The I-JSON Message Format
A restricted form of JSON that helps different software interpret the same message consistently.
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
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(http://trustee.ietf.org/license-info) in effect on the date of
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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/
ISO Plain Language
On this page
§1Why Define I-JSON
JSON is widely used in Internet protocols, but some allowed language idioms and text encodings can lead to interoperability problems or software breakage. These problems can arise when receiving software maps JSON automatically into programming-language structures or database records.
I-JSON, short for Internet JSON, is a profile of JSON with additional constraints. Every I-JSON message is also a JSON text as defined by RFC 7159. The added rules enforce practices intended to make messages more predictable.
I-JSON adds limits to JSON so programs are less likely to disagree about the meaning of a message. An I-JSON message is still JSON, but it must follow these extra rules.
§2.1Encoding and Allowed Characters
An I-JSON message MUST use UTF-8 encoding. Object member names and string values MUST NOT contain Unicode code points that identify Surrogates or Noncharacters.
This restriction applies whether a character appears directly in the UTF-8 text or as an escape sequence. The RFC gives � as an invalid unpaired surrogate, while 𐊭 is permitted.
Every I-JSON message must use UTF-8. Names and text values cannot include Unicode surrogate or noncharacter code points, whether written directly or as escapes.
§2.2Numbers and Exact Values
I-JSON senders SHOULD NOT include numbers with greater magnitude or precision than an IEEE 754-2008 binary64 (double-precision) number provides. A sender cannot expect a receiver to treat an integer as exact if its absolute value exceeds 9007199254740991.
When an application needs exact interchange of numbers beyond these limits, the RFC RECOMMENDS encoding them as JSON strings. The receiving program must understand the string's intended meaning. The RFC gives 64-bit integers as an example.
Programs may not preserve very large or very precise JSON numbers exactly. If exact values beyond the supported number range are needed, send them as strings and define how the receiver should read them.
§2.3Object Member Names
An I-JSON object MUST NOT contain duplicate member names. Two names are duplicates when, after escape sequences are processed, they contain the same sequence of Unicode characters.
Object member order does not change the meaning of an I-JSON message. A receiver MAY treat two messages as equivalent when their only difference is the order of object members.
An object cannot use the same name for two members, even if one name uses escaped characters. Changing the order of members does not change the message's meaning.
§3How Receivers Handle Messages
Receivers can reject or disregard messages that do not meet the I-JSON requirements. A protocol using I-JSON can require its receivers to reject nonconforming messages or, for security protocols, to avoid trusting them.
Protocol designers SHOULD provide a way for the receiver to report an error to the sender when it receives invalid data.
A receiver can reject a message that breaks the I-JSON rules instead of guessing what it means. Protocol designers should also give receivers a way to report the problem.
§4Recommendations for Protocol Designers
I-JSON messages may contain any JSON value at the top level. However, some older software accepts only a JSON object or array at the top level, so protocol designers SHOULD avoid other top-level values when they want maximum interoperability.
The RFC recommends a Must-Ignore policy: software MUST NOT treat an unrecognized protocol element as an error, and should process the rest of the transaction as if the element were absent. It also RECOMMENDS representing timestamps and durations as strings using RFC 3339 with uppercase letters, an explicit time zone, and trailing seconds even when they are 00. Arbitrary binary data is RECOMMENDED to be placed in a string using base64url encoding.
For compatibility, place a JSON object or array at the top level and have software ignore members it does not recognize. The RFC also recommends a consistent string format for times and base64url strings for binary data.
- Top-level valuePrefer an object or array for compatibility with older software.
- Unknown membersIgnore protocol elements that the implementation does not recognize.
- Time and dateUse strings following the RFC's RFC 3339-based recommendations.
- Binary dataUse a string encoded with base64url.
§5Security Considerations
All security considerations that apply to JSON in RFC 7159 also apply to I-JSON. The RFC identifies no additional security considerations specific to I-JSON.
Because I-JSON forbids some JSON practices that can cause unpredictable behavior in receiving software, the RFC says it may provide a more secure basis for Internet protocols and may suit protocol designers with special security needs.
I-JSON does not remove the security concerns that apply to JSON. Its stricter rules may reduce unpredictable interpretations, but protocol designers still need to address JSON security risks.