HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) M. Nottingham, Ed.
Intended status: Standards Track Akamai
Expires: August 27, 2013 J. Reschke, Ed.
greenbytes
February 23, 2013
Hypertext Transfer Protocol (HTTP/1.1): Caching
draft-ietf-httpbis-p6-cache-22
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information
systems. This document defines requirements on HTTP caches and the
associated header fields that control cache behavior or indicate
cacheable response messages.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in Appendix D.3.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on August 27, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Conformance and Error Handling . . . . . . . . . . . . . . 6
1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6
1.4.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6
2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6
3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7
3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 8
3.2. Storing Responses to Authenticated Requests . . . . . . . 9
4. Constructing Responses from Caches . . . . . . . . . . . . . . 9
4.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11
4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 12
4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12
4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 14
4.2. Validation Model . . . . . . . . . . . . . . . . . . . . . 15
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4.2.1. Freshening Responses with 304 Not Modified . . . . . . 16
4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 17
4.4. Combining Partial Content . . . . . . . . . . . . . . . . 17
5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 18
6. Request Methods that Invalidate . . . . . . . . . . . . . . . 19
7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 19
7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20
7.2.1. Request Cache-Control Directives . . . . . . . . . . . 20
7.2.2. Response Cache-Control Directives . . . . . . . . . . 22
7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26
7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 30
7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 30
7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 30
7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 30
7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 31
7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 31
7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 31
7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 31
8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32
9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 32
9.3. Header Field Registration . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35
Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38
Appendix D. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 39
D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 39
D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 39
D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 40
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing
response messages.
1.1. Purpose
An HTTP "cache" is a local store of response messages and the
subsystem that controls its message storage, retrieval, and deletion.
A cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server MAY employ a cache, though a cache
cannot be used by a server that is acting as a tunnel.
The goal of caching in HTTP/1.1 is to significantly improve
performance by reusing a prior response message to satisfy a current
request. A stored response is considered "fresh", as defined in
Section 4.1, if the response can be reused without "validation"
(checking with the origin server to see if the cached response
remains valid for this request). A fresh cache response can
therefore reduce both latency and network transfers each time it is
reused. When a cached response is not fresh, it might still be
reusable if it can be freshened by validation (Section 4.2) or if the
origin is unavailable.
1.2. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching.
"cache"
A conformant implementation of an HTTP cache. Note that this
implies an HTTP/1.1 cache; this specification does not define
conformance for HTTP/1.0 caches.
"shared cache"
A cache that stores responses to be reused by more than one user;
usually (but not always) deployed as part of an intermediary.
"private cache"
A cache that is dedicated to a single user.
"cacheable"
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A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints on whether a cache can use the stored copy to satisfy
a particular request.
"explicit expiration time"
The time at which the origin server intends that a stored response
no longer be used by a cache without further validation.
"heuristic expiration time"
An expiration time assigned by a cache when no explicit expiration
time is available.
"age"
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
"first-hand"
A response is first-hand if the freshness model is not in use;
i.e., its age is 0.
"freshness lifetime"
The length of time between the generation of a response and its
expiration time.
"fresh"
A response is fresh if its age has not yet exceeded its freshness
lifetime.
"stale"
A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).
"validator"
A protocol element (e.g., an entity-tag or a Last-Modified time)
that is used to find out whether a stored response is an
equivalent copy of a representation. See Section 2.1 of [Part4].
"strong validator"
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A validator that is defined by the origin server such that its
current value will change if the representation data changes;
i.e., an entity-tag that is not marked as weak (Section 2.3 of
[Part4]) or, if no entity-tag is provided, a Last-Modified value
that is strong in the sense defined by Section 2.2.2 of [Part4].
1.3. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [Part1].
1.4. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix B describes rules imported from other
documents. Appendix C shows the collected ABNF with the list rule
expanded.
1.4.1. Delta Seconds
The delta-seconds rule specifies a non-negative integer, representing
time in seconds.
delta-seconds = 1*DIGIT
If an implementation receives a delta-seconds value larger than the
largest positive integer it can represent, or if any of its
subsequent calculations overflows, it MUST consider the value to be
2147483648 (2^31). Recipients parsing a delta-seconds value MUST use
an arithmetic type of at least 31 bits of range, and senders MUST NOT
send delta-seconds with a value greater than 2147483648.
2. Overview of Cache Operation
Proper cache operation preserves the semantics of HTTP transfers
([Part2]) while eliminating the transfer of information already held
in the cache. Although caching is an entirely OPTIONAL feature of
HTTP, we assume that reusing the cached response is desirable and
that such reuse is the default behavior when no requirement or
locally-desired configuration prevents it. Therefore, HTTP cache
requirements are focused on preventing a cache from either storing a
non-reusable response or reusing a stored response inappropriately.
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Each "cache entry" consists of a cache key and one or more HTTP
responses corresponding to prior requests that used the same key.
The most common form of cache entry is a successful result of a
retrieval request: i.e., a 200 (OK) response to a GET request, which
contains a representation of the resource identified by the request
target (Section 4.3.1 of [Part2]). However, it is also possible to
cache permanent redirects, negative results (e.g., 404 (Not Found)),
incomplete results (e.g., 206 (Partial Content)), and responses to
methods other than GET if the method's definition allows such caching
and defines something suitable for use as a cache key.
