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

[Indan Zupancic]

Hello Jochem,

On 2022-06-01 11:08, Jochen Strohbeck wrote:
> What kind of problem may force lwip to send ACK instead of SYN+ACK?

When it hasn't seen the SYN yet (it didn't process all received packets
yet). Or when a packet got dropped and a new packet arrives, sending
the ACK is a way to let the other side know something needs to be 
resend.

Does the ACK also ack the SYN itself or only earlier data?
I would expect the latter.

> Are there any lwip or host parameters to make the TCP/IP timing
> more tolerant?

TCP has flow control for a reason. Hosts don't send more data than
you tell them you can handle. You could try reducing your receive
buffer sizes, check TCP_WND in your lwIP config. But it could also
be that the lwIP options are unbalanced and that causes packet
drop, either on the receive side or on the send side.

Greetings,

Indan