Internet Engineering Task Force (IETF) N. Cam-Winget Request for Comments: 7171 Cisco Systems Category: Standards Track P. Sangster ISSN: 2070-1721 Symantec Corporation May 2014
PT-EAP: Posture Transport (PT) Protocol for Extensible Authentication Protocol (EAP) Tunnel Methods
This document specifies PT-EAP, a Posture Transport (PT) protocol based on the Extensible Authentication Protocol (EAP) and designed to be used only inside an EAP tunnel method protected by Transport Layer Security (TLS). The document also describes the intended applicability of PT-EAP.
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.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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This document specifies PT-EAP, a Posture Transport (PT) protocol protected by a TLS-protected EAP tunnel method. The PT protocol in the Network Endpoint Assessment (NEA) architecture is responsible for transporting Posture Broker (PB-TNC [RFC5793]) batches, often containing Posture Attributes (PA-TNC [RFC5792]), across the network between the NEA Client and NEA Server. The PT-EAP protocol must be protected by an outer TLS-based EAP tunnel method to ensure the exchanged messages are protected from a variety of threats from hostile intermediaries.
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NEA protocols are intended to be used both for pre-admission assessment of endpoints joining the network and assessment of endpoints already present on the network. In order to support both usage models, two types of PT protocols are needed. One type of PT, PT-TLS [RFC6876], operates after the endpoint has an assigned IP address, layering on top of the IP protocol to carry a NEA exchange. The other type of PT operates before the endpoint gains any access to the IP network. This specification defines PT-EAP, the PT protocol used to assess endpoints before they gain access to the network.
PT-EAP is an inner EAP [RFC3748] method designed to be used inside a protected tunnel such as Tunnel EAP (TEAP) [RFC7170], EAP Flexible Authentication via Secure Tunneling (EAP-FAST) [RFC4851], or EAP Tunneled Transport Layer Security (EAP-TTLS) [RFC5281]. That is, an outer EAP method is typically a TLS-based EAP method that first establishes a protected tunnel by which other conversations, such as other EAP methods (e.g., "inner" EAP methods) can ensue under the tunnel protection.
This document does not define an architecture or reference model. Instead, it defines a protocol that works within the reference model described in the NEA Requirements specification [RFC5209]. The reader is assumed to be thoroughly familiar with that document.
This specification defines the syntax of PT-EAP 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.
Descriptions of 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.
This document reuses many terms defined in the NEA Requirements document [RFC5209], such as "Posture Transport Client" and "Posture Transport Server". The reader is assumed to have read that document and understood it.
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When defining the PT-EAP method, this specification does not use the terms "EAP peer" and "EAP authenticator". Instead, it uses the terms "NEA Client" and "NEA Server" since those are considered to be more familiar to NEA WG participants. However, these terms are equivalent for the purposes of this specification. The part of the NEA Client that terminates PT-EAP (generally in the Posture Transport Client) is the EAP peer for PT-EAP. The part of the NEA Server that terminates PT-EAP (generally in the Posture Transport Server) is the EAP authenticator for PT-EAP.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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.
PT-EAP is designed to encapsulate PB-TNC batches in a simple EAP method that can be carried within EAP tunnel methods. The EAP tunnel methods provide confidentiality and message integrity, so PT-EAP does not have to do so. Therefore, PT-EAP MUST be used inside a TLS-based EAP tunnel method that provides strong cryptographic authentication (possibly server only), message integrity, and confidentiality services.
The PT-EAP protocol operates between a Posture Transport Client and a Posture Transport Server, allowing them to send PB-TNC batches to each other over an EAP tunnel method. When PT-EAP is used, the Posture Transport Client in the NEA reference model acts as an EAP peer (terminating the PT-EAP method on the endpoint), and the Posture Transport Server acts as an EAP authenticator (terminating the PT-EAP method on the NEA Server).
This section describes and defines the PT-EAP method. First, it provides a protocol overview. Second, it describes specific features like version negotiation. Third, it gives a detailed packet
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description. Finally, it describes how the tls-unique channel binding [RFC5929] may be used to bind PA-TNC exchanges to the EAP tunnel method, defeating man-in-the-middle (MITM) attacks such as the Asokan attack [Asokan].
