As said, this is sendfile(). I do not have control on the size of the
chunks sent. sendfile is also using TCP.
So, sendfile cannot take advantage of PMTUD?
On Sun, Jun 4, 2023 at 5:33 PM Xiang Xiao <xiaoxiang781...@gmail.com> wrote:
> 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.
> > >>>> > > > >> >
> > >>>> > > > >>
> > >>>> > > > >
> > >>>> > > >
> > >>>> > >
> > >>>> >
> > >>>>
> > >>>
> >
>
