[lwip-devel] [bug #65630] issue to download file bigger than 6KB using using https protocol

[Stefano Oliveri]

Follow-up Comment #1, bug #65630 (group lwip):

Hello,

I was able, finally, to move forward and make my application works. 

The issue I noted (I am sure it depends on my integration of lwIP and Mbed TLS
in the application), analyzing the communication sniffed with Wireshark, was a
TCP Zero Window. I am trying to understand this mechanism, so any comment is
welcome.

This TCP window size depends on the TCP_WND. At the beginning it 7300 in my
case, and it worked fine without TLS. When I added the support the TCP window
start to shrink when I download "not small" file until the Zero Window problem
occurs. 
By increasing TCP_WND up to 21900 my application (seems to) works fine with
both http and https.

There are a couple of question in my mind I would like to understand:
1. is this analysis correct? Is that the root cause? Or the TCP/IP should be
able to work also with small TCP Window?
2. How to size correctly the TCP_WND option? 

Best regards,
Stefano


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