Internet-Draft | No-Vary-Search | September 2024 |
Denicola & Roman | Expires 31 March 2025 | [Page] |
A proposed HTTP header field for changing how URL search parameters impact caching.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://httpwg.org/http-extensions/draft-ietf-httpbis-no-vary-search.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-httpbis-no-vary-search/.¶
Discussion of this document takes place on the HTTP Working Group mailing list (mailto:ietf-http-wg@w3.org), which is archived at 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/no-vary-search.¶
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This Internet-Draft will expire on 31 March 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document also adopts some conventions and notation typical in WHATWG and W3C usage, especially as it relates to algorithms. See [WHATWG-INFRA].¶
The No-Vary-Search
HTTP header field is a structured field [STRUCTURED-FIELDS] whose value must be a dictionary (Section 3.2 of [STRUCTURED-FIELDS]).¶
It has the following authoring conformance requirements:¶
If present, the key-order
entry's value must be a boolean (Section 3.3.6 of [STRUCTURED-FIELDS]).¶
If present, the params
entry's value must be either a boolean (Section 3.3.6 of [STRUCTURED-FIELDS]) or an inner list (Section 3.1.1 of [STRUCTURED-FIELDS]).¶
If present, the except
entry's value must be an inner list (Section 3.1.1 of [STRUCTURED-FIELDS]).¶
The except
entry must only be present if the params
entry is also present, and the params
entry's value is the boolean value true.¶
The dictionary may contain entries whose keys are not one of key-order
, params
, and except
, but their meaning is not defined by this specification. Implementations of this specification will ignore such entries (but future documents may assign meaning to such entries).¶
A URL search variance consists of the following:¶
either the special value wildcard or a list of strings¶
either the special value wildcard or a list of strings¶
a boolean¶
The default URL search variance is a URL search variance whose no-vary params is an empty list, vary params is wildcard, and vary on key order is true.¶
The obtain a URL search variance algorithm (Section 4.2) ensures that all URL search variances obey the following constraints:¶
To parse a URL search variance given value:¶
If value is null, then return the default URL search variance.¶
Let result be a new URL search variance.¶
Set result's vary on key order to true.¶
If value["key-order
"] exists:¶
If value["params
"] exists:¶
If value["params
"] is a boolean:¶
Otherwise, if value["params
"] is an array:¶
If any item in value["params
"] is not a string, then return the default URL search variance.¶
Set result's no-vary params to the result of applying parse a key (Section 4.3) to each item in value["params
"].¶
Set result's vary params to wildcard.¶
Otherwise, return the default URL search variance.¶
If value["except
"] exists:¶
If value["params
"] is not true, then return the default URL search variance.¶
If value["except
"] is not an array, then return the default URL search variance.¶
If any item in value["except
"] is not a string, then return the default URL search variance.¶
Set result's vary params to the result of applying parse a key (Section 4.3) to each item in value["except
"].¶
Return result.¶
To obtain a URL search variance given a response response:¶
Let fieldValue be the result of getting a structured field value [FETCH] given `No-Vary-Search
` and "dictionary
" from response's header list.¶
Return the result of parsing a URL search variance (Section 4.1) given fieldValue. ¶
The following illustrates how various inputs are parsed, in terms of their impacting on the resulting no-vary params and vary params:¶
Input | Result |
---|---|
No-Vary-Search: params
|
no-vary params: wildcard vary params: (empty list) |
No-Vary-Search: params=("a")
|
no-vary params: « "a " »vary params: wildcard |
No-Vary-Search: params, except=("x")
|
no-vary params: wildcard vary params: « " x " » |
The following inputs are all invalid and will cause the default URL search variance to be returned:¶
No-Vary-Search: unknown-key
¶
No-Vary-Search: key-order="not a boolean"
¶
No-Vary-Search: params="not a boolean or inner list"
¶
No-Vary-Search: params=(not-a-string)
¶
No-Vary-Search: params=("a"), except=("x")
¶
No-Vary-Search: params=(), except=()
¶
No-Vary-Search: params=?0, except=("x")
¶
No-Vary-Search: params, except=(not-a-string)
¶
No-Vary-Search: params, except="not an inner list"
¶
No-Vary-Search: params, except=?1
¶
No-Vary-Search: except=("x")
¶
No-Vary-Search: except=()
¶
The following inputs are valid, but somewhat unconventional. They are shown alongside their more conventional form.¶
Input | Conventional form |
---|---|
No-Vary-Search: params=?1
|
No-Vary-Search: params
|
No-Vary-Search: key-order=?1
|
No-Vary-Search: key-order
|
No-Vary-Search: params, key-order, except=("x")
|
No-Vary-Search: key-order, params, except=("x")
|
No-Vary-Search: params=?0
|
(omit the header) |
No-Vary-Search: params=()
|
(omit the header) |
No-Vary-Search: key-order=?0
|
(omit the header) |
To parse a key given an ASCII string keyString:¶
Let keyBytes be the isomorphic encoding [WHATWG-INFRA] of keyString.¶
Replace any 0x2B (+) in keyBytes with 0x20 (SP).¶
Let keyBytesDecoded be the percent-decoding [WHATWG-URL] of keyBytes.¶
Let keyStringDecoded be the UTF-8 decoding without BOM [WHATWG-ENCODING] of keyBytesDecoded.¶
Return keyStringDecoded.¶
The parse a key algorithm allows encoding non-ASCII key strings in the ASCII structured header format, similar to how the application/x-www-form-urlencoded format [WHATWG-URL] allows encoding an entire entry list of keys and values in ASCII URL format. For example,¶
will result in a URL search variance whose vary params are « "é 気
" ». As explained in a later example, the canonicalization process during equivalence testing means this will treat as equivalent URL strings such as:¶
https://example.com/?é 気=1
¶
https://example.com/?é+気=2
¶
https://example.com/?%C3%A9%20気=3
¶
https://example.com/?%C3%A9+%E6%B0%97=4
¶
and so on, since they all are parsed [WHATWG-URL] to having the same key "é 気
".¶
Two URLs [WHATWG-URL] urlA and urlB are equivalent modulo search variance given a URL search variance searchVariance if the following algorithm returns true:¶
If the scheme, username, password, host, port, or path of urlA and urlB differ, then return false.¶
If searchVariance is equivalent to the default URL search variance, then:¶
In this case, even URL pairs that might appear the same after running the application/x-www-form-urlencoded parser [WHATWG-URL] on their queries, such as https://example.com/a
and https://example.com/a?
