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.
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.
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.
>>>> > > > >> >
>>>> > > > >>
>>>> > > > >
>>>> > > >
>>>> > >
>>>> >
>>>>
>>>
