rfc9211.original.xml   rfc9211.xml 
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<front> <front>
<title abbrev="Cache-Status Header">The Cache-Status HTTP Response Header Fi eld</title> <title abbrev="Cache-Status Header">The Cache-Status HTTP Response Header Fi eld</title>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-cache-header-10" /> <seriesInfo name="RFC" value="9211"/>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham"> <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Fastly</organization> <organization>Fastly</organization>
<address> <address>
<postal> <postal>
<city>Prahran</city> <postalLine>Prahran</postalLine>
<region>VIC</region> <postalLine>Australia</postalLine>
<country>Australia</country>
</postal> </postal>
<email>mnot@mnot.net</email> <email>mnot@mnot.net</email>
<uri>https://www.mnot.net/</uri> <uri>https://www.mnot.net/</uri>
</address> </address>
</author> </author>
<date/> <date year="2022" month="June" />
<area>Applications and Real-Time</area> <area>Applications and Real-Time</area>
<workgroup>HTTP</workgroup> <workgroup>HTTP</workgroup>
<keyword>http</keyword> <keyword>http</keyword>
<keyword>cache</keyword> <keyword>cache</keyword>
<keyword>debugging</keyword> <keyword>debugging</keyword>
<keyword>x-cache</keyword> <keyword>x-cache</keyword>
<abstract> <abstract>
<t>To aid debugging, HTTP caches often append header fields to a response explaining how they handled the request in an ad hoc manner. This specification defines a standard mechanism to do so that is aligned with HTTP's caching model. </t> <t>To aid debugging, HTTP caches often append header fields to a response, explaining how they handled the request in an ad hoc manner. This specification defines a standard mechanism to do so that is aligned with HTTP's caching model .</t>
</abstract> </abstract>
<note>
<name>Note to Readers</name>
<t><em>RFC EDITOR: please remove this section before publication</em></t>
<t>Discussion of this draft takes place on the HTTP working group mailing
list (ietf-http-wg@w3.org), which is archived at <eref target="https://lists.w3.
org/Archives/Public/ietf-http-wg/">https://lists.w3.org/Archives/Public/ietf-htt
p-wg/</eref>.</t>
<t>Working Group information can be found at <eref target="https://httpwg.
org/">https://httpwg.org/</eref>; source code and issues list for this draft can
be found at <eref target="https://github.com/httpwg/http-extensions/labels/cach
e-header">https://github.com/httpwg/http-extensions/labels/cache-header</eref>.<
/t>
</note>
</front> </front>
<middle> <middle>
<section anchor="introduction" numbered="true" toc="default"> <section anchor="introduction" numbered="true" toc="default">
<name>Introduction</name> <name>Introduction</name>
<t>To aid debugging (both by humans and automated tools), HTTP caches ofte <t>To aid debugging (both by humans and automated tools), HTTP caches ofte
n append header fields to a response explaining how they handled the request. Un n append header fields to a response explaining how they handled the request. Un
fortunately, the semantics of these headers are often unclear, and both the sema fortunately, the semantics of these header fields are often unclear, and both th
ntics and syntax used vary between implementations.</t> e semantics and syntax used vary between implementations.</t>
<t>This specification defines a new HTTP response header field, "Cache-Sta <t>This specification defines a new HTTP response header field, "Cache-Sta
tus" for this purpose, with standardized syntax and semantics.</t> tus", for this purpose with standardized syntax and semantics.</t>
<section anchor="notational-conventions" numbered="true" toc="default"> <section anchor="notational-conventions" numbered="true" toc="default">
<name>Notational Conventions</name> <name>Notational Conventions</name>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", " <t>
SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" i The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQU
n this document are to be interpreted as described in BCP 14 <xref target="RFC21 IRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOUL
19" format="default"/> <xref target="RFC8174" format="default"/> when, and only D</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>N
when, they appear in all capitals, as shown here.</t> OT RECOMMENDED</bcp14>",
<t>This document uses ABNF as defined in <xref target="RFC5234" format=" "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
default"/>, with rules prefixed with "sf-" and the "key" rule as defined in <xre be interpreted as
f target="STRUCTURED-FIELDS" format="default"/>. It uses terminology from <xref described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
target="HTTP" format="default"/> and <xref target="HTTP-CACHING" format="default when, and only when, they appear in all capitals, as shown here.
