Internet DRAFT - draft-doi-core-parameter-option
draft-doi-core-parameter-option
CoRE Y. Doi
Internet-Draft TOSHIBA Corporation
Intended status: Informational K. Lynn
Expires: February 9, 2014 Consultant
August 8, 2013
CoAP Content-Type Parameter Option
draft-doi-core-parameter-option-03
Abstract
Content-Types may have 'parameter' to make fine-grained description
of contents. The CoAP Accept Content-Type Parameter Option (Accept-
CT-Parameter Option) is CoAP options to add a parameter to a content
type specified in CoAP Accept Options.
Status of this Memo
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This Internet-Draft will expire on February 9, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use Cases of Option Parameter in CoAP . . . . . . . . . . . . . 3
2.1. Clarification on URI, Resource, and Representation . . . . 3
2.2. Parameter Coordination . . . . . . . . . . . . . . . . . . 3
2.3. Schema Negotiation of Schema-Informed EXI Communication . . 4
3. Accept Content-Type Parameter Option . . . . . . . . . . . . . 4
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Accept-CT-Parameter Option . . . . . . . . . . . . . . . . 5
3.2.1. Attribute ID . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Consideration on Compact Encodings of
Content-Type Parameter . . . . . . . . . . . . . . . . 8
Appendix B. ChangeLog . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
Content-Type field[RFC2045] have 'parameter' to make fine-grained
description of contents. The document proposes the CoAP Content-Type
Parameter Option that enables wide range of parameter description
over content types used in CoAP.
2. Use Cases of Option Parameter in CoAP
2.1. Clarification on URI, Resource, and Representation
In CoAP, a resource is specified by a CoAP URI. However, in some use
cases described in followings, an URI may correspond to multiple
versions of the resource, or a resource may have multiple
representations. As best practices, there are several ways to
identify a version and a representation on a resource pointed by an
URI. Some of discussions are given in
[W3C.Finding.alternatives-discovery].
One of the approaches commonly used is to parameterize contents with
content-type parameter and make a server-side content negotiation.
Basic specification of CoAP[I-D.ietf-core-coap] does not cover such
content negotiation and this draft is to propose a way to mimic
server-side content negotiation by making room for content type
parameters with a new option.
2.2. Parameter Coordination
If a resource has wide range of representations, a client may try to
specify what representation of the resource is requested by a GET
message. In HTTP, Accept: header and content type (media type)
parameter is used to coordinate parameters between clients and
servers. audio/rtp-midi[RFC6295] is an example of a content-type with
various parameters.
The audio use case seems not to be widely used in CoAP so far.
However, the same use case is applicable for sensor data. Sensor
data is time series data and it is possible to define a content type
with preferred sensing interval to avoid over/underquality of
sampling. In such cases, parameters with wildcard (rate=*) or range
(rate=10-20) is useful.
Similar parameter coordination is also proposed in
[I-D.wilde-atom-profile]. In the draft, several representations can
exist on a resource defined with a URI, and a client can negotiate
representation of the resource.
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2.3. Schema Negotiation of Schema-Informed EXI Communication
In some use case of Schema-Informed EXI [W3C.REC-exi-20110310], a
server and a client need to coordinate a XML schema to encode a
message. If there are some versions of XML schemas in an
application, a sender (may be server or client) needs to know schemas
a receiver has.
There are two choices. First choice is to define a content-type for
each version of XML schema. However, there are two problems. First,
the Content-type ID space is a global space and not suitable to
describe every minor revision. Second, Content-type ID per schema
cannot describe relation between a linage of schemas. XML schema
could be backward compatible and newer schema version can be applied
on older document validation and EXI encoding. Common practice on
this is to use (major).(minor) style versioning.
However, content-ID or other class of ID cannot describe which
version is compatible and which version is not compatible.
The other choice is to make use of content-type parameters. It seems
to be more acceptable because parameter is local to each content-
type. For example, an application may define 'application/
example-exi' as a content-type for the application. The application
may use 'sv' parameter as acceptable schema version. If the
application use backward-compatible approach, 'Accept: application/
example-exi;sv=1.4' from a client means the client can receive schema
version 1.0, 1.1, 1.2, 1.3, and 1.4.
Detailed discussion on EXI schema negotiation can be found in
[I-D.doi-exi-messaging-requirements].
3. Accept Content-Type Parameter Option
3.1. Requirements
In general, a content-type parameter has following notations.
parameter := ";" attribute "=" value
attribute := token ; case insensitive
value := token / quoted-string
In CoAP, a content-type parameter should have similar ability of
expression with regards to use cases. At the same time, a CoAP
option should be compact enough to fit in constrained environments.
As CoAP options do not have room for parameters, the content-type
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parameter option is designed to be an independent option that gives
additional description on a content-type in a CoAP message.
An attribute of CoAP option parameter should be fit in relatively
smaller set. The authors consider the attribute part could be
described in short integer (16 bits). On the other hand, the value
part should have higher degree of freedom for applications including
wildcards and range description. The author believes it is simple
and safe to have a string value in option parameter option.
3.2. Accept-CT-Parameter Option
To enable server-side content type negotiation, an option to describe
content type parameter is required. This document defines Accept-CT-
Parameter option for the purpose.
