Acknowledgement of unsupported settings

An endpoint that receives a SETTINGS frame must acknowledge it.

It still has to do this when the SETTINGS frame contains an unsupported setting parameter (either alone or combined with supported setting parameters), otherwise there is a loss of synchronization between SETTINGS frame and their acknowledgements.

This means that an endpoint has no way of stating whether it supports a new setting or not.

I think this is in line with the sense of the WG regarding extensions. But it should be made clear in the spec.