"/>.</t> </t>
<t>This document uses the following terminology from <xref section="3" section
Format="of" target="STRUCTURED-FIELDS"/> to specify syntax and parsing: List, St
ring, Token, Integer, and Boolean.</t>
<t>This document also uses terminology from <xref target="HTTP"/> and <x
ref target="HTTP-CACHING"/>.</t>
</section> </section>
</section> </section>
<section anchor="field" numbered="true" toc="default"> <section anchor="field" numbered="true" toc="default">
<name>The Cache-Status HTTP Response Header Field</name> <name>The Cache-Status HTTP Response Header Field</name>
<t>The Cache-Status HTTP response header field indicates how caches have h andled that response and its corresponding request. The syntax of this header fi eld conforms to <xref target="STRUCTURED-FIELDS" format="default"/>.</t> <t>The Cache-Status HTTP response header field indicates how caches have h andled that response and its corresponding request. The syntax of this header fi eld conforms to <xref target="STRUCTURED-FIELDS" format="default"/>.</t>
<t>Its value is a List (<xref section="3.1" sectionFormat="comma" target=" <t>Its value is a List. Each member of the List represents a cache that has ha
STRUCTURED-FIELDS" format="default"/>):</t> ndled the request. The first member represents the cache closest to the origin s
<sourcecode type="abnf"><![CDATA[ erver, and the last member represents the cache closest to the user (possibly in
Cache-Status = sf-list cluding the user agent's cache itself if it appends a value).</t>
]]></sourcecode>
<t>Each member of the list represents a cache that has handled the request
. The first member of the list represents the cache closest to the origin server
, and the last member of the list represents the cache closest to the user (poss
ibly including the user agent's cache itself, if it appends a value).</t>
<t>Caches determine when it is appropriate to add the Cache-Status header field to a response. Some might add it to all responses, whereas others might on ly do so when specifically configured to, or when the request contains a header field that activates a debugging mode. See <xref target="security" format="defau lt"/> for related security considerations.</t> <t>Caches determine when it is appropriate to add the Cache-Status header field to a response. Some might add it to all responses, whereas others might on ly do so when specifically configured to, or when the request contains a header field that activates a debugging mode. See <xref target="security" format="defau lt"/> for related security considerations.</t>
<t>An intermediary SHOULD NOT append a Cache-Status member to responses th <t>An intermediary <bcp14>SHOULD NOT</bcp14> append a Cache-Status member
at it generates locally, even if that intermediary contains a cache, unless the to responses that it generates locally, even if that intermediary contains a cac
generated response is based upon a stored response (e.g., 304 Not Modified and 2 he, unless the generated response is based upon a stored response (e.g., 304 (No
06 Partial Content are both based upon a stored response). For example, a proxy t Modified) and 206 (Partial Content) are both based upon a stored response). Fo
generating a 400 response due to a malformed request will not add a Cache-Status r example, a proxy generating a 400 response due to a malformed request will not
value, because that response was generated by the proxy, not the origin server. add a Cache-Status value, because that response was generated by the proxy, not
</t> the origin server.</t>
<t>When adding a value to the Cache-Status header field, caches SHOULD pre <t>When adding a value to the Cache-Status header field, caches <bcp14>SHO
serve the existing field value, to allow debugging of the entire chain of caches ULD</bcp14> preserve the existing field value, to allow debugging of the entire
handling the request.</t> chain of caches handling the request.</t>
<t>Each list member identifies the cache that inserted it and this identif <t>Each List member identifies the cache that inserted it, and this identifier <
ier MUST be a String or Token. Depending on the deployment, this might be a prod bcp14>MUST</bcp14> be a String or Token. Depending on the deployment, this might
uct or service name (e.g., ExampleCache or "Example CDN"), a hostname ("cache-3. be a product or service name (e.g., "ExampleCache" or "Example CDN"), a hostnam
example.com"), an IP address, or a generated string.</t> e ("cache-3.example.com"), an IP address, or a generated string.</t>
<t>Each member of the list can have parameters that describe that cache's
handling of the request. While these parameters are OPTIONAL, caches are encoura <t>Each member of the list can have parameters that describe that cache's
ged to provide as much information as possible.</t> handling of the request. While these parameters are <bcp14>OPTIONAL</bcp14>, cac
<t>This specification defines the following parameters:</t> hes are encouraged to provide as much information as possible.</t>
<sourcecode type="abnf"><![CDATA[ <t>This specification defines the following parameters.</t>
hit = sf-boolean
fwd = sf-token
fwd-status = sf-integer
ttl = sf-integer
stored = sf-boolean
collapsed = sf-boolean
key = sf-string
detail = sf-token / sf-string
]]></sourcecode>
<section anchor="the-hit-parameter" numbered="true" toc="default"> <section anchor="the-hit-parameter" numbered="true" toc="default">
<name>The hit parameter</name> <name>The hit Parameter</name>
<t>"hit", when true, indicates that the request was satisfied by the cac <t>The value of "hit" is a Boolean that, when true, indicates that the request
he; i.e., it was not forwarded, and the response was obtained from the cache.</t was satisfied by the cache; that is, it was not forwarded, and the response was
