draft-ietf-isms-tmsm-17.txt   draft-ietf-isms-tmsm-18.txt 
Network Working Group D. Harrington Network Working Group D. Harrington
Internet-Draft Huawei Technologies (USA) Internet-Draft Huawei Technologies (USA)
Updates: 3411,3412,3414,3417 J. Schoenwaelder Updates: 3411,3412,3414,3417 J. Schoenwaelder
(if approved) Jacobs University Bremen (if approved) Jacobs University Bremen
Intended status: Standards Track April 27, 2009 Intended status: Standards Track May 6, 2009
Expires: October 29, 2009 Expires: November 7, 2009
Transport Subsystem for the Simple Network Management Protocol (SNMP) Transport Subsystem for the Simple Network Management Protocol (SNMP)
draft-ietf-isms-tmsm-17 draft-ietf-isms-tmsm-18
Status of This Memo Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from IETF Standards Process. Without obtaining an adequate license from
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on October 29, 2009. This Internet-Draft will expire on November 7, 2009.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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such as SSH and TLS, using a subsystem will enable consistent design such as SSH and TLS, using a subsystem will enable consistent design
and modularity of such Transport Models. This document identifies and modularity of such Transport Models. This document identifies
and describes some key aspects that need to be considered for any and describes some key aspects that need to be considered for any
Transport Model for SNMP. Transport Model for SNMP.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. The Internet-Standard Management Framework . . . . . . . . 4 1.1. The Internet-Standard Management Framework . . . . . . . . 4
1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Conventions . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Where this Extension Fits . . . . . . . . . . . . . . . . 4 1.3. Where this Extension Fits . . . . . . . . . . . . . . . . 5
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements of a Transport Model . . . . . . . . . . . . . . 8 3. Requirements of a Transport Model . . . . . . . . . . . . . . 8
3.1. Message Security Requirements . . . . . . . . . . . . . . 8 3.1. Message Security Requirements . . . . . . . . . . . . . . 8
3.1.1. Security Protocol Requirements . . . . . . . . . . . . 8 3.1.1. Security Protocol Requirements . . . . . . . . . . . . 8
3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 8 3.2. SNMP Requirements . . . . . . . . . . . . . . . . . . . . 9
3.2.1. Architectural Modularity Requirements . . . . . . . . 9 3.2.1. Architectural Modularity Requirements . . . . . . . . 9
3.2.2. Access Control Requirements . . . . . . . . . . . . . 12 3.2.2. Access Control Requirements . . . . . . . . . . . . . 12
3.2.3. Security Parameter Passing Requirements . . . . . . . 13 3.2.3. Security Parameter Passing Requirements . . . . . . . 13
3.2.4. Separation of Authentication and Authorization . . . . 13 3.2.4. Separation of Authentication and Authorization . . . . 13
3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 14 3.3. Session Requirements . . . . . . . . . . . . . . . . . . . 14
3.3.1. No SNMP Sessions . . . . . . . . . . . . . . . . . . . 14 3.3.1. No SNMP Sessions . . . . . . . . . . . . . . . . . . . 14
3.3.2. Session Establishment Requirements . . . . . . . . . . 15 3.3.2. Session Establishment Requirements . . . . . . . . . . 15
3.3.3. Session Maintenance Requirements . . . . . . . . . . . 16 3.3.3. Session Maintenance Requirements . . . . . . . . . . . 16
3.3.4. Message security versus session security . . . . . . . 16 3.3.4. Message security versus session security . . . . . . . 16
4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 17 4. Scenario Diagrams and the Transport Subsystem . . . . . . . . 17
5. Cached Information and References . . . . . . . . . . . . . . 17 5. Cached Information and References . . . . . . . . . . . . . . 18
5.1. securityStateReference . . . . . . . . . . . . . . . . . . 18 5.1. securityStateReference . . . . . . . . . . . . . . . . . . 18
5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 18 5.2. tmStateReference . . . . . . . . . . . . . . . . . . . . . 18
5.2.1. Transport information . . . . . . . . . . . . . . . . 19 5.2.1. Transport information . . . . . . . . . . . . . . . . 19
5.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 20 5.2.2. securityName . . . . . . . . . . . . . . . . . . . . . 20
5.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 20 5.2.3. securityLevel . . . . . . . . . . . . . . . . . . . . 21
5.2.4. Session Information . . . . . . . . . . . . . . . . . 21 5.2.4. Session Information . . . . . . . . . . . . . . . . . 21
6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 21 6. Abstract Service Interfaces . . . . . . . . . . . . . . . . . 21
6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 22 6.1. sendMessage ASI . . . . . . . . . . . . . . . . . . . . . 22
6.2. Changes to RFC3411 Outgoing ASIs . . . . . . . . . . . . . 23 6.2. Changes to RFC3411 Outgoing ASIs . . . . . . . . . . . . . 23
6.2.1. Message Processing Subsystem Primitives . . . . . . . 23 6.2.1. Message Processing Subsystem Primitives . . . . . . . 23
6.2.2. Security Subsystem Primitives . . . . . . . . . . . . 24 6.2.2. Security Subsystem Primitives . . . . . . . . . . . . 24
6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 25 6.3. The receiveMessage ASI . . . . . . . . . . . . . . . . . . 25
6.4. Changes to RFC3411 Incoming ASIs . . . . . . . . . . . . . 26 6.4. Changes to RFC3411 Incoming ASIs . . . . . . . . . . . . . 26
6.4.1. Message Processing Subsystem Primitive . . . . . . . . 26 6.4.1. Message Processing Subsystem Primitive . . . . . . . . 26
6.4.2. Security Subsystem Primitive . . . . . . . . . . . . . 27 6.4.2. Security Subsystem Primitive . . . . . . . . . . . . . 27
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Non uppercased versions of the keywords should be read as in normal Non uppercased versions of the keywords should be read as in normal
English. They will usually, but not always, be used in a context English. They will usually, but not always, be used in a context
relating to compatibility with the RFC3411 architecture or the relating to compatibility with the RFC3411 architecture or the
subsystem defined here, but which might have no impact on on-the-wire subsystem defined here, but which might have no impact on on-the-wire
compatibility. These terms are used as guidance for designers of compatibility. These terms are used as guidance for designers of
proposed IETF models to make the designs compatible with RFC3411 proposed IETF models to make the designs compatible with RFC3411
subsystems and Abstract Service Interfaces (ASIs) (see section 3.2). subsystems and Abstract Service Interfaces (ASIs) (see section 3.2).
