Re: [lwip-users] How to root-cause duplicate ACKs?

[Indan Zupancic]

Hello,

Your application is slow in handling the data, which seems to cause a backlog.
Debug printing via UART is very slow, try not to do this while debugging this.

I can think of three explanations:

- Because of the extreme backlog between packet reception and lwIP handling,
  or because of the 4K packet size, which get split unfortunately into multiple
  pbufs, some bug in lwIP gets triggered.
- You have special ethernet hardware which splits that 4K packet into multiple
  smaller ones and one of them got dropped or corrupted.
- Somehow packets/pbufs are given to lwIP in re-ordered sequence (e.g. reverse
  order, then it only happens if there is a backlog).

Best regards,

Indan Zupancic



TT Vasumweg 150  |  1033 SH Amsterdam  |  The Netherlands
Phone: + 31 [0]20 482 56 32  |   Fax: + 31 [0]20 482 00 77  |  Email: 
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-----Original Message-----
From: lwip-users <lwip-users-bounces+indan.zupancic=address@hidden> On Behalf 
Of hondgm--- via lwip-users
Sent: Wednesday, 29 April 2020 14:39
To: address@hidden
Cc: address@hidden
Subject: [lwip-users] How to root-cause duplicate ACKs?

I'm using 2.1.2, no RTOS, and seeing frequent duplicate ACKs on Wireshark while 
lwIP is receiving a continuous audio stream. I've seen up to 6 duplicate ACKs 
but usually it's one or two.

I enabled:

#define TCP_OUTPUT_DEBUG                LWIP_DBG_ON

and here's a section of the output. The second number in each "sending ACK for" 
line is an addition of mine and is the micro's core timer for an idea of how 
quickly these are being sent. Each timer count is 13.3ns.

tcp_output: sending ACK for 1450257509  3252152102
tcp_output: nothing to send (0)
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450260429  3252196160
tcp_output: nothing to send (0)
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450261889  3269439860
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450261889  3288290782
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450261889  3288310973
tcp_output: nothing to send (0)
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450261889  3325967548
tcp_output: nothing to send (0)
tcp_output: nothing to send (0)
tcp_output: sending ACK for 1450262782  3333899283
tcp_output: nothing to send (0)


1450261889 was sent 4 times according to this, and there's actually quite a lot 
of these in the debug output. Is this normal? I've also attached a small 
section Wireshark trace which does not necessarily match up with the above 
debug trace as I can't determine what matches up. 192.168.0.120 is lwIP. 

How does one go about determining what is causing this? The system running lwIP 
is otherwise quite reliable and will stream the same TCP connection for hours, 
despite the duplicate ACKs and retransmissions even over my LAN.