[pilif]

>but thankfully the "extension" format is pretty flexible in that regard

Just like the version field.

I'm sure middlebox software is being updated as we speak to terminate connections with unknown versions in the „supported_versions“ extension.

[vitus]

If the extension field is anything like IP or TCP options, some middleboxes will also tamper the hell out of that field and strip unknown extensions, or just break connections.

Often-referenced paper in that field: http://conferences.sigcomm.org/imc/2011/docs/p181.pdf