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RFC 8322

Proposed STD
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Resource-Oriented Lightweight Information Exchange (ROLIE)

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Internet Engineering Task Force (IETF)                          J. Field
Request for Comments: 8322                                       Pivotal
Category: Standards Track                                    S. Banghart
ISSN: 2070-1721                                            D. Waltermire
                                                                    NIST
                                                           February 2018


       Resource-Oriented Lightweight Information Exchange (ROLIE)

Abstract

   This document defines a resource-oriented approach for security
   automation information publication, discovery, and sharing.  Using
   this approach, producers may publish, share, and exchange
   representations of software descriptors, security incidents, attack
   indicators, software vulnerabilities, configuration checklists, and
   other security automation information as web-addressable resources.
   Furthermore, consumers and other stakeholders may access and search
   this security information as needed, establishing a rapid and
   on-demand information exchange network for restricted internal use or
   public access repositories.  This specification extends the Atom
   Publishing Protocol and Atom Syndication Format to transport and
   share security automation resource representations.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8322.

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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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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 ....................................................3
   2. Terminology .....................................................4
   3. XML-Related Conventions .........................................5
      3.1. XML Namespaces .............................................5
      3.2. RELAX NG Compact Schema ....................................5
   4. Background and Motivation .......................................5
   5. ROLIE Requirements for the Atom Publishing Protocol .............7
      5.1. AtomPub Service Documents ..................................7
           5.1.1. Use of the "app:workspace" Element ..................8
           5.1.2. Use of the "app:collection" Element .................8
           5.1.3. Service Document Discovery ..........................9
      5.2. Category Documents .........................................9
      5.3. Transport Layer Security ..................................10
      5.4. User Authentication and Authorization .....................10
      5.5. "/" (Forward Slash) Resource URL ..........................11
      5.6. HTTP Methods ..............................................11
   6. ROLIE Requirements for the Atom Syndication Format .............11
      6.1. Use of the "atom:feed" Element ............................11
           6.1.1. Use of the "atom:category" Element .................13
           6.1.2. Use of the "atom:link" Element .....................14
           6.1.3. Use of the "atom:updated" Element ..................15
      6.2. Use of the "atom:entry" Element ...........................16
           6.2.1. Use of the "atom:content" Element ..................17
           6.2.2. Use of the "atom:link" Element .....................17
           6.2.3. Use of the "rolie:format" Element ..................18
           6.2.4. Use of the "rolie:property" Element ................19
           6.2.5. Requirements for a Standalone Entry ................20

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   7. Available Extension Points Provided by ROLIE ...................21
      7.1. The Category Extension Point ..............................21
           7.1.1. General Use of the "atom:category" Element .........22
           7.1.2. Identification of Security Automation
                  Information Types ..................................22
      7.2. The "rolie:format" Extension Point ........................24
      7.3. The Link Relation Extension Point .........................24
      7.4. The "rolie:property" Extension Point ......................24
   8. IANA Considerations ............................................26
      8.1. XML Namespaces and Schema URNs ............................26
      8.2. ROLIE URN Sub-namespace ...................................26
      8.3. ROLIE URN Parameters ......................................27
      8.4. ROLIE Information Types Registry ..........................29
   9. Security Considerations ........................................29
   10. Privacy Considerations ........................................31
   11. References ....................................................32
      11.1. Normative References .....................................32
      11.2. Informative References ...................................34
   Appendix A. RELAX NG Compact Schema for ROLIE .....................37
   Appendix B. Examples of Use .......................................37
     B.1. Service Discovery ..........................................37
     B.2. Feed Retrieval .............................................40
     B.3. Entry Retrieval ............................................42
   Acknowledgements ..................................................43
   Authors' Addresses ................................................43

1.  Introduction

   This document defines a resource-oriented approach to security
   automation information sharing that follows the Representational
   State Transfer (REST) architectural style [REST].  In this approach,
   computer security resources are maintained in web-accessible
   repositories structured as Atom Syndication Format [RFC4287] Feeds.
   Within a given Feed, which may be requested by the consumer,
   representations of specific types of security automation information
   are organized, categorized, and described.  Furthermore, all
   collections available to a given user are discoverable, allowing the
   consumer to search all available content they are authorized to view,
   and to locate and request the desired information resources.  Through
   the use of granular authentication and access controls, only
   authorized consumers may be permitted the ability to read or write to
   a given Feed.