> obtained from the cache.</t>
<t>A response that was originally produced by the origin but was modifie d by the cache (for example, a 304 or 206 status code) is still considered a hit , as long as it did not go forward (e.g., for validation).</t> <t>A response that was originally produced by the origin but was modifie d by the cache (for example, a 304 or 206 status code) is still considered a hit , as long as it did not go forward (e.g., for validation).</t>
<t>A response that was in cache but not able to be used without going fo rward (e.g., because it was stale, or partial) is not considered a hit. Note tha t a stale response that is used without going forward (e.g., because the origin server is not available) can be considered a hit.</t> <t>A response that was in cache but not able to be used without going fo rward (e.g., because it was stale or partial) is not considered a hit. Note that a stale response that is used without going forward (e.g., because the origin s erver is not available) can be considered a hit.</t>
<t>"hit" and "fwd" are exclusive; only one of them should appear on each list member.</t> <t>"hit" and "fwd" are exclusive; only one of them should appear on each list member.</t>
</section> </section>
<section anchor="the-fwd-parameter" numbered="true" toc="default"> <section anchor="the-fwd-parameter" numbered="true" toc="default">
<name>The fwd parameter</name> <name>The fwd Parameter</name>
<t>"fwd" indicates that the request went forward towards the origin, and <t>"fwd", when present, indicates that the request went forward towards the ori
why.</t> gin; its value is a Token that indicates why.</t>
<t>The following parameter values are defined to explain why the request went forward, from most specific to least:</t> <t>The following parameter values are defined to explain why the request went forward, from most specific to least:</t>
<ul spacing="normal"> <dl spacing="normal">
<li>bypass - The cache was configured to not handle this request</li> <dt>bypass:</dt>
<li>method - The request method's semantics require the request to be <dd>The cache was configured to not handle this request.</dd>
forwarded</li> <dt>method:</dt>
<li>uri-miss - The cache did not contain any responses that matched th <dd>The request method's semantics require the request to be forwarded
e request URI</li> .</dd>
<li>vary-miss - The cache contained a response that matched the reques <dt>uri-miss:</dt>
t URI, but could not select a response based upon this request's headers and sto <dd>The cache did not contain any responses that matched the request U
red Vary headers.</li> RI.</dd>
<li>miss - The cache did not contain any responses that could be used <dt>vary-miss:</dt>
to satisfy this request (to be used when an implementation cannot distinguish be <dd>The cache contained a response that matched the request URI, but i
tween uri-miss and vary-miss)</li> t could not select a response based upon this request's header fields and stored
<li>request - The cache was able to select a fresh response for the re Vary header fields.</dd>
quest, but the request's semantics (e.g., Cache-Control request directives) did <dt>miss:</dt>
not allow its use</li> <dd>The cache did not contain any responses that could be used to sati
<li>stale - The cache was able to select a response for the request, b sfy this request (to be used when an implementation cannot distinguish between u
ut it was stale</li> ri-miss and vary-miss).</dd>
<li>partial - The cache was able to select a partial response for the <dt>request:</dt>
request, but it did not contain all of the requested ranges (or the request was <dd>The cache was able to select a fresh response for the request, but
for the complete response)</li> the request's semantics (e.g., Cache-Control request directives) did not allow
</ul> its use.</dd>
<t>The most specific reason that the cache is aware of SHOULD be used, t <dt>stale:</dt>
o the extent that it is possible to implement. See also <xref section="4" sectio <dd>The cache was able to select a response for the request, but it wa
nFormat="comma" target="HTTP-CACHING" format="default"/>.</t> s stale.</dd>
<dt>partial:</dt>
<dd>The cache was able to select a partial response for the request, b
ut it did not contain all of the requested ranges (or the request was for the co
mplete response).</dd>
</dl>
<t>The most specific reason known to the cache <bcp14>SHOULD</bcp14> be
used, to the extent that it is possible to implement. See also <xref section="4"
sectionFormat="comma" target="HTTP-CACHING" format="default"/>.</t>
</section> </section>
<section anchor="the-fwd-status-parameter" numbered="true" toc="default"> <section anchor="the-fwd-status-parameter" numbered="true" toc="default">
<name>The fwd-status parameter</name> <name>The fwd-status Parameter</name>
<t>"fwd-status" indicates what status code (see <xref section="15" secti <t>The value of "fwd-status" is an Integer that indicates which status code (s
onFormat="comma" target="HTTP" format="default"/>) the next hop server returned ee <xref section="15" sectionFormat="comma" target="HTTP"/>) the next-hop server
in response to the forwarded request. Only meaningful when "fwd" is present; if returned in response to the forwarded request. The fwd-status parameter is only
"fwd-status" is not present but "fwd" is, it defaults to the status code sent in meaningful when fwd is present. If fwd-status is not present but the fwd parame
the response.</t> ter is, it defaults to the status code sent in the response.</t>
<t>This parameter is useful to distinguish cases when the next hop serve <t>This parameter is useful to distinguish cases when the next-hop serve
r sends a 304 Not Modified response to a conditional request, or a 206 Partial R r sends a 304 (Not Modified) response to a conditional request or a 206 (Partial
