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Versions: 00 01 02 03

Network Working Group                                           E. Wilde
Internet-Draft                                                       EMC
Updates: 4287 (if approved)                                July 29, 2013
Intended status: Informational
Expires: January 30, 2014


            Profile Support for the Atom Syndication Format
                      draft-wilde-atom-profile-02

Abstract

   The Atom syndication format is a generic XML format for representing
   collections.  Profiles are one way how Atom feeds can indicate that
   they support specific extensions.  To make this support visible on
   the media type level, this specification adds an optional "profile"
   media type parameter to the Atom media type.  This allows profiles to
   become visible at the media type level, so that servers as well as
   clients can indicate support for specific Atom profiles in
   conversations, for example when communicating via HTTP.  This
   specification updates RFC 4287 by adding the "profile" media type
   parameter to the application/atom+xml media type registration.

Note to Readers

   This draft should be discussed on the atom-syntax mailing list [10].

   Online access to all versions and files is available on github [11].

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
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   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 January 30, 2014.

Copyright Notice




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   Copyright (c) 2013 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Profiles for Alternatives . . . . . . . . . . . . . . . . . 4
     2.2.  Profiles for Specializations  . . . . . . . . . . . . . . . 4
     2.3.  Profile URI for AtomPub . . . . . . . . . . . . . . . . . . 5
   3.  Profile Parameter Definition  . . . . . . . . . . . . . . . . . 5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
     4.1.  Atom Media Type application/atom+xml  . . . . . . . . . . . 6
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  From -01 to -02 . . . . . . . . . . . . . . . . . . . . . . 7
     8.2.  From -00 to -01 . . . . . . . . . . . . . . . . . . . . . . 8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     9.2.  Non-Normative References  . . . . . . . . . . . . . . . . . 8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9
















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1.  Introduction

   The Atom Syndication Format "is an XML-based document format that
   describes lists of related information known as 'feeds'.  Feeds are
   composed of a number of items, known as 'entries', each with an
   extensible set of attached metadata.  For example, each entry has a
   title."  [1]

   Profiles "can be described as additional semantics that can be used
   to process a resource representation, such as constraints,
   conventions, extensions, or any other aspects that do not alter the
   basic media type semantics.  A profile MUST NOT change the semantics
   of the resource representation when processed without profile
   knowledge, so that clients both with and without knowledge of a
   profiled resource can safely use the same representation."  [2]

   Profiles are identified by URI, and their use can be indicated for a
   representation by adding a link with the registered "profile" link
   relation type, linking to the profile URI.  While this is sufficient
   to represent the fact that a certain representation is using a
   profile, it does not make that fact visible outside of this
   representation.  Ideally, peers communicating their media type, for
   example when communicating via Hypertext Transfer Protocol (HTTP)
   [5], should be able to indicate the support of certain profiles
   through the media type identifier itself, without changing the base
   media type.

   Because Atom supports generic links through its <link/> element,
   "profile" links can be easily added to a feed, indicating that this
   feed does adhere to a certain profile.  However, on the media type
   level, this feed would still be labeled as application/atom+xml,
   making the profile invisible on that level and thus not allowing it
   to be used in interactions such as content negotiation in HTTP.

   This specification adds a "profile" media type parameter to the
   application/atom+xml media type, thereby making it possible for
   profiles to be exposed at the media type level.  Apart from adding
   that one media type parameter, this specification does not change
   anything about the Atom format itself, or its media type
   registration.


2.  Examples

   Adding a "profile" parameter to the Atom media type adds visibility
   of profiles at the media type level, for example when alternative
   profiles are supported by a service.  It might also help to further
   "specialize" a media type in environments where such a



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   "specialization" is useful.  Two examples are intended to illustrate
   these two scenarios.

2.1.  Profiles for Alternatives

   For example, when linking to feeds of media-oriented services, it
   would be possible to expose two feeds, one using MediaRSS, and the
   other one using Podcasts.  Both formats roughly cover the same
   functionality as media-oriented feed-based extensions, but by having
   the ability to expose their capabilities at the media type level,
   HTTP mechanisms and conversations can be used to distinguish between
   these formats.

   In some cases it may be possible to support more than one profile,
   and then it is up for the service to decide whether these should be
   exposed in one representation (which can be exposed by linking to
   multiple profiles from the resource representation and/or in the
   media type parameter), or whether there should be two
   representations, one for each profile.  This decision will probably
   depend on implementation complexity, the trade-off between navigation
   complexity (two representations with one profile each) and processing
   complexity, and also the size of the profile data, because in
   particular in the case of overlapping profiles, there might be many
   redundancies.

   Thus, which way to go for multiple profiles is not a question that
   has one correct answer; it depends on the profiles, and on the
   services that are built around them.

2.2.  Profiles for Specializations

   Feed-based services may provide additional features in feeds that are
   represented using Atom's extension mechanisms.  These additional
   features might be useful only for those clients that support them,
   and otherwise might add volume to a feed that is of no value to
   general consumers.  In such a scenario, specialized clients might
   also request their specialized features via profile media type
   parameters, and will then get the feed being "enriched" with the
   additional features.  If clients do not request such a profile or
   request one that is not known to the server, the server responds with
   a generic feed, still allowing them to treat the feed as a generic
   feed (with no additional features being represented).

   Whether services respond with profiles by default or only for
   specific requests about a profile is a matter of policy, and will be
   influenced by factors such as the added volume when adding profile
   data, and the question whether profiles should only be exposed to
   those that specifically ask for them.  Since profiles are not allowed



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   to change the semantics of the media type itself, such a decision can
   depend on the trade-off being a matter of expressivity, and not
   whether it will break clients under some circumstances.

