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RFC 6876 - A Posture Transport Protocol over TLS (PT-TLS)


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Internet Engineering Task Force (IETF)                       P. Sangster
Request for Comments: 6876                          Symantec Corporation
Category: Standards Track                                  N. Cam-Winget
ISSN: 2070-1721                                               J. Salowey
                                                           Cisco Systems
                                                           February 2013

             A Posture Transport Protocol over TLS (PT-TLS)

Abstract

   This document specifies PT-TLS, a TLS-based Posture Transport (PT)
   protocol.  The PT-TLS protocol carries the Network Endpoint
   Assessment (NEA) message exchange under the protection of a Transport
   Layer Security (TLS) secured tunnel.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6876.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Prerequisites ..............................................4
      1.2. Message Diagram Conventions ................................4
      1.3. Conventions Used in This Document ..........................4
      1.4. Compatibility with Other Specifications ....................4
   2. Design Considerations ...........................................5
      2.1. Benefits of TCP/IP Connectivity ............................5
      2.2. Leveraging Proven TLS Security .............................6
      2.3. TLV-Based Message Encapsulation ............................6
      2.4. No Change to Base TLS Protocol .............................6
   3. PT-TLS Protocol .................................................7
      3.1. Initiating a PT-TLS Session ................................8
           3.1.1. Issues with Server-Initiated PT-TLS Sessions ........8
           3.1.2. Establish or Re-Use Existing PT-TLS Session .........9
      3.2. TCP Port Usage .............................................9
      3.3. Preventing MITM Attacks with Channel Bindings ..............9
      3.4. PT-TLS Message Flow .......................................10
           3.4.1. Assessment Triggers ................................10
           3.4.2. PT-TLS Message Exchange Phases .....................11
                  3.4.2.1. TLS Setup Phase ...........................12
                  3.4.2.2. PT-TLS Negotiation Phase ..................13
                  3.4.2.3. PT-TLS Data Transport Phase ...............14
           3.4.3. TLS Requirements ...................................14
      3.5. PT-TLS Message Format .....................................15
      3.6. IETF Namespace PT-TLS Message Types .......................18
      3.7. PT-TLS Version Negotiation ................................20
           3.7.1. Version Request Message ............................21
           3.7.2. Version Response Message ...........................22
      3.8. Client Authentication Using SASL ..........................22
           3.8.1. SASL Client Authentication Requirements ............23
           3.8.2. SASL in PT-TLS Overview ............................24
           3.8.3. SASL Authentication Flow ...........................24
           3.8.4. Aborting SASL Authentication .......................25
           3.8.5. Linkages to SASL Framework .........................25
                  3.8.5.1. SASL Service Name .........................25
                  3.8.5.2. SASL Authorization Identity ...............25
                  3.8.5.3. SASL Security Layer .......................25
                  3.8.5.4. Multiple Authentications ..................25
           3.8.6. SASL Channel Bindings ..............................25
           3.8.7. SASL Mechanisms ....................................26
           3.8.8. SASL Mechanism Selection ...........................26
           3.8.9. SASL Authentication Data ...........................27
           3.8.10. SASL Result .......................................28
      3.9. Error Message .............................................29

   4. Security Considerations ........................................32
      4.1. Trust Relationships .......................................32
           4.1.1. Posture Transport Client ...........................33
           4.1.2. Posture Transport Server ...........................34
      4.2. Security Threats and Countermeasures ......................35
           4.2.1. Message Theft ......................................35
           4.2.2. Message Fabrication ................................36
           4.2.3. Message Modification ...............................36
           4.2.4. Denial of Service ..................................37
           4.2.5. NEA Asokan Attacks .................................37
           4.2.6. Trust Anchors ......................................38
   5. Privacy Considerations .........................................38
   6. IANA Considerations ............................................38
      6.1. Designated Expert Guidelines ..............................39
      6.2. Registry for PT-TLS Message Types .........................40
      6.3. Registry for PT-TLS Error Codes ...........................41
   7. Acknowledgments ................................................41
   8. References .....................................................42
      8.1. Normative References ......................................42
      8.2. Informative References ....................................43

1.  Introduction

   The NEA architecture [RFC5209] defines a system for communicating
   posture between a client, where it is collected, and server, where it
   is assessed.  Posture is configuration and/or status of hardware or
   software on an endpoint as it pertains to an organization's security
   policy.  This document specifies PT-TLS, a TLS-based Posture
   Transport (PT) protocol protected by a TLS channel.

   NEA protocols are intended to be used for pre-admission assessment of
   endpoints joining the network and to assess endpoints already present
   on the network.  In order to support both usage models, two different
   types (or bindings) of PT protocols are necessary to operate before
   and after the endpoint has an assigned IP address and other network-
   layer information.  This specification focuses on the PT protocol
   used to assess endpoints already present on the network and thus is
   able to use TCP/IP-based transport protocols.  NEA has defined
   another protocol called PT-EAP [PT-EAP] to address assessment prior
   to the endpoint having an assigned IP address.

   The Posture Transport protocol in the NEA architecture [RFC5209] is
   responsible for transporting Posture Broker (PB-TNC [RFC5793])
   batches, often containing Posture Attributes (PA-TNC [RFC5792]) over
   the network between the Posture Transport Client component of the NEA
   Client and the Posture Transport Server component of the NEA Server.

   The PT protocol also offers strong security protections to ensure
   that the exchanged messages are protected from a variety of threats
   from hostile intermediaries.

1.1.  Prerequisites

   This document does not define an architecture or reference model.
   Instead, it defines one binding of the PT protocol that works within
   the reference model described in the NEA Overview and Requirements
   specification [RFC5209].  The reader is assumed to be thoroughly
   familiar with [RFC5209].  The NEA working group compared the
   functionality described in this specification with the requirements
   in [RFC5209] and found that each applicable requirement was
   addressed.

1.2.  Message Diagram Conventions

   This specification defines the syntax of PT-TLS messages using
   diagrams.  Each diagram depicts the format and size of each field in
   bits.  Implementations MUST send the bits in each diagram as they are
   shown, traversing the diagram from top to bottom and then from left
   to right within each line (which represents a 32-bit quantity).
   Multi-byte fields representing numeric values must be sent in network
   (big endian) byte order.

   Bit field (e.g., flag) values are described referring to the position
   of the bit within the field.  These bit positions are numbered from
   the most significant bit through the least significant bit, so a
   one-octet field with only bit 0 set has the value 0x80.

1.3.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119 [RFC2119].

1.4.  Compatibility with Other Specifications

   One of the goals of the NEA effort is to deliver a single set of
   endpoint assessment standards, agreed upon by all parties.  For this
   reason, the authors understand that the Trusted Computing Group (TCG)
   will be replacing its existing posture transport protocols with new
   versions that are equivalent to and interoperable with the NEA
   specifications.

2.  Design Considerations

   This section discusses some of the key design considerations for the
   PT protocol.  This document specifies the PT binding for use when
   performing an assessment or reassessment after the endpoint has been
   admitted to the network and is capable of using TCP/IP to communicate
   with the NEA Server.  If the endpoint does not yet have TCP/IP-layer
   access to the NEA Server (and vice versa), the endpoint can use the
   PT-EAP (Posture Transport (PT) Protocol for Extensible Authentication
   Protocol (EAP) Tunnel Methods) protocol when performing an
   assessment.

   Because the endpoint has TCP/IP access to the NEA Server (potentially
   on a restricted portion of the network), the NEA Client and NEA
   Server have the ability to establish (or re-use) a reliable TCP/IP
   connection in order to perform the assessment.  The TCP/IP connection
   enables the assessment to occur over a relatively high-performance,
   reliable channel capable of supporting multiple roundtrip message
   exchanges in a full-duplex manner.  These connection properties are
   very different from what is available when the endpoint is initially
   joining the network (e.g., during an 802.1X-based assessment);
   therefore, the design described in this specification follows a
   different path to maximize the benefits of the underlying TCP/IP
   connection.

2.1.  Benefits of TCP/IP Connectivity

   The PT protocol over TLS is typically able to offer to the NEA Client
   and NEA Server significantly higher quality of service and
   flexibility of operation than PT-EAP.  However, there may be some
   added risks when the endpoint is on the network prior to its initial
   assessment (if no admission time assessment had been performed).
   Because of these risks, the combined use of an EAP-based assessment
   during admission followed by reassessment using TCP/IP may be
   appropriate in some environments.

   Some of the benefits to having a TCP/IP-based transport during an
   assessment include:

   o  Full-Duplex Connectivity - used to support asynchronous initiation
      of posture exchanges within a single TLS connection (e.g.,
      triggered by alerts of posture or policy changes)

   o  High Bandwidth - potentially much higher bandwidth than other
      transports (e.g., EAP), allowing more in-band data (e.g.,
      remediation, verbose posture information)

   o  Large Messages - ability to send very large Posture Attribute
      messages without directly fragmenting them (underlying carrier
      protocol may introduce fragmentation)

   o  Bidirectional - NEA Client and NEA Server can initiate an
      assessment or reassessment

   o  Multiple Roundtrips - NEA Client and NEA Server can exchange
      numerous messages without fear of infrastructure timeouts.
      However, the entire exchange should be kept as brief as possible
      if the user has to wait for its completion.

2.2.  Leveraging Proven TLS Security

   All PT protocol bindings must be capable of providing strong
   authentication, integrity, and confidentiality protection for the
   PB-TNC batches.  Rather than define a new protocol over TCP/IP to
   provide adequate protection, this specification requires the use of
   Transport Layer Security [RFC5246] to secure the connection.  TLS was
   selected because it's a widely deployed protocol with parallel
   protections to a number of the EAP tunnel methods, and it meets all
   of the security requirements.