Implementers are free to implement differently. Some usages of these Implementers are free to implement differently. Some usages of these
lowercase terms are simply normal English usage. lowercase terms are simply normal English usage.
This document discusses an extension to the modular RFC3411
architecture; this is not a protocol document. An architectural MUST
is a really sharp constraint and to allow for the evolution of
technology, and to not unnecessarily constrain future models, often a
SHOULD or a should is more appropriate than a MUST in an architecure.
Future models MAY express tighter requirements for their own model-
specific processing.
For consistency with SNMP-related specifications, this document For consistency with SNMP-related specifications, this document
favors terminology as defined in STD62 rather than favoring favors terminology as defined in STD62 rather than favoring
terminology that is consistent with non-SNMP specifications that use terminology that is consistent with non-SNMP specifications that use
different variations of the same terminology. This is consistent different variations of the same terminology. This is consistent
with the IESG decision to not require the SNMPv3 terminology be with the IESG decision to not require the SNMPv3 terminology be
modified to match the usage of other non-SNMP specifications when modified to match the usage of other non-SNMP specifications when
SNMPv3 was advanced to Full Standard. SNMPv3 was advanced to Full Standard.
1.3. Where this Extension Fits 1.3. Where this Extension Fits
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part except identifying requirements and verifying the quality of part except identifying requirements and verifying the quality of
service being provided. service being provided.
The User-based Security Model (USM) as defined in [RFC3414] largely The User-based Security Model (USM) as defined in [RFC3414] largely
uses the first approach - it provides its own security. It utilizes uses the first approach - it provides its own security. It utilizes
existing mechanisms (e.g., SHA), but provides all the coordination. existing mechanisms (e.g., SHA), but provides all the coordination.
USM provides for the authentication of a principal, message USM provides for the authentication of a principal, message
encryption, data integrity checking, timeliness checking, etc. encryption, data integrity checking, timeliness checking, etc.
USM was designed to be independent of other existing security USM was designed to be independent of other existing security
infrastructures. USM therefore requires a separate principal and key infrastructures. USM therefore uses a separate principal and key
management infrastructure. Operators have reported that deploying management infrastructure. Operators have reported that deploying
another principal and key management infrastructure in order to use another principal and key management infrastructure in order to use
SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use SNMPv3 is a deterrent to deploying SNMPv3. It is possible to use
external mechanisms to handle the distribution of keys for use by external mechanisms to handle the distribution of keys for use by
USM. The more important issue is that operators wanted to leverage USM. The more important issue is that operators wanted to leverage
existing user base infrastructures that were not specific to SNMP. existing user base infrastructures that were not specific to SNMP.
A USM-compliant architecture might combine the authentication A USM-compliant architecture might combine the authentication
mechanism with an external mechanism, such as RADIUS [RFC2865] to mechanism with an external mechanism, such as RADIUS [RFC2865] to
provide the authentication service. Similarly it might be possible provide the authentication service. Similarly it might be possible
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problems through security layers at the transport layer or problems through security layers at the transport layer or
application layer, among them TLS [RFC5246], SASL [RFC4422], and SSH application layer, among them TLS [RFC5246], SASL [RFC4422], and SSH
[RFC4251] [RFC4251]
From an operational perspective, it is highly desirable to use From an operational perspective, it is highly desirable to use
security mechanisms that can unify the administrative security security mechanisms that can unify the administrative security
management for SNMPv3, command line interfaces (CLIs) and other management for SNMPv3, command line interfaces (CLIs) and other
management interfaces. The use of security services provided by management interfaces. The use of security services provided by
lower layers is the approach commonly used for the CLI, and is also lower layers is the approach commonly used for the CLI, and is also
the approach being proposed for other network management protocols, the approach being proposed for other network management protocols,
such as syslog [I-D.ietf-syslog-protocol] and NETCONF [RFC4741]. such as syslog [RFC5424] and NETCONF [RFC4741].
This document defines a Transport Subsystem extension to the RFC3411 This document defines a Transport Subsystem extension to the RFC3411
architecture based on the third approach. This extension specifies architecture based on the third approach. This extension specifies
how other lower layer protocols with common security infrastructures how other lower layer protocols with common security infrastructures
can be used underneath the SNMP protocol and the desired goal of can be used underneath the SNMP protocol and the desired goal of
unified administrative security can be met. unified administrative security can be met.
This extension allows security to be provided by an external protocol This extension allows security to be provided by an external protocol
connected to the SNMP engine through an SNMP Transport Model connected to the SNMP engine through an SNMP Transport Model
[RFC3417]. Such a Transport Model would then enable the use of [RFC3417]. Such a Transport Model would then enable the use of
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numerous problems. The advertised security characteristics of a numerous problems. The advertised security characteristics of a
protocol might depend on it being used as designed; when used in protocol might depend on it being used as designed; when used in
other ways, it might not deliver the expected security other ways, it might not deliver the expected security
characteristics. It is recommended that any proposed model include a characteristics. It is recommended that any proposed model include a
description of the applicability of the Transport Model. description of the applicability of the Transport Model.
A Transport Model SHOULD NOT require modifications to the underlying A Transport Model SHOULD NOT require modifications to the underlying
protocol. Modifying the protocol might change its security protocol. Modifying the protocol might change its security
characteristics in ways that could impact other existing usages. If characteristics in ways that could impact other existing usages. If
a change is necessary, the change SHOULD be an extension that has no a change is necessary, the change SHOULD be an extension that has no
impact on the existing usages. Any Transport Model SHOULD include a impact on the existing usages. Any Transport Model should include a
description of potential impact on other usages of the protocol. description of potential impact on other usages of the protocol.
Since multiple transport models can exist simultaneously within the Since multiple transport models can exist simultaneously within the
transport subsystem, transport models MUST be able to coexist with transport subsystem, transport models MUST be able to coexist with
each other. each other.