The default "cache key" consists of the request method and target
URI. However, since HTTP caches in common use today are typically
limited to caching responses to GET, many implementations simply
decline other methods and use only the URI as the key.
If a request target is subject to content negotiation, its cache
entry might consist of multiple stored responses, each differentiated
by a secondary key for the values of the original request's selecting
header fields (Section 4.3).
3. Storing Responses in Caches
A cache MUST NOT store a response to any request, unless:
o The request method is understood by the cache and defined as being
cacheable, and
o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 7.2) does not appear
in request or response header fields, and
o the "private" cache response directive (see Section 7.2.2.2) does
not appear in the response, if the cache is shared, and
o the Authorization header field (see Section 4.1 of [Part7]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 3.2), and
o the response either:
* contains an Expires header field (see Section 7.3), or
* contains a max-age response cache directive (see
Section 7.2.2.7), or
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* contains a s-maxage response cache directive and the cache is
shared, or
* contains a Cache Control Extension (see Section 7.2.3) that
allows it to be cached, or
* has a status code that is defined as cacheable (see
Section 4.1.2), or
* contains a public response cache directive (see
Section 7.2.2.1).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3.
In this context, a cache has "understood" a request method or a
response status code if it recognizes it and implements any cache-
specific behavior.
Note that, in normal operation, many caches will not store a response
that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses.
3.1. Storing Incomplete Responses
A response message is considered complete when all of the octets
indicated by the message framing ([Part1]) are received prior to the
connection being closed. If the request is GET, the response status
is 200 (OK), and the entire response header block has been received,
a cache MAY store an incomplete response message body if the cache
entry is recorded as incomplete. Likewise, a 206 (Partial Content)
response MAY be stored as if it were an incomplete 200 (OK) cache
entry. However, a cache MUST NOT store incomplete or partial content
responses if it does not support the Range and Content-Range header
fields or if it does not understand the range units used in those
fields.
A cache MAY complete a stored incomplete response by making a
subsequent range request ([Part5]) and combining the successful
response with the stored entry, as defined in Section 4.4. A cache
MUST NOT use an incomplete response to answer requests unless the
response has been made complete or the request is partial and
specifies a range that is wholly within the incomplete response. A
cache MUST NOT send a partial response to a client without explicitly
marking it as such using the 206 (Partial Content) status code.
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3.2. Storing Responses to Authenticated Requests
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.1 of [Part7]) to satisfy any
subsequent request unless a cache directive that allows such
responses to be stored is present in the response.
In this specification, the following Cache-Control response
directives (Section 7.2.2) have such an effect: must-revalidate,
public, s-maxage.
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale
(Section 4.1.4) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin
server.
4. Constructing Responses from Caches
For a presented request, a cache MUST NOT send a stored response,
unless:
o The presented effective request URI (Section 5.5 of [Part1]) and
that of the stored response match, and
o the request method associated with the stored response allows it
to be used for the presented request, and
o selecting header fields nominated by the stored response (if any)
match those presented (see Section 4.3), and
o the presented request does not contain the no-cache pragma
(Section 7.4), nor the no-cache cache directive (Section 7.2.1),
unless the stored response is successfully validated
(Section 4.2), and
o the stored response does not contain the no-cache cache directive
(Section 7.2.2.3), unless it is successfully validated
(Section 4.2), and
o the stored response is either:
* fresh (see Section 4.1), or
* allowed to be served stale (see Section 4.1.4), or
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* successfully validated (see Section 4.2).
Note that any of the requirements listed above can be overridden by a
cache-control extension; see Section 7.2.3.
When a stored response is used to satisfy a request without
validation, a cache MUST send a single Age header field (Section 7.1)
in the response with a value equal to the stored response's
current_age; see Section 4.1.3.
A cache MUST write through requests with methods that are unsafe
(Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not
allowed to generate a reply to such a request before having forwarded
the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already stored
responses; see Section 6.
When more than one suitable response is stored, a cache MUST use the
most recent response (as determined by the Date header field). It
can also forward a request with "Cache-Control: max-age=0" or "Cache-
Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT use stored
responses without revalidating them on every use.
4.1. Freshness Model
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using
either the Expires header field (Section 7.3) or the max-age response
cache directive (Section 7.2.2.7). Generally, origin servers will
assign future explicit expiration times to responses in the belief
that the representation is not likely to change in a semantically
significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will
normally validate a stale cached response before reusing it for
subsequent requests (see Section 4.1.4).
Since origin servers do not always provide explicit expiration times,
caches are also allowed to use a heuristic to determine an expiration
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time under certain circumstances (see Section 4.1.2).
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
The freshness_lifetime is defined in Section 4.1.1; the current_age
is defined in Section 4.1.3.
Clients can send the max-age or min-fresh cache directives in a
request to constrain or relax freshness calculations for the
corresponding response (Section 7.2.1).
Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a
resource. See Section 8 for an explanation of the difference between
caches and history mechanisms.
4.1.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of:
o If the cache is shared and the s-maxage response cache directive
(Section 7.2.2.8) is present, use its value, or
o If the max-age response cache directive (Section 7.2.2.7) is
present, use its value, or
o If the Expires response header field (Section 7.3) is present, use
its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see
Section 4.1.2.
Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server.
When there is more than one value present for a given directive
(e.g., two Expires header fields, multiple Cache-Control: max-age
directives), it is considered invalid. Caches are encouraged to
consider responses that have invalid freshness information to be
stale.
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4.1.2. Calculating Heuristic Freshness
Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other header field
values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results.
A cache MUST NOT use heuristics to determine freshness when an
explicit expiration time is present in the stored response. Because
of the requirements in Section 3, this means that, effectively,
heuristics can only be used on responses without explicit freshness
whose status codes are defined as cacheable, and responses without
explicit freshness that have been marked as explicitly cacheable
(e.g., with a "public" response cache directive).
If the response has a Last-Modified header field (Section 2.2 of
[Part4]), caches are encouraged to use a heuristic expiration value
that is no more than some fraction of the interval since that time.
A typical setting of this fraction might be 10%.
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the
response if its current_age is more than 24 hours and such a warning
is not already present.
Note: Section 13.9 of [RFC2616] prohibited caches from calculating
heuristic freshness for URIs with query components (i.e., those
containing '?'). In practice, this has not been widely
implemented. Therefore, servers are encouraged to send explicit
directives (e.g., Cache-Control: no-cache) if they wish to
preclude caching.
4.1.3. Calculating Age
The Age header field is used to convey an estimated age of the
response message when obtained from a cache. The Age field value is
the cache's estimate of the number of seconds since the response was
generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
The following data is used for the age calculation:
"age_value"
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The term "age_value" denotes the value of the Age header field
(Section 7.1), in a form appropriate for arithmetic operation; or
0, if not available.
"date_value"
The term "date_value" denotes the value of the Date header field,
in a form appropriate for arithmetic operations. See Section
7.1.1.2 of [Part2] for the definition of the Date header field,
and for requirements regarding responses without it.
"now"
The term "now" means "the current value of the clock at the host
performing the calculation". A host ought to use NTP ([RFC1305])
or some similar protocol to synchronize its clocks to Coordinated
Universal Time.
"request_time"
The current value of the clock at the host at the time the request
resulting in the stored response was made.
"response_time"
The current value of the clock at the host at the time the
response was received.
A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by
zero.
2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this
value relative to the time the request was initiated, not the
time that the response was received.
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
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These SHOULD be combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header field
(e.g., because there are no HTTP/1.0 hops in the Via header field),
in which case the corrected_age_value MAY be used as the
corrected_initial_age.
The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
Additionally, to avoid common problems in date parsing:
o Although all date formats are specified to be case-sensitive,
cache recipients SHOULD match day, week and timezone names case-
insensitively.
o If a cache recipient's internal implementation of time has less
resolution than the value of an HTTP-date, the recipient MUST
internally represent a parsed Expires date as the nearest time
equal to or earlier than the received value.
o Cache recipients MUST NOT allow local time zones to influence the
calculation or comparison of an age or expiration time.
o Cache recipients SHOULD consider a date with a zone abbreviation
other than "GMT" to be invalid for calculating expiration.
4.1.4. Serving Stale Responses
A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 4.1.
A cache MUST NOT send a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an
applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 7.2.2).
A cache MUST NOT send stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a
forward path) or doing so is explicitly allowed (e.g., by the max-
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stale request directive; see Section 7.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code
(see Section 7.5) to stale responses. Likewise, a cache SHOULD add
the 112 warn-code to stale responses if the cache is disconnected.
If a cache receives a first-hand response (either an entire response,
or a 304 (Not Modified) response) that it would normally forward to
the requesting client, and the received response is no longer fresh,
the cache can forward it to the requesting client without adding a
new Warning (but without removing any existing Warning header
fields). A cache shouldn't attempt to validate a response simply
because that response became stale in transit.
4.2. Validation Model
When a cache has one or more stored responses for a requested URI,
but cannot serve any of them (e.g., because they are not fresh, or
one cannot be selected; see Section 4.3), it can use the conditional
request mechanism [Part4] in the forwarded request to give the origin
server an opportunity to both select a valid stored response to be
used, and to update it. This process is known as "validating" or
"revalidating" the stored response.
When sending such a conditional request, a cache adds an If-Modified-
Since header field whose value is that of the Last-Modified header
field from the selected (see Section 4.3) stored response, if
available.
Additionally, a cache can add an If-None-Match header field whose
value is that of the ETag header field(s) from all responses stored
for the requested URI, if present. However, if any of the stored
responses contains only partial content, the cache shouldn't include
its entity-tag in the If-None-Match header field unless the request
is for a range that would be fully satisfied by that stored response.
Cache handling of a response to a conditional request is dependent
upon its status code:
o A 304 (Not Modified) response status code indicates that the
stored response can be updated and reused; see Section 4.2.1.
o A full response (i.e., one with a payload body) indicates that
none of the stored responses nominated in the conditional request
is suitable. Instead, the cache can use the full response to
satisfy the request and MAY replace the stored response(s).
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o However, if a cache receives a 5xx (Server Error) response while
attempting to validate a response, it can either forward this
response to the requesting client, or act as if the server failed
to respond. In the latter case, it can send a previously stored
response (see Section 4.1.4).