2.3.  TLV-Based Message Encapsulation

   The design of the PT-TLS protocol is based upon the use of a
   type-length-value (TLV)-oriented protocol message that identifies the
   type of message, the message's length, and a potentially variable-
   length payload value.  The use of a TLV-oriented encoding was chosen
   to match the Internet standard PA-TNC and PB-TNC protocols.  Because
   the PA-TNC, PB-TNC, and PT-TLS protocols are typically implemented
   inside the same process space, this allows a common set of message-
   parsing code to be used.  Similarly, creation of debugging tools is
   simplified by the common encoding methodologies.  TLV-based encoding
   was used in each of the NEA protocols in part because it enables a
   very space-efficient representation on the network and is simpler to
   parse than some other encodings to benefit lower-powered (or battery
   constrained) devices.

2.4.  No Change to Base TLS Protocol

   During the design of the PT-TLS protocol, several approaches were
   considered with different costs and benefits.  Several considered
   approaches involved integrating the PT protocol into the TLS
   handshake protocol.  Because the PT protocol requires the underlying
   TLS carrier to provide security protections, the PT protocol couldn't
   operate before the cipher suites were negotiated and in use.  One
   option was to integrate into the TLS handshake protocol after the

   ChangeCipherSpec phase, allowing the PT message to be protected.  The
   benefit of this approach is that the assessment protocol could
   operate below the application protocols, allowing for easier
   integration into applications.  However, making this change would
   require some extensions to the TLS handshake protocol standards and
   existing widely deployed TLS implementations, so it wasn't clear that
   the cost was warranted, particularly because the application
   independence can also be offered by a shim library between the
   application and TLS library that provides the PT protocol
   encapsulation/decapsulation.

   The other general approach considered was to have PT-TLS layer on top
   of TLS as an application protocol (using the standard
   application_data ContentType).  This has the advantage that existing
   TLS software could be used.  However, the PB-TNC traffic would need
   to be encapsulated/decapsulated by a new PT-TLS protocol layer before
   being passed to the TLS library.  This didn't seem like a significant
   issue, as PB-TNC is architected to layer on PT anyway.

   After considering the different options, it was determined that
   layering the PT protocol on top of the TLS protocol without requiring
   current TLS protocol implementations to change met all the
   requirements and offered the best path toward rapid adoption and
   deployment.  Therefore, the following sections describe a PT protocol
   that is carried on top of TLS.

3.  PT-TLS Protocol

   This section specifies the PT-TLS protocol, a Posture Transport (PT)
   protocol carried by the Transport Layer Security (TLS) protocol over
   a TCP/IP network.  As shown in Figure 1, this protocol runs directly
   on top of TLS as an application.  This means PT-TLS is encapsulated
   within the TLS Record Layer protocol using the standard ContentType
   for applications (application_data).

     +---------------------------------------------------------------+
     |             TLV Encapsulation of PB-PA message                |
     +---------------------------------------------------------------+
     |                             TLS                               |
     +---------------------------------------------------------------+
     |                             TCP                               |
     +---------------------------------------------------------------+

                     Figure 1.  PT-TLS Layering Model

3.1.  Initiating a PT-TLS Session

   The PT-TLS protocol may be initiated by a Posture Transport Client or
   a Posture Transport Server.  This flexibility supports different use
   cases.  For example, a Posture Transport Client that wishes to
   trigger a NEA assessment to determine whether its security posture is
   good can start up a PT-TLS session and request a posture assessment.
   On the other hand, when an endpoint requests access to a protected
   network or resource, a Posture Transport Server can start up a PT-TLS
   session and perform a posture assessment before deciding whether to
   grant access.

   The party that initiates a PT-TLS session is known as the "PT-TLS
   Initiator".  The other party in the session (which receives the
   request to open a PT-TLS session) is known as the "PT-TLS Responder".

3.1.1.  Issues with Server-Initiated PT-TLS Sessions

   In order for a NEA Server to establish a PT-TLS session, the NEA
   Client needs to be listening for a connection request on a TCP port
   known by the NEA Server.  In many deployments, the security policies
   of an endpoint (e.g., firewall software) or the security policies of
   a network (e.g., firewall devices) are designed to minimize the
   number of open inbound TCP/UDP ports that are available to the
   network to reduce the potential attack footprint.  This is one issue
   that makes it difficult for a NEA Server to initiate a PT-TLS
   session.

   Another issue with this scenario involves X.509 certificates.  When
   the NEA Server creates a TLS session to the NEA Client, the NEA
   Client is effectively acting as the TLS server during the TLS
   protocol exchange.  This means the NEA Client would typically need to
   possess an X.509 certificate to protect the initial portion of the
   TLS handshake.  In situations where the NEA Server initiates the
   creation of the TLS session, both the NEA Client and NEA Server MUST
   possess X.509 certificates to fully authenticate the session.  For
   many deployments, provisioning X.509 certificates to all NEA Clients
   has scalability and cost issues; therefore, it is recommended that
   the NEA Client not listen for connection requests from the NEA Server
   but instead establish and maintain a TLS session to the NEA Server
   proactively, so either party can initiate an assessment using the
   preexisting TLS session as required.

   In most cases, the traditional methods of server certificate ID
   validation will not apply when the NEA Server initiates the
   connection.  In this case, the NEA Client and Server need to follow
   the certificate path validation rules in RFC 5280 [RFC5280].  In

   addition, each side needs to be able to authorize its peer based upon
   matching Subject and SubjectAltName fields for certificates issued by
   a particular trust anchor.

   Therefore, NEA Clients SHOULD be capable of establishing and holding
   open a TLS session with the NEA Server immediately after obtaining
   network access.  A NEA Client MAY listen for connection requests from
   the NEA Server and establish a new PT-TLS session when one does not
   already exist.  Because of the potential added complexity, a NEA
   Client's support for accepting inbound PT-TLS connections is optional
   to implement.  Having an existing PT-TLS session allows either party
   to initiate an assessment without requiring the NEA Client to be
   listening for new connection requests.  In order to keep the TLS
   session alive, the NEA Client and NEA Server SHOULD be capable of
   supporting the TLS heartbeat protocol [RFC6520].

3.1.2.  Establish or Re-Use Existing PT-TLS Session

   A single PT-TLS session can support multiple NEA assessments, which
   can be started by either party (the PT-TLS Initiator or the PT-TLS
   Responder).  The party that starts a NEA assessment is known as the
   "assessment initiator", and the other party is known as the
   "assessment responder".

   If the assessment initiator already has a PT-TLS session to the
   assessment responder, the initiator can re-use this session;
   otherwise, a new PT-TLS session needs to be established.

3.2.  TCP Port Usage

   In order for a PT-TLS Initiator to establish a TCP connection to a
   PT-TLS Responder, the initiator needs to know the TCP port number on
   which the responder is listening for assessment requests.  The IANA
   has reserved TCP port number 271 for use by "pt-tls".

3.3.  Preventing MITM Attacks with Channel Bindings

   As described in "The Network Endpoint Assessment (NEA) Asokan Attack
   Analysis" [RFC6813], a sophisticated Man-in-the-Middle (MITM) attack
   can be mounted against NEA systems.  The attacker forwards PA-TNC
   messages from a healthy machine through an unhealthy one so that the
   unhealthy machine can gain network access.  Because there are easier
   attacks on NEA systems, like having the unhealthy machine lie about
   its configuration, this attack is generally only mounted against
   machines with an External Measurement Agent (EMA).  The EMA is a
   separate entity, difficult to compromise, that measures and attests
   to the configuration of the endpoint.

   To protect against NEA Asokan attacks, the Posture Broker Client on
   an EMA-equipped endpoint should pass the tls-unique channel binding
   [RFC5929] for PT-TLS's underlying TLS session to the EMA.  This value
   can then be included in the EMA's attestation, and the Posture
   Validator responsible for communicating with the EMA may then confirm
   that the value matches the tls-unique channel binding for its end of
   the connection.  If the values match, the posture sent by the EMA and
   NEA Client is from the same endpoint as the client side of the TLS
   connection (since the endpoint knows the tls-unique value), so no
   man-in-the-middle is forwarding posture.  If they differ, the Asokan
   attack has been detected.  The Posture Validator MUST fail its
   verification of the endpoint if the Asokan attack has been detected.

3.4.  PT-TLS Message Flow

   This section discusses the general flow of messages between the NEA
   Client's Posture Transport Client and the NEA Server's Posture
   Transport Server in order to perform NEA assessments using the PT-TLS
   protocol.

3.4.1.  Assessment Triggers

   Initially, the NEA Client or NEA Server will decide that an
   assessment is needed.  What stimulates the decision to perform an
   assessment is outside the scope of this specification, but some
   examples include:

   o  NEA Server becoming aware of suspicious behavior on an endpoint

   o  NEA Server receiving new policies requiring immediate action

   o  NEA Client noticing a change in local security posture

   o  NEA Client wishing to access a protected network or resource

   Because either the NEA Client or NEA Server can trigger the
   establishment of the TLS session and initiate the assessment, this
   document will use the terms "assessment initiator" and "assessment
   responder".  This nomenclature allows either NEA component to fill
   either of the PT-TLS roles.

3.4.2.  PT-TLS Message Exchange Phases

   The PT-TLS message exchange occurs in three distinct phases:

   o  TLS Setup (including TLS handshake protocol)

   o  PT-TLS Negotiation

   o  PT-TLS Data Transport

   The TLS Setup phase is responsible for the establishment of the TCP
   connection and the TLS protections for the PT-TLS messages.  The TLS
   Setup phase starts with the establishment of a TCP connection between
   the Posture Transport Client and Posture Transport Server.  The new
   connection triggers the TLS server to start the TLS handshake
   protocol to establish the cryptographic protections for the session.
   Once the TLS Setup phase has completed (after the TLS Finished
   messages), the TLS session MUST NOT be renegotiated.  TLS session
   renegotiation MAY be used before the TLS Setup phase ends and the
   PT-TLS Negotiation phase begins.  This phase also enables the
   establishment of the tls-unique shared secret.  The tls-unique shared
   secret can later be used by the PA protocol to protect against some
   forms of man-in-the-middle attacks.

