I've been seeing random ERR_CONNECTION_RESET errors in my browser (Chrome, Firefox or Edge running on different machines, Windows 10 1709) when I try to edit web pages on a remote Linux server running Ubuntu 16.04 LAMP. It's very hard to pinpoint but it seems to happen mainly when submitting large forms on SSL connections and it's been driving me crazy because I've been losing work.
Nothing in the error logs, no problem with ping or tracert between machines, firewalls disabled at both ends. It has all the symptoms of a networking problem but I can't be sure because it's so intermittent.
I suspect it's MTU related but I was shouted down and told modern OSes and routers should cope with any packet size and the problem must be elsewhere when I raised this last year.
I fired up Wireshark today and I see repeated lines like this which I think are coincident with the problem:
694 47.224124 192.168.1.90 149.210.138.83 TCP 1514 [TCP Retransmission] 53732 → 443 [ACK] Seq=3867 Ack=33955 Win=64512 Len=1460
192.168.1.90 is the Windows machine (browser) and 149.210.138.83 is the Linux server. The packet size is 1514 which I know is larger than my router (BT homehub 3) can pass. The "Do not fragment" bit is set. I would expect to see ICMP packets negotiating fragmentation but I don't. Yet pings are OK and firewalls are off.
A wireshark pcap of successful and unsuccessful form submissions and a ping is here (you don't need a OneDrive account, just download it as guest). https://1drv.ms/u/s!AvycuEtEj-isleIZMh8w_SHuoqYcoQ
If anyone can help shed light on what's going on I would be very grateful.
