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Keywords: CDN, Content Delivery Network, Caching

Internet Engineering Task Force (IETF) S. Ludin
Request for Comments: 9213 Akamai
Category: Standards Track M. Nottingham
ISSN: 2070-1721 Fastly
Y. Wu
Cloudflare
June 2022
Targeted HTTP Cache Control
Abstract
This specification defines a convention for HTTP response header
fields that allow cache directives to be targeted at specific caches
or classes of caches. It also defines one such header field, the
CDN-Cache-Control response header field, which is targeted at content
delivery network (CDN) caches.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9213.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Notational Conventions
2. Targeted Cache-Control Header Fields
2.1. Syntax
2.2. Cache Behavior
2.3. Interaction with HTTP Freshness
2.4. Defining Targeted Fields
3. The CDN-Cache-Control Targeted Field
3.1. Examples
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Authors' Addresses
1. Introduction
Modern deployments of HTTP often use multiple layers of caching. For
example, a website 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 CDNs that are distributed throughout
the Internet, and it might benefit from browser caching as well.
Because it is often desirable to control these different classes of
caches separately, some means of targeting cache directives at them
is necessary. For example, if a publisher has a mechanism to
invalidate the contents of a cache that it has a relationship with
(such as a CDN cache), they might be more comfortable assigning a
more generous caching policy to it while still wanting to restrict
the behavior of other caches.
The HTTP Cache-Control response header field (defined in Section 5.2
of [HTTP-CACHING]) is widely used to direct caching behavior.
However, it is relatively undifferentiated; while some cache
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
an 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 cache directives.
For example:
CDN-Cache-Control: max-age=60
is a targeted field that applies to CDNs, as defined in Section 3.
2.1. Syntax
Targeted fields are Dictionary Structured Fields (Section 3.2 of
[STRUCTURED-FIELDS]). Each member of the Dictionary is an HTTP cache
response directive (Section 5.2.2 of [HTTP-CACHING]) including
extension response directives (as per Section 5.2.3 of
[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. Section 5.2 of [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 of [STRUCTURED-FIELDS]). When the value is
a quoted-string, it will be mapped to a String (Section 3.3.3 of
[STRUCTURED-FIELDS]), and when it is a token, it will map to a Token
(Section 3.3.4 of [STRUCTURED-FIELDS]), an Integer (Section 3.3.1 of
[STRUCTURED-FIELDS]), or a Decimal (Section 3.3.2 of
[STRUCTURED-FIELDS]), depending on the content of the value.
For example, the max-age directive (Section 5.2.2.1 of
[HTTP-CACHING]) has an integer value; no-store (Section 5.2.2.5 of
[HTTP-CACHING]) always has a Boolean true value, and no-cache
(Section 5.2.2.4 of [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).
2.2. Cache Behavior
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:
[ExampleCDN-Cache-Control, CDN-Cache-Control]
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:
* 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 unrecognized extension
directives are ignored.
2.3. Interaction with HTTP Freshness
HTTP caching has a single, end-to-end freshness model defined in
Section 4.2 of [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:
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 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.
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" (<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.
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 behavior 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 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
(i.e., a cache with a target list of [CDN-Cache-Control]) to consider
the response fresh for 600 seconds, other shared caches to consider
the response fresh for 120 seconds, and any remaining caches to
consider the response fresh 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:
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
ignored.)
4. IANA Considerations
IANA has registered the following entry in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined by [HTTP]:
Field Name: CDN-Cache-Control
Status: permanent
Specification Document: RFC 9213
Comments: Cache directives targeted at content delivery networks
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, potentially resulting in unintentional reuse of responses
with sensitive information. For this reason, care must be exercised.
6. References
6.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/info/rfc9111>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
6.2. Informative References
[AGE-PENALTY]
Cohen, E. and H. Kaplan, "The age penalty and its effect
on cache performance", March 2001,
<https://dl.acm.org/doi/10.5555/1251440.1251447>.
Authors' Addresses
Stephen Ludin
Akamai
Email: sludin@ludin.org
Mark Nottingham
Fastly
Prahran
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
Yuchen Wu
Cloudflare
Email: me@yuchenwu.net