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IETF RFC 5793
PB-TNC: A Posture Broker (PB) Protocol Compatible with Trusted Network Connect (TNC)
Last modified on Tuesday, March 9th, 2010
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Internet Engineering Task Force (IETF) R. Sahita
Request for Comments: 5793 Intel
Category: Standards Track S. Hanna
ISSN: 2070-1721 Juniper
R. Hurst
Microsoft
K. Narayan
Cisco Systems
March 2010
PB-TNC: A Posture Broker (PB) Protocol Compatible
with Trusted Network Connect (TNC)
 Abstract
This document specifies PB-TNC, a Posture Broker protocol identical
to the Trusted Computing Group's IF-TNCCS 2.0 protocol. The document
then evaluates PB-TNC against the requirements defined in the NEA
Requirements specification.
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/RFC 5793.
Sahita, et al. Standards Track PAGE 1 
RFC 5793 PB-TNC March 2010
Copyright Notice
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it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................4
1.1. Prerequisites ..............................................4
1.2. Message Diagram Conventions ................................4
1.3. Terminology ................................................4
1.4. Conventions Used in This Document ..........................4
2. PB-TNC Design Considerations ....................................5
2.1. Message Addressing .........................................5
2.2. Vendor IDs .................................................7
2.3. Efficiency .................................................7
3. PB-TNC Protocol Description .....................................7
3.1. Protocol Overview ..........................................7
3.2. PB-TNC State Machine .......................................8
3.3. Layering on PT ............................................11
3.4. Example of PB-TNC Encapsulation ...........................12
4. PB-TNC Protocol Specification ..................................13
4.1. PB-TNC Header .............................................13
4.2. PB-TNC Message ............................................16
4.3. IETF Standard PB-TNC Message Types ........................19
4.4. PB-Experimental ...........................................19
Sahita, et al. Standards Track PAGE 2
RFC 5793 PB-TNC March 2010
4.5. PB-PA .....................................................20
4.6. PB-Assessment-Result ......................................25
4.7. PB-Access-Recommendation ..................................26
4.8. PB-Remediation-Parameters .................................28
4.9. PB-Error ..................................................32
4.10. PB-Language-Preference ...................................37
4.11. PB-Reason-String .........................................38
5. Security Considerations ........................................41
5.1. Threat Model ..............................................41
5.2. Countermeasures ...........................................42
6. IANA Considerations ............................................43
6.1. Designated Expert Guidelines ..............................44
6.2. Registry for PB-TNC Message Types .........................45
6.3. Registry for PA Subtypes ..................................45
6.4. Registry for PB-TNC Remediation Parameters Types ..........46
6.5. Registry for PB-TNC Error Codes ...........................46
7. Acknowledgments ................................................47
8. References .....................................................47
8.1. Normative References ......................................47
8.2. Informative References ....................................48
Appendix A. Use Cases .............................................49
A.1. Initial Client-Triggered Assessment .......................49
A.2. Server-Initiated Assessment with Remediation ..............54
A.3. Client-Triggered Reassessment .............................63
Appendix B. Evaluation against NEA Requirements ...................70
B.1. Evaluation against Requirement C-1 ........................70
B.2. Evaluation against Requirement C-2 ........................70
B.3. Evaluation against Requirement C-3 ........................70
B.4. Evaluation against Requirement C-4 ........................71
B.5. Evaluation against Requirement C-5 ........................71
B.6. Evaluation against Requirement C-6 ........................71
B.7. Evaluation against Requirement C-7 ........................72
B.8. Evaluation against Requirement C-8 ........................72
B.9. Evaluation against Requirement C-9 ........................72
B.10. Evaluation against Requirement C-10 ......................73
B.11. Evaluation against Requirement C-11 ......................73
B.12. Evaluation against Requirement PB-1 ......................74
B.13. Evaluation against Requirement PB-2 ......................74
B.14. Evaluation against Requirement PB-3 ......................74
B.15. Evaluation against Requirement PB-4 ......................75
B.16. Evaluation against Requirement PB-5 ......................75
B.17. Evaluation against Requirement PB-6 ......................76
Sahita, et al. Standards Track PAGE 3
RFC 5793 PB-TNC March 2010
1. Introduction
This document specifies PB-TNC, a Posture Broker (PB) protocol
identical to the Trusted Computing Group's IF-TNCCS 2.0 protocol [7].
The document then evaluates PB-TNC against the requirements defined
in the Network Endpoint Assessment (NEA) Requirements specification
[8].
1.1. Prerequisites
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 [8]. The reader is
assumed to be thoroughly familiar with that document. No familiarity
with TCG specifications is assumed.
1.2. Message Diagram Conventions
This specification defines the syntax of PB-TNC 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 1-octet field with only bit 0 set has the value 0x80.
1.3. Terminology
This document reuses the terminology defined in the NEA Requirements
document. One new term is defined in this section.
Batch - A group of PB-TNC messages sent over a Posture Transport (PT)
protocol at one time. Since the PB-TNC protocol needs to be able to
work over a half-duplex PT protocol, PB-TNC messages are grouped into
batches. The Posture Broker Client sends one batch to the Posture
Broker Server, which responds with a batch.
1.4. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Sahita, et al. Standards Track PAGE 4
RFC 5793 PB-TNC March 2010
2. PB-TNC Design Considerations
The primary purpose of the PB-TNC protocol is to carry Posture
Attribute (PA) messages between Posture Collectors and Posture
Validators. Also, PB-TNC must carry messages between the Posture
Broker Client and the Posture Broker Server (known as PB-TNC
messages) and manage the state of the assessment.
2.1. Message Addressing
The NEA Overview and Requirements document [8] describes in section
5.1.1.1 several ways that messages can be addressed and delivered to
the proper Posture Collector(s) and Posture Validator(s). Of the
techniques described in that section, PB-TNC supports dynamic
identifiers and message types.
2.1.1. Message Types
Message types are the simplest and most common way to handle message
delivery. Each PA message sent via PB-TNC has an associated PA
message type, composed of a PA Message Vendor ID and a PA subtype.
The PA-TNC specification [10] provides a list of IETF Standard PA
Subtypes, which are used with a PA Message Vendor ID of 0. These
include values such as Operating System and Anti-Virus, which are
used for messages relating to operating system and anti-virus
posture.
Vendor-specific PA message types may be indicated by placing the
defining vendor's Structure of Management Information (SMI) Private
Enterprise Number into the PA Message Vendor ID field and a PA
Subtype value assigned by that vendor in the PA Subtype field. This
allows each vendor to define its own set of PA Subtype values without
worrying about collisions with other vendors or with standard values.
The PA message type is somewhat analogous to a MIME type in that it
indicates the type of the PA message. Posture Collectors and Posture
Validators can use local APIs to indicate to the Posture Broker
Client and Posture Broker Server which PA message types they are
interested in receiving. For instance, a Posture Validator that
evaluates anti-virus posture might indicate that it would like to
receive PA messages with a PA Message Vendor ID of 0 and a PA Subtype
that matches the IETF Standard PA Subtype for Anti-Virus. It might
also indicate interest in some vendor-specific PA message types to
get additional vendor-specific information on anti-virus posture.
Sahita, et al. Standards Track PAGE 5
RFC 5793 PB-TNC March 2010
This type-based subscription model allows great flexibility in design
and implementation. One Posture Validator may be responsible for
evaluating several functions: anti-virus and host-based firewall, for
instance. Posture Collectors do not need to know which Posture
Validators are installed on the Posture Broker Server or what they
handle. The Posture Collector simply sends PA messages with message
types and the Posture Broker Server delivers them to the right
Posture Validators.
Because the Posture Broker Client and Posture Broker Server must have
access to the PA Message Vendor ID and PA Subtype fields and because
these are routing identifiers independent of the contents of the PA
messages, these fields are located in PB-TNC not inside the PA
messages themselves.
A similar type-based system is used to tag PB-TNC messages. In this
case, the extensibility benefits are not as essential as with PA-TNC
messages, but the ability to define IETF Standard PB-TNC Message
Types and vendor-specific PB-TNC message types is still valuable.
2.1.2. Dynamic Identifiers
The type-based message delivery model described above is not ideal
for all circumstances. Sometimes it is important for a Posture
Collector to deliver a message to a particular Posture Validator.
For example, a particular Posture Validator might send a remediation
message and the Posture Collector might need to send a response only
to that one Posture Validator. To handle this circumstance, PB-TNC
provides delivery based on dynamic identifiers.
When a Posture Broker Server loads a Posture Validator, it assigns it
a Posture Validator ID. Any PA messages sent by a Posture Validator
include that Posture Validator's Posture Validator ID in the Posture
Validator ID field of the PB-PA message. A Posture Collector that
receives such a message can send a message in response and request
exclusive delivery to the Posture Validator identified by that
Posture Validator ID.
Dynamic identifiers avoid problems caused by the multicast nature of
message types. Multiple Posture Collectors or Posture Validators may
be registered for the same message type, and this can cause confusion
if they all respond and the software designer did not consider that
possibility. The dynamic identifier system allows more directed
responses, but it does not work until at least one message has been
received (so that the dynamic identifiers can be received). Static
identifiers were considered as another alternative but rejected
because they result in a brittle system that only works with a
Sahita, et al. Standards Track PAGE 6
RFC 5793 PB-TNC March 2010
particular set of Posture Collectors and Posture Validators and
causes problems if two Posture Collectors or Posture Validators with
the same static identifier are installed.
2.2. Vendor IDs
In several places, PB-TNC needs to define a set of standard values
but also allow vendor-specific extensions. In each of these places
(PB-TNC Message Types, PA Subtypes, Remediation Parameters Types, and
Error Codes), the solution chosen was to preface the values with a
vendor ID. If a vendor ID is 0, the values in the next field are
registered in an IANA registry and their meanings defined in an RFC.
If a vendor ID is non-zero, the values in the next field are vendor
specific and defined by the vendor whose SMI Private Enterprise
Number matches the vendor ID. Vendor-specific messages that are not
understood by the recipient are ignored and skipped unless they have
the NOSKIP flag set, in which case an error code is returned.
2.3. Efficiency
PB-TNC needs to work with low bandwidth transports and low power
devices. Therefore, a simple, compact format was chosen for the PB-
TNC protocol: binary messages with a Type-Length-Value structure.
3. PB-TNC Protocol Description
3.1. Protocol Overview
The PB-TNC protocol carries batches of PB messages between a Posture
Broker Client and a Posture Broker Server. It encapsulates PA
messages and manages the NEA session. It runs over a PT protocol.
In order to work well over half-duplex PT protocols (such as those
based on EAP [9]), PB-TNC supports half-duplex protocol operation.
In this mode, the Posture Broker Client and Posture Broker Server
take turns sending a single batch of messages to each other. While
the half-duplex nature of PB-TNC could slow exchanges that require
many round trips or bidirectional multimedia exchanges, this is not a
problem in practice because endpoint assessments do not typically
involve multimedia or a large number of round trips. The benefit of
working over half-duplex transports outweighs any limitations
imposed.
PB-TNC also supports full-duplex protocol operation so that PB-TNC
exchanges can be re-initialized immediately when needed (e.g., if the
Posture Broker Server policy changes or if the Posture Broker Client
detects a suspicious event).
Sahita, et al. Standards Track PAGE 7
RFC 5793 PB-TNC March 2010
Each PB-TNC batch consists of a header followed by a sequence of PB-
TNC messages. Each PB-TNC message has a Type-Length-Value (TLV)
format with a few flags. The TLV format allows a recipient to skip
messages that it does not understand. The TLV format also provides a
standard way to mark messages as mandatory to ensure interoperability
between a Posture Broker Client and a Posture Broker Server.
This specification defines certain standard PB-TNC message types. It
also permits vendors to define their own vendor-specific message
types. One of the most important standard PB-TNC message types is
PB-PA. A message with this type contains a PA message and various
message routing information. A Posture Broker Client or Posture
Broker Server that receives such a message does not interpret the PA
message within. Instead, it delivers the PA message to the
appropriate set of Posture Collectors or Posture Validators, as
determined using the message routing information contained in the PB-
PA message.
A Posture Broker Server will often need to communicate with several
Posture Broker Clients at once. The reverse may also be true, as
when an endpoint has multiple network interfaces connected to
different networks. Each connection between a Posture Broker Server
and a Posture Broker Client is instantiated as a separate PB-TNC
session. There may be several simultaneous sessions between a single
Posture Broker Server and Posture Broker Client, but this is unusual.
3.2. PB-TNC State Machine
Figure 1 illustrates the PB-TNC state machine, showing the set of
states that a PB-TNC session can have and the possible transitions
among these states. The following paragraphs describe this state
machine in more detail.
Sahita, et al. Standards Track PAGE 8
RFC 5793 PB-TNC March 2010
Receive CRETRY SRETRY
or SRETRY +----------------+
+--+ | |
v | v |
+---------+ CRETRY +---------+
CDATA | Server |<---------| Decided | CLOSE
+----------->| Working |--------->| |-------+
| +---------+ RESULT +---------+ |
| ^ | | v
| | | +---------------------->=======
======== | | CLOSE " End "
" Init " CDATA| |SDATA =======
======== | | ^ ^
| | | v | |
| | SDATA +---------+ CLOSE | |
| +-------->| Client |----------------------+ |
| | Working | |
| +---------+ |
| | ^ |
| +--+ |
| Receive CRETRY |
| CLOSE |
+--------------------------------------------------+
Figure 1: PB-TNC state machine
In this diagram, states are indicated by rectangular boxes. The
initial and terminal states have double outlines (with = and ").
State transitions are indicated by unidirectional arrows marked with
the cause of the transition.
Many transitions (CDATA, SDATA, CRETRY, SRETRY, and RESULT) are
triggered by the transmission or reception of a PB-TNC batch of a
particular type. The type of a PB-TNC batch is indicated by the
contents of the Batch Type field in the PB-TNC header for that batch.
For brevity, this document says "a FOO batch" instead of "a PB-TNC
batch whose Batch Type field contains FOO". Other transitions are
triggered by receiving a PB-TNC batch of a particular type (e.g.,
Receive CRETRY). The CLOSE transition may be triggered by sending or
receiving a CLOSE batch but may also be triggered by termination of
the underlying PT connection.
A PB-TNC session starts in the Init state when the underlying
transport protocol (PT) establishes a connection between a Posture
Broker Client and a Posture Broker Server. If the Posture Broker
Client initiated the underlying transport session, it starts by
sending a CDATA batch to the Posture Broker Server, thus causing a
transition to the Server Working state. If the Posture Broker Server
Sahita, et al. Standards Track PAGE 9
RFC 5793 PB-TNC March 2010
initiated the transport session, it starts by sending a PB-TNC batch
of type SDATA to the Posture Broker Client, thus causing a transition
to the Client Working state.