PT-EAP has two phases that follow each other in strict sequence: negotiation and data transport.
The PT-EAP method begins with the negotiation phase. The NEA Server starts this phase by sending a PT-EAP Start message: an EAP Request message of type PT-EAP with the S (Start) flag set. The NEA Server also sets the Version field as described in Section 3.2. This is the only message in the negotiation phase.
The data transport phase is the only phase of PT-EAP where PB-TNC batches are allowed to be exchanged. This phase always starts with the NEA Client sending a PB-TNC batch to the NEA Server. The NEA Client and NEA Server then take turns sending a PB-TNC batch. The data transport phase always ends with an EAP Response message from the NEA Client to the NEA Server. The Data field of this message may have zero length if the NEA Server has just sent the last PB-TNC batch in the PB-TNC exchange.
Note that the success of PT-EAP does not mean the overall authentication (using the outer EAP tunnel method) will succeed. Neither does the failure of PT-EAP mean that the overall authentication will fail. Success of the overall authentication depends on the policy configured by the administrator.
At the end of the PT-EAP method, the NEA Server will indicate success or failure to the EAP tunnel method. Some EAP tunnel methods may provide explicit confirmation of inner method success; others may not. This is out of scope for the PT-EAP method specification. Successful completion of PT-EAP does not imply successful completion of the overall authentication nor does PT-EAP failure imply overall failure. This depends on the administrative policy in place.
The NEA Server and NEA Client may engage in an abnormal termination of the PT-EAP exchange at any time by simply stopping the exchange. This may also require terminating the EAP tunnel method, depending on the capabilities of the EAP tunnel method.
PT-EAP version negotiation takes place in the first PT-EAP message sent by the NEA Server (the Start message) and the first PT-EAP message sent by the NEA Client (the response to the Start message). The NEA Server MUST set the Version field in the Start message to the maximum PT-EAP version that the NEA Server supports and is willing to accept.
The NEA Client chooses the PT-EAP version to be used for the exchange and places this value in the Version field in its response to the Start message. The NEA Client SHOULD choose the value sent by the NEA Server if the NEA Client supports it. However, the NEA Client MAY set the Version field to a value less than the value sent by the NEA Server (for example, if the NEA Client only supports lesser PT-EAP versions). If the NEA Client only supports PT-EAP versions greater than the value sent by the NEA Server, the NEA Client MUST abnormally terminate the EAP negotiation.
If the version sent by the NEA Client is not acceptable to the NEA Server, the NEA Server MUST terminate the PT-EAP session immediately. Otherwise, the version sent by the NEA Client is the version of PT-EAP that MUST be used. Both the NEA Client and the NEA Server MUST set the Version field to the chosen version number in all subsequent PT-EAP messages in this exchange.
This specification defines version 1 of PT-EAP. Version 0 is reserved and MUST never be sent. New versions of PT-EAP (values 2-7) may be defined by Standards Action, as defined in [RFC5226].
This section provides a detailed description of the fields in a PT-EAP message. For a description of the diagram conventions used here, see Section 1.2. Since PT-EAP is an EAP method, the first four fields (e.g., Code, Identifier, Length, and Type as shown in Figure 1) in each message are mandated by and defined in EAP. The other fields, e.g., Flags, Version, and Data are specific to PT-EAP.
This flag MUST be set to 0 and ignored upon receipt.
This field is used for version negotiation, as described in Section 3.2.
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Variable length data. This field is processed by the PB layer and MUST include PB-TNC messages. For more information see PB-TNC [RFC5793].
The length of the Data field in a particular PT-EAP message may be determined by subtracting the length of the PT-EAP header fields from the value of the two-octet Length field.
3.4. Preventing MITM Attacks with Channel Bindings
As described in the NEA Asokan Attack Analysis [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. 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, it is necessary for the Posture Broker on an EMA-equipped endpoint to pass the tls-unique channel binding [RFC5929] from PT-EAP's tunnel method to the EMA. This value can then be included in the EMA's attestation so that the Posture Validator responsible may then confirm that the value matches the tls-unique channel binding for its end of the tunnel. If the tls-unique values of the NEA Client and NEA Server match and this is confirmed by the EMA, then the posture sent by a trustworthy EMA (and thus the 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 MITM is forwarding posture. If they differ, an attack has been detected, and the Posture Validator SHOULD fail its verification.