   The goal of this approach is to increase the communication and
   sharing of security information between providers and consumers that
   can be used to automate security processes (e.g., incident reports,
   vulnerability assessments, configuration checklists, and other
   security automation information).  Such sharing allows human

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   operators and computer systems to leverage this standardized
   communication system to gather information that supports the
   automation of security processes.

   To support new types of security automation information being used as
   time goes on, this specification defines a number of extension points
   that can be used either privately or globally.  These global
   extensions are IANA-registered by Resource-Oriented Lightweight
   Information Exchange (ROLIE) extension specifications and provide
   enhanced interoperability for new use cases and domains.  Sections 5
   and 6 of this document define the requirements for XML
   representations of ROLIE; other equivalent representations (e.g.
   JSON) may be described by other documents.  An overview of the
   extension system is provided in Section 7.  Implementers seeking to
   provide support for specific security automation information types
   should refer to the specification for that domain as described by the
   IANA registry found in Section 8.4.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The previous key words are used in this document to define only the
   requirements for implementations of this specification and are not
   used for recommendations or requirements for the usage of ROLIE.  (In
   other words, a programmer of a ROLIE server MUST implement a given
   feature, but a user of that ROLIE server needn't use that feature.)

   Definitions for some of the common computer-security-related
   terminology used in this document can be found in Section 2 of
   [RFC7970].

   The following term is unique to this specification:

   Information Type:  A class of security automation information having
      one or more associated data models.  Often, such security
      automation information is used in the automation of a security
      process.  See Section 7.1.2 for more information.

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3.  XML-Related Conventions

3.1.  XML Namespaces

   This specification uses XML namespaces [W3C.REC-xml-names-20091208]
   to uniquely identify XML element names.  It uses the following
   namespace prefix mappings for the indicated namespace URI:

   o  "app" is used for the "https://www.w3.org/2007/app" namespace
      defined in [RFC5023].

   o  "atom" is used for the "https://www.w3.org/2005/Atom" namespace
      defined in [RFC4287].

   o  "rolie" is used for the "urn:ietf:params:xml:ns:rolie:1.0"
      namespace defined in Section 8.1 of this specification.

3.2.  RELAX NG Compact Schema

   Some sections of this specification are illustrated with fragments of
   a non-normative RELAX NG Compact Schema [RELAX-NG].  The text of this
   specification provides the definition of conformance.  Schema for the
   "https://www.w3.org/2007/app" and "https://www.w3.org/2005/Atom"
   namespaces appear in Appendix B of [RFC5023] and Appendix B of
   [RFC4287], respectively.

   A complete informative RELAX NG Compact Schema for the new elements
   introduced by ROLIE is provided in Appendix A of this document.

4.  Background and Motivation

   In order to automate security processes, tools need access to
   sufficient sources of structured security information that can be
   used to drive security processes.  Thus, security information sharing
   is one of the core components of automating security processes.
   Vulnerabilities, configurations, software identification, security
   incidents, and patch data are just a few of the classes of
   information that are shared today to enable effective security on a
   wide scale.  However, as the scale of defense broadens as networks
   become larger and more complex, and the volume of information to
   process makes humans-in-the-loop difficult to scale, the need for
   automation and machine-to-machine communication becomes increasingly
   critical.

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   ROLIE seeks to address this need by providing four major information-
   sharing benefits:

   Extensible information type categories and format agnosticism:  ROLIE
      is not bound to any given data format or category of information.
      Instead, information categories are extensible, and Entries
      declare the format of the referenced data.  In cases where several
      formats or serializations are available, ROLIE can use link
      relations to communicate how a consumer can access these formats.
      For example, clients may request that a given resource
      representation be returned as XML, JSON, or in some other format
      or serialization.  This approach allows the provider to support
      multiple isomorphic formats, allowing the consumer to select the
      most suitable version.