   The PT-TLS Negotiation phase is only performed at the start of the
   first assessment on a TLS session.  During this phase, the NEA Client
   and NEA Server discover each other's PT-TLS capabilities and
   establish a context that will apply to all future PT-TLS messages
   sent over the TLS session.  The PT-TLS Negotiation phase MUST NOT be
   repeated after the session has entered the Data Transport phase.  NEA
   assessment messages (PB-TNC batches) MUST NOT be sent by the NEA
   Client or NEA Server prior to the completion of the PT-TLS
   Negotiation phase to ensure that the security protections for the
   session are properly established and applied to the NEA assessment
   messages.

   Finally, the Data Transport phase allows the NEA Client and NEA
   Server to exchange PT messages under the protection of the TLS
   session consistent with the capabilities established in earlier
   phases.  The exchanged messages can be a PT-TLS protected NEA
   assessment as described in this specification or other vendor-defined
   PT-TLS exchanged messages.

3.4.2.1.  TLS Setup Phase

   After a new TCP connection is established between the Posture
   Transport Client and Posture Transport Server, a standard TLS
   exchange is performed to negotiate a common security context for
   protecting subsequent communications.  As discussed in Section 3.1,
   the TCP connection establishment and/or the TLS handshake protocol
   could be initiated by either the NEA Client or NEA Server.  The most
   common situation would be for the assessment initiator to trigger the
   creation of the TCP connection and TLS handshake, so an assessment
   could begin when no session already exists.  When the NEA Server has
   initiated the TLS Setup, the NEA Server is acting as a TLS client and
   the NEA Client is the TLS server (accepting the inbound TLS session
   request).  The expected normal case is that the NEA Client initiates
   this phase, so that the NEA Server is acting as the TLS server and
   therefore the bootstrapping of the security of the TLS session is
   using the NEA Server's certificate.  Having the NEA Client initiate
   the TLS session avoids the need for the NEA Client to also possess a
   certificate.

   During the TLS Setup phase of PT-TLS, the PT-TLS Initiator contacts
   the listening port of the PT-TLS Responder and performs a TLS
   handshake.  The PT-TLS Responder MUST possess a trustworthy X.509
   certificate used to authenticate to the PT-TLS Initiator and used to
   bootstrap the security protections of the TLS session.  The PT-TLS
   Initiator MAY also use an X.509 certificate to authenticate to the
   PT-TLS Responder providing for a bidirectional authentication of the
   PT-TLS session.  The NEA Client MUST provide certificate validation
   according to the rules in RFC 5280 when evaluating the server
   certificate.  The NEA Client MAY perform certificate revocation
   checking on the NEA Server's certificate.  This specification allows
   the NEA Client implementation to decide on what certificate
   revocation technique is to be implemented.  If revocation information
   is not provided in a TLS handshake extension, then clients performing
   certificate validation will require some network access (e.g., HTTP)
   to be allowed during the assessment.  NEA Client network access to
   other non-essential services might be restricted during assessments
   in some situations.  If the client cannot access the status
   information, then its policy may prevent it from completing the TLS
   handshake.

   In addition, the NEA Client MUST follow the recommendations in
   RFC 6125 [RFC6125] when validating the NEA Server domain name against
   the contents of the server certificate, taking into consideration the
   following restrictions:

   o  Any SRV-IDs and URI-IDs in the certificate are ignored.

   o  Use of CN-IDs in certificates is NOT RECOMMENDED.

   o  Wildcards MUST NOT appear in the DNS-ID or CN-ID of a certificate
      identifying a PT-TLS server.

   Details for the reverse direction are given in Section 3.1.

   Due to deployment issues with issuing and distributing certificates
   to a potentially large number of NEA Clients, this specification
   allows the NEA Client to be authenticated during the PT-TLS
   Negotiation phase using other more cost-effective methods, as
   described in Section 3.8.1.  At the conclusion of a successful
   initial TLS Setup phase, the NEA Client and NEA Server have a
   protected session to exchange messages.  This allows the protocol to
   transition to the PT-TLS Negotiation phase.

3.4.2.2.  PT-TLS Negotiation Phase

   Once a TLS session has been established between a Posture Transport
   Client and Posture Transport Server, the PT-TLS Initiator sends a
   Version Request message indicating its supported PT-TLS protocol
   version range.  Next, the PT-TLS Responder sends a Version Response
   message, which selects a protocol version from within the range
   offered.  The PT-TLS Responder SHOULD select the preferred version
   offered if supported; otherwise, the highest version that the
   responder is able to support from the received Version Request
   message will be used.  If the PT-TLS Responder is unable or unwilling
   to support any of the versions included in the Version Request
   message, the responder SHOULD send a Version Not Supported error
   message.

   If no client-side authentication occurred during the TLS Setup phase,
   the Posture Transport Server can authenticate the client using PT-TLS
   client authentication messages as described in Section 3.8.  The NEA
   Server initiates the client authentication and indicates when the
   authentication is complete.

   When the NEA Client receives the Simple Authentication and Security
   Layer (SASL) [RFC4422] Mechanisms list, the NEA Client responds with
   a SASL Mechanism Selection TLV indicating the method of
   authentication to be used.  Upon selecting an appropriate SASL

   mechanism, the NEA Client and Server exchange SASL-mechanism-specific
   messages in order to authenticate the NEA Client.  When the client
   authentication successfully completes and no additional
   authentications are required (as indicated by the NEA Server sending
   an empty SASL Mechanisms list), the PT-TLS session transitions into
   the Data Transport phase, where it will remain for the duration of
   the session.  Note that the NEA Server could choose to not
   authenticate the client (indicated by only sending an empty SASL
   Mechanisms list) or to continue performing a posture assessment even
   if the authentication did not complete successfully.

3.4.2.3.  PT-TLS Data Transport Phase

   Once a PT-TLS session is available to carry NEA assessments, PT-TLS
   allows either side of the connection to send the first PB-TNC batch.
   The PB-TNC standard prescribes whether the Posture Broker Client or
   Posture Broker Server starts the assessment.  The assessment
   initiator first envelopes the PB-TNC batch in a PT-TLS message, then
   assigns a message identifier to the message and finally transmits it
   over the session.  The assessment responder validates the PT-TLS
   message and delivers the encapsulated PB-TNC batch to its upstream
   component (Posture Broker Client or Server).

   Most PT-TLS messages contain PB-TNC batches that house PA-TNC
   requests for posture information or a response containing the
   requested posture information.  The Posture Transport Client and
   Posture Transport Server may also exchange messages between them,
   such as a PT-TLS Error message indicating that a problem occurred
   processing a message.  During an assessment, the Posture Transport
   Client and Server merely encapsulate and exchange the PB-TNC batches
   and are unaware of the state of the assessment.

   The PT-TLS protocol allows either party to send a PT-TLS message at
   any time, reflecting the full-duplex nature of the underlying TLS
   session.  For example, an assessment initiator may send several
   PT-TLS messages prior to receiving any responses from the assessment
   responder.  All implementations of PT-TLS MUST support full-duplex
   PT-TLS message exchange.  However, some higher-layer NEA protocols
   (e.g., PB-TNC) may not be able to fully make use of the full-duplex
   message exchange.

3.4.3.  TLS Requirements

   In order to ensure that strong security is always available for
   deployers and to improve interoperability, this section discusses
   some requirements on the underlying TLS transport used by PT-TLS.
   Whenever TLS is used by this specification, the appropriate version
   (or versions) of TLS will vary over time, based on the widespread

   deployment and known security vulnerabilities.  At the time of this
   writing, TLS version 1.2 [RFC5246] is the most recent version, but it
   has a very limited deployment base and might not be readily available
   for implementation.  TLS version 1.0 [RFC2246] and version 1.1
   [RFC4346] are the most widely deployed versions and will provide the
   broadest interoperability.  Implementations of PT-TLS SHOULD support
   use of TLS 1.2.

   For each TLS version supported, implementations of the PT-TLS
   protocol MUST at least support the TLS_RSA_WITH_AES_128_CBC_SHA
   cipher suite.  This cipher suite requires the server to provide a
   certificate that can be used during the key exchange.
   Implementations SHOULD NOT include support for cipher suites that do
   not minimally offer PT-TLS Responder (typically Posture Transport
   Server) authentication, such as the anonymous Diffie-Hellman cipher
   suites (e.g., TLS_DH_anon_WITH_AES_128_CBC_SHA).  Implementations
   MUST support RFC 5746 [RFC5746].  Implementations MAY allow
   renegotiation to provide confidentiality for the client certificate.
   If renegotiation is allowed, implementations need to select the
   appropriate handshake messages as described in RFC 5929 for the
   tls-unique value.

3.5.  PT-TLS Message Format

   This section describes the format and semantics of the PT-TLS
   message.  Every message sent over a PT-TLS session MUST start with
   the PT-TLS header described in this section.

   The PT-TLS header format is as follows:

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reserved   |           Message Type Vendor ID              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Message Type                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Message Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Message Identifier                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Message Value (e.g., PB-TNC Batch) . . .       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Reserved

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Message Type Vendor ID

      This field indicates the owner of the namespace associated with
      the message type.  This is accomplished by specifying the 24-bit
      Structure of Management Information (SMI) Private Enterprise
      Number [PEN] (Vendor ID) of the party who owns the message type
      namespace.  Consistent with PA-TNC and PB-TNC, we depend on the
      PEN fitting in 24 bits, so if IANA were to register a wider PEN,
      then that PEN could not be used with NEA.  IETF namespace PT-TLS
      Message Types MUST use zero (0) in this field.  For more
      information about the intended use of NEA namespace identifiers,
      see the PA-TNC specification (RFC 5792), Sections 2.1 and 2.2.

