How to debug connection issues - Upload (speed-test) fails, slow connection to websites

[thni323]

Hi all!

I'm having trouble with the internet connection when using OpenMPTCPRouter and I'm currently a bit stuck in finding the root cause.

My setup:

• OpenMPTCP-Router running on a RPI 4 based Router-Board
• OpenMPTCP-Server running on a German VPS located in Berlin
• Two internet connections: Cable-Modem (1000mbits down, 50mbits up) and LTE (50mbits down, 25mbits up)

The problem:
In general the connection works quite well, I changed to V2Ray and bbr2 algorithm which gives me a download of about 750mbits and a upload of 70mbits.
However there are some connection issues.
Sometimes loading a web page takes very long or fails at all. The problem occurs during the "initial access" of that website (by typing the adress into the web browser or following a link to that side e.g. from google search). If the webside was loaded once, following links to sub-pages (mostly) works fine.
There are also some problems using common speed test websites. Sometimes the upload test fails or takes like forever.
Using the webside "speedtest.net" sometimes also the startup of the speedtest very long (test states "connecting"), than the download test runs well but the upload test fails (seems to timeout).
The problems seems to be related to the upload only outgoing traffic from the vps) - like if the VPS has problems reaching out to other servers.
I'm also using port forwarding to access a local server at home via the public IP of the VPS. That connection works well with no issues.
In many cases restarting OpenMPTCP (VPS or Router) seems to temporary fix the problem - meaning after a fresh connection all speed test websites work and loading web pages works well. The problem seems to occur after some time.
I was also thinking about a DNS issue, but I'm using a local (ad blocking) DNS server at home and when I use the Cable internet connection directly it works without any issues so I don't think that the problem is related to DNS resolution.

What I already did:

• Changing the VPS provider results in no significant change

I'm a bit stuck what I could do to further investigate the problem and would be very thankfull for any help.