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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-httpbis-cache-header-10" category="std" number="9211" obsoletes="" updates="" submissionType="IETF" category="std" consensus="true" tocInclude="true" sortRefs="true"  symRefs="true" xml:lang="en" version="3">
  <!-- xml2rfc v2v3 conversion 3.9.1 -->
  <front>
    <title abbrev="Cache-Status Header">The Cache-Status HTTP Response Header Field</title>
    <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-cache-header-10"/> name="RFC" value="9211"/>
    <author initials="M." surname="Nottingham" fullname="Mark Nottingham">
      <organization>Fastly</organization>
      <address>
        <postal>
          <city>Prahran</city>
          <region>VIC</region>
          <country>Australia</country>
          <postalLine>Prahran</postalLine>
          <postalLine>Australia</postalLine>
        </postal>
        <email>mnot@mnot.net</email>
        <uri>https://www.mnot.net/</uri>
      </address>
    </author>
    <date/>
    <date year="2022" month="June" />
    <area>Applications and Real-Time</area>
    <workgroup>HTTP</workgroup>
    <keyword>http</keyword>
    <keyword>cache</keyword>
    <keyword>debugging</keyword>
    <keyword>x-cache</keyword>
    <abstract>
      <t>To aid debugging, HTTP caches often append header fields to a response 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>
    <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-http-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/cache-header">https://github.com/httpwg/http-extensions/labels/cache-header</eref>.</t>
    </note>
  </front>
  <middle>
    <section anchor="introduction" numbered="true" toc="default">
      <name>Introduction</name>
      <t>To aid debugging (both by humans and automated tools), HTTP caches often append header fields to a response explaining how they handled the request. Unfortunately, the semantics of these headers header fields are often unclear, and both the semantics and syntax used vary between implementations.</t>
      <t>This specification defines a new HTTP response header field, "Cache-Status" "Cache-Status", for this purpose, purpose with standardized syntax and semantics.</t>
      <section anchor="notational-conventions" numbered="true" toc="default">
        <name>Notational Conventions</name>
        <t>The
        <t>
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and "OPTIONAL" "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119" format="default"/> target="RFC2119"/> <xref target="RFC8174" format="default"/> target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t> here.
        </t>

  <t>This document uses ABNF as defined in <xref target="RFC5234" format="default"/>, with rules prefixed with "sf-" and the "key" rule as defined in following terminology from <xref target="STRUCTURED-FIELDS" format="default"/>. It section="3" sectionFormat="of" target="STRUCTURED-FIELDS"/> to specify syntax and parsing: List, String, Token, Integer, and Boolean.</t>
        <t>This document also uses terminology from <xref target="HTTP" format="default"/> target="HTTP"/> and <xref target="HTTP-CACHING" format="default"/>.</t> target="HTTP-CACHING"/>.</t>
      </section>
    </section>
    <section anchor="field" numbered="true" toc="default">
      <name>The Cache-Status HTTP Response Header Field</name>
      <t>The Cache-Status HTTP response header field indicates how caches have handled that response and its corresponding request. The syntax of this header field conforms to <xref target="STRUCTURED-FIELDS" format="default"/>.</t>
  <t>Its value is a List (<xref section="3.1" sectionFormat="comma" target="STRUCTURED-FIELDS" format="default"/>):</t>
      <sourcecode type="abnf"><![CDATA[
Cache-Status   = sf-list
]]></sourcecode>
      <t>Each List. Each member of the list 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 (possibly including the user agent's cache itself, 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 only do so when specifically configured to, or when the request contains a header field that activates a debugging mode. See <xref target="security" format="default"/> for related security considerations.</t>
      <t>An intermediary SHOULD NOT <bcp14>SHOULD NOT</bcp14> append a Cache-Status member to responses that it generates locally, even if that intermediary contains a cache, unless the generated response is based upon a stored response (e.g., 304 Not Modified (Not Modified) and 206 Partial Content (Partial Content) are both based upon a stored response). For example, a proxy generating a 400 response due to a malformed request will not add a Cache-Status value, because that response was generated by the proxy, not the origin server.</t>
      <t>When adding a value to the Cache-Status header field, caches SHOULD <bcp14>SHOULD</bcp14> preserve the existing field value, to allow debugging of the entire chain of caches handling the request.</t>
<t>Each list List member identifies the cache that inserted it it, and this identifier MUST <bcp14>MUST</bcp14> be a String or Token. Depending on the deployment, this might be a product or service name (e.g., ExampleCache "ExampleCache" or "Example CDN"), a hostname ("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, <bcp14>OPTIONAL</bcp14>, caches are encouraged to provide as much information as possible.</t>
