A JSON Encoding for HTTP Field Values
draft-reschke-http-jfv-14
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Julian Reschke
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Network Working Group J. F. Reschke
Internet-Draft greenbytes
Intended status: Informational 22 April 2021
Expires: 24 October 2021
A JSON Encoding for HTTP Field Values
draft-reschke-http-jfv-14
Abstract
This document establishes a convention for use of JSON-encoded field
values in new HTTP fields.
Editorial Note
This note is to be removed before publishing as an RFC.
Distribution of this document is unlimited. Although this is not a
work item of the HTTPbis Working Group, comments should be sent to
the Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-
wg@w3.org (mailto:ietf-http-wg@w3.org), which may be joined by
sending a message with subject "subscribe" to ietf-http-wg-
request@w3.org (mailto:ietf-http-wg-
request@w3.org?subject=subscribe).
Discussions of the HTTPbis Working Group are archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
XML versions and latest edits for this document are available from
<http://greenbytes.de/tech/webdav/#draft-reschke-http-jfv>.
The changes in this draft are summarized in Appendix D.17.
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/.
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."
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This Internet-Draft will expire on 24 October 2021.
Copyright Notice
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
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 Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Relation to "Structured Field Values for HTTP"
(RFC8941) . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Data Model and Format . . . . . . . . . . . . . . . . . . . . 4
3. Sender Requirements . . . . . . . . . . . . . . . . . . . . . 5
3.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Recipient Requirements . . . . . . . . . . . . . . . . . . . 6
4.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Using this Format in Field Definitions . . . . . . . . . . . 7
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
7. Interoperability Considerations . . . . . . . . . . . . . . . 7
7.1. Encoding and Characters . . . . . . . . . . . . . . . . . 7
7.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.3. Object Constraints . . . . . . . . . . . . . . . . . . . 8
8. Internationalization Considerations . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
10.3. Specifications Using This Syntax (at some point of
time) . . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Comparison with Structured Fields . . . . . . . . . 10
A.1. Base Types . . . . . . . . . . . . . . . . . . . . . . . 10
A.2. Structures . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix B. Use of JSON Field Value Encoding in the Wild . . . . 11
B.1. W3C Reporting API Specification . . . . . . . . . . . . . 12
B.2. W3C Clear Site Data Specification . . . . . . . . . . . . 12
B.3. W3C Feature Policy Specification . . . . . . . . . . . . 12
Appendix C. Implementations . . . . . . . . . . . . . . . . . . 12
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 12
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D.1. Since draft-reschke-http-jfv-00 . . . . . . . . . . . . . 12
D.2. Since draft-reschke-http-jfv-01 . . . . . . . . . . . . . 12
D.3. Since draft-reschke-http-jfv-02 . . . . . . . . . . . . . 13
D.4. Since draft-reschke-http-jfv-03 . . . . . . . . . . . . . 13
D.5. Since draft-reschke-http-jfv-04 . . . . . . . . . . . . . 13
D.6. Since draft-ietf-httpbis-jfv-00 . . . . . . . . . . . . . 13
D.7. Since draft-ietf-httpbis-jfv-01 . . . . . . . . . . . . . 13
D.8. Since draft-ietf-httpbis-jfv-02 . . . . . . . . . . . . . 13
D.9. Since draft-reschke-http-jfv-05 . . . . . . . . . . . . . 13
D.10. Since draft-reschke-http-jfv-06 . . . . . . . . . . . . . 14
D.11. Since draft-reschke-http-jfv-07 . . . . . . . . . . . . . 14
D.12. Since draft-reschke-http-jfv-08 . . . . . . . . . . . . . 14
D.13. Since draft-reschke-http-jfv-09 . . . . . . . . . . . . . 14
D.14. Since draft-reschke-http-jfv-10 . . . . . . . . . . . . . 14
D.15. Since draft-reschke-http-jfv-11 . . . . . . . . . . . . . 14
D.16. Since draft-reschke-http-jfv-12 . . . . . . . . . . . . . 15
D.17. Since draft-reschke-http-jfv-13 . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Defining syntax for new HTTP fields ([HTTP], Section 5) is non-
trivial. Among the commonly encountered problems are:
* There is no common syntax for complex field values. Several well-
known fields do use a similarly looking syntax, but it is hard to
write generic parsing code that will both correctly handle valid
field values but also reject invalid ones.