4.2.1. Freshening Responses with 304 Not Modified
When a cache receives a 304 (Not Modified) response and already has
one or more stored 200 (OK) responses for the same cache key, the
cache needs to identify which of the stored responses are updated by
this new response and then update the stored response(s) with the new
information provided in the 304 response.
The stored response to update is identified by using the first match
(if any) of:
o If the new response contains a strong validator, then that strong
validator identifies the selected representation. All of the
stored responses with the same strong validator are selected. If
none of the stored responses contain the same strong validator,
then the new response MUST NOT be used to update any stored
responses.
o If the new response contains a weak validator and that validator
corresponds to one of the cache's stored responses, then the most
recent of those matching stored responses is selected.
o If the new response does not include any form of validator (such
as in the case where a client generates an If-Modified-Since
request from a source other than the Last-Modified response header
field), and there is only one stored response, and that stored
response also lacks a validator, then that stored response is
selected.
If a stored response is selected for update, the cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the 304 (Not Modified)
response to replace all instances of the corresponding header
fields in the stored response.
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4.3. Using Negotiated Responses
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 7.1.4 of [Part2]), it
MUST NOT use that response unless all of the selecting header fields
nominated by the Vary header field match in both the original request
(i.e., that associated with the stored response), and the presented
request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [Part1])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., re-ordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is
also absent there.
A Vary header field-value of "*" always fails to match, and
subsequent requests to that resource can only be properly interpreted
by the origin server.
The stored response with matching selecting header fields is known as
the selected response.
If multiple selected responses are available, the most recent
response (as determined by the Date header field) is used; see
Section 4.
If no selected response is available, the cache cannot satisfy the
presented request. Typically, it is forwarded to the origin server
in a (possibly conditional; see Section 4.2) request.
4.4. Combining Partial Content
A response might transfer only a partial representation if the
connection closed prematurely or if the request used one or more
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Range specifiers ([Part5]). After several such transfers, a cache
might have received several ranges of the same representation. A
cache MAY combine these ranges into a single stored response, and
reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client
requirements in Section 4.3 of [Part5].
When combining the new response with one or more stored responses, a
cache MUST:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the new response, aside from
Content-Range, to replace all instances of the corresponding
header fields in the stored response.
5. Updating Caches with HEAD Responses
A response to the HEAD method is identical to what an equivalent
request made with a GET would have been, except it lacks a body.
This property of HEAD responses is used to both invalidate and update
cached GET responses.
If one or more stored GET responses can be selected (as per
Section 4.3) for a HEAD request, and the Content-Length, ETag or
Last-Modified value of a HEAD response differs from that in a
selected GET response, the cache MUST consider that selected response
to be stale.
If the Content-Length, ETag and Last-Modified values of a HEAD
response (when present) are the same as that in a selected GET
response (as per Section 4.3), the cache SHOULD update the remaining
header fields in the stored response using the following rules:
o delete any Warning header fields in the stored response with warn-
code 1xx (see Section 7.5);
o retain any Warning header fields in the stored response with warn-
code 2xx; and,
o use other header fields provided in the response to replace all
instances of the corresponding header fields in the stored
response.
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6. Request Methods that Invalidate
Because unsafe request methods (Section 4.2.1 of [Part2]) such as
PUT, POST or DELETE have the potential for changing state on the
origin server, intervening caches can use them to keep their contents
up-to-date.
A cache MUST invalidate the effective Request URI (Section 5.5 of
[Part1]) as well as the URI(s) in the Location and Content-Location
response header fields (if present) when a non-error response to a
request with an unsafe method is received.
However, a cache MUST NOT invalidate a URI from a Location or
Content-Location response header field if the host part of that URI
differs from the host part in the effective request URI (Section 5.5
of [Part1]). This helps prevent denial of service attacks.
A cache MUST invalidate the effective request URI (Section 5.5 of
[Part1]) when it receives a non-error response to a request with a
method whose safety is unknown.
Here, a "non-error response" is one with a 2xx (Successful) or 3xx
(Redirection) status code. "Invalidate" means that the cache will
either remove all stored responses related to the effective request
URI, or will mark these as "invalid" and in need of a mandatory
validation before they can be sent in response to a subsequent
request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the
origin server might not have gone through the cache where a response
is stored.
7. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
7.1. Age
The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in
Section 4.1.3.
Age = delta-seconds
Age field-values are non-negative integers, representing time in
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seconds (see Section 1.4.1).
The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since
HTTP/1.0 caches might not implement the Age header field.
7.2. Cache-Control
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does
not imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control directives
defined in this section. See Section 7.2.3 for information about how
Cache-Control directives defined elsewhere are handled.
Note: HTTP/1.0 caches might not implement Cache-Control and might
only implement Pragma: no-cache (see Section 7.4).
A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their
significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is
not possible to target a directive to a specific cache.
Cache directives are identified by a token, to be compared case-
insensitively, and have an optional argument, that can use both token
and quoted-string syntax. For the directives defined below that
define arguments, recipients ought to accept both forms, even if one
is documented to be preferred. For any directive not defined by this
specification, recipients MUST accept both forms.
Cache-Control = 1#cache-directive
cache-directive = token [ "=" ( token / quoted-string ) ]
For the cache directives defined below, no argument is defined (nor
allowed) otherwise stated otherwise.