      <t>This specification defines the following parameters:</t>
      <sourcecode type="abnf"><![CDATA[
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> parameters.</t>
      <section anchor="the-hit-parameter" numbered="true" toc="default">
        <name>The hit parameter</name>
        <t>"hit", Parameter</name>
 <t>The value of "hit" is a Boolean that, when true, indicates that the request was satisfied by the cache; i.e., 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 modified 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 forward (e.g., because it was stale, stale or partial) is not considered a hit. Note that 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>"hit" and "fwd" are exclusive; only one of them should appear on each list member.</t>
      </section>
      <section anchor="the-fwd-parameter" numbered="true" toc="default">
        <name>The fwd parameter</name>
        <t>"fwd" Parameter</name>
 <t>"fwd", when present, indicates that the request went forward towards the origin, and origin; 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>
        <ul
        <dl spacing="normal">
          <li>bypass - The
          <dt>bypass:</dt>
          <dd>The cache was configured to not handle this request</li>
          <li>method - The request.</dd>
          <dt>method:</dt>
          <dd>The request method's semantics require the request to be forwarded</li>
          <li>uri-miss - The forwarded.</dd>
          <dt>uri-miss:</dt>
          <dd>The cache did not contain any responses that matched the request URI</li>
          <li>vary-miss - The URI.</dd>
          <dt>vary-miss:</dt>
          <dd>The cache contained a response that matched the request URI, but it could not select a response based upon this request's headers header fields and stored Vary headers.</li>
          <li>miss - The header fields.</dd>
          <dt>miss:</dt>
          <dd>The cache did not contain any responses that could be used to satisfy this request (to be used when an implementation cannot distinguish between uri-miss and vary-miss)</li>
          <li>request - The vary-miss).</dd>
          <dt>request:</dt>
          <dd>The cache was able to select a fresh response for the request, but the request's semantics (e.g., Cache-Control request directives) did not allow its use</li>
          <li>stale - The use.</dd>
          <dt>stale:</dt>
          <dd>The cache was able to select a response for the request, but it was stale</li>
          <li>partial - The stale.</dd>
          <dt>partial:</dt>
          <dd>The cache was able to select a partial response for the request, but it did not contain all of the requested ranges (or the request was for the complete response)</li>
        </ul> response).</dd>
        </dl>
        <t>The most specific reason that known to the cache is aware of SHOULD <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 anchor="the-fwd-status-parameter" numbered="true" toc="default">
        <name>The fwd-status parameter</name>
        <t>"fwd-status" Parameter</name>
  <t>The value of "fwd-status" is an Integer that indicates what which status code (see <xref section="15" sectionFormat="comma" target="HTTP" format="default"/>) target="HTTP"/>) the next hop next-hop server returned in response to the forwarded request. Only The fwd-status parameter is only meaningful when "fwd" fwd is present; if "fwd-status" present. If fwd-status is not present but "fwd" the fwd parameter is, it defaults to the status code sent in the response.</t>
        <t>This parameter is useful to distinguish cases when the next hop next-hop server sends a 304 Not Modified (Not Modified) response to a conditional request, request or a 206 Partial Response (Partial Content) response because of a range request.</t>
      </section>
      <section anchor="the-ttl-parameter" numbered="true" toc="default">
        <name>The ttl parameter</name>
        <t>"ttl" Parameter</name>
	<t>The value of "ttl" is an Integer that indicates the response's remaining freshness lifetime (see <xref section="4.2.1" sectionFormat="comma" target="HTTP-CACHING" format="default"/>) target="HTTP-CACHING"/>) as calculated by the cache, as an integer number of seconds, measured as closely as possible to when the response header section is sent by the cache. This includes freshness assigned by the cache; e.g., through cache through, for example, heuristics (see <xref section="4.2.2" sectionFormat="comma" target="HTTP-CACHING" format="default"/>), target="HTTP-CACHING"/>), local configuration, or other factors. May It may be negative, to indicate staleness.</t>
      </section>
      <section anchor="the-stored-parameter" numbered="true" toc="default">
        <name>The stored parameter</name>
        <t>"stored" Parameter</name>
<t>The value of "stored" is a Boolean that indicates whether the cache stored the response (see <xref section="3" sectionFormat="comma" target="HTTP-CACHING" format="default"/>); target="HTTP-CACHING"/>); a true value indicates that it did. Only The stored parameter is only meaningful when fwd is present.</t>
      </section>
