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rfc:rfc5209

Network Working Group P. Sangster Request for Comments: 5209 Symantec Category: Informational H. Khosravi

                                                            Intel
                                                          M. Mani
                                                            Avaya
                                                       K. Narayan
                                                    Cisco Systems
                                                         J. Tardo
                                                   Nevis Networks
                                                        June 2008
    Network Endpoint Assessment (NEA): Overview and Requirements

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Abstract

 This document defines the problem statement, scope, and protocol
 requirements between the components of the NEA (Network Endpoint
 Assessment) reference model.  NEA provides owners of networks (e.g.,
 an enterprise offering remote access) a mechanism to evaluate the
 posture of a system.  This may take place during the request for
 network access and/or subsequently at any time while connected to the
 network.  The learned posture information can then be applied to a
 variety of compliance-oriented decisions.  The posture information is
 frequently useful for detecting systems that are lacking or have
 out-of-date security protection mechanisms such as: anti-virus and
 host-based firewall software.  In order to provide context for the
 requirements, a reference model and terminology are introduced.

Sangster, et al. Informational [Page 1] RFC 5209 NEA Requirements June 2008

Table of Contents

 1. Introduction ....................................................3
    1.1. Requirements Language ......................................4
 2. Terminology .....................................................5
 3. Applicability ...................................................7
    3.1. Scope ......................................................7
    3.2. Applicability of Environments ..............................8
 4. Problem Statement ...............................................9
 5. Reference Model ................................................10
    5.1. NEA Client and Server .....................................12
         5.1.1. NEA Client .........................................12
                5.1.1.1. Posture Collector .........................12
                5.1.1.2. Posture Broker Client .....................14
                5.1.1.3. Posture Transport Client ..................15
         5.1.2. NEA Server .........................................15
                5.1.2.1. Posture Validator .........................15
                5.1.2.2. Posture Broker Server .....................17
                5.1.2.3. Posture Transport Server ..................18
    5.2. Protocols .................................................18
         5.2.1. Posture Attribute Protocol (PA) ....................18
         5.2.2. Posture Broker Protocol (PB) .......................19
         5.2.3. Posture Transport Protocol (PT) ....................19
    5.3. Attributes ................................................20
         5.3.1. Attributes Normally Sent by NEA Client: ............21
         5.3.2. Attributes Normally Sent by NEA Server: ............21
 6. Use Cases ......................................................22
    6.1. Initial Assessment ........................................22
         6.1.1. Triggered by Network Connection or Service
                Request ............................................22
                6.1.1.1. Example ...................................23
                6.1.1.2. Possible Flows and Protocol Usage .........23
                6.1.1.3. Impact on Requirements ....................25
         6.1.2. Triggered by Endpoint ..............................25
                6.1.2.1. Example ...................................25
                6.1.2.2. Possible Flows and Protocol Usage .........26
                6.1.2.3. Impact on Requirements ....................28
    6.2. Posture Reassessment ......................................28
         6.2.1. Triggered by NEA Client ............................28
                6.2.1.1. Example ...................................28
                6.2.1.2. Possible Flows & Protocol Usage ...........29
                6.2.1.3. Impact on Requirements ....................30
         6.2.2. Triggered by NEA Server ............................30
                6.2.2.1. Example ...................................30
                6.2.2.2. Possible Flows and Protocol Usage .........31
                6.2.2.3. Impact on Requirements ....................33

Sangster, et al. Informational [Page 2] RFC 5209 NEA Requirements June 2008

 7. Requirements ...................................................34
    7.1. Common Protocol Requirements ..............................34
    7.2. Posture Attribute (PA) Protocol Requirements ..............35
    7.3. Posture Broker (PB) Protocol Requirements .................36
    7.4. Posture Transport (PT) Protocol Requirements ..............38
 8. Security Considerations ........................................38
    8.1. Trust .....................................................39
         8.1.1. Endpoint ...........................................40
         8.1.2. Network Communications .............................41
         8.1.3. NEA Server .........................................42
    8.2. Protection Mechanisms at Multiple Layers ..................43
    8.3. Relevant Classes of Attack ................................43
         8.3.1. Man-in-the-Middle (MITM) ...........................44
         8.3.2. Message Modification ...............................45
         8.3.3. Message Replay or Attribute Theft ..................45
         8.3.4. Other Types of Attack ..............................46
 9. Privacy Considerations .........................................46
    9.1. Implementer Considerations ................................47
    9.2. Minimizing Attribute Disclosure ...........................49
 10. References ....................................................50
    10.1. Normative References .....................................50
    10.2. Informative References ...................................50
 11. Acknowledgments ...............................................51

1. Introduction

 Endpoints connected to a network may be exposed to a wide variety of
 threats.  Some protection against these threats can be provided by
 ensuring that endpoints conform to security policies.  Therefore, the
 intent of NEA is to assess these endpoints to determine their
 compliance with security policies so that corrective measures can be
 provided before they are exposed to those threats.  For example, if a
 system is determined to be out of compliance because it is lacking
 proper defensive mechanisms such as host-based firewalls, anti-virus
 software, or the absence of critical security patches, the NEA
 protocols provide a mechanism to detect this fact and indicate
 appropriate remediation actions to be taken.  Note that an endpoint
 that is deemed compliant may still be vulnerable to threats that may
 exist on the network.
 NEA typically involves the use of special client software running on
 the requesting endpoint that observes and reports on the
 configuration of the system to the network infrastructure.  The
 infrastructure has corresponding validation software that is capable
 of comparing the endpoint's configuration information with network
 compliance policies and providing the result to appropriate
 authorization entities that make decisions about network and
 application access.  Some endpoints may be incapable of running the

Sangster, et al. Informational [Page 3] RFC 5209 NEA Requirements June 2008

 NEA Client software (e.g., printer) or be unwilling to share
 information about their configuration.  This situation is outside the
 scope of NEA and is subject to local policies.
 The result of an endpoint assessment may influence an access decision
 that is provisioned to the enforcement mechanisms on the network
 and/or endpoint requesting access.  While the NEA Working Group
 recognizes there may be a link between an assessment and the
 enforcement of a resulting access decision, the mechanisms and
 protocols for enforcement are not in scope for this specification.
 Architectures, similar to NEA, have existed in the industry for some
 time and are present in shipping products, but do not offer adequate
 interoperability.  Some examples of such architectures include:
 Trusted Computing Group's Trusted Network Connect [TNC], Microsoft's
 Network Access Protection [NAP], and Cisco's Cisco Network Admission
 Control [CNAC].  These technologies assess the software and/or
 hardware configuration of endpoint devices for the purposes of
 monitoring or enforcing compliance to an organization's policy.
 The NEA Working Group is developing standard protocols that can be
 used to communicate compliance information between a NEA Client and a
 NEA Server.  This document provides the context for NEA including:
 terminology, applicability, problem statement, reference model, and
 use cases.  It then identifies requirements for the protocols used to
 communicate between a NEA Client and NEA server.  Finally, this
 document discusses some potential security and privacy considerations
 with the use of NEA.  The majority of this specification provides
 informative text describing the context of NEA.

1.1. Requirements Language

 Use of each capitalized word within a sentence or phrase carries the
 following meaning during the NEA WG's protocol selection process:
 MUST - indicates an absolute requirement
 MUST NOT - indicates something absolutely prohibited
 SHOULD - indicates a strong recommendation of a desired result
 SHOULD NOT - indicates a strong recommendation against a result
 MAY - indicates a willingness to allow an optional outcome
 Lower case use of "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and
 "MAY" carry their normal meaning and are not subject to these
 definitions.

Sangster, et al. Informational [Page 4] RFC 5209 NEA Requirements June 2008

2. Terminology

 This section defines a set of terms used throughout this document.
 In some cases these terms have been used in other contexts with
 different meanings so this section attempts to describe each term's
 meaning with respect to the NEA WG activities.
 Assessment - The process of collecting posture for a set of
    capabilities on the endpoint (e.g., host-based firewall) such that
    the appropriate validators may evaluate the posture against
    compliance policy.
 Assertion Attributes - Attributes that include reusable information
    about the success of a prior assessment of the endpoint.  This
    could be used to optimize subsequent assessments by avoiding a
    full posture reassessment.  For example, this classification of
    attribute might be issued specifically to a particular endpoint,
    dated and signed by the NEA Server allowing that endpoint to reuse
    it for a time period to assert compliance to a set of policies.
    The NEA Server might accept this in lieu of obtaining posture
    information.
 Attribute - Data element including any requisite meta-data describing
    an observed, expected, or the operational status of an endpoint
    feature (e.g., anti-virus software is currently in use).
    Attributes are exchanged as part of the NEA protocols (see section
    5.2).  NEA recognizes a variety of usage scenarios where the use
    of an attribute in a particular type of message could indicate:
       o previously assessed status (Assertion Attributes),
       o observed configuration or property (Posture Attributes),
       o request for configuration or property information (Request
         Attributes),
       o assessment decision (Result Attributes), or
       o repair instructions (Remediation Attributes).
    The NEA WG will standardize a subset of the attribute namespace
    known as standard attributes.  Those attributes not standardized
    are referred to in this specification as vendor-specific.
 Dialog - Sequence of request/response messages exchanged.

Sangster, et al. Informational [Page 5] RFC 5209 NEA Requirements June 2008

 Endpoint - Any computing device that can be connected to a network.
    Such devices normally are associated with a particular link layer
    address before joining the network and potentially an IP address
    once on the network.  This includes: laptops, desktops, servers,
    cell phones, or any device that may have an IP address.
 Message - Self contained unit of communication between the NEA Client
    and Server.  For example, a posture attribute message might carry
    a set of attributes describing the configuration of the anti-virus
    software on an endpoint.
 Owner - the role of an entity who is the legal, rightful possessor of
    an asset (e.g., endpoint).  The owner is entitled to maintain
    control over the policies enforced on the device even if the asset
    is not within the owner's possession.  The owner may permit user
    override or augmentation of control policies or may choose to not
    assert any policies limiting use of asset.
 Posture - Configuration and/or status of hardware or software on an
    endpoint as it pertains to an organization's security policy.
 Posture Attributes - Attributes describing the configuration or
    status (posture) of a feature of the endpoint.  For example, a
    Posture Attribute might describe the version of the operating
    system installed on the system.
 Request Attributes - Attributes sent by a NEA Server identifying the
    posture information requested from the NEA Client.  For example, a
    Request Attribute might be an attribute included in a request
    message from the NEA Server that is asking for the version
    information for the operating system on the endpoint.
 Remediation Attributes - Attributes containing the remediation
    instructions for how to bring an endpoint into compliance with one
    or more policies.  The NEA WG will not define standard remediation
    attributes, but this specification does describe where they are
    used within the reference model and protocols.
 Result Attributes - Attributes describing whether the endpoint is in
    compliance with NEA policy.  The Result Attribute is created by
    the NEA Server normally at the conclusion of the assessment to
    indicate whether or not the endpoint was considered compliant.
    More than one of these attributes may be used allowing for more
    granular feature level decisions to be communicated in addition to
    an overall, global assessment decision.

Sangster, et al. Informational [Page 6] RFC 5209 NEA Requirements June 2008

 Session - Stateful connection capable of carrying multiple message
    exchanges associated with (an) assessment(s) of a particular
    endpoint.  This document defines the term session at a conceptual
    level and does not describe the properties of the session or
    specify requirements for the NEA protocols to manage these
    sessions.
 User - Role of a person that is making use of the services of an
    endpoint.  The user may not own the endpoint so he or she might
    need to operate within the acceptable use constraints defined by
    the endpoint's owner.  For example, an enterprise employee might
    be a user of a computer provided by the enterprise (owner) for
    business purposes.

3. Applicability

 This section discusses the scope of the technologies being
 standardized and the network environments where it is envisioned that
 the NEA technologies might be applicable.