esponse because of a range request.</t> Content) response because of a range request.</t>
</section> </section>
<section anchor="the-ttl-parameter" numbered="true" toc="default"> <section anchor="the-ttl-parameter" numbered="true" toc="default">
<name>The ttl parameter</name> <name>The ttl Parameter</name>
<t>"ttl" indicates the response's remaining freshness lifetime (see <xre <t>The value of "ttl" is an Integer that indicates the response's remaini
f section="4.2.1" sectionFormat="comma" target="HTTP-CACHING" format="default"/> ng freshness lifetime (see <xref section="4.2.1" sectionFormat="comma" target="H
) as calculated by the cache, as an integer number of seconds, measured as close TTP-CACHING"/>) as calculated by the cache, as an integer number of seconds, mea
ly as possible to when the response header section is sent by the cache. This in sured as closely as possible to when the response header section is sent by the
cludes freshness assigned by the cache; e.g., through heuristics (see <xref sect cache. This includes freshness assigned by the cache through, for example, heuri
ion="4.2.2" sectionFormat="comma" target="HTTP-CACHING" format="default"/>), loc stics (see <xref section="4.2.2" sectionFormat="comma" target="HTTP-CACHING"/>),
al configuration, or other factors. May be negative, to indicate staleness.</t> local configuration, or other factors. It may be negative, to indicate stalenes
s.</t>
</section> </section>
<section anchor="the-stored-parameter" numbered="true" toc="default"> <section anchor="the-stored-parameter" numbered="true" toc="default">
<name>The stored parameter</name> <name>The stored Parameter</name>
<t>"stored" indicates whether the cache stored the response (see <xref s <t>The value of "stored" is a Boolean that indicates whether the cache stored th
ection="3" sectionFormat="comma" target="HTTP-CACHING" format="default"/>); a tr e response (see <xref section="3" sectionFormat="comma" target="HTTP-CACHING"/>)
ue value indicates that it did. Only meaningful when fwd is present.</t> ; a true value indicates that it did. The stored parameter is only meaningful wh
en fwd is present.</t>
</section> </section>
<section anchor="the-collapsed-parameter" numbered="true" toc="default"> <section anchor="the-collapsed-parameter" numbered="true" toc="default">
<name>The collapsed parameter</name> <name>The collapsed Parameter</name>
<t>"collapsed" indicates whether this request was collapsed together wit <t>The value of "collapsed" is a Boolean that indicates whether this request wa
h one or more other forward requests (see <xref section="4" sectionFormat="comma s collapsed together with one or more other forward requests (see <xref section=
" target="HTTP-CACHING" format="default"/>); if true, the response was successfu "4" sectionFormat="comma" target="HTTP-CACHING"/>). If true, the response was su
lly reused; if not, a new request had to be made. If not present, the request wa ccessfully reused; if not, a new request had to be made. If not present, the req
s not collapsed with others. Only meaningful when fwd is present.</t> uest was not collapsed with others. The collapsed parameter is only meaningful w
hen fwd is present.</t>
</section> </section>
<section anchor="the-key-parameter" numbered="true" toc="default"> <section anchor="the-key-parameter" numbered="true" toc="default">
<name>The key parameter</name> <name>The key Parameter</name>
<t>"key" conveys a representation of the cache key (see <xref section="2 <t>The value of "key" is a String that conveys a representation of the cache ke
" sectionFormat="comma" target="HTTP-CACHING" format="default"/>) used for the r y (see <xref section="2" sectionFormat="comma" target="HTTP-CACHING"/>) used for
esponse. Note that this may be implementation-specific.</t> the response. Note that this may be implementation specific.</t>
</section> </section>
<section anchor="the-detail-parameter" numbered="true" toc="default"> <section anchor="the-detail-parameter" numbered="true" toc="default">
<name>The detail parameter</name> <name>The detail Parameter</name>
<t>"detail" allows implementations to convey additional information not <t>The value of "detail" is either a String or a Token that allows imple
captured in other parameters; for example, implementation-specific states, or ot mentations to convey additional information not captured in other parameters, su
her caching-related metrics.</t> ch as implementation-specific states or other caching-related metrics.</t>
<t>For example:</t> <t>For example:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; detail=MEMORY Cache-Status: ExampleCache; hit; detail=MEMORY
]]></sourcecode> ]]></sourcecode>
<t>The semantics of a detail parameter are always specific to the cache that sent it; even if a member of details from another cache shares the same nam e, it might not mean the same thing.</t> <t>The semantics of a detail parameter are always specific to the cache that sent it; even if a details parameter from another cache shares the same val ue, it might not mean the same thing.</t>
<t>This parameter is intentionally limited. If an implementation's devel oper or operator needs to convey additional information in an interoperable fash ion, they are encouraged to register extension parameters (see <xref target="reg ister" format="default"/>) or define another header field.</t> <t>This parameter is intentionally limited. If an implementation's devel oper or operator needs to convey additional information in an interoperable fash ion, they are encouraged to register extension parameters (see <xref target="reg ister" format="default"/>) or define another header field.</t>
</section> </section>
</section> </section>
<section anchor="examples" numbered="true" toc="default"> <section anchor="examples" numbered="true" toc="default">
