HTTP                                                            S. Ludin
Internet-Draft                                                    Akamai
Intended status: Standards Track                           M. Nottingham
Expires: 24 March 2022                                            Fastly
                                                                   Y. Wu
                                                       20 September 2021

                      Targeted HTTP Cache Control


   This specification defines a convention for HTTP response header
   fields that allow directives controlling caching to be targeted at
   specific caches or classes of caches.  It also defines one such
   header field, targeted at Content Delivery Network (CDN) caches.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   The issues list for this draft can be found at

   The most recent (often, unpublished) draft is at

   See also the draft's current status in the IETF datatracker, at
   control/ (

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   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Targeted Cache-Control Header Fields  . . . . . . . . . . . .   3
     2.1.  Cache Behavior  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Interaction with HTTP Freshness . . . . . . . . . . . . .   6
     2.4.  Defining Targeted Fields  . . . . . . . . . . . . . . . .   7
   3.  The CDN-Cache-Control Targeted Field  . . . . . . . . . . . .   7
     3.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Modern deployments of HTTP often use multiple layers of caching with
   varying properties.  For example, a Web site might use a cache on the
   origin server itself; it might deploy a caching layer in the same
   network as the origin server, it might use one or more Content
   Delivery Networks (CDNs) that are distributed throughout the
   Internet, and it might utilise browser caching as well.

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   Because it is often desirable to control these different classes of
   caches separately, some means of targeting directives at them is

   The HTTP Cache-Control response header field is widely used to direct
   caching behavior.  However, it is relatively undifferentiated; while
   some directives (e.g., s-maxage) are targeted at a specific class of
   caches (for s-maxage, shared caches), targeting is not consistently
   available across all existing cache directives (e.g., stale-while-
   revalidate).  This is problematic, especially as the number of
   caching extensions grows, along with the number of potential targets.

   Some implementations have defined ad hoc control mechanisms to
   overcome this issue, but their interoperability is low.  Section 2
   defines a standard framework for targeted cache control using HTTP
   response headers, and Section 3 defines one such header: the CDN-
   Cache-Control response header field.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Targeted Cache-Control Header Fields

   A Targeted Cache-Control Header Field (hereafter, "targeted field")
   is a HTTP response header field that has the same semantics as the
   Cache-Control response header field ([HTTP-CACHING], Section 5.2).
   However, it has a distinct field name that indicates the target for
   its directives.

   For example:

   CDN-Cache-Control: max-age=60

   is a targeted field that applies to Content Delivery Networks (CDNs),
   as defined in Section 3.

2.1.  Cache Behavior

   A cache that implement this specification maintains a _target list_ -
   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.

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   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:

     [ExampleCDN-Cache-Control, CDN-Cache-Control]

   When a cache that implements this specification receives a response
   with one or more of 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 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

   Targeted fields that are not on a cache's target list MUST NOT change
   that cache's behaviour, and MUST be passed through.

   Caches that use a targeted field MUST implement the semantics of the
   following cache directives:

   *  max-age

   *  must-revalidate

   *  no-store

   *  no-cache

   *  private

   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 unrecognised extension
   directives are ignored.

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2.2.  Syntax

   Targeted fields are defined as Dictionary Structured Fields
   (Section 3.2 of [STRUCTURED-FIELDS]).  Each member of the dictionary
   is a cache directive from the Hypertext Transfer Protocol (HTTP)
   Cache Directive Registry.

   Because cache directives are not defined in terms of structured data
   types, it is necessary to map their values into the appropriate
   types.  Typically, they are mapped into a Boolean (Section 3.3.6 of
   [STRUCTURED-FIELDS]) when the member has no separate value, a Token
   (Section 3.3.4 of [STRUCTURED-FIELDS]) for alphanumeric values, a
   String (Section 3.3.3 of [STRUCTURED-FIELDS]) for quote-delimited
   values, or an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) for
   purely numeric values.

   For example, the max-age directive (Section of
   [HTTP-CACHING]) has an integer value; no-store (Section of
   [HTTP-CACHING]) always has a boolean true value, and no-cache
   (Section of [HTTP-CACHING]) has a value that can either be
   boolean true or a string containing a comma-delimited list of field

   Implementations MUST NOT generate and SHOULD NOT consume values that
   violate these inferred constraints on the directive's value (e.g.
   coerce a max-age with a decimal value into an integer).  Parameters
   received on directives are to be ignored, unless other handling is
   explicitly specified.

