Concise Binary Object Representation (CBOR) Tags for Time, Duration, and Period
draft-bormann-cbor-time-tag-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Carsten Bormann , Ben Gamari , Henk Birkholz | ||
| Last updated | 2017-03-13 | ||
| Replaced by | draft-ietf-cbor-time-tag | ||
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draft-bormann-cbor-time-tag-00
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational B. Gamari
Expires: September 13, 2017 Well-Typed
H. Birkholz
Fraunhofer SIT
March 12, 2017
Concise Binary Object Representation (CBOR) Tags for Time, Duration, and
Period
draft-bormann-cbor-time-tag-00
Abstract
The Concise Binary Object Representation (CBOR, RFC 7049) is a data
format whose design goals include the possibility of extremely small
code size, fairly small message size, and extensibility without the
need for version negotiation.
In CBOR, one point of extensibility is the definition of CBOR tags.
RFC 7049 defines two tags for time: CBOR tag 0 (RFC3339 time) and tag
1 (Posix time [TIME_T], int or float). Since then, additional
requirements have become known. The present document defines a CBOR
tag for time that allows a more elaborate representation of time, as
well as CBOR tags for duration and time period. It is intended as
the reference document for the IANA registration of the CBOR tags
defined.
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 http://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."
This Internet-Draft will expire on September 13, 2017.
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Copyright Notice
Copyright (c) 2017 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 . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Background . . . . . . . . . . . . . . . . . . . . . . . 3
2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Time Format . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Keys 0 and 1 . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Keys 4 and 5 . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Keys -3, -6, -9, -12 . . . . . . . . . . . . . . . . . . 5
3.4. Key -1 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.5. Key -2 . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.6. Key -4 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.7. Key -5 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.8. Key -7 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.9. Key -8 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.10. Key -10 . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Duration Format . . . . . . . . . . . . . . . . . . . . . . . 7
5. Period Format . . . . . . . . . . . . . . . . . . . . . . . . 7
6. CDDL typenames . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The Concise Binary Object Representation (CBOR, [RFC7049]) provides
for the interchange of structured data without a requirement for a
pre-agreed schema. RFC 7049 defines a basic set of data types, as
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well as a tagging mechanism that enables extending the set of data
types supported via an IANA registry.
(TBD: Expand on text from abstract here.)
1.1. Terminology
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].
The term "byte" is used in its now customary sense as a synonym for
"octet". Where bit arithmetic is explained, this document uses the
notation familiar from the programming language C (including C++14's
0bnnn binary literals), except that the operator "**" stands for
exponentiation.
1.2. Background
Additional information about the complexities of time representation
can be found in [TIME]. This specification uses a number of terms
that should be familiar to connoisseurs of precise time; references
for these may need to be added.
2. Objectives
For the time tag, the present specification addresses the following
objectives that go beyond the original tags 0 and 1:
o Indication of time scale. Tags 0 and 1 are for UTC; however, some
interchanges are better performed on TAI. Other time scales may
be registered once they become relevant (e.g., one of the proposed
successors to UTC that might no longer use leap seconds, or a
scale based on smeared leap seconds).
o Additional resolution for epoch-based time (as in tag 1). CBOR
tag 1 only provides for integer and up to binary64 floating point
representation of times, limiting resolution to approximately
microseconds at the time of writing (and progressively becoming
worse over time).
o Direct representation of natural platform time formats. Some
platforms use epoch-based time formats that require some
computation to convert them into the representations allowed by
tag 1; these computations can also lose precision and cause
ambiguities. (TBD: The present specification does not take a
position on whether tag 1 can be "fixed" to include, e.g., Decimal
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or BigFloat representations. It does define how to use these with
the extended time format.)
o Additional indication of intents about the interpretation of the
time given, in particular for future times. Intents might include
information about time zones, daylight savings times, etc. (TBD:
This is not yet a well-developed part of the spec; there needs to
be some effort to avoid the kitchen sink.)
The objectives for the duration and period tags are similar.
3. Time Format
An extended time is indicated by CBOR tag TBDET, which tags a map
data item (CBOR major type 5). The map may contain integer (major
types 0 and 1) or text string (major type 3) keys, with the value
type determined by each specific key. Implementations MUST ignore
key/value types they do not understand. (Discussion: Do we need
"critical" keys?)
The map must contain exactly one unsigned integer key, which
specifies the "base time", and may also contain one or more negative
integer or text-string keys, which may encode supplementary
information such as,
o a higher precision time offset to be added to the base time,
o a reference time scale,
o information about clock source and precision, accuracy, and
resolution
o intent information such as timezone and daylight savings time,
and/or possibly positioning coordinates, to express information
that would indicate a local time.
While this document does not define supplementary text keys, a number
of unsigned and negative-integer keys are defined below.
3.1. Keys 0 and 1
Keys 0 and 1 indicate values that are exactly like the data items
that would be tagged by CBOR tag 0 (RFC 3339 date/time string) or tag
1 (Posix time [TIME_T] as int or float), respectively.
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3.2. Keys 4 and 5
Keys 4 and 5 are like key 1, except that the data item is an array as
defined for CBOR tag 4 or 5, respectively. This can be used to
include a Decimal or Bigfloat epoch-based float [TIME_T] in an
extended time.
3.3. Keys -3, -6, -9, -12
The keys -3, -6, -9, -12 indicate additional decimal fractions by
giving an unsigned integer (major type 0) and scaling this with the
scale factor 1e-3, 1e-6, 1e-9, and 1e-12, respectively (see Table 1).
More than one of these keys MUST NOT be present in one extended time
data item. These additional fractions are added to a base time in
seconds [SI-SECOND] indicated by a Key 1, which then MUST also be
present and MUST have an integer value.