, or https://example.com/foo?a=b&&&c
and https://example.com/foo?a=b&c=
, will be treated as inequivalent.¶
Let searchParamsA and searchParamsB be empty lists.¶
If wrlA's query is not null, then set searchParamsA to the result of running the application/x-www-form-urlencoded parser [WHATWG-URL] given the isomorphic encoding [WHATWG-INFRA] of urlA's query.¶
If wrlB's query is not null, then set searchParamsB to the result of running the application/x-www-form-urlencoded parser [WHATWG-URL] given the isomorphic encoding [WHATWG-INFRA] of urlB's query.¶
If searchVariance's no-vary params is a list, then:¶
Otherwise, if searchVariance's vary params is a list, then:¶
If searchVariance's vary on key order is false, then:¶
Let keyLessThan be an algorithm taking as inputs two pairs (keyA, valueA) and (keyB, valueB), which returns whether keyA is code unit less than [WHATWG-INFRA] keyB.¶
Set searchParamsA to the result of sorting searchParamsA in ascending order with keyLessThan.¶
Set searchParamsB to the result of sorting searchParamsB in ascending order with keyLessThan.¶
If searchParamsA's size is not equal to searchParamsB's size, then return false.¶
Let i be 0.¶
While i < searchParamsA's size:¶
Return true.¶
Due to how the application/x-www-form-urlencoded parser canonicalizes query strings, there are some cases where query strings which do not appear obviously equivalent, will end up being treated as equivalent after parsing.¶
So, for example, given any non-default value for No-Vary-Search
, such as No-Vary-Search: key-order
, we will have the following equivalences:¶
https://example.com
https://example.com/?
https://example.com/?a=x
https://example.com/?%61=%78
https://example.com/?a=é
https://example.com/?a=%C3%A9
https://example.com/?a=%f6
https://example.com/?a=%ef%bf%bd
https://example.com/?a=x&&&&
https://example.com/?a=x
&
and discards empty strings¶
https://example.com/?a=
https://example.com/?a
a
¶
https://example.com/?a=%20
https://example.com/?a=+
https://example.com/?a= &
+
and %20
are both parsed as U+0020 SPACE¶
The main risk to be aware of is the impact of mismatched URLs. In particular, this could cause the user to see a response that was originally fetched from a URL different from the one displayed when they hovered a link, or the URL displayed in the URL bar.¶
However, since the impact is limited to query parameters, this does not cross the relevant security boundary, which is the origin [HTML]. (Or perhaps just the host, from the perspective of web browser security UI. [WHATWG-URL]) Indeed, we have already given origins complete control over how they present the (URL, reponse body) pair, including on the client side via technology such as history.replaceState() or service workers.¶
This proposal is adjacent to the highly-privacy-relevant space of navigational tracking, which often uses query parameters to pass along user identifiers. However, we believe this proposal itself does not have privacy impacts. It does not interfere with existing navigational tracking mitigations, or any known future ones being contemplated. Indeed, if a page were to encode user identifiers in its URL, the only ability this proposal gives is to reduce such user tracking by preventing server processing of such user IDs (since the server is bypassed in favor of the cache). [NAV-TRACKING-MITIGATIONS]¶
TODO IANA¶
TODO acknowledge.¶
Section 3, Paragraph 3; Section 4.1, Paragraph 2.1.1; Section 4.1, Paragraph 2.4.2.1.1; Section 4.1, Paragraph 2.5.2.2.2.1.1; Section 4.1, Paragraph 2.5.2.3.1; Section 4.1, Paragraph 2.6.2.1.1; Section 4.1, Paragraph 2.6.2.2.1; Section 4.1, Paragraph 2.6.2.3.1; Section 4.1, Paragraph 3.1; Section 4.2.1, Paragraph 3; Section 5, Paragraph 2.2.1¶
Section 2, Paragraph 5.1; Section 3, Paragraph 4; Section 4.2, Paragraph 1¶
Section 4.1, Paragraph 2.5.2.2.2.2.1; Section 4.1, Paragraph 2.6.2.4.1; Section 4.3, Paragraph 1; Section 4.3.1, Paragraph 1¶