7.2.1. Request Cache-Control Directives
7.2.1.1. no-cache
The "no-cache" request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
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7.2.1.2. no-store
The "no-store" request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from a
cache, the no-store request directive does not apply to the already
stored response.
7.2.1.3. max-age
Argument syntax:
delta-seconds (see Section 1.4.1)
The "max-age" request directive indicates that the client is
unwilling to accept a response whose age is greater than the
specified number of seconds. Unless the max-stale request directive
is also present, the client is not willing to accept a stale
response.
Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
quoted-string form.
7.2.1.4. max-stale
Argument syntax:
delta-seconds (see Section 1.4.1)
The "max-stale" request directive indicates that the client is
willing to accept a response that has exceeded its expiration time.
If max-stale is assigned a value, then the client is willing to
accept a response that has exceeded its expiration time by no more
than the specified number of seconds. If no value is assigned to
max-stale, then the client is willing to accept a stale response of
any age.
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Note: This directive uses the token form of the argument syntax;
e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use
the quoted-string form.
7.2.1.5. min-fresh
Argument syntax:
delta-seconds (see Section 1.4.1)
The "min-fresh" request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less than
its current age plus the specified time in seconds. That is, the
client wants a response that will still be fresh for at least the
specified number of seconds.
Note: This directive uses the token form of the argument syntax;
e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use
the quoted-string form.
7.2.1.6. no-transform
The "no-transform" request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1].
7.2.1.7. only-if-cached
The "only-if-cached" request directive indicates that the client only
wishes to obtain a stored response. If it receives this directive, a
cache SHOULD either respond using a stored response that is
consistent with the other constraints of the request, or respond with
a 504 (Gateway Timeout) status code. If a group of caches is being
operated as a unified system with good internal connectivity, a
member cache MAY forward such a request within that group of caches.
7.2.2. Response Cache-Control Directives
7.2.2.1. public
The "public" response directive indicates that any cache MAY store
the response, even if the response would normally be non-cacheable or
cacheable only within a non-shared cache. (See Section 3.2 for
additional details related to the use of public in response to a
request containing Authorization, and Section 3 for details of how
public affects responses that would normally not be stored, due to
their status codes not being defined as cacheable.)
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7.2.2.2. private
Argument syntax:
#field-name
The "private" response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared
cache. A private cache MAY store the response and reuse it for later
requests, even if the response would normally be non-cacheable.
If the private response directive specifies one or more field-names,
this requirement is limited to the field-values associated with the
listed response header fields. That is, a shared cache MUST NOT
store the specified field-names(s), whereas it MAY store the
remainder of the response message.
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-
insensitive.
Note: This usage of the word "private" only controls where the
response can be stored; it cannot ensure the privacy of the message
content. Also, private response directives with field-names are
often handled by implementations as if an unqualified private
directive was received; i.e., the special handling for the qualified
form is not widely implemented.
Note: This directive uses the quoted-string form of the argument
syntax. Senders SHOULD NOT use the token form (even if quoting
appears not to be needed for single-entry lists).
7.2.2.3. no-cache
Argument syntax:
#field-name
The "no-cache" response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without contacting
it, even by caches that have been configured to send stale responses.
If the no-cache response directive specifies one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, any header
fields in the response that have the field-name(s) listed MUST NOT be
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sent in the response to a subsequent request without successful
revalidation with the origin server. This allows an origin server to
prevent the re-use of certain header fields in a response, while
still allowing caching of the rest of the response.
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-
insensitive.
Note: Many HTTP/1.0 caches will not recognize or obey this directive.
Also, no-cache response directives with field-names are often handled
by implementations as if an unqualified no-cache directive was
received; i.e., the special handling for the qualified form is not
widely implemented.
Note: This directive uses the quoted-string form of the argument
syntax. Senders SHOULD NOT use the token form (even if quoting
appears not to be needed for single-entry lists).
7.2.2.4. no-store
The "no-store" response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a best-
effort attempt to remove the information from volatile storage as
promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for ensuring
privacy. In particular, malicious or compromised caches might not
recognize or obey this directive, and communications networks might
be vulnerable to eavesdropping.
7.2.2.5. must-revalidate
The "must-revalidate" response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy subsequent
requests without successful validation on the origin server.
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST generate
a 504 (Gateway Timeout) response.
The must-revalidate directive ought to be used by servers if and only
if failure to validate a request on the representation could result
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in incorrect operation, such as a silently unexecuted financial
transaction.
7.2.2.6. proxy-revalidate
The "proxy-revalidate" response directive has the same meaning as the
must-revalidate response directive, except that it does not apply to
private caches.
7.2.2.7. max-age
Argument syntax:
delta-seconds (see Section 1.4.1)
The "max-age" response directive indicates that the response is to be
considered stale after its age is greater than the specified number
of seconds.
Note: This directive uses the token form of the argument syntax;
e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the
quoted-string form.
7.2.2.8. s-maxage
Argument syntax:
delta-seconds (see Section 1.4.1)
The "s-maxage" response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum age
specified by either the max-age directive or the Expires header
field. The s-maxage directive also implies the semantics of the
proxy-revalidate response directive.
Note: This directive uses the token form of the argument syntax;
e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the
quoted-string form.
