The I-JSON Message Format
A strict profile of JSON that closes the loopholes where different programs read the same text differently.
Bray, T., Ed., "The I-JSON Message Format", RFC 7493, DOI 10.17487/RFC7493, March 2015, <https://www.rfc-editor.org/info/rfc7493>.
@misc{rfc7493,
series = {Request for Comments},
number = 7493,
howpublished = {RFC 7493},
publisher = {RFC Editor},
doi = {10.17487/RFC7493},
url = {https://www.rfc-editor.org/info/rfc7493},
author = {T. Bray},
title = {{The I-JSON Message Format}},
pagetotal = 6,
year = 2015,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) T. Bray, Ed.
Request for Comments: 7493 Textuality Services
Category: Standards Track March 2015
ISSN: 2070-1721
The I-JSON Message Format
Abstract
I-JSON (short for "Internet JSON") is a restricted profile of JSON
designed to maximize interoperability and increase confidence that
software can process it successfully with predictable results.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7493.
Copyright Notice
Copyright (c) 2015 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. I-JSON Messages . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Encoding and Characters . . . . . . . . . . . . . . . . . 3
2.2. Numbers . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.3. Object Constraints . . . . . . . . . . . . . . . . . . . 3
3. Software Behavior . . . . . . . . . . . . . . . . . . . . . . 4
4. Recommendations for Protocol Design . . . . . . . . . . . . . 4
4.1. Top-Level Constructs . . . . . . . . . . . . . . . . . . 4
4.2. Must-Ignore Policy . . . . . . . . . . . . . . . . . . . 4
4.3. Time and Date Handling . . . . . . . . . . . . . . . . . 5
4.4. Binary Data . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Normative References . . . . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
RFC 7159 describes the JSON data interchange format, which is widely
used in Internet protocols. For historical reasons, that
specification allows the use of language idioms and text encoding
patterns that are likely to lead to interoperability problems and
software breakage, particularly when a program receiving JSON data
uses automated software to map it into native programming-language
structures or database records. RFC 7159 describes practices that
may be used to avoid these interoperability problems.
This document specifies I-JSON, short for "Internet JSON". The unit
of definition is the "I-JSON message". I-JSON messages are also
"JSON texts" as defined in RFC 7159 but with certain extra
constraints that enforce the good interoperability practices
described in that specification.
1.1. Terminology
The terms "object", "member", "array", "number", "name", and "string"
in this document are to be interpreted as described in RFC 7159
[RFC7159].
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. I-JSON Messages
An I-JSON message is a JSON text, as defined by RFC 7159.
2.1. Encoding and Characters
I-JSON messages MUST be encoded using UTF-8 [RFC3629].
Object member names, and string values in arrays and object members,
MUST NOT include code points that identify Surrogates or
Noncharacters as defined by [UNICODE].
This applies both to characters encoded directly in UTF-8 and to
those which are escaped; thus, "\uDEAD" is invalid because it is an
unpaired surrogate, while "\uD800\uDEAD" would be legal.
2.2. Numbers
Software that implements IEEE 754-2008 binary64 (double precision)
numbers [IEEE754] is generally available and widely used.
Implementations that generate I-JSON messages cannot assume that
receiving implementations can process numeric values with greater
magnitude or precision than provided by those numbers. I-JSON
messages SHOULD NOT include numbers that express greater magnitude or
precision than an IEEE 754 double precision number provides, for
example, 1E400 or 3.141592653589793238462643383279.
An I-JSON sender cannot expect a receiver to treat an integer whose
absolute value is greater than 9007199254740991 (i.e., that is
outside the range [-(2**53)+1, (2**53)-1]) as an exact value.
For applications that require the exact interchange of numbers with
greater magnitude or precision, it is RECOMMENDED to encode them in
JSON string values. This requires that the receiving program
understand the intended semantic of the value. An example would be
64-bit integers, even though modern hardware can deal with them,
because of the limited scope of JavaScript numbers.
2.3. Object Constraints
Objects in I-JSON messages MUST NOT have members with duplicate
names. In this context, "duplicate" means that the names, after
processing any escaped characters, are identical sequences of Unicode
characters.
The order of object members in an I-JSON message does not change the
meaning of an I-JSON message. A receiving implementation MAY treat
two I-JSON messages as equivalent if they differ only in the order of
the object members.
3. Software Behavior
A major advantage of using I-JSON is that receivers can avoid
ambiguous semantics in the JSON messages they receive. This allows
receivers to reject or otherwise disregard messages that do not
conform to the requirements in this document for I-JSON messages.