Note that tls-unique, as opposed to invoking a mutual cryptographic binding, is used as there is no keying material being generated by PT-EAP (the method is defined to facilitate the transport of posture data and is not an authentication method). However, the NEA Client may host an EMA that can be used as the means to cryptographically bind the tls-unique content that may be validated by the Posture Validator interfacing with the EAP Server. The binding of the tls-unique to the client authentication prevents the client's message from being used in another context. This prevents a poorly configured client from unintentionally compromising the NEA system. Strong mutual authentication of the NEA Server and Client is still REQUIRED to prevent the disclosure of possibly sensitive NEA Client information to an attacker.
This section discusses the major threats and countermeasures provided by PT-EAP. As discussed throughout the document, the PT-EAP method is designed to run inside an EAP tunnel method that is capable of protecting the PT-EAP protocol from many threats. Since the EAP tunnel method will be specified separately, this section describes the considerations on the EAP tunnel method but does not evaluate its ability to meet those requirements. The security considerations and requirements for NEA can be found in [RFC5209].
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-EAP protocol. The following sub-sections discuss the trust properties associated with each portion of the NEA reference model directly involved with the processing of the PT-EAP protocol flowing inside an EAP tunnel.
Beyond the trusted relationships assumed in Section 4.1, the PT-EAP EAP method faces a number of potential security attacks that could require security countermeasures.
Generally, the PT protocol is responsible for providing strong security protections for all of the NEA protocols so any threats to PT's ability to protect NEA protocol messages could be very damaging
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to deployments. For the PT-EAP method, most of the cryptographic security is provided by the outer EAP tunnel method, and PT-EAP is encapsulated within the protected tunnel. Therefore, this section highlights the cryptographic requirements that need to be met by the EAP tunnel method carrying PT-EAP in order to meet the NEA PT requirements.
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.
When PT-EAP messages are sent over unprotected network links or span 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 an adversary. Messages observed by eavesdroppers could contain information that exposes potential weaknesses in the security of the endpoint or system fingerprinting information easing the ability of 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-EAP is carried by an EAP tunnel method that does not provide confidentiality protection, an adversary could observe the PA-TNC attributes included in the PB-TNC batch 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 EAP tunnel method carrying PT-EAP must provide strong cryptographic authentication, integrity, and confidentiality protection. The use of bidirectional authentication in the EAP tunnel method carrying PT-EAP ensures that only properly authenticated and authorized parties may be involved in an assessment message exchange. When PT-EAP is carried within a cryptographically protected EAP tunnel method like EAP-FAST or EAP-TTLS, all of the contents of PB-TNC and PA-TNC protocol messages are hidden from potential theft by intermediaries lurking on the network.
Attackers on the network or present within the NEA system could introduce fabricated PT-EAP messages intending to trick or create a denial of service against aspects of an assessment. For example, an adversary could attempt to insert a PT-EAP message to tell a NEA Server that the endpoint is totally infected. This could cause the
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device to be blocked from accessing a critical resource, which would be a denial of service.
The EAP tunnel method carrying a PT-EAP method needs to provide 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. See Section 5 for more details on the EAP tunnel requirements.
This attack could allow an active attacker capable of intercepting a message to modify a PT-EAP message or transported PA-TNC attribute to a desired value to ease the compromise of 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.
PT-EAP leverages the EAP tunnel method (e.g., TEAP, EAP-FAST, or EAP- TTLS) to provide strong authentication and integrity protections as a countermeasure to this threat. The bidirectional authentication prevents the attacker from acting as an active MITM to the protocol that could be used to modify the message exchange. The strong integrity protections offered by the TLS-based EAP tunnel method allow the PT-EAP message recipients to detect message alterations by other types of network-based adversaries. Because PT-EAP does not itself provide explicit integrity protection for the PT-EAP payload, an EAP tunnel method that offers strong integrity protection is needed to mitigate this threat.
A variety of types of denial-of-service attacks are possible against PT-EAP if the message exchange is 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-EAP method primarily relies on the outer EAP tunnel method to provide strong authentication (at least of one party), and deployers are expected to leverage other EAP methods to authenticate the other party (typically the client) within the protected tunnel. The use of a protected bidirectional authentication will prevent unauthorized parties from participating in a PT-EAP exchange.