      The PT-TLS Message Type Vendor ID 0xffffff is reserved.  Posture
      Transport Clients and Servers MUST NOT send PT-TLS messages in
      which the PT-TLS Message Type Vendor ID has this reserved value
      (0xffffff).  If a Posture Transport Client or Posture Transport
      Server receives a message containing this reserved value
      (0xffffff) in the PT-TLS Message Type Vendor ID, the recipient
      SHOULD respond with an Invalid Parameter error code in a PT-TLS
      Error message.

   Message Type

      This field defines the type of the PT-TLS message within the scope
      of the specified Message Type Vendor ID that is included in the
      Message Value field.  The specific IETF-defined values allowable
      in this field when the Message Type Vendor ID is the IETF SMI
      Private Enterprise Number value (0) are defined in Section 3.6.
      Recipients of a message containing a Message Type Vendor ID and a
      message type that is unrecognized SHOULD respond with a Type Not
      Supported error code in a PT-TLS Error message.

      Posture Transport Clients and Posture Transport Servers MUST NOT
      require support for particular vendor-defined PT-TLS Message Types
      in order to interoperate with other PT-TLS compliant
      implementations (although implementations MAY permit
      administrators to configure them to require support for specific
      vendor-defined PT-TLS Message Types).

      If the PT-TLS Message Type Vendor ID field has the value zero (0),
      then the PT-TLS Message Type field contains an IETF namespace
      PT-TLS Message Type, as listed in the IANA registry.  IANA
      maintains a registry of PT-TLS Message Types.  Entries in this
      registry are added following the IANA Specification Required
      policy, following the guidelines in Section 6.1.  Section 3.6 of
      this specification defines the initial set of IETF-defined PT-TLS
      Message Types.

      The PT-TLS Message Type 0xffffffff is reserved.  Posture Transport
      Clients and Posture Transport Servers MUST NOT send PT-TLS
      messages in which the PT-TLS Message Type has this reserved value
      (0xffffffff).  If a Posture Transport Client or Posture Transport
      Server receives a message in which the PT-TLS Message Type has
      this reserved value (0xffffffff), it SHOULD respond with an
      Invalid Parameter error code in a PT-TLS Error message.

   Message Length

      This field contains the length in octets of the entire PT-TLS
      message (including the entire header).  Therefore, this value MUST
      always be at least 16.  Any Posture Transport Client or Posture
      Transport Server that receives a message with a PT-TLS Message
      Length field whose value is less than 16 SHOULD respond with an
      Invalid Parameter PT-TLS Error Code.  Similarly, if a Posture
      Transport Client or Posture Transport Server receives a PT-TLS
      message for a Message Type that has a known Message Length and the
      Message Length indicates a different value (greater or less than
      the expected value), the recipient SHOULD respond with an Invalid
      Parameter PT-TLS Error Code.

   Message Identifier

      This field contains a value that uniquely identifies the PT-TLS
      message on a per message sender (Posture Transport Client or
      Server) basis.  This value is copied into the body of the PT-TLS
      Error message so the recipient can determine which message caused
      the error.

      The Message Identifier MUST be a monotonically increasing counter
      starting at zero indicating the number of the messages the sender
      has transmitted over the TLS session.  It is possible that a busy
      or long-lived session might exceed 2^32-1 messages sent, so the
      message sender MUST roll over to zero upon reaching the 2^32nd
      message, thus restarting the increasing counter.  During a
      rollover, it is feasible that the message recipient could be
      confused if it keeps track of every previously received Message
      Identifier, so recipients MUST be able to handle rollover
      situations without generating errors.

   Message Value

      The contents of this field vary depending on the particular
      Message Type Vendor ID and Message Type given in the PT-TLS header
      for this PT-TLS message.  This field most frequently contains a
      PB-TNC batch.  The contents of this field for each of the initial
      IETF namespace PT-TLS Message Types are defined in this
      specification.

3.6.  IETF Namespace PT-TLS Message Types

   This section defines the NEA standard PT-TLS Message Types used to
   carry PT-TLS messages and PB-TNC batches between the Posture
   Transport Client and Posture Transport Server.

   The following table summarizes the initial set of IETF-defined
   message type values, which are used with the PT-TLS Message Type
   Vendor ID field set to the IETF SMI PEN (0).  Note that the IANA
   administers a PEN value of 0 on behalf of the IETF.  These
   descriptions only apply to the IETF namespace.

          Value (Name)                          Definition
   ----------------------------  ---------------------------------------
   0 (Experimental)              Reserved for experimental use.  This
                                 type will not offer interoperability
                                 but allows for experimentation.  This
                                 message type MUST only be sent when the
                                 NEA Client and NEA Server are in the
                                 Data Transport phase and only on a
                                 restricted, experimental network.
                                 Compliant implementations MUST send an
                                 Invalid Message error code in a PT-TLS
                                 Error message if an Experimental
                                 message is received.

   1 (Version Request)           Version negotiation request including
                                 the range of versions supported by the
                                 sender.  This message type MUST only be
                                 sent by the TLS session initiator as
                                 the first PT-TLS message in the PT-TLS
                                 Negotiation phase.  Recipients MUST
                                 send an Invalid Message error code in a
                                 PT-TLS Error message if a Version
                                 Request is received at another time.

   2 (Version Response)          PT-TLS protocol version selected by the
                                 responder.  This message type MUST only
                                 be sent by the PT-TLS Responder as the
                                 second message in the PT-TLS
                                 Negotiation phase.  Recipients MUST
                                 send an Invalid Message error code in a
                                 PT-TLS Error message if a Version
                                 Response is received at another time.

   3 (SASL Mechanisms)           Sent by the NEA Server to indicate what
                                 SASL mechanisms it is willing to use
                                 for authentication on this session.
                                 This message type MUST only be sent by
                                 the NEA Server in the PT-TLS
                                 Negotiation phase.  The NEA Client MUST
                                 send an Invalid Message error code in a
                                 PT-TLS Error message if a SASL
                                 Mechanisms message is received at
                                 another time.

   4 (SASL Mechanism Selection)  Sent by the NEA Client to select a SASL
                                 mechanism from the list offered by the
                                 NEA Server.  This message type MUST
                                 only be sent by the NEA Client in the
                                 PT-TLS Negotiation phase.  The NEA
                                 Server MUST send an Invalid Message
                                 error code in a PT-TLS Error message if
                                 a SASL Mechanism Selection is received
                                 after the PT-TLS Negotiation phase.
                                 Once a SASL mechanism has been
                                 selected, it may not change until the
                                 mechanism completes either successfully
                                 or as a failure.

   5 (SASL Authentication Data)  Opaque octets exchanged between the NEA
                                 Client and NEA Server's SASL mechanisms
                                 to perform the client authentication.
                                 This message type MUST only be sent
                                 during the PT-TLS Negotiation phase.
                                 Recipients MUST send an Invalid Message
                                 error code in a PT-TLS Error message if
                                 a SASL Authentication Data message is
                                 received after the PT-TLS Negotiation
                                 phase.

   6 (SASL Result)               Indicates the result code of the SASL
                                 mechanism authentication as described
                                 in Section 3.8.10.  This message type
                                 MUST only be sent by the NEA Server
                                 when the NEA Client and NEA Server are
                                 in the PT-TLS Negotiation phase.  The
                                 NEA Client MUST send an Invalid Message
                                 error code in a PT-TLS Error message if
                                 a SASL Result is received after the
                                 PT-TLS Negotiation phase.

   7 (PB-TNC Batch)              Contains a PB-TNC batch.  For more
                                 information on PB-TNC batches, see
                                 RFC 5793 (PB-TNC) Section 4.  This
                                 message type MUST only be sent when the
                                 NEA Client and NEA Server are in the
                                 PT-TLS Data Transport phase.
                                 Recipients SHOULD send an Invalid
                                 Message error code in a PT-TLS Error
                                 message if a PB-TNC Batch is received
                                 outside of the Data Transport phase.

   8 (PT-TLS Error)              PT-TLS Error message as described in
                                 Section 3.9.  This message type may be
                                 used during any PT-TLS phase.

   9-4294967294 (Unassigned)     These values are for future allocation
                                 following guidelines defined in the
                                 IANA Considerations section (see
                                 Section 6.1).  Recipients of
                                 unsupported messages in the IETF
                                 namespace using a message type of 9 to
                                 4294967294 MUST respond with a Type Not
                                 Supported PT-TLS Error Code in a PT-TLS
                                 Error message.

   4294967295                    Reserved

3.7.  PT-TLS Version Negotiation

   This section describes the message format and semantics for the
   PT-TLS protocol version negotiation.  This exchange is used by the
   PT-TLS Initiator to trigger a version negotiation at the start of an
   assessment.  The PT-TLS Initiator MUST send a Version Request message
   as its first PT-TLS message and MUST NOT send any other PT-TLS
   messages on this connection until it receives a Version Response
   message or an Error message.  The PT-TLS Responder MUST complete the
   version negotiation (or cause an error) prior to sending or accepting

   reception of any additional messages.  After the successful
   completion of the version negotiation, both the Posture Transport
   Client and Posture Transport Server MUST only send messages compliant
   with the negotiated protocol version.  Subsequent assessments on the
   same session MUST use the negotiated version number and therefore
   MUST NOT send additional version negotiation messages.

3.7.1.  Version Request Message

   This message is sent by a PT-TLS Initiator as the first PT-TLS
   message in a PT-TLS session.  This message discloses the sender's
   supported versions of the PT-TLS protocol.  To ensure compatibility,
   this message MUST always be sent using version 1 of the PT-TLS
   protocol.  Recipients of this message MUST respond with a Version
   Response or with a PT-TLS Error message (Version Not Supported or
   Invalid Message).  The following diagram shows the format of the
   Version Request message:

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reserved   |    Min Vers   |    Max Vers   |   Pref Vers   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Reserved

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Min Vers

      This field contains the minimum version of the PT-TLS protocol
      supported by the sender.  This field MUST be set to 1 indicating
      support for the first version of PT-TLS.  However, future versions
      of this specification will probably remove this requirement, so
      PT-TLS Responders MUST be prepared to receive other values.