3.1. Scope

 The priority of the NEA Working Group is to develop standard
 protocols at the higher layers in the reference model (see section
 5): the Posture Attribute protocol (PA) and the Posture Broker
 protocol (PB).  PA and PB will be designed to be carried over a
 variety of lower layer transport (PT) protocols.  The NEA WG will
 identify standard PT protocol(s) that are mandatory to implement.  PT
 protocols may be defined in other WGs because the requirements may
 not be specific to NEA.  When used with a standard PT protocol (e.g.,
 Extensible Authentication Protocol (EAP), Transport Layer Security
 (TLS) [TLS]), the PA and PB protocols will allow interoperability
 between a NEA Client from one vendor and a NEA Server from another.
 This specification will not focus on the other interfaces between the
 functional components of the NEA reference model nor requirements on
 their internals.  Any discussion of these aspects is included to
 provide context for understanding the model and resulting
 requirements.
 Some tangent areas not shown in the reference model that are also out
 of scope for the NEA working group, and thus this specification,
 include:
    o Standardizing the protocols and mechanisms for enforcing
      restricted network access,
    o Developing standard protocols for remediation of non-compliant
      endpoints,

Sangster, et al. Informational [Page 7] RFC 5209 NEA Requirements June 2008

    o Specifying protocols used to communicate with remote portions of
      the NEA Client or Server (e.g., remote collectors or validators
      of posture),
    o Supporting a NEA Client providing posture for other endpoints
      (e.g., a NEA Client on an Intrusion Detection System (IDS)
      providing posture for an endpoint without a NEA Client),
    o Defining the set of events or situations that might trigger a
      NEA Client or NEA Server to request an assessment,
    o Detecting or handling lying endpoints (see section 8.1.1 for
      more information).

3.2. Applicability of Environments

 Because the NEA model is based on NEA-oriented software being present
 on the endpoint and in the network infrastructure, and due to the
 nature of the information being exposed, the use of NEA technologies
 may not apply in a variety of situations possible on the Internet.
 Therefore, this section discusses some of the scenarios where NEA is
 most likely to be applicable and some where it may not be.
 Ultimately, the use of NEA within a deployment is not restricted to
 just these scenarios.  The decision of whether to use NEA
 technologies lies in the hands of the deployer (e.g., network
 provider) based upon the expected relationship they have with the
 owners and users of potential endpoints.
 NEA technologies are largely focused on scenarios where the owner of
 the endpoint is the same as the owner of the network.  This is a very
 common model for enterprises that provide equipment to employees to
 perform their duties.  These employees are likely bound under an
 employment contract that outlines what level of visibility the
 employer expects to have into the employee's use of company assets
 and possibly activities during work hours.  This contract may
 establish the expectation that the endpoint needs to conform to
 policies set forth by the enterprise.
 Some other environments may be in a similar situation and thus find
 NEA technologies to be beneficial.  For example, environments where
 the endpoint is owned by a party (possibly even the user) that has
 explicitly expressed a desire to conform to the policies established
 by a network or service provider in exchange for being able to access
 its resources.  An example of this might be an independent contractor
 with a personal laptop, working for a company imposing NEA assessment
 policies on its employees, who may wish a similar level of access and
 is willing to conform to the company's policies.  NEA technologies
 may be applicable to this situation.

Sangster, et al. Informational [Page 8] RFC 5209 NEA Requirements June 2008

 Conversely, some environments where NEA is not expected to be
 applicable would be environments where the endpoint is owned by a
 user that has not agreed to conform to a network provider's policies.
 An example might include when the above contractor visits any public
 area like the local coffee shop that offers Internet access.  This
 coffee shop would not be expected to be able to use NEA technologies
 to assess the posture of the contractor's laptop.  Because of the
 potentially invasive nature of NEA technology, such an assessment
 could amount to an invasion of privacy of the contractor.
 It is more difficult to determine whether NEA is applicable in other
 environments, so the NEA WG will consider them to be out of scope for
 consideration and specification.  In order for an environment to be
 considered applicable for NEA, the owner or user of an endpoint must
 have established a clear expectation that it will comply with the
 policies of the owner and operator of the network.  Such an
 expectation likely includes a willingness to disclose appropriate
 information necessary for the network to perform compliance checks.

4. Problem Statement

 NEA technology may be used for a variety of purposes.  This section
 highlights some of the major situations where NEA technologies may be
 beneficial.
 One use is to facilitate endpoint compliance checking against an
 organization's security policy when an endpoint connects to the
 network.  Organizations often require endpoints to run an
 IT-specified Operating System (OS) configuration and have certain
 security applications enabled, e.g., anti-virus software, host
 intrusion detection/prevention systems, personal firewalls, and patch
 management software.  An endpoint that is not compliant with IT
 policy may be vulnerable to a number of known threats that might
 exist on the network.
 Without NEA technology, ensuring compliance of endpoints to corporate
 policy is a time-consuming and difficult task.  Not all endpoints are
 managed by a corporation's IT organization, e.g., lab assets and
 contractor machines.  Even for assets that are managed, they may not
 receive updates in a timely fashion because they are not permanently
 attached to the corporate network, e.g., laptops.  With NEA
 technology, the network is able to assess an endpoint as soon as it
 requests access to the network or at any time after joining the
 network.  This provides the corporation an opportunity to check
 compliance of all NEA-capable endpoints in a timely fashion and
 facilitate endpoint remediation potentially while quarantined when
 needed.

Sangster, et al. Informational [Page 9] RFC 5209 NEA Requirements June 2008

 NEA technology can be used to provide posture assessment for a range
 of ways of connecting to the network including (but not limited to)
 wired and wireless LAN access such as using 802.1X [802.1X], remote
 access via IPsec [IPSEC], or Secure Socket Layer (SSL) VPN, or
 gateway access.
 Endpoints that are not NEA-capable or choose not to share sufficient
 posture to evaluate compliance may be subject to different access
 policies.  The decision of how to handle non-compliant or
 non-participating endpoints can be made by the network administrator
 possibly based on information from other security mechanisms on the
 network (e.g., authentication).  For example, remediation
 instructions or warnings may be sent to a non-compliant endpoint with
 a properly authorized user while allowing limited access to the
 network.  Also, network access technologies can use the NEA results
 to restrict or deny access to an endpoint, while allowing
 vulnerabilities to be addressed before an endpoint is exposed to
 attack.  The communication and representation of NEA assessment
 results to network access technologies on the network is out of scope
 for this document.
 Reassessment is a second important use of NEA technology as it allows
 for additional assessments of previously considered compliant
 endpoints to be performed.  This might become necessary because
 network compliance policies and/or endpoint posture can change over
 time.  A system initially assessed as being compliant when it joined
 the network may no longer be in compliance after changes occur.  For
 example, reassessment might be necessary if a user disables a
 security protection (e.g., host-based firewall) required by policy or
 when the firewall becomes non-compliant after a firewall patch is
 issued and network policy is changed to require the patch.
 A third use of NEA technology may be to verify or supplement
 organization asset information stored in inventory databases.
 NEA technology can also be used to check and report compliance for
 endpoints when they try to access certain mission critical
 applications within an enterprise, employing service (application)
 triggered assessment.

5. Reference Model

 This section describes the reference model for Network Endpoint
 Assessment.  This model is provided to establish a context for the
 discussion of requirements and may not directly map to any particular
 product or deployment architecture.  The model identifies the major

Sangster, et al. Informational [Page 10] RFC 5209 NEA Requirements June 2008

 functionality of the NEA Client and Server and their relationships,
 as well as the protocols they use to communicate at various levels
 (e.g., PA is carried by the PB protocol).
 While the diagram shows 3 layered protocols, it is envisioned that PA
 is likely a thin message wrapper around a set of attributes and that
 it is batched and encapsulated in PB.  PB is primarily a lightweight
 message batching protocol, so the protocol stack is mostly the
 transport (PT).  The vertical lines in the model represent APIs
 and/or protocols between components within the NEA Client or Server.
 These interfaces are out of scope for standardization in the NEA WG.
  +-------------+                          +--------------+
  |  Posture    |   <--------PA-------->   |   Posture    |
  |  Collectors |                          |   Validators |
  |  (1 .. N)   |                          |   (1 .. N)   |
  +-------------+                          +--------------+
        |                                         |
        |                                         |
        |                                         |
  +-------------+                          +--------------+
  |   Posture   |                          |   Posture    |
  |   Broker    |   <--------PB-------->   |   Broker     |
  |   Client    |                          |   Server     |
  +-------------+                          +--------------+
        |                                         |
        |                                         |
  +-------------+                          +--------------+
  |   Posture   |                          |   Posture    |
  |   Transport |   <--------PT-------->   |   Transport  |
  |   Client    |                          |   Server     |
  |   (1 .. N)  |                          |   (1 .. N)   |
  +-------------+                          +--------------+
     NEA CLIENT                               NEA SERVER
               Figure 1: NEA Reference Model
 The NEA reference model does not include mechanisms for discovery of
 NEA Clients and NEA Servers.  It is expected that NEA Clients and NEA
 Servers are configured with information that allows them to reach
 each other.  The specific methods of referencing the configuration
 and establishing the communication channel are out of scope for the
 NEA reference model and should be covered in the specifications of
 candidate protocols such as the Posture Transport (PT) protocol.

Sangster, et al. Informational [Page 11] RFC 5209 NEA Requirements June 2008

5.1. NEA Client and Server

5.1.1. NEA Client

 The NEA Client is resident on an endpoint device and comprised of the
 following functionality:
    o Posture Collector(s)
    o Posture Broker Client
    o Posture Transport Client(s)
 The NEA Client is responsible for responding to requests for
 attributes describing the configuration of the local operating domain
 of the client and handling the assessment results including potential
 remediation instructions for how to conform to policy.  A NEA Client
 is not responsible for reporting on the posture of entities that
 might exist on the endpoint or over the network that are outside the
 domain of execution (e.g., in other virtual machine domains) of the
 NEA Client.
 For example, a network address translation (NAT) device might route
 communications for many systems behind it, but when the NAT device
 joins the network, its NEA Client would only report its own (local)
 posture.  Similarly, endpoints with virtualization capabilities might
 have multiple independent domains of execution (e.g., OS instances).
 Each NEA Client is only responsible for reporting posture for its
 domain of execution, but this information might be aggregated by
 other local mechanisms to represent the posture for multiple domains
 on the endpoint.  Such posture aggregation mechanisms are outside the
 focus of this specification.
 Endpoints lacking NEA Client software (which is out of NEA scope) or
 choosing not to provide the attributes required by the NEA Server
 could be considered non-compliant.  The NEA model includes
 capabilities to enable the endpoint to update its contents in order
 to become compliant.

5.1.1.1. Posture Collector

 The Posture Collector is responsible for responding to requests for
 posture information in Request Attributes from the NEA Server.  The
 Posture Collector is also responsible for handling assessment
 decisions in Result Attributes and remediation instructions in
 Remediation Attributes.  A single NEA Client can have several Posture
 Collectors capable of collecting standard and/or vendor-specific
 Posture Attributes for particular features of the endpoint.  Typical

Sangster, et al. Informational [Page 12] RFC 5209 NEA Requirements June 2008

 examples include Posture Collectors that provide information about
 Operating System (OS) version and patch levels, anti-virus software,
 and security mechanisms on the endpoint such as host-based Intrusion
 Detection System (IDS) or firewall.
 Each Posture Collector will be associated with one or more
 identifiers that enable it to be specified as the destination in a PA
 message.  The Posture Broker Client uses these identifiers to route
 messages to this Collector.  An identifier might be dynamic (e.g.,
 generated by the Posture Broker Client at run-time during
 registration) or more static (e.g., pre-assigned to the Posture
 Collector at install-time and passed to the Posture Broker Client
 during registration) or a function of the attribute messages the
 Collector desires to receive (e.g., message type for subscription).
 The NEA model allocates the following responsibilities to the Posture
 Collector:
    o Consulting with local privacy and security policies that may
      restrict what information is allowed to be disclosed to a given
      NEA Server.
    o Receiving Request Attributes from a Posture Validator and
      performing the local processing required to respond
      appropriately.  This may include:
  1. Collecting associated posture information for particular

features of the endpoint and returning this information in

         Posture Attributes.
  1. Caching and recognizing the applicability of recently issued

attributes containing reusable assertions that might serve to

         prove compliance and returning this attribute instead of
         posture information.
    o Receiving attributes containing remediation instructions on how
      to update functionality on the endpoint.  This could require the
      Collector to interact with the user, owner, and/or a remediation
      server.
    o Monitoring the posture of (a) particular features(s) on the
      endpoint for posture changes that require notification to the
      Posture Broker Client.
    o Providing cryptographic verification of the attributes received
      from the Validator and offering cryptographic protection to the
      attributes returned.

