json N. Williams
Internet-Draft Cryptonector
Intended status: Standards Track May 22, 2014
Expires: November 23, 2014
JavaScript Object Notation (JSON) Text Sequences
draft-ietf-json-text-sequence-03
Abstract
This document describes the JSON text sequence format and associated
media type.
Status of this Memo
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Table of Contents
1. Introduction and Motivation . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 3
2. JSON Text Sequence Format . . . . . . . . . . . . . . . . . 4
2.1. Requirements: . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use for Logfiles, or How to Resynchronize Following
Truncated entries . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction and Motivation
The JavaScript Object Notation (JSON) [RFC7159] is a very handy
serialization format. However, when serializing a large sequence of
values as an array, or a possibly indeterminate-length or never-
ending sequence of values, JSON becomes difficult to work with.
Consider a sequence of one million values, each possibly 1 kilobyte
when encoded, which would be roughly one gigabyte. If processing
such a dataset requires first parsing it entirely, then the result is
very inefficient and the processing will be limited by virtual
memory. "Online" (a.k.a., "streaming") parsers help, but they are
neither widely available or widely used, nor are they easy to use.
Ideally such datasets could be parsed and processed one element at a
time. Even if each element must be parsed in a not-online manner due
to local choice of parser, the result will usually be sufficiently
online: limited by the size of the biggest element in the sequence
rather than by the size of the sequence.
This document describes the concept and format of "JSON text
sequences", which are specifically not JSON texts themselves but are
composed of JSON texts.
1.1. Conventions used in this document
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 [RFC2119].
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2. JSON Text Sequence Format
The ABNF [RFC5234] for the JSON text sequence format is as given in
Figure 1. Note that this ABNF does not work if we assume greedy
matching. Therefore, in prose, a JSON text sequence is a sequence of
zero or more JSON texts, each surrounded by any number of JSON
whitespace characters and always followed by a newline.
JSON-sequence = ws *(JSON-text ws LF ws)
LF = <given by RFC5234>
ws = <given by RFC7159>
JSON-text = <given by RFC7159>
Figure 1: JSON text sequence ABNF
2.1. Requirements:
o JSON text sequence encoders MUST emit a newline after any JSON
text.
An input of 'truefalse' is not a valid sequence of two JSON values,
true and false! Neither is 'true0' a valid sequence of true and
zero. Some existing JSON parsers that might be used to construct
sequence parsers might in fact accept such sequences, resulting in
erroneous parsing of sequences of two or more numbers. E.g., a
sequence of two numbers, 4 and 2, encoded without the required
whitespace between them would parse incorrectly as the number 42.
This ambiguity is resolved by requiring that encoders emit a
whitespace separator (specifically: a newline) after each text.
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3. Use for Logfiles, or How to Resynchronize Following Truncated
entries
The JSON Text Sequence format is useful for logfiles, as those are
generally (and atomically) appended to on an ongoing basis. I.e.,
logfiles are of indeterminate length, at least right up until they
closed.
A problem comes up with this use case: it is difficult to guarantee
that append writes will complete. Therefore it's possible (if
unlikely) to end up with truncated log entries -which may fail to
parse as JSON texts- followed by other entries. The mechanics of
such failures are not explained here (but consider power failures).
Fortunately, as long as all texts in the logfile sequence are
followed by a newline, it is possible to detect a subsequent entry
written after an entry that fails to parse. Figure 2 shows an ABNF
rule for detecting the boundary between a non-truncated [and some
truncated] JSON text and the next JSON text in a sequence.
boundary = endchar *text-sep *ws startchar
text-sep = *(SP / HTAB / CR) LF ; these are from RFC5234
endchar = ( "}" / "]" / DQUOTE / "e" / "l" / DIGIT )
startchar = ( "{" / "[" / DQUOTE / "t" / "f" / "n" / "-" / DIGIT )
ws = <given by RFC7159>
Figure 2: ABNF for resynchronization
To resynchronize after failing to parse a JSON text, simply search
for a boundary as described in figure 2. A boundary found this way
might be the boundary between the truncated entry and the subsequent
entry, or it might be a subsequent boundary.
Scanning backwards for boundaries may not work reliably unless JSON
texts written to logfiles are stripped of internal newlines.
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4. Security Considerations
All the security considerations of JSON [RFC7159] apply.
There is no end of sequence indicator. This means that "end of
file", "end of transmission", and so on, can be indistinguishable
from a logical end of sequence. Applications where this matters
should denote end of sequence by convention (e.g., Content-Length in
HTTP).
JSON text sequence parsers based on non-incremental, non-online JSON
text parsers will not be able to efficiently parser JSON texts in
which newlines appear; attempting to parse such sequences with non-
incremental, non-online JSON text parsers creates a compute resource
exhaustion vulnerability.
The resynchronization heuristic for logfiles is imperfect and might
skip a valid entry following a truncated one. Purposefully appending
a truncated (or invalid) JSON text to a JSON text sequence logfile
can cause the subsequent entry to be invisible. Logfile writers
SHOULD validate (parse) any untrusted JSON text inputs and SHOULD
remove internal newlines from them, thus enabling reliable backwards
scanning for sequence element boundaries. Alternatively, logfile
writers might write texts in sequences of two texts, the first being
meaningless by convention. Of course, logfile writers SHOULD also
ensure that their writes are atomic, at least in so far as not
interleaving with other writers' writes.
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5. IANA Considerations
The MIME media type for JSON text sequences is application/json-seq.
Type name: application
Subtype name: json-seq
Required parameters: n/a
Optional parameters: n/a
Encoding considerations: binary
Security considerations: See <this document, once published>,
Section 4.
Interoperability considerations: Described herein.
Published specification: <this document, once published>.
Applicat<http://xml2rfc.tools.ietf.org/public/rfc/bibxml/
reference.RFC.2119.xml>ions that use this media type: JSON text
sequences have been used in applications written with the jq
programming language.
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6. Acknowledgements
Phillip Hallam-Baker proposed the use of JSON text sequences for
logfiles and pointed out the need for resynchronization. James
Manger contributed the ABNF for resynchronization.
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7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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