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. > 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/apache/nuttx/pull/9254 but this feature is disabled by default, you have to enable it manually.. > 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. > 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. >