* The HTTP message format allows field lines to repeat, so field
syntax needs to be designed in a way that these cases are either
meaningful, or can be unambiguously detected and rejected.
* HTTP does not define a character encoding scheme ([RFC6365],
Section 2), so fields are either stuck with US-ASCII ([RFC0020]),
or need out-of-band information to decide what encoding scheme is
used. Furthermore, APIs usually assume a default encoding scheme
in order to map from octet sequences to strings (for instance,
[XMLHttpRequest] uses the IDL type "ByteString", effectively
resulting in the ISO-8859-1 character encoding scheme [ISO-8859-1]
being used).
(See Section 16.3 of [HTTP] for a summary of considerations for new
fields.)
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This specification addresses the issues listed above by defining both
a generic JSON-based ([RFC8259]) data model and a concrete wire
format that can be used in definitions of new fields, where the goals
were:
* to be compatible with field recombination when field lines occur
multiple times in a single message (Section 5.3 of [HTTP]), and
* not to use any problematic characters in the field value (non-
ASCII characters and certain whitespace characters).
1.1. Relation to "Structured Field Values for HTTP" ([RFC8941])
"Structured Field Values for HTTP", an IETF RFC on the Standards
Track, is a different approach to this set of problems. It uses a
more compact notation, similar to what is used in existing header
fields, and avoids several potential interoperability problems
inherent to the use of JSON.
In general, that format is preferred for newly defined fields. The
JSON-based format defined by this document might however be useful in
case the data that needs to be transferred is already in JSON format,
or features not covered by "Structured Field Values" are needed.
See Appendix A for more details.
2. Data Model and Format
In HTTP, field lines with the same field name can occur multiple
times within a single message (Section 5.3 of [HTTP]). When this
happens, recipients are allowed to combine the field line values
using commas as delimiter, forming a combined "field value". This
rule matches nicely JSON's array format (Section 5 of [RFC8259]).
Thus, the basic data model used here is the JSON array.
Field definitions that need only a single value can restrict
themselves to arrays of length 1, and are encouraged to define error
handling in case more values are received (such as "first wins",
"last wins", or "abort with fatal error message").
JSON arrays are mapped to field values by creating a sequence of
serialized member elements, separated by commas and optionally
whitespace. This is equivalent to using the full JSON array format,
while leaving out the "begin-array" ('[') and "end-array" (']')
delimiters.
The ABNF character names and classes below are used (copied from
[RFC5234], Appendix B.1):
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CR = %x0D ; carriage return
HTAB = %x09 ; horizontal tab
LF = %x0A ; line feed
SP = %x20 ; space
VCHAR = %x21-7E ; visible (printing) characters
Characters in JSON strings that are not allowed or discouraged in
HTTP field values - that is, not in the "VCHAR" definition - need to
be represented using JSON's "backslash" escaping mechanism
([RFC8259], Section 7).
The control characters CR, LF, and HTAB do not appear inside JSON
strings, but can be used outside (line breaks, indentation etc.).
These characters need to be either stripped or replaced by space
characters (ABNF "SP").
Formally, using the HTTP specification's ABNF extensions defined in
Section 5.6.1 of [HTTP]:
json-field-value = #json-field-item
json-field-item = JSON-Text
; see [RFC8259], Section 2,
; post-processed so that only VCHAR characters
; are used
3. Sender Requirements
To map a JSON array to an HTTP field value, process each array
element separately by:
1. generating the JSON representation,
2. stripping all JSON control characters (CR, HTAB, LF), or
replacing them by space ("SP") characters,
3. replacing all remaining non-VSPACE characters by the equivalent
backslash-escape sequence ([RFC8259], Section 7).
The resulting list of strings is transformed into an HTTP field value
by combining them using comma (%x2C) plus optional SP as delimiter,
and encoding the resulting string into an octet sequence using the
US-ASCII character encoding scheme ([RFC0020]).
3.1. Example
With the JSON data below, containing the non-ASCII characters "ü"
(LATIN SMALL LETTER U WITH DIAERESIS, U+00FC) and "€" (EURO SIGN,
U+20AC):
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[
{
"destination": "Münster",
"price": 123,
"currency": "€"
}
]
The generated field value would be:
{ "destination": "M\u00FCnster", "price": 123, "currency": "\u20AC" }
4. Recipient Requirements
To map a set of HTTP field line values to a JSON array:
1. combine all field line values into a single field value as per
Section 5.3 of [HTTP],
2. add a leading begin-array ("[") octet and a trailing end-array
("]") octet, then
3. run the resulting octet sequence through a JSON parser.
