    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.

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    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].

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    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.

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       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

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       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>.

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       [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/

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