   Open and distributed information sharing:  Using the Atom Publishing
      Protocol (AtomPub), ROLIE Feeds can easily aggregate Feeds and
      accept information posted to them from other sources.  Webs of
      communicating ROLIE servers form ad hoc sharing communities,
      increasing data availability and the ability to correlate linked
      data across sources for participating consumers.  ROLIE servers
      needn't be distributed, however, as large ROLIE repositories can
      function as a central collection or federated collections.

   Stateless communication model:  ROLIE, as a RESTful system, is
      stateless.  That is, the server doesn't keep track of client
      sessions but rather uses link relations for state transitions.  In
      practice, this means that any consumer can find and share
      information at any organizational level and at any time without
      needing to execute a long series of requests.

   Information discovery and navigation:  ROLIE provides a number of
      mechanisms to allow clients to programmatically discover and
      navigate collections of information in order to dynamically
      discover new or revised content.  Extensible information types and
      other categories provide one way of determining content that is
      desirable.  Link elements, each with a target URI and an
      established relationship type, provide a means for ROLIE providers
      to link other information that is relevant to the current Entry
      or Feed.

   These benefits result in an information-sharing protocol that is
   lightweight, interactive, open, and, most importantly, machine
   readable.

   The requirements in this specification are broken into two major
   sections: extensions to AtomPub [RFC5023] and extensions to the Atom
   Syndication Format [RFC4287].  All normative requirements in AtomPub

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   and Atom Syndication are inherited from their respective
   specifications and apply here unless the requirement is explicitly
   overridden in this document.  In this way, this document may upgrade
   the requirement (e.g., make a "SHOULD" a "MUST") but will never
   downgrade a given requirement (e.g., make a "MUST" a "SHOULD").

5.  ROLIE Requirements for the Atom Publishing Protocol

   This section describes a number of restrictions of, and extensions
   to, AtomPub [RFC5023] that define the use of AtomPub in the context
   of a ROLIE-based solution.  The normative requirements in this
   section are generally oriented towards client and server
   implementations.  An understanding of the AtomPub specification
   [RFC5023] is helpful to understand the requirements in this section.

5.1.  AtomPub Service Documents

   As described in Section 8 of [RFC5023], a Service Document is an
   XML-based document format that allows a client to dynamically
   discover the Collections provided by a publisher.  A Service Document
   consists of one or more "app:workspace" elements that may each
   contain a number of "app:collection" elements.

   The general structure of a Service Document is as follows (from
   Section 4.2 of [RFC5023]):

        Service
           o- Workspace
           |    |
           |    o- Collection
           |    |     |
           |    |     o- URI, categories, media types
           |    |
           |    o- ...
           |
           o- Workspace
           |     |
           |     o- Collection
           |     |     |
           |     |     o- URI, categories, media types
           |     |
           |     o- ...
           |
           o- ...

   Note that the Internationalized Resource Identifiers (IRIs) in the
   original diagram have been replaced with URIs.

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5.1.1.  Use of the "app:workspace" Element

   In AtomPub, a workspace, represented by the "app:workspace" element,
   describes a group of one or more Collections.  Building on the
   AtomPub concept of a workspace, in ROLIE a workspace represents an
   aggregation of Collections pertaining to security automation
   information resources.  This specification does not restrict the
   number of workspaces that may be in a Service Document or the
   specific Collections to be provided within a given workspace.

   A ROLIE implementation can host Collections containing both public
   and private information Entries.  It is suggested that
   implementations segregate Collections into different "app:workspace"
   elements by their client access requirements.  With proper naming of
   workspaces, this reduces the amount of trial and error a human user
   would need to utilize to discover accessible Collections.

5.1.2.  Use of the "app:collection" Element

   In AtomPub, a Collection in a Service Document, represented by the
   "app:collection" element, provides metadata that can be used to point
   to a specific Atom Feed that contains information Entries that may be
   of interest to a client.  The association between a Collection and a
   Feed is provided by the "href" attribute of the "app:collection"
   element.  Building on the AtomPub concept of a Collection, in ROLIE a
   Collection represents a pointer to a group of security automation
   information resources pertaining to a given type of security
   automation information.  Collections are represented as Atom Feeds as
   per RFC 5023.  Requirements specific to Atom Feed are defined in
   Section 6.1.

