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