draft-ietf-httpbis-cache-header-10.txt   draft-ietf-httpbis-cache-header-latest.txt 
HTTP Working Group M. Nottingham HTTP Working Group M. Nottingham
Internet-Draft Fastly Internet-Draft Fastly
Intended status: Standards Track August 17, 2021 Intended status: Standards Track March 1, 2022
Expires: February 18, 2022 Expires: September 2, 2022
The Cache-Status HTTP Response Header Field The Cache-Status HTTP Response Header Field
draft-ietf-httpbis-cache-header-10 draft-ietf-httpbis-cache-header-latest
Abstract Abstract
To aid debugging, HTTP caches often append header fields to a To aid debugging, HTTP caches often append header fields to a
response explaining how they handled the request in an ad hoc manner. response, explaining how they handled the request in an ad hoc
This specification defines a standard mechanism to do so that is manner. This specification defines a standard mechanism to do so
aligned with HTTP's caching model. that is aligned with HTTP's caching model.
Note to Readers About This Document
_RFC EDITOR: please remove this section before publication_ This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Status information for this document may be found at
mailing list (ietf-http-wg@w3.org), which is archived at <https://datatracker.ietf.org/doc/draft-ietf-httpbis-cache-header/>.
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at https://httpwg.org/ [2]; Discussion of this document takes place on the HTTP Working Group
source code and issues list for this draft can be found at mailing list (<mailto:ietf-http-wg@w3.org>), which is archived at
https://github.com/httpwg/http-extensions/labels/cache-header [3]. <https://lists.w3.org/Archives/Public/ietf-http-wg/>. Working Group
information can be found at <https://httpwg.org/>.
Source for this draft and an issue tracker can be found at
<https://github.com/httpwg/http-extensions/labels/cache-header>.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 18, 2022. This Internet-Draft will expire on September 2, 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
1. Introduction 1. Introduction
To aid debugging (both by humans and automated tools), HTTP caches To aid debugging (both by humans and automated tools), HTTP caches
often append header fields to a response explaining how they handled often append header fields to a response explaining how they handled
the request. Unfortunately, the semantics of these headers are often the request. Unfortunately, the semantics of these header fields are
unclear, and both the semantics and syntax used vary between often unclear, and both the semantics and syntax used vary between
implementations. implementations.
This specification defines a new HTTP response header field, "Cache- This specification defines a new HTTP response header field, "Cache-
Status" for this purpose, with standardized syntax and semantics. Status", for this purpose with standardized syntax and semantics.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This document uses ABNF as defined in [RFC5234], with rules prefixed This document uses the following terminology from Section 3 of
with "sf-" and the "key" rule as defined in [STRUCTURED-FIELDS]. It [STRUCTURED-FIELDS] to specify syntax and parsing: List, String,
uses terminology from [HTTP] and [HTTP-CACHING]. Token, Integer, and Boolean.
This document also uses terminology from [HTTP] and [HTTP-CACHING].
2. The Cache-Status HTTP Response Header Field 2. The Cache-Status HTTP Response Header Field
The Cache-Status HTTP response header field indicates how caches have The Cache-Status HTTP response header field indicates how caches have
handled that response and its corresponding request. The syntax of handled that response and its corresponding request. The syntax of
this header field conforms to [STRUCTURED-FIELDS]. this header field conforms to [STRUCTURED-FIELDS].
Its value is a List ([STRUCTURED-FIELDS], Section 3.1): Its value is a List. Each member of the List represents a cache that
has handled the request. The first member represents the cache
Cache-Status = sf-list closest to the origin server, and the last member represents the
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 (possibly including the user agent's cache cache closest to the user (possibly including the user agent's cache
itself, if it appends a value). itself if it appends a value).
Caches determine when it is appropriate to add the Cache-Status Caches determine when it is appropriate to add the Cache-Status
header field to a response. Some might add it to all responses, header field to a response. Some might add it to all responses,
whereas others might only do so when specifically configured to, or whereas others might only do so when specifically configured to, or
when the request contains a header field that activates a debugging when the request contains a header field that activates a debugging
mode. See Section 6 for related security considerations. mode. See Section 6 for related security considerations.
