[This isn't done yet, but I've shown it to enough people that I should
share it with the list.]
1. Introduction and background
1.1. Motivation
Frequently, we find that very old versions of Tor should no longer be
supported on the network. To remove relays is easy enough: we
simply update the directory authorities to stop listing them.
But to disable clients is harder. We can do as we did with v1
directories in Tor 0.2.0.5-alpha, and make them know about no relays
at all.
...
1.2. History
Early versions of Tor checked the recommended-versions field in the
directory to see whether they should keep running. If they didn't
recognize
their own version, they would stop (what?).
...
3.2. Disabling versions prior to 0.2.4.11-alpha.
Starting in version Tor 0.2.4.11-alpha, we implemented proposal 214.
This proposal added a new link protocol, version 4. Earlier version
versions
of Tor never support it; newer versions of Tor always advertise their
support for it.
We could treat the presence of any link protocol of version 4 or
later as indicating the presence of 0.2.4.11-alpha. Upon receiving a
version 3 or earlier as indicating the presence of a version less than 0.2.4.11-alpha.
connection from such a version, we could throttle the bytes we give
it, drop its circuits, or stop reading.
[TODO: which?]