MILE Working Group                                              J. Field
Internet-Draft                                                   Pivotal
Intended status: Informational                          December 2, 2015                               S. Banghart
Expires: December 5, 2016                                           NIST
                                                            June 4, 3, 2016

           Resource-Oriented Lightweight Indicator Information Exchange
                        draft-ietf-mile-rolie-01
                        draft-ietf-mile-rolie-02

Abstract

   This document defines a resource-oriented approach to cyber security
   information sharing.  Using this approach, a CSIRT or other
   stakeholder operators may share and
   exchange representations of cyber security incidents, attack
   indicators, software vulnerabilities, and other related information
   as Web-
   addressable Web-addressable resources.  The transport protocol binding is specified  Furthermore, consumers and other
   stakeholders may access and search this security content as HTTP(S) with needed,
   establishing a MIME media type of Atom+XML.  An appropriate set of
   link relation types specific rapid and on-demand information exchange network for
   restricted internal use or public access repositories.  This
   specification builds on and extends the Atom Publishing Protocol and
   Atom Syndication Format to transport and share cyber security information sharing is
   defined.  The
   resource representations leverage representations.  This document leverages the existing
   representations IODEF
   [RFC5070] and RID [RFC6545] specifications where appropriate, and supports related
   cyber security representation standards.

Contributing to this document

   The source for this draft is being maintained in GitHub.  Suggested
   changes should be submitted as appropriate.
   Coexistence with deployments pull requests at
   <https://github.com/CISecurity/ROLIE>.  Instructions are on that conform page
   as well.  Editorial changes can be managed in GitHub, but any
   substantial issues need to existing specifications
   including RID [RFC6545] and Transport of Real-time Inter-network
   Defense (RID) Messages over HTTP/TLS [RFC6546] is supported via
   appropriate use of HTTP status codes. be discussed on the MILE mailing list.

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
   working documents as Internet-Drafts.  The list of current Internet-
   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 June 4, December 5, 2016.

Copyright Notice

   Copyright (c) 2015 2016 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
   (http://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 . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   3.  Background and Motivation . . . . . . . . . . . . . . . . . .   4
     3.1.  Message-oriented versus Resource-oriented Architecture  .   5
       3.1.1.  Message-oriented Architecture . . . . . . . . . . . .   5
       3.1.2.  Resource-Oriented Architecture  . . . . . . . . . . .   5
     3.2.  Authentication of Users .   6
   4.  Atom Publication Protocol and Atom Syndication Format TODO  .   7
   5.  Normative Requirements TODO . . . . . . . . . . . . . . .   7
     3.3.  Authorization Policy Enforcement . .   8
     5.1.  Atom Requirements . . . . . . . . . .   7
       3.3.1.  Enforcement at Destination System . . . . . . . . . .   8
       3.3.2.  Enforcement at Source System
     5.2.  Transport Layer Security  . . . . . . . . . . . .   9
   4.  RESTful Usage Model . . . .   8
     5.3.  Archiving and Paging  . . . . . . . . . . . . . . . . .   9
     4.1.  Dynamic Service Discovery versus Static URL Template .   8
     5.4.  Expectation and Impact Classes  .  10
     4.2.  Non-Normative Examples . . . . . . . . . . . .   8
     5.5.  User Authentication . . . . .  11
       4.2.1.  Service Discovery . . . . . . . . . . . . . .   9
     5.6.  User Authorization  . . . .  11
       4.2.2.  Feed Retrieval . . . . . . . . . . . . . . .   9
     5.7.  Content Model . . . .  14
       4.2.3.  Entry Retrieval . . . . . . . . . . . . . . . . . .   9
     5.8.  HTTP methods  .  16
       4.2.4.  Use of Link Relations . . . . . . . . . . . . . . . .  19
   5.  Requirements for RESTful (Atom+xml) Binding . . . . .  10
     5.9.  Service Discovery . . . .  29
     5.1.  Transport Layer Security . . . . . . . . . . . . . . . .  29
     5.2.  Archiving and Paging  . . . . . . . . . . . . . . . . . .  29
     5.3.  Expectation and Impact Classes  . . . . . . . . . . . . .  30
     5.4.  User Authentication . . . . . . . . . . . . . . . . . . .  30
     5.5.  User Authorization  . . . . . . . . . . . . . . . . . . .  30
     5.6.  Content Model . . . . . . . . . . . . . . . . . . . . . .  31
     5.7.  HTTP methods  . . . . . . . . . . . . . . . . . . . . . .  31
     5.8.  Service Discovery . . . . . . . . . . . . . . . . . . . .  32
       5.8.1.  11
       5.9.1.  Workspaces  . . . . . . . . . . . . . . . . . . . . .  32
       5.8.2.  11
       5.9.2.  Collections . . . . . . . . . . . . . . . . . . . . .  32
       5.8.3.  11
       5.9.3.  Service Document Security . . . . . . . . . . . . . .  33
     5.9.  11
     5.10. Category Mapping  . . . . . . . . . . . . . . . . . . . .  33
       5.9.1.  11
       5.10.1.  Collection Category  . . . . . . . . . . . . . . . . .  33
       5.9.2.  12
       5.10.2.  Entry Category . . . . . . . . . . . . . . . . . . .  33
     5.10.  12
     5.11. Entry ID  . . . . . . . . . . . . . . . . . . . . . . . .  34
     5.11.  12
     5.12. Entry Content . . . . . . . . . . . . . . . . . . . . . .  34
     5.12.  13
     5.13. Link Relations  . . . . . . . . . . . . . . . . . . . . .  34
       5.12.1.  13
       5.13.1.  Additional Link Relation Requirements  . . . . . . .  36

     5.13.  15
     5.14. Member Entry Forward Security . . . . . . . . . . . . . .  36
     5.14.  15
     5.15. Date Mapping  . . . . . . . . . . . . . . . . . . . . . .  37
     5.15.  16
     5.16. Search  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     5.16.  16
     5.17. / (forward slash) Resource URL  . . . . . . . . . . . . .  38  16
   6.  Security Considerations TODO  . . . . . . . . . . . . . . . . . . .  38  17
   7.  IANA Considerations TODO  . . . . . . . . . . . . . . . . . . . . .  40  19
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  40  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  40  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  40  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  42  20
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  43  21
   Appendix A.  Change Tracking  . . . . . . . . . . . . . . . . . .  43
   Appendix B.  Resource Authorization Model . . . . . . . . . . . .  43
   Author's Address  .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44  22

1.  Introduction

   This document defines a resource-oriented approach to cyber security
   information sharing that follows the REST (Architectural Styles and t
   he Design of Network-based Software Architectures) architectural
   style.  The resource representations leverage the existing IODEF
   [RFC5070] and RID [RFC6545] specifications as appropriate.  The
   transport protocol binding is specified as HTTP(S) with a media type
   of Atom+XML.  An appropriate set of link relation types specific to  In this approach, cyber security information sharing is defined.  Using this approach,
   a CSIRT or other stakeholder resources are maintained in
   web-accessible repositories structured as Atom Syndication Format
   [RFC4287] feeds.  Representations of content are categorized and
   organized into indexed collections, which are requested by the
   consumer.  As the set of resource collections are forward facing, the
   consumer may exchange search all available content for which they are
   authorized to view and request that which is desired.  Granular
   authentication and access controls permit only authorized consumers
   the ability to view, read, or write to a given feed.  This approach
   is in contrast to, and meant to improve on, the traditional point-to-
   point messaging system, in which consumers must request individual
   pieces of information from a server following a triggering event.
   This traditional approach creates a closed system of information
   sharing that encourages duplication of efforts and hinders automated
   cyber security incident
   and/or indicator information as Web-addressable resources. systems.

   The goal of this specification document is to define a loosely-coupled, agile the RESTful approach to cyber
   security situational awareness.  This approach has
   architectural advantages for some use case scenarios, such communication with the intent of increasing communication
   and sharing of incident reports, vulnerability assessments, and other
   security content between producers, operators, and consumers.

   In order to exchange information as when a
   CSIRT or web-addressable resources, the
   resource representations leverage the existing IODEF [RFC5070] and
   RID [RFC6545] specifications and other stakeholder representation standards as
   appropriate.  The transport protocol binding is required specified as HTTP(S)
   with a media type of Atom+XML.  An appropriate set of link relation
   types specific to share cyber security information broadly (e.g., at internet scale), or when an information sharing consortium requires support for asymmetric interactions
   amongst their stakeholders. is defined.

   Coexistence with deployments that conform to existing specifications
   including RID [RFC6545] and Transport of Real-time Inter-network
   Defense (RID) Messages over HTTP/TLS [RFC6546] is supported via
   appropriate use of HTTP status codes.

2.  Terminology

   The key words "MUST," "MUST NOT," "REQUIRED," "SHALL," "SHALL NOT,"
   "SHOULD," "SHOULD NOT," "RECOMMENDED," "MAY," and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].
   Definitions for some of the common computer security-related
   terminology used in this document can be found in Section 2 of
   [RFC5070].

3.  Background and Motivation

   It is well known that Internet security the field of threats are to computer security is
   evolving ever more rapidly, rapidly as time goes on.  As software increases in
   complexity, the number of vulnerabilities in our systems and are becoming ever more sophisticated than before.
   The threat networks
   increase exponentially.  Threat actors looking to exploit these
   vulnerabilities are frequently distributed making more frequent and are not constrained
   to operating within more widely distributed
   attacks across a fixed, closed consortium. large variety of systems.  The technical skills
   needed to perform effective analysis adoption of liberal
   information sharing amongst attackers creates a security incident, or to
   even recognize an indicator window of compromise are already specialized as little
   as a few hours between the discovery of a vulnerability and
   relatively scarce. attacks
   on the vulnerable system.  As threats continue to evolve, even an
   established network of CSIRT may find that it does not always have
   all of the skills and knowledge required to immediately
   identify and combat these attacks become more and more specialized,
   even a well established and secure system may find itself unable to
   quickly respond to every new an incident.  Effective identification of and
   response to a sophisticated, multi-stage sophisticated attack frequently depends
   upon requires open cooperation and collaboration, not only amongst the
   collaboration between defending
   CSIRTs, but also amongst other stakeholders, including, potentially,
   individual end users. operators, software vendors, and even
   end-users.