An intermediary SHOULD NOT append a Cache-Status member to responses An intermediary SHOULD NOT append a Cache-Status member to responses
that it generates locally, even if that intermediary contains a that it generates locally, even if that intermediary contains a
cache, unless the generated response is based upon a stored response cache, unless the generated response is based upon a stored response
(e.g., 304 Not Modified and 206 Partial Content are both based upon a (e.g., 304 (Not Modified) and 206 (Partial Content) are both based
stored response). For example, a proxy generating a 400 response due upon a stored response). For example, a proxy generating a 400
to a malformed request will not add a Cache-Status value, because response due to a malformed request will not add a Cache-Status
that response was generated by the proxy, not the origin server. value, because that response was generated by the proxy, not the
origin server.
When adding a value to the Cache-Status header field, caches SHOULD When adding a value to the Cache-Status header field, caches SHOULD
preserve the existing field value, to allow debugging of the entire preserve the existing field value, to allow debugging of the entire
chain of caches handling the request. chain of caches handling the request.
Each list member identifies the cache that inserted it and this Each List member identifies the cache that inserted it, and this
identifier MUST be a String or Token. Depending on the deployment, identifier MUST be a String or Token. Depending on the deployment,
this might be a product or service name (e.g., ExampleCache or this might be a product or service name (e.g., "ExampleCache" or
"Example CDN"), a hostname ("cache-3.example.com"), an IP address, or "Example CDN"), a hostname ("cache-3.example.com"), an IP address, or
a generated string. a generated string.
Each member of the list can have parameters that describe that Each member of the list can have parameters that describe that
cache's handling of the request. While these parameters are cache's handling of the request. While these parameters are
OPTIONAL, caches are encouraged to provide as much information as OPTIONAL, caches are encouraged to provide as much information as
possible. possible.
This specification defines the following parameters: This specification defines the following parameters.
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
2.1. The hit parameter 2.1. The hit Parameter
"hit", when true, indicates that the request was satisfied by the The value of "hit" is a Boolean that, when true, indicates that the
cache; i.e., it was not forwarded, and the response was obtained from request was satisfied by the cache; that is, it was not forwarded,
the cache. and the response was obtained from the cache.
A response that was originally produced by the origin but was A response that was originally produced by the origin but was
modified by the cache (for example, a 304 or 206 status code) is 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 still considered a hit, as long as it did not go forward (e.g., for
validation). validation).
A response that was in cache but not able to be used without going A response that was in cache but not able to be used without going
forward (e.g., because it was stale, or partial) is not considered a forward (e.g., because it was stale or partial) is not considered a
hit. Note that a stale response that is used without going forward hit. Note that a stale response that is used without going forward
(e.g., because the origin server is not available) can be considered (e.g., because the origin server is not available) can be considered
a hit. a hit.
"hit" and "fwd" are exclusive; only one of them should appear on each "hit" and "fwd" are exclusive; only one of them should appear on each
list member. list member.
2.2. The fwd parameter 2.2. The fwd Parameter
"fwd" indicates that the request went forward towards the origin, and "fwd", when present, indicates that the request went forward towards
why. the origin; its value is a Token that indicates why.
The following parameter values are defined to explain why the request The following parameter values are defined to explain why the request
went forward, from most specific to least: went forward, from most specific to least:
o bypass - The cache was configured to not handle this request bypass: The cache was configured to not handle this request.
o method - The request method's semantics require the request to be method: The request method's semantics require the request to be
forwarded forwarded.
o uri-miss - The cache did not contain any responses that matched uri-miss: The cache did not contain any responses that matched the
the request URI request URI.
o vary-miss - The cache contained a response that matched the vary-miss: The cache contained a response that matched the request
request URI, but could not select a response based upon this URI, but it could not select a response based upon this request's
request's headers and stored Vary headers. header fields and stored Vary header fields.
o miss - The cache did not contain any responses that could be used miss: The cache did not contain any responses that could be used to
to satisfy this request (to be used when an implementation cannot satisfy this request (to be used when an implementation cannot
distinguish between uri-miss and vary-miss) distinguish between uri-miss and vary-miss).
o request - The cache was able to select a fresh response for the request: The cache was able to select a fresh response for the
request, but the request's semantics (e.g., Cache-Control request request, but the request's semantics (e.g., Cache-Control request
directives) did not allow its use directives) did not allow its use.
o stale - The cache was able to select a response for the request, stale: The cache was able to select a response for the request, but
but it was stale it was stale.
o partial - The cache was able to select a partial response for the partial: The cache was able to select a partial response for the
request, but it did not contain all of the requested ranges (or request, but it did not contain all of the requested ranges (or
the request was for the complete response) the request was for the complete response).