3.2. SNMP Requirements 3.2. SNMP Requirements
3.2.1. Architectural Modularity Requirements 3.2.1. Architectural Modularity Requirements
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despite the fact there are multiple transport mappings already despite the fact there are multiple transport mappings already
defined for SNMP [RFC3417]. This document describes a Transport defined for SNMP [RFC3417]. This document describes a Transport
Subsystem that is compatible with the RFC3411 architecture. As work Subsystem that is compatible with the RFC3411 architecture. As work
is being done to use secure transports such as SSH and TLS, using a is being done to use secure transports such as SSH and TLS, using a
subsystem will enable consistent design and modularity of such subsystem will enable consistent design and modularity of such
Transport Models. Transport Models.
The design of this Transport Subsystem accepts the goals of the The design of this Transport Subsystem accepts the goals of the
RFC3411 architecture defined in section 1.5 of [RFC3411]. This RFC3411 architecture defined in section 1.5 of [RFC3411]. This
Transport Subsystem uses a modular design that permits Transport Transport Subsystem uses a modular design that permits Transport
Models (which may or may not be security-aware) to be "plugged into" Models (which might or might not be security-aware) to be "plugged
the RFC3411 architecture. Such Transport Models would be independent into" the RFC3411 architecture. Such Transport Models would be
of other modular SNMP components as much as possible. This design independent of other modular SNMP components as much as possible.
also permits Transport Models to be advanced through the standards This design also permits Transport Models to be advanced through the
process independently of other Transport Models. standards process independently of other Transport Models.
To encourage a basic level of interoperability, any Transport Model
SHOULD define one mandatory-to-implement security mechanism, but
should also be able to support additional existing and new
mechanisms.
The following diagram depicts the SNMPv3 architecture including the The following diagram depicts the SNMPv3 architecture including the
new Transport Subsystem defined in this document, and a new Transport new Transport Subsystem defined in this document, and a new Transport
Security Model defined in [I-D.ietf-isms-transport-security-model]. Security Model defined in [I-D.ietf-isms-transport-security-model].
+------------------------------+ +------------------------------+
| Network | | Network |
+------------------------------+ +------------------------------+
^ ^ ^ ^ ^ ^
| | | | | |
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perform similar security functions within the Transport Subsystem, perform similar security functions within the Transport Subsystem,
including the translation of transport security parameters to/from including the translation of transport security parameters to/from
security-model-independent parameters. security-model-independent parameters.
To accommodate this, an implementation-specific cache of transport- To accommodate this, an implementation-specific cache of transport-
specific information will be described (not shown), and the data specific information will be described (not shown), and the data
flows on this path will be extended to pass security-model- flows on this path will be extended to pass security-model-
independent values. This document amends some of the ASIs defined in independent values. This document amends some of the ASIs defined in
RFC 3411, and these changes are covered in section 6. RFC 3411, and these changes are covered in section 6.
New Security Models may be defined that understand how to work with New Security Models might be defined that understand how to work with
these modified ASIs and the transport-information cache. One such these modified ASIs and the transport-information cache. One such
Security Model, the Transport Security Model, is defined in Security Model, the Transport Security Model, is defined in
[I-D.ietf-isms-transport-security-model]. [I-D.ietf-isms-transport-security-model].
3.2.1.2. Changes to RFC3411 processing 3.2.1.2. Changes to RFC3411 processing
The introduction of secure transports affects the responsibilities The introduction of secure transports affects the responsibilities
and order of processing within the RFC3411 architecture. While the and order of processing within the RFC3411 architecture. While the
steps are the same, they may occur in a different order, and may be steps are the same, they might occur in a different order, and might
done by different subsystems. With the existing RFC3411 be done by different subsystems. With the existing RFC3411
architecture, security processing starts when the Message Processing architecture, security processing starts when the Message Processing
Model decodes portions of the encoded message to extract parameters Model decodes portions of the encoded message to extract parameters
that identify which Security Model should handle the security-related that identify which Security Model MUST handle the security-related
tasks. tasks.
A secure transport performs those security functions on the message, A secure transport performs those security functions on the message,
before the message is decoded. Some of these functions might then be before the message is decoded. Some of these functions might then be
repeated by the selected Security Model. repeated by the selected Security Model.
3.2.1.3. Passing Information between SNMP Engines 3.2.1.3. Passing Information between SNMP Engines
A secure Transport Model will establish an authenticated and possibly A secure Transport Model will establish an authenticated and possibly
encrypted tunnel between the Transport Models of two SNMP engines. encrypted tunnel between the Transport Models of two SNMP engines.
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mechanism-specific identity, and this mapping must be done for mechanism-specific identity, and this mapping must be done for
incoming messages by the Security Model before it passes securityName incoming messages by the Security Model before it passes securityName
to the Message Processing Model via the processIncoming ASI. to the Message Processing Model via the processIncoming ASI.
A Security Model is also responsible to specify, via the A Security Model is also responsible to specify, via the
securityLevel parameter, whether incoming messages have been securityLevel parameter, whether incoming messages have been
authenticated and encrypted, and to ensure that outgoing messages are authenticated and encrypted, and to ensure that outgoing messages are
authenticated and encrypted based on the value of securityLevel. authenticated and encrypted based on the value of securityLevel.
A Transport Model MAY provide suggested values for securityName and A Transport Model MAY provide suggested values for securityName and
securityLevel. A Security Model may have multiple sources for securityLevel. A Security Model might have multiple sources for
determining the principal and desired security services, and a determining the principal and desired security services, and a
particular Security Model may or may not utilize the values proposed particular Security Model might or might not utilize the values
by a Transport Model when deciding the value of securityName and proposed by a Transport Model when deciding the value of securityName
securityLevel. and securityLevel.
Documents defining a new transport domain MUST define a prefix that Documents defining a new transport domain MUST define a prefix that
MAY be prepended by the Security Model to all passed securityNames. MAY be prepended to all securityNames passed by the Security Model.