      <section anchor="the-collapsed-parameter" numbered="true" toc="default">
        <name>The collapsed parameter</name>
        <t>"collapsed" Parameter</name>
 <t>The value of "collapsed" is a Boolean that indicates whether this request was collapsed together with one or more other forward requests (see <xref section="4" sectionFormat="comma" target="HTTP-CACHING" format="default"/>); if target="HTTP-CACHING"/>). If true, the response was successfully reused; if not, a new request had to be made. If not present, the request was not collapsed with others. Only The collapsed parameter is only meaningful when fwd is present.</t>
      </section>
      <section anchor="the-key-parameter" numbered="true" toc="default">
        <name>The key parameter</name>
        <t>"key" Parameter</name>
 <t>The value of "key" is a String that conveys a representation of the cache key (see <xref section="2" sectionFormat="comma" target="HTTP-CACHING" format="default"/>) target="HTTP-CACHING"/>) used for the response. Note that this may be implementation-specific.</t> implementation specific.</t>
      </section>
      <section anchor="the-detail-parameter" numbered="true" toc="default">
        <name>The detail parameter</name>
        <t>"detail" Parameter</name>
	 <t>The value of "detail" is either a String or a Token that allows implementations to convey additional information not captured in other parameters; for example, parameters, such as implementation-specific states, states or other caching-related metrics.</t>
        <t>For example:</t>
        <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; detail=MEMORY
]]></sourcecode>
        <t>The semantics of a detail parameter are always specific to the cache that sent it; even if a member of details parameter from another cache shares the same name, value, it might not mean the same thing.</t>
        <t>This parameter is intentionally limited. If an implementation's developer or operator needs to convey additional information in an interoperable fashion, they are encouraged to register extension parameters (see <xref target="register" format="default"/>) or define another header field.</t>
      </section>
    </section>
    <section anchor="examples" numbered="true" toc="default">
      <name>Examples</name>
      <t>The most following is an example of a minimal cache hit:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit
]]></sourcecode>
      <t>... but
      <t>However, a polite cache will give some more information, e.g.:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; ttl=376
]]></sourcecode>
      <t>A stale hit just has negative freshness:</t> freshness, as in this example:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; hit; ttl=-412
]]></sourcecode>
      <t>Whereas this is an example of a complete miss is:</t> miss:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss
]]></sourcecode>
      <t>A
      <t>This is an example of a miss that successfully validated on the back-end backend server:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=stale; fwd-status=304
]]></sourcecode>
      <t>A
      <t>This is an example of a miss that was collapsed with another request:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss; collapsed
]]></sourcecode>
      <t>A
      <t>This is an example of a miss that the cache attempted to collapse, but couldn't:</t>
      <sourcecode type="http-message"><![CDATA[
Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0
]]></sourcecode>
      <t>Going
      <t>The following is an example of going through two separate layers of caching, 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[
Cache-Status: OriginCache; hit; ttl=1100,
              "CDN Company Here"; hit; ttl=545
]]></sourcecode>
      <t>Going
      <t>The following is an example of going through a three-layer caching system, where the closest to the origin is a reverse proxy (where the response was served from cache), 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), 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[
Cache-Status: ReverseProxyCache; hit
Cache-Status: ForwardProxyCache; fwd=uri-miss; collapsed; stored
Cache-Status: BrowserCache; fwd=uri-miss
]]></sourcecode>
    </section>
    <section anchor="register" numbered="true" toc="default">
      <name>Defining New Cache-Status Parameters</name>
      <t>New Cache-Status Parameters parameters can be defined by registering them in the HTTP Cache-Status Parameters "HTTP Cache-Status" registry.</t>
      <t>Registration requests are reviewed and approved by a Designated Expert, as designated expert, per <xref section="4.5" sectionFormat="comma" target="RFC8126" format="default"/>. A specification document is appreciated, appreciated but not required.</t>
      <t>The Expert(s) expert(s) should consider the following factors when evaluating requests:</t>
      <ul spacing="normal">
        <li>Community feedback</li>
        <li>If the value is sufficiently well-defined</li> well defined</li>
        <li>Generic parameters are preferred over vendor-specific, application-specific, 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., with a prefix that identifies the vendor, application application, or deployment).</li>
      </ul>