   ROLIE defines specialized data requirements for Collections, Feeds,
   and Entries containing data related to security automation.  The
   difference between a ROLIE Collection and a non-ROLIE Collection
   defined in a Service Document can be determined as follows:

   ROLIE Collection:  An app:collection is considered a ROLIE Collection
      when it contains an "app:categories" element that contains only
      one "atom:category" element with a "scheme" attribute value of
      "urn:ietf:params:rolie:category:information-type".  Further, this
      category has an appropriate "term" attribute value as defined in
      Section 7.1.1.  This ensures that a given Collection corresponds
      to a specific type of security automation information.

   Non-ROLIE Collection:  An app:collection is considered a non-ROLIE
      Collection when it does not contain an "atom:category" element
      with a "scheme" attribute value of
      "urn:ietf:params:rolie:category:information-type".

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   By distinguishing between ROLIE and non-ROLIE Collections in this
   way, implementations supporting ROLIE can host Collections pertaining
   to security automation information alongside Collections of other
   non-ROLIE information within the same AtomPub instance.

   The following are additional requirements on the use of the
   "app:collection" element for a ROLIE Collection:

   o  The child "atom:category" elements contained in the
      "app:categories" element MUST be the same set of "atom:category"
      elements used in the Atom Feed resource referenced by the
      "app:collection" element's "href" attribute value.  This ensures
      that the category metadata associated with the Collection and the
      associated Feed is discoverable in both of these resources.

   o  The "app:categories" element in an app:collection MAY include
      additional "atom:category" elements using a scheme other than
      "urn:ietf:params:rolie:category:information-type".  This allows
      other category metadata to be included.

5.1.3.  Service Document Discovery

   The Service Document serves as the "head" of a given ROLIE
   repository: from the Service Document, all other repository content
   can be discovered.  A client will need to determine the URL of this
   Service Document to discover the Collections provided by the
   repository.  The client might determine the URL from a web page,
   based on out-of-band communication, or through a "service" link
   relation in a Feed or Entry Document that the client has already
   retrieved.  The latter is a typical scenario if the client learns of
   a specific Feed or Entry through an out-of-band mechanism and wishes
   to discover additional information provided by the repository.

   This document does not provide a fully automated discovery mechanism.
   A mechanism may be defined in the future that allows automated
   clients to discover the URL to use to retrieve a ROLIE Service
   Document representing the head of the ROLIE repository.

5.2.  Category Documents

   As described in Section 7 of [RFC5023], a Category Document is an
   XML-based document format that allows a client to dynamically
   discover the categories used within AtomPub Service Documents, Atom
   Syndication Feeds, and Entry Documents provided by a publisher.  A
   Category Document consists of one "app:categories" element that
   contains a number of inline "atom:category" elements, or a URI
   referencing a Category Document.

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5.3.  Transport Layer Security

   ROLIE is intended to be handled with Transport Layer Security (TLS).
   TLS version 1.2 MUST be supported.  TLS 1.2 SHOULD be implemented
   according to all recommendations and best practices presented in
   [RFC7525].

   It is RECOMMENDED that the most recent published version of TLS be
   supported.  If this version is TLS 1.3 [TLS-1.3], it is suggested
   that 0-RTT (Zero Round-Trip Time Resumption) not be used, in order to
   prevent replay attacks.  Replay attacks on PUT, POST, or DELETE
   requests can disrupt repository operation by modifying data
   unexpectedly.

   For example, an automated ROLIE repository that updates very
   frequently may receive a PUT request against a given resource a few
   times an hour (or more).  An attacker may store an early PUT request,
   and at the end of the resumption window replay the PUT request,
   reverting the resource to an old version.  Not only could an attacker
   be doing this replay continuously to cause havoc on the server, but
   the client is completely unaware of the attack taking place.

   Given the potentially sensitive nature of data handled by ROLIE, all
   appropriate precautions should be taken at the transport layer to
   protect forward secrecy and user privacy.

   The server MUST implement certificate-based client authentication.
   This MAY be enabled on a workspace-by-workspace basis.