Table 1: List of options. U: proxy-Unsafe, C: Critical, R:
Repeatable
+----+---+---+---+---+------------------+--------+--------+---------+
| No | C | U | N | R | Name | Format | Length | Default |
| . | | | | | | | | |
+----+---+---+---+---+------------------+--------+--------+---------+
| TB | | | | x | Accept-CT-Parame | (see | 3-270B | (none) |
| D | | | | | ter | below) | | |
+----+---+---+---+---+------------------+--------+--------+---------+
The Accept-CT-Parameter option is used to attach a parameter on an
Accept option on the same CoAP message. The Accept-CT-parameter
options are proxy safe, elective.
An Accept-CT-Parameter option has two fields: attribute ID(aid), and
value.
|<--- option length ---->|
+---------+---------....-+
| aid | value |
+---------+---------....-+
|<2 Bytes>|<- optlen-2 ->|
:Figure 2: Structure of Accept-CT-Parameter Option
Attribute ID (aid) is a two-byte integer that describes the attribute
name (key) of the parameter. Details are described in later section
(see Table 2).
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A value is opaque octets (Unicode string in most cases) with the
length of the option length minus two (2) octets. A value may be
empty. Meanings of the values should be defined by the content-type
authority.
3.2.1. Attribute ID
Attribute ID is a 2-byte integer. An attribute is described in
2-byte integer as shown in the following table.
Table 2: List of Attribute IDs
+---------------+------------+-----------------+
| ID | Name | Reference |
+---------------+------------+-----------------+
| 0 | (reserved) | |
| 1 | charset | RFC2045 |
| 2 | version | RFC2045,RFC2046 |
| 3 | boundary | RFC2045 |
| 4 | type | RFC2046 |
| 5 | padding | RFC2046 |
| 6 | msgtype | RFC2616 |
| 7 | filename | RFC2616 |
| 8 | level | RFC2616 |
| 0xf000-0xffff | (reserved) | |
+---------------+------------+-----------------+
Other attribute ID may be managed and added by IANA, based on first-
come-first-serve basis for parameters defined in RFCs. Parameters
described in an internet draft or in proprietary extensions may be
added upon approval of core WG experts.
4. Security Considerations
Applications on CoAP servers should check the validity of parameters
before use. It may contain arbitrary string value.
5. IANA Considerations
This document requests a CoAP option ID assigned to Accept-CT-
Parameter option.
Attribute ID registry policy should be lined up with IANA
considerations of ()[#I-D.ietf-core-coap].
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6. Normative References
[I-D.doi-exi-messaging-requirements]
Doi, D., "EXI Messaging Requirements",
draft-doi-exi-messaging-requirements (work in progress),
October 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[I-D.wilde-atom-profile]
Wilde, E., "Profile Support for the Atom Syndication
Format", draft-wilde-atom-profile-01 (work in progress),
April 2013.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC6295] Lazzaro, J. and J. Wawrzynek, "RTP Payload Format for
MIDI", RFC 6295, June 2011.
[W3C.Finding.alternatives-discovery]
Raman, T., "On Linking Alternative Representations To
Enable Discovery And Publishing", World Wide Web
Consortium Finding Finding-alternatives-discovery,
November 2006, <http://www.w3.org/2001/tag/doc/
alternatives-discovery.html>.
[W3C.REC-exi-20110310]
Kamiya, T. and J. Schneider, "Efficient XML Interchange
(EXI) Format 1.0", World Wide Web Consortium
Recommendation REC-exi-20110310, March 2011,
<http://www.w3.org/TR/2011/REC-exi-20110310>.
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Appendix A. Consideration on Compact Encodings of Content-Type
Parameter
The use of 'value' part of parameter is up to the content-type. Some
content-type may use non-string integer or other format to describe
values in compact format, e.g. TLV, fixed-length integers, etc.
The specification that defines a content-type may define ASCII/UTF-8
notation for HTTP use and binary compact notation for CoAP. Anyway,
CoAP software/library will not need to understand content-type
parameter. The parameter should be transferred from/to application
without modification.
Appendix B. ChangeLog
o from draft-doi-core-parameter-option-01 to 02
* Removed content-type parameter of message content, and this
draft is now for content type parameter for Accept option
* Added description on relation between resource and
representation on RESTful architecture
* Added even some more use cases
o from draft-doi-core-parameter-option-00 to 01
* Added more use cases
* Parameter format has changed and now have clearly different
format for content-type and accept-content-type
o from draft-doi-core-option-parameter-option-00 to
draft-doi-core-ct-parameter-option-00
* Effect of the option is limited to Content-Type parameter (i.e.
Content-Type and Accept option).
* Name of the option has changed to 'Content-Type Parameter
Option'
* Removed Accept: preference use case (CoAP already defines
accept option order as preference)
* Removed Option Parameter Option 2 and 3.
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* Option Parameter Option is replaced with content-type parameter
option and accept content-type parameter option.
* Options are now considered to be proxy-safe (is it the right
assumption?)
* Some other unnecessary descriptions on option parameters (such
as option order constraint) are removed
Authors' Addresses
Yusuke Doi
TOSHIBA Corporation
Komukai Toshiba Cho 1
Saiwai-Ku
Kawasaki, Kanagawa 2128582
JAPAN
Phone: +81-45-342-7230
Email: yusuke.doi@toshiba.co.jp
Kerry Lynn
Consultant
Phone: +1 978 460 4253
Email: kerlyn@ieee.org
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