      <t>Registration requests should use the following template:</t>
      <ul
      <dl spacing="normal">
        <li>Name: [a
        <dt>Name:</dt>
        <dd>[a name for the Cache-Status Parameter that matches parameter's key; see <xref section="3.1.2" sectionFormat="of" target="STRUCTURED-FIELDS"/> for syntactic requirements]</dd>
	<dt>Type:</dt><dd>[the Structured Type of the 'key' ABNF rule]</li>
        <li>Description: [a parameter's value; see <xref section="3.1.2" sectionFormat="of" target="STRUCTURED-FIELDS"/>]</dd>
        <dt>Description:</dt>
        <dd>[a description of the parameter semantics and value]</li>
        <li>Reference: [to parameter's semantics]</dd>
        <dt>Reference:</dt>
        <dd>[to a specification defining this parameter, if available]</li>
      </ul> available]</dd>
      </dl>
      <t>See the registry at <eref target="https://iana.org/assignments/http-cache-status">https://iana.org/assignments/http-cache-status</eref> brackets="angle" target="https://www.iana.org/assignments/http-cache-status"></eref> for details on where to send registration requests.</t>
    </section>
    <section anchor="iana-considerations" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>Upon publication, please create
      <t>IANA has created the HTTP Cache-Status Parameters "HTTP Cache-Status" registry at <eref target="https://iana.org/assignments/http-cache-status">https://iana.org/assignments/http-cache-status</eref> brackets="angle" target="https://www.iana.org/assignments/http-cache-status"></eref> and populate populated it with the types defined in <xref target="field" format="default"/>; see <xref target="register" format="default"/> for its associated procedures.</t>
      <t>Also, please create
      <t>IANA has added the following entry in the Hypertext "Hypertext Transfer Protocol (HTTP) Field Name Registry Registry" defined in <xref section="18.4" sectionFormat="comma" target="HTTP" format="default"/>:</t>
      <ul spacing="normal">
        <li>Field name: Cache-Status</li>
        <li>Status: permanent</li>
        <li>Specification document: [this document]</li>
        <li>Comments:</li>
      </ul>
      <dl spacing="compact">
        <dt>Field name:</dt>
        <dd>Cache-Status</dd>
        <dt>Status:</dt>
        <dd>permanent</dd>
        <dt>Reference:</dt>
        <dd>RFC 9211</dd>
      </dl>
    </section>
    <section anchor="security" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>Attackers can use the information in Cache-Status to probe the behaviour behavior of the cache (and other components), components) and infer the activity of those using the cache. The Cache-Status header field may not create these risks on its own, but it can assist attackers in exploiting them.</t>
      <t>For example, knowing if a cache has stored a response can help an attacker execute a timing attack on sensitive data.</t>
      <t>Additionally, exposing the cache key can help an attacker understand modifications to the cache key, which may assist cache poisoning attacks. See <xref target="ENTANGLE" format="default"/> for details.</t>
      <t>The underlying risks can be mitigated with a variety of techniques (e.g., use of using encryption and authentication; authentication and avoiding the inclusion of attacker-controlled data in the cache key), depending on their exact nature. Note that merely obfuscating the key does not mitigate this risk.</t>
      <t>To avoid assisting such attacks, the Cache-Status header field can be omitted, only sent when the client is authorized to receive it, or only send sent with sensitive information (e.g., the key parameter) only when the client is authorized.</t>
    </section>
  </middle>
  <back>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>

        <reference anchor="RFC2119">
          <front>
            <title>Key words for use in RFCs to Indicate Requirement Levels</title>
            <author fullname="S. Bradner" initials="S." surname="Bradner">
              <organization/>
            </author>
            <date month="March" year="1997"/>
            <abstract>
              <t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  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 constants to identify various protocol parameters.  To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations 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 describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed.  This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.</t>
              <t>This is the third edition of this document; it obsoletes RFC 5226.</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"> anchor="STRUCTURED-FIELDS" target="https://www.rfc-editor.org/info/rfc8941">
          <front>
            <title>Structured Field Values for HTTP</title>
            <author fullname="M. Nottingham" initials="M." surname="Nottingham">
              <organization/>
            </author>
            <author fullname="P-H. Kamp" initials="P-H." surname="Kamp">
              <organization/>
            </author>
            <date month="February" year="2021"/>
            <abstract>