The Posture Broker Client and Posture Broker Server may now alternate
sending CDATA and SDATA batches to each other. Only the Posture
Broker Client can send a data batch when the session is in the Client
Working state, and only the Posture Broker Server can send a data
batch when the session is in the Server Working state.
The most common way to end an exchange is for the Posture Broker
Server to send a RESULT batch. This causes a transition into the
Decided state. This is not a terminal state. The PT session can
remain open and another exchange can be initiated by having the
Posture Broker Client send a CRETRY batch. This can be useful when
the Posture Broker Client (or more likely a Posture Collector)
discovers a suspicious condition on the endpoint, for example. If
the underlying transport protocol (PT) supports full-duplex
operation, the Posture Broker Server can also initiate another
exchange from this state by sending a SRETRY batch. This can be
useful when the policy changes on the server, for example.
Whether an SRETRY or CRETRY message or both are sent, the next state
is the Server Working State. From this state, the Posture Broker
Server sends an SDATA batch and the new exchange begins. The state
transitions marked Receive CRETRY and Receive CRETRY or SRETRY
indicate that it is permissible to receive such messages in the
indicated states, generally when the Posture Broker Client sent a
CRETRY message at roughly the same time as the Posture Broker Server
decided to send an SRETRY. In that case, a CRETRY message may be
received while in the Server Working or Client Working state. Also,
an SRETRY message may be received while in the Server Working state.
These messages are redundant and therefore ignored, as indicated by
the relevant transitions, which don't cause a state change.
The only terminal state is the End state. This state is reached if
the underlying PT connection closes. This can be caused by an action
of the Posture Broker Client or Posture Broker Server or it can be
caused by some external factor, such as pulling the network plug.
When possible, a CLOSE batch SHOULD be sent before the underlying PT
connection is terminated. However, there may be cases where the PT
connection is closed without notice. For example, a plug may be
pulled, a software program may fail, or a Posture Broker Client or
Posture Broker Server may be unable to send a CLOSE message due to
half-duplex limitations in the underlying PT protocol. In these
cases, the Posture Broker Client and Posture Broker Server will
generally receive some form of notification from the Posture
Transport Client and Posture Transport Server that the PT connection
Sahita, et al. Standards Track PAGE 10
RFC 5793 PB-TNC March 2010
has been closed. This notification can trigger the CLOSE transition.
However, the notification interaction is not standardized since the
vertical interfaces in the NEA Reference Model are not standardized.
In any case, the reception of the CLOSE batch or notification of
termination of the transport causes the transition to the End state.
Note that a Posture Broker Client and Posture Broker Server may not
always have exactly the same state for a given PB-TNC session. For
example, say that a session is in the Client Working state and the
Posture Broker Client transmits a CDATA batch. While this batch is
in transit (transmitted by the Posture Broker Client but not yet
received by the Posture Broker Server), the Posture Broker Client
will think that the session is in Server Working state but the
Posture Broker Server will think that the session is in Client
Working state. However, this is a temporary condition and does not
cause problems in practice. The only possible issue is that a
Posture Broker Client or Posture Broker Server does not know whether
the other party has received its message until it receives a response
from the other party.
If a half-duplex transport is used, note that the Posture Broker
Server cannot send a SRETRY batch when the session is in the Decided
state because the Posture Broker Server sent the most recent batch
(the RESULT batch) and this would violate the half-duplex nature of
the transport protocol. Instead, a server that wishes to initiate a
new exchange in the Decided state when a half-duplex transport is in
use should close the PT connection without sending a CLOSE batch and
start a new PB-TNC session. This limitation does not exist when a
full-duplex transport is used.
The Posture Broker Server and Posture Broker Client MUST follow the
state machine described in this section.
3.3. Layering on PT
PB-TNC batches are carried over protocol bindings of the PT protocol,
which provides the interaction between a Posture Transport Client and
a Posture Transport Server. PB-TNC counts on PT to provide a secure
transport. In particular, PT MUST support mutual authentication of
the Posture Transport Client and the Posture Transport Server,
confidentiality and integrity protection for PB-TNC batches, and
protection against replay attacks. PB-TNC is unaware of the
underlying transport protocols being used. PB-TNC operates directly
on PT; no further layer of PB-TNC is expected.
Sahita, et al. Standards Track PAGE 11
RFC 5793 PB-TNC March 2010
3.3.1. Posture Transport (PT) Protocol Requirements Addendum
RFC 5209 [8] describes normative requirements for the Posture
Transport protocol. This section specifies additional requirements
for the Posture Transport protocol. Candidate Posture Transport
protocols must indicate conformance to requirements specified in this
section as well as section 7.4 of RFC 5209.
The additional requirements for candidate PT protocols are:
PT-6 The PT protocol MUST be connection oriented; it MUST support
confirmed initiation and close down.
PT-7 The PT protocol MUST be able to carry binary data.
PT-8 The PT protocol MUST provide mechanisms for flow control and
congestion control.
PT-9 PT protocol specifications MUST describe the capabilities that
they provide for and limitations that they impose on the PB
protocol (e.g., half/full duplex, maximum message size).
3.4. Example of PB-TNC Encapsulation
This section shows how PA messages can be carried inside a PB-TNC
batch that is inside a PT protocol.
Within the PT protocol, the PB-TNC header is packaged next, followed
by two PB-PA messages that contain PA messages meant for the Posture
Collectors and Posture Validators on the platform.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PT Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-TNC Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-PA Message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-PA Message |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Example of PB-TNC message encapsulation
This figure is conceptual, of course, and not an exact byte-for-byte
replica.
Sahita, et al. Standards Track PAGE 12
RFC 5793 PB-TNC March 2010
4. PB-TNC Protocol Specification
This section defines the syntax and semantics of the PB-TNC protocol
fields. If a Posture Broker Client or Posture Broker Server receives
a batch that violates the requirements of this specification, it MUST
respond by sending a fatal Invalid Parameter error in a CLOSE batch
unless this document specifies otherwise.
4.1. PB-TNC Header
Every PB-TNC batch MUST start with the following header. A PB-TNC
batch MUST contain only one instance of this header followed by zero
or more PB-TNC messages. The PB-TNC messages are defined in
subsequent sections of this specification.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |D| Reserved | B-Type|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Batch Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version (8 bits)
This field indicates the version of the format for the PB-TNC
message. This version is intended to allow for evolution of the
PB-TNC protocol in a manner that can easily be detected by message
recipients.
This field MUST be set to 2 when the batch conforms to this
specification. Later versions of PB-TNC may define other values
for this field. The values 0x00, 0x09, 0x0a, 0x0d, 0x20, and 0x3c
are reserved and cannot be used for any version of PB-TNC to
ensure that PB-TNC can be easily distinguished from earlier
posture broker protocols already in use.
If a Posture Broker Client or Posture Broker Server receives a
Version value that it does not support, it MUST respond with a PB-
TNC batch with batch type CLOSE that contains only a fatal Version
Not Supported error code and whose Version header field has the
value 2. Implementations responding to a PB-TNC message
containing a supported version MUST use the same Version number to
minimize the risk of version incompatibility. PB-TNC message
initiators that support multiple PB-TNC protocol versions SHOULD
be able to alter which version of PB-TNC message they send based
on prior message exchanges with a particular peer Posture Broker
Client or Posture Broker Server.
Sahita, et al. Standards Track PAGE 13
RFC 5793 PB-TNC March 2010
Directionality (D) (1 bit)
When a Posture Broker Client is sending this message, the
Directionality bit MUST be set to 0. When a Posture Broker Server
is sending this message, the Directionality bit MUST be set to 1.
This helps avoid any situation where two Posture Broker Clients or
two Posture Broker Servers engage in a dialog. It also helps with
debugging.
Reserved (19 bits)
This field is reserved. For this version of this specification,
it MUST be set to 0 on transmission and ignored on reception.
Future versions of this specification may allow senders to set
some of these bits and recipients to interpret them.
B-Type (Batch Type) (4 bits)
This field is used to drive the state machine described in section
3.2. This field MUST have one of the values from the following
table. If any other value is received, the recipient MUST ignore
the contents of the batch and send a fatal Invalid Parameter error
code in a CLOSE batch. If the value received is not permitted for
the current state, according to the state machine in section 3.2.,
the recipient MUST ignore the contents of the batch and send a
fatal Unexpected Batch Type error code in a CLOSE batch.
Sahita, et al. Standards Track PAGE 14
RFC 5793 PB-TNC March 2010
Number Name Definition
------ ---- ----------
1 CDATA The Posture Broker Client may send a batch with
this Batch Type to convey messages to the
Posture Broker Server. A Posture Broker Server
MUST NOT send this Batch Type. A CDATA batch
may be empty (contain no messages) if the
Posture Broker Client has nothing to send.
2 SDATA The Posture Broker Server may send a batch with
this Batch Type to convey messages to the
Posture Broker Client. A Posture Broker Client
MUST NOT send this Batch Type. An SDATA batch
may be empty (contain no messages) if the
Posture Broker Server has nothing to send.
3 RESULT The Posture Broker Server may send a batch with
this Batch Type to indicate that it has
completed its evaluation. The batch MUST
include a PB-Assessment-Result message and MAY
include a PB-Access-Recommendation message.
4 CRETRY The Posture Broker Client may send a batch with
this Batch Type to indicate that it wishes to
restart an exchange. A Posture Broker Server
MUST NOT send this Batch Type. A CRETRY batch
may be empty (contain no messages) if the
Posture Broker Client has nothing else to send.
5 SRETRY The Posture Broker Server may send a batch with
this Batch Type to indicate that it wishes to
restart the exchange. A Posture Broker Client
MUST NOT send this Batch Type. A SRETRY batch
may be empty (contain no messages) if the
Posture Broker Server has nothing else to send.
6 CLOSE The Posture Broker Server or Posture Broker
Client may send a batch with this Batch Type to
indicate that it is about to terminate the
underlying PT connection. A CLOSE batch may be
empty (contain no messages) if there is nothing
to send. However, if the termination is due to a
fatal error, then the CLOSE batch MUST contain a
PB-Error message.
Sahita, et al. Standards Track PAGE 15
RFC 5793 PB-TNC March 2010
Batch Length (32 bits)
This length field contains the size of the full PB-TNC batch in
octets. This length includes the PB-TNC header and all the PB-TNC
messages in the batch. In other words, it includes the entire
contents of the batch. This field MUST contain at least the value
8 for the fixed-length fields in this header. Any Posture Broker
Client or Posture Broker Server that receives a PB-TNC message
with a PB-TNC Message Length field whose value is less than 8 MUST
respond with a fatal Invalid Parameter error code in a CLOSE
batch.
4.2. PB-TNC Message
All PB-TNC messages have the same overall structure, which is
described in this section. Of course, the format and semantics of
the PB-TNC Message Value field will vary, depending on the values of
the PB-TNC Vendor ID and PB-TNC Message Type fields.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | PB-TNC Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-TNC Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-TNC Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PB-TNC Message Value (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags (8 bits)
This field defines flags impacting the processing of this message.
Bit 0 of this Flags field (the most significant bit) is known as
the NOSKIP flag. If this flag is cleared (value 0), then the
recipient (a Posture Broker Client or Posture Broker Server) may
skip (ignore) this message if the message type is not understood
or the recipient cannot or will not process the message as
required in the definition of that message. If this flag is set
(value 1), then recipients MUST NOT skip this attribute.
This flag does not mean that all recipients must support this
message. Instead, any recipient that receives a message with this
flag set to 1 but cannot or will not process it as required MUST
NOT act on any part of the PB-TNC batch. Instead, the recipient
MUST respond with a fatal Unsupported Mandatory Message error code
Sahita, et al. Standards Track PAGE 16
RFC 5793 PB-TNC March 2010
in a CLOSE batch. In order to avoid taking action on some
messages in a batch only to later find an unsupported NOSKIP
flagged message, recipients of a PB-TNC batch might choose to scan
all of the messages in the batch prior to acting upon any of the
messages, checking to determine whether one of them is an
unsupported message with the NOSKIP flag set.
The other bits in this Flags field are reserved. For this version
of PB-TNC, they MUST be set to 0 on transmission and ignored on
reception.
PB-TNC Vendor ID (24 bits)
The PB-TNC Vendor ID field identifies a vendor by using the SMI
Private Enterprise Number (PEN). Any organization can receive its
own unique PEN from IANA, the Internet Assigned Numbers Authority.
This Vendor ID qualifies the PB-TNC Message Type field so that
each vendor has 2^32-1 separate message types available for their
use.
Message types standardized by the IETF use zero (0) in this field.
The Vendor ID 0xffffff is reserved. Posture Broker Clients and
Posture Broker Servers MUST NOT send messages in which the Vendor
ID has this reserved value (0xffffff). If a Posture Broker Client
or Posture Broker Server receives a message in which the PB-TNC
Vendor ID has this reserved value (0xffffff), it MUST respond with
a fatal Invalid Parameter error code in a CLOSE batch.
PB-TNC Message Type (32 bits)
The PB-TNC Message Type field identifies the type of the PB-TNC
message contained in the PB-TNC Message Value field. The PB-TNC
message type 0xffffffff is reserved. Posture Broker Clients and
Posture Broker Servers MUST NOT send messages in which the PB-TNC
Message Type field has this reserved value (0xffffffff). If a
Posture Broker Client or Posture Broker Server receives a message
in which the PB-TNC Message Type field has this reserved value
(0xffffffff), it MUST respond with a fatal Invalid Parameter error
code in a CLOSE batch. Unless otherwise prohibited in the
definition of a particular PB-TNC message type (e.g., PB-Language-
Preference), a single PB-TNC batch may contain multiple messages
with the same message type and/or vendor ID.
The IETF and any other organization with a PEN can define 2^32-1
unique PB-TNC message types, as long as the organization's PEN is
placed in the PB-TNC Vendor ID field of the message. Since the
PB-TNC message type is qualified by the vendor ID, there is no
Sahita, et al. Standards Track PAGE 17
RFC 5793 PB-TNC March 2010
risk of conflicts as long as each organization uses its own PEN
for the vendor ID and manages its own set of 2^32-1 message type
values.
This document defines certain PB-TNC message types that, when used
with the IETF SMI PEN (0), have standard meanings. These are
known as IETF Standard PB-TNC Message Types. Some of these PB-TNC
message types are mandatory and therefore MUST be implemented by
all Posture Broker Client and Posture Broker Server
implementations that claim compliance with this specification.
For details on which PB-TNC message types are mandatory, see the
description of these message types later in section 4.
IANA maintains a registry of PB-TNC message types. Entries in
this registry are added by Expert Review with Specification
Required, following the guidelines in section 6.1.
