> sendfile should return an error in this case, but senfile should only be
> used with TCP, not UDP, since sendfile doesn't have any logic to ack or
> retry..

Sorry if this wasn't clear. This last test was with plain old `send()`...

I opened a UDP socket, and used `send()` to transmit a buffer larger than
the MTU.
Instead of getting an error, the application hangs indefinitely.
`devif_send()` is called periodically, but of course it always fails.



On Mon, May 29, 2023 at 1:13 PM Xiang Xiao <xiaoxiang781...@gmail.com>
wrote:

> On Mon, May 29, 2023 at 5:02 PM Fotis Panagiotopoulos <f.j.pa...@gmail.com
> >
> wrote:
>
> > > You need to enable IP fragmentation in this case, which is also added
> > > recently and disabled by default:
> > > https://github.com/apache/nuttx/pull/8059
> > <https://github.com/apache/nuttx/pull/8059>
> > > Otherwise, any packet bigger than MTU will be dropped silently.
> >
> > Yes, this is the expected behavior.
> > But, instead of dropping the packet, the system hangs because the
> semaphore
> > is never posted.
> > It just tries endlessly to call devif_send() which always fails.
> >
> >
> sendfile should return an error in this case, but senfile should only be
> used with TCP, not UDP, since sendfile doesn't have any logic to ack or
> retry..
>
>
> >
> >
> > On Mon, May 29, 2023 at 11:42 AM Xiang Xiao <xiaoxiang781...@gmail.com>
> > wrote:
> >
> > > On Sun, May 28, 2023 at 11:55 PM Fotis Panagiotopoulos <
> > > f.j.pa...@gmail.com>
> > > wrote:
> > >
> > > > While experimenting with MTU, and checking the stability of my
> system,
> > I
> > > > noticed the following.
> > > >
> > > > I try to send a UDP datagram that is larger than the configured MTU.
> > > > In this case, the offending thread seems to hang indefinitely (or at
> > > least
> > > > waiting for a very long timeout?)
> > > >
> > >
> > > You need to enable IP fragmentation in this case, which is also added
> > > recently and disabled by default:
> > > https://github.com/apache/nuttx/pull/8059
> > > Otherwise, any packet bigger than MTU will be dropped silently.
> > >
> > >
> > > > The problem seems to be this line:
> > > >
> > > >
> > >
> >
> https://github.com/apache/nuttx/blob/master/net/udp/udp_sendto_unbuffered.c#L197
> > > > `devif_send()` fails because the datagram is too large, but
> > > > `pstate->st_sem` is never posted (the code returns immediately).
> > > >
> > > > This leaves the sending task to be blocked here:
> > > >
> > > >
> > >
> >
> https://github.com/apache/nuttx/blob/master/net/udp/udp_sendto_unbuffered.c#L469
> > > >
> > > > Shouldn't this failure also post the semaphore?
> > > > And let the code proceed returning an error in `send()`?
> > > >
> > > >
> > > > On Sun, May 28, 2023 at 5:26 PM Fotis Panagiotopoulos <
> > > f.j.pa...@gmail.com
> > > > >
> > > > wrote:
> > > >
> > > > >
> > > > > On Sat, May 27, 2023 at 5:35 PM Xiang Xiao <
> > xiaoxiang781...@gmail.com>
> > > > > wrote:
> > > > >
> > > > >> On Sat, May 27, 2023 at 8:19 PM Fotis Panagiotopoulos <
> > > > >> f.j.pa...@gmail.com>
> > > > >> 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.
> > > > >>
> > > > >
> > > > > I am not very familiar with MTU negotiation, so it seems that it
> > > doesn't
> > > > > happen in the network layer that I thought...
> > > > >
> > > > >
> > > > >>
> > > > >>
> > > > >> > 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/apachey/nuttx/pull/9254
> > > > >> <https://github.com/apache/nuttx/pull/9254>
> > > > >> but this feature is disabled by default, you have to enable it
> > > > manually..
> > > > >>
> > > > >
> > > > > I will definitely take a look at this. Thank you.
> > > > >
> > > > >
> > > > >>
> > > > >>
> > > > >
> > > > >> > 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.
> > > > >>
> > > > >> Disregard this. I was confused with delayed ACKs. Which is a
> > > receiver's
> > > > > functionality, not a sender's...
> > > > >
> > > > >
> > > > >>
> > > > >> > 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.
> > > > >> >
> > > > >>
> > > > >
> > > >
> > >
> >
>

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