Retrofit Structured Fields for HTTP
draft-nottingham-http-structure-retrofit-00
The information below is for an old version of the document.
| Document | Type |
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|---|---|---|---|
| Author | Mark Nottingham | ||
| Last updated | 2021-10-06 | ||
| Replaced by | draft-ietf-httpbis-retrofit | ||
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draft-nottingham-http-structure-retrofit-00
Network Working Group M. Nottingham
Internet-Draft 7 October 2021
Intended status: Informational
Expires: 10 April 2022
Retrofit Structured Fields for HTTP
draft-nottingham-http-structure-retrofit-00
Abstract
This specification defines how a selection of existing HTTP fields
can be handled as Structured Fields.
Note to Readers
_RFC EDITOR: please remove this section before publication_
The issues list for this draft can be found at
https://github.com/mnot/I-D/labels/http-structure-retrofit
(https://github.com/mnot/I-D/labels/http-structure-retrofit).
The most recent (often, unpublished) draft is at
https://mnot.github.io/I-D/http-structure-retrofit/
(https://mnot.github.io/I-D/http-structure-retrofit/).
Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
pages/http-structure-retrofit (https://github.com/mnot/I-D/commits/
gh-pages/http-structure-retrofit).
See also the draft's current status in the IETF datatracker, at
https://datatracker.ietf.org/doc/draft-nottingham-http-structure-
retrofit/ (https://datatracker.ietf.org/doc/draft-nottingham-http-
structure-retrofit/).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 April 2022.
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Copyright (c) 2021 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
1.2. Compatible Fields . . . . . . . . . . . . . . . . . . . . 4
1.3. Mapped Fields . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1. URLs . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3.2. Dates . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3.3. ETags . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.4. Links . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.5. Cookies . . . . . . . . . . . . . . . . . . . . . . . 8
1.4. IANA Considerations . . . . . . . . . . . . . . . . . . . 9
2. Security Considerations . . . . . . . . . . . . . . . . . . . 10
3. Normative References . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Data Supporting Field Compatibility . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
Structured Field Values for HTTP [STRUCTURED-FIELDS] introduced a
data model with associated parsing and serialisation algorithms for
HTTP field values. Header fields that are defined as Structured
Fields can realise a number of benefits, including:
* Improved interoperability and security: precisely defined parsing
and serialisation algorithms are typically not available for
fields defined with just ABNF and/or prose.
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* Reuse of common implementations: many parsers for other fields are
specific to a single field or a small family of fields
* Canonical form: because a deterministic serialisation algorithm is
defined for each type, Structure Fields have a canonical
representation
* Enhanced API support: a regular data model makes it easier to
expose field values as a native data structure in implementations
* Alternative serialisations: While [STRUCTURED-FIELDS] defines a
textual serialisation of that data model, other, more efficient
serialisations of the underlying data model are also possible.
However, a field needs to be defined as a Structured Field for these
benefits to be realised. Many existing fields are not, making up the
bulk of header and trailer fields seen in HTTP traffic on the
Internet.
This specification defines how a selection of existing HTTP fields
can be handled as Structured Fields, so that these benefits can be
realised -- thereby making them Retrofit Structured Fields.
It does so using two techniques. Section 1.2 lists compatible fields
-- those that can be handled as if they were Structured Fields due to
the similarity of their defined syntax to that in Structured Fields.
Section 1.3 lists mapped fields -- those whose syntax needs to be
transformed into an underlying data model which is then mapped into
that defined by Structured Fields.
While implementations can parse and serialise Compatible Fields as
Structured Fields subject to the caveats in Section 1.2, a sender
cannot generate mapped fields from Section 1.3 and expect them to be
understood and acted upon by the recipient without prior negotiation.
This specification does not define such a mechanism.
1.1. Notational Conventions
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.
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1.2. Compatible Fields
HTTP fields with the following names can usually have their values
handled as Structured Fields according to the listed parsing and
serialisation algorithms in [RFC8941], subject to the listed caveats.