7.2.2.9. no-transform
The "no-transform" response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT transform the
payload, as defined in Section 5.7.2 of [Part1].
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7.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value.
Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that
applications that do not understand the new directive will default to
the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without
requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.
For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any private
cache, any cache that is shared only by members of the community
named within its value is allowed to cache the response. An origin
server wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI"
A cache seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe
behavior.
A cache MUST ignore unrecognized cache directives; it is assumed that
any cache directive likely to be unrecognized by an HTTP/1.1 cache
will be combined with standard directives (or the response's default
cacheability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s).
New extension directives ought to consider defining:
o What it means for a directive to be specified multiple times,
o When the directive does not take an argument, what it means when
an argument is present,
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o When the directive requires an argument, what it means when it is
missing.
The HTTP Cache Directive Registry defines the name space for the
cache directives.
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
.
7.3. Expires
The "Expires" header field gives the date/time after which the
response is considered stale. See Section 4.1 for further discussion
of the freshness model.
The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The Expires value is an HTTP-date timestamp, as defined in Section
7.1.1.1 of [Part2].
Expires = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
A cache recipient MUST interpret invalid date formats, especially the
value "0", as representing a time in the past (i.e., "already
expired").
If a response includes a Cache-Control field with the max-age
directive (Section 7.2.2.7), a recipient MUST ignore the Expires
field. Likewise, if a response includes the s-maxage directive
(Section 7.2.2.8), a shared cache recipient MUST ignore the Expires
field. In both these cases, the value in Expires is only intended
for recipients that have not yet implemented the Cache-Control field.
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An origin server without a clock MUST NOT generate an Expires field
unless its value represents a fixed time in the past (always expired)
or its value has been associated with the resource by a system or
user with a reliable clock.
Historically, HTTP required the Expires field-value to be no more
than a year in the future. While longer freshness lifetimes are no
longer prohibited, extremely large values have been demonstrated to
cause problems (e.g., clock overflows due to use of 32-bit integers
for time values), and many caches will evict a response far sooner
than that.
7.4. Pragma
The "Pragma" header field allows backwards compatibility with
HTTP/1.0 caches, so that clients can specify a "no-cache" request
that they will understand (as Cache-Control was not defined until
HTTP/1.1). When the Cache-Control header field is also present and
understood in a request, Pragma is ignored.
In HTTP/1.0, Pragma was defined as an extensible field for
implementation-specified directives for recipients. This
specification deprecates such extensions to improve interoperability.
Pragma = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
When the Cache-Control header field is not present in a request, the
no-cache request pragma-directive MUST have the same effect on caches
as if "Cache-Control: no-cache" were present (see Section 7.2.1).
When sending a no-cache request, a client ought to include both the
pragma and cache-control directives, unless Cache-Control: no-cache
is purposefully omitted to target other Cache-Control response
directives at HTTP/1.1 caches. For example:
GET / HTTP/1.1
Host: www.example.com
Cache-Control: max-age=30
Pragma: no-cache
will constrain HTTP/1.1 caches to serve a response no older than 30
seconds, while precluding implementations that do not understand
Cache-Control from serving a cached response.
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Note: Because the meaning of "Pragma: no-cache" in responses is
not specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in them.
7.5. Warning
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be
reflected in the message. This information is typically used to warn
about possible incorrectness introduced by caching operations or
transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures.
Warning header fields can in general be applied to any message,
however some warn-codes are specific to caches and can only be
applied to response messages.
Warning = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding
; the Warning header field, for use in debugging
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number, only differing in warn-text.
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response.
Systems that generate multiple Warning header fields are encouraged
to order them with this user agent behavior in mind. New Warning
header fields are added after any existing Warning header fields.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored
response after validation:
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o 1xx Warnings describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation.
o 2xx Warnings describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after
validation, unless a full response is sent, in which case they
MUST be.
If an implementation sends a message with one or more Warning header
fields to a receiver whose version is HTTP/1.0 or lower, then the
sender MUST include in each warning-value a warn-date that matches
the Date header field in the message.
If a system receives a message with a warning-value that includes a
warn-date, and that warn-date is different from the Date value in the
response, then that warning-value MUST be deleted from the message
before storing, forwarding, or using it. (preventing the consequences
of naive caching of Warning header fields.) If all of the warning-
values are deleted for this reason, the Warning header field MUST be
deleted as well.
The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning.
7.5.1. 110 Response is Stale
A cache SHOULD generate this whenever the sent response is stale.
7.5.2. 111 Revalidation Failed
A cache SHOULD generate this when sending a stale response because an
attempt to validate the response failed, due to an inability to reach
the server.
7.5.3. 112 Disconnected Operation
A cache SHOULD generate this if it is intentionally disconnected from
the rest of the network for a period of time.
7.5.4. 113 Heuristic Expiration
A cache SHOULD generate this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater than
24 hours.
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7.5.5. 199 Miscellaneous Warning
The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT
take any automated action, besides presenting the warning to the
user.
7.5.6. 214 Transformation Applied
MUST be added by a proxy if it applies any transformation to the
representation, such as changing the content-coding, media-type, or
modifying the representation data, unless this Warning code already
appears in the response.