Sangster, et al. Informational [Page 13] RFC 5209 NEA Requirements June 2008

 The above list describes the model's view of the possible
 responsibilities of the Posture Collector.  Note that this is not a
 set of requirements for what each Posture Collector implementation
 must support, nor is it an exhaustive list of all the things a
 Posture Collector may do.

5.1.1.2. Posture Broker Client

 The Posture Broker Client is both a PA message multiplexer and a
 de-multiplexer.  The Posture Broker Client is responsible for
 de-multiplexing the PB message received from the NEA Server and
 distributing each encapsulated PA message to the corresponding
 Posture Collector(s).  The model also allows for the posture
 information request to be pre-provisioned on the NEA Client to
 improve performance by allowing the NEA Client to report posture
 without receiving a request for particular attributes from the NEA
 Server.
 The Posture Broker Client also multiplexes the responses from the
 Posture Collector(s) and returns them to the NEA Server.  The Posture
 Broker Client constructs one or more PB messages using the PA
 message(s) it obtains from the Posture Collector(s) involved in the
 assessment.  The quantity and ordering of Posture Collector responses
 (PA message(s)) multiplexed into the PB response message(s) can be
 determined by the Posture Broker Client based on many factors
 including policy or characteristics of the underlying network
 transport (e.g., MTU).  A particular NEA Client will have one Posture
 Broker Client.
 The Posture Broker Client also handles the global assessment decision
 from the Posture Broker Server and may interact with the user to
 communicate the global assessment decision and aid in any necessary
 remediation steps.
 The NEA model allocates the following responsibilities to the Posture
 Broker Client:
    o Maintaining a registry of known Posture Collectors and allowing
      for Posture Collectors to dynamically register and deregister.
    o Multiplexing and de-multiplexing attribute messages between the
      NEA Server and the relevant Posture Collectors.
    o Handling posture change notifications from Posture Collectors
      and triggering reassessment.
    o Providing user notification about the global assessment decision
      and other user messages sent by the NEA Server.

Sangster, et al. Informational [Page 14] RFC 5209 NEA Requirements June 2008

5.1.1.3. Posture Transport Client

 The Posture Transport Client is responsible for establishing a
 reliable communication channel with the NEA Server for the message
 dialog between the NEA Client and NEA Server.  There might be more
 than one Posture Transport Client on a particular NEA Client
 supporting different transport protocols (e.g., 802.1X, VPN).
 Certain Posture Transport Clients may be configured with the address
 of the appropriate Posture Transport Server to use for a particular
 network.
 The NEA model allocates the following responsibilities to the Posture
 Transport Client:
    o  Initiating and maintaining the communication channel to the NEA
       Server.  The Posture Transport Client hides the details of the
       underlying carrier that could be a Layer 2 or Layer 3 protocol.
    o  Providing cryptographic protection for the message dialog
       between the NEA Client and NEA Server.

5.1.2. NEA Server

 The NEA Server is typically comprised of the following NEA
 functionality:
    o Posture Validator(s)
    o Posture Broker Server
    o Posture Transport Server(s)
 The Posture Validators might be located on a separate server from the
 Posture Broker Server, requiring the Posture Broker Server to deal
 with both local and remote Posture Validators.

5.1.2.1. Posture Validator

 A Posture Validator is responsible for handling Posture Attributes
 from corresponding Posture Collector(s).  A Posture Validator can
 handle Posture Attributes from one or more Posture Collectors and
 vice-versa.  The Posture Validator performs the posture assessment
 for one or more features of the endpoint (e.g., anti-virus software)
 and creates the result and, if necessary, the remediation
 instructions, or it may choose to request additional attributes from
 one or more Collectors.

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 Each Posture Validator will be associated with one or more
 identifiers that enable it to be specified as the destination in a PA
 message.  The Posture Broker Server uses this identifier to route
 messages to this Validator.  This identifier might be dynamic (e.g.,
 generated by the Posture Broker Server at run-time during
 registration) or more static (e.g., pre-assigned to a Posture
 Validator at install-time and passed to the Posture Broker Server
 during registration) or a function of the attribute messages the
 Validator desires to receive (e.g., message type for subscription).
 Posture Validators can be co-located on the NEA Server or can be
 hosted on separate servers.  A particular NEA Server is likely to
 need to handle multiple Posture Validators.
 The NEA model allocates the following responsibilities to the Posture
 Validator:
    o Requesting attributes from a Posture Collector.  The request may
      include:
  1. Request Attributes that indicate to the Posture Collector to

fetch and provide Posture Attributes for particular

         functionality on the endpoint.
    o Receiving attributes from the Posture Collector.  The response
      from the Posture Collector may include:
  1. Posture Attributes collected for the requested functionality.
  1. Assertion Attributes that indicate the compliance result from

a prior assessment.

    o Assessing the posture of endpoint features based on the
      attributes received from the Collector.
    o Communicating the posture assessment result to the Posture
      Broker Server.
    o Communicating the posture assessment results to the Posture
      Collector; this attribute message may include:
  1. Result Attributes that communicate the posture assessment

result.

  1. Remediation Attributes that communicate the remediation

instructions to the Posture Collector.

    o Monitoring out-of-band updates that trigger reassessment and
      require notifications to be sent to the Posture Broker Server.

Sangster, et al. Informational [Page 16] RFC 5209 NEA Requirements June 2008

    o Providing cryptographic protection for attributes sent to the
      Posture Collector and offering cryptographic verification of the
      attributes received from the Posture Collector.
 The above list describes the model's view of the possible
 responsibilities of the Posture Validator.  Note that this is not a
 set of requirements for what each Posture Validator implementation
 must support, nor is it an exhaustive list of all the things a
 Posture Validator may do.

5.1.2.2. Posture Broker Server

 The Posture Broker Server acts as a multiplexer and a de-multiplexer
 for attribute messages.  The Posture Broker Server parses the PB
 messages received from the NEA Client and de-multiplexes them into PA
 messages that it passes to the associated Posture Validators.  The
 Posture Broker Server multiplexes the PA messages (e.g., messages
 containing (a) Request Attribute(s) from the relevant Posture
 Validator(s)) into one or more PB messages and sends them to the NEA
 Client via the Posture Transport protocol.  The quantity and ordering
 of Posture Validator responses (PA messages) and global assessment
 decision multiplexed into the PB response message(s) can be
 determined by the Posture Broker Server based on many factors
 including policy or characteristics of the underlying network
 transport (e.g., MTU).
 The Posture Broker Server is also responsible for computing the
 global assessment decision based on individual posture assessment
 results from the various Posture Validators.  This global assessment
 decision is sent back to the NEA Client in Result Attributes within a
 PB message.  A particular NEA Server will have one Posture Broker
 Server, and this Posture Broker Server will handle all the local and
 remote Posture Validators.
 The NEA model allocates the following responsibilities to the Posture
 Broker Server:
    o Maintaining a registry of Posture Validators and allowing for
      Posture Validators to register and deregister.
    o Multiplexing and de-multiplexing posture messages from and to
      the relevant Posture Validators.
    o Computing the global assessment decision based on posture
      assessment results from the various Posture Validators and
      compliance policy.  This assessment decision is sent to the
      Posture Broker Client in a PB message.

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5.1.2.3. Posture Transport Server

 The Posture Transport Server is responsible for establishing a
 reliable communication channel with the NEA Client for the message
 dialog between the NEA Client and NEA Server.  There might be more
 than one Posture Transport Server on a particular NEA Server to
 support different transport protocols.  A particular Posture
 Transport Server will typically handle requests from several Posture
 Transport Clients and may require local configuration describing how
 to reach the NEA Clients.
 The NEA model allocates the following responsibilities to the Posture
 Transport Server:
    o Initiating and maintaining a communication channel with,
      potentially, several NEA Clients.
    o Providing cryptographic protection for the message dialog
      between the NEA Client and NEA Server.

5.2. Protocols

 The NEA reference model includes three layered protocols (PA, PB, and
 PT) that allow for the exchange of attributes across the network.
 While these protocols are intended to be used together to fulfill a
 particular role in the model, they may offer overlapping
 functionality.  For example, each protocol should be capable of
 protecting its information from attack (see section 8.2 for more
 information).

5.2.1. Posture Attribute Protocol (PA)

 PA is a protocol that carries one or more attributes between Posture
 Collectors and their associated Posture Validator.  The PA protocol
 is a message-oriented lightweight wrapper around a set of attributes
 being exchanged.  This wrapper may indicate the purpose of attributes
 within the message.  Some of the types of messages expected include:
 requests for posture information (Request Attributes), posture
 information about the endpoint (Posture Attributes), results of an
 assessment (Result Attributes), reusable compliance assertions
 (Assertion Attributes), and instructions to remediate non-compliant
 portions of the endpoint (Remediation Attributes).  The PA protocol
 also provides the requisite encoding and cryptographic protection for
 the Posture Attributes.

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5.2.2. Posture Broker Protocol (PB)

 PB is a protocol that carries aggregate attribute messages between
 the Posture Collectors on the NEA Client and the corresponding
 Posture Validators on the NEA Server involved in a particular
 assessment.  The PB protocol provides a session allowing for message
 dialogs for every assessment.  This PB session is then used to bind
 multiple Posture Attribute requests and responses from the different
 Posture Collectors and Posture Validators involved in a particular
 assessment.  The PB protocol may also carry the global assessment
 decision in the Result Attribute from the Posture Broker Server to
 the Posture Broker Client.  PB may be used to carry additional types
 of messages for use by the Posture Broker Client and Server (e.g.,
 information about user preferred interface settings such as
 language).

5.2.3. Posture Transport Protocol (PT)

 PT is a transport protocol between the NEA Client and the NEA Server
 responsible for carrying the messages generated by the PB protocol.
 The PT protocol(s) transport(s) PB messages during the network
 connection request or after network connectivity has been
 established.
 In scenarios where an initial assessment needs to occur during the
 network connection, the PT protocol (e.g., EAP within 802.1X) may
 have constrained use of the network, so deployments may choose to
 limit the amount and/or size of the attributes exchanged.  The NEA
 Client and NEA Server should be able to detect when a potentially
 constrained situation exists prior to the assessment based upon
 properties of the underlying network protocol.  Using this
 information, NEA policy could dictate what aspects of the endpoint to
 include in the initial assessment and potentially limit the PA
 message attributes exchanged.  This could be followed up by a full
 reassessment after the endpoint is placed on the network.
 Alternatively, deployments can choose not to limit their assessment
 by configuring their network access technology to temporarily grant
 restricted IP connectivity prior to the assessment and use an
 unconstrained, high bandwidth IP-based transport during the
 assessment.  Some of the constraints that may exist for protocols
 involved in the network connection phase include:
    o Limited maximum transmission unit (MTU) size and ability to
      negotiate larger MTUs,
    o Inability to perform multiple roundtrips,
    o Lack of support for piggybacking attributes for other protocols,

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    o Low bandwidth or high latency limitations precluding exchanges
      of large amounts of data,
    o Inability of servers to initiate messages except during the
      network connection phase.
 The PT protocol selection process needs to consider the impact of
 selecting a particular PT and set of underlying protocols on the
 deployment needs of PA and PB.  PA and PB will be selected prior to
 PT so the needs of PA and PB will be known.  Certain underlying
 protocol stacks may be too constrained to support adequate NEA
 assessments during network connection.
 The PT protocol provides reliable message delivery, mutual
 authentication, and cryptographic protection for the PB messages as
 specified by local policy.