Protocols that use I-JSON messages can be written so that receiving
implementations are required to reject (or, as in the case of
security protocols, not trust) messages that do not satisfy the
constraints of I-JSON.
Designers of protocols that use I-JSON messages SHOULD provide a way,
in this case, for the receiver of the erroneous data to signal the
problem to the sender.
4. Recommendations for Protocol Design
I-JSON is designed for use in Internet protocols. The following
recommendations apply to the use of I-JSON in such protocols.
4.1. Top-Level Constructs
An I-JSON message can be any JSON value. However, there are software
implementations, coded to the older specification [RFC4627], which
only accept JSON objects or JSON arrays at the top level of JSON
texts. For maximum interoperability with such implementations,
protocol designers SHOULD NOT use top-level JSON texts that are
neither objects nor arrays.
4.2. Must-Ignore Policy
It is frequently the case that changes to protocols are required
after they have been put in production. Protocols that allow the
introduction of new protocol elements in a way that does not disrupt
the operation of existing software have proven advantageous in
practice.
This can be referred to as a "Must-Ignore" policy, meaning that when
an implementation encounters a protocol element that it does not
recognize, it should treat the rest of the protocol transaction as if
the new element simply did not appear, and in particular, the
implementation MUST NOT treat this as an error condition. The
converse "Must-Understand" policy does not tolerate the introduction
of new protocol elements, and while this has proven necessary in
certain protocol designs, in general it has been found to be overly
restrictive and brittle.
A good way to support the use of Must-Ignore in I-JSON protocol
designs is to require that top-level protocol elements must be JSON
objects, and to specify that members whose names are unrecognized
MUST be ignored.
4.3. Time and Date Handling
Protocols often contain data items that are designed to contain
timestamps or time durations. It is RECOMMENDED that all such data
items be expressed as string values in ISO 8601 format, as specified
in [RFC3339], with the additional restrictions that uppercase rather
than lowercase letters be used, that the timezone be included not
defaulted, and that optional trailing seconds be included even when
their value is "00". It is also RECOMMENDED that all data items
containing time durations conform to the "duration" production in
Appendix A of RFC 3339, with the same additional restrictions.
4.4. Binary Data
When it is required that an I-JSON protocol element contain arbitrary
binary data, it is RECOMMENDED that this data be encoded in a string
value in base64url; see Section 5 of [RFC4648].
5. Security Considerations
All the security considerations that apply to JSON (see RFC 7159)
apply to I-JSON. There are no additional security considerations
specific to I-JSON.
Since I-JSON forbids the use of certain JSON idioms that can lead to
unpredictable behavior in receiving software, it may prove a more
secure basis for Internet protocols and may be a good choice for
protocol designers with special security needs.
6. Normative References
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic", IEEE
754-2008, 2008, <http://grouper.ieee.org/groups/754/>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003,
<http://www.rfc-editor.org/info/rfc3629>.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006,
<http://www.rfc-editor.org/info/rfc4627>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014,
<http://www.rfc-editor.org/info/rfc7159>.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
<http://www.unicode.org/versions/latest/>.
Acknowledgements
I-JSON is entirely dependent on the design of JSON, largely due to
Douglas Crockford. The specifics were strongly influenced by the
contributors to the design of RFC 7159 in the IETF JSON Working
Group.
Author's Address
Tim Bray (editor)
Textuality Services
EMail: tbray@textuality.com
URI: https://www.tbray.org/
AI ELI5
On this page
§1Why a Profile of JSON
RFC 7159 describes JSON, which is widely used in Internet protocols. For historical reasons it allows language idioms and text encoding patterns that are likely to cause interoperability problems and software breakage. The trouble is worst when a program receiving JSON uses automated software to map it into native programming-language structures or database records.
I-JSON (short for Internet JSON) is the answer. The unit of definition is the I-JSON message. Every I-JSON message is also a JSON text as defined in RFC 7159, but with extra constraints that enforce the good interoperability practices RFC 7159 already describes. The terms object, member, array, number, name and string keep their RFC 7159 meanings, and the key words MUST, SHOULD and so on follow RFC 2119.
JSON lets you write the same idea in several slightly different ways, and computers can get confused. I-JSON picks the safe ways and says to use only those.