2.3.  Profile URI for AtomPub

   The Atom Publishing Protocol (AtomPub) [6] builds on Atom and defines
   additional interactions with feeds, such as the ability to POST an
   entry to a collection URI as a request to create a new entry in that
   collection.  AtomPub uses Atom's media type for representing feeds
   and entries (and introduces its own media type for representing
   category and service documents, but these are not relevant for this
   discussion).

   When requesting a collection URI from an AtomPub server, clients will
   GET a feed document with no indication that the server supports
   AtomPub.  Clients are supposed to have knowledge about AtomPub
   support, so that they know whether POST requests to the collection
   URI might succeed.  It is possible that clients send an OPTIONS
   request to the collection URI to find out about the allowed methods,
   but this requires an additional roundtrip, and since the AtomPub spec
   does not explicitly mention OPTIONS, it may be the case that
   implementations do not generally support this discovery mechanism.

   To make AtomPub support of a collection explicit in a feed document,
   the profile URI urn:ietf:rfc:5023 is suggested.  When including this
   profile URI in a feed, a server indicates AtomPub support:
   <?xml version="1.0" ?>
   <feed xmlns="http://www.w3.org/2005/Atom">
     <link rel="profile" href="urn:ietf:rfc:5023">

   When used with the profile parameter of the Atom media type, this
   profile URI MAY be used to indicate that the resource is advertising
   AtomPub support.  It should be noted that AtomPub servers are not
   required to use the AtomPub profile URI in any way (because it is not
   a part of the AtomPub specification), but that supporting it may make
   it easier for clients to discover the AtomPub capabilities of
   available resources.


3.  Profile Parameter Definition

   The profile parameter for the application/atom+xml media type allows
   one or more profile URIs to be specified.  These profile URIs have
   the identifier semantics defined in [2], and when appearing as media
   type parameter, they have the same semantics as if they had been
   associated with the resource URI through other means, such as using
   one or more <link profile="" href=""/> elements as children of the



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   <feed> element.

   As a general rule, media type parameters must be quoted unless they
   are tokens.  For the "profile" media type parameter defined here,
   this means that is must be quoted.  It contains a non-empty list of
   space-separated URIs (the profile URIs).
   profile-param = "profile=" profile-value
   profile-value = <"> profile-URI 0*( 1*SP profile-URI ) <">
   profile-URI   = URI

   The "URI" in the above grammar refers to the "URI" as defined in
   Section 3 of [3]


4.  IANA Considerations

   This specification updates an existing media type according to the
   registry mechanism described in [7].

4.1.  Atom Media Type application/atom+xml

   The Internet media type for Atom (application/atom+xml) should be
   updated by adding the following optional media type parameter:

4.1.1.  Optional Parameters

   profile: This parameter indicates that one or more profiles are used
   in the feed, according to the definition of profiles in [2].  The
   parameter syntax is specified in Section 3 of RFC XXXX


5.  Implementation Status

   Note to RFC Editor: Please remove this section before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 6982 [8].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.




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   According to RFC 6982, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

      ...


6.  Security Considerations

   There are no known security considerations for adding this optional
   media type parameter to the application/atom+xml media type.


7.  Open Issues

   Note to RFC Editor: Please remove this section before publication.

   o  Monitor how the proposal for a "Profile URI Registry" [9] is
      coming along.  If it is successful, then the proposed AtomPub
      Profile URI Section 2.3 should be included in the IANA
      Considerations Section 4.


8.  Change Log

   Note to RFC Editor: Please remove this section before publication.

8.1.  From -01 to -02

   o  Added "Implementation Status" section (Section 5)."

   o  Added example and suggested URI for an AtomPub Profile
      (Section 2.3)

   o  Changed IANA section to only request adding a "profile" media type
      parameter (instead of providing a complete media type registration
      template).

   o  Added "Open Issues" section (Section 7) and reminder to check the
      progress of the "Profile URI Registry" draft.

   o  Updating "Implementation Status" section to refer to RFC 6982 [8].

   o  Adding "Security Considerations" section (Section 6)




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8.2.  From -00 to -01

   o  Fixed typos.

   o  Removed the requirement to percent-encode URIs in the profile
      parameter.

   o  Added example for media type specialization.


9.  References

9.1.  Normative References

   [1]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom Syndication
        Format", RFC 4287, December 2005.

   [2]  Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
        March 2013.

   [3]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
        Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
        January 2005.

   [4]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
        RFC 3023, January 2001.

9.2.  Non-Normative References

   [5]  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.

   [6]  Gregorio, J. and B. de hOra, "The Atom Publishing Protocol",
        RFC 5023, October 2007.

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

   [8]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code:
        The Implementation Status Section", RFC 6982, July 2013.

   [9]  Lanthaler, M., "The IETF Profile URI Registry",
        draft-lanthaler-profile-registry-02 (work in progress),
        June 2013.

URIs



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   [10]  <http://www.imc.org/atom-syntax/>

   [11]  <https://github.com/dret/I-D/tree/master/atom-profile>


Appendix A.  Acknowledgements

   Thanks for comments and suggestions provided by Markus Lanthaler and
   Peter Rushforth.


Author's Address

   Erik Wilde
   EMC
   6801 Koll Center Parkway
   Pleasanton, CA 94566
   U.S.A.

   Phone: +1-925-6006244
   Email: erik.wilde@emc.com
   URI:   http://dret.net/netdret/





























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