The prefix MUST include from one to four ASCII characters, not The prefix MUST include from one to four US-ASCII alpha-numeric
including a ":" (ASCII 0x3a) character. If a prefix is used, a characters, not including a ":" (US-ASCII 0x3a) character. If a
securityName is constructed by concatenating the prefix and a ":" prefix is used, a securityName is constructed by concatenating the
(ASCII 0x3a) character followed by a non-empty identity in an prefix and a ":" (US-ASCII 0x3a) character followed by a non-empty
snmpAdminString compatible format. Transport domains and their identity in an snmpAdminString compatible format. The prefix can be
used by SNMP applications to distinguish "alice" authenticated by SSH
from "alice" authenticated by TLS. Transport domains and their
corresponding prefixes are coordinated via the IANA registry "SNMP corresponding prefixes are coordinated via the IANA registry "SNMP
Transport Domains". Transport Domains".
3.2.3. Security Parameter Passing Requirements 3.2.3. Security Parameter Passing Requirements
A Message Processing Model might unpack SNMP-specific security A Message Processing Model might unpack SNMP-specific security
parameters from an incoming message before calling a specific parameters from an incoming message before calling a specific
Security Model to handle the security-related processing of the Security Model to handle the security-related processing of the
message. When using a secure Transport Model, some security message. When using a secure Transport Model, some security
parameters might be extracted from the transport layer by the parameters might be extracted from the transport layer by the
Transport Model before the message is passed to the Message Transport Model before the message is passed to the Message
Processing Subsystem. Processing Subsystem.
This document describes a cache mechanism (see Section 5), into which This document describes a cache mechanism (see Section 5), into which
the Transport Model puts information about the transport and security the Transport Model puts information about the transport and security
parameters applied to a transport connection or an incoming message, parameters applied to a transport connection or an incoming message,
and a Security Model may extract that information from the cache. A and a Security Model might extract that information from the cache.
tmStateReference is passed as an extra parameter in the ASIs between A tmStateReference is passed as an extra parameter in the ASIs
the Transport Subsystem, the Message Processing and Security between the Transport Subsystem, the Message Processing and Security
Subsystems, to identify the relevant cache. This approach of passing Subsystems, to identify the relevant cache. This approach of passing
a model-independent reference is consistent with the a model-independent reference is consistent with the
securityStateReference cache already being passed around in the securityStateReference cache already being passed around in the
RFC3411 ASIs. RFC3411 ASIs.
3.2.4. Separation of Authentication and Authorization 3.2.4. Separation of Authentication and Authorization
The RFC3411 architecture defines a separation of authentication and The RFC3411 architecture defines a separation of authentication and
the authorization to access and/or modify MIB data. A set of model- the authorization to access and/or modify MIB data. A set of model-
independent parameters (securityModel, securityName, and independent parameters (securityModel, securityName, and
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To reduce redundancy, this document describes aspects that are To reduce redundancy, this document describes aspects that are
expected to be common to all Transport Model sessions. expected to be common to all Transport Model sessions.
3.3.1. No SNMP Sessions 3.3.1. No SNMP Sessions
The architecture defined in [RFC3411] and the Transport Subsystem The architecture defined in [RFC3411] and the Transport Subsystem
defined in this document do not support SNMP sessions or include a defined in this document do not support SNMP sessions or include a
session selector in the Abstract Service Interfaces. session selector in the Abstract Service Interfaces.
The Transport Subsystem may support transport sessions. However, the The Transport Subsystem might support transport sessions. However,
transport subsystem does not have access to the pduType (i.e., the the transport subsystem does not have access to the pduType (i.e.,
SNMP operation type), so cannot select a given transport session for the SNMP operation type), so cannot select a given transport session
particular types of traffic. for particular types of traffic.
Certain parameters of the Abstract Service Interfaces might be used Certain parameters of the Abstract Service Interfaces might be used
to guide the selection of an appropriate transport session to use for to guide the selection of an appropriate transport session to use for
a given request by an application. a given request by an application.
The transportDomain and transportAddress identify the transport The transportDomain and transportAddress identify the transport
connection to a remote network node. Elements of the transport connection to a remote network node. Elements of the transport
address (such as the port number) might be used by an application to address (such as the port number) might be used by an application to
send a particular PDU type to a particular transport address. For send a particular PDU type to a particular transport address. For
example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are example, the SNMP-TARGET-MIB and SNMP-NOTIFICATION-MIB [RFC3413] are
used to configure notification originators with the destination port used to configure notification originators with the destination port
to which SNMPv2-Trap PDUs or Inform PDUs should be sent, but the to which SNMPv2-Trap PDUs or Inform PDUs are to be sent, but the
transport subsystem never looks inside the PDU. transport subsystem never looks inside the PDU.
The securityName identifies which security principal to communicate The securityName identifies which security principal to communicate
with at that address (e.g., different NMS applications), and the with at that address (e.g., different NMS applications), and the
securityLevel might permit selection of different sets of security securityLevel might permit selection of different sets of security
properties for different purposes (e.g., encrypted SETs vs. non- properties for different purposes (e.g., encrypted SETs vs. non-
encrypted GETs). encrypted GETs).
However, because the handling of transport sessions is specific to However, because the handling of transport sessions is specific to
each transport model, some transport models MAY restrict selecting a each transport model, some transport models MAY restrict selecting a
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Implementations SHOULD be able to maintain some reasonable number of Implementations SHOULD be able to maintain some reasonable number of
concurrent transport sessions, and MAY provide non-standard internal concurrent transport sessions, and MAY provide non-standard internal
mechanisms to select transport sessions. mechanisms to select transport sessions.
3.3.2. Session Establishment Requirements 3.3.2. Session Establishment Requirements
SNMP applications provide the transportDomain, transportAddress, SNMP applications provide the transportDomain, transportAddress,
securityName, and securityLevel to be used to create a new session. securityName, and securityLevel to be used to create a new session.
If the Transport Model cannot provide at least the requested level of If the Transport Model cannot provide at least the requested level of
security, the Transport Model SHOULD discard the message and SHOULD security, the Transport Model should discard the message and should
notify the dispatcher that establishing a session and sending the notify the dispatcher that establishing a session and sending the
message failed. Similarly, if the session cannot be established, message failed. Similarly, if the session cannot be established,
then the message SHOULD be discarded and the dispatcher notified. then the message should be discarded and the dispatcher notified.