<name>Examples</name> <name>Examples</name>
<t>The most minimal cache hit:</t> <t>The following is an example of a minimal cache hit:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit Cache-Status: ExampleCache; hit
]]></sourcecode> ]]></sourcecode>
<t>... but a polite cache will give some more information, e.g.:</t> <t>However, a polite cache will give some more information, e.g.:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; ttl=376 Cache-Status: ExampleCache; hit; ttl=376
]]></sourcecode> ]]></sourcecode>
<t>A stale hit just has negative freshness:</t> <t>A stale hit just has negative freshness, as in this example:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; ttl=-412 Cache-Status: ExampleCache; hit; ttl=-412
]]></sourcecode> ]]></sourcecode>
<t>Whereas a complete miss is:</t> <t>Whereas this is an example of a complete miss:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss Cache-Status: ExampleCache; fwd=uri-miss
]]></sourcecode> ]]></sourcecode>
<t>A miss that successfully validated on the back-end server:</t> <t>This is an example of a miss that successfully validated on the backend server:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=stale; fwd-status=304 Cache-Status: ExampleCache; fwd=stale; fwd-status=304
]]></sourcecode> ]]></sourcecode>
<t>A miss that was collapsed with another request:</t> <t>This is an example of a miss that was collapsed with another request:</ t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss; collapsed Cache-Status: ExampleCache; fwd=uri-miss; collapsed
]]></sourcecode> ]]></sourcecode>
<t>A miss that the cache attempted to collapse, but couldn't:</t> <t>This is an example of a miss that the cache attempted to collapse, but couldn't:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0 Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0
]]></sourcecode> ]]></sourcecode>
<t>Going through two separate layers of caching, where the cache closest t o the origin responded to an earlier request with a stored response, and a secon d cache stored that response and later reused it to satisfy the current request: </t> <t>The following is an example of going through two separate layers of cac hing, where the cache closest to the origin responded to an earlier request with a stored response, and a second cache stored that response and later reused it to satisfy the current request:</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: OriginCache; hit; ttl=1100, Cache-Status: OriginCache; hit; ttl=1100,
"CDN Company Here"; hit; ttl=545 "CDN Company Here"; hit; ttl=545
]]></sourcecode> ]]></sourcecode>
<t>Going through a three-layer caching system, where the closest to the or igin is a reverse proxy (where the response was served from cache), the next is a forward proxy interposed by the network (where the request was forwarded becau se there wasn't any response cached with its URI, the request was collapsed with others, and the resulting response was stored), and the closest to the user is a browser cache (where there wasn't any response cached with the request's URI): </t> <t>The following is an example of going through a three-layer caching syst em, where the closest to the origin is a reverse proxy (where the response was s erved from cache); the next is a forward proxy interposed by the network (where the request was forwarded because there wasn't any response cached with its URI, the request was collapsed with others, and the resulting response was stored); and the closest to the user is a browser cache (where there wasn't any response cached with the request's URI):</t>
<sourcecode type="http-message"><![CDATA[ <sourcecode type="http-message"><![CDATA[
Cache-Status: ReverseProxyCache; hit Cache-Status: ReverseProxyCache; hit
Cache-Status: ForwardProxyCache; fwd=uri-miss; collapsed; stored Cache-Status: ForwardProxyCache; fwd=uri-miss; collapsed; stored
Cache-Status: BrowserCache; fwd=uri-miss Cache-Status: BrowserCache; fwd=uri-miss
]]></sourcecode> ]]></sourcecode>
</section> </section>
<section anchor="register" numbered="true" toc="default"> <section anchor="register" numbered="true" toc="default">
<name>Defining New Cache-Status Parameters</name> <name>Defining New Cache-Status Parameters</name>
<t>New Cache-Status Parameters can be defined by registering them in the H <t>New Cache-Status parameters can be defined by registering them in the "
TTP Cache-Status Parameters registry.</t> HTTP Cache-Status" registry.</t>
<t>Registration requests are reviewed and approved by a Designated Expert, <t>Registration requests are reviewed and approved by a designated expert,
as per <xref section="4.5" sectionFormat="comma" target="RFC8126" format="defau per <xref section="4.5" sectionFormat="comma" target="RFC8126" format="default"
lt"/>. A specification document is appreciated, but not required.</t> />. A specification document is appreciated but not required.</t>
<t>The Expert(s) should consider the following factors when evaluating req <t>The expert(s) should consider the following factors when evaluating req
uests:</t> uests:</t>
<ul spacing="normal"> <ul spacing="normal">
<li>Community feedback</li> <li>Community feedback</li>
<li>If the value is sufficiently well-defined</li> <li>If the value is sufficiently well defined</li>
<li>Generic parameters are preferred over vendor-specific, application-s <li>Generic parameters are preferred over vendor-specific, application-s
pecific, or deployment-specific values. If a generic value cannot be agreed upon pecific, or deployment-specific values. If a generic value cannot be agreed upon
in the community, the parameter's name should be correspondingly specific (e.g. in the community, the parameter's name should be correspondingly specific (e.g.