§2.1Encoding and Characters
I-JSON messages MUST be encoded using UTF-8 (RFC 3629). Object member names, and string values in arrays and object members, MUST NOT include code points that identify Surrogates or Noncharacters as defined by the Unicode Standard.
The rule covers characters written directly in UTF-8 and characters written as escapes. The RFC gives an example: \uDEAD is invalid because it is an unpaired surrogate, while \uD800\uDEAD would be legal.
Everybody must write their letters using the same alphabet code, and some special letter pieces may never be used on their own, like a puzzle piece that only works as half of a pair.
§2.2Numbers
Software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used. A sender therefore cannot assume that a receiver can handle numbers with greater magnitude or precision. I-JSON messages SHOULD NOT include numbers that express more magnitude or precision than an IEEE 754 double precision number provides. The RFC's examples are 1E400 and 3.141592653589793238462643383279.
A sender also cannot expect a receiver to treat an integer whose absolute value is greater than 9007199254740991 as exact. That is the range from -(253)+1 to (253)-1.
When an application needs exact numbers beyond those limits, it is RECOMMENDED to encode them in JSON string values. This works only if the receiving program understands what the string means. The RFC's example is 64-bit integers, which modern hardware handles but JavaScript numbers do not, given their limited scope.
A ruler only has so many tiny marks. If you write a length that needs more marks than the ruler has, the reader will round it. For exact big numbers, write them out as words in a string instead.
§2.3Object Constraints
Objects in I-JSON messages MUST NOT have members with duplicate names. Here, duplicate means that the names, after processing any escaped characters, are identical sequences of Unicode characters. Two names that look different on the page but decode to the same characters still count as duplicates.
The order of members in an object does not change the meaning of an I-JSON message. A receiving implementation MAY treat two I-JSON messages as equivalent if they differ only in the order of the object members.
In a list of labeled boxes, no two boxes may have the same label, or nobody knows which one you mean. It also does not matter which box sits first.
§3How Receivers Behave
A major advantage of I-JSON is that receivers can avoid ambiguous semantics in the messages they get. Receivers can reject or otherwise disregard messages that do not meet the I-JSON requirements.
Protocols that use I-JSON can be written so that receivers are required to reject messages that break the constraints, or, in the case of security protocols, not to trust them. Designers of such protocols SHOULD provide a way for the receiver of the erroneous data to signal the problem to the sender.
If a note is written in a confusing way, the reader is allowed to hand it back and say so, instead of guessing what it means.
§4Recommendations for Protocol Design
I-JSON is designed for use in Internet protocols, and the RFC adds four recommendations for protocol designers.
- Top level. An I-JSON message can be any JSON value, but some software written to the older RFC 4627 accepts only objects or arrays at the top level. For maximum interoperability, designers SHOULD NOT use top-level texts that are neither objects nor arrays.
- Must-Ignore. When an implementation meets a protocol element it does not recognize, it treats the rest of the transaction as if the element did not appear, and MUST NOT treat this as an error. The converse Must-Understand policy has proven overly restrictive and brittle in general. A good way to get Must-Ignore is to require top-level elements to be objects and to say that members with unrecognized names MUST be ignored.
- Time and date. Timestamps and durations are RECOMMENDED to be strings in ISO 8601 format as specified in RFC 3339, with extra restrictions: uppercase letters, an explicit time zone, and optional trailing seconds included even when they are
00. Durations are RECOMMENDED to follow thedurationproduction in Appendix A of RFC 3339, with the same restrictions. - Binary data. Arbitrary binary data is RECOMMENDED to be encoded in a string value using base64url, as in Section 5 of RFC 4648.
The RFC also gives tips for people who build things with I-JSON: put your data in a box, ignore labels you do not know instead of panicking, write dates one careful way, and turn pictures and files into plain letters first.
- Objects or arrays on topAvoid top-level texts that are neither, for older parsers.
- Must-IgnoreSkip unrecognized members instead of failing.
- RFC 3339 stringsTimes and durations as strings, uppercase, with time zone.
- base64urlBinary data goes in a string using base64url.
§5Security Considerations
All the security considerations that apply to JSON in RFC 7159 apply to I-JSON. The RFC says there are no additional security considerations specific to I-JSON.
Because I-JSON forbids certain JSON idioms that can lead to unpredictable behavior in receiving software, the RFC suggests it may prove a more secure basis for Internet protocols and may be a good choice for designers with special security needs.
Everything that is risky about JSON is still risky here. But fewer confusing choices means fewer chances for a sneaky trick to slip through.