New vendor-specific PB-TNC message types (those used with a non-
zero PB-TNC vendor ID) may be defined and employed by vendors
without IETF or IANA involvement. However, Posture Broker Clients
and Posture Broker Servers MUST NOT require support for particular
vendor-specific PB-TNC message types and MUST interoperate with
other parties despite any differences in the set of vendor-
specific PB-TNC message types supported (although they MAY permit
administrators to configure them to require support for specific
PB-TNC message types).
Note that the PB-TNC Message Type field is completely separate
from the PA Subtype field. The same value (e.g., 0) may have
different meanings as a PB-TNC message type and as a PA subtype.
PB-TNC Message Length (32 bits)
This field specifies the length of this PB-TNC message in octets.
It includes this header (the fields Flags, PB-TNC Vendor ID, PB-
TNC Message Type, and PB-TNC Message Length). Therefore, this
value MUST always be at least 12. Any Posture Broker Client or
Posture Broker Server that receives a message with a PB-TNC
Message Length field whose value is less than 12 MUST respond with
a fatal Invalid Parameter error code in a CLOSE batch.
PB-TNC Message Value (variable length)
The syntax and semantics of this field vary, depending on the
values in the PB-TNC Vendor ID and PB-TNC Message Type fields.
The syntax and semantics of several standard messages are defined
in subsequent sections of this specification.
Sahita, et al. Standards Track PAGE 18
RFC 5793 PB-TNC March 2010
4.3. IETF Standard PB-TNC Message Types
The following table provides a reference list with brief descriptions
of the IETF Standard PB-TNC Message Types defined in this
specification. These PB-TNC message types must be used with a PB-TNC
vendor ID of zero (0). If these PB-TNC message type values are used
with a different PB-TNC vendor ID, they have a completely different
meaning that is not defined in this specification.
For more details on these message types, see the remainder of section
4. For IETF Standard PA Subtypes (which are completely different
from PB-TNC message types), please refer to the PA-TNC specification
[10].
Message Type Definition
------------ ----------
0 PB-Experimental - reserved for experimental use
1 PB-PA - contains a PA message
2 PB-Assessment-Result - the overall assessment result
computed by the Posture Broker Server
3 PB-Access-Recommendation - includes Posture Broker
Server access recommendation
4 PB-Remediation-Parameters - includes Posture Broker
Server remediation parameters
5 PB-Error - error indicator
6 PB-Language-Preference - sender's preferred
language(s) for human-readable strings
7 PB-Reason-String - string explaining reason for
Posture Broker Server access recommendation
4.4. PB-Experimental
The PB-Experimental PB-TNC message type is a PB-TNC message type
(value 0) that has been set aside for experimental purposes. It may
be used to test code or for other experimental purposes. It MUST NOT
be used in a production environment or in a product. This meaning
for this PB-TNC message type only applies if the PB-TNC Vendor ID
field in the PB-TNC Message Header contains the value zero (0). If a
different Vendor ID is contained in that field, the PB-TNC message
type 0 has a completely different meaning not defined in this
specification.
The contents of the PB-TNC Message Length and PB-TNC Message Value
fields for this PB-TNC message type are not specified. They may have
almost any value, depending on what experiments are being conducted.
Similarly, the Flags field for this message may have the NOSKIP bit
set or cleared, depending on what experiments are being conducted.
However, note that the PB-TNC Message Length field must have a value
Sahita, et al. Standards Track PAGE 19
RFC 5793 PB-TNC March 2010
of at least 12 since that is the total of the length of the fixed-
length fields at the start of the PB-TNC message (the fields Flags,
PB-TNC Vendor ID, PB-TNC Message Type, and PB-TNC Message Length).
Any Posture Broker Client or Posture Broker Server that receives a
message with a PB-TNC Message Length field whose value is invalid
MUST respond with a fatal Invalid Parameter error code in a CLOSE
batch.
A Posture Broker Client or Posture Broker Server implementation
intended for production use MUST NOT send a message with this Message
Type with the value zero (0) as the vendor ID. If it receives a
message with this message type and with the value zero (0) as the
vendor ID, it MUST ignore the message unless the NOSKIP bit is set,
in which case it MUST respond with a fatal Unsupported Mandatory
Message error code in a CLOSE batch.
4.5. PB-PA
The PB-TNC message type named PB-PA (value 1) contains one PA
message. Many batches will contain several PB-PA messages, but some
batches may not contain any messages of this type.
All Posture Broker Client and Posture Broker Server implementations
MUST implement support for this PB-TNC message type. Generally, this
support will consist of forwarding the enclosed PA message to the
appropriate Posture Collectors and Posture Validators. Specific
requirements are contained later in the description of this message
type.
The type of the PA message contained in a PB-PA message is indicated
by the PA Message Vendor ID and PA Subtype fields, as described later
in this section. The PA-TNC specification [10] describes several
standard PA message types that can be identified by the PA Message
Vendor ID and PA Subtype values listed in the PA-TNC specification.
Other PA message types may also be defined, as described in the
description of the PA Subtype field later in this section.
The NOSKIP flag in the PB-TNC Message Header MUST be set for this
message type. Any Posture Broker Client or Posture Broker Server
that receives a PB-PA message with the NOSKIP flag not set MUST
ignore the message and MUST respond with a fatal Invalid Parameter
error code in a CLOSE batch.
For the PB-PA message type, the PB-TNC Vendor ID field MUST contain
the value zero (0) and the PB-TNC Message Type field MUST contain 1.
If a non-zero value is contained in the PB-TNC Vendor ID field,
message type 1 has a completely different meaning not defined in this
specification.
Sahita, et al. Standards Track PAGE 20
RFC 5793 PB-TNC March 2010
The PB-TNC Message Length field MUST contain the length of the entire
PB-TNC message, including the fixed-length fields at the start of the
PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC Message
Type, and PB-TNC Message Length), the fixed-length fields listed
below (Flags, PA Message Vendor ID, PA Subtype, Posture Collector
Identifier, and Posture Validator Identifier), and the PA Message
Body. Since the PA Message Body is variable length, the value in the
PB-TNC Message Length field will vary also. However, it MUST always
be at least 24 to cover the fixed-length fields listed in the
preceding sentences. Any Posture Broker Client or Posture Broker
Server that receives a PB-PA message with a PB-TNC Message Length
field that has an invalid value MUST respond with a fatal Invalid
Parameter error code in a CLOSE batch.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | PA Message Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PA Subtype |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Posture Collector Identifier | Posture Validator Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PA Message Body (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags (8 bits)
This field contains flags relating to the PA message.
Bit 0 of this flags field (the most significant bit) is known as
the EXCL flag (for exclusive). If the EXCL bit is cleared (value
0), the Posture Broker Client or Posture Broker Server that
receives this PB-TNC message SHOULD deliver the PA message
contained in this PB-TNC message to all Posture Collectors or
Posture Validators that have expressed an interest in PA messages
with this PA Message Vendor ID and PA subtype. If a Posture
Broker Client receives a message with the EXCL flag set (value 1),
the Posture Broker Client SHOULD deliver the PA message contained
in this PB-TNC message only to the Posture Collector identified by
the Posture Collector Identifier field. However, if the
identified Posture Collector has not expressed an interest in PA
messages with this PA Message Vendor ID and PA subtype, the PA
Sahita, et al. Standards Track PAGE 21
RFC 5793 PB-TNC March 2010
message should be silently discarded. Analogous requirements
apply to a Posture Broker Server that receives a message with the
EXCL flag set.
The EXCL bit allows, for example, a Posture Validator to handle
the circumstance where there are two Posture Collectors on the
endpoint that are interested in a particular kind of PA messages
and the Posture Validator has remediation instructions that only
apply to one of those Posture Collectors.
The other bits in this Flags field are reserved. For this version
of PB-TNC, they MUST be set to 0 on transmission and ignored on
reception.
PA Message Vendor ID (24 bits)
The PA Message Vendor ID field identifies a vendor by using the
SMI Private Enterprise Number (PEN). Any organization can receive
its own unique PEN from IANA, the Internet Assigned Numbers
Authority. The PA Message Vendor ID qualifies the PA Subtype
field so that each vendor has 2^32-1 separate PA subtypes
available for its use. PA subtypes standardized by the IETF are
always used with a PA Message Vendor ID of the value zero (0) in
this field. The PA Message Vendor ID 0xffffff is reserved. A
Posture Broker Client or Posture Broker Server MUST NOT send
messages in which the PA Message Vendor ID field has this reserved
value (0xffffff). If a Posture Broker Client or Posture Broker
Server receives a message in which the PA Message Vendor ID has
this reserved value (0xffffff), it MUST respond with a fatal
Invalid Parameter error code in a CLOSE batch.
PA Subtype (32 bits)
The PA Subtype field identifies the type of the PA message
contained in the PA Message Body field. The PA subtype 0xffffffff
is reserved. A Posture Broker Client or Posture Broker Server
MUST NOT send messages in which the PA Subtype field has this
reserved value (0xffffffff). If a Posture Broker Client or
Posture Broker Server receives a message in which the PA Subtype
has this reserved value (0xffffffff), it MUST respond with a fatal
Invalid Parameter error code in a CLOSE batch. A Posture Broker
Client or Posture Broker Server MUST support having multiple PA
messages in a single PB-TNC batch that have the same PA subtype
and/or PA Message Vendor ID.
IANA maintains a registry of PA subtypes. Entries in this
registry are added by Expert Review with Specification Required,
following the guidelines in section 6.1. No PA subtypes are
Sahita, et al. Standards Track PAGE 22
RFC 5793 PB-TNC March 2010
defined in this specification. Definitions of IETF Standard PA
Subtypes are contained in the PA-TNC specification [10] and other
specifications. IETF Standard PA Subtypes are always used with a
PA Message Vendor ID of zero (0).
New vendor-specific PA subtypes (those used with a non-zero PA
Message Vendor ID) may be defined and employed by vendors without
IETF or IANA involvement. However, Posture Broker Clients and
Posture Broker Servers MUST NOT require support for particular
vendor-specific PA subtypes and MUST interoperate with other
parties despite any differences in the set of vendor-specific PA
subtypes supported (although they MAY permit administrators to
configure them to require support for specific PA subtypes).
Note that the PB-TNC Message Type field is completely separate
from the PA Subtype field. The same value (e.g., 0) may have
different meanings as a PB-TNC message type and as a PA subtype.
Posture Collector Identifier (16 bits)
The Posture Collector Identifier field contains the identifier of
the Posture Collector associated with this PA message.
The Posture Broker Client is responsible for assigning one or more
Posture Collector Identifier values (but not 0xffff) to each
Posture Collector involved in a message exchange. Multiple
Posture Collector identifiers are required for appropriate
correlation in cases where there are multiple components of the
same type handled by a single Posture Collector, e.g., an endpoint
with two VPN client implementations handled by a single VPN
Posture Collector. Please refer to section 3.3 of the PA-TNC
specification for an example that illustrates the use of multiple
Posture Collector Identifiers. The Posture Collector Identifier
value(s) assigned to a Posture Collector by a Posture Broker
Client MUST NOT change during the course of a PT session. This
identifier is used to identify a unique Posture Collector
communicating with the Posture Broker Client on the endpoint
during a NEA exchange, and is used by the Posture Validator to
send response attributes to a specific Posture Collector component
if required.
When a Posture Broker Server sets the EXCL flag for a PA message,
the Posture Broker Server MUST set the Posture Collector
Identifier field to the identifier of the Posture Collector that
should receive the PA message. If the EXCL flag is not set, a
Posture Broker Server MAY still set the Posture Collector
Identifier value for PA messages that it sends to indicate that
the PA message is intended as a response to a message sent by the
Sahita, et al. Standards Track PAGE 23
RFC 5793 PB-TNC March 2010
Posture Collector associated with the specified Posture Collector
Identifier. If the Posture Broker Server does not wish to
indicate any Posture Collector in this manner, it SHOULD set this
field to the reserved value 0xffff.
Posture Validator Identifier (16 bits)
The Posture Validator Identifier field contains the identifier of
the Posture Validator associated with this PA message.
The Posture Broker Server MUST assign a unique Posture Validator
Identifier value (but not 0xffff) to each Posture Validator
involved in a message exchange and include this Posture Validator
identifier in this field for any PA messages sent by that Posture
Validator. The Posture Validator Identifier value assigned to a
Posture Validator by a Posture Broker Server MUST NOT change
during the course of a PT session. This identifier is used to
identify a unique Posture Validator communicating with the Posture
Broker Server endpoint during a NEA exchange, and is used by the
Posture Collector to send attributes to a specific Posture
Validator if required.
When a Posture Broker Client sets the EXCL flag for a PA message,
the Posture Broker Client MUST set the Posture Validator
Identifier field to the identifier of the Posture Validator that
should receive the PA message. If the EXCL flag is not set, a
Posture Broker Client MAY still set the Posture Validator
Identifier value for PA messages that it sends to indicate that
the PA message is intended as a response to a message sent by the
Posture Validator associated with the specified Posture Validator
Identifier. If the Posture Broker Client does not wish to
indicate any Posture Validator in this manner, it SHOULD set this
field to the reserved value 0xffff.
PA Message Body (variable length)
The PA Message Body field contains the body of the PA message that
is being carried in this PB-TNC message. The length of this field
can be determined by subtracting the length of the fixed-length
fields at the start of the PB-TNC message (the fields Flags, PB-
TNC Vendor ID, PB-TNC Message Type, and PB-TNC Message Length) and
the fixed-length fields at the start of the PB-PA message (Flags,
PA Message Vendor ID, PA Subtype, Posture Collector Identifier,
and Posture Validator Identifier) from the message length
contained in the PB-TNC Message Length field. The length of these
fixed-length fields is 24 octets. Therefore, any Posture Broker
Client or Posture Broker Server that receives a PB-PA message with
Sahita, et al. Standards Track PAGE 24
RFC 5793 PB-TNC March 2010
a PB-TNC Message Length field whose value is less than 24 MUST
respond with a fatal Invalid Parameter error code in a CLOSE
batch.
4.6. PB-Assessment-Result
The PB-TNC message type named PB-Assessment-Result (value 2) is used
by the Posture Broker Server to provide the assessment result after
the Posture Broker Server has completed the assessment of the
endpoint. The Posture Broker Server will typically compute the
assessment result as a cumulative of the individual assessment
results received from the various Posture Validators; the algorithm
for computation of assessment result at the Posture Broker layer is
implementation specific and can also change based on policies in a
specific deployment. The Posture Broker Server MUST include one
message of this type in any batch of type RESULT and MUST NOT include
a message of this type in any other type of batch. The Posture
Broker Client MUST NOT send a PB-TNC message with this message type.
If a Posture Broker Server receives a PB-TNC message with this
message type, it MUST respond with a fatal Invalid Parameter error in
a CLOSE batch. The Posture Broker Client MUST implement and process
this message and MUST ignore any message with this message type that
is not part of a batch of type RESULT.