, with a prefix that identifies the vendor, application or deployment).</li> , with a prefix that identifies the vendor, application, or deployment).</li>
</ul> </ul>
<t>Registration requests should use the following template:</t> <t>Registration requests should use the following template:</t>
<ul spacing="normal"> <dl spacing="normal">
<li>Name: [a name for the Cache-Status Parameter that matches the 'key' <dt>Name:</dt>
ABNF rule]</li> <dd>[a name for the Cache-Status parameter's key; see <xref section="3.1
<li>Description: [a description of the parameter semantics and value]</l .2" sectionFormat="of" target="STRUCTURED-FIELDS"/> for syntactic requirements]<
i> /dd>
<li>Reference: [to a specification defining this parameter, if available <dt>Type:</dt><dd>[the Structured Type of the parameter's value; see <xre
]</li> f section="3.1.2" sectionFormat="of" target="STRUCTURED-FIELDS"/>]</dd>
</ul> <dt>Description:</dt>
<t>See the registry at <eref target="https://iana.org/assignments/http-cac <dd>[a description of the parameter's semantics]</dd>
he-status">https://iana.org/assignments/http-cache-status</eref> for details on <dt>Reference:</dt>
where to send registration requests.</t> <dd>[to a specification defining this parameter, if available]</dd>
</dl>
<t>See the registry at <eref brackets="angle" target="https://www.iana.org
/assignments/http-cache-status"></eref> for details on where to send registratio
n requests.</t>
</section> </section>
<section anchor="iana-considerations" numbered="true" toc="default"> <section anchor="iana-considerations" numbered="true" toc="default">
<name>IANA Considerations</name> <name>IANA Considerations</name>
<t>Upon publication, please create the HTTP Cache-Status Parameters regist <t>IANA has created the "HTTP Cache-Status" registry at <eref brackets="an
ry at <eref target="https://iana.org/assignments/http-cache-status">https://iana gle" target="https://www.iana.org/assignments/http-cache-status"></eref> and pop
.org/assignments/http-cache-status</eref> and populate it with the types defined ulated it with the types defined in <xref target="field" format="default"/>; see
in <xref target="field" format="default"/>; see <xref target="register" format= <xref target="register" format="default"/> for its associated procedures.</t>
"default"/> for its associated procedures.</t> <t>IANA has added the following entry in the "Hypertext Transfer Protocol
<t>Also, please create the following entry in the Hypertext Transfer Proto (HTTP) Field Name Registry" defined in <xref section="18.4" sectionFormat="comma
col (HTTP) Field Name Registry defined in <xref section="18.4" sectionFormat="co " target="HTTP" format="default"/>:</t>
mma" target="HTTP" format="default"/>:</t> <dl spacing="compact">
<ul spacing="normal"> <dt>Field name:</dt>
<li>Field name: Cache-Status</li> <dd>Cache-Status</dd>
<li>Status: permanent</li> <dt>Status:</dt>
<li>Specification document: [this document]</li> <dd>permanent</dd>
<li>Comments:</li> <dt>Reference:</dt>
</ul> <dd>RFC 9211</dd>
</dl>
</section> </section>
<section anchor="security" numbered="true" toc="default"> <section anchor="security" numbered="true" toc="default">
<name>Security Considerations</name> <name>Security Considerations</name>
<t>Attackers can use the information in Cache-Status to probe the behaviou r of the cache (and other components), and infer the activity of those using the cache. The Cache-Status header field may not create these risks on its own, but can assist attackers in exploiting them.</t> <t>Attackers can use the information in Cache-Status to probe the behavior of the cache (and other components) and infer the activity of those using the c ache. The Cache-Status header field may not create these risks on its own, but i t can assist attackers in exploiting them.</t>
<t>For example, knowing if a cache has stored a response can help an attac ker execute a timing attack on sensitive data.</t> <t>For example, knowing if a cache has stored a response can help an attac ker execute a timing attack on sensitive data.</t>
<t>Additionally, exposing the cache key can help an attacker understand mo difications to the cache key, which may assist cache poisoning attacks. See <xre f target="ENTANGLE" format="default"/> for details.</t> <t>Additionally, exposing the cache key can help an attacker understand mo difications to the cache key, which may assist cache poisoning attacks. See <xre f target="ENTANGLE" format="default"/> for details.</t>
<t>The underlying risks can be mitigated with a variety of techniques (e.g <t>The underlying risks can be mitigated with a variety of techniques (e.g
., use of encryption and authentication; avoiding the inclusion of attacker-cont ., using encryption and authentication and avoiding the inclusion of attacker-co
rolled data in the cache key), depending on their exact nature. Note that merely ntrolled data in the cache key), depending on their exact nature. Note that mere
obfuscating the key does not mitigate this risk.</t> ly obfuscating the key does not mitigate this risk.</t>
<t>To avoid assisting such attacks, the Cache-Status header field can be o <t>To avoid assisting such attacks, the Cache-Status header field can be o
mitted, only sent when the client is authorized to receive it, or only send sens mitted, only sent when the client is authorized to receive it, or sent with sens
itive information (e.g., the key parameter) when the client is authorized.</t> itive information (e.g., the key parameter) only when the client is authorized.<
/t>
</section> </section>
</middle> </middle>
<back> <back>
<references> <references>
<name>References</name> <name>References</name>
<references> <references>
<name>Normative References</name> <name>Normative References</name>
<reference anchor="RFC2119"> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
<front> FC.2119.xml"/>
<title>Key words for use in RFCs to Indicate Requirement Levels</tit <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
le> FC.8126.xml"/>
<author fullname="S. Bradner" initials="S." surname="Bradner">
<organization/> <reference anchor="STRUCTURED-FIELDS" target="https://www.rfc-editor.org
</author> /info/rfc8941">
<date month="March" year="1997"/>
<abstract>
<t>In many standards track documents several words are used to sig
nify the requirements in the specification. These words are often capitalized.