              <t>This document describes a set of data types and associated algorithms that are intended to make it easier and safer to define and handle HTTP header and trailer fields, known as "Structured Fields", "Structured Headers", or "Structured Trailers". It is intended for use by specifications of new HTTP fields that wish to use a common syntax that is more restrictive than traditional HTTP field values.</t>
            </abstract>
          </front>
          <seriesInfo name="RFC" value="8941"/>
          <seriesInfo name="DOI" value="10.17487/RFC8941"/>
        </reference>
        <reference anchor="HTTP">
          <front>
            <title>HTTP Semantics</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 application-
   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

<!-- [HTTP] [I.D.ietf-httpbis-semantics] in AUTH48; companion 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>
          <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-semantics-17"/>
        </reference> 9110 -->
        <reference anchor="HTTP-CACHING"> anchor='HTTP' target="https://www.rfc-editor.org/info/rfc9110">
          <front>
            <title>HTTP Caching</title>
            <author fullname="Roy T. Fielding">
              <organization>Adobe</organization>
            </author> Semantics</title>
            <author fullname="Mark Nottingham">
              <organization>Fastly</organization> initials='R' surname='Fielding' fullname='Roy Fielding' role="editor">
              <organization />
            </author>
            <author fullname="Julian Reschke">
              <organization>greenbytes GmbH</organization> initials='M' surname='Nottingham' fullname='Mark Nottingham' role="editor">
              <organization />
            </author>
            <date day="25" month="July" year="2021"/>
            <abstract>
              <t>   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   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>
            </abstract>
          </front>
          <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-cache-17"/>
        </reference>
        <reference anchor="RFC8174">
          <front>
            <title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
            <author fullname="B. Leiba" initials="B." surname="Leiba">
              <organization/> initials='J' surname='Reschke' fullname='Julian Reschke' role="editor">
              <organization />
            </author>
            <date month="May" year="2017"/>
            <abstract>
              <t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t>
            </abstract> year='2022' month='June' />
          </front>
	  <seriesInfo name="BCP" value="14"/> name="STD" value="97"/>
          <seriesInfo name="RFC" value="8174"/> value="9110"/>
          <seriesInfo name="DOI" value="10.17487/RFC8174"/> value="10.17487/RFC9110"/>
        </reference>

<!-- [HTTP-CACHING] [I.D.ietf-httpbis-cache] in AUTH48; companion document RFC 9111 -->
        <reference anchor="RFC5234"> anchor='HTTP-CACHING' target="https://www.rfc-editor.org/info/rfc9111">
          <front>
            <title>Augmented BNF for Syntax Specifications: ABNF</title>
            <title>HTTP Caching</title>
            <author fullname="D. Crocker" initials="D." role="editor" surname="Crocker">
              <organization/> initials='R' surname='Fielding' fullname='Roy Fielding' role="editor">
              <organization />
            </author>
            <author fullname="P. Overell" initials="P." surname="Overell">
              <organization/> initials='M' surname='Nottingham' fullname='Mark Nottingham' role="editor">
              <organization />
            </author>
            <author initials='J' surname='Reschke' fullname='Julian Reschke' role="editor">
              <organization />
            </author>
            <date month="January" year="2008"/>
            <abstract>
              <t>Internet technical specifications often need to define a formal syntax.  Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications.  The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power.  The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges.  This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications.  [STANDARDS-TRACK]</t>
            </abstract> year='2022' month='June' />
          </front>
	  <seriesInfo name="STD" value="68"/> value="98"/>
          <seriesInfo name="RFC" value="5234"/> value="9111"/>
          <seriesInfo name="DOI" value="10.17487/RFC5234"/> value="10.17487/RFC9111"/>
        </reference>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>

      </references>
      <references>
        <name>Informative References</name>
        <reference anchor="ENTANGLE" target="https://i.blackhat.com/USA-20/Wednesday/us-20-Kettle-Web-Cache-Entanglement-Novel-Pathways-To-Poisoning-wp.pdf"> target="https://portswigger.net/research/web-cache-entanglement">
          <front>
            <title>Web Cache Entanglement: Novel Pathways to Poisoning</title>
            <author initials="J." surname="Kettle" fullname="James Kettle">
              <organization>PortSwigger</organization>
            </author>
            <date month="September" year="2020"/>
          </front>
        </reference>
      </references>
    </references>
  </back>
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