The most specific reason that the cache is aware of SHOULD be used, The most specific reason known to the cache SHOULD be used, to the
to the extent that it is possible to implement. See also extent that it is possible to implement. See also [HTTP-CACHING],
[HTTP-CACHING], Section 4. Section 4.
2.3. The fwd-status parameter 2.3. The fwd-status Parameter
"fwd-status" indicates what status code (see [HTTP], Section 15) the The value of "fwd-status" is an Integer that indicates which status
next hop server returned in response to the forwarded request. Only code (see [HTTP], Section 15) the next-hop server returned in
meaningful when "fwd" is present; if "fwd-status" is not present but response to the forwarded request. The fwd-status parameter is only
"fwd" is, it defaults to the status code sent in the response. meaningful when fwd is present. If fwd-status is not present but the
fwd parameter is, it defaults to the status code sent in the
response.
This parameter is useful to distinguish cases when the next hop This parameter is useful to distinguish cases when the next-hop
server sends a 304 Not Modified response to a conditional request, or server sends a 304 (Not Modified) response to a conditional request
a 206 Partial Response because of a range request. or a 206 (Partial Content) response because of a range request.
2.4. The ttl parameter 2.4. The ttl Parameter
"ttl" indicates the response's remaining freshness lifetime (see The value of "ttl" is an Integer that indicates the response's
[HTTP-CACHING], Section 4.2.1) as calculated by the cache, as an remaining freshness lifetime (see [HTTP-CACHING], Section 4.2.1) as
integer number of seconds, measured as closely as possible to when calculated by the cache, as an integer number of seconds, measured as
the response header section is sent by the cache. This includes closely as possible to when the response header section is sent by
freshness assigned by the cache; e.g., through heuristics (see the cache. This includes freshness assigned by the cache through,
[HTTP-CACHING], Section 4.2.2), local configuration, or other for example, heuristics (see [HTTP-CACHING], Section 4.2.2), local
factors. May be negative, to indicate staleness. configuration, or other factors. It may be negative, to indicate
staleness.
2.5. The stored parameter 2.5. The stored Parameter
"stored" indicates whether the cache stored the response (see The value of "stored" is a Boolean that indicates whether the cache
[HTTP-CACHING], Section 3); a true value indicates that it did. Only stored the response (see [HTTP-CACHING], Section 3); a true value
meaningful when fwd is present. indicates that it did. The stored parameter is only meaningful when
fwd is present.
2.6. The collapsed parameter 2.6. The collapsed Parameter
"collapsed" indicates whether this request was collapsed together The value of "collapsed" is a Boolean that indicates whether this
with one or more other forward requests (see [HTTP-CACHING], request was collapsed together with one or more other forward
Section 4); if true, the response was successfully reused; if not, a requests (see [HTTP-CACHING], Section 4). If true, the response was
new request had to be made. If not present, the request was not successfully reused; if not, a new request had to be made. If not
collapsed with others. Only meaningful when fwd is present. present, the request was not collapsed with others. The collapsed
parameter is only meaningful when fwd is present.
2.7. The key parameter 2.7. The key Parameter
"key" conveys a representation of the cache key (see [HTTP-CACHING], The value of "key" is a String that conveys a representation of the
Section 2) used for the response. Note that this may be cache key (see [HTTP-CACHING], Section 2) used for the response.
implementation-specific. Note that this may be implementation specific.
2.8. The detail parameter 2.8. The detail Parameter
"detail" allows implementations to convey additional information not The value of "detail" is either a String or a Token that allows
captured in other parameters; for example, implementation-specific implementations to convey additional information not captured in
states, or other caching-related metrics. other parameters, such as implementation-specific states or other
caching-related metrics.
For example: For example:
Cache-Status: ExampleCache; hit; detail=MEMORY Cache-Status: ExampleCache; hit; detail=MEMORY
The semantics of a detail parameter are always specific to the cache 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 that sent it; even if a details parameter from another cache shares
the same name, it might not mean the same thing. the same value, it might not mean the same thing.
This parameter is intentionally limited. If an implementation's This parameter is intentionally limited. If an implementation's
developer or operator needs to convey additional information in an developer or operator needs to convey additional information in an
interoperable fashion, they are encouraged to register extension interoperable fashion, they are encouraged to register extension
parameters (see Section 4) or define another header field. parameters (see Section 4) or define another header field.
