This is a great read. As a long-time (unfortunately) IPv6 tunnel user, I’m always amazed at how frequently Akamai can cock up PMTU within its network. Thumbs up to CloudFlare (thumbs down on their captcha showing up on every site I visit from Tor, but that’s another issue).
Additional reading: http://en.wikipedia.org/wiki/Path_MTU_Discovery
Disclaimer: I work at CloudFlare, and I’m tangentially involved in our
Tor working group.
We’ve been working to improve the Tor stuff, but it’s a more difficult
problem than it seems at first. Quite recently, we’ve been pushing some
changes to make the Tor experience better, for what it’s worth.