Transport session establishment might require provisioning Transport session establishment might require provisioning
authentication credentials at an engine, either statically or authentication credentials at an engine, either statically or
dynamically. How this is done is dependent on the transport model dynamically. How this is done is dependent on the transport model
and the implementation. and the implementation.
3.3.3. Session Maintenance Requirements 3.3.3. Session Maintenance Requirements
A Transport Model can tear down sessions as needed. It might be A Transport Model can tear down sessions as needed. It might be
necessary for some implementations to tear down sessions as the necessary for some implementations to tear down sessions as the
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an implementation determines that an operation has completed is an implementation determines that an operation has completed is
implementation-dependent. While it is possible to tear down each implementation-dependent. While it is possible to tear down each
transport session after processing for each message has completed, transport session after processing for each message has completed,
this is not recommended for performance reasons. this is not recommended for performance reasons.
The elements of procedure describe when cached information can be The elements of procedure describe when cached information can be
discarded, and the timing of cache cleanup might have security discarded, and the timing of cache cleanup might have security
implications, but cache memory management is an implementation issue. implications, but cache memory management is an implementation issue.
If a Transport Model defines MIB module objects to maintain session If a Transport Model defines MIB module objects to maintain session
state information, then the Transport Model MUST define what SHOULD state information, then the Transport Model MUST define what happens
happen to the objects when a related session is torn down, since this to the objects when a related session is torn down, since this will
will impact interoperability of the MIB module. impact interoperability of the MIB module.
3.3.4. Message security versus session security 3.3.4. Message security versus session security
A Transport Model session is associated with state information that A Transport Model session is associated with state information that
is maintained for its lifetime. This state information allows for is maintained for its lifetime. This state information allows for
the application of various security services to multiple messages. the application of various security services to multiple messages.
Cryptographic keys associated with the transport session SHOULD be Cryptographic keys associated with the transport session SHOULD be
used to provide authentication, integrity checking, and encryption used to provide authentication, integrity checking, and encryption
services, as needed, for data that is communicated during the services, as needed, for data that is communicated during the
session. The cryptographic protocols used to establish keys for a session. The cryptographic protocols used to establish keys for a
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might add significant overhead to processing of the messages. might add significant overhead to processing of the messages.
Some Transport Models might support only specific authentication and Some Transport Models might support only specific authentication and
encryption services, such as requiring all messages to be carried encryption services, such as requiring all messages to be carried
using both authentication and encryption, regardless of the security using both authentication and encryption, regardless of the security
level requested by an SNMP application. A Transport Model MAY level requested by an SNMP application. A Transport Model MAY
upgrade the security level requested by a transport-aware security upgrade the security level requested by a transport-aware security
model, i.e. noAuthNoPriv and authNoPriv might be sent over an model, i.e. noAuthNoPriv and authNoPriv might be sent over an
authenticated and encrypted session. A Transport Model MUST NOT authenticated and encrypted session. A Transport Model MUST NOT
downgrade the security level requested by a transport-aware security downgrade the security level requested by a transport-aware security
model, and SHOULD discard any message where this would occur. model, and SHOULD discard any message where this would occur. This
is a SHOULD rather than a MUST only to permit the potential
development of models that can perform error-handling in a manner
that is less severe than discarding the message. However, any model
that does not discard the message in this circumstance should have a
clear justification for why not discarding will not create a security
vulnerability.
4. Scenario Diagrams and the Transport Subsystem 4. Scenario Diagrams and the Transport Subsystem
RFC3411 section 4.6.1 and 4.6.2 provide scenario diagrams to RFC3411 section 4.6.1 and 4.6.2 provide scenario diagrams to
illustrate how an outgoing message is created, and how an incoming illustrate how an outgoing message is created, and how an incoming
message is processed. RFC3411 does not define ASIs for the "Send message is processed. RFC3411 does not define ASIs for the "Send
SNMP Request Message to Network", "Receive SNMP Response Message from SNMP Request Message to Network", "Receive SNMP Response Message from
Network", "Receive SNMP Message from Network" and "Send SNMP message Network", "Receive SNMP Message from Network" and "Send SNMP message
to Network" arrows in these diagrams. to Network" arrows in these diagrams.
This document defines two ASIs corresponding to these arrows: a This document defines two ASIs corresponding to these arrows: a
sendMessage ASI to send SNMP messages to the network, and a sendMessage ASI to send SNMP messages to the network, and a
receiveMessage ASI to receive SNMP messages from the network. These receiveMessage ASI to receive SNMP messages from the network. These
ASIs are used for all SNMP messages, regardless of pduType. ASIs are used for all SNMP messages, regardless of pduType.
5. Cached Information and References 5. Cached Information and References
When performing SNMP processing, there are two levels of state When performing SNMP processing, there are two levels of state
information that may need to be retained: the immediate state linking information that might need to be retained: the immediate state
a request-response pair, and potentially longer-term state relating linking a request-response pair, and potentially longer-term state
to transport and security. relating to transport and security.
The RFC3411 architecture uses caches to maintain the short-term The RFC3411 architecture uses caches to maintain the short-term
message state, and uses references in the ASIs to pass this message state, and uses references in the ASIs to pass this
information between subsystems. information between subsystems.
This document defines the requirements for a cache to handle This document defines the requirements for a cache to handle
additional short-term message state and longer-term transport state additional short-term message state and longer-term transport state
information, using a tmStateReference parameter to pass this information, using a tmStateReference parameter to pass this
information between subsystems. information between subsystems.
To simplify the elements of procedure, the release of state To simplify the elements of procedure, the release of state
information is not always explicitly specified. As a general rule, information is not always explicitly specified. As a general rule,
if state information is available when a message being processed gets if state information is available when a message being processed gets
discarded, the state related to that message SHOULD also be discarded, the state related to that message should also be
discarded. If state information is available when a relationship discarded. If state information is available when a relationship
between engines is severed, such as the closing of a transport between engines is severed, such as the closing of a transport
session, the state information for that relationship SHOULD also be session, the state information for that relationship should also be
discarded. discarded.
Since the contents of a cache are meaningful only within an Since the contents of a cache are meaningful only within an
implementation, and not on-the-wire, the format of the cache is implementation, and not on-the-wire, the format of the cache is
implementation-specific. implementation-specific.
