Re: [lwip-users] ACK missing after SYN, strange high Ack Num

[Indan Zupancic]

Hello Jochen,

On 2022-06-07 15:55, Jochen Strohbeck wrote:
> In Wireshark I see that after the data transfer the client sends out
> FIN+ACK and the http server responds only with ACK, no further FIN sent
> out by the server.

This is normal if the server hasn't sent all data yet. I would expect
it to send more data and to set FIN on the last packet. If it doesn't
then it's a bug in the server.

> Is this considered a 'half closed connection'?

Yes.

> Do you think the server also close the connection too in order to avoid
> the RST on a 'pending connection'?

If the client is starting a new connection while it didn't receive
a FIN for the previous connection yet, then I'd say that's a bug in
the network stack on the client side.

> The problem is that the client application gets an CONNECTION_TIMEOUT
> error if RST happens.

> How to distinguish the 'RST on pending connection' from a 'server 
> unreachable' error?

Use a better OS? (I assume you meant 'Connection timed out' error.)

Look, if you OS resets a connection and forgets about it, and then 
re-used
the port for a new connection, that will get reset and it better 
silently
retry with a different random port instead of bothering the application
with a bogus error.

Just retry in your application on CONNECTION_TIMEOUT errors and it 
should
work, there's not much else you can do. If there's a real timeout the 
user
would have gotten impatient way before it triggered, otherwise they want
to keep trying.

Greetings,

Indan