processed and get
access to socket/webserver?]

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