Status Code for Negative Acknowledgements

[brianraymor]

I'm separating this out from #17.

There is a 5XX status code placeholder in the design for negative acknowledgements.

@martinthomson wrote:

OK, here's the problem: there is no status code that works for this scenario.

We could try to be clever and attach different semantics to 404 and 410, or we could mint a new status code ...

We could use 418 (I'm A Teapot), but IANA still don't have it registered.

...

The server (the push service in this case) is not acting as a gateway or proxy. It is the authority for the resource, so using 504 would be nonsensical. A status code for "the contents of the resource expired" are more appropriate.

[brianraymor]

Discussed in Yokohama. @brianraymor to raise discussion on httpbis mailing list.

[brianraymor]

@darshakthakore suggested a variation based on:
Expect header and a corresponding 417 – Expectation Failed status code