   Max Vers

      This field contains the maximum version of the PT-TLS protocol
      supported by the sender.  This field MUST be set to 1 indicating
      support for the first version of PT-TLS.  However, future versions
      of this specification will probably remove this requirement, so
      PT-TLS Responders MUST be prepared to receive other values.

   Pref Vers

      This field contains the sender's preferred version of the PT-TLS
      protocol.  This is a hint to the recipient that the sender would
      like this version selected if supported.  The value of this field
      MUST fall within the range of Min Vers to Max Vers.  This field
      MUST be set to 1 indicating support for the first version of
      PT-TLS.  However, future versions of this specification will
      probably remove this requirement, so PT-TLS Responders MUST be
      prepared to receive other values.

3.7.2.  Version Response Message

   This message is sent in response to receiving a Version Request
   message at the start of a new assessment session.  If a recipient
   receives a Version Request after a successful version negotiation has
   occurred on the session, the recipient MUST send an Invalid Message
   error code in a PT-TLS Error message and have TLS close the session.
   This message MUST be sent using the syntax, semantics, and
   requirements of the protocol version specified in this message.

                           1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Reserved                      |    Version    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Reserved

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Version

      This field contains the version selected by the sender of this
      message.  The version selected MUST be within the Min Vers to Max
      Vers inclusive range sent in the Version Request message.  If a
      PT-TLS Initiator receives a message with an invalid Version
      selected, the PT-TLS Initiator MUST respond with a Version Not
      Supported PT-TLS Error message.

3.8.  Client Authentication Using SASL

   This section includes a description of the message format and
   semantics necessary to perform client authentication (authentication
   of the NEA Client) over PT-TLS.  Client authentication could be
   necessary if the NEA Server requires such an authentication and it
   was not performed during the TLS handshake.  The general model used

   for performing an authentication of the client using PT-TLS messages
   is to integrate the Simple Authentication and Security Layer (SASL)
   [RFC4422] framework.  SASL provides a number of standards-based
   authentication mechanisms capable of authenticating the NEA Client
   using a variety of base technologies.

   Client authentication could occur during the TLS handshake using TLS-
   defined authentication techniques.  Because this client
   authentication is optional, the NEA Server's policy might require the
   client to be authenticated by PT-TLS before performing the
   assessment.  Similarly, the NEA Server may require a PT-TLS
   authentication even if the NEA Client was authenticated during the
   TLS handshake (e.g., to allow a user authentication after a system-
   level authentication occurred during the TLS handshake).  The
   decision of whether a SASL client authentication is to occur is left
   to the NEA Server's policy.

   As discussed in Section 3.1.1, it is possible that the NEA Server may
   initiate the TLS session to the NEA Client, thus causing it to fill
   the role of TLS client during the TLS handshake.  Because the NEA
   Server is required to possess an X.509 certificate for those times
   when it is acting as the TLS server (normal case), PT-TLS requires
   that the NEA Server MUST use its X.509 certificate for TLS client
   authentication during the TLS handshake to authenticate itself even
   when it is acting as the TLS client.  In this case, the NEA Client
   and NEA Server will authenticate using certificates during the TLS
   handshake, so the PT-TLS SASL client authentication might not be
   required unless NEA Server policy required an additional
   authentication of the NEA Client.  Therefore, the normal usage for
   the SASL messages is when the NEA Client acted as the TLS client and
   did not authenticate during the TLS handshake.

3.8.1.  SASL Client Authentication Requirements

   Implementations compliant with the PT-TLS specification MUST
   implement the PT-TLS client authentication messages described in this
   section.  These PT-TLS client authentication messages are capable of
   carrying a variety of SASL authentication mechanisms' exchanges.  In
   order to ensure interoperability, all PT-TLS implementations
   compliant with this specification MUST at least support the PLAIN
   SASL mechanism [RFC4616].  Similarly, implementations MUST provide
   the EXTERNAL SASL mechanism if both parties are authenticated during
   the TLS establishment.  In order to be able to take advantage of
   other strong, widely deployed authentication technologies such as
   Kerberos and support for channel bindings, implementations MAY

   include support for GS2 (the second Generic Security Service
   Application Program Interface (GSS-API) bridge for SASL) [RFC5801].
   GS2 includes negotiable support for channel binding for use with SASL
   (see Section 5 of RFC 5801).

   Implementations MUST also support the case where no client
   authentication is required.

3.8.2.  SASL in PT-TLS Overview

   Mechanism negotiation is initiated by the NEA Server sending the SASL
   Mechanisms TLV to the NEA Client to indicate the zero or more SASL
   mechanisms that the NEA Server's policy is willing to use with the
   NEA Client.  The NEA Client selects one SASL mechanism from the list
   and sends a SASL Mechanism Selection TLV completing the negotiation.
   Subsequent challenges and responses are carried within the SASL
   Authentication Data TLV carrying the authentication data for the
   selected mechanism.  The authentication outcome is communicated in a
   SASL Result TLV containing a status code.  If additional
   authentications are required, the NEA Server could trigger the next
   authentication by sending another SASL Mechanisms TLV after sending
   the SASL Result TLV for the current authentication mechanism.

3.8.3.  SASL Authentication Flow

   The SASL client authentication starts when the NEA Server enters the
   PT-TLS Negotiation phase and its policy indicates that an
   authentication of the NEA Client is necessary, such as if it was not
   performed during the TLS handshake protocol.  The NEA Server is
   responsible for triggering the client authentication by sending the
   SASL Mechanisms TLV to the NEA Client listing the set of SASL
   mechanisms the server is willing to use based upon its policy.

   The NEA Client selects a SASL mechanism from the list proposed by the
   NEA Server or sends a PT-TLS Invalid Message error code indicating
   that it is unable or unwilling to perform any of the mechanisms that
   were offered.  If the NEA Server receives a SASL Mechanism Selection
   TLV that contains an unacceptable SASL mechanism, the NEA Server
   would respond with a SASL Mechanism Error in a PT-TLS Error TLV.

   In situations where the NEA Server does not require a client
   authentication, the NEA Server MUST send a SASL Mechanisms TLV with
   no mechanisms included (only the PT-TLS header) indicating that the
   connection should transition to the PT-TLS Data Transport phase.  The
   same mechanism is employed to indicate that a SASL authentication
   already performed in this session is adequate to permit transition to
   the PT-TLS Data Transport phase.  So the NEA Server MUST always send

   a SASL Mechanisms TLV with no mechanisms as the last message in the
   PT-TLS Negotiation phase, and the NEA Client MUST NOT transition to
   the PT-TLS Data Transport phase until it receives such a message.

   If the NEA Server receives a NEA assessment message before the
   completion of the client authentication, the NEA Server MUST send an
   Authentication Required PT-TLS Error message indicating to the NEA
   Client that an authentication exchange is required prior to entering
   the PT-TLS Data Transport phase.

3.8.4.  Aborting SASL Authentication

   The NEA Server may abort the authentication exchange by sending the
   SASL Result TLV with a status code of Abort.  The NEA Client may
   abort the authentication exchange by sending a PT-TLS Error message
   with an Error Code of SASL Mechanism Error.

3.8.5.  Linkages to SASL Framework

3.8.5.1.  SASL Service Name

   The service name for PT-TLS is "nea-pt-tls".

3.8.5.2.  SASL Authorization Identity

   The PT-TLS protocol does not make use of a SASL authorization
   identity string as described in RFC 4422.

3.8.5.3.  SASL Security Layer

   The NEA PT-TLS protocol always runs under the protection of TLS.
   SASL security layers are not used and thus MUST be negotiated off
   during SASL authentication.

3.8.5.4.  Multiple Authentications

   Only one SASL mechanism authentication may be in progress at any one
   time.  Once a SASL mechanism completes (successfully or
   unsuccessfully), the NEA Server MAY trigger an additional
   authentication by sending a SASL Mechanisms TLV.

3.8.6.  SASL Channel Bindings

   SASL channel bindings are used to bind the SASL authentication to the
   outer TLS tunnel to ensure that the authenticating endpoints are the
   same as the TLS endpoints.  For SASL mechanisms that support channel
   bindings, the tls-unique value defined in RFC 5929 is carried by the
   SASL mechanism.  For most mechanisms, this means including the

   tls-unique value with the appropriate prefix defined in RFC 5929 in
   the application data portion of the SASL Mechanism channel-binding
   data.  If the validation of the channel binding fails, then the
   connection MUST be aborted.

3.8.7.  SASL Mechanisms

   This TLV is sent by the NEA Server to indicate the list of SASL
   mechanisms that it is willing and able to use to authenticate the NEA
   Client.  Each mechanism name consists of a length followed by a name.
   The total length of the list is determined by the TLV Length field.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Rsvd| Mech Len|            Mechanism Name (1-20 bytes)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Rsvd| Mech Len|            Mechanism Name (1-20 bytes)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      . . . . . . . . . . .                    |

   Rsvd (Reserved)

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Mech Len (Mechanism Name Length)

      The length of the Mechanism Name field in octets.

   Mechanism Name

      SASL mechanism name adhering to the rules defined in RFC 4422 with
      no padding.

3.8.8.  SASL Mechanism Selection

   This TLV is sent by the NEA Client in order to select a SASL
   mechanism for use on this session.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Rsvd| Mech Len|            Mechanism Name (1-20 bytes)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Optional Initial Mechanism Response              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Rsvd (Reserved)

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Mech Len (Mechanism Name Length)

      The length of the Mechanism Name field in octets.