5.1. securityStateReference 5.1. securityStateReference
The securityStateReference parameter is defined in RFC3411. Its The securityStateReference parameter is defined in RFC3411. Its
primary purpose is to provide a mapping between a request and the primary purpose is to provide a mapping between a request and the
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For each transport session, information about the transport security For each transport session, information about the transport security
is stored in a tmState cache or datastore, that is referenced by a is stored in a tmState cache or datastore, that is referenced by a
tmStateReference. The tmStateReference parameter is used to pass tmStateReference. The tmStateReference parameter is used to pass
model-specific and mechanism-specific parameters between the model-specific and mechanism-specific parameters between the
Transport subsystem and transport-aware Security Models. Transport subsystem and transport-aware Security Models.
In general, when necessary, the tmState is populated by the security In general, when necessary, the tmState is populated by the security
model for outgoing messages and by the transport model for incoming model for outgoing messages and by the transport model for incoming
messages. However, in both cases, the model populating the tmState messages. However, in both cases, the model populating the tmState
may have incomplete information, and the missing information might be might have incomplete information, and the missing information might
populated by the other model when the information becomes available. be populated by the other model when the information becomes
available.
The tmState might contain both long-term and short-term information. The tmState might contain both long-term and short-term information.
The session information typically remains valid for the duration of The session information typically remains valid for the duration of
the transport session, might be used for several messages, and might the transport session, might be used for several messages, and might
be stored in a local configuration datastore. Some information has a be stored in a local configuration datastore. Some information has a
shorter lifespan, such as tmSameSecurity and tmRequestedSecurityLevel shorter lifespan, such as tmSameSecurity and tmRequestedSecurityLevel
which are associated with a specific message. which are associated with a specific message.
Since this cache is only used within an implementation, and not on- Since this cache is only used within an implementation, and not on-
the-wire, the precise contents and format of the cache are the-wire, the precise contents and format of the cache are
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representing this transport identity. This value is transport- representing this transport identity. This value is transport-
and implementation-specific, and is only used (directly) by the and implementation-specific, and is only used (directly) by the
Transport and Security Models. Transport and Security Models.
o securityName: a human-readable name (in snmpAdminString format) o securityName: a human-readable name (in snmpAdminString format)
representing the SNMP principal in a model-independent manner. representing the SNMP principal in a model-independent manner.
This value is used directly by SNMP Applications, the access This value is used directly by SNMP Applications, the access
control subsystem, the message processing subsystem, and the control subsystem, the message processing subsystem, and the
security subsystem. security subsystem.
The transport principal may or may not be the same as the The transport principal might or might not be the same as the
tmSecurityName. Similarly, the tmSecurityName may or may not be the tmSecurityName. Similarly, the tmSecurityName might or might not be
same as the securityName as seen by the Application and Access the same as the securityName as seen by the Application and Access
Control subsystems. In particular, a non-transport-aware Security Control subsystems. In particular, a non-transport-aware Security
Model will ignore tmSecurityName completely when determining the SNMP Model will ignore tmSecurityName completely when determining the SNMP
securityName. securityName.
However it is important that the mapping between the transport However it is important that the mapping between the transport
principal and the SNMP securityName (for transport-aware Security principal and the SNMP securityName (for transport-aware Security
Models) is consistent and predictable, to allow configuration of Models) is consistent and predictable, to allow configuration of
suitable access control and the establishment of transport suitable access control and the establishment of transport
connections. connections.
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this should be a SHOULD architecturally, and it is a security-model- this should be a SHOULD architecturally, and it is a security-model-
specific decision whether to REQUIRE this. specific decision whether to REQUIRE this.
o tmSameSecurity: this flag is used by a transport-aware Security o tmSameSecurity: this flag is used by a transport-aware Security
Model to indicate whether the Transport Model MUST enforce this Model to indicate whether the Transport Model MUST enforce this
restriction. restriction.
o tmSessionID: in order to verify whether the session has changed, o tmSessionID: in order to verify whether the session has changed,
the Transport Model must be able to compare the session used to the Transport Model must be able to compare the session used to
receive the original request with the one to be used to send the receive the original request with the one to be used to send the
response. This typically requires some form of session response. This typically needs some form of session identifier.
identifier. This value is only ever used by the Transport Model, This value is only ever used by the Transport Model, so the format
so the format and interpretation of this field are model-specific and interpretation of this field are model-specific and
and implementation-dependent. implementation-dependent.
When processing an outgoing message, if tmSameSecurity is true, then When processing an outgoing message, if tmSameSecurity is true, then
the tmSessionID MUST match the current transport session, otherwise the tmSessionID MUST match the current transport session, otherwise
the message MUST be discarded, and the dispatcher notified that the message MUST be discarded, and the dispatcher notified that
sending the message failed. sending the message failed.
6. Abstract Service Interfaces 6. Abstract Service Interfaces
Abstract service interfaces have been defined by RFC 3411 to describe Abstract service interfaces have been defined by RFC 3411 to describe
the conceptual data flows between the various subsystems within an the conceptual data flows between the various subsystems within an
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1) The release of state information is not always explicitly 1) The release of state information is not always explicitly
specified in a transport model. As a general rule, if state specified in a transport model. As a general rule, if state
information is available when a message gets discarded, the message- information is available when a message gets discarded, the message-
state information should also be released, and if state information state information should also be released, and if state information
is available when a session is closed, the session state information is available when a session is closed, the session state information
should also be released. Keeping sensitive security information should also be released. Keeping sensitive security information
longer than necessary might introduce potential vulnerabilities to an longer than necessary might introduce potential vulnerabilities to an
implementation. implementation.
2)An error indication in statusInformation will typically include the 2)An error indication in statusInformation will typically include the
OID and value for an incremented error counter. This may be OID and value for an incremented error counter. This might be
accompanied by values for contextEngineID and contextName for this accompanied by values for contextEngineID and contextName for this
counter, a value for securityLevel, and the appropriate state counter, a value for securityLevel, and the appropriate state
reference if the information is available at the point where the reference if the information is available at the point where the
error is detected. error is detected.