The result of the parsing operation is either an error (in which case
the field values needs to be considered invalid), or a JSON array.
4.1. Example
An HTTP message containing the field lines:
Example: "\u221E"
Example: {"date":"2012-08-25"}
Example: [17,42]
would be parsed into the JSON array below:
[
"∞",
{
"date": "2012-08-25"
},
[
17,
42
]
]
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5. Using this Format in Field Definitions
Specifications defining new HTTP fields need to take the
considerations listed in Section 16.3 of [HTTP] into account. Many
of these will already be accounted for by using the format defined in
this specification.
Readers of HTTP-related specifications frequently expect an ABNF
definition of the field value syntax. This is not really needed
here, as the actual syntax is JSON text, as defined in Section 2 of
[RFC8259].
A very simple way to use this JSON encoding thus is just to cite this
specification - specifically the "json-field-value" ABNF production
defined in Section 2 - and otherwise not to talk about the details of
the field syntax at all.
An alternative approach is just to repeat the ABNF-related parts from
Section 2.
This frees the specification from defining the concrete on-the-wire
syntax. What's left is defining the field value in terms of a JSON
array. An important aspect is the question of extensibility, e.g.
how recipients ought to treat unknown field names. In general, a
"must ignore" approach will allow protocols to evolve without
versioning or even using entire new field names.
6. Deployment Considerations
This JSON-based syntax will only apply to newly introduced fields,
thus backwards compatibility is not a problem. That being said, it
is conceivable that there is existing code that might trip over
double quotes not being used for HTTP's quoted-string syntax
(Section 5.6.4 of [HTTP]).
7. Interoperability Considerations
The "I-JSON Message Format" specification ([RFC7493]) addresses known
JSON interoperability pain points. This specification borrows from
the requirements made over there:
7.1. Encoding and Characters
This specification requires that field values use only US-ASCII
characters, and thus by definition uses a subset of UTF-8
(Section 2.1 of [RFC7493]).
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Furthermore, escape sequences in JSON strings (Section 7 of
[RFC8259]) - both in object member names and string values - are not
allowed to represent non-Unicode code points such as unpaired
surrogates or Noncharacters (see "General Structure" in [UNICODE]).
7.2. Numbers
Be aware of the issues around number precision, as discussed in
Section 2.2 of [RFC7493].
7.3. Object Constraints
As described in Section 4 of [RFC8259], JSON parser implementations
differ in the handling of duplicate object names. Therefore, senders
are not allowed to use duplicate object names, and recipients are
advised to either treat field values with duplicate names as invalid
(consistent with [RFC7493], Section 2.3) or use the lexically last
value (consistent with [ECMA-262], Section 24.3.1.1).
Furthermore, ordering of object members is not significant and can
not be relied upon.
8. Internationalization Considerations
In current versions of HTTP, field values are represented by octet
sequences, usually used to transmit ASCII characters, with
restrictions on the use of certain control characters, and no
associated default character encoding, nor a way to describe it
([HTTP], Section 5).
This specification maps all characters which can cause problems to
JSON escape sequences, thereby solving the HTTP field
internationalization problem.
Future specifications of HTTP might change to allow non-ASCII
characters natively. In that case, fields using the syntax defined
by this specification would have a simple migration path (by just
stopping to require escaping of non-ASCII characters).
9. Security Considerations
Using JSON-shaped field values is believed to not introduce any new
threads beyond those described in Section 12 of [RFC8259], namely the
risk of recipients using the wrong tools to parse them.
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Other than that, any syntax that makes extensions easy can be used to
smuggle information through field values; however, this concern is
shared with other widely used formats, such as those using parameters
in the form of name/value pairs.
10. References
10.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", Work in Progress, Internet-Draft,
draft-ietf-httpbis-semantics-15, 30 March 2021,
<https://tools.ietf.org/html/draft-ietf-httpbis-semantics-
15>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[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>.
