[mtud]

> During the TLS handshake, the client tells the server which treeheads it has.

I don’t love the idea of giving every server I connect to via TLS the ability to fingerprint me by how recently (or not) I’ve fetched MTC treeheads. Even worse if this is in client hello, where anyone on the network path can view it either per connection or for my DoH requests to bootstrap encrypted client hello.

[bwesterb]

If your browser is online on an unrestricted network, then the tree heads will be kept up to date, and this will leak nothing. If you had your laptop closer for a weekend, open it up immediately and visit a website before your browser had a chance to update, well, you leak for maybe a minute or two you had your laptop closed for a weekend. So it's not that much. But we'll want to see how we can reduce this as much as possible.

[mtud]

It can't possibly be updating continuously in real time, can it? Especially for battery devices, a constant background thread polling for updates seems untenable.