The NOSKIP flag in the PB-TNC Message Header MUST be set for this
message type. The PB-TNC Vendor ID field MUST contain the value zero
(0) and the PB-TNC Message Type field MUST contain 2. If a non-zero
value is contained in the PB-TNC Vendor ID field, message type 2 has
a completely different meaning not defined in this specification.
The PB-TNC Message Length field MUST contain the value 16 since that
is the total of the length of the fixed-length fields at the start of
the PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC
Message Type, and PB-TNC Message Length) along with the Assessment
Result field described below. Any Posture Broker Client or Posture
Broker Server that receives a PB-Assessment-Result message with a PB-
TNC Message Length field that does not have a value of 16 MUST
respond with a fatal Invalid Parameter error code in a CLOSE batch.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assessment Result |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sahita, et al. Standards Track PAGE 25
RFC 5793 PB-TNC March 2010
Assessment Result
This 32-bit field MUST contain one of the following values
Value Description
----- -----------
0 Posture Broker Server assessed the endpoint to be
compliant with policy.
1 Posture Broker Server assessed the endpoint to be non-
compliant with policy but the difference from compliance
was minor.
2 Posture Broker Server assessed the endpoint to be non-
compliant with policy and the assessed difference from
compliance was very significant.
3 Posture Broker Server was unable to determine policy
compliance due to an error.
4 Posture Broker Server was unable to determine whether the
assessed endpoint is compliant with policy based on the
attributes provided by endpoint.
If a Posture Broker Client receives an Assessment Result value
other than the five values described above, it MUST respond with a
fatal Invalid Parameter error in a CLOSE batch. Other values may
be defined in future versions of PB-TNC but only if the PB-TNC
version number is changed. Therefore, there is no need for an
IANA registry for Assessment Result values.
4.7. PB-Access-Recommendation
The PB-TNC message type named PB-Access-Recommendation (value 3) is
used by the Posture Broker Server to provide an access recommendation
after the Posture Broker Server has completed some assessment of the
endpoint. The PB-Assessment-Result and the PB-Access-Recommendation
attribute together constitute the global assessment decision for an
endpoint. The PB-Access-Recommendation is not authoritative, and the
network and host-based access control systems would typically use
additional information to determine the network access that is
granted to the endpoint. The Posture Broker Server MAY include one
message of this type in any batch of type RESULT and MUST NOT include
a message of this type in any other type of batch. Posture Broker
Clients MUST NOT send a PB-TNC message with this message type. If a
Posture Broker Server receives a PB-TNC message with this message
type, it MUST respond with a fatal Invalid Parameter error in a CLOSE
Sahita, et al. Standards Track PAGE 26
RFC 5793 PB-TNC March 2010
batch. The Posture Broker Client MUST implement and process this
message and MUST ignore any message with this message type that is
not part of a batch of type RESULT.
The NOSKIP flag in the PB-TNC Message Header MUST NOT be set for this
message type. Any Posture Broker Client or Posture Broker Server
that receives a PB-Access-Recommendation message with the NOSKIP flag
set MUST ignore the message and MUST respond with a fatal Invalid
Parameter error code in a CLOSE batch. The PB-TNC Vendor ID field
MUST contain the value zero (0) and the PB-TNC Message Type field
MUST contain 3. If a non-zero value is contained in the PB-TNC
Vendor ID field, message type 3 has a completely different meaning
not defined in this specification. The PB-TNC Message Length field
MUST contain the value 16 since that is the total of the length of
the fixed-length fields at the start of the PB-TNC message (the
fields Flags, PB-TNC Vendor ID, PB-TNC Message Type, and PB-TNC
Message Length) along with the Access Recommendation field described
below. Any Posture Broker Client or Posture Broker Server that
receives a PB-Access-Recommendation message with a PB-TNC Message
Length field that does not have a value of 16 MUST respond with a
fatal Invalid Parameter error code in a CLOSE batch.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
0 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 | Access Recommendation Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved (16 bits)
These Reserved bits MUST be set to 0 on transmission and ignored
on reception.
Access Recommendation Code (16 bits)
The Access Recommendation Code field identifies the Access
Recommendation that the Posture Broker Server has made for this
Posture Broker Client at this time. This field MUST have one of
these three values: 1 for Access Allowed (full access), 2 for
Access Denied (no access), or 3 for Quarantined (partial access).
If a Posture Broker Client receives an Access Recommendation Code
value other than these three values, it MUST respond with a fatal
Invalid Parameter error code in a CLOSE batch. Other values may
Sahita, et al. Standards Track PAGE 27
RFC 5793 PB-TNC March 2010
be defined in future versions of PB-TNC but only if the PB-TNC
version number is changed. Therefore, there is no need for an
IANA registry for Access Recommendation Codes.
4.8. PB-Remediation-Parameters
The PB-TNC message type named PB-Remediation-Parameters (value 4) is
used by the Posture Broker Server to provide global (not Posture
Validator-specific) remediation parameters after the Posture Broker
Server has completed some assessment of the endpoint. The Posture
Broker Server MAY include one or more messages of this type in any
batch of any type, but this message type is most useful in batches of
type RESULT.
The Posture Broker Client MUST NOT send a PB-TNC message with this
message type. If a Posture Broker Server receives a PB-TNC message
with this message type, it MUST respond with a fatal Invalid
Parameter error in a CLOSE batch. The Posture Broker Client may
implement and process this message but is not required to do so. It
may skip this message. Even if the Posture Broker Client implements
this message type, it is not obligated to act on it.
The NOSKIP flag in the PB-TNC Message Header MUST NOT be set for this
message type. The PB-TNC Vendor ID field MUST contain the value zero
(0) and the PB-TNC Message Type field MUST contain 4. If a non-zero
value is contained in the PB-TNC Vendor ID field, message type 4 has
a completely different meaning not defined in this specification.
The PB-TNC Message Length field MUST contain the length of the entire
PB-TNC message, including the fixed-length fields at the start of the
PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC Message
Type, and PB-TNC Message Length), the fixed-length fields listed
below (Reserved, Remediation Parameters Vendor ID, and Remediation
Parameters Type), and the Remediation Parameters. Since the
Remediation Parameters field is variable length, the value in the PB-
TNC Message Length field will vary also. However, it MUST always be
at least 20 to cover the fixed-length fields listed in the preceding
sentences. Any Posture Broker Client that receives a PB-Remediation-
Parameters message with a PB-TNC Message Length field that contains
an invalid value (e.g., less than 20) MUST respond with a fatal
Invalid Parameter error code in a CLOSE batch.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
Sahita, et al. Standards Track PAGE 28
RFC 5793 PB-TNC March 2010
0 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 | Remediation Parameters Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remediation Parameters Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remediation Parameters (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved (8 bits)
These Reserved bits MUST be set to 0 on transmission and ignored
on reception.
Remediation Parameters Vendor ID (24 bits)
The Remediation Parameters Vendor ID field identifies a vendor by
using the SMI Private Enterprise Number (PEN). Any organization
can receive its own unique PEN from IANA, the Internet Assigned
Numbers Authority. The Remediation Parameters Vendor ID qualifies
the Remediation Parameters Type field so that each vendor has 2^32
separate Remediation Parameters Types available for its use.
Remediation Parameters Types standardized by the IETF are always
used with the value zero (0) in this field.
Remediation Parameters Type (32 bits)
The Remediation Parameters Type field identifies the type of
remediation parameters contained in the Remediation Parameters
field. A Posture Broker Client or Posture Broker Server MUST
support having multiple Remediation Parameters messages contained
in a single PB-TNC batch that have the same Remediation Parameters
Type and/or Remediation Parameters Vendor ID.
IANA maintains a registry of PB-TNC Remediation Parameters Types.
Entries in this registry are added by Expert Review with
Specification Required, following the guidelines in section 6.1.
A list of IETF Standard PB-TNC Remediation Parameters Types
defined in this specification appears later in this section.
New vendor-specific Remediation Parameters Types (those used with
a non-zero Remediation Parameters vendor ID) may be defined and
employed by vendors without IETF or IANA involvement. However,
Posture Broker Clients and Posture Broker Servers MUST NOT require
support for particular vendor-specific Remediation Parameters
Types and MUST interoperate with other parties despite any
differences in the set of vendor-specific Remediation Parameters
Sahita, et al. Standards Track PAGE 29
RFC 5793 PB-TNC March 2010
Types supported (although they MAY permit administrators to
configure them to require support for specific Remediation
Parameters Types).
Note that the Remediation Parameters Type is completely separate
from the PB-TNC Message Type and the PA Subtype fields. The same
value (e.g., 0) may have different meanings in each of these
fields.
Remediation Parameters (variable length)
The Remediation Parameters field contains the actual remediation
parameters carried in this PB-TNC message. The length of this
field can be determined by subtracting the length of the fixed-
length fields at the start of the PB-TNC message (the fields
Flags, PB-TNC Vendor ID, PB-TNC Message Type, and PB-TNC Message
Length) and the fixed-length fields at the start of the PB-
Remediation-Parameters message (Reserved, Remediation Parameters
Vendor ID, and Remediation Parameters Type) from the message
length contained in the PB-TNC Message Length field. The length
of these fixed-length fields is 20 octets. Therefore, any Posture
Broker Client that receives a PB-Remediation-Parameters message
with a PB-TNC Message Length field whose value is less than 20
MUST consider this a malformed message. The Posture Broker Client
MUST respond with a fatal Invalid Parameter error code in a CLOSE
batch.
4.8.1. IETF Standard PB-TNC Remediation Parameters Types
This subsection defines several Remediation Parameters Types that
have been standardized by the IETF.
Remediation-URI
This Remediation Parameters Type is employed by creating a PB-
Remediation-Parameters message with a Remediation Parameters
Vendor ID equal to the value zero (0) and a Remediation Parameters
Type of 1. The Remediation Parameters field in the PB-
Remediation-Parameters message MUST contain a URI, as described in
RFC 3986 [2]. This URI contains instructions and resources for
remediation. The Posture Broker Client MAY load the URI and
display the resulting web page to the user. The Posture Broker
Client MAY also ignore the URI or take another action with it.
The Posture Broker Server and any other parties involved in
configuring this remediation URI should consider the likely
capabilities of the Posture Broker Client when creating the URI
Sahita, et al. Standards Track PAGE 30
RFC 5793 PB-TNC March 2010
and the content referenced by the URI. For example, they should
consider the Posture Broker Client's language preferences as
expressed in the PB-Language-Preference message.
Remediation-String
This Remediation Parameters Type is employed by creating a PB-
Remediation-Parameters message with a Remediation Parameters
Vendor ID equal to the value zero (0) and a Remediation Parameters
Type of 2. The Remediation Parameters field in the PB-
Remediation-Parameters message MUST contain the structure defined
below, which contains human-readable instructions for remediation.
The Posture Broker Client MAY display the instructions to the
user. The Posture Broker Client MAY also ignore the instructions
or take another action with them. The Posture Broker Server and
any other parties involved in configuring these instructions
should consider the likely capabilities of the Posture Broker
Client when creating the instructions. For example, they should
consider the Posture Broker Client's language preferences as
expressed in the PB-Language-Preference message.
The following diagram illustrates the format and contents of the
Remediation Parameters field when carrying a Remediation-String
parameter. The text after this diagram describes the fields shown
here.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remediation String Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remediation String (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lang Code Len | Remediation String Lang Code (Variable Len) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Remediation String Length (32 bits)
The Remediation String Length contains the length of the
Remediation String field in octets.
Remediation String (variable length)
The Remediation String field MUST contain a UTF-8 [6] encoded
string. This string contains human-readable instructions for
remediation that MAY be displayed to the user by the Posture
Broker Client. NUL termination MUST NOT be included. If a
Sahita, et al. Standards Track PAGE 31
RFC 5793 PB-TNC March 2010
Posture Broker Client receives a Reason String that does contain a
NUL termination, it MUST respond with a fatal Invalid Parameter
error in a CLOSE batch.
Lang Code Len (8 bits)
The Lang Code Len field contains the length of the Remediation
String Lang Code field in octets. This value may be set to zero
to indicate that the language code for the Remediation String
field is not known.
Remediation String Lang Code (variable length)
The Remediation String Lang Code field contains a US-ASCII string
composed of a well-formed RFC 4646 [3] language tag that indicates
the language(s) used in the Remediation String in the Remediation
Parameters field. A zero-length string may be sent for this field
(essentially omitting this field) to indicate that the language
code for the Remediation String field is not known.
4.9. PB-Error
The PB-TNC message type named PB-Error (value 5) is used by the
Posture Broker Client or Posture Broker Server to indicate that an
error has occurred. The Posture Broker Client or Posture Broker
Server MAY include one or more messages of this type in any batch of
any type. Other messages may also be included in the same batch.
The party that receives a PB-Error message MAY log it or take other
action as deemed appropriate. If the FATAL flag is set (value 1),
the recipient MUST terminate the PB-TNC session after processing the
batch without sending any messages in response. Every Posture Broker
Client and Posture Broker Server MUST implement this message type.
The NOSKIP flag in the PB-TNC Message Header MUST be set for this
message type. The PB-TNC Vendor ID field MUST contain the value zero
(0) and the PB-TNC Message Type field MUST contain 5. If a non-zero
value is contained in the PB-TNC Vendor ID field, message type 5 has
a completely different meaning not defined in this specification.
The PB-TNC Message Length field MUST contain the length of the entire
PB-TNC message, including the fixed-length fields at the start of the
PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC Message
Type, and PB-TNC Message Length), the fixed-length fields listed
below (Flags, Error Code Vendor ID, Error Code, and Reserved), and
the Error Parameters. Since the Error Parameters field is variable
length, the value in the PB-TNC Message Length field will vary also.
Sahita, et al. Standards Track PAGE 32
RFC 5793 PB-TNC March 2010
However, it MUST always be at least 20 to cover the fixed-length
fields listed in the preceding sentences. Any Posture Broker Client
or Posture Broker Server that receives a PB-Error message with a PB-
TNC Message Length field that contains an invalid value (e.g., less
than 20) MUST respond with a fatal Invalid Parameter error code in a
CLOSE batch. Any PB-Error message generated while processing a PB-
Error message MUST be a fatal error to avoid the chance of generating
an infinite loop of errors.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Error Code Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Parameters (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flags (8 bits)
This field defines flags relating to the error.
Bit 0 of this flags field (the most significant bit) is known as
the FATAL flag. If the FATAL bit is cleared (value 0), the
Posture Broker Client or Posture Broker Server that receives this
PB-TNC message SHOULD process this error and then continue with
the exchange. If the FATAL flag is set (value 1), the Posture
Broker Client or Posture Broker Server that receives this PB-TNC
message MUST terminate the exchange after processing the error.