   Existing approaches to cyber security information sharing are based
   upon message exchange patterns that are point-to-point, and event-
   driven.  Sometimes, information that may be useful to, and sharable
   with multiple peers is only made available to peers after they have
   specifically requested it.  Unfortunately, a sharing peer may not
   know, a priori, what information to request from another peer.
   Sending unsolicited RID reports does provide a mechanism for
   alerting, however these reports are again sent point-to-point, and
   must be reviewed for relevance and then prioritized for action by the
   recipient.  Thus, distribution of some relevant incident and
   indicator information may exhibit significant latency.

   In order to appropriately adequately combat the evolving threats, the defending
   CSIRTs computer security
   resource producers should be enabled to operate in a more agile manner, sharing share selected cyber security information proactively, if and
   proactively as appropriate.  Proactive sharing greatly aids knowledge
   dissemination as well as improving on response times and usability.

   For example, a CSIRT cyber security analyst would benefit by having the
   ability to search a comprehensive collection of attack indicators
   that has have been published by a government agency, or by another member
   of a sharing consortium.  The representation of each indicator may
   include links to the related resources, enabling an appropriately
   authenticated and authorized analyst to freely navigate the
   information space of indicators, incidents, vulnerabilities, and
   other cyber security domain concepts, as needed.  In general, a more
   Web-centric sharing approach will enable a more dynamic and agile
   collaboration amongst a broader, and varying constituency.

   The following sections discuss additional specific technical issues
   that motivate the development of an alternative approach.

3.1.  Message-oriented versus Resource-oriented Architecture

   The existing approaches to cyber security information sharing are
   based upon message-oriented interactions.  The following paragraphs
   explore some of the architectural constraints associated with
   message-oriented interactions and consider the relative merits of an
   alternative model based on a Resource-oriented architecture for use
   in some use case scenarios.

   ROLIE specifies a resource-oriented architecture.

3.1.1.  Message-oriented Architecture

   In general, message-based integration architectures may be based upon
   either an RPC-style or a document-style binding.  The message types
   defined by RID represent an example of an RPC-style request.  This
   approach imposes implied requirements for conversational state
   management on both of the communicating RID endpoint(s).  Experience
   has shown that this state management frequently becomes the limiting
   factor with respect to the runtime scalability of an RPC-style
   architecture.

   In addition, the practical scalability of a peer-to-peer message-
   based approach will be limited by the administrative procedures
   required to manage O(N^2) trust relationships and at least O(N)
   policy groups.

   As long as the number of CSIRTs participating entities in an information
   sharing consortium is limited to a relatively smaller small number of nodes
   (i.e., O(2^N), where N < 5), these scalability constraints may not
   represent a critical concern.  However, when there is a requirement
   to support a significantly larger number of participating peers, a
   different architectural approach will be required.  One alternative
   to the message-based approach that has demonstrated scalability is
   the REST [REST] architectural style.

3.1.2.  Resource-Oriented Architecture

   Applying the REST architectural style to the problem domain of cyber
   security information sharing would take the approach of exposing
   incidents, indicators, and any other relevant types as simple Web-
   addressable resources.  By using this approach, a CSIRT or other an organization can
   more quickly and easily share relevant incident and indicator
   information with a much larger and potentially more diverse
   constituency.  A client consumer may leverage virtually any available HTTP
   user agent in order to make requests of the service provider.  This
   improved ease of use could enable more rapid adoption and broader
   participation, thereby improving security for everyone.

   A key interoperability aspect of any RESTful Web service will be the
   choices regarding the available resource representations.  For
   example, clients may request that a given resource representation be
   returned as either XML or JSON.  In order to enable back-
   compatibility and interoperability with existing CSIRT implementations,
   IODEF [RFC5070] is specified for this transport binding as a
   mandatory to implement (MTI) data representation for incident and
   indicator resources.  In addition to the REQUIRED representation, an
   implementation MAY support additional representations if and as
   needed such as IODEF extensions, the RID schema, or other schemas.
   For example, an implementation may choose to provide support for
   returning a JSON representation of an incident resource.

   Finally, an important principle of the REST architectural style is
   the use of hypertext links as the embodiment of application state
   (HATEOAS).  Rather than the server maintaining conversational state
   for each client context, the server will instead include a suitable
   set of hyperlinks in the resource representation that is returned to
   the client.  In this way, the server remains stateless with respect
   to a series of client requests.  The included hyperlinks provide the
   client with a specific set of permitted state transitions.  Using
   these links the client may perform an operation, such as updating or
   deleting the resource representation.  The client may also be
   provided with hypertext links that can be used to navigate to any
   related resource.  For example, the resource representation for an
   incident object may contain links to the related indicator
   resource(s).

   This document specifies the use of Atom Syndication Format [RFC4287]
   and Atom Publishing Protocol [RFC5023] as the mechanism for
   representing the required hypertext links.

3.1.2.1.  A Resource-Oriented Use Case: "Mashup"

   In this section we consider a non-normative example use case scenario
   for creating a cyber security "mashup".

   Any CSIRT operator can enable authorize any authenticated and authorized client that is
   a member or all members of the sharing
   community to quickly and easily navigate through any of the cyber
   security information that that provider is willing to share.  An authenticated and authorized
   analyst may then make HTTP(S) requests to collect incident and indicator vulnerability
   information known at one CSIRT with producer and threat actor data being made
   available from another CSIRT. producer.  The resulting correlations may
   yield new insights that enable a more timely and effective defensive
   response.  Of course, this report may, in turn, be made available to
   others as a new Web-addressable resource, reachable via another URL.
   By employing the RESTful Web service approach the effectiveness of
   the collaboration amongst a consortium of CSIRTs and their cyber security stakeholders
   can be greatly improved.

3.2.  Authentication of Users

   In the store-and-forward, message-based model for information sharing
   client authentication is provided via a Public Key Infrastructure
   (PKI) -based trust

4.  Atom Publication Protocol and mutually authenticated TLS between the
   messaging system endpoints.  There is no provision to support
   authentication of a client by another means. Atom Syndication Format TODO

   As a result,
   participation described in the sharing community Atom Publishing Protocol [RFC5023], an Atom Service
   Document is limited to those
   organizations an XML-based document format that have sufficient resources and capabilities to
   manage allows a PKI.

   A CSIRT may apply XML Security client to
   dynamically discover the content of a message, however
   the contact information collections provided within the message body represents by a
   self-asserted identity, and there is no guarantee publisher.

   As described in Atom Syndication Format [RFC4287], Atom is an XML-
   based document format that the contact describes lists of related information will be recognized by the peer.  As
   items known as collections, or "feeds".  Each feed document contains
   a result, the audit
   trail and the granularity collection of any authorization policies is limited zero or more related information items called "member
   entries" or "entries".

   When applied to the identity problem domain of the peer CSIRT organization.

   A CSIRT implementing this specification MUST implement server-
   authenticated TLS.  The CSIRT cyber security information
   sharing, an Atom feed may choose to authenticate its client
   users via any suitable authentication scheme that can be implemented
   via HTTP(S).  A participating CSIRT MAY choose used to support more than
   one authentication method.  Support for use represent any meaningful
   collection of information resources such as a Federated Identity
   approach is RECOMMENDED.  Establishing a specific end user identity
   prior to processing set of incidents, or
   indicators.  Each entry in a request is RECOMMENDED.  Doing so will enable
   the source system feed could then represent an individual
   incident, or indicator, or some other resource, as appropriate.
   Additional feeds could be used to maintain a more complete audit trail represent other meaningful and
   useful collections of exactly
   what cyber security incident and indicator information has been
   shared, when, and with whom.

3.3.  Authorization Policy Enforcement resources.  A key aspect of feed may be
   categorized, and any cyber security feed may contain information sharing arrangement is
   assigning the responsibility for authorization policy enforcement. from zero or more
   categories.  The authorization policy must be enforced either at naming scheme and the destination
   system, or semantic meaning of the source system, or both.  The following sections
   discuss these alternatives in greater detail.

3.3.1.  Enforcement at Destination System

   The store-and-forward, message-based approach terms
   used to cyber security
   information sharing requires that the origin system delegate
   authorization policy enforcement to the destination system.  The
   origin system may leverage XML Encryption and DigitalSignature to
   protect the message content.  In addition, the origin system assigns
   a number of policy-related attribute values, including a
   "restriction" attribute, before the message is sent.  These labels
   indicate the sender's expectation for confidentiality enforcement and
   appropriate handling at the destination.  Section 9.1 of RFC6545
   provides specific guidance to implementers on use of the XML security
   standards in order to achieve the required levels of security for the
   exchange of incident information.

   Once the message has been received at the destination system, the XML
   encryption and digital signature protections on the message will be
   processed, and based upon the pre-established PKI-based trust
   relationships, the message content is validated and decrypted.
   Typical implementations will then pass the cleartext data to an
   internal Incident Handling System (IHS) for further review and/or
   action by a human operator or analyst.  Regardless of where in the
   deployment architecture the XML message-level security is being
   handled, eventually the message content will be made available as
   cleartext for handling by human systems analysts and other
   operational staff.

   The authorization policy enforcement of the message contents must
   then be provided by the destination IHS.  It is the responsibility of
   the destination system to honor the intent of the policy restriction
   labels assigned by the origin system.  Ideally, these policy labels
   would serve as part of a distributed Mandatory Access Control scheme.
   However, in practice a typical IHS will employ a Discretionary Access
   Control (DAC) model rather than a MAC model and so the policy related
   attributes are defined to represent handling "hints" and provide no
   guarantee of enforcement at the destination.

   As a result, ensuring that the destination system or counterparty
   will in fact correctly enforce the intended authorization policies
   becomes a key issue when entering into any information sharing
   agreements.  The origin CSIRT must accept a non-zero risk of
   information leakage, and therefore must rely upon legal recourse as a
   compensating control.  Establishing such legal sharing agreements can
   be a slow and difficult process, as it assumes a high level of trust
   in the peer, with respect to both intent and also technical
   capabilities.

3.3.2.  Enforcement at Source System

   In this model, the required authorization policy enforcements are
   implemented entirely within the source system.  Enforcing the
   required authorization policy controls at the source system
   eliminates the risk of subsequent information leakage at the
   destination system due to inadequate or incomplete implementation of
   the expected controls.  The destination system is not expected to
   perform any additional authorization enforcements.  Authorization
   enforcement at the source system may be based on, e.g.  Role-based
   Access Controls applied in the context of an established user
   identity.  The source system may use any appropriate authentication
   mechanism in order to determine the user identity of the requestor,
   including, e.g. federated identity.  An analyst or operator at a
   CSIRT may request specific information on a given incident or
   indicator from a peer CSIRT, and the source system will return a
   suitable representation of that resource based upon the specific role
   of the requestor.  A different authenticated user (perhaps from the
   same destination CSIRT) may receive a different representation of the
   same resource, based upon the source system applying suitable Role-
   based Access Control policy enforcements for the second user
   identity.