This document defines these words as they should be interpreted in IETF document
s. This document specifies an Internet Best Current Practices for the Internet
Community, and requests discussion and suggestions for improvements.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
<seriesInfo name="DOI" value="10.17487/RFC2119"/>
</reference>
<reference anchor="RFC8126">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs
</title>
<author fullname="M. Cotton" initials="M." surname="Cotton">
<organization/>
</author>
<author fullname="B. Leiba" initials="B." surname="Leiba">
<organization/>
</author>
<author fullname="T. Narten" initials="T." surname="Narten">
<organization/>
</author>
<date month="June" year="2017"/>
<abstract>
<t>Many protocols make use of points of extensibility that use con
stants to identify various protocol parameters. To ensure that the values in th
ese fields do not have conflicting uses and to promote interoperability, their a
llocations are often coordinated by a central record keeper. For IETF protocols
, that role is filled by the Internet Assigned Numbers Authority (IANA).</t>
<t>To make assignments in a given registry prudently, guidance des
cribing the conditions under which new values should be assigned, as well as whe
n and how modifications to existing values can be made, is needed. This documen
t defines a framework for the documentation of these guidelines by specification
authors, in order to assure that the provided guidance for the IANA Considerati
ons is clear and addresses the various issues that are likely in the operation o
f a registry.</t>
<t>This is the third edition of this document; it obsoletes RFC 52
26.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="26"/>
<seriesInfo name="RFC" value="8126"/>
<seriesInfo name="DOI" value="10.17487/RFC8126"/>
</reference>
<reference anchor="STRUCTURED-FIELDS">
<front> <front>
<title>Structured Field Values for HTTP</title> <title>Structured Field Values for HTTP</title>
<author fullname="M. Nottingham" initials="M." surname="Nottingham"> <author fullname="M. Nottingham" initials="M." surname="Nottingham">
<organization/> <organization/>
</author> </author>
<author fullname="P-H. Kamp" initials="P-H." surname="Kamp"> <author fullname="P-H. Kamp" initials="P-H." surname="Kamp">
<organization/> <organization/>
</author> </author>
<date month="February" year="2021"/> <date month="February" year="2021"/>
<abstract>
<t>This document describes a set of data types and associated algo
rithms that are intended to make it easier and safer to define and handle HTTP h
eader and trailer fields, known as "Structured Fields", "Structured Headers", or
"Structured Trailers". It is intended for use by specifications of new HTTP fie
lds that wish to use a common syntax that is more restrictive than traditional H
TTP field values.</t>
</abstract>
</front> </front>
<seriesInfo name="RFC" value="8941"/> <seriesInfo name="RFC" value="8941"/>
<seriesInfo name="DOI" value="10.17487/RFC8941"/> <seriesInfo name="DOI" value="10.17487/RFC8941"/>
</reference> </reference>
<reference anchor="HTTP">
<!-- [HTTP] [I.D.ietf-httpbis-semantics] in AUTH48; companion document RFC 9110
-->
<reference anchor='HTTP' target="https://www.rfc-editor.org/info/rfc9110
">
<front> <front>
<title>HTTP Semantics</title> <title>HTTP Semantics</title>
<author fullname="Roy T. Fielding"> <author initials='R' surname='Fielding' fullname='Roy Fielding' role
<organization>Adobe</organization> ="editor">
<organization />
</author> </author>
<author fullname="Mark Nottingham"> <author initials='M' surname='Nottingham' fullname='Mark Nottingham'
<organization>Fastly</organization> role="editor">
<organization />
</author> </author>
<author fullname="Julian Reschke"> <author initials='J' surname='Reschke' fullname='Julian Reschke' rol
<organization>greenbytes GmbH</organization> e="editor">
<organization />
</author> </author>
<date day="25" month="July" year="2021"/> <date year='2022' month='June' />
<abstract>
<t> The Hypertext Transfer Protocol (HTTP) is a stateless applic
ation-
level protocol for distributed, collaborative, hypertext information
systems. This document describes the overall architecture of HTTP,
establishes common terminology, and defines aspects of the protocol
that are shared by all versions. In this definition are core
protocol elements, extensibility mechanisms, and the "http" and
"https" Uniform Resource Identifier (URI) schemes.