5.3. Attributes

 The PA protocol is responsible for the exchange of attributes between
 a Posture Collector and Posture Validator.  The PB protocol may also
 carry the global assessment decision attributes from the Posture
 Broker Server.  Attributes are effectively the reserved word 'nouns'
 of the posture assessment.  The NEA Server is only able to ask for
 information that has a corresponding attribute, thus bounding what
 type of posture can be obtained.  The NEA WG will define a common
 (standard) set of attributes that are expected to be widely
 applicable to Posture Collectors and thus used for maximum
 interoperability, but Posture Collectors may support additional
 vendor-specific attributes when necessary.
 Depending on the deployment scenario, the purpose of the attributes
 exchanged may be different (e.g., posture information vs. asserted
 compliance).  This section discusses the originator and expected
 situation resulting in the use of each classification of attributes
 in a PA message.  These classifications are not intended to dictate
 how the NEA WG will specify the attributes when defining the
 attribute namespace or schema.

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5.3.1. Attributes Normally Sent by NEA Client:

    o Posture Attributes - Attributes and values sent to report
      information about a particular aspect (based on semantic of the
      attribute) of the system.  These attributes are typically sent
      in response to Request Attributes from the NEA Server.  For
      example, a set of Posture Attributes might describe the status
      of the host-based firewall (e.g., if running, vendor, version).
      The NEA Server would base its decision on comparing this type of
      attribute against policy.
    o Assertion Attributes - Attributes stating recent prior
      compliance to policy in hopes of avoiding the need to recollect
      the posture and send it to the NEA Server.  Examples of
      assertions include (a) NEA Server provided attributes (state)
      describing a prior evaluation (e.g., opaque to endpoint, signed,
      time stamped items stating specific results) or (b) NEA Client
      identity information used by the NEA Server to locate state
      about prior decisions (e.g., system-bound cookie).  These might
      be returned in lieu of, or in addition to, Posture Attributes.

5.3.2. Attributes Normally Sent by NEA Server:

    o Request Attributes - Attributes that define the specific posture
      information desired by the NEA Server.  These attributes might
      effectively form a template that the Posture Collector fills in
      (subject to local policy restrictions) with the specific value
      corresponding to each attribute.  The resulting attributes are
      typically Posture or Assertion Attributes from the NEA Client.
    o Result Attributes - Attributes that contain the decisions of the
      Posture Validators and/or Posture Broker Server.  The level of
      detail provided may vary from which individual attributes were
      compliant or not through just the global assessment decision.
    o Remediation Attributes - Attributes that explain to the NEA
      Client and its user how to update the endpoint to become
      compliant with the NEA Server policies.  These attributes are
      sent when the global assessment decision was that the endpoint
      is not currently compliant.  Remediation and Result Attributes
      may both exist within a NEA Server attribute message.
    o Assertion Attributes - Attributes containing NEA Server
      assertions of compliance to a policy for future use by the NEA
      Client.  See section 5.3.1 for more information.

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6. Use Cases

 This section discusses several of the NEA use cases with intent to
 describe and collectively bound the NEA problem space under
 consideration.  The use cases provide a context and general rationale
 for the defined requirements.  In order to ease understanding of each
 use case and how it maps to the reference model, each use case will
 be accompanied by a simple example and a discussion of how this
 example relates to the NEA protocols.  It should be emphasized that
 the provided examples are not intended to indicate the only approach
 to addressing the use case but rather are included to ease
 understanding of how the flows might occur and impact the NEA
 protocols.
 We broadly classify the use cases into two categories, each with its
 own set of trigger events:
    o Initial assessment - evaluation of the posture of an endpoint
      that has not recently been assessed and thus is not in
      possession of any valid proof that it should be considered
      compliant.  This evaluation might be triggered by a request to
      join a network, a request to use a service, or a desire to
      understand the posture of a system.
    o Reassessment - evaluation of the posture of an endpoint that has
      previously been assessed.  This evaluation could occur for a
      variety of reasons including the NEA Client or Server
      recognizing an occurrence affecting the endpoint that might
      raise the endpoint's risk level.  This could be as simple as it
      having been a long time since the endpoint's prior reassessment.

6.1. Initial Assessment

 An initial assessment occurs when a NEA Client or Server event occurs
 that causes the evaluation of the posture of the endpoint for the
 first time.  Endpoints do not qualify for this category of use case
 if they have been recently assessed and the NEA Client or Server has
 maintained state (or proof) that the endpoint is compliant and
 therefore does not need to have its posture evaluated again.

6.1.1. Triggered by Network Connection or Service Request

 This use case focuses on assessments performed at the time an
 endpoint attempts to join a network or request use of a service that
 requires a posture evaluation.  This use case is particularly
 interesting because it allows the NEA Server to evaluate the posture
 of an endpoint before allowing it access to the network or service.

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 This approach could be used to help detect endpoints with known
 vulnerabilities and facilitate their repair before they are admitted
 to the network and potentially exposed to threats on the network.
 A variety of types of endpoint actions could result in this class of
 assessment.  For example, an assessment could be triggered by the
 endpoint trying to access a highly protected network service (e.g.,
 financial or HR application server) where heightened security
 checking is required.  A better known example could include
 requesting entrance to a network that requires systems to meet
 compliance policy.  This example is discussed in more detail in the
 following section.

6.1.1.1. Example

 An IT employee returning from vacation boots his office desktop
 computer that generates a request to join the wired enterprise
 network.  The network's security policy requires the system to
 provide posture information in order to determine whether the
 desktop's security features are enabled and up to date.  The desktop
 sends its patch, firewall, and anti-virus posture information.  The
 NEA Server determines that the system is lacking a recent security
 patch designed to fix a serious vulnerability and the system is
 placed on a restricted access network.  The desktop follows the
 provided remediation instructions to download and install the
 necessary patch.  Later, the desktop requests again to join the
 network and this time is provided full access to the enterprise
 network after a full assessment.

6.1.1.2. Possible Flows and Protocol Usage

 The following describes typical message flows through the NEA
 reference model for this example use case:
    1. The IT employee's desktop computer connects to the network
       through an access gateway in the wired enterprise network.
    2. The Posture Broker Server on the NEA Server is instructed to
       assess the endpoint joining the wired network.
    3. Based upon compliance policy, the Posture Broker Server
       contacts the operating system patch, host-based firewall, and
       anti-virus Posture Validators to request the necessary posture.
       Each Posture Validator creates a PA message containing the
       desired attributes to be requested for assessment from the
       desktop system.

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    4. The Posture Broker Server aggregates the PA messages from the
       Posture Validators into a PB message.  The Posture Broker
       Server passes the PB message to the Posture Transport Server
       that uses the PT protocol to send the PB message to the NEA
       Client on the desktop computer.
    5. The Posture Transport Client receives the message from the NEA
       Server and passes it to the Posture Broker Client for message
       delivery.
    6. The Posture Broker Client de-multiplexes the PB message and
       delivers the PA messages with the requests for attributes to
       the firewall, operating system patch, and anti-virus Posture
       Collectors.
    7. Each Posture Collector involved consults local privacy policy
       to determine what information is allowed to be disclosed and
       then returns the requested attributes that are authorized in a
       PA message to the Posture Broker Client.
    8. The Posture Broker Client aggregates these PA messages into a
       single PB message and sends it to the Posture Broker Server
       using the Posture Transport Client to Server session.
    9. The Posture Transport Server provides the PB message to the
       Posture Broker Server that de-multiplexes the message and sends
       the appropriate attributes to the corresponding Posture
       Validator.
   10. Each Posture Validator compares the values of the attributes it
       receives with the expected values defined in its policy.
   11. The anti-virus and firewall Posture Validators return
       attributes to the Posture Broker Server stating the desktop
       computer is compliant, but the operating system patch Posture
       Validator returns non-compliant.  The operating system patch
       Posture Validator creates a PA message that contains attributes
       with remediation instructions in addition to the attribute
       indicating non-compliance result.
   12. The Posture Broker Server aggregates the PA messages and sends
       them in a PB message to the Posture Broker Client via the
       Posture Transport Server and Posture Transport Client.

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   13. The Posture Broker Client delivers the PA messages with the
       results from the various Posture Validators to the Posture
       Collectors including the PA message containing attributes with
       remediation instructions to the operating system patch Posture
       Collector.  This Posture Collector then interacts with the user
       to download and install the needed patches, potentially while
       the endpoint remains quarantined.
   14. Upon completion of the remediation, the above steps 1-10 are
       repeated (triggered by the NEA Client repeating its request to
       join the network).
   15. This time each involved Posture Validator (including the
       operating system patch Posture Validator) returns a compliant
       status and the Posture Broker Server returns a compliant result
       indicating a global success.
   16. The Posture Broker Client receives the compliant result and the
       IT employee's desktop is now on the network.

6.1.1.3. Impact on Requirements

 The following are several different aspects of the use case example
 that potentially need to be factored into the requirements.
    o Posture assessment before endpoint allowed on network
    o Endpoint sends attributes containing posture information
    o NEA Server sends remediation instructions
    o NEA Client causes a reassessment after remediation

6.1.2. Triggered by Endpoint

 This use case highlights that an endpoint (possibly at the request of
 a user) may wish to trigger an assessment of its posture to determine
 whether its security protective mechanisms are running and up to
 date.

6.1.2.1. Example

 A student goes to the terminal room to work on a project.  The
 terminal room contains shared systems owned by the school that are on
 the network.  These systems have been previously used by other
 students so their security posture is unknown.  The student wishes to
 check whether a system is currently in compliance with the school's
 security policies prior to doing work, so she requests a posture

Sangster, et al. Informational [Page 25] RFC 5209 NEA Requirements June 2008

 assessment.  The NEA Server performs an initial assessment of the
 system and determines it is compliant but the anti-virus protection
 is not in use.  The student receives an advisory response indicating
 the system's anti-virus software is turned off but that otherwise it
 complies with the school's policy.  The student turns on the
 anti-virus software, initiates a scan, and upon completion decides to
 trust the system with her work.

6.1.2.2. Possible Flows and Protocol Usage

 The following describes the message flows through the NEA reference
 model for the student using a terminal room shared system example:
    1. Student triggers the Posture Broker Client on the computer
       system in the terminal room to initiate a posture assessment.
    2. The Posture Broker Client establishes a session with the
       Posture Broker Server that causes an assessment to be
       triggered.
    3. The Posture Broker Server detects the new session and consults
       policy to determine that Posture Validators to involve in the
       assessment.  The Posture Broker Server decides to employ
       several Posture Validators including the anti-virus Posture
       Validator.
    4. The Posture Validators involved create PA messages containing
       requests for particular attributes containing information about
       the desired terminal room computer based on the school's
       security policy.
    5. The Posture Broker Server assembles a PB message including each
       of the PA messages from the Posture Validators.
    6. The Posture Transport Server sends the PB message to the
       Posture Transport Client where it is passed on to the Posture
       Broker Client.
    7. The Posture Broker Client on the student's computer
       de-multiplexes the PA messages and delivers them to the
       corresponding Posture Collectors.
    8. The Posture Collectors consult privacy policy to decide what
       information to share with the Server.  If allowable, the
       Collectors each return a PA message containing the requested
       posture to the Posture Broker Client.

