IPv6 Tunnel very, very slow - is it Virgin Media or something else?

[JenkinsNET]

Like many people I have been suffering with this problem (very low throughput over my tunnel compared to native IPv4) for years. I too am on Virgin Media (currently 350 Mbit service). I can easily saturate my connection using IPv4 but a similar test over the tunnel rarely exceeds 10-20 Mbit/s  :( Many people have suggested that maybe VM are somehow traffic shaping or otherwise 'managing' protocol 41.

Now, my tunnel terminates in my router (Apple AirPort Extreme [802.11ac tower model] with latest firmware) which seems to do a good job... or does it?

As an experiment I connected one of my Macs directly to my cable modem (which runs in modem mode) and created a tunnel in macOS. This gives me a single /128 endpoint on the Mac (the tunnel client address, so no routing occurring on my end in this simple test). Now I am easily able to saturate my connection over both IPv4 and IPv6!

If this issue was simply due to VM slowing/managing/filtering protocol 41 then I should have seen the same issue for this test, but I don't. It seems to me on the basis of this evidence that the issue is either in the AirPort Extreme (poor tunnel implementation and/or poor IPv6 routing implementation) - possible but seems unlikely it could be *this* bad? Or there is some more subtle issue at work.

[cholzhauer]

It seems like there's a problem with the Extreme...can you prove the point again by connecting it to a Linux/Windows machine and re-running your test?

[PhilBZ]

I suggest it's a problem with Virgin Media.  See https://twitter.com/ispreview/status/1291269456979808261 which links to https://www.ispreview.co.uk/index.php/2020/08/virgin-media-uk-move-to-fix-20mbps-speed-cap-on-ipv6-tunnels.html

I've noticed it in the last few weeks, where my alleged 100Mbps connection (which, to be fair, can do that on a good day over IPv4), struggles to hit 5Mbps over my HE6 link, which was previously okay.  The only realistic option to manage this is to filter heavy IPv6 usage so it doesn't hit the IPv6 tunnel, but that migth not be realistic for most users.

The bottom line is that VM don't really care....