7.5.7. 299 Miscellaneous Persistent Warning
The warning text can include arbitrary information to be presented to
a human user, or logged. A system receiving this warning MUST NOT
take any automated action.
7.5.8. Warn Code Extensions
The HTTP Warn Code Registry defines the name space for warn codes.
A registration MUST include the following fields:
o Warn Code (3 digits)
o Short Description
o Pointer to specification text
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
.
8. History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
retrieved earlier in a session.
The freshness model (Section 4.1) does not necessarily apply to
history mechanisms. I.e., a history mechanism can display a previous
representation even if it has expired.
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This does not prohibit the history mechanism from telling the user
that a view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store).
9. IANA Considerations
9.1. Cache Directive Registry
The registration procedure for HTTP Cache Directives is defined by
Section 7.2.3 of this document.
The HTTP Cache Directive Registry shall be created at
and be
populated with the registrations below:
+------------------------+----------------------------------+
| Cache Directive | Reference |
+------------------------+----------------------------------+
| max-age | Section 7.2.1.3, Section 7.2.2.7 |
| max-stale | Section 7.2.1.4 |
| min-fresh | Section 7.2.1.5 |
| must-revalidate | Section 7.2.2.5 |
| no-cache | Section 7.2.1.1, Section 7.2.2.3 |
| no-store | Section 7.2.1.2, Section 7.2.2.4 |
| no-transform | Section 7.2.1.6, Section 7.2.2.9 |
| only-if-cached | Section 7.2.1.7 |
| private | Section 7.2.2.2 |
| proxy-revalidate | Section 7.2.2.6 |
| public | Section 7.2.2.1 |
| s-maxage | Section 7.2.2.8 |
| stale-if-error | [RFC5861], Section 4 |
| stale-while-revalidate | [RFC5861], Section 3 |
+------------------------+----------------------------------+
9.2. Warn Code Registry
The registration procedure for HTTP Warn Codes is defined by
Section 7.5.8 of this document.
The HTTP Warn Code Registry shall be created at
and be
populated with the registrations below:
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+-----------+----------------------------------+---------------+
| Warn Code | Short Description | Reference |
+-----------+----------------------------------+---------------+
| 110 | Response is Stale | Section 7.5.1 |
| 111 | Revalidation Failed | Section 7.5.2 |
| 112 | Disconnected Operation | Section 7.5.3 |
| 113 | Heuristic Expiration | Section 7.5.4 |
| 199 | Miscellaneous Warning | Section 7.5.5 |
| 214 | Transformation Applied | Section 7.5.6 |
| 299 | Miscellaneous Persistent Warning | Section 7.5.7 |
+-----------+----------------------------------+---------------+
9.3. Header Field Registration
The Message Header Field Registry located at shall be
updated with the permanent registrations below (see [BCP90]):
+-------------------+----------+----------+-------------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-------------+
| Age | http | standard | Section 7.1 |
| Cache-Control | http | standard | Section 7.2 |
| Expires | http | standard | Section 7.3 |
| Pragma | http | standard | Section 7.4 |
| Warning | http | standard | Section 7.5 |
+-------------------+----------+----------+-------------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
10. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP/1.1 caching.
More general security considerations are addressed in HTTP messaging
[Part1] and semantics [Part2].
Caches expose additional potential vulnerabilities, since the
contents of the cache represent an attractive target for malicious
exploitation. Because cache contents persist after an HTTP request
is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the
network. Therefore, cache contents need to be protected as sensitive
information.
Furthermore, the very use of a cache can bring about privacy
concerns. For example, if two users share a cache, and the first one
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browses to a site, the second may be able to detect that the other
has been to that site, because the resources from it load more
quickly, thanks to the cache.
Implementation flaws might allow attackers to insert content into a
cache ("cache poisoning"), leading to compromise of clients that
trust that content. Because of their nature, these attacks are
difficult to mitigate.
Likewise, implementation flaws (as well as misunderstanding of cache
operation) might lead to caching of sensitive information (e.g.,
authentication credentials) that is thought to be private, exposing
it to unauthorized parties.
Note that the Set-Cookie response header [RFC6265] does not inhibit
caching; a cacheable response with a Set-Cookie header can be (and
often is) used to satisfy subsequent requests to caches. Servers who
wish to control caching of these responses are encouraged to emit
appropriate Cache-Control response headers.
11. Acknowledgments
See Section 9 of [Part1].
12. References
12.1. Normative References
[Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-22 (work in progress),
February 2013.
[Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-22 (work in progress),
February 2013.
[Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-22 (work in progress),
February 2013.
[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
draft-ietf-httpbis-p5-range-22 (work in progress),
February 2013.
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[Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-22 (work in progress),
February 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
12.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale
Content", RFC 5861, April 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
Appendix A. Changes from RFC 2616
Caching-related text has been substantially rewritten for clarity.
The algorithm for calculating age is now less conservative.
(Section 4.1.3)
Caches are now required to handle dates with timezones as if they're
invalid, because it's not possible to accurately guess.
(Section 4.1.3)
The Content-Location response header field is no longer used to
determine the appropriate response to use when validating.