   Consistent with HTTP [RFC2616] a user's request MAY be denied with a
   resulting HTTP status code value of 4xx such as 401 Unauthorized, 403
   Forbidden, or 404 Not Found, or 405 Method Not Allowed, if and as
   appropriate.

4.  RESTful Usage Model

   This section describes the basic use of Atom Syndication Format
   [RFC4287] and Atom Publishing Protocol [RFC5023] as a RESTful
   transport binding and dynamic discovery protocol, respectively, for
   cyber security information sharing.

   As described in Atom Publishing Protocol [RFC5023], an Atom Service
   Document is an XML-based document format that allows a client to
   dynamically discover the collections provided by a publisher.

   As described in Atom Syndication Format [RFC4287], Atom is an XML-
   based document format that describes lists of related information
   items known as collections, or "feeds".  Each feed document contains
   a collection of zero or more related information items called "member
   entries" or "entries".

   When applied to the problem domain of cyber security information
   sharing, an Atom feed may be used to represent any meaningful
   collection of information resources such as a set of incidents, or
   indicators.  Each entry in a feed could then represent an individual
   incident, or indicator, or some other resource, as appropriate.
   Additional feeds could be used to represent other meaningful and
   useful collections of cyber security resources.  A feed may be
   categorized, and any feed may contain information from zero or more
   categories.  The naming scheme and the semantic meaning of the terms
   used to identify an Atom category are application-defined.

4.1.  Dynamic Service Discovery versus Static URL Template

   In order to specify a protocol for cyber security information sharing
   using the REST architectural style it is necessary to define the set
   of resources to be modeled, and how these resources are related.
   Based on this interface contract, clients will then interact with the
   REST service by navigating the modeled entities, and their
   relationships.  The interface contract between the client and the
   server may either be statically bound or dynamically bound.

   In the statically bound case, the clients have a priori knowledge of
   the resources that are supported.  In the REST architectural style
   this static interface contract takes the form of a URL template.
   This approach is not appropriate for the cyber security information
   sharing domain for at least two reasons.

   First, there is no standard for a cyber security domain model.  While
   information security practitioners can generally agree on some of the
   basic concepts that are important to modeling the cyber security
   domain -- such as "indicator," "incident," or "attacker," -- there is
   no single domain model that can been referenced as the basis for
   specifying a standardized RESTful URI Template.  Second, the use of
   static URL templates creates a tighter coupling between the client
   implementation and the server implementation.  Security threats on
   the internet are evolving ever more rapidly, and it will never be
   possible to establish a statically defined resource model and URL
   Template.  Even if there were an initial agreement on an appropriate
   URL template, it would eventually need to change.  If and when a
   CSIRT finds that it needs to change the URL template, then any
   existing deployed clients would need to be upgraded.

   Thus, rather than attempting to define a fixed set of resources via a
   URI Template, this document has instead specified an approach based
   on dynamic discovery of resources via an Atom Publishing Protocol
   Service Document.  By using this approach, it is possible to
   standardize the RESTful usage model, without needing to standardize
   on the definitions of specific, strongly-typed resources.  A client
   can dynamically discover what resources are provided by a given
   CSIRT, and then navigate that domain model accordingly A specific
   server implementation may still embody a particular URL template,
   however the client does not need a priori knowledge of the format of
   the links, and the URL itself is effectively opaque to the client.
   Clients are not bound to any particular server's interface.

   The following paragraphs provide a number of non-normative examples
   to illustrate the use of Atom Publishing Protocol for basic cyber
   security information sharing service discovery, as well as the use of
   Atom Syndication Format as a mechanism to publish cyber security
   information feeds.

   Normative requirements are defined below, in Section 5.

4.2.  Non-Normative Examples

4.2.1.  Service Discovery

   This section provides a non-normative example of a client doing
   service discovery.

   An Atom service document enables a client to dynamically discover
   what feeds a particular publisher makes available.  Thus, a CSIRT may
   use an Atom service document to enable clients of the CSIRT to
   determine what specific cyber security information the CSIRT makes
   available to the community.  The service document could be made
   available at any well known location, such as via a link from the
   CSIRT's home page.  One common technique is to include a link in the
   <HEAD> section of the organization's home page, as shown below:

   Example of bootstrapping Service Document discovery:

      <link rel="introspection"  type="application/atomsvc+xml" title="Atom Publishing Protocol Service Document" href="/csirt/svcdoc.xml" />

   A client may then format an HTTP GET request to retrieve the service
   document:

   GET /csirt/svcdoc.xml
   Host: www.example.org
   Accept: application/atomsvc+xml

   Notice the use of the HTTP Accept: request header, indicating the
   MIME type for Atom service discovery.  The response to this GET
   request will be an XML document that contains information on the
   specific feed collections that are provided by the CSIRT.

   Example HTTP GET response:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:09:11 GMT
      Content-Length: 570
      Content-Type: application/atomsvc+xml;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <service xmlns="http://www.w3.org/2007/app"
               xmlns:atom="http://www.w3.org/2005/Atom">
          <workspace xml:lang="en-US" xmlns:xml="http://www.w3.org/XML/1998/namespace">
            <atom:title type="text">Incidents</atom:title>
            <collection  href="http://example.org/csirt/incidents">
               <atom:title type="text">Incidents Feed</atom:title>
               <accept>application/atom+xml; type=entry</accept>
            </collection>
          </workspace>
      </service>

   This simple Service Document example shows that this CSIRT provides
   one workspace, named "Incidents."  Within that workspace, the CSIRT
   makes one feed collection available.  When attempting to GET or POST
   entries to that feed collection, the client must indicate a content
   type of application/atom+xml.

   A CSIRT may also offer a number of different feeds, each containing
   different types of cyber security information.  In the following
   example, the feeds have been categorized.  This categorization will
   help the clients to decide which feeds will meet their needs.

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:10:11 GMT
      Content-Length: 1912
      Content-Type: application/atomsvc+xml;charset="utf-8"

       <?xml version="1.0" encoding='utf-8'?>
          <service xmlns="http://www.w3.org/2007/app"
            xmlns:atom="http://www.w3.org/2005/Atom">
            <workspace>
              <atom:title>Cyber Security Information Sharing</atom:title>
              <collection href="http://example.org/csirt/public/indicators" >
                <atom:title>Public Indicators</atom:title>
                <categories fixed="yes">
                  <atom:category scheme="http://example.org/csirt/restriction" term="public" />
                  <atom:category scheme="http://example.org/csirt/purpose" term="reporting" />
                </categoies>
                <accept>application/atom+xml; type=entry</accept>
              </collection>
              <collection href="http://example.org/csirt/public/incidents" >
                <atom:title>Public Incidents</atom:title>
                <categories fixed="yes">
                  <atom:category scheme="http://example.org/csirt/restriction" term="public" />
                  <atom:category scheme="http://example.org/csirt/purpose" term="reporting" />
                </categoies>
                <accept>application/atom+xml; type=entry</accept>
            </collection>
            </workspace>
            <workspace>
              <atom:title>Private Consortium Sharing</atom:title>
              <collection href="http://example.org/csirt/private/incidents" >
                <atom:title>Incidents</atom:title>
                <accept>application/atom+xml;type=entry</accept>
                <categories fixed="yes">
                  <atom:category scheme="http://example.org/csirt/purpose" term="traceback, mitigation, reporting" />
                  <atom:category scheme="http://example.org/csirt/restriction" term="private, need-to-know" />
                </categories>
              </collection>
            </workspace>
          </service>

   In this example, the CSIRT is providing a total of three feed
   collections, organized into two different workspaces.  The first
   workspace contains two feeds, consisting of publicly available
   indicators and publicly available incidents, respectively.  The
   second workspace provides one additional feed, for use by a sharing
   consortium.  The feed contains incident information containing
   entries related to three purposes: traceback, mitigation, and
   reporting.  The entries in this feed are categorized with a
   restriction of either "Need-to-Know" or "private".  An appropriately
   authenticated and authorized client may then proceed to make GET
   requests for one or more of these feeds.  The publicly provided
   incident information may be accessible with or without
   authentication.  However, users accessing the feed targeted to the
   private sharing consortium would be expected to authenticate, and
   appropriate authorization policies would subsequently be enforced by
   the feed provider.

4.2.2.  Feed Retrieval

   This section provides a non-normative example of a client retrieving
   an incident feed.

   Having discovered the available cyber security information sharing
   feeds, an authenticated and authorized client who is a member of the
   private sharing consortium may be interested in receiving the feed of
   known incidents.  The client may retrieve this feed by performing an
   HTTP GET operation on the indicated URL.

   Example HTTP GET request for a Feed:

   GET /csirt/private/incidents
   Host: www.example.org
   Accept: application/atom+xml

   The corresponding HTTP response would be an XML document containing
   the incidents feed:

   Example HTTP GET response for a Feed:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:20:11 GMT
      Content-Length: 2882
      Content-Type: application/atom+xml;type=feed;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <feed xmlns="http://www.w3.org/2005/Atom"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.w3.org/2005/Atom file:/C:/schemas/atom.xsd
                              urn:ietf:params:xml:ns:iodef-1.0 file:/C:/schemas/iodef-1.0.xsd"
          xml:lang="en-US">

          <generator version="1.0" xml:lang="en-US">emc-csirt-iodef-feed-service</generator>
          <id xml:lang="en-US">http://www.example.org/csirt/private/incidents</id>
          <title type="text" xml:lang="en-US">Atom formatted representation of a feed of IODEF documents</title>
          <updated xml:lang="en-US">2012-05-04T18:13:51.0Z</updated>
          <author>
              <email>csirt@example.org</email>
              <name>EMC CSIRT</name>
          </author>

          <!-- By convention there is usually a self link for the feed -->
          <link href="http://www.example.org/csirt/private/incidents" rel="self"/>

          <entry>
              <id>http://www.example.org/csirt/private/incidents/123456</id>
              <title>Sample Incident</title>
              <link href="http://www.example.org/csirt/private/incidents/123456" rel="self"/>       <!-- by convention -->
              <link href="http://www.example.org/csirt/private/incidents/123456" rel="alternate"/>  <!-- required by Atom spec -->
              <published>2012-08-04T18:13:51.0Z</published>
              <updated>2012-08-05T18:13:51.0Z</updated>
              <!-- The category is based upon IODEF purpose and restriction attributes -->
              <category term="traceback" scheme="purpose" label="trace back" />
              <category term="need-to-know" scheme="restriction" label="need to know" />
              <summary>A short description of this incident, extracted from the IODEF Incident class, <description> element. </summary>
          </entry>

          <entry>
              <!-- ...another entry... -->
          </entry>

      </feed>

   This feed document has two atom entries, one of which has been
   elided.  The completed entry illustrates an Atom <entry> element that
   provides a summary of essential details about one particular
   incident.  Based upon this summary information and the provided
   category information, a client may choose to do an HTTP GET operation
   to retrieve the full details of the incident.  This example provides
   a RESTful alterntive to the RID investigation request messaage, as
   described in sections 6.1 and 7.2 of RFC6545.