In addition, any Posture Broker Client or Posture Broker Server
that sends a fatal error MUST NOT process the batch that caused
the error and MUST terminate the exchange after sending the batch
containing the error report. A PB-Error message with the FATAL
flag set MUST always be sent in a CLOSE batch since the sender
will be terminating the exchange immediately after sending the
batch.
The FATAL bit allows a Posture Broker Client or Posture Broker
Server to signal a fatal error (like an invalid batch type) and/or
a non-fatal error (like an invalid language tag for a preferred
language).
Sahita, et al. Standards Track PAGE 33
RFC 5793 PB-TNC March 2010
The other bits in this Flags field are reserved. For this version
of PB-TNC, they MUST be set to 0 on transmission and ignored on
reception.
Error Code Vendor ID (24 bits)
The Error Code Vendor ID field identifies a vendor by using the
SMI Private Enterprise Number (PEN). Any organization can receive
its own unique PEN from IANA, the Internet Assigned Numbers
Authority. The Error Code Vendor ID qualifies the Error Code
field so that each vendor has 2^16 separate Error Codes available
for its use. Error codes standardized by the IETF are always used
with the value zero (0) in this field. For detailed descriptions
of those messages, see the next few subsections.
Error Code (16 bits)
The Error Code field identifies the type of error being signaled
with this message. The format of the Error Parameters field
depends on the value of the Error Code Vendor ID and the Error
Code. However, any recipient that does not understand a
particular error code can process the error fairly well by using
the FATAL flag to determine whether the error is fatal and the PB-
TNC Message Length to skip over the Error Parameters field (or log
it).
IANA maintains a registry of PB-TNC Error Codes. Entries in this
registry are added by Expert Review with Specification Required,
following the guidelines in section 6.1. A list of IETF Standard
PB-TNC Error Codes defined in this specification appears later in
section 4.9.1.
New vendor-specific error codes (those used with a non-zero error
code vendor ID) may be defined and employed by vendors without
IETF or IANA involvement. Posture Broker Clients and Posture
Broker Servers that receive an unknown error code MUST process
this error code gracefully by ignoring or logging it if it is not
marked as fatal and terminating the exchange if it is marked as
fatal.
Reserved (16 bits)
The Reserved bits MUST be set to 0 on transmission and ignored on
reception.
Sahita, et al. Standards Track PAGE 34
RFC 5793 PB-TNC March 2010
4.9.1. IETF Standard PB-TNC Error Codes
The following error codes are IETF Standard PB-TNC Error Codes, hence
the Error Code Vendor ID MUST be the value zero (0). The following
table defines the 16-bit error code. Vendor-specific error codes may
be defined by setting the Error Code Vendor ID to the defining
vendor's SMI PEN and setting the Error Code field to whatever error
code(s) that vendor has defined. The format, length, and meaning of
the Error Parameters field varies, based on the Error Code Vendor ID
and Error Code. Subsequent sections of this document define the
format, length, and meaning of the Error Parameters for the IETF
Standard PB-TNC Error Codes defined in this section.
Error Code Definition
---------- ----------
0 Unexpected Batch Type. Error Parameters are empty.
1 Invalid Parameter. Error Parameters has offset where
invalid value was found.
2 Local Error. Error Parameters are empty.
3 Unsupported Mandatory Message. Error Parameters has
offset of offending PB-TNC Message
4 Version Not Supported. Error Parameters has information
about which versions are supported.
4.9.2. Error Parameters Structures for IETF Standard PB-TNC Error Codes
This section defines the format, length, and meaning of the Error
Parameters field for the IETF Standard PB-TNC Error Codes defined in
this specification.
The Error Parameters field is zero length for the IETF Standard PB-
TNC Error Code 0. The FATAL flag MUST be set for this error code.
The Error Parameters field has the following structure for the IETF
Standard PB-TNC Error Code 1. The Offset field is the offset in
octets from the start of the PB-TNC batch to the invalid value. The
FATAL flag may be either set or cleared for this error code.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sahita, et al. Standards Track PAGE 35
RFC 5793 PB-TNC March 2010
The Error Parameters field is zero length for the IETF Standard PB-
TNC Error Code 2. The FATAL flag MUST be set for this error code.
The Error Parameters field has the following structure for the IETF
Standard PB-TNC Error Code 3. The Offset field is the offset in
octets from the start of the PB-TNC batch to the PB-TNC message whose
message type was not recognized (and where the NOSKIP flag was set).
The FATAL flag MUST be set for this error code.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Error Parameters field has the following structure for the IETF
Standard PB-TNC Error Code 4. The FATAL flag MUST be set for this
error code.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bad Version | Max Version | Min Version | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Bad Version field is the version number that was received and is
not supported. The Max Version and Min Version fields indicate which
PB-TNC version numbers are supported by the sender of the error code.
The sender MUST support all PB-TNC versions between the Min Version
and the Max Version, inclusive (i.e., including the Min Version and
the Max Version) but excluding the reserved versions listed in
section 4.1. The Reserved field MUST be set to 0 on transmission and
ignored upon reception. When possible, recipients of this error code
SHOULD send future messages to the Posture Broker Server or Posture
Broker Client that originated this error message with a PB-TNC
version number within the stated range.
Any party that is sending the Version Not Supported error code MUST
include that error code as the only PB-TNC message in a PB-TNC CLOSE
batch with version number 2. All parties that send PB-TNC batches
SHOULD be able to properly process a batch that meets this
description, even if they cannot process any other aspect of PB-TNC
version 2. This ensures that a PB-TNC version exchange can proceed
properly, no matter what versions of PB-TNC the parties implement.
Sahita, et al. Standards Track PAGE 36
RFC 5793 PB-TNC March 2010
4.10. PB-Language-Preference
The PB-TNC message type named PB-Language-Parameters (value 6) is
used by the Posture Broker Client to indicate which language or
languages it would prefer for any human-readable strings that might
be sent to it. This allows the Posture Broker Server and Posture
Validators to adapt any messages they may send to the Posture Broker
Client's preferences (probably determined by the language preferences
of the endpoint's users).
The Posture Broker Server may also send this message type to the
Posture Broker Client to indicate the Posture Broker Server's
language preferences, but this is not very useful since the Posture
Broker Client rarely sends human-readable strings to the Posture
Broker Server and, if it does, rarely can adapt those strings to the
preferences of the Posture Broker Server.
No Posture Broker Client or Posture Broker Server is required to send
or implement this message type. However, a Posture Broker Server
SHOULD attempt to adapt to user language preferences by implementing
this message type, passing the language preference information to
Posture Validators, and allowing administrators to configure human-
readable languages in whatever languages are preferred by their
users.
A Posture Broker Client or Posture Broker Server may include a
message of this type in any batch of any type. However, it is
suggested that this message be included in the first batch sent by
the Posture Broker Client or Posture Broker Server in a PB-TNC
session so that the recipient can start adapting its human-readable
messages as soon as possible. If one PB-Language-Parameters message
is received and then another one is received in a later batch for the
same PB-TNC session, the value included in the later message should
be considered to replace the value in the earlier message.
A Posture Broker Client or Posture Broker Server MUST NOT include
more than one message of this type in a single batch. If a Posture
Broker Client or Posture Broker Server receives more than one message
of this type in a single batch, it should ignore all but the last
one.
The NOSKIP flag in the PB-TNC Message Header MUST NOT be set for this
message type. The PB-TNC Vendor ID field MUST contain the value zero
(0) and the PB-TNC Message Type field MUST contain 6. If a non-zero
value is contained in the PB-TNC Vendor ID field, message type 6 has
a completely different meaning not defined in this specification.
Sahita, et al. Standards Track PAGE 37
RFC 5793 PB-TNC March 2010
The PB-TNC Message Length field MUST contain the length of the entire
PB-TNC message, including the fixed-length fields at the start of the
PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC Message
Type, and PB-TNC Message Length) and the Language Preference field.
Since the Language Preference field is variable length, the value in
the PB-TNC Message Length field will vary also. However, it MUST
always be at least 12 to cover the fixed-length fields listed in the
preceding sentences.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Language Preference (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Language Preference (variable length)
The Language Preference field contains an Accept-Language header,
as described in RFC 3282 [4] (using the RFC 2234 ABNF definition
of Accept-Language included in that RFC, US-ASCII only, no control
characters allowed, no comments, no NUL termination). Any Posture
Broker Client or Posture Broker Server that sends a PB-Language-
Preference message MUST ensure that the Language Preference field
conforms to this format. For example, one acceptable value would
be "Accept-Language: fr, en" (without the quote marks).
A zero-length Language Preference field indicates that no language
preference information is available. Generally, there's no need
to send a PB-Language-Preference message with a zero-length
Language Preference field since this is equivalent to sending no
PB-Language-Preference message at all, but it may be useful to
send a zero-length Language Preference field if a PB-Language-
Preference message with a non-zero-length Language Preference
field was sent in an earlier batch but these preferences no longer
apply.
4.11. PB-Reason-String
The PB-TNC message type named PB-Reason-String (value 7) is used by
the Posture Broker Server to provide a human-readable explanation for
the global assessment decision conveyed in the PB-Assessment-Result &
PB-Access-Recommendation messages. Therefore, a PB-Reason-String
Sahita, et al. Standards Track PAGE 38
RFC 5793 PB-TNC March 2010
message SHOULD only be included in the same batch as the PB-
Assessment-Result and PB-Access-Recommendation message. The Posture
Broker Client MUST NOT ever send a PB-Reason-String message.
The Posture Broker Client is not required to implement this message
type and the Posture Broker Server is not required to send it.
However, there is some benefit to doing so since users are often
curious about why the endpoint was considered non-compliant. The
manner in which a Posture Broker Client uses this field is up to the
implementer and not specified here. The Posture Broker Client MAY
display the message to the user, log it, ignore it, or take any other
action that is not inconsistent with the requirements of this
specification. Since the strings contained in this message are
human-readable, the Posture Broker Server SHOULD adapt them to the
Posture Broker Client's language preferences as expressed in any PB-
Language-Preference message sent by the Posture Broker Client in this
PB-TNC session.
A Posture Broker Server MAY include more than one message of this
type in any batch of any type. However, it is suggested that this
message be included in the same batch as the PB-Assessment-Result and
PB-Access-Recommendation message. If more than one PB-Reason-String
message is included in a single batch, the Posture Broker Client
SHOULD consider the strings included in these messages to be
equivalent in meaning. This allows the Posture Broker Server to
return multiple equivalent reason strings in different languages,
which may help if the Posture Broker Server is not able to
accommodate the Posture Broker Client's language preferences.
The NOSKIP flag in the PB-TNC Message Header MUST NOT be set for this
message type. The PB-TNC Vendor ID field MUST contain the value zero
(0) and the PB-TNC Message Type field MUST contain 7. If a non-zero
value is contained in the PB-TNC Vendor ID field, message type 7 has
a completely different meaning not defined in this specification.
The PB-TNC Message Length field MUST contain the length of the entire
PB-TNC message, including the fixed-length fields at the start of the
PB-TNC message (the fields Flags, PB-TNC Vendor ID, PB-TNC Message
Type, and PB-TNC Message Length), the fixed-length fields listed
below (Reason String Length and Lang Code Len), and the Reason String
and Reason String Language Code fields. Since the Reason String and
Reason String Language Code fields are variable length, the value in
the PB-TNC Message Length field will vary also. However, it MUST
always be at least 17 to cover the fixed-length fields listed in the
preceding sentences. In fact, the PB-TNC Message Length field MUST
be exactly the sum of 17 (for the fixed-length fields) and the values
Sahita, et al. Standards Track PAGE 39
RFC 5793 PB-TNC March 2010
of the Reason String Length and Lang Code Len fields. If this is not
the case, the recipient MUST respond with a fatal Invalid Parameter
error code in a CLOSE batch.
The following diagram illustrates the format and contents of the PB-
TNC Message Value field for this message type. The text after this
diagram describes the fields shown here.
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason String Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason String (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lang Code Len | Reason String Language Code (Variable Length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reason String Length (32 bits)
The Reason String Length field contains the length of the Reason
String field in octets.
Reason String (variable length)
The Reason String field contains a UTF-8 encoded string that
provides a human-readable reason for the Posture Broker Server's
assessment decision. NUL termination MUST NOT be included. If a
Posture Broker Client receives a Reason String that does contain a
NUL termination, it MUST respond with a fatal Invalid Parameter
error code in a CLOSE batch. A zero-length string MUST NOT be
sent since this is the same as sending no reason string at all,
leaving the reason unspecified.
Lang Code Len (8 bits)
The Lang Code Len field contains the length of the Reason String
Language Code field in octets.
Reason String Language Code (variable length)
The Reason String Language Code field contains a US-ASCII string
containing a well-formed RFC 4646 [3] language tag that indicates
the language(s) used in the Reason String in this message. NUL
termination MUST NOT be included in this field. A zero-length
string MAY be sent for this field (essentially omitting this
field) to indicate that the language code for the reason string is
not known.
Sahita, et al. Standards Track PAGE 40
RFC 5793 PB-TNC March 2010
5. Security Considerations
PT is required and assumed to provide reliable and secure transport
for the PB-TNC protocol (including authentication, confidentiality,
integrity protection, and replay protection). Still, it is useful to
describe the possible threats to PB-TNC and the countermeasures that
are or can be employed. This section does that.
5.1. Threat Model
There are several possible threats to the PB-TNC protocol.
Untrusted intermediaries on the network between the NEA Client and
the NEA Server may attempt to observe data sent between the Posture
Broker Client and the Posture Broker Server via PB-TNC, modify this
data in transit, reorder it, or replay it. They may also attempt to
mount a denial-of-service attack against either party or truncate the
exchange prematurely. If successful, these attacks may result in
improper assessment decisions relating to the NEA Client, failure to
reassess these decisions in light of changed circumstances, improper
remediation instructions sent to the NEA Client (which could lead to
the compromise of the NEA Client), unauthorized access to
confidential information about the NEA Client's health and/or
identity, improper reason strings or other messages that might be
displayed to the user, access to reusable credentials such as posture
assertions, denial of service on the NEA Client, and even complete
denial of access to the network (if a denial-of-service attack
against the NEA Server was successful and the network required
permission from the NEA Server to grant network access).
Trusted intermediaries between the Posture Broker Client and the
Posture Broker Server include the Posture Transport Client and the
Posture Transport Server. These parties are considered trusted
because they are responsible for properly implementing the security
protections provided by PT. If they fail to do so properly, these
security protections may be diminished or eliminated altogether. The
possible attacks are the same as those listed in the previous
paragraph. To give one fairly likely example, if a Posture Transport
Client fails to properly authenticate and authorize the Posture
Transport Server (whether through implementation error or through
user configuration to "trust anyone"), the improperly authorized
Posture Transport Server may mount any of the previously described
attacks against the NEA Client.