5.4.  User Authentication and Authorization

   Implementations MUST support user authentication.  However, a given
   implementation MAY allow user authentication to be disabled on a
   Feed-by-Feed or workspace-by-workspace basis.

   It is recommended that servers participating in an information-
   sharing consortium and supporting interactive user logins by members
   of the consortium support client authentication via a federated
   identity scheme.

   This document does not mandate the use of any specific user
   authorization mechanisms.  However, service implementers SHOULD
   support appropriate authorization checking for all resource accesses,
   including individual Atom Entries, Atom Feeds, and Atom Service
   Documents.

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5.5.  "/" (Forward Slash) Resource URL

   The "/" resource MAY be supported for compatibility with existing
   deployments that are using [RFC6546] ("Transport of Real-time
   Inter-network Defense (RID) Messages over HTTP/TLS").  The following
   requirements apply only to implementations that support both RFC 6546
   and the "/" resource as described above:

   o  Consistent with Erratum ID 3267 [Err3267] for [RFC6546], a client
      requesting a GET on the "/" resource SHOULD receive an HTTP status
      code 405 ("Method Not Allowed").

   o  An implementation MAY provide full support for [RFC6546] such that
      a POST to the "/" resource containing a recognized RID message is
      handled correctly as a RID request.  Alternatively, a client
      requesting a POST to "/" MAY receive an HTTP status code 307
      ("Temporary Redirect").  In this case, the location header in the
      HTTP response will provide the URL of the appropriate RID
      endpoint, and the client may repeat the POST method at the
      indicated location.

   If RFC 6546 is unsupported, then a request for the "/" resource may
   be handled as deemed appropriate by the server.

5.6.  HTTP Methods

   Servers MAY accept request methods beyond those specified in this
   document.

   Clients MUST be capable of recognizing and processing any standard
   HTTP status code, as defined in Section 5 of [RFC5023].

6.  ROLIE Requirements for the Atom Syndication Format

   This section describes a number of restrictions of, and extensions
   to, the Atom Syndication Format [RFC4287] that define the valid use
   of the format in the context of a ROLIE implementation.  An
   understanding of the Atom Syndication Format specification [RFC4287]
   is helpful to understand the requirements in this section.

6.1.  Use of the "atom:feed" Element

   As described in Section 4.1.1 of [RFC4287], an Atom Feed is an
   XML-based document format that describes a list of related
   information items.  The Atom Feeds provided by a ROLIE service are
   listed in the service's Service Document through one or more
   "app:collection" elements.  Each Feed Document, represented using the
   "atom:feed" element, contains a listing of zero or more Entries.

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   When applied to the problem domain of security automation information
   sharing, an Atom Feed may be used to represent any meaningful
   collection of security automation information resources.  Each Entry
   in a Feed represents an individual resource (e.g., a specific
   checklist, a software vulnerability record).  Additional Feeds can be
   used to represent other collections of security automation resources.

   As discussed in Section 5.1.2, ROLIE defines specialized data
   requirements for Feeds containing data related to security
   automation.  The difference between a ROLIE Feed and a non-ROLIE Feed
   can be determined as follows:

   ROLIE Feed:  For an "atom:feed" to be considered a ROLIE Feed, the
      "atom:feed" MUST contain only one child "atom:category" element
      with a "scheme" attribute value of
      "urn:ietf:params:rolie:category:information-type".  This category
      MUST have an appropriate "term" attribute value as defined in
      Section 7.1.1.  This ensures that a given Feed corresponds to a
      specific type of security automation information.

   Non-ROLIE Feed:  For an "atom:feed" to be considered a non-ROLIE
      Feed, the "atom:feed" MUST NOT contain an "atom:category" element
      with a "scheme" attribute value of
      "urn:ietf:params:rolie:category:information-type".

   By distinguishing between ROLIE and non-ROLIE Feeds in this way,
   implementations supporting ROLIE can host Feeds pertaining to
   security automation information alongside Feeds of other non-ROLIE
   information within the same AtomPub instance.  This is parallel to
   the handling of Collections as discussed earlier in this
   specification (Section 5.1.2).

