NEA Working Group                                       P. Sangster  
 Internet Draft                                             Symantec  
 Expires: Jan 2008                                       H. Khosravi  
                                                               Intel  
                                                             M. Mani  
                                                               Avaya  
                                                          K. Narayan  
                                                       Cisco Systems  
                                                            J. Tardo  
                                                      Nevis Networks  
                                                                      
                                                         August 2007  
    
    
                  Network Endpoint Assessment (NEA):  
                      Overview and Requirements          
                  draft-ietf-nea-requirements-04.txt  
    
    
    
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 Abstract  
     

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   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   systems that are lacking or have out of date security protective  
   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.  
      
      
 Table of Contents  
  
   1. Introduction....................................................3  
      1.1 Conventions Used in This Document...........................4  
   2. Terminology.....................................................4  
   3. Applicability...................................................7  
      3.1 Scope.......................................................7  
      3.2 Applicability of Environments...............................7  
   4. Problem Statement...............................................9  
   5. Reference Model................................................10  
      5.1 NEA Client and Server......................................12  
          5.1.1 NEA Client...........................................12  
          5.1.2 NEA Server...........................................15  
      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.................................................19  
          5.3.1 Attributes Normally Sent by NEA Client:..............20  
          5.3.2 Attributes Normally Sent by NEA Server:..............20  
   6. Use Cases......................................................21  
      6.1 Initial Assessment.........................................21  
          6.1.1 Triggered by Network Connection or Service Request...22  
          6.1.2 Triggered by Endpoint................................24  
      6.2 Posture Reassessment.......................................26  
          6.2.1 Triggered by NEA Client..............................26  
          6.2.2 Triggered by NEA Server..............................28  
   7. Requirements (Only Normative Section)..........................31  
      7.1 Common Protocol Requirements...............................31  
      7.2 Posture Attribute (PA) Protocol Requirements...............33  
      7.3 Posture Broker (PB) Protocol Requirements..................34  
      7.4 Posture Transport (PT) Protocol Requirements...............35  
   8. Security Considerations........................................36  
      8.1 Trust......................................................36  
          8.1.1 Endpoint.............................................37  
   
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          8.1.2 Network Communications...............................38  
          8.1.3 NEA Server...........................................39  
      8.2 Protection Mechanisms at Multiple Layers...................39  
      8.3 Relevant Classes of Attack.................................40  
          8.3.1 Man-in-the-Middle (MITM).............................41  
          8.3.2 Message Modification.................................41  
          8.3.3 Message Replay or Attribute Theft....................42  
          8.3.4 Other Types of Attack................................42  
   9. Privacy Considerations.........................................43  
      9.1 Implementer Considerations.................................44  
      9.2 Minimizing Attribute Disclosure............................45  
   10. References....................................................47  
      10.1 Normative References......................................47  
      10.2 Informative References....................................47  
   Acknowledgments...................................................48  
   Authors' Addresses................................................48        
     
 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 policy and providing the result to  
    appropriate authorization entities that make decisions about  
    network and application access.  Some endpoints may be incapable  
    of running the 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.  
      

   
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    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], Cisco's  
    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.  The only normative portion of  
    the document exists in the Requirements section.  
  
 1.1 Conventions Used in This Document  
       
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL  
    NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and  
    "OPTIONAL" in this document are to be interpreted as described  
    in [KEYWORDS].  
  
 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  

   
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      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 operational status of an  
      endpoint feature (e.g. anti-virus software).  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 name  
      space known as standard attributes.  Those attributes not  
      standardized are referred to in this specification as vendor- 
      specific.  
      
    Dialog - Sequence of request/response messages exchanged   
      
    Endpoint - Any host 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  
   
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      even if the asset is not within the owner's possession.  The  
      owner may permit user override or augmentation of control  
      policies or may 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 a quality or status  
      (posture) of an 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.  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 the endpoint was considered  
      compliant or not.  Multiple of these attributes may be used  
      allowing for more granular feature level decisions to be  
      communicated in addition to an overall, global assessment  
      decision.  
      
    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 an entity that is making use of the services of  
      an endpoint.  The user may not own the endpoint so 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.  
   
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 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  
    intends to 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, 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 non-normative and 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,   
      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 IDS providing posture  
        for an endpoint without a NEA Client),  
      o Detecting or handling lying endpoints (see section 8.1.1  
        for more information).  
      
 3.2 Applicability of Environments  

   
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    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.  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 which provide  
    equipment to employees to perform their duties.  These employees  
    are likely bound under an employment contract which 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) which 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.   
      