4.2.3.  Entry Retrieval

   This section provides a non-normative example of a client retrieving
   an incident as an Atom entry.

   Having retrieved the feed of interest, the client may then decide
   based on the description and/or category information that one of the
   entries in the feed is of further interest.  The client may retrieve
   this incident Entry by performing an HTTP GET operation on the
   indicated URL.

   Example HTTP GET request for an Entry:

   GET /csirt/private/incidents/123456
   Host: www.example.org
   Accept: application/atom+xml

   The corresponding HTTP response would be an XML document containing
   the incident:

   Example HTTP GET response for an Entry:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:30:11 GMT
      Content-Length: 4965
      Content-Type: application/atom+xml;type=entry;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <entry>
        <id>http://www.example.org/csirt/private/incidents/123456</id>
        <title>Sample Incident</title>
        <link href="http://www.example.org/csirt/private/incidents/123456" rel="self"/>       <!-- by convention -->
        <link href="http://www.example.org/csirt/private/incidents/123456" rel="alternate"/>  <!-- required by Atom spec -->
        <published>2012-08-04T18:13:51.0Z</published>
        <updated>2012-08-05T18:13:51.0Z</updated>
        <!-- The category is based upon IODEF purpose and restriction attributes -->
        <category term="traceback" scheme="purpose" label="trace back" />
        <category term="need-to-know" scheme="restriction" label="need to know" />
        <summary>A short description of this incident, extracted from the IODEF Incident class, <description> element. </summary>
        <!-- Refer to section 5.9 for the list of supported (cyber information-specific) link relationships -->
        <!-- Typical operations that can be performed on this IODEF message include edit -->
        <link href="http://www.example.org/csirt/private/incidents/123456" rel="edit"/>

        <!-- the next and previous are just sequential access, may not map to anything related to this IODEF Incident ID -->
        <link href="http://www.example.org/csirt/private/incidents/123457" rel="next"/>
        <link href="http://www.example.org/csirt/private/incidents/123455" rel="previous"/>

        <!-- navigate up to the full collection.  Might also be rel="collection" as per IANA registry -->
        <link href="http://www.example.org/csirt/private/incidents" rel="up"/>

        <content type="application/xml">
          <iodef:IODEF-Document lang="en" xmlns:iodef="urn:ietf:params:xml:ns:iodef-1.0">
            <iodef:Incident purpose="traceback" restriction="need-to-know">

              <!-- Note that the ID is assigned using a namespace that is our base URL, so that it can also be leveraged as an Atom link -->
              <iodef:IncidentID name="http://www.example.org/csirt/private/incidents">123456</iodef:IncidentID>

              <iodef:DetectTime>2004-02-02T22:49:24+00:00</iodef:DetectTime>
              <iodef:StartTime>2004-02-02T22:19:24+00:00</iodef:StartTime>
              <iodef:ReportTime>2004-02-02T23:20:24+00:00</iodef:ReportTime>
              <iodef:Description>
                Host involved in DoS attack
              </iodef:Description>
              <iodef:Assessment>
                <iodef:Impact completion="failed" severity="low" type="dos"/>
              </iodef:Assessment>
              <iodef:Contact role="creator" type="organization">
                <iodef:ContactName>Constituency-contact for 192.0.2.35
                </iodef:ContactName>
                <iodef:Email>Constituency-contact@192.0.2.35</iodef:Email>
              </iodef:Contact>
              <iodef:EventData>
                <iodef:Flow>
                  <iodef:System category="source">
                    <iodef:Node>
                      <iodef:Address category="ipv4-addr">192.0.2.35
                      </iodef:Address>
                    </iodef:Node>
                    <iodef:Service ip_protocol="6">
                      <iodef:Port>38765</iodef:Port>
                    </iodef:Service>
                  </iodef:System>
                  <iodef:System category="target">
                    <iodef:Node>
                      <iodef:Address category="ipv4-addr">192.0.2.67
                      </iodef:Address>
                    </iodef:Node>
                    <iodef:Service ip_protocol="6">
                      <iodef:Port>80</iodef:Port>
                    </iodef:Service>
                  </iodef:System>
                </iodef:Flow>
                <iodef:Expectation action="rate-limit-host" severity="high">
                  <iodef:Description>
                    Rate-limit traffic close to source
                  </iodef:Description>
                </iodef:Expectation>
                <iodef:Record>
                  <iodef:RecordData>
                    <iodef:Description>
                      The IPv4 packet included was used in the described attack
                    </iodef:Description>
                    <iodef:RecordItem dtype="ipv4-packet">450000522ad9
                      0000ff06c41fc0a801020a010102976d0050103e020810d9
                      4a1350021000ad6700005468616e6b20796f7520666f7220
                      6361726566756c6c792072656164696e6720746869732052
                      46432e0a
                    </iodef:RecordItem>
                  </iodef:RecordData>
                </iodef:Record>
              </iodef:EventData>
            </iodef:Incident>
          </iodef:IODEF-Document>
        </content>
      </entry>

   As can be seen in the example response, above, an IODEF document is
   contained within the Atom <content> element.  The client may now
   process the IODEF document as needed.

   Note also that, as described previously, the content of the Atom
   <category> element is application-defined.  In the present context,
   the Atom categories have been assigned based on a mapping of the
   <restriction> and <purpose> attributes, as defined in the IODEF
   schema.  In addition, the IODEF <incidentID> element has been
   judiciously chosen so that the associated name attribute, as well as
   the corresponding incidentID value, can be concatenated in order to
   easily create the corresponding <id> element for the Atom entry.
   These and other mappings are normatively defined in Section 5, below.

   Finally, it should be noted that in order to optimize the client
   experience, and avoid an additional round trip, a feed provider may
   choose to include the entry content inline, as part of the feed
   document.  That is, an Atom <entry> element within a Feed document
   may contain an Atom <content> element as a child.  In this case, the
   client will receive the full content of the entries within the feed.
   The decision of whether to include the entry content inline or to
   include it as a link is a design choice left to the feed provider
   (e.g. based upon local environmental factors such as the number of
   entries contained in a feed, the available network bandwidth, the
   available server compute cycles, the expected client usage patterns,
   etc.).

4.2.4.  Use of Link Relations

   As noted previously, a key benefit of using the RESTful architectural
   style is the ability to enable the client to navigate to related
   resources through the use of hypermedia links.  In the Atom
   Syndication Format, the type of the related resource identified in a
   <link> element is indicated via the "rel" attribute, where the value
   of this attribute identifies the kind of related resource available
   at the corresponding "href" attribute.  Thus, in lieu of a well-known
   URI template the URI itself is effectively opaque to the client, and
   therefore the client must understand the semantic meaning of the
   "rel" attribute in order to successfully navigate.  Broad
   interoperability may be based upon a sharing consortium defining a
   well-known set of Atom Link Relation types.  These Link Relation
   types may either be registered with IANA, or held in a private
   registry.

   Individual CSIRTs may always define their own link relation types in
   order to support specific use cases, however support for a core set
   of well-known link relation types is encouraged as this will maximize
   interoperability.

   In addition, it may be beneficial to define use case profiles that
   correspond to specific groupings of supported link relationship
   types.  In this way, a CSIRT may unambiguously specify the classes of
   use cases for which a client can expect to find support.

   The following sections provide NON-NORMATIVE examples of link
   relation usage.  Four distinct cyber security information sharing use
   case scenarios are described.  In each use case, the unique benefits
   of adopting a resource-oriented approach to information sharing are
   illustrated.  It is important to note that these use cases are
   intended to be a small representative set and is by no means meant to
   be an exhaustive list.  The intent is to illustrate how the use of
   link relationship types will enable this resource-oriented approach
   to cyber security information sharing to successfully support the
   complete range of existing use cases, and also to motivate an initial
   list of well-defined link relationship types.

4.2.4.1.  Use Case: Incident Sharing

   This section provides a non-normative example of an incident sharing
   use case.

   In this use case, a member CSIRT shares incident information with
   another member CSIRT in the same consortium.  The client CSIRT
   retreives a feed of incidents, and is able to identify one particular
   entry of interest.  The client then does an HTTP GET on that entry,
   and the representation of that resource contains link relationships
   for both the associated "indicators" and the incident "history", and
   so on.  The client CSIRT recognizes that some of the indicator and
   history may be relevant within her local environment, and can respond
   proactively.

   Example HTTP GET response for an incident entry:

      <?xml version="1.0" encoding="UTF-8"?>
      <entry>
        <id>http://www.example.org/csirt/private/incidents/123456</id>
        <title>Sample Incident</title>
        <link href="http://www.example.org/csirt/private/incidents/123456" rel="self"/>       <!-- by convention -->
        <link href="http://www.example.org/csirt/private/incidents/123456" rel="alternate"/>  <!-- required by Atom spec -->
        <published>2012-08-04T18:13:51.0Z</published>
        <updated>2012-08-05T18:13:51.0Z</updated>

        <link href="http://www.example.org/csirt/private/incidents/123456" rel="edit"/>

        <!-- The links to indicators related to this incident, and the history of this incident, and so on.... -->
        <link href="http://www.example.org/csirt/private/incidents/123456/relationships/indicators" rel="indicators"/>
        <link href="http://www.example.org/csirt/private/incidents/1234456/relationships/history" rel="history"/>
        <link href="http://www.example.org/csirt/private/incidents/1234456/relationships/campaign" rel="campaign"/>

        <!-- navigate up to the full collection.  Might also be rel="collection" as per IANA registry -->
        <link href="http://www.example.org/csirt/private/incidents" rel="up"/>

        <content type="application/xml">
          <iodef:IODEF-Document lang="en" xmlns:iodef="urn:ietf:params:xml:ns:iodef-1.0">
            <iodef:Incident purpose="traceback" restriction="need-to-know">
              <iodef:IncidentID name="http://www.example.org/csirt/private/incidents">123456</iodef:IncidentID>
              <!-- ...additional incident data.... -->
              </iodef:Incident>
          </iodef:IODEF-Document>
        </content>
      </entry>

   As can be seen in the example response, the Atom <link> elements
   enable the client to navigate to the related indicator resources,
   and/or the history entries associated with this incident.