   Mechanism Name

      SASL mechanism name adhering to the rules defined in RFC 4422.

   Optional Initial Mechanism Response

      Initial set of authentication information required from the NEA
      Client to kick-start the authentication.  This data is optional
      and if not provided would be solicited by the NEA Server in the
      first SASL Authentication Data TLV request.

3.8.9.  SASL Authentication Data

   This TLV carries an opaque (to PT-TLS) blob of octets being exchanged
   between the NEA Client and the NEA Server.  This TLV facilitates
   their communications without interpreting any of the bytes.  The SASL
   Authentication Data TLV MUST NOT be sent until a SASL mechanism has
   been established for a session.  The SASL Authentication Data TLV
   associated with the current authentication mechanism MUST NOT be sent
   after a SASL Result is sent with a Result Code of Success.
   Additional SASL Authentication Data TLVs would be sent if the PT-TLS
   Initiator and Responder desire a subsequent SASL authentication to
   occur but only after another SASL mechanism selection exchange
   occurs.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                SASL Mechanism Data (Variable Length)          ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   SASL Mechanism Data

      Opaque, variable-length set of bytes exchanged between the PT-TLS
      Initiator's SASL mechanism and its peer PT-TLS Responder's SASL
      mechanism.  These bytes MUST NOT be interpreted by the PT-TLS
      layer.

3.8.10.  SASL Result

   This TLV is sent by the NEA Server at the conclusion of the SASL
   exchange to indicate the authentication result.  Upon reception of a
   SASL Result TLV indicating an Abort, the NEA Client MUST terminate
   the current authentication conversation.  The recipient may retry the
   authentication in the event of an authentication failure.  Similarly,
   the NEA Server may request that additional SASL authentication(s) be
   performed after the completion of a SASL mechanism by sending another
   SASL Mechanisms TLV including any mechanisms dictated by its policy.

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Result Code         |    Optional Result Data       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      . . . . . . . . . . .                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Result Code

      This field contains the result of the SASL authentication
      exchange.

           Value (Name)                         Definition
      ---------------------  -------------------------------------------
      0 (Success)            SASL authentication was successful, and
                             identity was confirmed.

      1 (Failure)            SASL authentication failed.  This might be
                             caused by the client providing an invalid
                             user identity and/or credential pair.  Note
                             that this is not the same as a mechanism's
                             failure to process the authentication as
                             reported by the Mechanism Failure code.

      2 (Abort)              SASL authentication exchange was aborted by
                             the sender.

      3 (Mechanism Failure)  SASL "mechanism failure" during the
                             processing of the client's authentication
                             (e.g., not related to the user's input).
                             For example, this could occur if the
                             mechanism is unable to allocate memory
                             (e.g., malloc) that is needed to process a
                             received SASL mechanism message.

   Optional Result Data

      This field contains a variable-length set of additional data for a
      successful result.  This field MUST be zero length unless the NEA
      Server is returning a Result Code of Success and has more data to
      return.  For more information on the additional data with success
      in SASL, see RFC 4422.

3.9.  Error Message

   This section describes the format and contents of the PT-TLS Error
   message sent by the NEA Client or NEA Server when it detects a
   PT-TLS-level protocol error.  Each error message contains an error
   code indicating the error that occurred, followed by a copy of the
   message that caused the error.

   When a PT-TLS error is received, the recipient MUST NOT respond with
   a PT-TLS error, because this could result in an infinite loop of
   error messages being sent.  Instead, the recipient MAY log the error,
   modify its behavior to avoid future errors, ignore the error,
   terminate the assessment, or take other action as appropriate (as
   long as it is consistent with the requirements of this
   specification).

   The Message Value portion of a PT-TLS Error message contains the
   following information:

                          1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reserved   |               Error Code Vendor ID            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Error Code                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Copy of Original Message (Variable Length)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           . . . . . . .                       |

   Reserved

      Reserved for future use.  This field MUST be set to 0 on
      transmission and ignored upon reception.

   Error Code Vendor ID

      This field contains the IANA-assigned SMI Private Enterprise
      Number for the vendor whose Error Code namespace is being used in
      the message.  For IETF namespace Error Code values, this field
      MUST be set to zero (0).  For other vendor-defined Error Code
      namespaces, this field MUST be set to the SMI Private Enterprise
      Number of the vendor.

   Error Code

      This field contains the error code.  This error code exists within
      the scope of Error Code Vendor ID in this message.  Posture
      Transport Clients and Posture Transport Servers MUST NOT require
      support for particular vendor-specific PT-TLS Error Codes in order
      to interoperate with other PT-TLS-compliant implementations
      (although implementations MAY permit administrators to configure
      them to require support for specific PT-TLS Error Codes).

      When the Error Code Vendor ID is set to the IETF Private
      Enterprise Number, the following table lists the initial supported
      IETF-defined numeric error codes:

           Value (Name)                         Definition
      -------------------------  ---------------------------------------
      0 (Reserved)               Reserved value indicates that the
                                 PT-TLS Error message SHOULD be ignored
                                 by all recipients.  This MAY be used
                                 for debugging purposes to allow a
                                 sender to see a copy of the message
                                 that was received.

      1 (Malformed Message)      PT-TLS message unrecognized or
                                 unsupported.  This error code SHOULD be
                                 sent when the basic message content
                                 sanity test fails.  The sender of this
                                 error code MUST consider it a fatal
                                 error and abort the assessment.

      2 (Version Not Supported)  This error SHOULD be sent when a PT-TLS
                                 Responder receives a PT-TLS Version
                                 Request message containing a range of
                                 version numbers that doesn't include
                                 any version numbers that the recipient
                                 is willing and able to support on the
                                 session.  All PT-TLS messages carrying
                                 the Version Not Supported error code
                                 MUST use a version number of 1.  All

                                 parties that receive or send PT-TLS
                                 messages MUST be able to properly
                                 process an error message that meets
                                 this description, even if they cannot
                                 process any other aspect of PT-TLS
                                 version 1.  The sender and receiver of
                                 this error code MUST consider it a
                                 fatal error and close the TLS session
                                 after sending or receiving this PT-TLS
                                 message.

      3 (Type Not Supported)     PT-TLS Message Type unknown or not
                                 supported.  When a recipient receives a
                                 PT-TLS Message Type that it does not
                                 support, it MUST send back this error,
                                 ignore the message, and proceed.  For
                                 example, this could occur if the sender
                                 used a Vendor ID for the Message Type
                                 that is not supported by the recipient.
                                 This error message does not indicate
                                 that a fatal error has occurred, so the
                                 assessment is allowed to continue.

      4 (Invalid Message)        PT-TLS message received was invalid
                                 based on the protocol state.  For
                                 example, this error would be sent if a
                                 recipient receives a message associated
                                 with the PT-TLS Negotiation Phase (such
                                 as Version messages) after the protocol
                                 has reached the PT-TLS Data Transport
                                 Phase.  The sender and receiver of this
                                 error code MUST consider it a fatal
                                 error and close the TLS session after
                                 sending or receiving this PT-TLS
                                 message.

      5 (SASL Mechanism Error)   A fatal error occurred while trying to
                                 perform the client authentication.  For
                                 example, the NEA Client is unable to
                                 support any of the offered SASL
                                 mechanisms.  The sender and receiver of
                                 this error code MUST consider it a
                                 fatal error and close the TLS session
                                 after sending or receiving this PT-TLS
                                 message.

      6 (Invalid Parameter)      The PT-TLS Error message sender has
                                 received a message with an invalid or
                                 unsupported value in the PT-TLS header.
                                 This could occur if the NEA Client
                                 receives a PT-TLS message from the NEA
                                 Server with a Message Length of zero
                                 (see Section 3.5 for details).  The
                                 sender and receiver of this error code
                                 MUST consider it a fatal error and
                                 close the TLS session after sending or
                                 receiving this PT-TLS message.

   Copy of Original Message

      This variable-length value MUST contain a copy (up to 1024 bytes)
      of the original PT-TLS message that caused the error.  If the
      original message is longer than 1024 bytes, only the initial 1024
      bytes will be included in this field.  This field is included so
      the error recipient can determine which message sent caused the
      error.  In particular, the recipient can use the Message
      Identifier field from the Copy of Original Message data to
      determine which message caused the error.

4.  Security Considerations

   This section discusses the major threats potentially faced by each
   binding of the PT protocol and countermeasures provided by the PT-TLS
   protocol.

4.1.  Trust Relationships

   In order to understand where security countermeasures are necessary,
   this section starts with a discussion of where the NEA architecture
   envisions some trust relationships between the processing elements of
   the PT-TLS protocol.  Implementations or deployments where these
   trust relationships are not present would need to provide additional
   countermeasures to ensure the proper operation and security of PT-TLS
   (which relies upon these relationships to be trustworthy).  The
   following subsections discuss the trust properties associated with
   each portion of the NEA reference model directly involved with the
   processing of the PT-TLS protocol.