[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 8259, DOI 10.17487/RFC8259,
December 2017, <https://www.rfc-editor.org/info/rfc8259>.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
10.2. Informative References
[ECMA-262] Ecma International, "ECMA-262 6th Edition, The ECMAScript
2015 Language Specification", Standard ECMA-262, June
2015, <http://www.ecma-international.org/ecma-262/6.0/>.
[ISO-8859-1]
International Organization for Standardization,
"Information technology -- 8-bit single-byte coded graphic
character sets -- Part 1: Latin alphabet No. 1", ISO/
IEC 8859-1:1998, 1998.
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[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<https://www.rfc-editor.org/info/rfc6365>.
[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>.
[XMLHttpRequest]
WhatWG, "XMLHttpRequest", <https://xhr.spec.whatwg.org/>.
10.3. Specifications Using This Syntax (at some point of time)
[CLEARSITE]
West, M., "Clear Site Data", W3C Working Draft WD-clear-
site-data-20171130, 30 November 2017,
<https://www.w3.org/TR/2017/WD-clear-site-data-20171130/>.
Latest version available at <https://www.w3.org/TR/clear-
site-data/>.
[FEATUREPOL]
Clelland, I., "Feature Policy", W3C Editor's Draft ,
<https://w3c.github.io/webappsec-feature-policy/>.
[REPORTING]
Creager, D., Grigorik, I., Meyer, P., and M. West,
"Reporting API", W3C Working Draft WD-reporting-
1-20180925, 25 September 2018,
<https://www.w3.org/TR/2018/WD-reporting-1-20180925/>.
Latest version available at <https://www.w3.org/TR/
reporting-1/>.
Appendix A. Comparison with Structured Fields
A.1. Base Types
+==========+==================================+==================+
| Type | in Structured Fields | in JSON-based |
| | | Fields |
+==========+==================================+==================+
| Integer | [RFC8941], Section 3.3.1 | [RFC8259], |
| | | Section 6 |
| +----------------------------------+------------------+
| | (restricted to 15 digits) | |
+==========+----------------------------------+------------------+
| Decimal | [RFC8941], Section 3.3.2 | [RFC8259], |
| | | Section 6 |
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| +----------------------------------+------------------+
| | (a fixed point decimal | |
| | restricted to 12 + 3 digits) | |
+==========+----------------------------------+------------------+
| String | [RFC8941], Section 3.3.3 | [RFC8259], |
| | | Section 7 |
| +----------------------------------+------------------+
| | (only ASCII supported, non-ASCII | |
| | requires using Byte Sequences) | |
+==========+----------------------------------+------------------+
| Token | [RFC8941], Section 3.3.4 | not available |
+==========+----------------------------------+------------------+
| Byte | [RFC8941], Section 3.3.5 | not available |
| Sequence +----------------------------------+------------------+
| | | (usually mapped |
| | | to strings using |
| | | base64 encoding) |
+==========+----------------------------------+------------------+
| Boolean | [RFC8941], Section 3.3.6 | [RFC8259], |
| | | Section 3 |
+==========+----------------------------------+------------------+
Table 1
Structured Fields provide more data types (such as "token" or "byte
sequence"). Numbers are restricted, avoiding the JSON interop
problems described in Section 7.2. Strings are limited to ASCII,
requiring the use of byte sequences should non-ASCII characters be
needed.
A.2. Structures
Structured Fields define Lists ([RFC8941], Section 3.1), similar to
JSON arrays ([RFC8259], Section 5), and Dictionaries ([RFC8941],
Section 3.2), similar to JSON objects ([RFC8259], Section 4).
In addition, most items in Structured Fields can be parametrized
([RFC8941], Section 3.1.2), attaching a dictionary-like structure to
the value. To emulate this in JSON based field, an additional
nesting of objects would be needed.
Finally, nesting of data structures is intentionally limited to two
levels (see Appendix A.1 of [RFC8941] for the motivation).
Appendix B. Use of JSON Field Value Encoding in the Wild
This section is to be removed before publishing as an RFC.
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Since work started on this document, various specifications have
adopted this format. At least one of these moved away after the HTTP
Working Group decided to focus on [RFC8941] (see thread starting at
<https://lists.w3.org/Archives/Public/ietf-http-
wg/2016OctDec/0505.html>).
The sections below summarize the current usage of this format.