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    9. The Posture Broker Client aggregates the returned PA messages
       into a PB message and hands it to the Posture Transport Client
       for transmission to the Posture Transport Server.
   10. The Posture Broker Server separates and distributes the Posture
       Collector PA messages to the associated Posture Validators.
   11. The Posture Validators determine whether the attributes
       containing the posture included in the PA message are compliant
       with their policies and returns a posture assessment decision
       to the Posture Broker Server.  In this case, the anti-virus
       Posture Validator returns a PA message indicating a
       non-compliant result because the anti-virus software is not
       running and includes attributes describing how to activate the
       software.
   12. The Posture Broker Server determines the overall compliance
       decision based on all of the Validators' assessment results and
       sends a PB message containing an attribute expressing the
       global assessment decision and the anti-virus Validator's PA
       message.  In this case, the global assessment decision
       indicates the system is compliant (despite the anti-virus
       Validator's result) because the Posture Broker Server policy
       allowed for the anti-virus to not be running as long as the
       system was properly patched and running a firewall (which was
       the case according to the other Posture Validators).
   13. The Posture Transport Server sends the PB message to the
       Posture Transport Client that provides the message to the
       Posture Broker Client.
   14. The Posture Broker Client on the terminal room computer
       examines the PB message's global assessment decision attribute
       and reports to the student that the system was deemed to be
       compliant, but that an advisory was included.
   15. The Posture Broker Client provides the PA message with the
       remediation attributes to the anti-virus Posture Collector that
       interacts with the user to explain how to turn on anti-virus to
       improve the local protections.
   16. The student turns on the anti-virus software and on completion
       steps 1-10 are repeated.
   17. This time the anti-virus Posture Validator returns a success
       status and the Posture Broker Server returns a successful
       global assessment decision in the PB message.

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   18. The Posture Broker Client receives the successful global
       assessment decision in the PB message and the student now uses
       the computer for her assignment.

6.1.2.3. Impact on Requirements

 The following are several different aspects of the use case example
 that potentially need to be factored into the requirements.
    o Voluntary endpoint requested initial assessment,
    o Successful (compliant) global assessment decision included in PB
      message with a PA message containing an advisory set of
      attributes for remediation.

6.2. Posture Reassessment

 Reassessment(s) of endpoints can happen anytime after being admitted
 to the network after a successful initial NEA assessment.  These
 reassessments may be event-based, such as driven by posture changes
 detected by the NEA Client, or changes detected by network
 infrastructure such as detection of suspicious behavior or network
 policy updates on the NEA Server.  They may also be periodic (timer-
 driven) to reassess the health of the endpoint.

6.2.1. Triggered by NEA Client

 This use case allows for software on the endpoint or a user to
 determine that a reassessment of the system is required.  There are a
 variety of reasons why such a reassessment might be beneficial
 including: changes in its previously reported posture, detection of
 potentially suspicious behavior, or even to enable the system to
 periodically poll the NEA Server to assess its condition relative to
 the latest policies.

6.2.1.1. Example

 The desktops within a company's HR department have a history of poor
 security practices and eventual compromise.  The HR department
 administrator decides to deploy software on each desktop to monitor
 the use of security protective mechanisms to assure their use.  One
 day, an HR person accidentally turns off the desktop firewall.  The
 monitoring process detects the lack of a firewall and contacts the
 NEA Server to request a reassessment of the firewall compliance.  The
 NEA Server returns a decision that the firewall must be reactivated
 to stay on the network.  The NEA Client explains the decision to the
 user and how to reactivate the firewall.  The HR person restarts the
 firewall and initiates a request to rejoin the network.

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6.2.1.2. Possible Flows & Protocol Usage

 The following describes the message flows through the NEA reference
 model for the HR department example:
    1. The desktop monitoring software that typically might act as a
       Posture Collector triggers the Posture Broker Client to
       initiate a posture reassessment.  The Posture Broker Client
       creates a PB message that contains a PA message indicating the
       desktop firewall has been disabled.
    2. The Posture Broker Client sends the PB message to the Posture
       Broker Server.
    3. The Posture Transport Client sends the PB message to the
       Posture Transport Server over the PT protocol.
    4. The Posture Broker Server receives the PB message and forwards
       the PA message to the firewall Posture Validator for
       evaluation.
    5. The firewall Posture Validator determines that the endpoint is
       no longer compliant because its firewall has been disabled.
    6. The Posture Validator generates a PA message that contains
       attributes indicating a non-compliant posture assessment result
       and remediation instructions for how to reactivate the
       firewall.
    7. The Posture Validator communicates the PA message with the
       posture assessment result to the Posture Broker Server to
       respond back to the NEA Client.
    8. The Posture Broker Server generates a PB message including a
       global assessment decision of non-compliant and the PA message
       from the firewall Posture Validator.
    9. The Posture Transport Server transports the PB message to the
       Posture Transport Client where it is passed to the Posture
       Broker Client.
   10. The Posture Broker Client processes the attribute containing
       the global assessment decision received from the NEA Server and
       displays the non-compliance messages to the user.

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   11. The Posture Broker Client forwards the PA message to the
       firewall Posture Collector; the Posture Collector displays the
       remediation instructions for how to enable the desktop
       firewall.
   12. The user is prompted to initiate a reassessment after
       completing the remediation.
   13. Upon completion of the remediation, the NEA Client reinitiates
       a request for reassessment and steps 1-4 are repeated.  This
       time the firewall Posture Validator determines the endpoint is
       compliant and returns a successful posture assessment decision.
   14. The Posture Broker Server generates a PB message with a global
       assessment decision of compliant and returns this to the NEA
       Client.

6.2.1.3. Impact on Requirements

 The following are several different aspects of the use case example
 that potentially need to be factored into the requirements.
    o Voluntary, endpoint (software) initiated posture reassessment
      request
    o NEA Server requests specific firewall-oriented Posture
      Attributes
    o NEA Client (firewall Posture Collector) interacts with user to
      remediate problem

6.2.2. Triggered by NEA Server

 In many cases, especially for reassessment, the NEA Server may
 initiate specific or complete reassessment of one or more endpoints
 triggered by:
    o Time (periodic)
    o Event occurrence
    o Policy updates

6.2.2.1. Example

 An enterprise requires employees on the network to always stay up to
 date with security critical operating system patches.  A marketing
 employee joins the network and performs an initial assessment.  The
 assessment determines the employee's laptop is compliant.  Several

Sangster, et al. Informational [Page 30] RFC 5209 NEA Requirements June 2008

 hours later, a major operating system vendor releases a set of
 patches preventing a serious vulnerability that is being exploited on
 the Internet.
 The enterprise administrators make available the patches and change
 the network policy to require them to be installed by 5 PM.  This
 policy change causes the NEA Server to request a reassessment to
 determine which endpoints are impacted and lacking the patches.  The
 marketing employee's laptop is reassessed and determined to need the
 patches.  A remediation advisory is sent and presented to the
 employee explaining how to obtain the patches and that they must be
 installed by 5 PM.  The marketing employee immediately downloads and
 installs the patches and obtains an assertion that all patches are
 now installed.
 At 5 PM, the enterprise performs another reassessment of all impacted
 endpoints to determine if they are now in compliance.  The marketing
 employee's laptop is reassessed and presents the assertion that it
 has the patches installed and thus is determined to be compliant.

6.2.2.2. Possible Flows and Protocol Usage

 The following describes the message flows through the NEA reference
 model for the above example:
    1. Marketing employee joins network and completes an initial
       assessment resulting in a compliant decision.
    2. The Enterprise Administrator configures an operating system
       patch policy indicating that recent patches are required on all
       endpoints by 5 PM to prevent serious vulnerabilities.
    3. The NEA Server's operating system patch Posture Validator
       becomes aware of this policy change and creates a PA message
       requesting attributes describing OS patches in use and triggers
       the Posture Broker Server to initiate a posture reassessment of
       all endpoints connected to the network.
    4. The Posture Broker creates a PB message that includes the PA
       message from the operating system patch Posture Validator.
    5. The Posture Broker Server gradually establishes a session with
       each available NEA Client.
    6. The Posture Broker Server sends the PB message to the Posture
       Broker Client.

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    7. The Posture Transport Server carries the PB message to the
       Posture Transport Client over the PT protocol.
    8. The Posture Broker Client receives the PB message and forwards
       the PA message to the operating system patch Posture Collector.
    9. The operating system patch Posture Collector determines the OS
       patches present on the endpoint and if authorized by its
       disclosure policy creates a PA message containing the patch
       information attributes.
   10. The Posture Broker Client sends a PB message that includes the
       operating system patch PA message.
   11. The Posture Transport Client transports the PB message to the
       Posture Transport Server where it is passed to the Posture
       Broker Server.
   12. The Posture Broker Server receives the PB message and delivers
       the PA message to the operating system patch Posture Validator.
   13. The operating system patch Posture Validator extracts the
       attributes describing the current OS patches from the PA
       message and uses the values to determine whether the endpoint
       is compliant with the new policy.  The Posture Validator
       determines that the endpoint is not compliant since it does not
       have the new OS patches installed.
   14. The Posture Validator generates a PA message that includes
       attributes stating the posture assessment decision is
       non-compliant and attributes containing the remediation
       instructions to enable the endpoint to download the required OS
       patches.
   15. The Posture Validator communicates the posture assessment
       result to the Posture Broker Server along with its PA message.
   16. The Posture Broker Server generates a global assessment
       decision and sends a PB message with the decision and the
       operating system patch Posture Validator's PA message.
   17. The Posture Transport Server transports the PB message to the
       Posture Transport Client where it is passed to the Posture
       Broker Client.
   18. The Posture Broker Client processes the Result Attribute
       received from the NEA Server and displays the non-compliance
       decision to the user.

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   19. The Posture Broker Client forwards the PA message containing
       the remediation instructions to the operating system patch
       Posture Collector; the Posture Collector guides the user with
       instructions on how to become compliant that include
       downloading the appropriate OS patches to prevent the
       vulnerability.
   20. The marketing employee installs the required patches and now is
       in compliance.
   21. The NEA Client triggers a reassessment of the operating system
       patches that causes a repeat of many of the steps above.  This
       time, in step 13 the operating system patch Posture Validator
       determines the marketing employee's laptop is compliant.  It
       returns a reusable (e.g., signed and dated) set of attributes
       that assert OS patch compliance to the latest policy.  These OS
       patch compliance assertions can be used in a future PA message
       from the operating system patch Collector instead of
       determining and providing the specific patch set posture as
       before.
   22. This time when the operating system patch Posture Collector
       receives the PA message that contains reusable attributes
       asserting compliance, it caches those attributes for future
       use.
   23. Later at 5 PM, the NEA Server triggers a gradual reassessment
       to determine compliance to the patch advisory.  When the
       operating system patch Posture Collector receives the request
       for posture information (like in step 9 above) it returns the
       cached set of assertions (instead of specific OS patch
       information) to indicate that the patches have been installed
       instead of determining all the patches that have been installed
       on the system.
   24. When the operating system patch Posture Validator receives the
       PA message containing the assertions, it is able to determine
       that they are authentic and acceptable assertions instead of
       specific posture.  It returns a posture assessment decision of
       compliant thus allowing the laptop to remain on the network.

6.2.2.3. Impact on Requirements

 The following are several different aspects of the use case example
 that potentially need to be factored into the requirements.
    o Server-initiated reassessment required due to urgent patch
      availability

Sangster, et al. Informational [Page 33] RFC 5209 NEA Requirements June 2008

    o NEA Client submits reusable assertion attributes instead of
      posture that patch is installed
    o NEA Server capable of recognizing previously issued assertion
      attributes are sufficient instead of posture

7. Requirements

 This section describes the requirements that will be used by the NEA
 WG to assess and compare candidate protocols for PA, PB, and PT.
 These requirements frequently express features that a candidate
 protocol must be capable of offering so that a deployer can decide
 whether to make use of that feature.  This section does not state
 requirements about what features of each protocol must be used during
 a deployment.
 For example, a requirement (MUST, SHOULD, or MAY) might exist for
 cryptographic security protections to be available from each protocol
 but this does not require that a deployer make use of all or even any
 of them should they deem their environment to offer other protections
 that are sufficient.

