Hello list, I have a site-to-site ipsec vpn with strongswan. It was working well for 5-6 months then a day ago I have noticed something strange, that from Site-A to Site-B (tunnel mode) only the upload bandwidth is capped down to 20-30kbit/s inside the VPN. I have tried various apps like ftp, scp on different ports it was the same result. I also ran speedtest/wget on both endpoints just to make sure that not the entire connection of those networks are capped. Since outside parties cannot see anything from what's going on inside the tunnel, first I was thinking that they started limiting the traffic based on port (4500 udp) or based on protocol (ESP), that is easy to do. In older versions of strongswan it's not possible to change the charon nat port (probably wouldn't work anyway since most of the traffic should be ESP (protocol 50)). I have restarted the strongswan daemon on both endpoints multiple times it did not change the situation (the bandwidth limiting was still present). So my last idea was to make new vpn certificates. For my biggest surprise with the new certificates the capping was gone and the bandwidth went back to normal. I hope I don't have to put the old certs back from backup just to make a point. One of the ISPs must started tagging the ipsec traffic based on the certificate and then do traffic shaping (QoS) on it to throttle down the bandwidth. How is this even possible? I was thinking that an ipsec connection is encrypted and random from the beginning. How can they define a pattern to their whatever device to be able to mark this specific traffic? Is there a part at the beginning of the connection sequence which is always the same with using the same certificate? Do I have to worry about here that my vpn keys got compromised? Anybody ever experienced this? Thanks!