Compromise of any of the trusted parties (the Posture Broker Client,
the Posture Transport Client, the Posture Broker Server, or the
Posture Transport Server) may result in failures that are equivalent
to those listed in the first paragraph. These failures may be even
Sahita, et al. Standards Track PAGE 41
RFC 5793 PB-TNC March 2010
more dangerous since they will not be detectable by observing network
traffic or by examining and comparing audit logs. Failure to
properly secure communications between the Posture Broker Client and
the Posture Transport Client or between the Posture Broker Server and
the Posture Transport Server is usually indistinguishable from
compromise of those parties. Compromise of the operating system or
other critical software, firmware, or hardware components on the NEA
Client or NEA Server will typically result in an equivalent result.
And an attacker's ability to gain privileged access to the NEA Client
or NEA Server (even for a brief time, long enough to disable or
misconfigure security settings) is generally equivalent as well. If
the NEA Client or NEA Server are dependent on other services for
their proper operation (including Posture Collectors, Posture
Validators, directories, and patch management services), compromise
of those services may result in compromise or failure of the
dependent parties. Of course, compromise or failure of NEA Server
components is most serious since this would probably affect a large
number of NEA Clients while the effects of NEA Client compromise
might well be limited to a single machine.
5.2. Countermeasures
The primary countermeasure against attacks by untrusted network
intermediaries is the security provided by the PT protocol. Any
candidate PT protocols should be carefully examined to ensure that
all the threats described above are adequately addressed.
As noted above, compromise or erroneous operation of any of the
trusted parties is a serious matter with substantial security
implications. This includes the Posture Broker Client, the Posture
Broker Server, the Posture Transport Client, and the Posture
Transport Server. These are all security-sensitive components so
they should be built and managed in accordance with best practices
for security devices. This is especially important for the NEA
Server and its components since a compromise of this device would
affect the security and availability of the entire network (similar
to compromise of a AAA server). Communications between the trusted
parties must also be secured. For example, if the Posture Broker
Server and the Posture Transport Server are separate components,
their communications must be secured.
Since the NEA Client may be a mobile device with little physical
security (such as a laptop computer or even a public telephone), it
should generally be assumed that some proportion of Access NEA
Clients will be compromised and therefore hostile. The NEA Server
should be designed to be robust against hostile NEA Clients. Once a
Sahita, et al. Standards Track PAGE 42
RFC 5793 PB-TNC March 2010
compromised NEA Client is detected, it can be treated in a manner
equivalent to an untrusted party and should pose no greater threat
than any other untrusted party.
Countermeasures against a compromised NEA Server (or a component
thereof such as a Posture Broker Server or a Posture Transport
Server) include prevention of compromise, detection of compromise,
and mitigation of the effects of compromise. For prevention, the NEA
Server and its components and dependencies should be implemented
using secure implementation techniques (e.g., secure coding and
minimization) and managed using secure practices (e.g., strong
authentication and separation of duty). For detection, the behavior
of the NEA Server should be monitored (e.g., via logging especially
of remediation instructions, intrusion detection systems, and probes
that impersonate a valid NEA Client and record NEA Server behavior)
and any anomalies analyzed. For mitigation, NEA Clients should not
blindly follow remediation instructions received from a trusted NEA
Server. At least for patches and other dangerous actions, they
should validate these actions (e.g., via user confirmation) before
proceeding. It should not be possible to configure a NEA Client to
trust all NEA Servers without proper authentication and
authorization.
6. IANA Considerations
Four new IANA registries are defined by this specification: PB-TNC
Message Types, PA Subtypes, PB-TNC Remediation Parameters Types, and
PB-TNC Error Codes. This section explains how these registries work.
All of these registries support IETF standard values and vendor-
defined values. To explain this phenomenon, we will use the PB-TNC
Message Type as an example but the other three registries work the
same way. Whenever a PB-TNC 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
PB-TNC Message Type is an IETF standard value listed in the IANA
registry for PB-TNC Message Types and its meaning is defined in the
specification listed for that PB-TNC Message Type in that registry.
If the vendor ID is not zero, the meaning of the PB-TNC 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 PB-TNC
Message Types used with their vendor ID and publish a specification
for each of these values.
This delegation of namespace is analogous to the technique used for
OIDs. It can result in interoperability problems if vendors require
support for particular vendor-specific values. However, such
Sahita, et al. Standards Track PAGE 43
RFC 5793 PB-TNC March 2010
behavior is explicitly prohibited by this specification, which
dictates that "Posture Broker Clients and Posture Broker Servers MUST
NOT require support for particular vendor-specific PB-TNC message
types and MUST interoperate with other parties despite any
differences in the set of vendor-specific PB-TNC message types
supported (although they MAY permit administrators to configure them
to require support for specific PB-TNC message types)." Similar
requirements are included for PA Subtypes, Remediation Parameters
Types, and PB-TNC Error Codes.
6.1. Designated Expert Guidelines
For all of the four IANA registries defined by this specification,
new values are added to the registry by Expert Review with
Specification Required, using the Designated Expert process defined
in RFC 5226 [5].
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 the same number of values for its use. The
only exception is the registry for PB-TNC Error Codes where 2^16
values are available for the IETF and 2^16 values for each vendor.
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 MUST be documented
in a specification that is permanently and publicly available. IETF
standard 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 standard
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
MUST 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.
Sahita, et al. Standards Track PAGE 44
RFC 5793 PB-TNC March 2010
6.2. Registry for PB-TNC Message Types
The name for this registry is "PB-TNC 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 2^32-2, and
a reference to a specification where the contents of this message
type are defined. This specification must define the meaning of this
PB-TNC message type and the format and semantics of the PB-TNC
Message Value field for PB-TNC messages that include the designated
numeric value in the PB-TNC Message Type field and the designated
Private Enterprise Number in the PB-TNC Vendor ID field.
Entries to this registry are added by Expert Review with
Specification Required, following the guidelines in section 6.1.
The following entries for this registry are defined in this document.
They are the initial entries in the registry for PB-TNC Message
Types.
PEN Integer Name Defining Specification
--- ------- ---- ----------------------
0 0 PB-Experimental RFC 5793
0 1 PB-PA RFC 5793
0 2 PB-Assessment-Result RFC 5793
0 3 PB-Access-Recommendation RFC 5793
0 4 PB-Remediation-Parameters RFC 5793
0 5 PB-Error RFC 5793
0 6 PB-Language-Preference RFC 5793
0 7 PB-Reason-String RFC 5793
0 0xffffffff Reserved RFC 5793
6.3. Registry for PA Subtypes
The name for this registry is "PA Subtypes". Each entry in this
registry should include a human-readable name, an SMI Private
Enterprise Number, a decimal integer value between 0 and 2^32-2, and
a reference to a specification where the contents of this PA subtype
are defined. This specification must define the meaning of this PA
subtype and the format and semantics of the PA Message Body field for
PB-TNC messages that have a PB-TNC Vendor ID of 0, a PB-TNC Message
Type of PB-PA, the designated numeric value in the PA Subtype field,
and the designated Private Enterprise Number in the PA Message Vendor
ID field.
Entries to this registry are added by Expert Review with
Specification Required, following the guidelines in section 6.1.
Sahita, et al. Standards Track PAGE 45
RFC 5793 PB-TNC March 2010
This document does not define any initial entries for this registry.
Therefore, this registry should initially be empty. Subsequent RFCs
(such as PA-TNC) will define entries in this registry.
6.4. Registry for PB-TNC Remediation Parameters Types
The name for this registry is "PB-TNC Remediation Parameters Types".
Each entry in this registry should include a human-readable name, an
SMI Private Enterprise Number, a decimal integer value between 0 and
2^32-1, and a reference to a specification where the contents of this
remediation parameters type are defined. This specification must
define the meaning of this remediation parameters type value and the
format and semantics of the Remediation Parameters field for PB-TNC
messages that have a PB-TNC Vendor ID of 0, a PB-TNC Message Type of
PB-Remediation-Parameters, the designated numeric value in the
Remediation Parameters Type field, and the designated Private
Enterprise Number in the Remediation Parameters Vendor ID field.
Entries to this registry are added by Expert Review with
Specification Required, following the guidelines in section 6.1.
The following entries for this registry are defined in this document.
They are the initial entries in the registry for PB-TNC Remediation
Parameters Types.
PEN Integer Name Defining Specification
--- ------- ---- ----------------------
0 1 Remediation-URI RFC 5793
0 2 Remediation-String RFC 5793
6.5. Registry for PB-TNC Error Codes
The name for this registry is "PB-TNC 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 2^16-1, and
a reference to a specification where this error code is defined.
This specification must define the meaning of this error code and the
format and semantics of the Error Parameters field for PB-TNC
messages that have a PB-TNC Vendor ID of 0, a PB-TNC Message Type of
PB-Error, the designated numeric value in the Error Code field, and
the designated Private Enterprise Number in the Error Code Vendor ID
field.
Entries to this registry are added by Expert Review with
Specification Required, following the guidelines in section 6.1.
The following entries for this registry are defined in this document.
They are the initial entries in the registry for PB-TNC Error Codes.
Sahita, et al. Standards Track PAGE 46
RFC 5793 PB-TNC March 2010
PEN Integer Name Defining Specification
--- ------- ---- ----------------------
0 0 Unexpected Batch Type RFC 5793
0 1 Invalid Parameter RFC 5793
0 2 Local Error RFC 5793
0 3 Unsupported Mandatory Message RFC 5793
0 4 Version Not Supported RFC 5793
7. Acknowledgments
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: Bernard Aboba,
Amit Agarwal, Morteza Ansari, Diana Arroyo, Stuart Bailey, Boris
Balacheff, Gene Chang, Roger Chickering, Scott Cochrane, Pasi Eronen,
Aman Garg, Sandilya Garimella, Lauren Giroux, Mudit Goel, Charles
Goldberg, Thomas Hardjono, Chris Hessing, Hidenobu Ito, John Jerrim,
Meenakshi Kaushik, Greg Kazmierczak, Scott Kelly, Tom Kelnar, Bryan
Kingsford, PJ Kirner, Houcheng Lee, Sung Lee, Lisa Lorenzin,
Mahalingam Mani, Paul Mayfield, Michael McDaniels, Bipin Mistry, Rod
Murchison, Barbara Nelson, Kazuaki Nimura, Ron Pon, Ivan Pulleyn,
Alex Romanyuk, Chris Salter, Mauricio Sanchez, Paul Sangster, Dean
Sheffield, Curtis Simonson, Jeff Six, Ned Smith, Michelle Sommerstad,
Joseph Tardo, Lee Terrell, Chris Trytten, Brad Upson, Ram Vadali,
Guha Prasad Venataraman, John Vollbrecht, Jun Wang, and Han Yin.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[3] Phillips, A., Ed., and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[4] Alvestrand, H., "Content Language Headers", RFC 3282, May
2002.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
Sahita, et al. Standards Track PAGE 47
RFC 5793 PB-TNC March 2010
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
8.2. Informative References
[7] Hanna, S., Hurst, R. and R. Sahita, "TNC IF-TNCCS: TLV
Binding", Trusted Computing Group, February 2008.
[8] Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
Tardo, "Network Endpoint Assessment (NEA): Overview and
Requirements", RFC 5209, June 2008.
[9] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol (EAP)",
RFC 3748, June 2004.
[10] Sangster, P., and K. Narayan, "PA-TNC: A Posture Attribute
(PA) Protocol Compatible with Trusted Network Connect (TNC)",
RFC 5792, March 2010.
Sahita, et al. Standards Track PAGE 48
RFC 5793 PB-TNC March 2010
Appendix A. Use Cases
A.1. Initial Client-Triggered Assessment
This scenario involves the assessment of an endpoint initiated during
network join. The assessment is triggered by the Posture Broker
Client (PBC) and involves collection of patch information from both
Standard Operating System (OS) Posture Collector and vendor-specific
Patch Posture Collector (PC). The assessment by both the vendor-
specific Patch Posture Validator (PV) and Standard OS Posture
Validator result in a compliant assessment decision that results in a
compliant System Assessment Decision to be returned by the Posture
Broker Server (PBS).
+--------+ +-------+ +---------+ +--------+ +-------++--------+
| Vndr. X| | Std. | | Std. | | Std. | | Std. || Vndr. X|
|Patch PC| | OS PC | | PBC | | PBS | | OS PV ||Patch PV|
+----+---+ +---+---+ +-----+---+ +---+----+ +---+----++---+---+
| | N/W Join| | | |
| | ----->| | | |
| | Req Post. | | | |
| +<----------+ | | |
| | Req Post. | | | |
+<--------------------| | | |
|Vndr X Patch Posture | | | |
|-------------------->| | | |
| |OS Posture | | | |
| |---------->| | | |
| | | Posture | | |
| | | Report | | |
| | +-------->| | |
Sahita, et al. Standards Track PAGE 49
RFC 5793 PB-TNC March 2010
| | | | Verify | |
| | | | Posture | |
| | | |---------> |
| | | | | Verify |
| | | | | Posture |
| | | |------------------->|
| | | | OS Reslt | |
| | | |<---------| |
| | | | VndrX Patch Result |
| | | Assess |<-------------------|
| | | Result | |
| | <---------| | |
| | OS PRslt | | | |
| |<----------| | | |
| VndrX Patch PResult | | | |
|<--------------------| | | |
A.1.1. Message Contents
This section shows the contents of the key fields in each of the PA
messages exchanged in this use case. When necessary, additional
commentary is provided to explain why certain fields contain the
shown values. Note that many of the flows shown are between
components on the same system so no message contents are shown.
A.1.1.1. N/W Join
This flow represents the event that causes the PBC to decide to start
an assessment of the endpoint in order to gain access to the network.
This is merely an event and doesn't include a message being sent.
Sahita, et al. Standards Track PAGE 50
RFC 5793 PB-TNC March 2010
A.1.1.2. Request Posture (Req Post.)
This flow illustrates an invocation of the OS and Patch Posture
Collectors requesting particular posture attributes to be sent.
Because this use case is triggered locally, NEA doesn't specify the
contents of this flow.
A.1.1.3. Vendor X Patch Posture (VndrX Patch Posture)
This flow contains the PA message from the Vendor X Patch Posture
Collector; the message content is described in the PA-TNC
specification.
A.1.1.4. OS Posture
This flow contains the PA message from the OS Posture Collector; the
message content is described in the PA-TNC specification.
