A Targeted Cache-Control Header Field (hereafter "targeted field") is an HTTP response header field that has the same semantics as the Cache-Control response header field ([HTTP-CACHING
]). However, it has a distinct field name that indicates the target for its cache directives.
is a targeted field that applies to CDNs, as defined in Section 3
Targeted fields are Dictionary Structured Fields (Section 3.2
]). Each member of the Dictionary is an HTTP cache response directive ([HTTP-CACHING
]) including extension response directives (as per [HTTP-CACHING
]). Note that while targeted fields often have the same syntax as Cache-Control fields, differences in error handling mean that using a Cache-Control parser rather than a Structured Fields parser can introduce interoperability issues.
Because cache directives are not defined in terms of structured data types, it is necessary to map their values into the appropriate types. [HTTP-CACHING
] defines cache directive values to be either absent, a quoted-string, or a token.
This means that cache directives that have no value will be mapped to a Boolean (Section 3.3.6
]). When the value is a quoted-string, it will be mapped to a String (Section 3.3.3
]), and when it is a token, it will map to a Token (Section 3.3.4
]), an Integer (Section 3.3.1
]), or a Decimal (Section 3.3.2
]), depending on the content of the value.
For example, the max-age directive ([HTTP-CACHING
]) has an integer value; no-store ([HTTP-CACHING
]) always has a Boolean true value, and no-cache ([HTTP-CACHING
]) has a value that can be either Boolean true or a string containing a comma-delimited list of field names.
Implementations MUST NOT
generate values that violate these inferred constraints on the cache directive's value. In particular, string values whose first character is not alphabetic or "*" MUST
be generated as Strings so that they are not mistaken for other types.
Implementations SHOULD NOT
consume values that violate these inferred constraints. For example, a consuming implementation that coerces a max-age with a decimal value into an integer would behave differently than other implementations, potentially causing interoperability issues.
Parameters received on cache directives are to be ignored, unless other handling is explicitly specified.
If a targeted field in a given response is empty, or a parsing error is encountered, that field MUST
be ignored by the cache (i.e., it behaves as if the field were not present, likely falling back to other cache-control mechanisms present).
A cache that implements this specification maintains a target list. A target list is an ordered list of the targeted field names that it uses for caching policy, with the order reflecting priority from most applicable to least. The target list might be fixed, user configurable, or generated per request, depending upon the implementation.
For example, a CDN cache might support both CDN-Cache-Control and a header specific to that CDN, ExampleCDN-Cache-Control, with the latter overriding the former. Its target list would be:
When a cache that implements this specification receives a response with one or more of the header field names on its target list, the cache MUST
select the first (in target-list order) field with a valid, non-empty value and use its value to determine the caching policy for the response, and it MUST
ignore the Cache-Control and Expires header fields in that response, unless no valid, non-empty value is available from the listed header fields.
Note that this occurs on a response-by-response basis; if no member of the cache's target list is present, valid, and non-empty, a cache falls back to other cache control mechanisms as required by HTTP [HTTP-CACHING
Targeted fields that are not on a cache's target list MUST NOT
change that cache's behavior and MUST
be passed through.
Caches that use a targeted field MUST
implement the semantics of the following cache directives:
Furthermore, they SHOULD
implement other cache directives (including extension cache directives) that they support in the Cache-Control response header field.
The semantics and precedence of cache directives in a targeted field are the same as those in Cache-Control. In particular, no-store and no-cache make max-age inoperative, and unrecognized extension directives are ignored.
HTTP caching has a single, end-to-end freshness model defined in [HTTP-CACHING
]. When additional freshness mechanisms are only available to some caches along a request path (for example, using targeted fields), their interactions need to be carefully considered. In particular, a targeted cache might have longer freshness lifetimes available to it than other caches, causing it to serve responses that appear to be prematurely (or even immediately) stale to those other caches, negatively impacting cache efficiency.
For example, a response stored by a CDN cache might be served with the following headers:
From the CDN's perspective, this response is still fresh after being cached for 30 minutes, while from the standpoint of other caches, this response is already stale. See [AGE-PENALTY
] for more discussion.
When the targeted cache has a strong coherence mechanism (e.g., the origin server has the ability to proactively invalidate cached responses), it is often desirable to mitigate these effects. Some techniques seen in deployments include:
Removing the Age header field
Updating the Date header field value to the current time
Updating the Expires header field value to the current time, plus any Cache-Control: max-age value
This specification does not place any specific requirements on implementations to mitigate these effects, but definitions of targeted fields can do so.
A targeted field for a particular class of cache can be defined by requesting registration in the "Hypertext Transfer Protocol (HTTP) Field Name Registry" (<https://www.iana.org/assignments/http-fields/
Registration requests can use this document as the specification document; in which case, the Comments field should clearly define the class of caches that the targeted field applies to. Alternatively, if other documentation for the field has been created, it can be used as the specification document.
By convention, targeted fields have the suffix "-Cache-Control", e.g., "ExampleCDN-Cache-Control". However, this suffix MUST NOT
be used on its own to identify targeted fields; it is only a convention.