B.1. W3C Reporting API Specification
Defined in W3C Working Draft "Reporting API" (Section 3.1 of
[REPORTING]). Still in use in latest working draft dated September
2018.
B.2. W3C Clear Site Data Specification
Used in earlier versions of "Clear Site Data". The current version
replaces the use of JSON with a custom syntax that happens to be
somewhat compatible with an array of JSON strings (see Section 3.1 of
[CLEARSITE] and <https://lists.w3.org/Archives/Public/ietf-http-
wg/2017AprJun/0214.html> for feedback).
B.3. W3C Feature Policy Specification
Originally defined in W3C document "Feature Policy" ([FEATUREPOL]),
but switched to use of Structured Header Fields ([RFC8941]).
Appendix C. Implementations
This section is to be removed before publishing as an RFC.
See <https://github.com/reschke/json-fields> for a proof-of-concept
(in development).
Appendix D. Change Log
This section is to be removed before publishing as an RFC.
D.1. Since draft-reschke-http-jfv-00
Editorial fixes + working on the TODOs.
D.2. Since draft-reschke-http-jfv-01
Mention slightly increased risk of smuggling information in header
field values.
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D.3. Since draft-reschke-http-jfv-02
Mention Kazuho Oku's proposal for abbreviated forms.
Added a bit of text about the motivation for a concrete JSON subset
(ack Cory Benfield).
Expand I18N section.
D.4. Since draft-reschke-http-jfv-03
Mention relation to KEY header field.
D.5. Since draft-reschke-http-jfv-04
Between June and December 2016, this was a work item of the HTTP
working group (see <https://datatracker.ietf.org/doc/draft-ietf-
httpbis-jfv/>). Work (if any) continues now on
<https://datatracker.ietf.org/doc/draft-reschke-http-jfv/>.
Changes made while this was a work item of the HTTP Working Group:
D.6. Since draft-ietf-httpbis-jfv-00
Added example for "Accept-Encoding" (inspired by Kazuho's feedback),
showing a potential way to optimize the format when default values
apply.
D.7. Since draft-ietf-httpbis-jfv-01
Add interop discussion, building on I-JSON and ECMA-262 (see
<https://github.com/httpwg/http-extensions/issues/225>).
D.8. Since draft-ietf-httpbis-jfv-02
Move non-essential parts into appendix.
Updated XHR reference.
D.9. Since draft-reschke-http-jfv-05
Add meat to "Using this Format in Header Field Definitions".
Add a few lines on the relation to "Key".
Summarize current use of the format.
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D.10. Since draft-reschke-http-jfv-06
RFC 5987 is obsoleted by RFC 8187.
Update CLEARSITE comment.
D.11. Since draft-reschke-http-jfv-07
Update JSON and HSTRUCT references.
FEATUREPOL doesn't use JSON syntax anymore.
D.12. Since draft-reschke-http-jfv-08
Update HSTRUCT reference.
Update notes about CLEARSITE and FEATUREPOL.
D.13. Since draft-reschke-http-jfv-09
Update HSTRUCT and FEATUREPOL references.
Update note about REPORTING.
Changed category to "informational".
D.14. Since draft-reschke-http-jfv-10
Update HSTRUCT reference.
D.15. Since draft-reschke-http-jfv-11
Update HSTRUCT reference.
Update note about FEATUREPOL (now using Structured Fields).
Reference [HTTP] instead if RFC723* and adjust (header) field
terminology accordingly.
Remove discussion about the relation to KEY (as that spec is dormant:
<https://datatracker.ietf.org/doc/draft-ietf-httpbis-key/>).
Remove appendices "Examples" and "Discussion".
Mark "Use of JSON Field Value Encoding in the Wild" for removal in
RFC.
Reschke Expires 24 October 2021 [Page 14]
Internet-Draft JSON Encoding for HTTP Field Values April 2021
D.16. Since draft-reschke-http-jfv-12
Update HTTP reference and update terminology some more.
Update HSTRUCT reference (now RFC 8941).
D.17. Since draft-reschke-http-jfv-13
Update HTTP reference.
Mention test implementation.
Clarify that Unicode unpaired surrogates or Noncharacters must not be
sent.
Rewrite text about [RFC8941], add appendix comparing both formats.
And send/receive examples.
Acknowledgements
Thanks go to the Hypertext Transfer Protocol Working Group
participants.
Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
48155 Münster
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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