4.2.4.2.  Use Case: Collaborative Investigation

   This section provides a non-normative example of a collaborative
   investigation use case.

   In this use case, two member CSIRTs that belong to a closed sharing
   consortium are collaborating on an incident investigation.  The
   initiating CSIRT performs an HTTP GET to retrieve the service
   document of the peer CSIRT, and determines the collection name to be
   used for creating a new investigation request.  The initiating CSIRT
   then POSTs a new incident entry to the appropriate collection URL.

   The target CSIRT acknowledges the request by responding with an HTTP
   status code 201 Created.

   Example HTTP GET response for the service document:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:09:11 GMT
      Content-Length: 934
      Content-Type: application/atomsvc+xml;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <service xmlns="http://www.w3.org/2007/app"
               xmlns:atom="http://www.w3.org/2005/Atom">
          <workspace xml:lang="en-US" xmlns:xml="http://www.w3.org/XML/1998/namespace">
            <atom:title type="text">RID Use Case Requests</atom:title>
            <collection  href="http://www.example.org/csirt/RID/InvestigationRequests">
               <atom:title type="text">Investigation Requests</atom:title>
               <accept>application/atom+xml; type=entry</accept>  <!-- perhaps we should have a more specific media type -->
            </collection>
            <collection  href="http://www.example.org/csirt/RID/TraceRequests">
               <atom:title type="text">Trace Requests</atom:title>
               <accept>application/atom+xml; type=entry</accept>
            </collection>
            <!-- ...and so on.... -->
          </workspace>
      </service>

   As can be seen in the example response, the Atom <collection>
   elements enable the client to determine the appropriate collection
   URL to request an investigation or a trace.

   The client CSIRT then POSTs a new entry to the appropriate feed
   collection.  Note that the <content> element of the new entry may
   contain a RID message of type "InvestigationRequest" if desired,
   however this would NOT be required.  The entry content itself need
   only be an IODEF document, with the choice of the target collection
   resource URL indicating the callers intent.  A CSIRT would be free to
   use any URI template to accept investigationRequests.

POST /csirt/RID/InvestigationRequests HTTP/1.1
Host: www.example.org
Content-Type: application/atom+xml;type=entry
Content-Length: 852

<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
  <title>New Investigation Request</title>
  <id>http://www.example2.org/csirt/private/incidents/123456</id>  <!-- id and updated not guranteed to be preserved -->
  <updated>2012-08-12T11:08:22Z</updated>                         <!-- may want to profile that behavior in this document -->
  <author><name>Name of peer CSIRT</name></author>
  <content type="application/xml">
    <iodef:IODEF-Document lang="en" xmlns:iodef="urn:ietf:params:xml:ns:iodef-1.0">
      <iodef:Incident purpose="traceback" restriction="need-to-know">
      <iodef:IncidentID name="http://www.example2.org/csirt/private/incidents">123</iodef:IncidentID>
        <!-- ...additional incident data.... -->
      </iodef:Incident>
    </iodef:IODEF-Document>
  </content>
</entry>

   The receiving CSIRT acknowledges the request with HTTP return code
   201 Created.

HTTP/1.1 201 Created
Date: Fri, 24 Aug 2012 19:17:11 GMT
Content-Length: 906
Content-Type: application/atom+xml;type=entry
Location: http://www.example.org/csirt/RID/InvestigationRequests/823
ETag: "8a9h9he4qphqh"

<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns="http://www.w3.org/2005/Atom">
  <title>New Investigation Request</title>
  <id>http://www.example.org/csirt/RID/InvestigationRequests/823</id>  <!-- id and updated not guranteed to be preserved -->
  <updated>2012-08-12T11:08:30Z</updated>                              <!-- may want to profile that behavior in this document -->
  <published>2012-08-12T11:08:30Z</published>
  <author><name>Name of peer CSIRT</name></author>
  <content type="application/xml">
    <iodef:IODEF-Document lang="en" xmlns:iodef="urn:ietf:params:xml:ns:iodef-1.0">
      <iodef:Incident purpose="traceback" restriction="need-to-know">
      <iodef:IncidentID name="http://www.example.org/csirt/private/incidents">123</iodef:IncidentID>
        <!-- ...additional incident data.... -->
      </iodef:Incident>
    </iodef:IODEF-Document>
  </content>
</entry>

   Consistent with HTTP/1.1 RFC, the location header indicates the URL
   of the newly created InvestigationRequest.  If for some reason the
   request were not authorized, the client would receive an HTTP status
   code 403 Unauthorized.  In this case the HTTP response body may
   contain additional details, if an as appropriate.

4.2.4.3.  Use Case: Search (Query)

   This section provides a non-normative example of a search use case.

   The following example provides a RESTful alternative to the RID Query
   messaage, as described in sections 6.5 and 7.4 of RFC6545.  Note that
   in the RESTful approach described herein there is no requirement to
   define a query language specific to RID queries.  Instead, CSIRTs may
   provide support for search operations via existing search facilities,
   and advertise these capabilities via an appropriate URL template.
   Clients dynamically retrieve the search description document, and
   invoke specific searches via an instantiated URL template.

   An HTTP response body may include a link relationship of type
   "search."  This link provides a reference to an OpenSearch
   description document.

   Example HTTP response that includes a "search" link:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:20:11 GMT
      Content-Length: nnnn
      Content-Type: application/atom+xml;type=feed;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <feed xmlns="http://www.w3.org/2005/Atom"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.w3.org/2005/Atom file:/C:/schemas/atom.xsd
                              urn:ietf:params:xml:ns:iodef-1.0 file:/C:/schemas/iodef-1.0.xsd"
          xml:lang="en-US">

          <link href="http://www.example.org/opensearchdescription.xml" rel="search"
                  type="application/opensearchdescription+xml"
                  title="CSIRT search facility" />

          <!-- ...other links... -->

          <entry>
              <!-- ...zero or more entries... -->
          </entry>

      </feed>

   The OpenSearch Description document contains the information needed
   by a client to request a search.  An example of an Open Search
   description document is shown below:

   Example HTTP response that includes a "search" link:

              <?xml version="1.0" encoding="UTF-8"?>
              <OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
                <ShortName>CSIRT search example</ShortName>
                <Description>Cyber security information sharing consortium search interface</Description>
                <Tags>example csirt indicator search</Tags>
                <Contact>admin@example.org</Contact>
                <!-- ...optionally, other elements, as per OpenSearch specification... -->
                <Url type="application/opensearchdescription+xml" rel="self" template="http://www.example.com/csirt/opensearchdescription.xml"/>
                <Url type="application/atom+xml" rel="results" template="http://www.example.org/csirt?q={searchTerms}&amp;format=Atom+xml"/>
                <LongName>www.example.org CSIRT search</LongName>
                <Query role="example" searchTerms="incident" />
                <Language>en-us</Language>
                <OutputEncoding>UTF-8</OutputEncoding>
                <InputEncoding>UTF-8</InputEncoding>
              </OpenSearchDescription>

   The OpenSearch Description document shown above contains two <Url>
   elements that contain parameterized URL templates.  These templates
   provide a representation of how the client should make search
   requests.  The exact format of the query string, including the
   parameterization is specified by the feed provider.

   This OpenSearch Description Document also contains an example of a
   <Query> element.  Each <Query> element describes a specific search
   request that can be made by the client.  Note that the parameters of
   the <Query> element correspond to the URL template parameters.  In
   this way, a provider may fully describe the search interface
   available to the clients.  Section 5.12, below, provides specific
   NORMATIVE requirements for the use of Open Search.

4.2.4.4.  Use Case: Cyber Data Repository

   This section provides a non-normative example of a cyber security
   data repository use case.

   In this use case a client accesses a persistent repository of cyber
   security data via a RESTful usage model.  Retrieving a feed
   collection is analogous to an SQL SELECT statement producing a result
   set.  Retrieving an individual Atom Entry is analogous to a SQL
   SELECT statement based upon a primary key producing a unique record.
   The cyber security data contained in the repository may include
   different data types, including indicators, incidents, becnmarks, or
   any other related resources.  In this use case, the repository is
   queried via HTTP GET, and the results that are returned to the client
   may optionally contain URL references to other cyber security
   resources that are known to be related.  These related resources may
   also be persisted locally, or they may exist at another (remote)
   cyber data respository.

   Example HTTP GET request to a persistent repository for any resources
   representing Distributed Denial of Service (DDOS) attacks:

   GET /csirt/repository/ddos
   Host: www.example.org
   Accept: application/atom+xml

   The corresponding HTTP response would be identify an XML document containing
   the DDOS feed.

   Example HTTP GET response for a DDOS feed:

      HTTP/1.1 200 OK
      Date: Fri, 24 Aug 2012 17:20:11 GMT
      Content-Length: nnnn
      Content-Type: application/atom+xml;type=feed;charset="utf-8"

      <?xml version="1.0" encoding="UTF-8"?>
      <feed xmlns="http://www.w3.org/2005/Atom"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://www.w3.org/2005/Atom file:/C:/schemas/atom.xsd
                              urn:ietf:params:xml:ns:iodef-1.0 file:/C:/schemas/iodef-1.0.xsd"
          xml:lang="en-US">

          <generator version="1.0" xml:lang="en-US">emc-csirt-iodef-feed-service</generator>
          <id xml:lang="en-US">http://www.example.org/csirt/repository/ddos</id>
          <title type="text" xml:lang="en-US">Atom formatted representation of a feed of known ddos resources.</title>
          <updated xml:lang="en-US">2012-05-04T18:13:51.0Z</updated>
          <author>
              <email>csirt@example.org</email>
              <name>EMC CSIRT</name>
          </author>

          <!-- By convention there is usually a self link for the feed -->
          <link href="http://www.example.org/csirt/repository/ddos" rel="self"/>

          <entry>
              <id>http://www.example.org/csirt/repository/ddos/123456</id>
              <title>Sample DDOS Incident</title>
              <link href="http://www.example.org/csirt/repository/ddos/123456" rel="self"/>          <!-- by convention -->
              <link href="http://www.example.org/csirt/repository/ddos/123456" rel="alternate"/>     <!-- required by Atom spec -->
              <link href="http://www.example.org/csirt/repository/ddos/987654" rel="related"/>       <!-- link to a related DDOS resource in this repository -->
              <link href="http://www.cyber-agency.gov/repository/indicators/1a2b3c" rel="related"/>  <!-- link to a related DDOS resource in another repository -->
              <published>2012-08-04T18:13:51.0Z</published>
              <updated>2012-08-05T18:13:51.0Z</updated>
              <!-- The category is based upon IODEF purpose and restriction attributes -->
              <category term="traceback" scheme="purpose" label="trace back" />
              <category term="need-to-know" scheme="restriction" label="need to know" />
              <category term="ddos" scheme="ttp" label="tactics, techniques, and procedures"/>
              <summary>A short description of this DDOS attack, extracted from the IODEF Incident class, <description> element. </summary>
          </entry>

          <entry>
              <!-- ...another entry... -->
          </entry>

      </feed> are application-defined.