This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC
7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions
of RFC 7230.
</t>
</abstract>
</front> </front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-semantics- <seriesInfo name="STD" value="97"/>
17"/> <seriesInfo name="RFC" value="9110"/>
<seriesInfo name="DOI" value="10.17487/RFC9110"/>
</reference> </reference>
<reference anchor="HTTP-CACHING">
<front>
<title>HTTP Caching</title>
<author fullname="Roy T. Fielding">
<organization>Adobe</organization>
</author>
<author fullname="Mark Nottingham">
<organization>Fastly</organization>
</author>
<author fullname="Julian Reschke">
<organization>greenbytes GmbH</organization>
</author>
<date day="25" month="July" year="2021"/>
<abstract>
<t> The Hypertext Transfer Protocol (HTTP) is a stateless applic
ation-
level protocol for distributed, collaborative, hypertext information
systems. This document defines HTTP caches and the associated header
fields that control cache behavior or indicate cacheable response
messages.
This document obsoletes RFC 7234.
</t> <!-- [HTTP-CACHING] [I.D.ietf-httpbis-cache] in AUTH48; companion document RFC 9
</abstract> 111 -->
</front> <reference anchor='HTTP-CACHING' target="https://www.rfc-editor.org/info
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-cache-17"/ /rfc9111">
>
</reference>
<reference anchor="RFC8174">
<front> <front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</ti <title>HTTP Caching</title>
tle> <author initials='R' surname='Fielding' fullname='Roy Fielding' role
<author fullname="B. Leiba" initials="B." surname="Leiba"> ="editor">
<organization/> <organization />
</author> </author>
<date month="May" year="2017"/> <author initials='M' surname='Nottingham' fullname='Mark Nottingham'
<abstract> role="editor">
<t>RFC 2119 specifies common key words that may be used in protoco <organization />
l specifications. This document aims to reduce the ambiguity by clarifying tha
t only UPPERCASE usage of the key words have the defined special meanings.</t>
</abstract>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="8174"/>
<seriesInfo name="DOI" value="10.17487/RFC8174"/>
</reference>
<reference anchor="RFC5234">
<front>
<title>Augmented BNF for Syntax Specifications: ABNF</title>
<author fullname="D. Crocker" initials="D." role="editor" surname="C
rocker">
<organization/>
</author> </author>
<author fullname="P. Overell" initials="P." surname="Overell"> <author initials='J' surname='Reschke' fullname='Julian Reschke' rol
<organization/> e="editor">
<organization />
</author> </author>
<date month="January" year="2008"/> <date year='2022' month='June' />
<abstract>
<t>Internet technical specifications often need to define a formal
syntax. Over the years, a modified version of Backus-Naur Form (BNF), called A
ugmented BNF (ABNF), has been popular among many Internet specifications. The c
urrent specification documents ABNF. It balances compactness and simplicity with
reasonable representational power. The differences between standard BNF and AB
NF involve naming rules, repetition, alternatives, order-independence, and value
ranges. This specification also supplies additional rule definitions and encod
ing for a core lexical analyzer of the type common to several Internet specifica
tions. [STANDARDS-TRACK]</t>
</abstract>
</front> </front>
<seriesInfo name="STD" value="68"/> <seriesInfo name="STD" value="98"/>
<seriesInfo name="RFC" value="5234"/> <seriesInfo name="RFC" value="9111"/>
<seriesInfo name="DOI" value="10.17487/RFC5234"/> <seriesInfo name="DOI" value="10.17487/RFC9111"/>
</reference> </reference>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.8174.xml"/>
</references> </references>
<references> <references>
<name>Informative References</name> <name>Informative References</name>
<reference anchor="ENTANGLE" target="https://i.blackhat.com/USA-20/Wedne sday/us-20-Kettle-Web-Cache-Entanglement-Novel-Pathways-To-Poisoning-wp.pdf"> <reference anchor="ENTANGLE" target="https://portswigger.net/research/we b-cache-entanglement">
<front> <front>
<title>Web Cache Entanglement: Novel Pathways to Poisoning</title> <title>Web Cache Entanglement: Novel Pathways to Poisoning</title>
<author initials="J." surname="Kettle" fullname="James Kettle"> <author initials="J." surname="Kettle" fullname="James Kettle">
<organization>PortSwigger</organization> <organization>PortSwigger</organization>
</author> </author>
<date year="2020"/> <date month="September" year="2020"/>
</front> </front>
</reference> </reference>
</references> </references>
</references> </references>
</back> </back>
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