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

From: Indan Zupancic
Subject: Re: [lwip-users] ACK missing after SYN, strange high Ack Num
Date: Wed, 01 Jun 2022 11:38:34 +0200
User-agent: Roundcube Webmail/1.3.16

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.



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