   This feed document has two atom entries, one of which has been
   elided.  The completed entry illustrates an Atom <entry> element that
   provides a summary of essential details about one particular DDOS
   incident.  Based upon this summary information and the provided
   category information, a client may choose to do an HTTP GET operation
   to retrieve the full details of the DDOS incident.  This example
   shows how a persistent repository may provide links to additional
   resources, both local and remote.

   Note that the provider of a persistent repostory is not obligated to
   follow any particular URL template scheme.  The repository available
   at the hypothetical provider "www.example.com" uses a different URL
   pattern than the hypothetical repository available at "www.cyber-
   agency.gov".  When a client de-references a link to resource that is
   located in a remote repository the client may be challenged for
   authentication credentials acceptable to assumes that provider.  If the two
   repository providers choose to support a federated identity scheme or
   some other form of single-sign-on technology, then the user
   experience can be improved for interactive clients (e.g., a human
   user at a browser).  However, this is not required and is an
   implementation choice that is out reader has an understanding of scope for both
   Atom documents.  Further discussion of Atom's application to this specification.
   domain a well of examples of its use are provided in the BCG
   document.

5.  Normative Requirements for RESTful (Atom+xml) Binding TODO

   This section provides the NORMATIVE requirements for using Atom
   format and Atom Pub as a RESTful binding for cyber security
   information sharing.

5.1.  Atom Requirements

   Implementations of this specification MUST implement all requirements
   specified in Atom Publishing Protocol and the Atom Syndication
   Format.  (TODO: work on a more normative and perhaps constrained
   requirement.)

5.2.  Transport Layer Security

   Servers implementing this specification

   Implementations MUST support server-
   authenticated server-authenticated TLS.

   Servers

   Implementations MAY support mutually authenticated TLS.

5.2.

5.3.  Archiving and Paging

   A feed may can contain an arbitrary number of entries.  In some cases,
   the complete response to a given query may consist of a logical
   result set that contains a large number of entries.  As a practical
   matter, the full result set may will likely need to be divided into more
   managable
   manageable portions.  For example, a query may produce a full result
   set that may need to be grouped into logical pages, for purposes of
   rendering on a user interface.

   An historical feed may need to be stable, and/or divided into some
   defined epochs.

   Use cases that require capabilities for paging and archiving of feeds  Implementations SHOULD support the mechanisms
   described in Feed Paging and Archiving
   [RFC5005].

5.3. [RFC5005] to provide
   capabilities for paging and archiving of feeds.

5.4.  Expectation and Impact Classes

   It is frequently the case that a CSIRT an organization will need to triage
   their investigation and response activities based upon, e.g., the
   state of the current threat environment, or simply as a result of
   having limited resources.

   In order to enable CSIRTs operators to effectively prioritize their response
   activity, it is RECOMMENDED that feed implementors implementers provide Atom
   categories that correspond to the IODEF Expectation and Impact
   classes.  The availability of these feed categories will enable
   clients to more easily retrieve and prioritize cyber security
   information that has already been identified as having a specific
   potential impact, or having a specific expectation.

   Support for these catagories categories may also enable efficiencies for
   organizations that already have established (or plan to establish)
   operational processes and workflows that are based on these IODEF
   classes.

5.4.

5.5.  User Authentication

   Servers

   Implementations MUST require support user authentication.

   Servers  User
   authentication MAY be enabled for specific feeds.

   Implementations MAY support more than one client authentication
   method.

   Servers participating in an information sharing consotium consortium and
   supporting interactive user logins by members of the consortium
   SHOULD support client authentication via a federated identity scheme
   as per SAML 2.0.

   Servers

   Implementations MAY support client authenticated TLS.

5.5.

5.6.  User Authorization

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

   Authorization for a resource MAY be adjudicated based on the value(s)
   of the associated Atom <category> element(s).

   When the content model for the Atom <content> element of an Atom
   Entry contains an <IODEF-Document>, then authorization MUST be
   adjudicated based upon the Atom <category> element(s), whose values
   have been mapped as per Section 5.9. 5.10.

   Any use of the <category> element(s) as an input to an authorization
   policy decision MUST include both the "scheme" and "term" attributes
   contained therein.  As described in Section 5.9 5.10 below, the namespace
   of the "term" attribute is scoped by the associated "scheme"
   attribute.

5.6.

5.7.  Content Model

   Member entry resources providing a representation of an incident
   resource (e.g., as specified in the link relation type) MUST use the
   IODEF schema as the content model for the Atom Entry <content>
   element.

   Member Entry resources providing a representation of an indicator
   resource (e.g., as specified in the link relation type) MUST use the
   IODEF schema as the content model for the Atom Entry <content>
   element.

   The resource representation MAY include an appropriate indicator
   schema type within the <AdditionalData> element of the IODEF Incident
   class.  Supported indicator schema types SHALL be registered via an
   IANA table (todo: IANA registration/review).

   Member Entry resources providing a representation of a RID report
   resource (e.g., as specified in the link relation type) MUST use the
   RID schema as the content model for the Atom Entry <content> element.

   Member Entry resources providing representation of other types,
   SHOULD use the IODEF schema appropriate for their data category as the
   content model for the Atom Entry <content> element.

   If the member entry content model is not IODEF, then the  These data
   categories SHALL be registered via an IANA table.

   The <content> element of the Atom entry MUST contain an appropriate
   XML namespace declaration.

5.7.

5.8.  HTTP methods

   The following table defines the HTTP [RFC2616] [RFC7235] uniform interface
   methods supported by this specification:

   +--------+----------------------------------------------------------+
   | HTTP   | Description                                              |
   | method |                                                          |
   +--------+----------------------------------------------------------+
   | GET    | Returns a representation of an individual member entry   |
   |        | resource, or a feed collection.                          |
   | PUT    | Replaces the current representation of the specified     |
   |        | member entry resource with the representation provided   |
   |        | in the HTTP request body.                                |
   | POST   | Creates a new instance of a member entry resource. The   |
   |        | representation of the new resource is provided in the    |
   |        | HTTP request body.                                       |
   | DELETE | Removes the indicated member entry resource, or feed     |
   |        | collection.                                              |
   | HEAD   | Returns metadata about the member entry resource, or     |
   |        | feed collection, contained in HTTP response headers.     |
   | PATCH  | Support TBD.                                             |
   +--------+----------------------------------------------------------+

       Table 1: Uniform Interface for Resource-Oriented Lightweight
                            Indicator Exchange

   Clients MUST be capable of recognizing and prepared to process any
   standard HTTP status code, as defined in [RFC2616]

5.8. [RFC7235]

5.9.  Service Discovery

   This specification requires that a CSIRT implementation MUST publish an
   Atom Service Document that describes the set of cyber security
   information sharing feeds that are provided.

   The service document SHOULD be discoverable via the CSIRT organization's
   Web home page or another well-known public resource.

5.8.1.

5.9.1.  Workspaces

   The service document MAY include multiple workspaces.  Any CSIRT producer
   providing both public feeds and private consortium feeds MUST place
   these different classes of feeds into different workspaces, and
   provide appropriate descriptions and naming conventions to indicate
   the intended audience of each workspace.

5.8.2.

5.9.2.  Collections

   A CSIRT

   An implementation MAY provide any number of collections within a
   given Workspace.  It is RECOMMENDED that each collection appear in
   only a single Workspace.  It is RECOMMENDED that at least one
   collection be provided that accepts new incident reports from users.
   At least one collection MUST provide a feed of incident information
   for which the content model for the entries uses the IODEF schema.
   The title of this collection SHOULD be "Incidents".

5.8.3.

5.9.3.  Service Document Security

   Access to the service document MUST be protected via server-
   authenticated TLS and a server-side certificate.

   When deploying a service document for use by a closed consortium, the
   service document MAY also be digitally signed and/or encrypted, using
   XML DigSig and/or XML Encryption, respectively.

5.9.

5.10.  Category Mapping

   This section defines normative requirements for mapping IODEF
   metadata to corresponding Atom category elements. (todo: decide
   between IANA registration of scheme, or use a full URI).

5.9.1.

5.10.1.  Collection Category

   An Atom collection MAY hold entries from one or more categories.  The
   collection category set MUST contain at least the union of all the
   member entry categories.  A collection MAY have additional category
   metadata that are unique to the collection, and not applicable to any
   individual member entry.  A collection containing IODEF incident
   content MUST contain at least two <category> elements.  One category
   MUST be specified with the value of the "scheme" attribute as
   "restriction".  One category MUST be specified with the value of the
   "scheme" attribute as "purpose".  The value of the "fixed" attribute
   for both of these category elements MUST be "yes".  When the category
   scheme="restriction", the allowable values for the "term" attribute
   are constrained as per section 3.2 of IODEF, e.g. public, need-to-
   know, private, default.  When the category scheme="purpose", the
   allowable values for the "term" attribute are constrained as per
   section 3.2 of IODEF, e.g. traceback, mitigation, reporting, other.

5.9.2.

5.10.2.  Entry Category

   An Atom entry containing IODEF content MUST contain at least two
   <category> elements.  One category MUST be specified with the value
   of the "scheme" attribute as "restriction".  One category MUST be
   specified with the value of the "scheme" attribute as "purpose".
   When the category scheme="restriction", the value of the "term"
   attribute must be exactly one of ( public, need-to-know, private,
   default).  When the category scheme="purpose", the value of the
   "term" attribute must be exactly one of (traceback, mitigation,
   reporting, other).  When the purpose is "other"....