The listed types are chosen for compatibility with the defined syntax
of the field as well as with actual Internet traffic (see
Appendix A). However, not all instances of these fields will
successfully parse. This might be because the field value is clearly
invalid, or it might be because it is valid but not parseable as a
Structured Field.
As such, an application using this specification will need to
consider how to handle these field values. Depending on its
requirements, it might be advisable to reject such values, treat them
as opaque strings, or attempt to recover a structured value from them
in an ad hoc fashion.
* Accept - List
* Accept-Encoding - List
* Accept-Language - List
* Accept-Patch - List
* Accept-Ranges - List
* Access-Control-Allow-Credentials - Item
* Access-Control-Allow-Headers - List
* Access-Control-Allow-Methods - List
* Access-Control-Allow-Origin - Item
* Access-Control-Expose-Headers - List
* Access-Control-Max-Age - Item
* Access-Control-Request-Headers - List
* Access-Control-Request-Method - Item
* Age - Item
* Allow - List
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* ALPN - List
* Alt-Svc - Dictionary
* Alt-Used - Item
* Cache-Control - Dictionary
* Connection - List
* Content-Encoding - List
* Content-Language - List
* Content-Length - List
* Content-Type - Item
* Cross-Origin-Resource-Policy - Item
* Expect - Item
* Expect-CT - Dictionary
* Host - Item
* Keep-Alive - Dictionary
* Origin - Item
* Pragma - Dictionary
* Prefer - Dictionary
* Preference-Applied - Dictionary
* Retry-After - Item
* Surrogate-Control - Dictionary
* TE - List
* Timing-Allow-Origin: List
* Trailer - List
* Transfer-Encoding - List
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* Vary - List
* X-Content-Type-Options - Item
* X-Frame-Options - Item
* X-XSS-Protection - List
Note the following caveats:
Parameters: HTTP parameter names are case-insensitive (as per
Section 5.6.6 of [HTTP]), but Structured Fields require them to be
all-lowercase. Although the vast majority of parameters seen in
typical traffic are all-lowercase, compatibility can be improved
by force-lowercasing parameters when encountered.
Empty Field Values: Empty and whitespace-only field values are
considered errors in Structured Fields. For compatible fields, an
empty field indicates that the field should be silently ignored.
Alt-Svc: Some ALPN tokens (e.g., h3-Q43) do not conform to key's
syntax. Since the final version of HTTP/3 uses the h3 token, this
shouldn't be a long-term issue, although future tokens may again
violate this assumption.
Cache-Control, Expect-CT, Pragma, Prefer, Preference-Applied,
Surrogate-Control: These Dictionary-based fields consider the key to
be case-insensitive, but Structured Fields requires keys to be
all-lowercase. Although the vast majority of values seen in
typical traffic are all-lowercase, compatibility can be improved
by force-lowercasing these Dictionary keys when encountered.
Content-Length: Content-Length is defined as a List because it is
not uncommon for implementations to mistakenly send multiple
values. See Section 8.6 of [HTTP] for handling requirements.
Retry-After: Only the delta-seconds form of Retry-After is
supported; a Retry-After value containing a http-date will need to
be either converted into delta-seconds or represented as a raw
value.
1.3. Mapped Fields
HTTP fields with the following names can have their values
represented in Structured Fields by mapping them into its data types
and then serialising the result using an alternative field name.
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For example, the Date HTTP header field carries a string representing
a date:
Date: Sun, 06 Nov 1994 08:49:37 GMT
Its value is more efficiently represented as an integer number of
delta seconds from the Unix epoch (00:00:00 UTC on 1 January 1970,
minus leap seconds). Thus, the example above would be mapped as:
SF-Date: 784072177
As in Section 1.2, these fields are unable to represent values that
are not Structured Fields, and so an application using this
specification will need to how to support such values. Typically,
serialising them using the original field name is sufficient.
Each field name listed below indicates a replacement field name and a
means of mapping its original value into a Structured Field.
1.3.1. URLs
The following field names (paired with their replacement field names)
have values that can be represented as Structured Fields by
considering the original field's value as a string.