A.1.1.5. Posture Report
This flow contains the PB message containing the PA messages from the
Patch and OS Posture Collectors:
PB Envelope {
HDR {
D bit=0 (Posture Broker Client is originator)
Batch Type=CDATA
Batch Length
}
PB Message 1 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=0 (Standard)
PA-subtype=1 (OS)
Sahita, et al. Standards Track PAGE 51
RFC 5793 PB-TNC March 2010
OS Posture PA Message
}
}
PB Message 2 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=1 (Vendor X)
PA-subtype=1 (Vendor X PA sub-type for patch management)
Vendor X Patch Posture PA Message
}
}
}
A.1.1.6. Verify Posture
This flow illustrates an invocation of the OS and Patch Posture
Validators requesting verification of the posture attributes
received. Because this flow happens locally within the NEA server,
NEA doesn't specify the message content.
A.1.1.7. OS Posture Result (OS Reslt)
This flow contains the PA message (Posture Assessment Result) from
the OS Posture Validator; the message content is described in the PA-
TNC specification.
A.1.1.8. Vendor X Patch Posture Result (VndrX Patch Result)
This flow contains the PA message (Posture Assessment Result) from
the Vendor X Patch Posture Validator; the message content is
described in the PA-TNC specification.
Sahita, et al. Standards Track PAGE 52
RFC 5793 PB-TNC March 2010
A.1.1.9. Assessment Result (Assess Result)
This flow contains the PB message containing the system assessment
result computed by the Posture Broker Server and the PA messages from
the Patch and OS Posture Validators:
PB Envelope {
HDR {
D bit=1 (Posture Broker Server is originator)
Batch Type=RESULT
Batch Length
}
PB Message 1 {
Vendor-id=0,
Type =3 (Access-Recommendation)
Length
Value = {
System-Evaluation-Result=0 (Compliant)
}
}
PB Message 2 {
Vendor-id=0,
Type=2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=0
PA-subtype=1 (OS)
Sahita, et al. Standards Track PAGE 53
RFC 5793 PB-TNC March 2010
OS Posture Result PA Message
}
}
PB Message 3 {
Vendor-id=0,
Type=2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=1 (Vendor X)
PA-subtype=1 (Vendor X PA sub-type for patch management)
Vendor X Patch Posture Result PA Message
}
}
}
A.1.1.10. Posture Result (OS PRslt & Vndr X Post PResult)
These flows illustrate an invocation of the OS and Vendor X Patch
Posture Collectors to receive the posture assessment results.
Because this flow is triggered locally, NEA doesn't specify the
contents of this flow.
A.2. Server-Initiated Assessment with Remediation
This scenario involves the assessment of an endpoint initiated by the
NEA server. The assessment is triggered by the Posture Broker Server
and involves collection of Anti-Virus attributes for two Anti-Virus
components running on the endpoint. The endpoint is assessed to be
compliant by one of the vendor (Vendor X) anti-virus posture
validators and non-compliant by the other vendor (Vendor Y) anti-
virus posture validator. This results in a non-compliant System
Assessment Decision to be returned by the Posture Broker Server. The
Posture Broker Server also returns remediation instructions for the
endpoint as part of the response.
Sahita, et al. Standards Track PAGE 54
RFC 5793 PB-TNC March 2010
+--------+ +-------+ +---------+ +--------+ +-------+ +--------+
| Vndr Y | | Vndr X| | Std. | | Std. | | Vndr X| | Vndr Y |
| AV PC | | AV PC | | PBC | | PBS | | AV PV | | AV PV |
+----+---+ +---+---+ +-----+---+ +---+----+ +---+---+ +----+---+
| | | N/W Join| | |
| | | ----->| | |
| | | | Create | |
| | | |Post. Req | |
| | | |--------->| |
| | | |Create Posture Req |
| | | |----------+--------->|
| | | |Vndr Y AV Posture Req|
| | | |<---------+----------|
| | | |Vndr X AV | |
| | | |Post. Req | |
| | | Posture |<---------| |
| | | Request | | |
| | Vndr X AV |<--------| | |
| | Post. Req | | | |
| |<----------| | | |
| Vndr Y AV | | | |
| Posture Req | | | |
+<---------+-----------| | | |
| Vndr Y AV Posture | | | |
Sahita, et al. Standards Track PAGE 55
RFC 5793 PB-TNC March 2010
+----------+---------->| | | |
| | Vndr X AV | | | |
| | Posture | | | |
| |---------->| Posture | | |
| | |Response | | |
| | |-------->| | |
| | | | Verify | |
| | | | Posture | |
| | | |--------->| |
| | | | Verify Posture |
| | | |----------+--------->|
| | | |Vndr Y Posture Result|
| | | |<---------+----------|
| | | |Vndr X AV | |
| | | |Post Reslt| |
| | | Assess |<---------| |
| | | Result | | |
| | Vndr X AV |<--------| | |
| |Post Reslt |<--------| | |
| |<----------| | | |
| Vndr Y AV Post Reslt | | | |
+<---------+-----------| | | |
| | | | | |
Sahita, et al. Standards Track PAGE 56
RFC 5793 PB-TNC March 2010
A.2.1. Message Contents
This section shows the contents of the key fields in each of the PA
messages exchanged in this use case. When necessary, additional
commentary is provided to explain why certain fields contain the
shown values. Note that many of the flows shown are between
components on the same system so no message contents are shown.
A.2.1.1. N/W Join
This flow represents the event that causes the PBS to decide to start
an assessment of the endpoint in order to gain access to the network.
This is merely an event and doesn't include a message being sent.
A.2.1.2. Create Posture Request (Create Posture Req)
This flow illustrates an invocation of the Vendor X and Vendor Y
Anti-Virus posture validators requesting posture requests to be
created. Because this use case is triggered locally, NEA doesn't
specify the contents of this flow.
A.2.1.3. Vendor X Anti-Virus Posture Request (Vndr X AV Post. Req)
This flow contains the PA message (Posture Request) from the Vendor X
Anti-Virus Posture Validator; the message content is described in the
PA-TNC specification.
A.2.1.4. Vendor Y Anti-Virus Posture Request
This flow contains the PA message (Posture Request) from the Vendor Y
Anti-Virus Posture Validator; the message content is described in the
PA-TNC specification.
A.2.1.5. Posture Request
This flow contains the PB message containing the PA messages from the
Vendor X and Vendor Y Anti-Virus Posture Validators:
PB Envelope {
HDR {
D bit=1 (Posture Broker Server is originator)
Batch Type=SDATA
Batch Length
Sahita, et al. Standards Track PAGE 57
RFC 5793 PB-TNC March 2010
}
PB Message 1 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=1 (Vendor X)
PA-subtype=2 (Vendor X PA sub-type for Anti-Virus)
Vendor X AV Posture Request PA Message
}
}
PB Message 2 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=2 (Vendor Y)
PA-subtype=1 (Vendor Y PA sub-type for Anti-Virus)
Vendor Y AV Posture Request PA Message
}
}
}
Sahita, et al. Standards Track PAGE 58
RFC 5793 PB-TNC March 2010
A.2.1.6. Process Posture Request (Vndr X AV Post Req & Vndr Y AV
Posture Req)
This flow illustrates an invocation of the Vendor X and Vendor Y
Anti-Virus Posture Collectors to process the Posture Request and
return particular posture attributes requested. Because this use
case is triggered locally, NEA doesn't specify the contents of this
flow.
A.2.1.7. Vendor Y Anti-Virus Posture (Vndr Y AV Posture)
This flow contains the PA message (response to the Posture Request)
from the Vendor Y Anti-Virus Posture Collector; the message content
is described in the PA-TNC specification.
A.2.1.8. Vendor X Anti-Virus Posture (Vndr X AV Posture)
This flow contains the PA message (response to the Posture Request)
from the Vendor X Anti-Virus Posture Collector; the message content
is described in the PA-TNC specification.
A.2.1.9. Posture Response
This flow contains the PB message containing the PA messages from the
Vendor X and Vendor Y Anti-Virus Posture Collectors:
PB Envelope {
HDR {
D bit=0 (Posture Broker Client is originator)
Batch Type=CDATA
Batch Length
}
PB Message 1 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
Sahita, et al. Standards Track PAGE 59
RFC 5793 PB-TNC March 2010
PA-Msg-vendor-id=1 (Vendor X)
PA-subtype=2 (Vendor X PA sub-type for Anti-Virus)
Vendor X AV Posture PA Message
}
}
PB Message 2 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=2 (Vendor Y)
PA-subtype=1 (Vendor Y PA sub-type for Anti-Virus)
Vendor Y AV Posture PA Message
}
}
}
A.2.1.10. Verify Posture
This flow illustrates an invocation of the Vendor X and Vendor Y
Anti-Virus Posture Validators requesting verification of the posture
attributes received. Because this flow happens locally within the
NEA server, NEA doesn't specify the message contents.
A.2.1.11. Vendor Y Anti-Virus Posture Result (Vndr Y AV Post Result)
This flow contains the PA message (Posture Assessment Result) from
the Vendor Y Anti-Virus Posture Validator; the message content is
described in the PA-TNC specification.
Sahita, et al. Standards Track PAGE 60
RFC 5793 PB-TNC March 2010
A.2.1.12. Vendor X Anti-Virus Posture Result (Vndr Y AV Post Result)
This flow contains the PA message (Posture Assessment Result) from
the Vendor X Anti-Virus Posture Validator; the message content is
described in the PA-TNC specification.
A.2.1.13. Assessment Result (Assess Result)
This flow contains the PB message containing the system assessment
result computed by the Posture Broker Server and the PA messages from
the Patch and OS Posture Validators:
PB Envelope {
HDR {
D bit=1 (Posture Broker Server is originator)
Batch Type=RESULT
Batch Length
}
PB Message 1 {
Vendor-id=0,
Type=3 (Access-Recommendation)
Length
Value = {
PB-Assessment-Result=1 (Non-Compliant)
}
}
PB Message 2 {
Vendor-id=0,
Type=4 (Remediation-Parameters)
Length
Sahita, et al. Standards Track PAGE 61
RFC 5793 PB-TNC March 2010
Value = {
Remediation-Param-Vendor-ID=0
Remediation-Param-Type=1 (Remediation-URI)
Remediation-Param=''http://xyz''
}
}
PB Message 3 {
Vendor-id=0,
Type=4 (Remediation-Parameters)
Length
Value = {
Remediation-Param-Vendor-ID=0
Remediation-Param-Type=2 (Remediation-String)
Remediation-Param=''Try Step1, Step2,...''
}
}
PB Message 4 {
Vendor-id=0,
Type=2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=1 (Vendor X)
PA-subtype=2 (Vendor X PA sub-type for Anti-Virus)
Vendor X AV Posture Result PA Message
Sahita, et al. Standards Track PAGE 62
RFC 5793 PB-TNC March 2010
}
}
PB Message 5 {
Vendor-id=0,
Type=2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=2 (Vendor Y)
PA-subtype=1 (Vendor Y PA sub-type for Anti-Virus)
Vendor Y AV Posture Result PA Message
}
}
}
A.2.1.14. Posture Result (Vndr X AV Post Reslt & Vndr Y AV Post Reslt)
These flows illustrate an invocation of the Vendor X and Vendor Y
Anti-Virus Posture Collectors to receive the posture assessment
results. Because this flow is triggered locally, NEA doesn't specify
the contents of this flow.
A.3. Client-Triggered Reassessment
This scenario involves the reassessment of an endpoint as a result of
enabling a software component on the endpoint. The endpoint has two
VPN client software components, one from vendor X for the user's home
network and other from vendor Y for the network that the endpoint is
currently accessing. The assessment is triggered when the user tries
to use the Vendor X VPN client; this is a violation of the posture
policy. The Posture Broker Client triggers the posture assessment
when it receives a notification from the Standard VPN Posture
Collector about the change to the operational state of the VPN
component on the endpoint. Note that the VPN Posture Collector
supports standard attributes and some vendor-defined attributes from
vendor X's and vendor Y's namespaces. This use case doesn't leverage
vendor-defined attributes. The assessment involves verification of
Sahita, et al. Standards Track PAGE 63
RFC 5793 PB-TNC March 2010
the standard VPN posture attributes by the Standard VPN Posture
Validator that results in a non-compliant assessment result. This
use case relies on the use of a virtual Posture Collector concept
described in section 3.3 of the PA-TNC specification. As illustrated
in this example, the Posture Broker Client will assign two Posture
Collector IDs to a single Posture Collector (Standard VPN PC), and
the Posture Collector will generate two separate PA messages to
report the posture for Vendor X and Vendor Y VPN Clients. The
Posture Broker Client will use the assigned IDs in the PB message
sent to the NEA Server. This entire behavior will be completely
opaque to the NEA Server, which will handle the PB message as if
there were two VPN Posture Collectors on the NEA Client.
+--------+ +-------+ +---------+ +--------+ +--------+ +--------+
|Vndr X | |Vndr Y | |Standard | |Standard| |Standard| |Standard|
|VPNClnt | |VPNClnt| | VPN PC | | PBC | | PBS | | VPN PV |
+----+---+ +---+---+ +-----+---+ +---+----+ +---+----+ +----+---+
Enble| | | | | |
---->| | | | | |
| VPN Status Change | | | |
|--------------------->| Posture | | |
| | | Change | | |
| | |-------->| | |
| | |Req. Post| | |
| | |<--------| | |
| |Ins/Rq Info| | | |
| |<----------| | | |
| Inspect/Request Info | | | |
|<---------+-----------|VPNX Post| | |
| | |-------->| | |
| | |VPNY Post| | |
Sahita, et al. Standards Track PAGE 64
RFC 5793 PB-TNC March 2010
| | |-------->| | |
| | | | Posture | |
| | | | Report | |
| | | |--------->| |
| | | | |Vrfy Post. |
| | | | |---------->|
| | | | |VPN PRslt |
| | | | Assess |<----------|
| | | | Result | |
| | | |<---------| |
| | |VPN PRslt| | |
| | |<--------| | |
A.3.1. Message Contents
This section shows the contents of the key fields in each of the PA
messages exchanged in this use case. When necessary, additional
commentary is provided to explain why certain fields contain the
shown values. Note that many of the flows shown are between
components on the same system so no message contents are shown.
A.3.1.1. Enable VPN Client (Enble)
This flow represents the end user triggered event of starting the VPN
Client software from Vendor X. This is merely an event and doesn't
include a message being sent.
A.3.1.2. Notify Status Change (VPN Status Change)
This flow represents the detection of the active state of the Vendor
X VPN Client software by the Standard VPN Posture Collector. This is
merely an event and doesn't include a message being sent.
Sahita, et al. Standards Track PAGE 65
RFC 5793 PB-TNC March 2010
A.3.1.3. Notify Posture Change (Posture Change)
This flow represents the notification of the VPN Posture change sent
from the VPN Posture Collector to the Standard Posture Broker Client.
This is merely an event and doesn't include a message being sent.
