Internet DRAFT - draft-bormann-lpwan-cbor-template
draft-bormann-lpwan-cbor-template
Network Working Group C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Informational January 17, 2018
Expires: July 21, 2018
Concise Binary Object Representation (CBOR) Tag for CBOR Templates
draft-bormann-lpwan-cbor-template-02
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.
The present document makes use of this extensibility to define a CBOR
tag for a variable within a CBOR data item, which then could be
filled in by a separate process (e.g., from another CBOR data item).
Status of This Memo
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This Internet-Draft will expire on July 21, 2018.
Copyright Notice
Copyright (c) 2018 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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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
2. Variable . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. CDDL typename . . . . . . . . . . . . . . . . . . . . . . . . 3
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . 5
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
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
well as a tagging mechanism that enables extending the set of data
types supported via an IANA registry.
With the work on static compression for CoAP [RFC7252] and CoAP
application protocols [I-D.ietf-core-comi] on LPWANs
[I-D.ietf-lpwan-coap-static-context-hc], there appears to be a need
for defining CBOR data items that have within them some open
positions in them that can later be filled in from a separate source
(such as another CBOR data item). The anchor points for this
substitution are called "variables" in this specification.
This document defines a CBOR tag for a variable in a CBOR data item.
It is intended as the reference document for the IANA registration of
the tag defined.
2. Variable
A variable is a CBOR data item that typically is included as a part
of a larger data item (the "CBOR template"). In a process that is
outside the scope of this specification, the variable is then
substituted by an actual value in order to yield an instance from the
template.
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A variable is identified by the data item within the tag, the
"variable identifier". Typically, variables are numbered by
integers. Some applications may also benefit from the use of strings
as identifiers. The specification of the tag does not make a
restriction on the type of the identifier; however note that very
complex variable identifiers may benefit from canonicalization to
enable their comparison, cf. section 3.9 of [RFC7049].
We term a CBOR data item that contains one or more variables as a
"CBOR template"; generally processes that accept CBOR templates with
variables will also accept CBOR data items without variables, so we
accept this as a degenerate case for "CBOR template". Note that a
template may use the same variable (i.e., a variable with the same
identifier) in multiple positions, leading to multiple substitutions
of the same value.
2.1. Example
An example for a CBOR template in diagnostic notation:
{ "name": "Carsten Bormann",
"place": 42(0) }
When this template undergoes substitution, with the variable 0 set to
the value "Bremen", this would result in the data item:
{ "name": "Carsten Bormann",
"place": "Bremen" }
3. CDDL typename
CDDL [I-D.greevenbosch-appsawg-cbor-cddl] definitions will typically
describe the structure of a data item after substitution.
However, when the CDDL definition needs to explicitly identify the
positions where substitutions can occur, the typename defined in
Figure 1 is recommended:
variable<varid> = #6.42(varid)
Figure 1: Recommended typenames for CDDL
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4. IANA Considerations
RFC-Editor note: Please replace "42" throughout this document by the
actual tag allocated and delete this note.
IANA is requested to allocate the tag 42 as in Table 1, with the
present document as the specification reference.
+-----+-----------+---------------+
| Tag | Data Item | Semantics |
+-----+-----------+---------------+
| 42 | any | CBOR variable |
+-----+-----------+---------------+
Table 1: Values for Tags
5. Security Considerations
The security considerations of RFC 7049 apply; the tag introduced
here not expected to raise security considerations beyond those.
Obviously, any process for performing variable substitution as
outlined in Section 2 needs to ensure that all of its inputs are
derived considering the security objectives, and that the inputs are
actually intended to fit together for this substitution.
For example, if a signed value includes the variables to be
substituted, but does not contain a unique identifier for the
template, then the structure resulting in substituting the variables
in the template must not be considered as having been signed.
Identifying templates by a hash, or by an identifier (that needs to
be specific to the actual value and version of the template) is
recommended.
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6. References
6.1. Normative References
[I-D.greevenbosch-appsawg-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-greevenbosch-appsawg-
cbor-cddl-11 (work in progress), July 2017.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
6.2. Informative References
[I-D.ietf-core-comi]
Veillette, M., Stok, P., Pelov, A., and A. Bierman, "CoAP
Management Interface", draft-ietf-core-comi-02 (work in
progress), December 2017.
[I-D.ietf-lpwan-coap-static-context-hc]
Minaburo, A. and L. Toutain, "LPWAN Static Context Header
Compression (SCHC) for CoAP", draft-ietf-lpwan-coap-
static-context-hc-02 (work in progress), September 2017.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
Contributors
Laurent Toutain suggested the creation of a mechanism for indicating
which parts of a CBOR data item are not yet available or subject to
change.
Jim Schaad contributed to the security considerations.
Acknowledgements
TBD
Author's Address
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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