6.1. sendMessage ASI 6.1. sendMessage ASI
The sendMessage ASI is used to pass a message from the Dispatcher to The sendMessage ASI is used to pass a message from the Dispatcher to
the appropriate Transport Model for sending. In the diagram in the appropriate Transport Model for sending. In the diagram in
section 4.6.1 of RFC 3411, the sendMessage ASI defined in this section 4.6.1 of RFC 3411, the sendMessage ASI defined in this
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In section 4.6.2, the sendMessage ASI replaces the text "Send SNMP In section 4.6.2, the sendMessage ASI replaces the text "Send SNMP
Message to Network" Message to Network"
If present and valid, the tmStateReference refers to a cache If present and valid, the tmStateReference refers to a cache
containing transport-model-specific parameters for the transport and containing transport-model-specific parameters for the transport and
transport security. How a tmStateReference is determined to be transport security. How a tmStateReference is determined to be
present and valid is implementation-dependent. How the information present and valid is implementation-dependent. How the information
in the cache is used is transport-model-dependent and implementation- in the cache is used is transport-model-dependent and implementation-
dependent. dependent.
This may sound underspecified, but a transport model might be This might sound underspecified, but a transport model might be
something like SNMP over UDP over IPv6, where no security is something like SNMP over UDP over IPv6, where no security is
provided, so it might have no mechanisms for utilizing a provided, so it might have no mechanisms for utilizing a
tmStateReference cache. tmStateReference cache.
statusInformation = statusInformation =
sendMessage( sendMessage(
IN destTransportDomain -- transport domain to be used IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length IN outgoingMessageLength -- its length
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3411, the receiveMessage ASI replaces the text "Receive SNMP Response 3411, the receiveMessage ASI replaces the text "Receive SNMP Response
Message from Network". In section 4.6.2, the receiveMessage ASI Message from Network". In section 4.6.2, the receiveMessage ASI
replaces the text "Receive SNMP Message from Network". replaces the text "Receive SNMP Message from Network".
When a message is received on a given transport session, if a cache When a message is received on a given transport session, if a cache
does not already exist for that session, the Transport Model might does not already exist for that session, the Transport Model might
create one, referenced by tmStateReference. The contents of this create one, referenced by tmStateReference. The contents of this
cache are discussed in section 5. How this information is determined cache are discussed in section 5. How this information is determined
is implementation- and transport-model-specific. is implementation- and transport-model-specific.
"Might create one" may sound underspecified, but a transport model "Might create one" might sound underspecified, but a transport model
might be something like SNMP over UDP over IPv6, where transport might be something like SNMP over UDP over IPv6, where transport
security is not provided, so it might not create a cache. security is not provided, so it might not create a cache.
The Transport Model does not know the securityModel for an incoming The Transport Model does not know the securityModel for an incoming
message; this will be determined by the Message Processing Model in a message; this will be determined by the Message Processing Model in a
message-processing-model-dependent manner. message-processing-model-dependent manner.
statusInformation = statusInformation =
receiveMessage( receiveMessage(
IN transportDomain -- origin transport domain IN transportDomain -- origin transport domain
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IN tmStateReference -- reference to transport state IN tmStateReference -- reference to transport state
) )
6.4. Changes to RFC3411 Incoming ASIs 6.4. Changes to RFC3411 Incoming ASIs
The tmStateReference parameter has also been added to some of the The tmStateReference parameter has also been added to some of the
incoming ASIs defined in RFC3411. How or if a Message Processing incoming ASIs defined in RFC3411. How or if a Message Processing
Model or Security Model uses tmStateReference is message-processing- Model or Security Model uses tmStateReference is message-processing-
and security-model-specific. and security-model-specific.
This may sound underspecified, but a message processing model might This might sound underspecified, but a message processing model might
have access to all the information from the cache and from the have access to all the information from the cache and from the
message. The Message Processing Model might determine that the USM message. The Message Processing Model might determine that the USM
Security Model is specified in an SNMPv3 message header; the USM Security Model is specified in an SNMPv3 message header; the USM
Security Model has no need of values in the tmStateReference cache to Security Model has no need of values in the tmStateReference cache to
authenticate and secure the SNMP message, but an application might authenticate and secure the SNMP message, but an application might
have specified to use a secure transport such as that provided by the have specified to use a secure transport such as that provided by the
SSH Transport Model to send the message to its destination. SSH Transport Model to send the message to its destination.
6.4.1. Message Processing Subsystem Primitive 6.4.1. Message Processing Subsystem Primitive
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) -- information, needed for response ) -- information, needed for response
7. Security Considerations 7. Security Considerations
This document defines an architectural approach that permits SNMP to This document defines an architectural approach that permits SNMP to
utilize transport layer security services. Each proposed Transport utilize transport layer security services. Each proposed Transport
Model should discuss the security considerations of that Transport Model should discuss the security considerations of that Transport
Model. Model.
It is considered desirable by some industry segments that SNMP It is considered desirable by some industry segments that SNMP
Transport Models should utilize transport layer security that Transport Models utilize transport layer security that addresses
addresses perfect forward secrecy at least for encryption keys. perfect forward secrecy at least for encryption keys. Perfect
Perfect forward secrecy guarantees that compromise of long term forward secrecy guarantees that compromise of long term secret keys
secret keys does not result in disclosure of past session keys. Each does not result in disclosure of past session keys. Each proposed
proposed Transport Model should include a discussion in its security Transport Model should include a discussion in its security
considerations of whether perfect forward security is appropriate for considerations of whether perfect forward security is appropriate for
that Transport Model. that Transport Model.
The Denial of Service characteristics of various transport models and The Denial of Service characteristics of various transport models and
security protocols will vary and should be evaluated when determining security protocols will vary and should be evaluated when determining
the applicability of a transport model to a particular deployment the applicability of a transport model to a particular deployment
situation. situation.
Since the cache will contain security-related parameters, Since the cache will contain security-related parameters,
implementers should store this information (in memory or in implementers SHOULD store this information (in memory or in
persistent storage) in a manner to protect it from unauthorized persistent storage) in a manner to protect it from unauthorized
disclosure and/or modification. disclosure and/or modification.
