Hello everybody,
As allready posted in the "Windows 10 IT Pro > Windows 10 Networking" forum:
We're currently investigating some Networking Issues at our workplace.
After countless testing scenarios and network-dumps it seems that the problems we're facing are due to hyper-v breaking path mtu discovery.
System: Win 10 Enterprise Version 1709 (Build 16299.1146) - (We also tested Version 1809 in hopes it would fix this issue)
NW Card: Intel(R) Ethernet Connection I219-LM, Driver Version 12.15.25.6712 (It also happened using different NW-Cards and Driver Versions)
Requirements to reproduce the issue:
Some network service to talk to (e.g. a Webserver etc.)
A Win 10 Machine having hyper-v enabled/installed and PMTU discovery enabled
A network device in between which has a lower mtu size configured than the Client PC (1280 in our case, 1500 on the PC)
Symptoms:
As soon as the PC sends a packet with DF flag set > the configured mtu it gets ICMP responses (Type 3, Code4) telling it should lower the mtu to a given value. So far expected PMTUD behaviour...
However the Win 10 Machine keeps retransmitting the "oversized" packet without ever changing the MTU size until the connection is reset.
Crosscheck:
Uninstall/Deactivate hyper-v. (Also uninstall all created virtual switches!)
Now try to retrieve the same resource -> Works flawlessly (NW dump shows 1 ICMP Type 3, Code 4 packet. After that packet size to this destination never exceeds the lowest MTU in the path)
Update:
We found out that the problem does not exist on Win 10 v1703 and that it has something to do with the hyper-v standard switch.
Does anyone of you experience the same issue or is able to confirm our findings? Maybe does even know a solution? ;)
Thanks in advance!