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   The following Atom Feed definition represents a stricter definition
   of the "atom:feed" element defined in [RFC4287] when used as a
   ROLIE Feed.  Any element not specified here inherits its definition
   and requirements from [RFC4287].

      atomFeed =
         element atom:feed {
            atomCommonAttributes,
            (atomAuthor*
             & atomCategory+
             & atomContributor*
             & atomGenerator?
             & atomIcon?
             & atomId
             & atomLink+
             & atomLogo?
             & atomRights?
             & atomSubtitle?
             & atomTitle
             & atomUpdated
             & extensionElement*),
            atomEntry*
         }

   The following subsections contain requirements for a ROLIE Feed.

6.1.1.  Use of the "atom:category" Element

   An "atom:feed" can contain one or more "atom:category" elements.  In
   Atom, the naming scheme and the semantic meaning of the terms used to
   identify an Atom category are application defined.

   The following are additional requirements on the use of the
   "atom:category" element when used in a ROLIE Feed:

   o  All member Entries in the Feed MUST represent security automation
      information records of the provided information type category.

   o  The "atom:feed" MAY include additional "atom:category" elements
      using a scheme other than
      "urn:ietf:params:rolie:category:information-type".  This allows
      other category metadata to be included.

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6.1.2.  Use of the "atom:link" Element

   Link relations defined by the "atom:link" element are used to
   represent state transitions using a stateless approach.  In Atom, a
   type of link relationship can be defined using the "rel" attribute.

   A ROLIE Feed MUST contain one or more "atom:link" elements with
   rel="service" and an "href" attribute whose value is a URI that
   points to an Atom Service Document associated with the Feed.  If a
   client accesses a Feed without first accessing the service's Service
   Document, a link with the "service" relationship provides a means to
   discover additional security automation information.  The "service"
   link relationship is defined in the IANA "Link Relations" registry at
   <https://www.iana.org/assignments/link-relations/>.

   A Feed can contain an arbitrary number of Entries.  In some cases, a
   complete Feed may consist of a large number of Entries.
   Additionally, as new and updated Entries are ordered at the beginning
   of a Feed, a client may only be interested in retrieving the first N
   Entries in a Feed to process only the Entries that have changed since
   the last retrieval of the Feed.  As a practical matter, a large set
   of Entries will likely need to be divided into more manageable
   portions, or pages.  Based on Section 3 of [RFC5005], link elements
   SHOULD be included in all Feeds to support paging using the following
   link relation types:

   o  "first" - Indicates that the "href" attribute value of the link
      identifies a resource URI for the furthest preceding page of
      the Feed.

   o  "last" - Indicates that the "href" attribute value of the link
      identifies a resource URI for the furthest following page of
      the Feed.

   o  "previous" - Indicates that the "href" attribute value of the link
      identifies a resource URI for the immediately preceding page of
      the Feed.

   o  "next" - Indicates that the "href" attribute value of the link
      identifies a resource URI for the immediately following page of
      the Feed.

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   For example:

     <?xml version="1.0" encoding="UTF-8"?>
     <feed xmlns="https://www.w3.org/2005/Atom">
         <id>b7f65304-b63b-4246-88e2-c104049c5fd7</id>
         <title>Paged Feed</title>
         <link rel="self" href="https://example.org/feedA?page=5"/>
         <link rel="first" href="https://example.org/feedA?page=1"/>
         <link rel="prev" href="https://example.org/feedA?page=4"/>
         <link rel="next" href="https://example.org/feedA?page=6"/>
         <link rel="last" href="https://example.org/feedA?page=10"/>
         <updated>2012-05-04T18:13:51.0Z</updated>

         <!-- remainder of the Feed's elements -->
     </feed>

                            Example Paged Feed

   A reference to a historical Feed may need to be stable, and/or a Feed
   may need to be divided into a series of defined epochs.
   Implementations SHOULD support the mechanisms described in Section 4
   of [RFC5005] to provide link-based state transitions for maintaining
   the archiving of Feeds.

   A Feed MAY include additional link relationships not specified in
   this document.  If a client encounters an unknown link relationship
   type, the client MUST ignore the unrecognized link and continue
   processing as if the unrecognized link element did not appear.  The
   definition of new link relations that provide additional state
   transition extensions is discussed in Section 7.3.