   Any member entry MAY have any number of additional categories.

5.10.

5.11.  Entry ID

   The ID element for an Atom entry SHOULD be established via the
   concatenation of the value of the name attribute from the IODEF
   <IncidentID> element and the corresponding value of the <IncidentID>
   element.  This requirement ensures a simple and direct one-to-one
   relationship between an IODEF incident ID and a corresponding Feed
   entry ID and avoids the need for any system to maintain a persistent
   store of these identity mappings.

   (todo: Note that this implies a constraint on the IODEF document that
   is more restrictive than the current IODEF schema.  IODEF section 3.3
   requires only that the name be a STRING type.  Here we are stating
   that name must be an IRI.  Possible request to update IODEF to
   constrain, or to support a new element or attribute).

5.11.

5.12.  Entry Content

   The <content> element of an Atom <entry> SHOULD include an IODEF
   document.  The <entry> element SHOULD include an appropriate XML
   namespace declaration for the IODEF schema.  If the content model of
   the <entry> element does not follow the IODEF schema, then the
   <entry> element MUST include an appropriate XML namespace
   declaration.

   A client MAY ignore content that is not using the IODEF schema.

5.12.

5.13.  Link Relations

   In addition to the standard Link Relations defined by the Atom
   specification, this specification defines the following additional
   Link Relation terms, which are introduced specifically in support of
   the Resource-Oriented Lightweight Indicator Information Exchange protocol.

   +-----------------------+-----------------------------+-------------+
   | Name                  | Description                 | Conformance |
   +-----------------------+-----------------------------+-------------+
   | service               | Provides a link to an atom  | MUST        |
   |                       | service document associated |             |
   |                       | with the collection feed.   |             |
   | search                | Provides a link to an       | MUST        |
   |                       | associated Open Search      |             |
   |                       | document that describes a   |             |
   |                       | URL template for search     |             |
   |                       | queries.                    |             |
   | history               | Provides a link to a        | MUST        |
   |                       | collection of zero or more  |             |
   |                       | historical entries that are |             |
   |                       | associated with the         |             |
   |                       | resource.                   |             |
   | incidents             | Provides a link to a        | MUST        |
   |                       | collection of zero or more  |             |
   |                       | instances of actual cyber incident       |             |
   |                       | representations associated  |             |
   |                       | with the resource.          |             |
   | indicators            | Provides a link to a        | MUST        |
   |                       | collection of zero or more  |             |
   |                       | instances of cyber security event(s) |             |
   |                       | indicators that are         |             |
   |                       | associated with the         |             |
   |                       | resource.                   |             |
   | indicators information           | Provides a link to a        | MUST        |
   |                       | collection of zero or more  |             |
   |                       | instances of cyber security |             |
   |                       | indicators information that are is         |             |
   |                       | associated with the         |             |
   |                       | resource.                   |             |
   | evidence              | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that provides     |             |
   |                       | some proof of attribution   |             |
   |                       | for an incident. The        |             |
   |                       | evidence may or may not     |             |
   |                       | have any identified chain   |             |
   |                       | of custody.                 |             |
   | campaign              | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that provides a   |             |
   |                       | representation of the       |             |
   |                       | associated cyber attack     |             |
   |                       | campaign.                   |             |
   | attacker              | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that provides a   |             |
   |                       | representation of the       |             |
   |                       | attacker.                   |             |
   | vector                | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that provides a   |             |
   |                       | representation of the       |             |
   |                       | method used by the          |             |
   |                       | attacker.                   |             |
   | assessments           | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that represent    |             |
   |                       | the results of executing a  |             |
   |                       | benchmark.                  |             |
   | reports               | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that represent    |             |
   |                       | RID reports.                |             |
   | traceRequests         | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that represent    |             |
   |                       | RID traceRequests.          |             |
   | investigationRequests | Provides a link to a        | SHOULD      |
   |                       | collection of zero or more  |             |
   |                       | resources that represent    |             |
   |                       | RID investigationRequests.  |             |
   +-----------------------+-----------------------------+-------------+
    Table 2: Link Relations for Resource-Oriented Lightweight Indicator
                                 Exchange

   Unless specifically registered with IANA these short names MUST be
   fully qualified via concatenation with a base-uri.  An appropriate
   base-uri could be established via agreement amongst the members of an
   information sharing consortium.  For example, the rel="indicators"
   relationship would become
   rel="http://www.example.org/csirt/incidents/relationships/
   rel="http://www.example.org/rolie/incidents/relationships/
   indicators."

5.12.1.

5.13.1.  Additional Link Relation Requirements

   An IODEF document that is carried in an Atom Entry SHOULD NOT contain
   a <relatedActivity> element.  Instead, the related activity SHOULD be
   available via a link rel=related.

   An IODEF document that is carried in an Atom Entry SHOULD NOT contain
   a <history> element.  Instead, the related history SHOULD be
   available via a link rel="history" (todo: or a fully qualified link
   rek name).  The associated href MAY leverage OpenSearch to specify
   the required query.

   An Atom Entry MAY include additional link relationships not specified
   here.  If a client encounters a link relationship of an unkown unknown type
   the client MUST ignore the offending link and continue processing the
   remaining resource representation as if the offending link element
   did not appear.

5.13.

5.14.  Member Entry Forward Security

   As described in Authorization Policy Enforcement
   (Authorization Policy Enforcement) a RESTful model for
   cyber security information sharing requires that all of the required
   security enforcement for feeds and entries MUST be enforced at the
   source system, at the point the representation of the given
   resource(s) is created.  A CSIRT provider SHALL NOT return any feed content
   or member entry content for which the client identity has not been
   specifically authenticated, authorized, and audited.

   Sharing communities that have a requirement for forward message
   security (such that client systems are required to participate in
   providing message level security and/or distributed authorization
   policy enforcement), MUST use the RID schema as the content model for
   the member entry <content> element.

5.14.

5.15.  Date Mapping

   The Atom feed <updated> element MUST be populated with the current
   time at the instant the feed representation was generated.  The Atom
   entry <published> element MUST be populated with the same time value
   as the <reportTime> element from the IODEF document.

5.15.

5.16.  Search

   Implementers MUST support OpenSearch 1.1 [opensearch] as the
   mechanism for describing how clients may form search requests.

   Implementers MUST provide a link with a relationship type of
   "search".  This link SHALL return an Open Search Description Document
   as defined in OpenSearch 1.1.

   Implementers MUST support an OpenSearch 1.1 compliant search URL
   template that enables a search query via Atom Category, including the
   scheme attribute and terms attribute as search parameters.

   Implementers SHOULD support search based upon the IODEF AlternativeID
   class as a search parameter.

   Implementers SHOULD support search based upon the four timestamp
   elements of the IODEF Incident class: <startTime>, <EndTime>,
   <DetectTime>, and <ReportTime>.

   Implementers MAY support additional search capabilities based upon
   any of the remaining elements of the IODEF Incident class, including
   the <Description> element.

   Collections that support use of the RID schema as a content model in
   the Atom member entry <content> element (e.g. in a report resource
   representation reachable via the "report" link relationship) MUST
   support search operations that include the RID MessageType as a
   search parameter, in addition to the aforementioned IODEF schema
   elements, as contained within the <ReportSchema> element.

   Implementers MUST fully qualify all OpenSearch URL template parameter
   names using the defined IODEF or RID XML namespaces, as appropriate.

5.16.

5.17.  / (forward slash) Resource URL

   The "/" resource MAY be provided for compatibility with existing
   deployments that are using Transport of Real-time Inter-network
   Defense (RID) Messages over HTTP/TLS [RFC6546].  Consistent with
   RFC6546 errata, a client requesting a GET on "/" MUST receive an HTTP
   status code 405 Method Not Allowed.  An implementation MAY provide
   full support for RFC6546 such that a POST to "/" containing a
   recognized RID message type just works.  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.
   This resource could also leverage the new draft by reschke that
   proposes HTTP status code 308 (cf: draft-reschke-http-status-
   308-07.txt).

6.  Security Considerations TODO

   This document defines a resource-oriented approach to lightweight
   indicator
   information exchange using HTTP, TLS, Atom Syndicate Format, and Atom
   Publishing Protocol.  As such, implementers must understand the
   security considerations described in those specifications.

   In addition, there are a number of additional security considerations
   that are unique to this specification.

   As described above in the section Authentication of Users
   (Section 3.2), the

   The approach described herein is based upon all policy enforcements
   being implemented at the point when a resource representation is
   created.  As such, CSIRTS producers sharing cyber security information using
   this specification must take care to authenticate their HTTP clients
   using a suitably strong user authentication mechanism.  Sharing
   communities that are exchanging information on well-known indicators
   and incidents for purposes of public education may choose to rely
   upon, e.g.  HTTP Authentication, or similar.  However, sharing
   communities that are engaged in sensitive collaborative analysis and/or and/
   or operational response for indicators and incidents targeting high
   value information systems should adopt a suitably stronger user
   authentication solution, such as TLS client certificates, or a risk-based risk-
   based or multi-factor approach.  In general, trust in the sharing
   consortium will depend upon the members maintaining adequate user
   authentication mechanisms.

   Collaborating consortiums may benefit from the adoption of a
   federated identity solution, such as those based upon SAML-core
   [SAML-core] and SAML-bind [SAML-bind] and SAML-prof [SAML-prof] for
   Web-based authentication and cross-organizational single sign-on.
   Dependency on a trusted third party identity provider implies that
   appropriate care must be exercised to sufficiently secure the
   Identity provider.  Any attacks on the federated identity system
   would present a risk to the CISRT, as a relying party.  Potential
   mitigations include deployment of a federation-aware identity
   provider that is under the control of the information sharing
   consortium, with suitably stringent technical and management
   controls.

   As discussed above in the section

   Authorization Policy Enforcement
   (Section 3.3), authorization of resource representations is the responsibility of
   the source system, i.e. based on the authenticated user identity
   associated with an HTTP(S) request.  The required authorization
   policies that are to be enforced must therefore be managed by the
   security administrators of the source system.  Various authorization
   architectures would be suitable for this purpose, such as RBAC [1]
   and/or ABAC, as embodied in XACML [XACML].  In particular,
   implementers adopting XACML may benefit from the capability to
   represent their authorization policies in a standardized,
   interoperable format.