   Sending implementations MUST generate valid Structured Fields.
   Receiving implementations SHOULD use a Structured Fields parser, but
   MAY reuse an existing parser for the Cache-Control field value
   (Section 5.2 of [HTTP-CACHING]).  Those that do SHOULD implement the
   following constraints, to aid in a smooth transition to a full
   Structured Field parser and prevent interoperability issues:

   *  Directive names are all lowercase (e.g., "MAX-AGE=60" is
      considered an error).

   *  If a directive is repeated in the field value (e.g., "max-age=30,
      max-age=60"), the last value 'wins' (60, in this case).

   *  Members of the directives can have parameters (e.g., "max-
      age=30;a=b;c=d"), which are ignored unless specified.

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   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).

2.3.  Interaction with HTTP Freshness

   HTTP caching has a single, end-to-end freshness model defined in
   Section 4.2 of [I-D.ietf-httpbis-cache].  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 them, negatively impacting cache efficiency.

   For example, a response stored by a CDN cache might be served with
   the following headers:

   Age: 1800
   Cache-Control: max-age=600
   CDN-Cache-Control: max-age=3600

   From the CDN's perspective, this response is still fresh after being
   cached for 30 minutes, while from other caches' standpoint, 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.

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2.4.  Defining Targeted Fields

   A targeted field for a particular class of cache can be defined by
   requesting registration in the Hypertext Transfer Protocol (HTTP)
   Field Name Registry
   (, listing this
   specification as the specification document.  The Comments field of
   the registration should clearly define the class of caches that the
   targeted field applies to.

   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.

3.  The CDN-Cache-Control Targeted Field

   The CDN-Cache-Control response header field is a targeted field
   (Section 2) that allows origin servers to control the behaviour of
   CDN caches interposed between them and clients, separately from other
   caches that might handle the response.

   It applies to caches that are part of a distributed network that
   operate on behalf of an origin server (commonly called a Content
   Delivery Network or CDN).

   CDN caches that use CDN-Cache-Control will typically forward this
   header so that downstream CDN caches can use it as well.  However,
   they MAY remove it when this is undesirable (for example, when
   configured to do so because it is known not to be used downstream).

3.1.  Examples

   For example, the following header fields would instruct a CDN cache
   to consider the response fresh for 600 seconds, other shared caches
   for 120 seconds and any remaining caches for 60 seconds:

   Cache-Control: max-age=60, s-maxage=120
   CDN-Cache-Control: max-age=600

   These header fields would instruct a CDN cache to consider the
   response fresh for 600 seconds, while all other caches would be
   prevented from storing it:

   CDN-Cache-Control: max-age=600
   Cache-Control: no-store

   Because CDN-Cache-Control is not present, this header field would
   prevent all caches from storing the response:

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   Cache-Control: no-store

   Whereas these would prevent all caches except for CDN caches from
   storing the response:

   Cache-Control: no-store
   CDN-Cache-Control: none

   (note that 'none' is not a registered cache directive; it is here to
   avoid sending a header field with an empty value, which would be

4.  IANA Considerations

   Please register the following entry in the Hypertext Transfer
   Protocol (HTTP) Field Name Registry defined by [HTTP]:

   *  Field Name: CDN-Cache-Control

   *  Status: permanent

   *  Specification Document: [this document]

   *  Comments: Cache-Control directives targeted at Content Delivery

5.  Security Considerations

   The security considerations of HTTP caching [HTTP-CACHING] apply.

   The ability to carry multiple caching policies on a response can
   result in confusion about how a response will be cached in different
   systems, if not used carefully.  This might result in unintentional
   reuse of responses with sensitive information.

6.  References

6.1.  Normative References

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,

              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-

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              httpbis-cache-19, 12 September 2021,

              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Caching", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-cache-19, 12 September 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

6.2.  Informative References

              Cohen, E. and H. Kaplan, "The age penalty and its effect
              on cache performance", March 2001,

Authors' Addresses

   Stephen Ludin


   Mark Nottingham


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   Yuchen Wu


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