* Content-Location - SF-Content-Location
* Location - SF-Location
* Referer - SF-Referer
For example, a Location field could be represented as:
SF-Location: "https://example.com/foo"
1.3.2. Dates
The following field names (paired with their replacement field names)
have values that can be represented as Structured Fields by parsing
their payload according to [RFC7231], Section 7.1.1.1, and
representing the result as an integer number of seconds delta from
the Unix Epoch (00:00:00 UTC on 1 January 1970, minus leap seconds).
* Date - SF-Date
* Expires - SF-Expires
* If-Modified-Since - SF-IMS
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* If-Unmodified-Since - SF-IUS
* Last-Modified - SF-LM
For example, an Expires field could be represented as:
SF-Expires: 1571965240
1.3.3. ETags
The following field names (paired with their replacement field names)
have values that can be represented as Structured Fields by
representing the entity-tag as a string, and the weakness flag as a
boolean "w" parameter on it, where true indicates that the entity-tag
is weak; if 0 or unset, the entity-tag is strong.
* ETag - SF-ETag
For example:
SF-ETag: "abcdef"; w=?1
If-None-Match is a list of the structure described above.
* If-None-Match - SF-INM
For example:
SF-INM: "abcdef"; w=?1, "ghijkl"
1.3.4. Links
The field-value of the Link header field [RFC8288] can be represented
as a Structured Field by representing the URI-Reference as a string,
and link-param as parameters.
* Link: SF-Link
For example:
SF-Link: "/terms"; rel="copyright"; anchor="#foo"
1.3.5. Cookies
The field-values of the Cookie and Set-Cookie fields [RFC6265] can be
represented in Structured Fields as a List with parameters and a
Dictionary, respectively.
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The serialisation is almost identical, except that the Expires
parameter is always a string (as it can contain a comma), multiple
cookie-strings can appear in Set-Cookie, and cookie-pairs are
delimited in Cookie by a comma, rather than a semicolon.
* Set-Cookie: SF-Set-Cookie
* Cookie: SF-Cookie
SF-Set-Cookie: lang=en-US; expires="Wed, 09 Jun 2021 10:18:14 GMT";
samesite=Strict
SF-Cookie: SID=31d4d96e407aad42, lang=en-US
* ISSUE: explicitly convert Expires to an integer?
https://github.com/mnot/I-D/issues/308 (https://github.com/mnot/I-
D/issues/308)
* ISSUE: dictionary keys cannot contain UC alpha.
https://github.com/mnot/I-D/issues/312 (https://github.com/mnot/I-
D/issues/312)
* ISSUE: explicitly allow non-string content.
https://github.com/mnot/I-D/issues/313 (https://github.com/mnot/I-
D/issues/313)
1.4. IANA Considerations
IANA is asked to register the following entries in the HTTP Field
Name Registry with a status of "permanent" and referring to this
document:
* SF-Content-Location
* SF-Location
* SF-Referer
* SF-Date
* SF-Expires
* SF-IMS
* SF-IUS
* SF-LM
* SF-ETag
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* SF-INM
* SF-Link
* SF-Set-Cookie
* SF-Cookie
2. Security Considerations
Section 1.2 identifies existing HTTP fields that can be parsed and
serialised with the algorithms defined in [STRUCTURED-FIELDS].
Variances from other implementations might be exploitable,
particularly if they allow an attacker to target one implementation
in a chain (e.g., an intermediary). However, given the considerable
variance in parsers already deployed, convergence towards a single
parsing algorithm is likely to have a net security benefit in the
longer term.
Section 1.3 defines alternative representations of existing fields.
Because downstream consumers might interpret the message differently
based upon whether they recognise the alternative representation,
implementations are prohibited from generating such fields unless
they have negotiated support for them with their peer. This
specification does not define such a mechanism, but any such
definition needs to consider the implications of doing so carefully.
