Re: [lwip-users] Performance drop due port numbers reused too fast

[Indan Zupancic]

Hello Jochem,

On 2022-10-20 14:44, Jochen Strohbeck wrote:
> The server will sent ACK instead SYN+ACK for 50118 and 50130 because
> these port numbers have been previously used less than a second before.

Was the connection closed properly by both sides when this happens?

If the sequence number is totally wrong it should send a RST I think,
but if it seems from the future it should send an ACK for the last
received packet (not ACKing the SYN itself!). TCP originally allowed
connection re-use, so continuing after FIN. You could argue that lwIP
should always send a RST after closing a connection and getting new 
data.

> It depends. First I used a single timeout parameter e.g. 10s for the
> http request. This results in multiple retransmit (using backoff) until
> 10s are over. Because the retransmit is using the same port number, 
> each
> retry fails.

The retransmits should be solved if lwIP sends a RST instead of an ACK.
Personally I've only seen lwIP sending RST, never an ACK for re-used 
ports.
What lwIP version are you using and is the ACK problem gone with master?

> Yes, but I guess there is something else I can do to make the
> performance drop less painful. Maybe some other windows parameter?

Try newest lwIP version and see if the problem persists: If RST is sent
on port re-use the problem should be much severe, assuming the 
application
reconnects immediately.

Greetings,

Indan