+-----+--------------+-----------------+
| Key | meaning | example usage |
+-----+--------------+-----------------+
| -3 | milliseconds | Java time |
| -6 | microseconds | (old) UNIX time |
| -9 | nanoseconds | (new) UNIX time |
| -12 | picoseconds | Haskell time |
+-----+--------------+-----------------+
Table 1: Key for decimally scaled Fractions
3.4. Key -1
Key -1 is used to indicate a time scale. The value 0 indicates UTC,
the value 1 indicates TAI. If key -1 is not present, time scale
value 0 is implied. Additional values can be registered in the (TBD
define name for time scale registry); values MUST be integers or text
strings.
(Note that there should be no time scale "GPS" - instead, the time
should be converted to TAI using a single subtraction.)
3.5. Key -2
Key -2 can be used to indicate the quality of the point in time: The
value 0 indicates a time obtained from a clock (past or "current"
time). The value -1 indicates a future time that has been scheduled
by a human. The value 1 indicates a time derived from a time
obtained from a clock (such as the timestamp of a record in a log
file). (TBD: Is this well-defined enough? What other cases should
be considered here?)
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If key -2 is not present, no information is available about the
quality of the time.
3.6. Key -4
Key -4 can be used to indicate the resolution of the time provided
[RESOLUTION]: "The minimum time interval that a clock can measure or
whose passage a timer can detect." The value is expressed in SI
seconds [SI-SECOND] and can be any positive number, such as an
integer, a floating point value (major type 7 or Tag 5), or a decimal
value (Tag 4).
3.7. Key -5
Key -5 can be used to indicate the accuracy of the time
[IEEE1588-2008]: "The mean of the time or frequency error between the
clock under test and a perfect reference clock, over an ensemble of
measurements." The value is expressed in SI seconds [SI-SECOND] and
can be any positive number, such as an integer, a floating point
value (major type 7 or Tag 5), or a decimal value (Tag 4).
(This could be extended into more information about the way the clock
source is synchronized, e.g. manually, GPS, NTP, PTP, roughtime, ...)
3.8. Key -7
Key -7 can be used to indicate the time zone that would best fit for
displaying the time given to humans. (TBD: Format for the time zone
information; possibly including DST information. No default;
generally, the time can by default be presented as UTC/"Zulu time".)
3.9. Key -8
Key -8 can be used to indicate the location in which the time given
should be interpreted (e.g., for deriving time zone information).
(TBD: Format for the coordinate information; may need to contain the
Datum information.)
3.10. Key -10
Key -10 can be used to indicate the calendar that would best fit for
displaying the time given to humans. (TBD: Format for the calendar
information. This should probably default to Gregorian.)
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4. Duration Format
(TBD; this can probably use most of the same keys as for time.
Clearly, ISO8601 durations are a bit different.)
5. Period Format
(TBD; this could be a pair of times, a time and a duration, a
duration and a time or v.v., or a RFC 3339 period.)
6. CDDL typenames
For the use with the CBOR Data Definition Language, CDDL
[I-D.greevenbosch-appsawg-cbor-cddl], the type names defined in
Figure 1 are recommended:
etime = #6.TBDET({* (int/tstr) => any})
duration = #6.TBDED({* (int/tstr) => any})
period = #6.TBDEP({* (int/tstr) => any}) ; ?
Figure 1: Recommended type names for CDDL
7. IANA Considerations
IANA is requested to allocate the tags in Table 2, with the present
document as the specification reference.
+-------+-----------+-------------------------+
| Tag | Data Item | Semantics |
+-------+-----------+-------------------------+
| TBDET | map | [RFCthis] extended time |
| TBDED | map | [RFCthis] duration |
| TBDEP | map (?) | [RFCthis] period |
+-------+-----------+-------------------------+
Table 2: Values for Tags
(TBD: Add registry for time scales. Add registry for map keys and
allocation policies for additional keys.)
RFC editor note: Please replace TBDET, TBDED, and TBDEP by the tag
numbers allocated by IANA throughout the document and delete this
note.
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8. Security Considerations
The security considerations of RFC 7049 apply; the tags introduced
here are not expected to raise security considerations beyond those.
Time, of course, has significant security considerations; these
include the exploitation of ambiguities where time is security
relevant (e.g., for freshness or in a validity span) or the
disclosure of characteristics of the emitting system (e.g., time
zone, or clock resolution and wall clock offset).
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9. References
9.1. Normative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Vigano, C. and H. Birkholz, "CBOR data definition language
(CDDL): a notational convention to express CBOR data
structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
in progress), September 2016.
[IEEE1588-2008]
IEEE, "1588-2008 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", July 2008,
<http://standards.ieee.org/findstds/
standard/1588-2008.html>.
[RESOLUTION]
The Open Group Base Specifications, "Vol. 1: Base
Definitions, Issue 7", Section 3.328 '(Time) Resolution',
IEEE Std 1003.1-2008, 2016 Edition, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap03.html#tag_03_328>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[SI-SECOND]
International Organization for Standardization (ISO),
"Quantities and units -- Part 3: Space and time",
ISO 80000-3, March 2006.
[TIME_T] The Open Group Base Specifications, "Vol. 1: Base
Definitions, Issue 7", Section 4.15 'Seconds Since the
Epoch', IEEE Std 1003.1-2008, 2016 Edition, 2016,
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/
V1_chap04.html#tag_04_16>.
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9.2. Informative References
[TIME] Touch, J., "... time ...", unpublished manuscript, n.d..
Acknowledgements
Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Ben Gamari
Well-Typed
Henk Birkholz
Fraunhofer Institute for Secure Information Technology
Rheinstrasse 75
Darmstadt 64295
Germany
Email: henk.birkholz@sit.fraunhofer.de
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