Care must be taken to ensure that a SNMP engine is sending packets Care must be taken to ensure that a SNMP engine is sending packets
out over a transport using credentials that are legal for that engine out over a transport using credentials that are legal for that engine
to use on behalf of that user. Otherwise an engine that has multiple to use on behalf of that user. Otherwise an engine that has multiple
transports open might be "tricked" into sending a message through the transports open might be "tricked" into sending a message through the
wrong transport. wrong transport.
A Security Model may have multiple sources from which to define the A Security Model might have multiple sources from which to define the
securityName and securityLevel. The use of a secure Transport Model securityName and securityLevel. The use of a secure Transport Model
does not imply that the securityName and securityLevel chosen by the does not imply that the securityName and securityLevel chosen by the
Security Model represent the transport-authenticated identity or the Security Model represent the transport-authenticated identity or the
transport-provided security services. The securityModel, transport-provided security services. The securityModel,
securityName, and securityLevel parameters are a related set, and an securityName, and securityLevel parameters are a related set, and an
administrator should understand how the specified securityModel administrator should understand how the specified securityModel
selects the corresponding securityName and securityLevel. selects the corresponding securityName and securityLevel.
7.1. Coexistence, Security Parameters, and Access Control 7.1. Coexistence, Security Parameters, and Access Control
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RFC3412), regardless of the transport mapping or transport model RFC3412), regardless of the transport mapping or transport model
used. If the SNMPv3 message specifies the User-based Security used. If the SNMPv3 message specifies the User-based Security
Model (USM), with noAuthNoPriv, then the access controls will be Model (USM), with noAuthNoPriv, then the access controls will be
based on the unauthenticated USM user. based on the unauthenticated USM user.
o For outgoing messages, if a secure transport model is selected in o For outgoing messages, if a secure transport model is selected in
combination with a security model that does not populate a combination with a security model that does not populate a
tmStateReference, the secure transport model SHOULD detect the tmStateReference, the secure transport model SHOULD detect the
lack of a valid tmStateReference and fail. lack of a valid tmStateReference and fail.
However, in times of network stress, a secure transport model may not In times of network stress, a secure transport model might not work
work properly if its underlying security mechanisms (e.g., Network properly if its underlying security mechanisms (e.g., Network Time
Time Protocol (NTP) or Authentication, Authorization, and Accounting Protocol (NTP) or Authentication, Authorization, and Accounting (AAA)
(AAA) protocols or certificate authorities) are not reachable. The protocols or certificate authorities) are not reachable. The User-
User-based Security Model was explicitly designed to not depend upon based Security Model was explicitly designed to not depend upon
external network services, and provides its own security services. external network services, and provides its own security services.
It is RECOMMENDED that operators provision authPriv USM as a fallback It is RECOMMENDED that operators provision authPriv USM as a fallback
mechanism to supplement any security model or transport model that mechanism to supplement any security model or transport model that
has external dependencies, so that secure SNMP communications can has external dependencies, so that secure SNMP communications can
continue when the external network service is not available. continue when the external network service is not available.
8. IANA Considerations 8. IANA Considerations
IANA is requested to create a new registry in the Simple Network IANA is requested to create a new registry in the Simple Network
Management Protocol (SNMP) Number Spaces. The new registry should be Management Protocol (SNMP) Number Spaces. The new registry should be
called "SNMP Transport Domains". This registry will contain ASCII called "SNMP Transport Domains". This registry will contain US-ASCII
strings of one to four characters to identify prefixes for alpha-numeric strings of one to four characters to identify prefixes
corresponding SNMP transport domains. Each transport domain requires for corresponding SNMP transport domains. Each transport domain MUST
an OID assignment under snmpDomains [RFC2578] . Values are to be have an OID assignment under snmpDomains [RFC2578] . Values are to
assigned via [RFC5226] "Specification Required". be assigned via [RFC5226] "Specification Required".
The registry should be populated with the following initial entries: The registry should be populated with the following initial entries:
Registry Name: SNMP Transport Domains Registry Name: SNMP Transport Domains
Reference: [RFC2578] [RFC3417] [XXXX] Reference: [RFC2578] [RFC3417] [XXXX]
Registration Procedures: Specification Required Registration Procedures: Specification Required
Each domain is assigned a MIB-defined OID under snmpDomains Each domain is assigned a MIB-defined OID under snmpDomains
Prefix snmpDomains Reference Prefix snmpDomains Reference
------- ----------------------------- --------- ------- ----------------------------- ---------
skipping to change at page 34, line 4 skipping to change at page 34, line 4
Alvestrand, "Guidelines for Alvestrand, "Guidelines for
Writing an IANA Writing an IANA
Considerations Section in Considerations Section in
RFCs", BCP 26, RFC 5226, RFCs", BCP 26, RFC 5226,
May 2008. May 2008.
[I-D.ietf-isms-transport-security-model] Harrington, D. and W. [I-D.ietf-isms-transport-security-model] Harrington, D. and W.
Hardaker, "Transport Hardaker, "Transport
Security Model for SNMP", d Security Model for SNMP", d
raft-ietf-isms-transport- raft-ietf-isms-transport-
security-model-12 (work in security-model-13 (work in
progress), March 2009. progress), April 2009.
[I-D.ietf-isms-secshell] Harrington, D., Salowey, [I-D.ietf-isms-secshell] Harrington, D., Salowey,
J., and W. Hardaker, J., and W. Hardaker,
"Secure Shell Transport "Secure Shell Transport
Model for SNMP", Model for SNMP",
draft-ietf-isms-secshell-15 draft-ietf-isms-secshell-16
(work in progress), (work in progress),
March 2009. April 2009.
[I-D.ietf-syslog-protocol] Gerhards, R., "The syslog [RFC5424] Gerhards, R., "The Syslog
Protocol", draft-ietf- Protocol", RFC 5424,
syslog-protocol-23 (work in March 2009.
progress), September 2007.
Appendix A. Why tmStateReference? Appendix A. Why tmStateReference?
This appendix considers why a cache-based approach was selected for This appendix considers why a cache-based approach was selected for
passing parameters. passing parameters.
There are four approaches that could be used for passing information There are four approaches that could be used for passing information
between the Transport Model and a Security Model. between the Transport Model and a Security Model.
1. one could define an ASI to supplement the existing ASIs, or 1. one could define an ASI to supplement the existing ASIs, or
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