Question about iperf TCP test performance issue (MSS related?)

Hello All,

From time to time it happens that you have no more ideas on what to do with the issues. Apparently, this often happens to me with MSS/MTU shenanigans...

Short intro. We have an infrastructure in some data center. There are some servers, switches and a core router. Core router is connected to a couple of ISPs and a couple of exchanges for blended traffic.

Issue. We were testing the speed of our network to outside locations (outside our infrastructure) and noticed that a single Iperf connection is only achieving miserable 2-3 Mbps. In that case, all of our equipment was connected with 10G or more. As an example, other tests, like transatlantic and so on, reach 50 Mpbs for a single connection. We also did a test connection via other ISP and speed was good.

So upon further packet dump inspection, I found that in a faulty connection Iperf test fails to raise TCP segment size: https://imgur.com/QhZVut5

While testing on other ISP everything was fine: https://imgur.com/TLHew5a

What comes to mind that there are some MTU/MSS related issues. However, no packet loss or anything else is reported.

I've sent a ticket to the ISP, but it will probably take some time for them to respond.

Would be thankful for any ideas or thoughts about this issue.