(Section 4.2)
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The algorithm for selecting a cached negotiated response to use has
been clarified in several ways. In particular, it now explicitly
allows header-specific canonicalization when processing selecting
header fields. (Section 4.3)
Requirements regarding denial of service attack avoidance when
performing invalidation have been clarified. (Section 6)
Cache invalidation only occurs when a successful response is
received. (Section 6)
The conditions under which an authenticated response can be cached
have been clarified. (Section 3.2)
The one-year limit on Expires header field values has been removed;
instead, the reasoning for using a sensible value is given.
(Section 7.3)
The Pragma header field is now only defined for backwards
compatibility; future pragmas are deprecated. (Section 7.4)
Cache directives are explicitly defined to be case-insensitive.
(Section 7.2)
Handling of multiple instances of cache directives when only one is
expected is now defined. (Section 7.2)
The qualified forms of the private and no-cache cache directives are
noted to not be widely implemented; e.g., "private=foo" is
interpreted by many caches as simply "private". Additionally, the
meaning of the qualified form of no-cache has been clarified.
(Section 7.2.2)
The "no-store" cache request directive doesn't apply to responses;
i.e., a cache can satisfy a request with no-store on it, and does not
invalidate it. (Section 7.2.1.2)
The "no-cache" response cache directive's meaning has been clarified.
(Section 7.2.2.3)
New status codes can now define that caches are allowed to use
heuristic freshness with them. (Section 4.1.2)
Caches are now allow to calculate heuristic freshness for URLs with
query components. (Section 4.1.2)
Some requirements regarding production of the Warning header have
been relaxed, as it is not widely implemented. (Section 7.5)
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The Warning header field no longer uses RFC 2047 encoding, nor allows
multiple languages, as these aspects were not implemented.
(Section 7.5)
This specification introduces the Cache Directive and Warn Code
Registries, and defines considerations for new cache directives.
(Section 7.2.3 and Section 7.5.8)
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
character).
The rules below are defined in [Part1]:
OWS =
field-name =
quoted-string =
token =
port =
pseudonym =
uri-host =
The rules below are defined in other parts:
HTTP-date =
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Appendix C. Collected ABNF
Age = delta-seconds
Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS
cache-directive ] )
Expires = HTTP-date
HTTP-date =
OWS =
Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS
pragma-directive ] )
Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ]
)
cache-directive = token [ "=" ( token / quoted-string ) ]
delta-seconds = 1*DIGIT
extension-pragma = token [ "=" ( token / quoted-string ) ]
field-name =
port =
pragma-directive = "no-cache" / extension-pragma
pseudonym =
quoted-string =
token =
uri-host =
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date
]
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Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the first Working Group Last Call draft are summarized
in .
D.1. Since draft-ietf-httpbis-p6-cache-19
Closed issues:
o : "untangle
Cache-Control ABNF"
o : "Multiple
values in Cache-Control header fields"
o : "Case
sensitivity of header fields in CC values"
o : "Spurious
'MAYs'"
o : "enhance
considerations for new cache control directives"
o : "ABNF
requirements for recipients"
o : "note
introduction of new IANA registries as normative changes"
o : "broken prose
in description of 'Vary'"
D.2. Since draft-ietf-httpbis-p6-cache-20
Closed issues:
o : "'Most
Conservative'"
Other changes:
o Conformance criteria and considerations regarding error handling
are now defined in Part 1.
o Move definition of "Vary" header field into Part 2.
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o Add security considerations with respect to cache poisoning and
the "Set-Cookie" header field.
D.3. Since draft-ietf-httpbis-p6-cache-21
Closed issues:
o : "Allowing
heuristic caching for new status codes"
o : "304 without
validator"
o : "No-Transform"
o : "Revert prior
change to the meaning of the public cache response directive.
Index
1
110 Response is Stale (warn code) 30
111 Revalidation Failed (warn code) 30
112 Disconnected Operation (warn code) 30
113 Heuristic Expiration (warn code) 30
199 Miscellaneous Warning (warn code) 31
2
214 Transformation Applied (warn code) 31
299 Miscellaneous Persistent Warning (warn code) 31
A
age 5
Age header field 19
C
cache 4
cache entry 6
cache key 6
Cache-Control header field 20
cacheable 4
E
Expires header field 27
explicit expiration time 5
F
first-hand 5
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fresh 5
freshness lifetime 5
G
Grammar
Age 19
Cache-Control 20
cache-directive 20
delta-seconds 6
Expires 27
extension-pragma 28
Pragma 28
pragma-directive 28
warn-agent 29
warn-code 29
warn-date 29
warn-text 29
Warning 29
warning-value 29
H
heuristic expiration time 5
M
max-age (cache directive) 21, 25
max-stale (cache directive) 21
min-fresh (cache directive) 22
must-revalidate (cache directive) 24
N
no-cache (cache directive) 20, 23
no-store (cache directive) 21, 24
no-transform (cache directive) 22, 25
O
only-if-cached (cache directive) 22
P
Pragma header field 28
private (cache directive) 23
private cache 4
proxy-revalidate (cache directive) 25
public (cache directive) 22
S
s-maxage (cache directive) 25
shared cache 4
stale 5
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strong validator 5
V
validator 5
strong 5
W
Warning header field 29
Authors' Addresses
Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
EMail: fielding@gbiv.com
URI: http://roy.gbiv.com/
Mark Nottingham (editor)
Akamai
EMail: mnot@mnot.net
URI: http://www.mnot.net/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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