3. Examples 3. Examples
The most minimal cache hit: The following is an example of a minimal cache hit:
Cache-Status: ExampleCache; hit Cache-Status: ExampleCache; hit
... but a polite cache will give some more information, e.g.: However, a polite cache will give some more information, e.g.:
Cache-Status: ExampleCache; hit; ttl=376 Cache-Status: ExampleCache; hit; ttl=376
A stale hit just has negative freshness: A stale hit just has negative freshness, as in this example:
Cache-Status: ExampleCache; hit; ttl=-412 Cache-Status: ExampleCache; hit; ttl=-412
Whereas a complete miss is: Whereas this is an example of a complete miss:
Cache-Status: ExampleCache; fwd=uri-miss Cache-Status: ExampleCache; fwd=uri-miss
A miss that successfully validated on the back-end server: This is an example of a miss that successfully validated on the
backend server:
Cache-Status: ExampleCache; fwd=stale; fwd-status=304 Cache-Status: ExampleCache; fwd=stale; fwd-status=304
A miss that was collapsed with another request: This is an example of a miss that was collapsed with another request:
Cache-Status: ExampleCache; fwd=uri-miss; collapsed Cache-Status: ExampleCache; fwd=uri-miss; collapsed
This is an example of a miss that the cache attempted to collapse,
A miss that the cache attempted to collapse, but couldn't: but couldn't:
Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0 Cache-Status: ExampleCache; fwd=uri-miss; collapsed=?0
Going through two separate layers of caching, where the cache closest The following is an example of going through two separate layers of
to the origin responded to an earlier request with a stored response, caching, where the cache closest to the origin responded to an
and a second cache stored that response and later reused it to earlier request with a stored response, and a second cache stored
satisfy the current request: that response and later reused it to satisfy the current request:
Cache-Status: OriginCache; hit; ttl=1100, Cache-Status: OriginCache; hit; ttl=1100,
"CDN Company Here"; hit; ttl=545 "CDN Company Here"; hit; ttl=545
Going through a three-layer caching system, where the closest to the The following is an example of going through a three-layer caching
origin is a reverse proxy (where the response was served from cache), system, where the closest to the origin is a reverse proxy (where the
the next is a forward proxy interposed by the network (where the response was served from cache); the next is a forward proxy
request was forwarded because there wasn't any response cached with interposed by the network (where the request was forwarded because
its URI, the request was collapsed with others, and the resulting there wasn't any response cached with its URI, the request was
response was stored), and the closest to the user is a browser cache collapsed with others, and the resulting response was stored); and
(where there wasn't any response cached with the request's URI): the closest to the user is a browser cache (where there wasn't any
response cached with the request's URI):
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
4. Defining New Cache-Status Parameters 4. Defining New Cache-Status Parameters
New Cache-Status Parameters can be defined by registering them in the New Cache-Status parameters can be defined by registering them in the
HTTP Cache-Status Parameters registry. "HTTP Cache-Status" registry.
Registration requests are reviewed and approved by a Designated Registration requests are reviewed and approved by a designated
Expert, as per [RFC8126], Section 4.5. A specification document is expert, per [RFC8126], Section 4.5. A specification document is
appreciated, but not required. appreciated but not required.
The Expert(s) should consider the following factors when evaluating The expert(s) should consider the following factors when evaluating
requests: requests:
o Community feedback o Community feedback
o If the value is sufficiently well-defined o If the value is sufficiently well defined
o Generic parameters are preferred over vendor-specific, o Generic parameters are preferred over vendor-specific,
application-specific, or deployment-specific values. If a generic application-specific, or deployment-specific values. If a generic
value cannot be agreed upon in the community, the parameter's name value cannot be agreed upon in the community, the parameter's name
should be correspondingly specific (e.g., with a prefix that should be correspondingly specific (e.g., with a prefix that
identifies the vendor, application or deployment). identifies the vendor, application, or deployment).
Registration requests should use the following template: Registration requests should use the following template:
o Name: [a name for the Cache-Status Parameter that matches the Name: [a name for the Cache-Status parameter's key; see
'key' ABNF rule] Section 3.1.2 of [STRUCTURED-FIELDS] for syntactic requirements]
o Description: [a description of the parameter semantics and value] Type: [the Structured Type of the parameter's value; see
Section 3.1.2 of [STRUCTURED-FIELDS]]
o Reference: [to a specification defining this parameter, if Description: [a description of the parameter's semantics]
Reference: [to a specification defining this parameter, if
available] available]
See the registry at https://iana.org/assignments/http-cache-status See the registry at <https://www.iana.org/assignments/http-cache-
[4] for details on where to send registration requests. status> for details on where to send registration requests.