    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 which offers  
    Internet access.  This coffee shop would not be expected to be  
   
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    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.  
      
    Other environments are more difficult to determine whether NEA  
    is applicable, 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 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.   
      
    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  
   
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    802.1X[801.1X], remote access via IPsec[IPSEC] or SSL[TLS] 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 functionality of the NEA Client and Server and  
    their relationships, as well as the protocols they use to  


   
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    communicate at various levels (e.g. PA is carried by the PB  
    protocol).    
      
    While the diagram shows 3 layered protocols, it's envisioned that  
    PA is likely a thin message wrapper around a set of attributes that  
    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  
   
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    scope for the NEA reference model and should be covered in the  
    specifications of candidate protocols for the Posture Transport  
    (PT) protocol.  
      
 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  
    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 NAT device might route communications for many  
    systems behind it, but when the NAT device is joining 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 on 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 and subject to  
    different access policies.  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  
   
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    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 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  
    Posture Collector at install-time and passed to 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:  
           - Collecting associated posture information for  
             particular features(s) of the endpoint and returning  
             this information in Posture Attributes.  
           - 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 particular features(s) on the  
        endpoint for posture changes that require notification to  
        the Posture Broker Client.  
        
   
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      o Providing cryptographic verification of the attributes  
        received from the Validator and offering cryptographic  
        protection to the attributes returned.  
     
   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.  
      
 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/un-register.  
      
      o Multiplexing and de-multiplexing attribute messages  
        between the NEA Server and the relevant Posture  
        Collectors.  
      

   
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      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.  
      
 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, IPSec).  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 which 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  
    Collector(s) and vice-versa.  The Posture Validator performs the  
   
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    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 may choose to request  
    additional attributes from one or more collectors.  The response  
    from the Posture Collector could be a set of attributes or a set  
    of assertions in case of NEA Client based evaluation.   
      
    Each Posture Validator will be associated with one or more  
    identifiers that enable it to be the 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:  
           - 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:  
           - Posture Attributes collected for the requested  
             functionality.  
           - 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:  
           - Result Attributes that communicates the posture  
             assessment result.  

   
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           - 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.  
        
      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.  
  
 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 which it passes to the  
    associated Posture Validators.  The Posture Broker Server  
    multiplexes the PA messages (e.g. messages containing 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.  
      

   
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      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.  
      
 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 endpoint  
    (Posture Attributes), results of an assessment (Result  

   
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    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.  
      
 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) transports PB messages during the  
    network connection request or after network connectivity has  
    been established.  The PT protocol provides reliable message  
    delivery, mutual authentication and cryptographic protection for  
    the PB message 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.    
      

   
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    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 name space or schema.     
      
 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 addition to  
        Posture Attributes.  
  
 5.3.2 Attributes Normally Sent by NEA Server:  
  
      o Request Attributes - Attributes which 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 thru 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  
  
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        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.  
  
 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, request to use a service or  
        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  
        which might raise the endpoint's risk level.   This could  
        be as simple as it having been a long time since its last  
        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.  

   
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 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  
    which 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.  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  
    requirements are required.  A better known example could include  
    requesting entrance to a network which 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 which 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 allowed on 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.  

   
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     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.  
     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, which 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 request 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 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 which 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.  
    13. The Posture Broker Client delivers the PA messages with  
        results from the various Posture Validators to the  
        appropriate Posture Collectors including the PA message  
        containing attributes with remediation instructions to the  
        operating system patch Posture Collector which interacts  
        with the user to download and install the needed patches,  
        potentially while quarantined.  

   
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    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 the 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 requests a posture assessment.  The NEA Server performs  
    an initial assessment of the system and determines it's  
    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:  
     
   
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    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 which causes an assessment to be  
       triggered.  
    3. The Posture Broker Server detects the new session and  
       consults policy to determine which 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.  
    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  
   
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       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 which 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  
       which 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.  
   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 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  

   
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    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.  
      
 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 which 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.  

   
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    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.  
   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  
  
 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  
   
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    compliant.  Several 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  
    5PM.  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 patch and that it must be installed by 5PM.  The  
    marketing employee immediately downloads and installs the patch  
    and obtains an assertion that the patches are now installed.  
      
    At 5PM 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 5PM 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.  
    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  
   
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       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.  
   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 includes  
       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 which 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.  
   
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   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 5PM, 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 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  
      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 (Only Normative Section)  
  
    This section describes the normative 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 which are sufficient.  
  
 7.1 Common Protocol Requirements  
  
   
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    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 MUST provide protection against active and  
         passive attacks by intermediaries and endpoints including  
         protection to prevent 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, PANA, TLS and  
         IKE/IPsec).    
       
