Mozilla has proposed
immutable, an extension, to
Cache-Control HTTP response header that can be very useful in the IPFS context where the content has infinite guaranteed immutable lifetime. Though, this will only be true when the hash reference is used in the URI. An IPNS mapping on the other hand is not considered immutable.
Here is the abstract of the proposed standard:
The immutable HTTP response Cache-Control extension allows servers to
identify resources that will not be updated during their freshness
lifetime. This assures that a client never needs to revalidate a
cached fresh resource to be certain it has not been modified.