5. IANA Considerations 5. IANA Considerations
Upon publication, please create the HTTP Cache-Status Parameters IANA has created the "HTTP Cache-Status" registry at
registry at https://iana.org/assignments/http-cache-status [5] and <https://iana.org/assignments/http-cache-status> and populated it
populate it with the types defined in Section 2; see Section 4 for with the types defined in Section 2; see Section 4 for its associated
its associated procedures. procedures.
Also, please create the following entry in the Hypertext Transfer
Protocol (HTTP) Field Name Registry defined in [HTTP], Section 18.4:
o Field name: Cache-Status IANA has added the following entry in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined in [HTTP], Section 18.4:
o Status: permanent Field name: Cache-Status
o Specification document: [this document] Status: permanent
o Comments: Reference: RFC 9211
6. Security Considerations 6. Security Considerations
Attackers can use the information in Cache-Status to probe the Attackers can use the information in Cache-Status to probe the
behaviour of the cache (and other components), and infer the activity behavior of the cache (and other components) and infer the activity
of those using the cache. The Cache-Status header field may not of those using the cache. The Cache-Status header field may not
create these risks on its own, but can assist attackers in exploiting create these risks on its own, but it can assist attackers in
them. exploiting them.
For example, knowing if a cache has stored a response can help an For example, knowing if a cache has stored a response can help an
attacker execute a timing attack on sensitive data. attacker execute a timing attack on sensitive data.
Additionally, exposing the cache key can help an attacker understand Additionally, exposing the cache key can help an attacker understand
modifications to the cache key, which may assist cache poisoning modifications to the cache key, which may assist cache poisoning
attacks. See [ENTANGLE] for details. attacks. See [ENTANGLE] for details.
The underlying risks can be mitigated with a variety of techniques The underlying risks can be mitigated with a variety of techniques
(e.g., use of encryption and authentication; avoiding the inclusion (e.g., using encryption and authentication and avoiding the inclusion
of attacker-controlled data in the cache key), depending on their of attacker-controlled data in the cache key), depending on their
exact nature. Note that merely obfuscating the key does not mitigate exact nature. Note that merely obfuscating the key does not mitigate
this risk. this risk.
To avoid assisting such attacks, the Cache-Status header field can be To avoid assisting such attacks, the Cache-Status header field can be
omitted, only sent when the client is authorized to receive it, or omitted, only sent when the client is authorized to receive it, or
only send sensitive information (e.g., the key parameter) when the sent with sensitive information (e.g., the key parameter) only when
client is authorized. the client is authorized.
7. References 7. References
7.1. Normative References 7.1. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP [HTTP] Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Semantics", draft-ietf-httpbis-semantics-17 (work in Semantics", draft-ietf-httpbis-semantics-19 (work in
progress), July 2021. progress), September 2021.
[HTTP-CACHING] [HTTP-CACHING]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Fielding, R., Nottingham, M., and J. Reschke, "HTTP
Caching", draft-ietf-httpbis-cache-17 (work in progress), Caching", draft-ietf-httpbis-cache-19 (work in progress),
July 2021. September 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017, RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>. <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>. <https://www.rfc-editor.org/info/rfc8941>.
7.2. Informative References 7.2. Informative References
[ENTANGLE] [ENTANGLE]
Kettle, J., "Web Cache Entanglement: Novel Pathways to Kettle, J., "Web Cache Entanglement: Novel Pathways to
Poisoning", 2020, <https://i.blackhat.com/USA- Poisoning", September 2020,
20/Wednesday/us-20-Kettle-Web-Cache-Entanglement-Novel- <https://portswigger.net/research/web-cache-entanglement>.
Pathways-To-Poisoning-wp.pdf>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/cache-header
[4] https://iana.org/assignments/http-cache-status
[5] https://iana.org/assignments/http-cache-status
Author's Address Author's Address
Mark Nottingham Mark Nottingham
Fastly Fastly
Prahran, VIC Prahran
Australia Australia
Email: mnot@mnot.net Email: mnot@mnot.net
URI: https://www.mnot.net/ URI: https://www.mnot.net/
 End of changes. 82 change blocks. 
189 lines changed or deleted 174 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/