On Sat, May 27, 2023 at 8:19 PM Fotis Panagiotopoulos
wrote:
> Hello,
>
> I encounter some problems using sendfile().
>
> I am using sendfile to... send a file to a remote server, with my own
> implementation of an FTP client.
> sendfile() indeed starts to transmit chunks of the file, but as I see in
> Wireshark, I get an ICMP response "Destination unreachable (Fragmentation
> needed)".
> I have verified that the Ethrenet MTU is correctly set to 1500.
>
> I tried lowering the MTU a lot (1000 bytes), and the problem is solved.
> Communication succeeds.
>
> This raises some questions, and indicates some potential bugs:
>
> 1. Why is there a problem with MTU in the first place? Shouldn't MTU be
> negotiated? (Is this functionality available in NuttX?)
>
MTU isn't negotiated but a physical attribute of your transport(netdev). On
the other hand, PMTU could be discovered from ICMP.
> 2. Why is the ICMP response not handled? It seems that sendfile() just
> ignores it and continues to send chunks, nevertheless.
>
It is handled by the recent addition here:
https://github.com/apache/nuttx/pull/9254
but this feature is disabled by default, you have to enable it manually..
> 3. Why sendfile() sends TCP segments without receiving any ACKs back?
> AFAIK, depending on the configuration, TCP allows at most two pending
> segments on the wire. But I see dozens of them, till sendfile finally
> fails.
>
>
Why only two segments? TCP can send packages until the slide window is full.
> This last point is also verified in my MQTT client.
> I have seen NuttX TCP allowing sending lots of TCP segments without ACKing
> the previous data.
>
> So, is there any insight on the above?
> Is my configuration wrong, or is there anything wrong with TCP?
>
> Thank you.
>