After the cryptographic authentication by the EAP tunnel method, the session can be protected cryptographically to provide confidentiality and source authenticity. Such protection prevents undetected
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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.
As described in Section 3.4 and in the NEA Asokan Attack Analysis [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 3.4 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 an 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-EAP implementers may not know whether an EMA will be used with their implementation. Therefore, PT-EAP implementers SHOULD support these countermeasures 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. If the tls-unique channel binding is implemented, it must be verified before any other attestations are evaluated.
This section discusses how PT-EAP is used within various EAP tunnel methods to meet the PT requirements in Section 5.
TEAP [RFC7170], EAP-FAST [RFC4851], and EAP-TTLS [RFC5281] make use of TLS [RFC5246] to protect the transport of information between the NEA Client and NEA Server. Each of these EAP tunnel methods has two phases. In the first phase, a TLS tunnel is established between the NEA Client and NEA Server. In the second phase, the tunnel is used to pass other information. PT-EAP requires that establishing this tunnel include at least an authentication of the NEA Server by the NEA Client.
The phase two dialog may include authentication of the user by doing other EAP methods or, in the case of EAP-TTLS, by using EAP or non- EAP authentication dialogs. PT-EAP is also carried by the phase two tunnel, allowing the NEA assessment to be within an encrypted and integrity-protected transport.
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With all these methods (e.g., TEAP [RFC7170], EAP-FAST [RFC4851], and EAP-TTLS [RFC5281]), a cryptographic key is derived from the authentication that may be used to secure later transmissions. Each of these methods employs at least a NEA Server authentication using an X.509 certificate. Within each EAP tunnel method will exist a set of inner EAP methods. These inner methods may perform additional security handshakes including more granular authentications or exchanges of integrity information (such as PT-EAP). At some point after the conclusion of each inner EAP method, some of the methods will export the established secret keys to the outer tunnel method. It's expected that the outer method will cryptographically mix these keys into any keys it is currently using to protect the session and perform a final operation to determine whether both parties have arrived at the same mixed key. This cryptographic binding of the inner method results to the outer method's keys is essential for detection of conventional (non-NEA) Asokan attacks.
TEAP [RFC7170] is the mandatory-to-implement EAP tunnel method.
This section summarizes the security claims for this specification, as required by [RFC3748], Section 7.2:
Auth. mechanism: None Ciphersuite negotiation: No Mutual authentication: No Integrity protection: No Replay protection: No Confidentiality: No Key derivation: No Key strength: N/A Dictionary attack resistant: N/A Fast reconnect: No Crypt. binding: N/A Session independence: N/A Fragmentation: No Channel binding: No
Because the PT-EAP inner method described in this specification relies on the outer EAP tunnel method for a majority of its security protections, this section reiterates the PT requirements that MUST be met by the IETF standard EAP tunnel method for use with PT-EAP.
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TEAP [RFC7170] is a Standards Track EAP tunnel method that satisfies NEA's requirements and is the mandatory-to-implement EAP tunnel method.
The security requirements described in this specification MUST be implemented in any product claiming to be PT-EAP compliant. The decision of whether a particular deployment chooses to use these protections is a deployment issue. A customer may choose to avoid potential deployment issues or performance penalties associated with the use of cryptography when the required protection has been achieved through other mechanisms (e.g., physical isolation). If security mechanisms may be deactivated by policy, an implementation SHOULD offer an interface to query how a message will be (or was) protected by PT so higher-layer NEA protocols can factor this into their decisions.
RFC 5209 [RFC5209] includes the following requirement that is to be applied during the selection of the EAP tunnel method(s) used in conjunction with PT-EAP:
PT-2: The PT protocol MUST be capable of supporting mutual authentication, integrity, confidentiality, and replay protection of the PB messages between the Posture Transport Client and the Posture Transport Server.
Note that mutual authentication could be achieved by a combination of a strong authentication of one party (e.g., server authentication while establishing the TLS-based tunnel) by the EAP tunnel method in conjunction with a second authentication of the other party (e.g., client authentication inside the protected tunnel) by another EAP method running prior to PT-EAP.
