As I see, sendfile uses MSS instead of MTU.
Wouldn't it be better to scale also MSS along with MTU when PMTUD is
enabled?
On Sun, Jun 4, 2023 at 5:40 PM Fotis Panagiotopoulos <f.j.pa...@gmail.com>
wrote:
> 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.
>> > >>>> > > > >> >
>> > >>>> > > > >>
>> > >>>> > > > >
>> > >>>> > > >
>> > >>>> > >
>> > >>>> >
>> > >>>>
>> > >>>
>> >
>>
>