   Additional security requirements such as enforcing message-level
   security at the destination system could supplement the security
   enforcements performed at the source system, however these
   destination-provided policy enforcements are out of scope for this
   specification.  Implementers requiring this capability should
   consider leveraging, e.g. the <RIDPolicy> element in the RID schema.
   Refer to RFC6545 section 9 for more information.

   When security policies relevant to the source system are to be
   enforced at both the source and destination systems, implementers
   must take care to avoid unintended interactions of the separately
   enforced policies.  Potential risks will include unintended denial of
   service and/or unintended information leakage.  These problems may be
   mitigated by avoiding any dependence upon enforcements performed at
   the destination system.  When distributed enforcement is unavoidable,
   the usage of a standard language (e.g.  XACML) for the expression of
   authorization policies will enable the source and destination systems
   to better coordinate and align their respective policy expressions.

   Adoption of the information sharing approach described in this
   document will enable users to more easily perform correlations across
   separate, and potentially unrelated, cyber security information
   providers.  A client may succeed in assembling a data set that would
   not have been permitted within the context of the authorization
   policies of either provider when considered individually.  Thus,
   providers may face a risk of an attacker obtaining an access that
   constitutes an undetected separation of duties (SOD) violation.  It
   is important to note that this risk is not unique to this
   specification, and a similar potential for abuse exists with any
   other cyber security information sharing protocol.  However, the wide
   availability of tools for HTTP clients and Atom feed handling implies
   that the resources and technical skills required for a successful
   exploit may be less than it was previously.  This risk can be best
   mitigated through appropriate vetting of the client at account
   provisioning time.  In addition, any increase in the risk of this
   type of abuse should be offset by the corresponding increase in
   effectiveness that that this specification affords to the defenders.

   While it is a goal of this specification to enable more agile cyber
   security information sharing across a broader and varying
   constituency, there is nothing in this specification that necessarily
   requires this type of deployment.  A cyber security information
   sharing consortium may chose to adopt this specification while
   continuing to operate as a gated community with strictly limited
   membership.

7.  IANA Considerations

   This document does not require any actions from IANA. TODO

   TODO.

8.  Acknowledgements

   The author gratefully acknowledges the valuable contributions of Tom
   Maguire, Kathleen Moriarty, and Vijayanand Bharadwaj.  These
   individuals provided detailed review comments on earlier drafts, and
   many suggestions that have helped to improve this document .

9.  References

9.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2616]

   [RFC7235]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Ed. and T. Berners-Lee, J. Reschke, Ed., "Hypertext Transfer
              Protocol -- HTTP/1.1", (HTTP/1.1): Authentication", RFC 2616, 7235,
              DOI 10.17487/RFC2616, 10.17487/RFC7235, June 1999,
              <http://www.rfc-editor.org/info/rfc2616>. 2014,
              <http://www.rfc-editor.org/info/rfc7235>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <http://www.rfc-editor.org/info/rfc4287>.

   [RFC5005]  Nottingham, M., "Feed Paging and Archiving", RFC 5005,
              DOI 10.17487/RFC5005, September 2007,
              <http://www.rfc-editor.org/info/rfc5005>.

   [RFC5023]  Gregorio, J., Ed. and B. de hOra, Ed., "The Atom
              Publishing Protocol", RFC 5023, DOI 10.17487/RFC5023,
              October 2007, <http://www.rfc-editor.org/info/rfc5023>.

   [RFC5070]  Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
              Object Description Exchange Format", RFC 5070,
              DOI 10.17487/RFC5070, December 2007,
              <http://www.rfc-editor.org/info/rfc5070>.

   [RFC6545]  Moriarty, K., "Real-time Inter-network Defense (RID)",
              RFC 6545, DOI 10.17487/RFC6545, April 2012,
              <http://www.rfc-editor.org/info/rfc6545>.

   [opensearch]
              Clinton, D., "OpenSearch 1.1 draft 5 specification", 2011,
              <http://www.opensearch.org/Specifications/OpenSearch/1.1>.

   [SAML-core]
              Cantor, S., Kemp, J., Philpott, R., and E. Mahler,
              "Assertions and Protocols for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard , March 2005,
              <http://docs.oasis-open.org/security/saml/v2.0/
              saml-core-2.0-os.pdf>.

   [SAML-prof]
              Hughes, J., Cantor, S., Hodges, J., Hirsch, F., Mishra,
              P., Philpott, R., and E. Mahler, "Profiles for the OASIS
              Security Assertion Markup Language (SAML) V2.0", OASIS
              Standard , March 2005, <http://docs.oasis-
              open.org/security/saml/v2.0/saml-profiles-2.0-os.pdf>.

   [SAML-bind]
              Cantor, S., Hirsch, F., Kemp, J., Philpott, R., and E.
              Mahler, "Bindings for the OASIS Security Assertion Markup
              Language (SAML) V2.0", OASIS Standard , March 2005,
              <http://docs.oasis-open.org/security/saml/v2.0/
              saml-bindings-2.0-os.pdf>.

9.2.  Informative References

   [XMLencrypt]
              Imaura, T., Dillaway, B., and E. Simon, "XML Encryption
              Syntax and Processing", W3C Recommendation , December
              2002, <http://www.w3.org/TR/xmlenc-core/>.

   [XMLsig]   Bartel, M., Boyer, J., Fox, B., LaMaccia, B., and E.
              Simon, "XML-Signature Syntax and Processing", W3C
              Recommendation Second Edition, June 2008,
              <http://www.w3.org/TR/xmldsig-core/>.

   [XACML]    Rissanen, E., "eXtensible Access Control Markup Language
              (XACML) Version 3.0", August 2010, <http://docs.oasis-
              open.org/xacml/3.0/xacml-3.0-core-spec-cs-01-en.pdf>.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", 2000,
              <http://www.ics.uci.edu/~fielding/pubs/dissertation/
              top.htm>.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              DOI 10.17487/RFC2396, August 1998,
              <http://www.rfc-editor.org/info/rfc2396>.

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", RFC 2822,
              DOI 10.17487/RFC2822, April 2001,
              <http://www.rfc-editor.org/info/rfc2822>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6546]  Trammell, B., "Transport of Real-time Inter-network
              Defense (RID) Messages over HTTP/TLS", RFC 6546,
              DOI 10.17487/RFC6546, April 2012,
              <http://www.rfc-editor.org/info/rfc6546>.

9.3.  URIs

   [1] http://csrc.nist.gov/groups/SNS/rbac/

Appendix A.  Change Tracking

   Changes since -02 draft-field-mile-rolie-01 version, August 15, 2013 to December 2, 2015:

   o  Added section specifying the use of RFC5005 for Archive and Paging
      of feeds.  See: Section 5.2

   o  Added section describing use of atom categories that correspond to
      IODEF expectation class and impact classes.  See: Section 5.3

   o  Dropped references December, 2015 to adoption of a MILE-specific HTTP media type
      parameter.
   May 27, 2016:

   o  Updated IANA Considerations  Spun section to clarify that no IANA
      actions are required.

Appendix B.  Resource Authorization Model

   As described in Section 3.3.2 above, ROLIE assumes that all
   authorization policy enforcement is provided at the source server.
   The implementation details of the authorization scheme chosen by a
   ROLIE-compliant provider are out of scope for this specification.
   Implementers are free to choose any suitable authorization mechanism
   that is capable of fulfilling the policy enforcement requirements
   relevant to their consortium and/or organization.

   It is well known that one of the major barriers to information
   sharing is ensuring acceptable use of the information shared.  In the
   case of ROLIE, one way to lower that barrier may be to develop a
   XACML profile.  Use of XACML would allow a ROLIE-compliant provider
   to express their information sharing authorization policies in a
   standards-compliant, and machine-readable format.

   This improved interoperability may, in turn, enable more agile
   interactions in the cyber security sharing community.  For example, a
   peer CSIRT, or another interested stakeholder such as an auditor,
   would be able to review 4 and compare CSIRT sharing policies using
   appropriate tooling. some related contextual information into its
      own document see TODO:Add reference

   o  Recast document into a more general use perspective.  The XACML 3.0 standard is based upon the notion that authorization
   policies are defined in terms
      implication of predicate logic expressions written
   against CSIRTs as the attributes associated with one or more defacto end-user has been removed
      where ever possible.  All of the following
   four entities:

   o  SUBJECT

   o  ACTION original CSIRT based use cases
      remain completely supported by this document, it has been opened
      up to supported many other use cases.

   o  RESOURCE  Changed the content model to broaden support of representation

   o  ENVIRONMENT

   Thus, a suitable approach  Edited and rewrote much of sections 1,2 and 3 in order to
      accomplish a XACML 3.0 profile for ROLIE
   authorization policies could begin by using the 3-tuple of [SUBJECT,
   ACTION, RESOURCE] where: broader scope and greater readability

   o  SUBJECT is  Removed any requirements from the suitably authenticated identity of Background section and, if not
      already stated, placed them in the requestor. requirements section

   o  ACTION is  Re-formatted the associated HTTP method, GET, PUT, POST, DELETE,
      HEAD, (PATCH).

   o  RESOURCE is an XPath expression requirements section to make it clearer that uniquely identifies it
      contains the
      instance or type lions-share of the ROLIE resource being requested.

   Implementers who have a need may also choose to evaluate based upon requirements of the additional ENVIRONMENT factors, such as current threat level, and
   so on.  One could also write policy specification

   Changes made in draft-ietf-mile-rolie-01 since draft-field-mile-
   rolie-02 version, August 15, 2013 to consider the CVSS score
   associated with the resource, or December 2, 2015:

   o  Added section specifying the lifecycle phase use of the resource
   (vulnerability unverified, confirmed, patch available, etc.), and so
   on.

   Having these policies expressed in a standards-compliant RFC5005 for Archive and machine-
   readable format could improve the agility Paging
      of feeds.  See: Section 5.3

   o  Added section describing use of atom categories that correspond to
      IODEF expectation class and effectiveness impact classes.  See: Section 5.4

   o  Dropped references to adoption of a
   cyber security information sharing group or consortium, and enable
   better cyber defenses.

Author's Address MILE-specific HTTP media type
      parameter.

   o  Updated IANA Considerations section to clarify that no IANA
      actions are required.

Authors' Addresses

   John P. Field
   Pivotal Software, Inc.
   625 Avenue of the Americas
   New York, New York
   USA

   Phone: (646)792-5770
   Email: jfield@pivotal.io

   Stephen A. Banghart
   National Institute of Standards and Technology
   100 Bureau Drive
   Gaithersburg, Maryland
   USA

   Phone: (301)975-4288
   Email: sab3@nist.gov