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I have 2 location with fibre connection. Location A has ~400M down/up whereas Location B has 1Gbps (up/down) shared pipe between 4 offices (total of 15 computers). Location A has a Windows 2019 Hyper-V VM, host connected via Ethernet cable. Location B has Windows 10 computers, all connected via Ethernet cable. There is a IPSec VPN connection between the 2 locations.

The NIC speed on all the computers and server is set to Auto and actively shows 1G. Both locations have Gig switches with LACP disabled.

The Ookla speedtest at both locations shows speed close to the bandwidth that they are offered. However, the data transfer speed between the 2 location is abysmal. It ranges from ~500 kbps to 4 mbps. I have tried SMB/CIFS data transfer through the tunnel and FTP transfer outside of the tunnel and both of them show similar transfer speed.

I did an iperf3 test and it shows that when using TCP protocol, the data transfer speed is extremely minimal but when using UDP connection, the speed is between 890-950 mbps with around 50% datagram loss.

Is there anything that you think, that I can do to improve the speed?

Iperf3 test TCP

TCP

UDP

UDP

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  • Perhaps using traceroute between the two locations can discover the bottleneck. What does ping show?
    – harrymc
    Commented May 15, 2023 at 19:03
  • So, I use WinMTR for that and I found that there are 3 hops where there is extreme loss (close to 90%) but when I spoke to my ISP about it, they said "their backbone may have set an ICMP limit to avoid possible DDOS", which kinda makes sense.
    – Dhrumit Patel
    Commented May 15, 2023 at 20:02
  • @DhrumitPatel - Do you have KB5025305 installed? There is a known issue associated with IPSec and this cumulative update
    – Ramhound
    Commented May 17, 2023 at 12:09
  • @Ramhound - I checked and the update is for Windows 11 computers whereas the workstations are on Windows 10 and server is 2019.
    – Dhrumit Patel
    Commented May 18, 2023 at 14:59

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