7.1. Common Protocol Requirements

 The following are the common requirements that apply to the PA, PB,
 and PT protocols in the NEA reference model:
 C-1  NEA protocols MUST support multiple round trips between the NEA
      Client and NEA Server in a single assessment.
 C-2  NEA protocols SHOULD provide a way for both the NEA Client and
      the NEA Server to initiate a posture assessment or reassessment
      as needed.
 C-3  NEA protocols including security capabilities MUST be capable of
      protecting against active and passive attacks by intermediaries
      and endpoints including prevention from replay based attacks.
 C-4  The PA and PB protocols MUST be capable of operating over any PT
      protocol.  For example, the PB protocol must provide a transport
      independent interface allowing the PA protocol to operate
      without change across a variety of network protocol environments
      (e.g., EAP/802.1X, TLS, and Internet Key Exchange Protocol
      version 2 (IKEv2)).

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 C-5  The selection process for NEA protocols MUST evaluate and prefer
      the reuse of existing open standards that meet the requirements
      before defining new ones.  The goal of NEA is not to create
      additional alternative protocols where acceptable solutions
      already exist.
 C-6  NEA protocols MUST be highly scalable; the protocols MUST
      support many Posture Collectors on a large number of NEA Clients
      to be assessed by numerous Posture Validators residing on
      multiple NEA Servers.
 C-7  The protocols MUST support efficient transport of a large number
      of attribute messages between the NEA Client and the NEA Server.
 C-8  NEA protocols MUST operate efficiently over low bandwidth or
      high latency links.
 C-9  For any strings intended for display to a user, the protocols
      MUST support adapting these strings to the user's language
      preferences.
 C-10 NEA protocols MUST support encoding of strings in UTF-8 format
      [UTF8].
 C-11 Due to the potentially different transport characteristics
      provided by the underlying candidate PT protocols, the NEA
      Client and NEA Server MUST be capable of becoming aware of and
      adapting to the limitations of the available PT protocol.  For
      example, some PT protocol characteristics that might impact the
      operation of PA and PB include restrictions on: which end can
      initiate a NEA connection, maximum data size in a message or
      full assessment, upper bound on number of roundtrips, and
      ordering (duplex) of messages exchanged.  The selection process
      for the PT protocols MUST consider the limitations the candidate
      PT protocol would impose upon the PA and PB protocols.

7.2. Posture Attribute (PA) Protocol Requirements

 The Posture Attribute (PA) protocol defines the transport and data
 model to carry posture and validation information between a
 particular Posture Collector associated with the NEA Client and a
 Posture Validator associated with a NEA Server.  The PA protocol
 carries collections of standard attributes and vendor-specific
 attributes.  The PA protocol itself is carried inside the PB
 protocol.

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 The following requirements define the desired properties that form
 the basis for comparison and evaluation of candidate PA protocols.
 These requirements do not mandate the use of these properties, but
 merely that the candidate protocols are capable of offering the
 property if it should be needed.
 PA-1 The PA protocol MUST support communication of an extensible set
      of NEA standards defined attributes.  These attributes will be
      distinguishable from non-standard attributes.
 PA-2 The PA protocol MUST support communication of an extensible set
      of vendor-specific attributes.  These attributes will be
      segmented into uniquely identified vendor-specific namespaces.
 PA-3 The PA protocol MUST enable a Posture Validator to make one or
      more requests for attributes from a Posture Collector within a
      single assessment.  This enables the Posture Validator to
      reassess the posture of a particular endpoint feature or to
      request additional posture including from other parts of the
      endpoint.
 PA-4 The PA protocol MUST be capable of returning attributes from a
      Posture Validator to a Posture Collector.  For example, this
      might enable the Posture Collector to learn the specific reason
      for a failed assessment and to aid in remediation and
      notification of the system owner.
 PA-5 The PA protocol SHOULD provide authentication, integrity, and
      confidentiality protection for attributes communicated between a
      Posture Collector and Posture Validator.  This enables
      end-to-end security across a NEA deployment that might involve
      traversal of several systems or trust boundaries.
 PA-6 The PA protocol MUST be capable of carrying attributes that
      contain non-binary and binary data including encrypted content.

7.3. Posture Broker (PB) Protocol Requirements

 The PB protocol supports multiplexing of Posture Attribute messages
 (based on PA protocol) between the Posture Collectors on the NEA
 Client to and from the Posture Validators on the NEA Server (in
 either direction).
 The PB protocol carries the global assessment decision made by the
 Posture Broker Server, taking into account the results of the Posture
 Validators involved in the assessment, to the Posture Broker Client.

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 The PB protocol also aggregates and transports advisories and
 notifications such as remediation instructions (e.g., patch
 references) from one or more Posture Validators.
 The requirements for the PB protocol are:
 PB-1 The PB protocol MUST be capable of carrying attributes from the
      Posture Broker Server to the Posture Broker Client.  This
      enables the Posture Broker Client to learn the posture
      assessment decision and if appropriate to aid in remediation and
      notification of the endpoint owner.
 PB-2 The PB protocol MUST NOT interpret the contents of PA messages
      being carried, i.e., the data it is carrying must be opaque to
      it.
 PB-3 The PB protocol MUST carry unique identifiers that are used by
      the Posture Brokers to route (deliver) PA messages between
      Posture Collectors and Posture Validators.  Such message routing
      should facilitate dynamic registration or deregistration of
      Posture Collectors and Validators.  For example, a dynamically
      registered anti-virus Posture Validator should be able to
      subscribe to receive messages from its respective anti-virus
      Posture Collector on NEA Clients.
 PB-4 The PB protocol MUST be capable of supporting a half-duplex PT
      protocol.  However this does not preclude PB from operating
      full-duplex when running over a full-duplex PT.
 PB-5 The PB protocol MAY support authentication, integrity and
      confidentiality protection for the attribute messages it carries
      between a Posture Broker Client and Posture Broker Server.  This
      provides security protection for a message dialog of the
      groupings of attribute messages exchanged between the Posture
      Broker Client and Posture Broker Server.  Such protection is
      orthogonal to PA protections (which are end to end) and allows
      for simpler Posture Collector and Validators to be implemented,
      and for consolidation of cryptographic operations possibly
      improving scalability and manageability.
 PB-6 The PB protocol MUST support grouping of attribute messages
      optimize transport of messages and minimize round trips.

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7.4. Posture Transport (PT) Protocol Requirements

 The Posture Transport (PT) protocol carries PB protocol messages
 between the Posture Transport Client and the Posture Transport
 Server.  PT is responsible for providing a protected transport for
 the PB protocol.  The PT protocol may itself be transported by one or
 more concatenated sessions using lower layer protocols, such as
 802.1X, RADIUS [RADIUS], TLS, or IKE.
 This section defines the requirements that candidate PT protocols
 must be capable of supporting.
 PT-1 The PT protocol MUST NOT interpret the contents of PB messages
      being transported, i.e., the data it is carrying must be opaque
      to it.
 PT-2 The PT protocol MUST be capable of supporting mutual
      authentication, integrity, confidentiality, and replay
      protection of the PB messages between the Posture Transport
      Client and the Posture Transport Server.
 PT-3 The PT protocol MUST provide reliable delivery for the PB
      protocol.  This includes the ability to perform fragmentation
      and reassembly, detect duplicates, and reorder to provide
      in-sequence delivery, as required.
 PT-4 The PT protocol SHOULD be able to run over existing network
      access protocols such as 802.1X and IKEv2.
 PT-5 The PT protocol SHOULD be able to run between a NEA Client and
      NEA Server over TCP or UDP (similar to Lightweight Directory
      Access Protocol (LDAP)).

8. Security Considerations

 This document defines the functional requirements for the PA, PB, and
 PT protocols used for Network Endpoint Assessment.  As such, it does
 not define a specific protocol stack or set of technologies, so this
 section will highlight security issues that may apply to NEA in
 general or to particular aspects of the NEA reference model.
 Note that while a number of topics are outside the scope of the NEA
 WG and thus this specification (see section 3.1), it is important
 that those mechanisms are protected from attack.  For example, the
 methods of triggering an assessment or reassessment are out of scope
 but should be appropriately protected from attack (e.g., an attacker
 hiding the event indicating a NEA Server policy change has occurred).

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 NEA intends to facilitate detection and corrective actions for
 cooperating endpoints to become compliant with network compliance
 policies.  For example, it is envisioned that these policies will
 allow deployers to detect out-of-date, inactive, or absent security
 mechanisms on the endpoint that might leave it more vulnerable to
 known attacks.  If an endpoint is more vulnerable to compromise, then
 it is riskier to have this endpoint present on the network with other
 valuable assets.  By proactively assessing cooperating endpoints
 before their entrance to the network, deployers can improve their
 resilience to attack prior to network access.  Similarly,
 reassessments of cooperating endpoints on the network may be helpful
 in assuring that security mechanisms remain in use and are up to date
 with the latest policies.
 NEA fully recognizes that not all endpoints will be cooperating by
 providing their valid posture (or any posture at all).  This might
 occur if malware is influencing the NEA Client or policies, and thus
 a trustworthy assessment isn't possible.  Such a situation could
 result in the admission of an endpoint that introduces threats to the
 network and other endpoints despite passing the NEA compliance
 assessment.

8.1. Trust

 Network Endpoint Assessment involves assessing the posture of
 endpoints entering or already on the network against compliance
 policies to assure they are adequately protected.  Therefore, there
 must be an implied distrusting of endpoints until there is reason to
 believe (based on posture information) that they are protected from
 threats addressed by compliance policy and can be trusted to not
 propagate those threats to other endpoints.  On the network provider
 side, the NEA Client normally is expected to trust the network
 infrastructure systems to not misuse any disclosed posture
 information (see section 9) and any remediation instructions provided
 to the endpoint.  The NEA Client normally also needs to trust that
 the NEA Server will only request information required to determine
 whether the endpoint is safe to access the network assets.
 Between the NEA Client and Server there exists a network that is not
 assumed to be trustworthy.  Therefore, little about the network is
 implicitly trusted beyond its willingness and ability to transport
 the exchanged messages in a timely manner.  The amount of trust given
 to each component of the NEA reference model is deployment specific.
 The NEA WG intends to provide security mechanisms to reduce the
 amount of trust that must be assumed by a deployer.  The following
 sections will discuss each area in more detail.

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8.1.1. Endpoint

 For NEA to properly operate, the endpoint needs to be trusted to
 accurately represent the requested security posture of the endpoint
 to the NEA Server.  By NEA WG charter, the NEA reference model does
 not explicitly specify how to detect or prevent lying endpoints that
 intentionally misrepresent their posture.  Similarly, the detection
 of malware (e.g., root kits) that are able to trick the Posture
 Collectors into returning incorrect information is the subject for
 research and standardization outside the IETF (e.g., Trusted
 Computing Group [TCG]) and is not specifically addressed by the
 model.  However, if such mechanisms are used in a deployment, the NEA
 reference model should be able to accommodate these technologies by
 allowing them to communicate over PA to Posture Validators or work
 orthogonally to protect the NEA Client from attack and assure the
 ability of Posture Collectors to view the actual posture.
 Besides having to trust the integrity of the NEA Client and its
 ability to accurately collect and report Posture Attributes about the
 endpoint, we try to limit other assumed trust.  Most of the usage
 models for NEA expect the posture information to be sent to the NEA
 Server for evaluation and decision making.  When PA and/or PT level
 security protections are used, the endpoint needs to trust the
 integrity and potentially confidentiality of the trust anchor
 information (e.g., public key certificates) used by the Posture
 Collector and/or Posture Transport Client.  However, NEA
 implementations may choose to send or pre-provision some policies to
 the endpoint for evaluation that would assume more trust in the
 endpoint.  In this case, the NEA Server must trust the endpoint's
 policy storage, evaluation, and reporting mechanisms to not falsify
 the results of the posture evaluation.
 Generally the endpoint should not trust network communications (e.g.,
 inbound connection requests) unless this trust has been specifically
 authorized by the user or owner defined policy or action.  The NEA
 reference model assumes the entire NEA Client is local to the
 endpoint.  Unsolicited communications originating from the network
 should be inspected by normal host-based security protective
 mechanisms (e.g., firewalls, security protocols, Intrusion
 Detection/Prevention System (IDS/IPS), etc.).  Communications
 associated with a NEA assessment or reassessment requires some level
 of trust particularly when initiated by the NEA Server
 (reassessment).  The degree of trust can be limited by use of strong
 security protections on the messages as dictated by the network
 deployer and the endpoint user/owner policy.