3. Normative References
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://www.ietf.org/archive/id/draft-ietf-httpbis-
semantics-19.txt>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
[STRUCTURED-FIELDS]
Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
Appendix A. Data Supporting Field Compatibility
To help guide decisions about compatible fields, the HTTP response
headers captured by the HTTP Archive https://httparchive.org
(https://httparchive.org) in September 2021 (representing more than
528,000,000 HTTP exchanges) were parsed as Structured Fields using
the types listed in Section 1.2, with the indicated number of
successful header instances, failures, and the resulting failure
rate:
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accept 9,099 / 34 = 0.372%*
accept-encoding 116,708 / 58 = 0.050%*
accept-language 127,710 / 95 = 0.074%*
accept-patch 281 / 0 = 0.000%
accept-ranges 289,341,375 / 7,776 = 0.003%
access-control-allow-credentials 36,159,371 / 2,671 = 0.007%
access-control-allow-headers 25,980,519 / 23,181 = 0.089%
access-control-allow-methods 32,071,437 / 17,424 = 0.054%
access-control-allow-origin 165,719,859 / 130,247 = 0.079%
access-control-expose-headers 20,787,683 / 1,973 = 0.009%
access-control-max-age 9,549,494 / 9,846 = 0.103%
access-control-request-headers 165,882 / 503 = 0.302%*
access-control-request-method 346,135 / 30,680 = 8.142%*
age 107,395,872 / 36,649 = 0.034%
allow 579,822 / 281 = 0.048%
alt-svc 56,773,977 / 4,914,119 = 7.966%
cache-control 395,402,834 / 1,146,080 = 0.289%
connection 112,017,641 / 3,491 = 0.003%
content-encoding 225,568,224 / 237 = 0.000%
content-language 3,339,291 / 1,744 = 0.052%
content-length 422,415,406 / 126 = 0.000%
content-type 503,950,894 / 507,133 = 0.101%
cross-origin-resource-policy 102,483,430 / 799 = 0.001%
expect 0 / 53 = 100.000%*
expect-ct 54,129,244 / 80,333 = 0.148%
host 57,134 / 1,486 = 2.535%*
keep-alive 50,606,877 / 1,509 = 0.003%
origin 32,438 / 1,396 = 4.126%*
pragma 66,321,848 / 97,328 = 0.147%
preference-applied 189 / 0 = 0.000%
referrer-policy 14,274,787 / 8,091 = 0.057%
retry-after 523,533 / 7,585 = 1.428%
surrogate-control 282,846 / 976 = 0.344%
te 1 / 0 = 0.000%
timing-allow-origin 91,979,983 / 8 = 0.000%
trailer 1,171 / 0 = 0.000%
transfer-encoding 15,098,518 / 0 = 0.000%
vary 246,483,644 / 69,607 = 0.028%
x-content-type-options 166,063,072 / 237,255 = 0.143%
x-frame-options 56,863,322 / 1,014,464 = 1.753%
x-xss-protection 132,739,109 / 347,133 = 0.261%
Note that this data set only includes response headers, although some
request headers are present, indicated with an asterisk (because, the
Web). Also, Dictionary and Parameter keys have not been force-
lowercased, with the result that any values containing uppercase keys
are considered to fail.
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The top thirty header fields in that data set that were not
considered compatible are (* indicates that the field is mapped in
Section 1.3):
* *date: 524,810,577
* server: 470,777,294
* *last-modified: 383,437,099
* *expires: 292,109,781
* *etag: 255,788,799
* strict-transport-security: 111,993,787
* x-cache: 70,713,258
* via: 55,983,914
* cf-ray: 54,556,881
* p3p: 54,479,183
* report-to: 54,056,804
* cf-cache-status: 53,536,789
* nel: 44,815,769
* x-powered-by: 37,281,354
* content-security-policy-report-only: 33,104,387
* *location: 30,533,957
* x-amz-cf-pop: 28,549,182
* x-amz-cf-id: 28,444,359
* content-security-policy: 25,404,401
* x-served-by: 23,277,252
* x-cache-hits: 21,842,899
* *link: 20,761,372
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* x-timer: 18,780,130
* content-disposition: 18,516,671
* x-request-id: 16,048,668
* referrer-policy: 15,596,734
* x-cdn: 10,153,756
* x-amz-version-id: 9,786,024
* x-amz-request-id: 9,680,689
* x-dc: 9,557,728
Author's Address
Mark Nottingham
Prahran
VIC
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/
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