     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.  
  
   
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 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.  
      
    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 the standard  
          attributes.  
       
     PA-2 The PA protocol MUST support communication of vendor-specific  
          attributes.  
       
     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.  This enables 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 MUST provide authentication, integrity, and  
          confidentiality of 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.  
       



   
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     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. 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 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.  
   
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     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  
          to optimize transport of messages and minimize round trips.    
     
 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.  
  

   
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     PT-5 The PT protocol SHOULD be able to run between a NEA Client  
          and NEA Server over TCP or UDP (similar to 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.  
      
    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 more risky 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  
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    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.  
      
 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.  However, NEA  
    implementations may choose to send or pre-provision some policies  
    to the endpoint for evaluation which 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, IDS/IPS,  
   
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    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.  
  
 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  
    may have the opportunity to observe and change the message dialogs.   
      
    The intermediary devices may fall into a few major categories which  
    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. 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's 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 which monitor and take corrective action when suspicious  
    behavior is observed on the network.  These devices may have a  
    relationship with the NEA Server which 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 which are present to  
    service other network functions beside NEA.  These devices might be  
    capable of passively eavesdropping on the network, archiving  
   
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    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's 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.  
      
 8.1.3 NEA Server  
      
    The NEA Server (including potentially remote systems providing  
    posture validation services) are generally trusted to apply the  
    specified assessment policies and must be protected from  
    compromise.  It's 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 we 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 which 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.   
  
 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  

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

   
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 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 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.  
      
 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 an HMAC [HMAC] of the message) so any change to  
    a message would be detected.  PA includes a similar requirement to  
   
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    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 can not 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  
    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's 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's 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  
   
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    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.  
      
    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  

   
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    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's  
    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 Collectors are  
    allowed to disclose.  
      
    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  
   
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    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's 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.  
      
 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  

   
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    disclosed attribute set is established.  Each model has benefits  
    and issues and these descriptions are provided to seed  
    discussion as to whether each is feasible and whether a  
    preferred approach will emerge potentially impacting the  
    protocols and general privacy of NEA.    
      
    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  
    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 the Linux ask for a different set of  
    attributes specific to Linux then it would if the endpoint was a  
   
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    Windows system.  It's envisioned that this 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  
  
    [KEYWORDS]     S. Bradner, "Keywords for use in RFCs to  
                   Indicate Requirement Levels", RFC2119 (BCP),  
                   IETF, March 1997.  
      
      
     
 10.2 Informative References  
     
    [802.1X]       IEEE Port Based Network Access Control,  
                   http://www.ieee802.org/1/pages/802.1x.html  
      
    [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, "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., Seo K., "Security Architecture for the  
                   Internet Protocol", RFC 4301, December 2005.  
      


   
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    [NAP]          Microsoft, Network Access Protection Main Web  
                   Site, http://www.microsoft.com/nap  
      
    [RADIUS]       Rigney, C., Willens, S., Rubens, A., and  
                   Simpson, W., "Remote Authentication Dial In User  
                   Service (RADIUS)", RFC 2865, June 2000.  
      
    [TLS]          Dierks, T., Rescorla, E., "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/net 
                   work/  
     
 Acknowledgments  
  
    The authors of this draft would like to acknowledge the NEA working  
    group members who have contributed to previous requirements and  
    problem statement drafts that influenced the direction of this  
    specification: Kevin Amorin, Diana Arroyo, Uri Blumenthal, Alan  
    DeKok, Lauren Giroux, Steve Hanna, Thomas Hardjono, Tim Polk, Ravi  
    Sahita, Joe Salowey, Chris Salter, Mauricio Sanchez, Jeff Six,  
    Susan Thompson, John Vollbrecht, Nancy Winget, Han Yin, Hao Zhou.  
      
 Authors' Addresses  
  
   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  
     
   
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   Kaushik Narayan  
   Cisco Systems Inc.  
   10 West Tasman Drive  
   San Jose, CA 95134  
   Phone: +1 408 526-8168  
   Email: kaushik@cisco.com  
     
   Paul Sangster  
   Symantec Corporation  
   6825 Citrine Dr  
   Carlsbad, CA 92009 USA  
   Phone: +1 760 438-5656  
   Email: Paul_Sangster@symantec.com  
      
   Joseph Tardo  
   Nevis Networks  
   295 N. Bernardo Ave., Suite 100  
   Mountain View, CA 94043 USA  
   Email: joseph.tardo@nevisnetworks.com  
       
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