Internet DRAFT - draft-nottingham-http-structure-retrofit
draft-nottingham-http-structure-retrofit
Network Working Group M. Nottingham
Internet-Draft 6 January 2022
Intended status: Informational
Expires: 10 July 2022
Retrofit Structured Fields for HTTP
draft-nottingham-http-structure-retrofit-02
Abstract
This specification defines how a selection of existing HTTP fields
can be handled as Structured Fields.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-nottingham-http-structure-
retrofit/.
information can be found at https://mnot.github.io/I-D/.
Source for this draft and an issue tracker can be found at
https://github.com/mnot/I-D/labels/http-structure-retrofit.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 10 July 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
2. Compatible Fields . . . . . . . . . . . . . . . . . . . . . . 3
3. Mapped Fields . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. URLs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Dates . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. ETags . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Cookies . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. 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
use by new 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.
* 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
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* 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 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 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 2, a sender
cannot generate mapped fields from Section 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.
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 [STRUCTURED-FIELDS], 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.
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An application using this specification will need to consider how to
handle such 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
* ALPN - List
* Alt-Svc - Dictionary
* Alt-Used - Item
* Cache-Control - Dictionary
* Connection - List
* Content-Encoding - List
* Content-Language - List
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* 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
* Vary - List
* X-Content-Type-Options - Item
* X-Frame-Options - Item
* X-XSS-Protection - List
Note the following caveats:
Parameter names: HTTP parameter names are case-insensitive (as per
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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.
3. Mapped Fields
Some HTTP fields 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.
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
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As in Section 2, these fields are unable to represent values that are
not parseable, and so an application using this specification will
need to how to support such values. Typically, handling 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.
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"
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 Section 5.6.7 of [HTTP] 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
* If-Unmodified-Since - SF-IUS
* Last-Modified - SF-LM
For example, an Expires field could be represented as:
SF-Expires: 1571965240
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3.3. ETags
The field value of the ETag header field can be represented as a
String Structured Field 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.
For example:
SF-ETag: "abcdef"; w=?1
If-None-Match's field value can be represented as SF-INM, which is a
List of the structure described above.
For example:
SF-INM: "abcdef"; w=?1, "ghijkl"
3.4. Links
The field value of the Link header field [RFC8288] can be represented
in the SF-Link List Structured Field by representing the URI-
Reference as a string, and link-param as parameters.
For example:
SF-Link: "/terms"; rel="copyright"; anchor="#foo"
3.5. Cookies
The field values of the Cookie and Set-Cookie fields [RFC6265] can be
represented in the SF-Cookie Structured Field (a List) and SF-Set-
Cookie Structured Field (a Dictionary), respectively.
In each case, cookie names are serialized as tokens, whereas their
values are serialised as Strings, unless they can be represented
accurately and unambiguously using the textual representation of
another structured types (e.g., an Integer or Decimal).
Set-Cookie parameters map to parameters on the appropriate SF-Set-
Cookie member, with the parameter name being forced to lowercase.
Set-Cookie parameter values are Strings unless a specific type is
defined. This specification defines the following parameter types:
* Max-Age: Integer
* Secure: Boolean
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* HttpOnly: Boolean
* SameSite: Token
Note that cookies in both fields are separated by commas, not
semicolons, and multiple cookies can appear in each field.
For example:
SF-Set-Cookie: lang=en-US; expires="Wed, 09 Jun 2021 10:18:14 GMT";
samesite=Strict
SF-Cookie: SID=31d4d96e407aad42, lang=en-US
4. IANA Considerations
Please add the following note to the HTTP Field Name Registry:
The "Structured Type" column indicates the type of the field as
per RFC8941, if any, and may be "Dictionary", "List" or "Item". A
prefix of "*" indicates that it is a retrofit type (i.e., not
natively Structured); see [this specification].
Then, add a new column, "Structured Type", with the values from
Section 2 assigned to the nominated registrations, prefixing each
with "*" to indicate that it is a retrofit type.
Then, add the following field names into the HTTP Field Name
Registry, with the corresponding Structured Type as indicated, a
status of "permanent" and referring to this document:
* SF-Content-Location - String
* SF-Location - String
* SF-Referer - String
* SF-Date - Integer
* SF-Expires - Integer
* SF-IMS - Integer
* SF-IUS - Integer
* SF-LM - Integer
* SF-ETag - Item
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* SF-INM - List
* SF-Link - List
* SF-Set-Cookie - Dictionary
* SF-Cookie - List
5. Security Considerations
Section 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 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.
6. 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://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
[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/rfc/rfc2119>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/rfc/rfc6265>.
[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/rfc/rfc8174>.
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[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/rfc8288>.
[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/rfc/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 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 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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