Having the Posture Transport Client always authenticate the Posture Transport Server provides assurance to the NEA Client that the NEA Server is authentic (not a rogue or MITM) prior to disclosing secret or potentially privacy-sensitive information about what is running or configured on the endpoint. However, the NEA Server's policy may allow for the delay of the authentication of the NEA Client until a suitable protected channel has been established allowing for non- cryptographic NEA Client credentials (e.g., username/password) to be used. Whether the communication channel is established with mutual or server-side-only authentication, the resulting channel needs to provide strong integrity and confidentiality protection to its contents. These protections are to be bound to at least the authentication of the NEA Server by the NEA Client, so the session is cryptographically bound to a particular authentication event.
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The EAP tunnel method carrying PT-EAP MUST provide strong cryptographic authentication, integrity, and confidentiality protection to protect against NEA assessment message theft as described in Section 4.2.1. The cryptographically protected EAP tunnel ensures that all of the PA-TNC and PB-TNC protocol messages are hidden from intermediaries wanting to steal NEA data.
To support countermeasures against NEA Asokan attacks as described in Section 3.4, the EAP tunnel method used with PT-EAP will need to support generation of the tls-unique value to be used with the higher layers of the NEA reference model. This should not be a high bar since all EAP tunnel methods currently support this, but not all implementations of those methods may do so.
The role of PT-EAP is to act as a secure transport for PB-TNC over a network before the endpoint has been admitted to the network. As a transport protocol, PT-EAP does not directly utilize or require direct knowledge of any personally identifiable information (PII). PT-EAP will typically be used in conjunction with other EAP methods that provide for the user authentication (if bidirectional authentication is used), so the user's credentials are not directly seen by the PT-EAP inner method.
While PT-EAP does not provide cryptographic protection for the PB-TNC batches, it is designed to operate within an EAP tunnel method that provides strong authentication, integrity, and confidentiality services. Therefore, it is important for deployers to leverage these protections in order to prevent disclosure of PII potentially contained within PA-TNC or PB-TNC within the PT-EAP payload.
This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the PT-EAP protocol, in accordance with BCP 26 [RFC5226].
The EAP Method type for PT-EAP has been assigned value 54, i.e., the assignment for Type in Section 3.3.
+-------+----------------------------+-----------+ | Value | Description | Reference | +-------+----------------------------+-----------+ | 54 | EAP Method Type for PT-EAP | [RFC7171] | +-------+----------------------------+-----------+
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This document also defines one new IANA top-level registry: "PT-EAP Versions". This section explains how this registry works. Because only eight (8) values are available in this registry, a high bar is set for new assignments. The only way to register new values in this registry is through Standards Action (via an approved Standards Track RFC).
The name for this registry is "PT-EAP Versions". Each entry in this registry includes a decimal integer value between 1 and 7 identifying the version and also includes a reference to the RFC where the version is defined.
The following entries are defined in this document and are the initial entries in the registry. Additional entries to this registry are added by Standards Action, as defined in RFC 5226 [RFC5226].
Thanks to the Trusted Computing Group for contributing the initial text upon which this document was based.
The authors of this document would like to acknowledge the following people who have contributed to or provided substantial input on the preparation of this document or predecessors to it: Amit Agarwal, Morteza Ansari, Diana Arroyo, Stuart Bailey, Boris Balacheff, Uri Blumenthal, Gene Chang, Scott Cochrane, Pasi Eronen, Aman Garg, Sandilya Garimella, David Grawrock, Stephen Hanna, Thomas Hardjono, Chris Hessing, Ryan Hurst, Hidenobu Ito, John Jerrim, Meenakshi Kaushik, Greg Kazmierczak, Scott Kelly, Bryan Kingsford, PJ Kirner, Sung Lee, Lisa Lorenzin, Mahalingam Mani, Bipin Mistry, Seiji Munetoh, Rod Murchison, Barbara Nelson, Kazuaki Nimura, Ron Pon, Ivan Pulleyn, Alex Romanyuk, Ravi Sahita, Joseph Salowey, Chris Salter, Mauricio Sanchez, Dean Sheffield, Curtis Simonson, Jeff Six, Ned Smith, Michelle Sommerstad, Joseph Tardo, Lee Terrell, Susan Thomson, Chris Trytten, and John Vollbrecht.