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8.1.2. Network Communications

 Between the NEA Client and Server, there may exist a variety of types
 of devices to facilitate the communication path.  Some of the devices
 may serve as intermediaries (e.g., simple L2 switches) so they may
 have the opportunity to observe and change the message dialogs.
 The intermediary devices may fall into a few major categories that
 impact our degree of trust in their operation.  First, some
 intermediary devices may act as message forwarders or carriers for PT
 (e.g., L2 switches, L3 routers).  For these devices we trust them not
 to drop the messages or actively attempt to disrupt (e.g., denial of
 service (DoS)) the NEA deployment.
 Second, some intermediary devices may be part of the access control
 layer of the network and as such, we trust them to enforce policies
 including remediation, isolation, and access controls given to them
 as a result on a NEA assessment.  These devices may also fill other
 types of roles described in this section.
 Third, some devices may act as a termination point or proxy for the
 PT carrier protocol.  Frequently, it is expected that the carrier
 protocol for PT will terminate on the NEA Client and Server so will
 be co-resident with the PT endpoints.  If this expectation is not
 present in a deployment, we must trust the termination device to
 accurately proxy the PT messages without alteration into the next
 carrier protocol (e.g., if inner EAP method messages are transitioned
 from an EAP [EAP] tunnel to a RADIUS session).
 Fourth, many networks include infrastructure such as IDS/IPS devices
 that monitor and take corrective action when suspicious behavior is
 observed on the network.  These devices may have a relationship with
 the NEA Server that is not within scope for this specification.
 Devices trusted by the NEA Server to provide security information
 that might affect the NEA Server's decisions are trusted to operate
 properly and not cause the NEA Server to make incorrect decisions.
 Finally, other types of intermediary devices may exist on the network
 between the NEA Client and Server that are present to service other
 network functions beside NEA.  These devices might be capable of
 passively eavesdropping on the network, archiving information for
 future purposes (e.g., replay or privacy invasion), or more actively
 attacking the NEA protocols.  Because these devices do not play a
 role in facilitating NEA, it is essential that NEA deployers not be
 forced to trust them for NEA to reliably operate.  Therefore, it is
 required that NEA protocols offer security protections to assure
 these devices can't steal, alter, spoof or otherwise damage the
 reliability of the message dialogs.

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8.1.3. NEA Server

 The NEA Server (including potentially remote systems providing
 posture validation services) is generally trusted to apply the
 specified assessment policies and must be protected from compromise.
 It is essential that NEA Server deployments properly safeguard these
 systems from a variety of attacks from the network and endpoints to
 assure their proper operation.
 While there is a need to trust the NEA Server operation to some
 degree, rigorous security architecture, analysis, monitoring, and
 review should assure its network footprint and internal workings are
 protected from attack.  The network footprint would include
 communications over the network that might be subject to attack such
 as policy provisioning from the policy authoring systems and general
 security and system management protocols.  Some examples of internal
 workings include protections from malware attacking the intra-NEA
 Server communications, NEA Server internal logic, or policy stores
 (particularly those that would change the resulting decisions or
 enforcements).  The NEA Server needs to trust the underlying NEA and
 lower layer network protocols to properly behave and safeguard the
 exchanged messages with the endpoint.  The NEA reference model does
 not attempt to address integrity protection of the operating system
 or other software supporting the NEA Server.
 One interesting example is where some components of the NEA Server
 physically reside in different systems.  This might occur when a
 Posture Validator (or a remote backend server used by a local Posture
 Validator) exists on another system from the Posture Broker Server.
 Similarly, the Posture Broker Server might exist on a separate system
 from the Posture Transport Server.  When there is a physical
 separation, the communications between the remote components of the
 NEA Server must ensure that the PB session and PA message dialogs are
 resistant to active and passive attacks, in particular, guarded
 against eavesdropping, forgery and replay.  Similarly, the Posture
 Validators may also wish to minimize their trust in the Posture
 Broker Server beyond its ability to properly send and deliver PA
 messages.  The Posture Validators could employ end-to-end PA security
 to verify the authenticity and protect the integrity and/or
 confidentiality of the PA messages exchanged.
 When PA security is used, each Posture Validator must be able to
 trust the integrity and potentially confidentiality of its trust
 anchor policies.

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8.2. Protection Mechanisms at Multiple Layers

 Inherent in the requirements is a desire for NEA candidate protocols
 throughout the reference model to be capable of providing strong
 security mechanisms as dictated by the particular deployment.  In
 some cases, these mechanisms may appear to provide overlapping or
 redundant protections.  These apparent overlaps may be used in
 combination to offer a defense in depth approach to security.
 However, because of the layering of the protocols, each set of
 protections offers slightly different benefits and levels of
 granularity.
 For example, a deployer may wish to encrypt traffic at the PT layer
 to protect against some forms of traffic analysis or interception by
 an eavesdropper.  Additionally, the deployer may also selectively
 encrypt messages containing the posture of an endpoint to achieve
 end-to-end confidentiality to its corresponding Posture Validator.
 In particular, this might be desired when the Posture Validator is
 not co-located with the NEA Server so the information will traverse
 additional network segments after the PT protections have been
 enforced or so that the Posture Validator can authenticate the
 corresponding Posture Collector (or vice versa).
 Different use cases and environments for the NEA technologies will
 likely influence the selection of the strength and security
 mechanisms employed during an assessment.  The goal of the NEA
 requirements is to encourage the selection of technologies and
 protocols that are capable of providing the necessary protections for
 a wide variety of types of assessment.

8.3. Relevant Classes of Attack

 A variety of attacks are possible against the NEA protocols and
 assessment technologies.  This section does not include a full
 security analysis, but wishes to highlight a few attacks that
 influenced the requirement definition and should be considered by
 deployers selecting use of protective mechanisms within the NEA
 reference model.
 As discussed, there are a variety of protective mechanisms included
 in the requirements for candidate NEA protocols.  Different use cases
 and environments may cause deployers to decide not to use some of
 these mechanisms; however, this should be done with an understanding
 that the deployment may become vulnerable to some classes of attack.
 As always, a balance of risk vs. performance, usability,
 manageability, and other factors should be taken into account.

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 The following types of attacks are applicable to network protocols
 defined in the reference model and thus should be considered by
 deployers.

8.3.1. Man-in-the-Middle (MITM)

 MITM attacks against a network protocol exist when a third party can
 insert itself between two communicating entities without detection
 and gain benefit from involvement in their message dialog.  For
 example, a malware infested system might wish to join the network
 replaying posture observed from a clean endpoint entering the
 network.  This might occur by the system inserting itself into and
 actively proxying an assessment message dialog.  The impact of the
 damage caused by the MITM can be limited or prevented by selection of
 appropriate protocol protective mechanisms.
 For example, the requirement for PT to be capable of supporting
 mutual authentication prior to any endpoint assessment message
 dialogs prevents the attacker from inserting itself as an active
 participant (proxy) within the communications without detection
 (assuming the attacker lacks credentials convincing either party it
 is legitimate).  Reusable credentials should not be exposed on the
 network to assure the MITM doesn't have a way to impersonate either
 party.  The PT requirement for confidentiality-protected (encrypted)
 communications linked to the above authentication prevents a passive
 MITM from eavesdropping by observing the message dialog and keeping a
 record of the conformant posture values for future use.  The PT
 requirement for replay prevention stops a passive MITM from later
 establishing a new session (or hijacking an existing session) and
 replaying previously observed message dialogs.
 If a non-compliant, active MITM is able to trick a clean endpoint to
 give up its posture information, and the MITM has legitimate
 credentials, it might be able to appear to a NEA Server as having
 compliant posture when it does not.  For example, a non-compliant
 MITM could connect and authenticate to a NEA Server and as the NEA
 Server requests posture information, the MITM could request the same
 posture from the clean endpoint.  If the clean endpoint trusts the
 MITM to perform a reassessment and is willing to share the requested
 posture, the MITM could obtain the needed posture from the clean
 endpoint and send it to the NEA Server.  In order to address this
 form of MITM attack, the NEA protocols would need to offer a strong
 (cryptographic) binding between the posture information and the
 authenticated session to the NEA Server so the NEA Server knows the
 posture originated from the endpoint that authenticated.  Such a
 strong binding between the posture's origin and the authenticating
 endpoint may be feasible so should be preferred by the NEA WG.

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8.3.2. Message Modification

 Without message integrity protection, an attacker capable of
 intercepting a message might be capable of modifying its contents and
 causing an incorrect decision to be made.  For example, the attacker
 might change the Posture Attributes to always reflect incorrect
 values and thus prevent a compliant system from joining the network.
 Unless the NEA Server could detect this change, the attacker could
 prevent admission to large numbers of clean systems.  Conversely, the
 attacker could allow a malware infested machine to be admitted by
 changing the sent Posture Attributes to reflect compliant values,
 thus hiding the malware from the Posture Validator.  The attacker
 could also infect compliant endpoints by sending malicious
 remediation instructions that, when performed, would introduce
 malware on the endpoint or deactivate security mechanisms.
 In order to protect against such attacks, the PT includes a
 requirement for strong integrity protection (e.g., including a
 protected hash like a Hashed Message Authentication Code (HMAC)
 [HMAC] of the message) so any change to a message would be detected.
 PA includes a similar requirement to enable end-to-end integrity
 protection of the attributes, extending the protection all the way to
 the Posture Validator even if it is located on another system behind
 the NEA Server.
 It is important that integrity protection schemes leverage fresh
 secret information (not known by the attacker) that is bound to the
 authenticated session such as an HMAC using a derived fresh secret
 associated with the session.  Inclusion of freshness information
 allows the parties to protect against some forms of message replay
 attacks using secret information from prior sessions.

8.3.3. Message Replay or Attribute Theft

 An attacker might listen to the network, recording message dialogs or
 attributes from a compliant endpoint for later reuse to the same NEA
 Server or just to build an inventory of software running on other
 systems watching for known vulnerabilities.  The NEA Server needs to
 be capable of detecting the replay of posture and/or the model must
 assure that the eavesdropper cannot obtain the information in the
 first place.  For this reason, the PT protocol is required to provide
 confidentiality and replay prevention.
 The cryptographic protection from disclosure of the PT, PB, or PA
 messages prevents the passive listener from observing the exchanged
 messages and thus prevents theft of the information for future use.
 However, an active attacker might be able to replay the encrypted
 message if there is no strong link to the originating party or

Sangster, et al. Informational [Page 45] RFC 5209 NEA Requirements June 2008

 session.  By linking the encrypted message dialog to the
 authentication event and leveraging per-transaction freshness and
 keying exchanges, this prevents a replay of the encrypted
 transaction.

