Re: [lwip-users] Browser finishes every HTTP GET request with a ACK RST, why ?

[Indan Zupancic]

Hello,

On 2021-09-15 15:37, Info wrote:
> In the attached Wireshark capture file I can see many frames (from the
> web browser) with the set ACK RST flags. For me it not clear why the
> web browser sends this frames and if it is the reason for the broken
> periodic communication ? Any help and maybe some explanation would be
> very welcome.

I ran my own test for 8 hours, first few hours a HTTP request every 100 
ms,
the others every 10 seconds. We're using our own webserver code without
any issues for a few years now.

In those 8 hours I get 8 resets, all in the first few hours during the 
100ms test.
(This is with Windows 10 as host.)

All these resets happened when the port was re-used by the browser after 
2 seconds
of a previous transaction. Our TIME WAIT is configured for 2 seconds 
(TCP_MSL 1000).

What I see happening is that lwIP sends an ACK with the same sequence 
number
as previously, instead of a SYN, ACK. The other side of course doesn't 
like
that and sends a RST.

This does not fully explain everything in your Wireshark log, but maybe 
it is related.

Looking at the code, this behaviour is intentional and done in 
tcp_timewait_input().
Reading the RFCs this behaviour is by design and lwIP does nothing 
wrong.

The reason why ports are re-used so quickly is probably because they're 
chosen randomly.
If the host just resends a new SYN and does not return an error to the 
browser this
behaviour is fine.

Greetings,

Indan