A.3.1.4. Request Posture (Req. Post)
This flow illustrates an invocation of the VPN Posture Collector
requesting particular posture attributes to be sent. Because this
use case is triggered locally, the contents of this flow aren't
specified by NEA.
A.3.1.5. Inspect/Request Information (Ins/Rq Info)
This flow illustrates the acquisition of the posture attributes by
the Standard VPN Posture Collector from the Vendor X and Vendor Y VPN
Client components. Because this flow is triggered locally, NEA
doesn't specify the message contents.
A.3.1.6. Vendor X VPN Posture (VPNX Post.)
This flow contains the PA message from the VPN Posture Collector for
Vendor X VPN Client posture; the message content is described in the
PA-TNC specification.
A.3.1.7. Vendor Y VPN Posture (VPNY Post.)
This flow contains the PA message from the VPN Posture Collector for
Vendor Y VPN Client posture; the message content is described in the
PA-TNC specification.
A.3.1.8. Posture Report (Post. Rpt.)
This flow contains the PB message containing the PA message from the
VPN Posture Collector:
PB Envelope {
HDR {
D bit=0 (Posture Broker Client is originator)
Batch Type=CRETRY
Batch Length
}
Sahita, et al. Standards Track PAGE 66
RFC 5793 PB-TNC March 2010
PB Message 1 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=0
PA-subtype=7 (VPN)
Posture-Collector-ID=1 //Virtual Posture Collector ID for
Vendor X VPN Client
Vendor X VPN Posture PA Message
}
}
PB Message 2 {
Vendor-id=0
Type =2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=0
PA-subtype=7 (VPN)
Posture-Collector-ID=2 //Virtual Posture Collector ID for
Vendor Y VPN Client
Vendor Y VPN Posture PA Message
}
}
Sahita, et al. Standards Track PAGE 67
RFC 5793 PB-TNC March 2010
A.3.1.9. Verify Posture (Vrfy Post.)
This flow illustrates an invocation of the VPN Posture Validator
requesting verification of the posture attributes received. Because
this flow happens locally within the NEA server, NEA doesn't specify
the message contents.
A.3.1.10. VPN Posture Result (VPN PRslt)
This flow contains the PA message (Posture Assessment Result) from
the VPN Posture Validator; the message content is described in the
PA-TNC specification.
A.3.1.11. Assessment Result (Assess Result)
This flow contains the PB message containing the system assessment
result computed by the Posture Broker Server and the PA messages from
the VPN Posture Validator:
PB Envelope {
HDR {
D bit=1 (Posture Broker Server is originator)
Batch Type=RESULT
Batch Length
}
PB Message 1 {
Vendor-id=0,
Type =3 (Access-Recommendation)
Length
Value = {
PB-Assessment-Result=1 (Non-Compliant)
}
}
Sahita, et al. Standards Track PAGE 68
RFC 5793 PB-TNC March 2010
PB Message 2 {
Vendor-id=0,
Type=2 (PB-PA)
Length
Value = {
PA-Msg-vendor-id=0
PA-subtype=7 (VPN)
VPN Posture Result PA Message
}
}
A.3.1.12. Posture Result (VPN PRslt)
This flow illustrate an invocation of the VPN Posture Collectors to
receive the posture assessment result. Because this flow is
triggered locally, NEA doesn't specify the contents of this flow.
Sahita, et al. Standards Track PAGE 69
RFC 5793 PB-TNC March 2010
Appendix B. Evaluation against NEA Requirements
This section evaluates the PB-TNC protocol against the requirements
defined in the NEA Requirements document. Each subsection considers
a separate requirement from the NEA Requirements document. Only
common requirements (C-1 through C-11) and PB requirements (PB-1
through PB-6) are considered, since these are the only ones that
apply to PB.
B.1. Evaluation against Requirement C-1
Requirement C-1 says:
C-1 NEA protocols MUST support multiple round trips between the NEA
Client and NEA Server in a single assessment.
PB-TNC meets this requirement. It allows an unlimited number of
round trips between the NEA Client and NEA Server.
B.2. Evaluation against Requirement C-2
Requirement C-2 says:
C-2 NEA protocols SHOULD provide a way for both the NEA Client and
the NEA Server to initiate a posture assessment or reassessment
as needed.
PB-TNC meets this requirement. Either the NEA Client or the NEA
Server can initiate a posture assessment or reassessment.
There is one limitation on this support. If a NEA Server wishes to
initiate a reassessment after it has sent a RESULT batch, it must
close the underlying transport session and initiate a new assessment.
For half-duplex transports, this is unavoidable unless a constant
exchange of messages is maintained, which would be very wasteful.
For full-duplex transports, it would be possible to allow the Posture
Broker Server to send an SRETRY batch even in the Decided state. If
the NEA working group reaches consensus that this change should be
made, it will be.
B.3. Evaluation against Requirement C-3
Requirement C-3 says:
C-3 NEA protocols including security capabilities MUST be capable
of protecting against active and passive attacks by
intermediaries and endpoints including prevention from replay-
based attacks.
Sahita, et al. Standards Track PAGE 70
RFC 5793 PB-TNC March 2010
PB-TNC does not include any security capabilities. It depends on PT
to supply a secure transport. This addresses all the necessary
threats without adding an extra layer of security. Since this
requirement only applies to NEA protocols that include security
capabilities, PB-TNC meets this requirement.
B.4. Evaluation against Requirement C-4
Requirement C-4 says:
C-4 The PA and PB protocols MUST be capable of operating over any
PT protocol. For example, the PB protocol must provide a
transport-independent interface allowing the PA protocol to
operate without change across a variety of network protocol
environments (e.g., EAP/802.1X, PANA, TLS, and IKE/IPsec).
PB-TNC meets this requirement. PB-TNC can operate over any PT
protocol that meets the requirements for PT stated in the NEA
Requirements document. Also, PB-TNC insulates the PA protocol from
any specifics of the PT protocol. With PB-TNC, all PT protocols are
equivalent from the perspective of the PA protocol.
B.5. Evaluation against Requirement C-5
Requirement C-5 says:
C-5 The selection process for NEA protocols MUST evaluate and
prefer the reuse of existing open standards that meet the
requirements before defining new ones. The goal of NEA is not
to create additional alternative protocols where acceptable
solutions already exist.
Based on this requirement, PB-TNC should receive a strong preference.
PB-TNC is equivalent with IF-TNCCS 2.0, an open TCG specification.
IF-TNCCS 2.0 is an extension of the existing IF-TNCCS 1.X protocols,
which have been implemented by dozens of vendors and open source
projects.
B.6. Evaluation against Requirement C-6
Requirement C-6 says:
C-6 NEA protocols MUST be highly scalable; the protocols MUST
support many Posture Collectors on a large number of NEA
Clients to be assessed by numerous Posture Validators residing
on multiple NEA Servers.
Sahita, et al. Standards Track PAGE 71
RFC 5793 PB-TNC March 2010
PB-TNC meets this requirement. PB-TNC supports up to 2^16-1 Posture
Collectors and an equal number of Posture Validators in a given PB-
TNC session. It also supports an unlimited number of NEA Clients and
NEA Servers.
The scalability of PB-TNC extends into other areas as well. For
example, PB-TNC supports an unlimited number of batches and each
batch can contain up to 2^32-1 octets and about 2^24 PA messages.
Each PA message can contain up to 2^32-1 octets. Of course, sending
this much data in a NEA assessment is not generally advisable, but
the point is that PB-TNC is highly scalable.
B.7. Evaluation against Requirement C-7
Requirement C-7 says:
C-7 The protocols MUST support efficient transport of a large
number of attribute messages between the NEA Client and the NEA
Server.
PB-TNC meets this requirement. Each PB-TNC batch can contain about
2^24 PA messages. Since PB-TNC supports an unlimited number of
batches in a session, this number is actually unlimited (except
perhaps by PT protocols, user patience, or other external factors).
As for efficiency, PB-TNC adds only 24 octets of overhead per PA
message. PA-TNC can include many attributes in a single PA message
so this overhead is diluted further.
B.8. Evaluation against Requirement C-8
Requirement C-8 says:
C-8 NEA protocols MUST operate efficiently over low bandwidth or
high latency links.
PB-TNC meets this requirement. A minimal PB-TNC exchange can be as
small as 72 octets and one round trip. Even if privacy policies or
other factors require multiple round trips, PB-TNC generally imposes
an overhead of only 8 octets per batch and 24 octets per PA message.
B.9. Evaluation against Requirement C-9
Requirement C-9 says:
C-9 For any strings intended for display to a user, the protocols
MUST support adapting these strings to the user's language
preferences.
Sahita, et al. Standards Track PAGE 72
RFC 5793 PB-TNC March 2010
PB-TNC meets this requirement. It defines a standard way for
the NEA Client and NEA Server to send their language
preferences to each other, leveraging the widely implemented
Accept-Language format defined in RFC 3282.
B.10. Evaluation against Requirement C-10
Requirement C-10 says:
C-10 NEA protocols MUST support encoding of strings in UTF-8 format.
PB-TNC meets this requirement. All strings in the PB-TNC protocol
are encoded in UTF-8 format. This allows the protocol to support a
wide range of languages efficiently.
B.11. Evaluation against Requirement C-11
Requirement C-11 says:
C-11 Due to the potentially different transport characteristics
provided by the underlying candidate PT protocols, the NEA
Client and NEA Server MUST be capable of becoming aware of and
adapting to the limitations of the available PT protocol. For
example, some PT protocol characteristics that might impact the
operation of PA and PB include restrictions on which end can
initiate a NEA connection, maximum data size in a message or
full assessment, upper bound on number of round trips, and
ordering (duplex) of messages exchanged. The selection process
for the PT protocols MUST consider the limitations the
candidate PT protocol would impose upon the PA and PB
protocols.
PB-TNC meets this requirement. The PB-TNC protocol is designed to be
flexible enough to operate with a variety of underlying PT protocols,
including those that may have limitations on message or assessment
size, number of round trips, and duplex. Local APIs can allow
Posture Collectors and Posture Validators to discover when they are
operating in a less constrained deployment and then make use of more
verbose attributes. Similarly, Posture Collectors could choose not
to send or use smaller attributes (including assertions from previous
assessments) when faced with a very constrained network connection.
Sahita, et al. Standards Track PAGE 73
RFC 5793 PB-TNC March 2010
B.12. Evaluation against Requirement PB-1
Requirement PB-1 says:
PB-1 The PB protocol MUST be capable of carrying attributes from the
Posture Broker Server to the Posture Broker Client. This
enables the Posture Broker Client to learn the posture
assessment decision and if appropriate to aid in remediation
and notification of the endpoint owner.
PB-TNC meets this requirement. It can carry attributes from the
Posture Broker Client to the Posture Broker Server and back in an
unlimited number of round trips. Furthermore, PB-TNC provides
explicit attribute support for posture decision and remediation aid
notification.
B.13. Evaluation against Requirement PB-2
Requirement PB-2 says:
PB-2 The PB protocol MUST NOT interpret the contents of PA messages
being carried; i.e., the data it is carrying must be opaque to
it.
PB-TNC meets this requirement. It does not parse or interpret PA
messages in any way.
B.14. Evaluation against Requirement PB-3
Requirement PB-3 says:
PB-3 The PB protocol MUST carry unique identifiers that are used by
the Posture Brokers to route (deliver) PA messages between
Posture Collectors and Posture Validators. Such message
routing should facilitate dynamic registration or
deregistration of Posture Collectors and Validators. For
example, a dynamically registered anti-virus Posture Validator
should be able to subscribe to receive messages from its
respective anti-virus Posture Collector on NEA Clients.
PB-TNC meets this requirement. PB-TNC tags each PA message with a PA
subtype that the Posture Brokers can use to deliver the PA messages
to the proper Posture Collectors and Posture Validators. By tagging
messages according to their content, PB-TNC allows Posture Collectors
and Posture Validators to be dynamically registered and deregistered,
ensuring that each one receives the proper data. PB-TNC also
supports exclusive delivery, which allows messages to be targeted at
a particular Posture Collector or Posture Validator.
Sahita, et al. Standards Track PAGE 74
RFC 5793 PB-TNC March 2010
B.15. Evaluation against Requirement PB-4
Requirement PB-4 says:
PB-4 The PB protocol MUST be capable of supporting a half-duplex PT
protocol. However, this does not preclude PB from operating
full-duplex when running over a full-duplex PT.
PB-TNC meets this requirement. In order to insulate PA from any
differences between half-duplex and full-duplex PT protocols, PB-TNC
always operates in a half-duplex mode, regardless of the capabilities
of the PT protocol. While this could in theory slow assessments that
require many round trips or bidirectional multimedia exchanges, this
is not a problem in practice because endpoint assessments do not
typically involve multimedia or a large number of round trips.
B.16. Evaluation against Requirement PB-5
Requirement PB-5 says:
PB-5 The PB protocol MAY support authentication, integrity, and
confidentiality protection for the attribute messages it
carries between a Posture Broker Client and Posture Broker
Server. This provides security protection for a message dialog
of the groupings of attribute messages exchanged between the
Posture Broker Client and Posture Broker Server. Such
protection is orthogonal to PA protections (which are end to
end) and allows for simpler Posture Collector and Validators to
be implemented, and for consolidation of cryptographic
operations possibly improving scalability and manageability.
PB-TNC does not address this optional requirement. It leaves
security to PT (which is required to address it) and PA (which SHOULD
do so). There seems to be minimal benefit in adding a third layer of
security to the NEA protocol stack. However, if the NEA working
group determines that PB should include support for authentication,
integrity protection, and confidentiality protection, then this could
be added to PB in a similar manner to the way that the PA-TNC
security is done.
Sahita, et al. Standards Track PAGE 75
RFC 5793 PB-TNC March 2010
B.17. Evaluation against Requirement PB-6
Requirement PB-6 says:
PB-6 The PB protocol MUST support grouping of attribute messages to
optimize transport of messages and minimize round trips.
PB-TNC meets this requirement. Multiple attribute messages can be
conveyed in a single PA message. In fact, that's how PA-TNC works.
Authors' Addresses
Ravi Sahita
Intel Corporation
2200 Mission College Blvd.
Santa Clara, CA 95054 USA
EMail: Ravi.Sahita@intel.com
Steve Hanna
Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, CA 94089 USA
EMail: shanna@juniper.net
Ryan Hurst
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052 USA
EMail: Ryan.Hurst@microsoft.com
Kaushik Narayan
Cisco Systems Inc.
10 West Tasman Drive
San Jose, CA 95134 USA
EMail: kaushik@cisco.com
Sahita, et al. Standards Track PAGE 76
PB-TNC: A Posture Broker (PB) Protocol Compatible with Trusted Network Connect (TNC)
RFC TOTAL SIZE: 169140 bytes
PUBLICATION DATE: Tuesday, March 9th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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