Appsawg                                                        Z. Shelby
Internet-Draft                                      Micro:bit Foundation
Intended status: Informational                               O. Bergmann
Expires: August 26, 2017                                      C. Bormann
                                                 Universitaet Bremen TZI
                                                       February 22, 2017

                       The +exi Media Type Suffix


   Efficient XML Interchange (EXI) is an XML representation technique
   specified by the W3C to provide a time and space efficient encoding
   for XML documents.  This document defines a new Structured Syntax
   Suffix "+exi" for use in a specific class of protocols, where "exi"
   content-type encoding or the generic "application/exi" media type are
   not applicable.

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
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   This Internet-Draft will expire on August 26, 2017.

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   document authors.  All rights reserved.

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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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  When to Use the +exi Suffix . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Efficient XML Interchange (EXI) [W3C.REC-exi-20140211] is an XML
   representation technique specified by the W3C to provide a time and
   space efficient binary encoding alternative to the standard text XML
   representation.  EXI is not a generic compression technique like gzip
   or deflate, but an encoding technique specifically for XML structured
   documents, which uses either learned or pre-informed schema

   [W3C.REC-exi-20140211] defines a generic media type for documents
   encoded in EXI, "application/exi"; this does not provide a way to
   indicate more information about structure and semantics of the EXI-
   encoded XML.  Also, [W3C.REC-exi-20140211] defines an HTTP content
   encoding, "exi", that can be used to indicate EXI coding in
   combination with an existing XML media type.

   This document defines a new Structured Syntax Suffix "+exi" for use
   in media types for a specific class of protocols, where the "exi"
   content-type encoding or the generic "application/exi" media type are
   not viable.  In particular, the Constrained Application Protocol
   (CoAP) [RFC7252] combines the media type and its encoding in a single
   option value.  Thus, a client would include an _Accept_ option in a
   "GET" request to indicate its capability of processing, e.g., "text/
   plain" in UTF-8 encoding, or "application/exi", while the actual
   media type and encoding of a transferred payload would be described
   by the _Content-Format_ option.  CoAP servers can provide a
   description of their hosted resources as specified in Section 7.2 of
   [RFC7252].  A description usually contains an attribute "ct" that
   lists the Content-Format codes the server offers for a respective

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   Since EXI-encoded documents may or may not contain explicit
   information on the schema that is applicable to this document, the
   receiver of an EXI document would have to inspect its contents to
   decide if it can continue processing.  The structure syntax suffix
   specified in this document can be used by a sender to provide
   explicit information about the media types and encodings it can

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  When to Use the +exi Suffix

   The EXI standard already defines both an "exi" content-type encoding
   and an "application/exi" media type.  This section discusses when it
   is appropriate to use the new "+exi" structured syntax suffix when
   registering a media type.

   Appendix F.1 of [W3C.REC-exi-20140211] clearly describes when the exi
   content-type encoding should be used: "Protocols that can identify
   and negotiate the content coding of XML information independent of
   its media type, SHOULD use the content coding "exi" (case-
   insensitive) to convey the acceptance or actual use of EXI encoding
   for XML information."

   Thus when a protocol depends on the media type to identify that the
   payload is EXI, it can make use of the "application/exi" media type
   defined in Appendix F.2 of [W3C.REC-exi-20140211].  This works
   particularly well for applications using EXI in a generic way, and in
   particular in schema-less EXI streams, where protocol specific
   information such as the XML schema used is not needed to process the
   payload, or where the EXI stream contains the "schemaId" option to
   reference an applicable XML schema.  In these cases it is RECOMMENDED
   to use either the "application/exi" media type or "exi" content-type
   encoding with an existing media type.

   The "+exi" structure syntax suffix is appropriate for use in either
   of the following cases:

   1.  In protocols that have no means of separately transferring the
       media type and content coding information, the "+exi" suffix can
       be used to inform the recipient of a payload that the EXI
       serialization for the given media type has been used.  This
       SHOULD be used if and only if the EXI payload does not contain a
       "schemaId" option and the EXI payload has been produced using the

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       XML schema that is registered with the respective media type.
       This is typically the case for protocols that use EXI as a native
       encoding (without the use of character-based XML as an

   2.  To list the available combinations of media types and encodings
       in a Web Linking attribute [RFC5988].  CoAP [RFC7252], for
       example, defines the attribute "ct" as a list of Content-Format
       codes.  The Content-Format aggregates the media type and coding

   Both application areas address a very specific set of use cases where
   the media type "application/exi" or the content coding "exi" do not
   provide sufficient information for a receiver to decide if it is able
   to process the respective payload.

3.  Security Considerations

   Security considerations are discussed in Section 4.

4.  IANA Considerations

   This document requests registration of the Structured Syntax Suffix
   "+exi" as follows, following the registration template from
   Section 4.2.8 of [RFC6838]

   Name:  Efficient XML Interchange

   +suffix:  "+exi"

   References:  The EXI standard is defined in [W3C.REC-exi-20140211],
      in particular schema-informed grammars are defined in Section 8.5
      and the "applicatio/exi" media type is defined in Appendix F.2.

   Encoding considerations:  Binary

   Interoperability considerations:  The registration of a media type
      using this suffix MUST describe how to determine the XML Schema
      that is used to encode/decode a payload identified by that media
      type.  In particular this description defines how to determine the
      schema used to encode a payload using the "schemaId" option of the
      EXI header, if present.  The format of the identifier to be used
      in the "schemaId", a reference to where the corresponding schema
      is defined, and a description of how future versions of such
      schemas will be handled MUST be included.  A default schema
      version in the absence of the "schemaId" field MAY be defined.

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   Security considerations:  The "+exi" suffix shares the same security
      considerations as XML, described in [RFC7303], Section 10.  In
      addition, the security considerations discussed in the media type
      registration for "application/exi" apply as defined in
      Appendix F.2 of [W3C.REC-exi-20140211]}.

   Contact:  Applications and Real-Time Area (ART) General Applications
      Working Group (

   Author/Change controller:  The ART General Applications Area Working
      Group has change control over this registration.

5.  Acknowledgments

   This draft is the result of discussions on the former Apps Area
   Working Group mailing list.  Thanks to Carine Bournez and Guido
   Moritz for their helpful comments.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

              Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
              "Efficient XML Interchange (EXI) Format 1.0 (Second
              Edition)", World Wide Web Consortium Recommendation REC-
              exi-20140211, February 2014,

6.2.  Informative References

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,

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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,

Authors' Addresses

   Zach Shelby
   Micro:bit Foundation

   Phone: +358 407796297

   Olaf Bergmann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63904

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921

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