On Sun, Jun 4, 2023 at 10:23 PM Fotis Panagiotopoulos <f.j.pa...@gmail.com>
wrote:
> I am trying this new PMTUD functionality, but it seems it doesn't work.
>
> I have configured:
> CONFIG_NET_ICMP_PMTU_ENTRIES = 10
> CONFIG_NET_ICMP_PMTU_TIMEOUT = 10
>
> Again I see lots of segments being sent (all with size 1400), and all of
> them are responded with the same ICMP reply.
> A couple of retransmissions are attempted, and then the connection is
> reset.
>
The option can discover the minimal MTU from the source to the destination.
You need to use PMTU to split your data into small packets(<= PMTU) by
yourself to improve the efficiency.
>
> I try again the same procedure (now that the system had the chance to
> discover the maximum PMTU), but it still fails.
> Again all segments have a size of 1400, instead of less.
>
If you don't split your package to fit MTU by yourself, you have
to enable NET_IPFRAG. But I would suggest that.you switch UDP to TCP
because the protocol you implement on top of UDP is likely very inefficient.
>
> I am using sendfile() in case this matters.
>
> On Tue, May 30, 2023 at 12:28 AM Fotis Panagiotopoulos <
> f.j.pa...@gmail.com>
> wrote:
>
> > I can see that there are two different problems with MTU.
> > They are completely independent from each other, so let's start with the
> > simple case first.
> >
> > I am testing on an STM32F427, using Ethernet.
> >
> > As previously noted, the following code will cause the running task to
> > hang.
> >
> > netlib_set_mtu(CONFIG_NETIF_DEV_NAME, 1500);
> >
> > int sd = socket(AF_INET, SOCK_DGRAM, 0);
> >
> > struct sockaddr_in server;
> > server.sin_family = AF_INET;
> > server.sin_port = 1000;
> > server.sin_addr.s_addr = inet_addr("192.168.1.235");
> >
> > uint8_t * data = malloc(2048);
> > memset(data, 0xAA, 2048);
> >
> > sendto(sd, data, 2048, 0, (struct sockaddr*)&server, sizeof(server));
> >
> > close(sd);
> >
> > As you can see, the MTU is set to 1500, and then I try to send a UDP
> > datagram with a larger size (2048).
> > Indeed `devif_send()` fails, and the aforementioned semaphore is never
> > posted.
> >
> > (This is without buffering in UTP, in case this is important).
> >
> > This draft PR, provides a solution to the issue.
> > https://github.com/apache/nuttx/pull/9423
> >
> > If this is correct, I will also check buffered UDP, and other uses of
> > devif_send().
> >
> > Alternatively, devif_send() may be changed to actually return an error
> > code (instead of returning void), so improved error handling can take
> place.
> >
> >
> > On Mon, May 29, 2023 at 1:41 PM Fotis Panagiotopoulos <
> f.j.pa...@gmail.com>
> > wrote:
> >
> >> The failure scenario is a bit more complicated...
> >>
> >> Give me some time and I will provide a correct and reproducible example,
> >> with a clear explanation.
> >>
> >> On Mon, May 29, 2023, 13:27 Fotis Panagiotopoulos <f.j.pa...@gmail.com>
> >> wrote:
> >>
> >>> > 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.
> >>>> > > > >> >
> >>>> > > > >>
> >>>> > > > >
> >>>> > > >
> >>>> > >
> >>>> >
> >>>>
> >>>
>