6.1.3.  Use of the "atom:updated" Element

   The "atom:updated" element identifies the date and time that a Feed
   was last updated.

   The "atom:updated" element MUST be populated with the current time at
   the instant the Feed was last updated by adding, updating, or
   deleting an Entry, or by changing any metadata for the Feed.

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6.2.  Use of the "atom:entry" Element

   Each Entry in an Atom Feed, represented by the "atom:entry" element,
   describes a single referenced information record, along with
   descriptive information about its format, media type, and other
   publication metadata.  The following "atom:entry" schema definition
   represents a stricter representation of the "atom:entry" element
   defined in [RFC4287] for use in a ROLIE-based Atom Feed as defined in
   Section 6.1.1.

     atomEntry =
       element atom:entry {
         atomCommonAttributes,
         (atomAuthor*
         & atomCategory*
         & atomContent
         & atomContributor*
         & atomId
         & atomLink*
         & atomPublished?
         & atomRights?
         & atomSource?
         & atomSummary?
         & atomTitle
         & atomUpdated
         & rolieFormat?
         & rolieProperty*
         & extensionElement*)
     }

   The notable changes from [RFC4287] are the addition of "rolieFormat"
   and "rolieProperty" elements.  Also, the "atomContent" element is
   restricted to the atomOutOfLineContent formulation and is now
   REQUIRED.

   The following subsections contain requirements for Entries in a
   ROLIE Feed.

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6.2.1.  Use of the "atom:content" Element

   An "atom:content" element associates its containing Entry with a
   content resource identified by the "src" attribute.

   There MUST be exactly one "atom:content" element in the Entry.  The
   "atom:content" element MUST adhere to this definition, which is a
   stricter representation of the "atom:content" element defined in
   [RFC4287]:

     atomContent =
       element atom:content {
         atomCommonAttributes,
         attribute type { atomMediaType },
         attribute src { atomUri },
         empty
     }

   This restricts atomContent in ROLIE to the atomOutOfLineContent
   formulation presented in [RFC4287].

   The "type" attribute MUST identify the serialization type of the
   content -- for example, "application/xml" or "application/json".  A
   prefixed media type MAY be used to reflect a specific model used with
   a given serialization approach (e.g., "application/rdf+xml").  The
   "src" attribute MUST be a URI that can be dereferenced to retrieve
   the related content data.

6.2.2.  Use of the "atom:link" Element

   Link relations can be included in an Entry to represent state
   transitions to and from the Entry, as well as to provide links to
   related information.

   If there is a need to provide the same information in different data
   models and/or serialization formats, separate Entry instances can be
   included in the same Feed or a different Feed.  Such an alternate
   content representation can be indicated using an "atom:link" having a
   "rel" attribute with the value "alternate".

   A Feed MAY include additional link relationships not specified in
   this document.  If a client encounters an unknown link relationship
   type, the client MUST ignore the unrecognized link and continue
   processing as if the unrecognized link element did not appear.  The
   definition of new link relations that provide additional state
   transition extensions is discussed in Section 7.3.

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6.2.3.  Use of the "rolie:format" Element

   As mentioned in Sections 1 and 4, a key goal of this specification is
   to allow a consumer to review a set of published security automation
   information resources and then identify and retrieve any resources of
   interest.  The format of the data is a key criteria to consider when
   deciding what information to retrieve.  For a given type of security
   automation information, it is expected that a number of different
   formats may be used to represent this information.  To support this
   use case, both the serialization format and the specific data model
   expressed in that format must be known by the consumer.

   In the Atom Syndication Format, a media type can be defined using the
   "type" attribute of the "atom:content" element of an "atom:entry".
   The media type can be fully descriptive of the format of the linked
   document, such as "application/atom+xml".  In some cases, however, a
   format-specific media type may not be defined.  An example might be
   when "application/xml" is used because there is no defined specific
   media type for the content.  In such a case, the exact data model of
   the content cannot be known without first retrieving the content.

