Did you try native IPv6? Did you Call Your ISP? Very likely the case. You can try the hacky clamp-to-MSS trick to confirm (this is kinda what Google does for instance, but they hack and guess it server side). Note that that only covers TCP data and lots of stuff is also UDP. At least you know what the problem is then. tracepath[6] or scamper can also show PMTU blackholes. Note that it might just be a ECMP load balancer that causes this problem. Good article to read and point people to: https://blog.cloudflare.com/path-mtu-discovery-in-practice/ Contact the webmaster of the site, contact the ISP that hosts the site etc... Provide sufficient details (traceroute, pcaps etc) along with a link to a page describing MTU issues (see link above). As we do not operate the remote service, little we can do. We do not have the resources to try and contact ISPs all over the world and keep on telling them their stuff is broken, have been trying to explain that for 15+ years already...