Profile Support for the Atom Syndication Format
draft-wilde-atom-profile-00

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 re-registers the Atom media type, and adds a "profile" media type parameter. 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.

Note to Readers

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

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

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on October 01, 2013.

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Table of Contents

• 1. Introduction
• 2. Examples
• 3. Profile Parameter Definition
• 4. IANA Considerations
• 4.1. Atom Media Type application/atom+xml
• 5. References
• 5.1. Normative References
• 5.2. Non-Normative References
• Author's Address

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." [RFC4287]

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." [RFC6906]

Profiles are identified by URI, and can be added to 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 should be able to indicate the support of certain profiles through the media type 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 the Hypertext Transfer Protocol (HTTP). [RFC2616].

This specification adds a "profile" 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.

2. Examples

Adding a "profile" parameter to the Atom media type adds visibility of profiles at the media type level. For example, when linking to feeds of media-oriented services, it would be possible to expose two feeds, one using MediaRSS, and the other 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 redundacies.

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

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 [RFC6906], 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=""/> element as a child of the <feed> element.

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, and contains a non-empty list of space-separated URI-encoded URIs.

profile-param = "profile=" profile-value
profile-value = <"> profile-URI 0*( 1*SP profile-URI ) <">
profile-URI   = percent-encoded-URI

When processing "mprofile" media type parameters, it is therefore important to apply URI-decoding before processing the URI (such as comparing it to known profiles). This is particularly important since profile URIs in Atom content (in <link profile="" href=""/> links) will not be URI-encoded, and thus properly encoding and decoding URIs in these two location is essential to implement correct processing.

4. IANA Considerations

The Internet media type [RFC6838] for an Atom document is application/atom+xml.

4.1. Atom Media Type application/atom+xml

This specification requests the registration of an XML-based media type for the eXtensible Access Control Markup Language (XACML).

4.1.1. Media Type Name

application

4.1.2. Subtype Name

atom+xml

4.1.3. Required Parameters

none

4.1.4. Optional Parameters

charset: This parameter has semantics identical to the charset parameter of the "application/xml" media type as specified in [RFC3023].

profile: This parameter indicates that one or more profiles are used in the feed, according to the definition of profiles in [RFC6906]. The parameter syntax is specified in of RFC XXXX [profile-definition]

4.1.5. Encoding Considerations

Identical to those of "application/xml" as described in [RFC3023], Section 3.2.

4.1.6. Security Considerations

As defined in [RFC4287]. In addition, as this media type uses the "+xml" convention, it shares the same security considerations as described in [RFC3023], Section 10.

4.1.7. Interoperability Considerations

There are no known interoperability issues.

4.1.8. Published Specification

[RFC4287]

4.1.9. Applications which use this media type

Many. Atom has become a common foundation for many syndication-oriented scenarios, and also has become a commonly used representation for collection contents.

4.1.10. Magic number(s)

As specified for "application/xml" in [RFC3023], Section 3.2.

4.1.11. File extension(s)

.atom

4.1.12. Fragment Identifiers

As specified for "application/xml" in [RFC3023], Section 5.

4.1.13. Base URI

As specified in [RFC3023], Section 6.

4.1.14. Macintosh File Type Code(s)

TEXT

4.1.15. Person & email address to contact for further information

Mark Nottingham <mnot@mnot.net> and Erik Wilde <erik.wilde@emc.com>

4.1.16. Intended Usage

Common

4.1.17. Author/Change Controller

IESG

5. References

5.1. Normative References

5.2. Non-Normative References

Author's Address