   In cases where a specific media type does not exist, the
   "rolie:format" element is used to describe the data model used to
   express the information referenced in the "atom:content" element.
   The "rolie:format" element also allows a schema to be identified that
   can be used when parsing the content to verify or better understand
   the structure of the content.

   When it appears, the "rolie:format" element MUST adhere to this
   definition:

     rolieFormat =
       element rolie:format {
         atomCommonAttributes,
         attribute ns { atomUri },
         attribute version { text } ?,
         attribute schema-location { atomUri } ?,
         attribute schema-type { atomMediaType } ?,
         empty
     }

   The "rolie:format" element MUST provide a "ns" attribute that
   identifies the data model of the resource referenced by the
   "atom:content" element.  For example, the namespace used may be an
   XML namespace URI or an identifier that represents a serialized JSON
   model.  The URI used for the "ns" attribute MUST be absolute.  The
   resource identified by the URI need not be resolvable.

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   The "rolie:format" element MAY provide a "version" attribute that
   identifies the version of the format used for the related
   "atom:content" element.

   The "rolie:format" element MAY provide a "schema-location" attribute,
   which is a URI that identifies a schema resource that can be used to
   validate the related "atom:content" element.

   The "rolie:format" element MAY provide a "schema-type" attribute,
   which is a media type (as described in [RFC2045]) identifying the
   format of the schema resource identified by the "schema-location"
   attribute.

   The following nominal example shows how these attributes describe the
   format of the content:

<rolie:format ns="urn:ietf:params:xml:ns:iodef-2.0"
  version="2.0"
  schema-location=
    "https://www.iana.org/assignments/xml-registry/schema/iodef-2.0.xsd"
  schema-type="text/xml"/>

   The previous element provides an indication that the content of the
   given Entry is using the Incident Object Description Exchange Format
   (IODEF) v2 format.

6.2.4.  Use of the "rolie:property" Element

   An "atom:category" element provides a way to associate a name/value
   pair of categorical information using the "scheme" and "term"
   attributes to represent the name and using the "label" attribute to
   represent the value.  When used in this way, an "atom:category"
   allows a specific label to be selected from a finite set of possible
   label values that can be used to further classify a given Entry or
   Feed.  Within ROLIE, there may be a need to associate additional
   metadata with an Entry.  In such a case, the use of an
   "atom:category" is not practical to represent name/value data for
   which the allowed values are unbounded.  Instead, ROLIE introduces a
   new "rolie:property" element that can represent non-categorical
   metadata as name/value pairs.  Examples include content-specific
   identifiers, naming data, and other properties that allow for
   unbounded values.

   There MAY be zero or more "rolie:property" elements in an
   "atom:entry".

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   The element MUST adhere to this definition:

     rolieProperty =
       element rolie:property {
         atomCommonAttributes,
         attribute name { atomUri },
         attribute value { text },
         empty
     }

   The "name" attribute provides a URI that identifies the namespace and
   name of the property as a URI.

   The "value" attribute is text that provides a value for the property
   identified by the "name" attribute.

   For example, the nominal element <rolie:property
   name="urn:ietf:params:rolie:property:content-id" value="12345"/>
   would expose an IODEF ID value contained in a given Entry's content.
   The name used in the example also demonstrates the use of a
   registered ROLIE property extension, which is described in
   Section 7.4.

   Implementations MAY use locally defined and namespaced elements in an
   Entry in order to provide additional information.  Clients that do
   not recognize a property with an unregistered "name" attribute MUST
   ignore the "rolie:property" element; that is, the client MUST NOT
   fail parsing content that contains an unrecognized property.

6.2.5.  Requirements for a Standalone Entry

   If an Entry is ever shared as a standalone resource, separate from
   its containing Feed, then the following additional requirements
   apply:

   o  The Entry MUST have an "atom:link" element with rel="collection"
      and href="[URI of the containing Collection]".  This allows the
      Feed or Feeds of which the Entry is a member to be discovered,
      along with the related information the Feed may contain.  In the
      case where the Entry has multiple containing Feeds, the Entry MUST
      have one "atom:link" for each related Feed.

   o  The Entry MUST declare the information type of the content
      resource referenced by the Entry (see Section 7.1.2).


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