4.1.1.  Posture Transport Client

   The Posture Transport Client is trusted by the Posture Broker
   Client to:

   o  Not observe, fabricate, or alter the contents of the PB-TNC
      batches received from the network

   o  Not observe, fabricate, or alter the PB-TNC batches passed down
      from the Posture Broker Client for transmission on the network

   o  Transmit on the network any PB-TNC batches passed down from the
      Posture Broker Client

   o  Deliver properly security protected messages received from the
      network that are destined for the Posture Broker Client

   o  Provide configured security protections (e.g., authentication,
      integrity, and confidentiality) for the Posture Broker Client's
      PB-TNC batches sent on the network

   o  Expose the authenticated identity of the Posture Transport Server
      only to the PB-TNC layer within the NEA Client

   o  Verify the security protections placed upon messages received from
      the network to ensure that the messages are authentic and
      protected from attacks on the network

   o  Provide a secure, reliable, "in-order delivery", full-duplex
      transport for the Posture Broker Client's messages

   The Posture Transport Client is trusted by the Posture Transport
   Server to:

   o  Not send malicious traffic intending to harm (e.g., denial of
      service) the Posture Transport Server

   o  Not send malformed messages (e.g., messages lacking a PT-TLS
      header)

   o  Not send invalid or incorrect responses to messages (e.g., errors
      when no error is warranted)

   o  Not ignore or drop messages when such an action would cause issues
      for the protocol processing (e.g., dropping PT-TLS SASL
      Authentication Data messages)

   o  Verify the security protections placed upon messages received from
      the network to ensure that the messages are authentic and
      protected from attacks on the network

4.1.2.  Posture Transport Server

   The Posture Transport Server is trusted by the Posture Broker
   Server to:

   o  Not observe, fabricate, or alter the contents of the PB-TNC
      batches received from the network

   o  Not observe, fabricate, or alter the PB-TNC batches passed down
      from the Posture Broker Server for transmission on the network

   o  Transmit on the network any PB-TNC batches passed down from the
      Posture Broker Server

   o  Deliver properly security protected messages received from the
      network that are destined for the Posture Broker Server

   o  Provide configured security protections (e.g., authentication,
      integrity, and confidentiality) for the Posture Broker Server's
      messages sent on the network

   o  Expose the authenticated identity of the Posture Transport Client
      only to the PB-TNC layer within the NEA Server

   o  Verify the security protections placed upon messages received from
      the network to ensure that the messages are authentic and
      protected from attacks on the network

   o  Provide a secure, reliable, "in-order delivery", full-duplex
      transport for the Posture Broker Server's messages

   The Posture Transport Server is trusted by the Posture Transport
   Client to:

   o  Not send malicious traffic intending to harm (e.g., denial of
      service) the Posture Transport Client

   o  Not send malformed messages (e.g., messages lacking a PT-TLS
      header)

   o  Not send invalid or incorrect responses to messages (e.g., errors
      when no error is warranted)

   o  Not ignore or drop messages when such an action would cause issues
      for the protocol processing (e.g., dropping PT-TLS SASL Result
      messages)

   o  Verify the security protections placed upon messages received from
      the network to ensure that the messages are authentic and
      protected from attacks on the network

4.2.  Security Threats and Countermeasures

   Beyond the trust relationships assumed in Section 4.1, the PT-TLS
   protocol faces a number of potential security attacks that could
   require security countermeasures.

   Generally, the PT-TLS protocol is responsible for offering strong
   security protections for all of the NEA protocols, so any threats to
   its ability to protect NEA protocol messages could be very damaging
   to deployments.  Once the message is delivered to the Posture Broker
   Client or Posture Broker Server, the posture brokers are trusted to
   properly and safely process the messages.

4.2.1.  Message Theft

   When PT-TLS messages are sent over unprotected network links or
   spanning local software stacks that are not trusted, the contents of
   the messages may be subject to information theft by an intermediary
   party.  This theft could result in information being recorded for
   future use or analysis by the adversary.  Messages observed by
   eavesdroppers could contain information that exposes potential
   weaknesses in the security of the endpoint, or system fingerprinting
   information; this information would make it easier for the attacker
   to employ attacks more likely to be successful against the endpoint.
   The eavesdropper might also learn information about the endpoint or
   network policies that either singularly or collectively is considered
   sensitive information.  For example, if PT-TLS does not provide
   confidentiality protection, an adversary could observe the PA-TNC
   attributes included in the PT-TLS message and determine that the
   endpoint is lacking patches or that particular sub-networks have more
   lenient policies.

   In order to protect against NEA assessment message theft, the PT-TLS
   protocol provides strong cryptographic authentication, integrity, and
   confidentiality protection.  Deployers are strongly encouraged to
   employ "best practice of the day" TLS ciphers to ensure that the
   information remains safe despite advances in technology and
   discovered cipher weaknesses.  The use of bidirectional
   authentication of the assessment transport session ensures that only
   properly authenticated and authorized parties may be involved in an

   assessment dialog.  The PT-TLS protocol also provides strong
   cryptography for all of the PB-TNC and PA-TNC protocol messages
   traveling over the network, allowing the message contents to be
   hidden from potential theft by the adversary even if the attacker is
   able to observe the encrypted PT-TLS session.

4.2.2.  Message Fabrication

   Attackers on the network or present within the NEA system could
   introduce fabricated PT-TLS messages intending to trick or create a
   denial of service against aspects of an assessment.  For example, an
   adversary could attempt to insert into the message exchange fake
   PT-TLS Error Codes in order to disrupt communications.

   The PT-TLS protocol provides strong security protections for the
   complete message exchange over the network.  These security
   protections prevent an intermediary from being able to insert fake
   messages into the assessment.  In particular, TLS's use of hashing
   algorithms provides strong integrity protections that allow for
   detection of any changes in the content of the message stream.
   Additionally, adversaries are unable to observe the PT-TLS protocol
   exchanges because they are encrypted by the TLS ciphers and so would
   have difficulty determining where to insert the falsified message,
   since the attacker is unable to determine where the message
   boundaries exist.  Even if a successful message insertion did occur,
   the recipient would be able to detect it due to failure of the TLS
   cipher suite's integrity check.

4.2.3.  Message Modification

   This attack could allow an active attacker capable of intercepting a
   message to modify a PT-TLS message or transported PA-TNC attribute to
   a desired value to make it easier to compromise an endpoint.  Without
   the ability for message recipients to detect whether a received
   message contains the same content as what was originally sent, active
   attackers can stealthily modify the attribute exchange.

   The PT-TLS protocol leverages the TLS protocol to provide strong
   authentication and integrity protections as a countermeasure to this
   threat.  The bidirectional authentication prevents the attacker from
   acting as an active man-in-the-middle to the protocol that could be
   used to modify the message exchange.  The strong integrity protection
   (e.g., hashing) offered by TLS allows PT-TLS message recipients to
   detect message alterations by other types of network-based
   adversaries.

4.2.4.  Denial of Service

   A variety of types of denial-of-service attacks are possible against
   the PT-TLS protocol if the message exchanges are left unprotected
   while traveling over the network.  The Posture Transport Client and
   Posture Transport Server are trusted not to participate in the denial
   of service of the assessment session, leaving the threats to come
   from the network.

   The PT-TLS protocol provides bidirectional authentication
   capabilities in order to prevent a man-in-the-middle on the network
   from becoming an undetected active proxy of PT-TLS messages.  Because
   the PT-TLS protocol runs after the TLS handshake and thus cipher
   establishment/use, all of the PT-TLS messages are protected from
   undetected modification that could create a denial-of-service
   situation.  However, it is possible for an adversary to alter the
   message flows, causing each message to be rejected by the recipient
   because it fails the integrity checking.

   The PT-TLS protocol operates as an application protocol on top of TLS
   and thus TCP/IP protocols, so is subject to denial-of-service attacks
   against the TLS, TCP, and IP protocols.

4.2.5.  NEA Asokan Attacks

   As described in Section 3.3 and in [RFC6813], a sophisticated MITM
   attack can be mounted against NEA systems.  The attacker forwards
   PA-TNC messages from a healthy machine through an unhealthy one so
   that the unhealthy machine can gain network access.  Section 3.3 and
   [RFC6813] provide a detailed description of this attack and of the
   countermeasures that can be employed against it.

   Because lying endpoint attacks are much easier than Asokan attacks
   and the only known effective countermeasure against lying endpoint
   attacks is the use of an External Measurement Agent (EMA),
   countermeasures against an Asokan attack are not necessary unless an
   EMA is in use.  However, PT-TLS implementers may not know whether an
   EMA will be used with their implementation.  Therefore, PT-TLS
   implementers SHOULD support the Asokan attack countermeasures
   described in Section 3.3 by providing the value of the tls-unique
   channel binding to higher layers in the NEA reference model: Posture
   Broker Clients, Posture Broker Servers, Posture Collectors, and
   Posture Validators.

   The Asokan attack can also apply to authentication mechanisms carried
   within TLS.  SASL mechanisms providing channel bindings use the
   tls-unique channel-binding data as defined in Section 3.3 to protect
   against the attack.

4.2.6.  Trust Anchors

   The TLS protocol bases its trust decision about the signer of the
   certificates received during the TLS authentication on using a set of
   trust anchor certificates.  It is essential that these trust anchor
   certificates are integrity protected from unauthorized modification.
   Many common software components (e.g., browsers, operating systems,
   security protocols) include a set of trust anchor certificates that
   are relevant to their operation.  The PT-TLS SHOULD use a PT-TLS-
   specific set of trust anchor certificates in order to limit what
   Certificate Authorities are authorized to issue certificates for use
   with NEA.

5.  Privacy Considerations

   The role of PT-TLS is to act as a secure transport for PB-TNC and
   other higher-layer protocols.  As such, PT-TLS does not directly
   utilize personally identifiable information (PII) except when client
   authentication is enabled.  When client authentication is being used,
   the NEA Client will be asked to use SASL, which may disclose a local
   identifier (e.g., username) associated with the endpoint and an
   authenticator (e.g., password) to authenticate that identity.
   Because the identity and authenticator are potentially privacy-
   sensitive information, the NEA Client MUST offer a mechanism to
   restrict which NEA Servers will be sent this information.  Similarly,
   the NEA Client SHOULD provide an indication to the person being
   identified that a request for their identity has been made in case
   they choose to opt out of the authentication to remain anonymous
   unless no user interface is available.  PT-TLS provides cryptographic
   peer authentication, message integrity, and data confidentiality
   protections to higher-layer NEA protocols that may exchange data
   potentially including PII.  These security services can be used to
   protect any PII involved in an assessment from passive and active
   attackers on the network.  Endpoints sending potentially privacy-
   sensitive information SHOULD ensure that the PT-TLS security
   protections (TLS cipher suites) negotiated for an assessment of the
   endpoint are adequate to avoid interception and off-line attacks of
   any long-term privacy-sensitive information unless other network
   protections are already present.

