Compression: Vary Header not returned when content gzipped

[spboyer]

Response header should contain

Vary: Accept-Encoding

when content is gzipped. IIS compression currently adds header.

[Tratcher]

There's some debate over who should be setting this. Right now the cache headers are set by the resource (e.g. the ResponseCache attribute on the MVC action).

I guess you could consider this an opt-out header and compare it to the CookieAuth middleware always opting out of all caching.

Does IIS add the Vary even for non-cacheable responses?

[spboyer]

My understanding, would need confirmation from IIS team, is that if the Accept-Encoding header is sent then IIS responds with the Vary, otherwise no.