[RustyRussell]

These days I generally advise that you interpret the version number as odd and even bits: odd means it's compatible with readers, even means it isn't.

[quag]

That sounds interesting. Can you say a little more about how this works?

[RustyRussell]

It's a trick I stole from ext2, and simplified. In that filesystem there are three bitsets: one for reading, one for writing, one for fsck. If you don't understand a bit you can't do that action.

For most protocols there's only reading and writing, so you can use odd bits to mean "backwards compatible features, you can read even if you don't understand" and even for "stop, we broke compat".