6.  IANA Considerations

   Per this specification, two new IANA registries have been created and
   a TCP port number has been assigned.  IANA has permanently reserved
   the early allocated TCP port number 271 for use with the PT-TLS
   protocol.

   This section defines the contents of two new IANA registries, PT-TLS
   Message Types and PT-TLS Error Codes, and explains how these
   registries work.

   Each of the registries defined in this document support IETF-defined
   values and vendor-defined values.  To explain this phenomenon, we
   will use the PT-TLS Message Type as an example, but the other
   registry works the same way.

   Whenever a PT-TLS Message Type appears on a network, it is always
   accompanied by an SMI Private Enterprise Number (PEN), also known as
   a vendor ID.  If this vendor ID is zero, the accompanying PT-TLS
   Message Type is an IETF namespace value listed in the IANA registry
   for PT-TLS Message Types, and its meaning is defined in the
   specification listed for that PT-TLS Message Type in that registry.
   If the vendor ID is not zero, the meaning of the PT-TLS Message Type
   is defined by the vendor identified by the vendor ID (as listed in
   the IANA registry for SMI PENs).  The identified vendor is encouraged
   but not required to register with IANA some or all of the PT-TLS
   Message Types used with their vendor ID and publish a specification
   for each of these values.

6.1.  Designated Expert Guidelines

   For each of the IANA registries defined by this specification, new
   values are added to the registry by following the IANA Specification
   Required policy [RFC5226].

   This section provides guidance to designated experts so that they may
   make decisions using a philosophy appropriate for these registries.

   The registries defined in this document have plenty of values.  In
   most cases, the IETF has approximately 2^32 values available for it
   to define, and each vendor has the same number of values for its use.
   Because there are so many values available, designated experts should
   not be terribly concerned about exhausting the set of values.

   Instead, designated experts should focus on the following
   requirements.  All values in these IANA registries are required to be
   documented in a specification that is permanently and publicly
   available.  IETF namespace values must also be useful not harmful to
   the Internet, and defined in a manner that is clear and likely to
   ensure interoperability.

   Designated experts should encourage vendors to avoid defining similar
   but incompatible values and instead agree on a single IETF-reviewed
   approach and value.  However, it is beneficial to document existing
   practice.

   There are several ways to ensure that a specification is permanently
   and publicly available.  It may be published as an RFC.
   Alternatively, it may be published in another manner that makes it
   freely available to anyone.  However, in this latter case, the vendor
   will need to supply a copy to the IANA and authorize the IANA to
   archive this copy and make it freely available to all if at some
   point the document becomes no longer freely available to all through
   other channels.

   The following two sections provide guidance to the IANA in creating
   and managing the new IANA registries defined by this specification.

6.2.  Registry for PT-TLS Message Types

   The name for this registry is "PT-TLS Message Types".  Each entry in
   this registry should include a human-readable name, an SMI Private
   Enterprise Number, a decimal integer value between 0 and 4294967294,
   and a reference to the specification where the contents of this
   message type are defined.  This specification must define the meaning
   of the PT-TLS Message Type and the format and semantics of the PT-TLS
   Message Value field that include the designated Private Enterprise
   Number in the PT-TLS Message Type Vendor ID field and the designated
   numeric value in the PT-TLS Message Type field.

   The following entries for this registry are defined in this document.
   Additional entries to this registry are added by following the IANA
   Specification Required policy, consistent with the guidelines in
   Section 6.1.

   PEN   Value                 Name             Reference
   ---  --------      ------------------------  ---------
    0      0          Experimental              RFC 6876
    0      1          Version Request           RFC 6876
    0      2          Version Response          RFC 6876
    0      3          SASL Mechanisms           RFC 6876
    0      4          SASL Mechanism Selection  RFC 6876
    0      5          SASL Authentication Data  RFC 6876
    0      6          SASL Result               RFC 6876
    0      7          PB-TNC Batch              RFC 6876
    0      8          PT-TLS Error              RFC 6876
    0   9-4294967294  Unassigned
    0   4294967295    Reserved                  RFC 6876

   The PEN 0 (IETF) PT-TLS Message Type values between 9 and 4294967294
   inclusive are allocated for future assignment by the IANA.

6.3.  Registry for PT-TLS Error Codes

   The name for this registry is "PT-TLS Error Codes".  Each entry in
   this registry should include a human-readable name, an SMI Private
   Enterprise Number, a decimal integer value between 0 and 4294967295,
   and a reference to the specification where this error code is
   defined.  This specification must define the meaning of this error
   code, a PT-TLS Message Type of PT-TLS Error, the designated Private
   Enterprise Number in the PT-TLS Error Code Vendor ID field, and the
   designated numeric value in the PT-TLS Error Code field.

   The following entries for this registry are defined in this document.
   Additional entries to this registry are added following the IANA
   Specification Required policy, consistent with the guidelines in
   Section 6.1.

     PEN     Value             Name            Reference
     ---  ------------  ---------------------  ---------
      0      0          Reserved               RFC 6876
      0      1          Malformed Message      RFC 6876
      0      2          Version Not Supported  RFC 6876
      0      3          Type Not Supported     RFC 6876
      0      4          Invalid Message        RFC 6876
      0      5          SASL Mechanism Error   RFC 6876
      0      6          Invalid Parameter      RFC 6876
      0   7-4294967295  Unassigned

   The PEN 0 (IETF) PT-TLS Error Codes between 7 and 4294967295
   inclusive are allocated for future assignment by the IANA.

7.  Acknowledgments

   Thanks to the Trusted Computing Group for contributing the initial
   text upon which this document was based [IFT-TLS].

   The authors of this document would also like to acknowledge the
   following people who have contributed to or provided substantial
   input on the preparation of this document or predecessors to it: Syam
   Appala, Stuart Bailey, Lauren Giroux, Steve Hanna, Josh Howlett,
   Scott Kelly, Carolin Latze, Sung Lee, Lisa Lorenzin, Alexey Melnikov,
   Ravi Sahita, Subbu Srinivasan, Susan Thomson, and Mark Townsend.

8.  References

8.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4422]   Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
               Authentication and Security Layer (SASL)", RFC 4422,
               June 2006.

   [RFC4616]   Zeilenga, K., Ed., "The PLAIN Simple Authentication and
               Security Layer (SASL) Mechanism", RFC 4616, August 2006.

   [RFC5226]   Narten, T. and H. Alvestrand, "Guidelines for Writing an
               IANA Considerations Section in RFCs", BCP 26, RFC 5226,
               May 2008.

   [RFC5246]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
               Housley, R., and W. Polk, "Internet X.509 Public Key
               Infrastructure Certificate and Certificate Revocation
               List (CRL) Profile", RFC 5280, May 2008.

   [RFC5746]   Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
               "Transport Layer Security (TLS) Renegotiation Indication
               Extension", RFC 5746, February 2010.

   [RFC5792]   Sangster, P. and K. Narayan, "PA-TNC: A Posture Attribute
               (PA) Protocol Compatible with Trusted Network Connect
               (TNC)", RFC 5792, March 2010.

   [RFC5793]   Sahita, R., Hanna, S., Hurst, R., and K. Narayan,
               "PB-TNC: A Posture Broker (PB) Protocol Compatible with
               Trusted Network Connect (TNC)", RFC 5793, March 2010.

   [RFC5929]   Altman, J., Williams, N., and L. Zhu, "Channel Bindings
               for TLS", RFC 5929, July 2010.

   [RFC6125]   Saint-Andre, P. and J. Hodges, "Representation and
               Verification of Domain-Based Application Service Identity
               within Internet Public Key Infrastructure Using X.509
               (PKIX) Certificates in the Context of Transport Layer
               Security (TLS)", RFC 6125, March 2011.

   [RFC6520]   Seggelmann, R., Tuexen, M., and M. Williams, "Transport
               Layer Security (TLS) and Datagram Transport Layer
               Security (DTLS) Heartbeat Extension", RFC 6520,
               February 2012.

8.2.  Informative References

   [IFT-TLS]   Trusted Computing Group, "TNC IF-T: Binding to TLS",
               <http://www.trustedcomputinggroup.org/>, May 2009.

   [PEN]       IANA Private Enterprise Numbers (PEN) registry,
               <http://www.iana.org/assignments/enterprise-numbers>.

   [PT-EAP]    Cam-Winget, N. and P. Sangster, "PT-EAP: Posture
               Transport (PT) Protocol For EAP Tunnel Methods", Work in
               Progress, January 2013.

   [RFC2246]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
               RFC 2246, January 1999.

   [RFC4346]   Dierks, T. and E. Rescorla, "The Transport Layer Security
               (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC5209]   Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
               Tardo, "Network Endpoint Assessment (NEA): Overview and
               Requirements", RFC 5209, June 2008.

   [RFC5801]   Josefsson, S. and N. Williams, "Using Generic Security
               Service Application Program Interface (GSS-API)
               Mechanisms in Simple Authentication and Security Layer
               (SASL): The GS2 Mechanism Family", RFC 5801, July 2010.

   [RFC6813]   Salowey, J. and S. Hanna, "The Network Endpoint
               Assessment (NEA) Asokan Attack Analysis", RFC 6813,
               December 2012.

Authors' Addresses

   Paul Sangster
   Symantec Corporation
   6825 Citrine Dr.
   Carlsbad, CA  92009

   EMail: paul_sangster@symantec.com

   Nancy Cam-Winget
   Cisco Systems
   80 West Tasman Drive
   San Jose, CA  95134
   US

   EMail: ncamwing@cisco.com

   Joseph Salowey
   Cisco Systems
   2901 Third Avenue
   Seattle, WA  98121
   US

   EMail: jsalowey@cisco.com

 

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