8.3.4. Other Types of Attack

 This section doesn't claim to present an exhaustive list of attacks
 against the NEA reference model.  Several types of attack will become
 easier to understand and analyze once the NEA WG has created
 specifications describing the specific selected technologies and
 protocols to be used within NEA.  One such area is Denial of Service
 (DoS).  At this point in time, it is not practical to try to define
 all of the potential exposures present within the NEA protocols, so
 such an analysis should be included in the Security Considerations
 sections of the selected NEA protocols.
 However, it is important that the NEA Server be resilient to DoS
 attacks as an outage might affect large numbers of endpoints wishing
 to join or remain on the network.  The NEA reference model expects
 that the PT protocol would have some amount of DoS resilience and
 that the PA and PB protocols would need to build upon that base with
 their own protections.  To help narrow the window of attack by
 unauthenticated parties, it is envisioned that NEA Servers would
 employ PT protocols that enable an early mutual authentication of the
 requesting endpoint as one technique for filtering out attacks.
 Attacks occurring after the authentication would at least come from
 sources possessing valid credentials and could potentially be held
 accountable.  Similarly, NEA protocols should offer strong replay
 protection to prevent DoS-based attacks based on replayed sessions
 and messages.  Posture assessment should be strongly linked with the
 Posture Transport authentications that occurred to assure the posture
 came from the authenticated party.  Cryptographic mechanisms and
 other potentially resource intensive operations should be used
 sparingly until the validity of the request can be established.  This
 and other resource/protocol based attacks can be evaluated once the
 NEA technologies and their cryptographic use have been selected.

9. Privacy Considerations

 While there are a number of beneficial uses of the NEA technology for
 organizations that own and operate networks offering services to
 similarly owned endpoints, these same technologies might enhance the
 potential for abuse and invasion of personal privacy if misused.
 This section will discuss a few of the potential privacy concerns
 raised by the deployment of this technology and offer some guidance
 to implementers.

Sangster, et al. Informational [Page 46] RFC 5209 NEA Requirements June 2008

 The NEA technology enables greater visibility into the configuration
 of an endpoint from the network.  Such transparency enables the
 network to take into consideration the strength of the endpoint's
 security mechanisms when making access control decisions to network
 resources.  However, this transparency could also be used to enforce
 restrictive policies to the detriment of the user by limiting their
 choice of software or prying into past or present uses of the
 endpoint.
 The scope of the NEA WG was limited to specifying protocols targeting
 the use cases where the endpoints and network are owned by the same
 party or the endpoint owner has established a clear expectation of
 disclosure/compliance with the network owner.  This is a familiar
 model for governments, institutions, and a wide variety of
 enterprises that provide endpoints to their employees to perform
 their jobs.  In many of these situations, the endpoint is purchased
 and owned by the enterprise and they often reserve the right to audit
 and possibly dictate the allowable uses of the device.  The NEA
 technologies allow them to automate the inspection of the contents of
 an endpoint and this information may be linked to the access control
 mechanisms on the network to limit endpoint use should the endpoint
 not meet minimal compliance levels.
 In these environments, the level of personal privacy the employee
 enjoys may be significantly reduced subject to local laws and
 customs.  However, in situations where the endpoint is owned by the
 user or where local laws protect the rights of the user even when
 using endpoints owned by another party, it is critical that the NEA
 implementation enable the user to control what endpoint information
 is shared with the network.  Such controls imposed by the user might
 prevent or limit their ability to access certain networks or
 protected resources, but this must be a user choice.

9.1. Implementer Considerations

 The NEA WG is not defining NEA Client policy content standards nor
 defining requirements on aspects of an implementation outside of the
 network protocols; however, the following guidance is provided to
 encourage privacy friendly implementations for broader use than just
 the enterprise-oriented setting described above.
 NEA Client implementations are encouraged to offer an opt-in policy
 to users prior to sharing their endpoint's posture information.  The
 opt-in mechanism should be on a per-user, per-NEA Server basis so
 each user can control which networks can access any posture
 information on their system.  For those networks that are allowed to
 assess the endpoint, the user should be able to specify granular
 restrictions on what particular types and specific attributes Posture

Sangster, et al. Informational [Page 47] RFC 5209 NEA Requirements June 2008

 Collectors are allowed to disclose.  Posture Validator
 implementations are discouraged from having the default behavior of
 using wild carded requests for posture potentially leading to
 overexposure of information (see section 9.2).  Instead Posture
 Validators, by default, should only request the specific attributes
 that are required to perform their assessment.
 Requests for attributes that are not explicitly allowed (or
 specifically disallowed) to be shared should result in a user
 notification and/or log record so the user can assess whether the
 service is doing something undesirable or whether the user is willing
 to share this additional information in order to gain access.  Some
 products might consider policy-driven support for prompting the user
 for authorization with a specific description of the posture
 information being requested prior to sending it to the NEA Server.
 It is envisioned that the owner of the endpoint is able to specify
 disclosure policies that may override or influence the user's
 policies on the attributes visible to the network.  If the owner
 disclosure policy allows for broader posture availability than the
 user policy, the implementation should provide a feedback mechanism
 to the user so they understand the situation and can choose whether
 to use the endpoint in those circumstances.
 In such a system, it is important that the user's policy authoring
 interface is easy to understand and clearly articulates the current
 disclosure policy of the system including any influences from the
 owner policy.  Users should be able to understand what posture is
 available to the network and the general impact of this information
 being known.  In order to minimize the list of restrictions
 enumerated, use of a conservative default disclosure policy such as
 "that which is not explicitly authorized for disclosure is not
 allowed" might make sense to avoid unintentional leakage of
 information.
 NEA Server implementations should provide newly subscribing endpoints
 with a disclosure statement that clearly states:
    o What information is required
    o How this information will be used and protected
    o What local privacy policies are applicable
 This information will empower subscribing users to decide whether the
 disclosure of this information is acceptable considering local laws
 and customs.

Sangster, et al. Informational [Page 48] RFC 5209 NEA Requirements June 2008

9.2. Minimizing Attribute Disclosure

 One important issue in the design of the NEA reference model and
 protocols is enabling endpoints to disclose minimal information
 required to establish compliance with network policies.  There are
 several models that could be considered as to how the disclosed
 attribute set is established.  Each model has privacy related
 benefits and issues that should be considered by product developers.
 This section summarizes three potential models for how attribute
 disclosure might be provided within NEA products and some privacy
 implications potentially associated with each model.
 The first model is easy to implement and deploy but has privacy and
 potentially latency and scalability implications.  This approach
 effectively defaults the local policy to send all known NEA Posture
 Attributes when an assessment occurs.  While this might simplify
 deployment, it exposes a lot of information that is potentially not
 relevant to the security assessment of the system and may introduce
 privacy issues.  For example, is it really important that the
 enterprise know whether Firefox is being used on a system instead of
 other browsers during the security posture assessment?
 The second model involves an out-of-band provisioning of the
 disclosure policy to all endpoints.  This model may involve the
 enterprise establishing policy that a particular list of attributes
 must be provided when a NEA exchange occurs.  Endpoint privacy policy
 may filter this attribute list, but such changes could cause the
 endpoint not to be given network or resource access.  This model
 simplifies the network exchange as the endpoint always sends the
 filtered list of attributes when challenged by a particular network.
 However, this approach requires an out-of-band management protocol to
 establish and manage the NEA disclosure policies of all systems.
 The third model avoids the need for pre-provisioning of a disclosure
 policy by allowing the NEA Server to specifically request what
 attributes are required.  This is somewhat analogous to the policy
 being provisioned during the NEA exchanges so is much easier to
 manage.  This model allows for the NEA Server to iteratively ask for
 attributes based on the values of prior attributes.  Note, even in
 this model the NEA protocols are not expected to be a general purpose
 query language, but rather allow the NEA Server to request specific
 attributes as only the defined attributes are possible to request.
 For example, an enterprise might ask about the OS version in the
 initial message dialog and after learning the system is running Linux
 ask for a different set of attributes specific to Linux than it would
 if the endpoint was a Windows system.  It is envisioned that this

Sangster, et al. Informational [Page 49] RFC 5209 NEA Requirements June 2008

 approach might minimize the set of attributes sent over the network
 if the assessment is of a complex system (such as trying to
 understand what patches are missing from an OS).
 In each model, the user could create a set of per-network privacy
 filter policies enforced by the NEA Client to prevent the disclosure
 of attributes felt to be personal in nature or not relevant to a
 particular network.  Such filters would protect the privacy of the
 user but might result in the user not being allowed access to the
 desired asset (or network) or being provided limited access.

10. References

10.1. Normative References

 [UTF8]   Yergeau, F., "UTF-8, a transformation format of ISO 10646",
          STD 63, RFC 3629, November 2003.

10.2. Informative References

 [802.1X] IEEE Standards for Local and Metropolitan Area Networks:
          Port based Network Access Control, IEEE Std 802.1X-2001,
          June 2001.
 [CNAC]   Cisco, Cisco's Network Admission Control Main Web Site,
          http://www.cisco.com/go/nac
 [EAP]    Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
          Levkowetz, Ed., "Extensible Authentication Protocol (EAP)",
          RFC 3748, June 2004.
 [HMAC]   Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
          Hashing for Message Authentication", RFC 2104, February
          1997.
 [IPSEC]  Kent, S. and K. Seo, "Security Architecture for the Internet
          Protocol", RFC 4301, December 2005.
 [NAP]    Microsoft, Network Access Protection Main Web Site,
          http://www.microsoft.com/nap
 [RADIUS] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote
          Authentication Dial In User Service (RADIUS)", RFC 2865,
          June 2000.

Sangster, et al. Informational [Page 50] RFC 5209 NEA Requirements June 2008

 [TLS]    Dierks, T. and E. Rescorla, "The Transport Layer Security
          (TLS) Protocol Version 1.1", RFC 4346, April 2006.
 [TCG]    Trusted Computing Group, Main TCG Web Site,
          http://www.trustedcomputinggroup.org/
 [TNC]    Trusted Computing Group, Trusted Network Connect Main Web
          Site, https://www.trustedcomputinggroup.org/groups/network/

11. Acknowledgments

 The authors of this document would like to acknowledge the NEA
 Working Group members who have contributed to previous requirements
 and problem statement documents that influenced the direction of this
 specification: Kevin Amorin, Parvez Anandam, Diana Arroyo, Uri
 Blumenthal, Alan DeKok, Lauren Giroux, Steve Hanna, Thomas Hardjono,
 Tim Polk, Ravi Sahita, Joe Salowey, Chris Salter, Mauricio Sanchez,
 Yaron Sheffer, Jeff Six, Susan Thompson, Gary Tomlinson, John
 Vollbrecht, Nancy Winget, Han Yin, and Hao Zhou.

Sangster, et al. Informational [Page 51] RFC 5209 NEA Requirements June 2008

Authors' Addresses

 Paul Sangster
 Symantec Corporation
 6825 Citrine Dr
 Carlsbad, CA 92009 USA
 Phone: +1 760 438-5656
 EMail: Paul_Sangster@symantec.com
 Hormuzd Khosravi
 Intel
 2111 NE 25th Avenue
 Hillsboro, OR 97124 USA
 Phone: +1 503 264 0334
 EMail: hormuzd.m.khosravi@intel.com
 Mahalingam Mani
 Avaya Inc.
 1033 McCarthy Blvd.
 Milpitas, CA 95035 USA
 Phone: +1 408 321-4840
 EMail: mmani@avaya.com
 Kaushik Narayan
 Cisco Systems Inc.
 10 West Tasman Drive
 San Jose, CA 95134
 Phone: +1 408 526-8168
 EMail: kaushik@cisco.com
 Joseph Tardo
 Nevis Networks
 295 N. Bernardo Ave., Suite 100
 Mountain View, CA 94043 USA
 EMail: joseph.tardo@nevisnetworks.com

Sangster, et al. Informational [Page 52] RFC 5209 NEA Requirements June 2008

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